[48] Deletion of Crtc1 leads to hippocampal neuroenergetic impairments associated with depressive-like behavior

A. Cherix; C. Poitry-Yamate; B. Lanz; O. Zanoletti; J. Grosse  et al.

[47] Hyperpolarized (1-13C)Alaninamide Is a Multifunctional In Vivo Sensor of Aminopeptidase N Activity, pH, and CO2

A. Radaelli; D. Ortiz; A. Michelotti; M. Roche; R. Hata  et al.

[46] Segmenting electroencephalography wires reduces radiofrequency shielding artifacts in simultaneous electroencephalography and functional magnetic resonance imaging at 7 T

T. P. Lê; R. Gruetter; J. Jorge; Ö. Ipek

[45] [13C]bicarbonate labelled from hyperpolarized [1-13C]pyruvate is an in vivo marker of hepatic gluconeogenesis in fasted state

E. Can; J. A. M. Bastiaansen; D.-L. Couturier; R. Gruetter; H. A. I. Yoshihara  et al.

[44] Central nervous system and systemic oxidative stress interplay with inflammation in a bile duct ligation rat model of type C hepatic encephalopathy

K. Pierzchala; D. Simicic; A. Sienkiewicz; D. Sessa; S. Mitrea  et al.

[43] Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

C. C. Zanella; A. Capozzi; H. A. Yoshihara; A. Radaelli; A. L. C. Mackowiak  et al.

[42] Hyperpolarized C-13-glucose magnetic resonance highlights reduced aerobic glycolysis in vivo in infiltrative glioblastoma

M. Mishkovsky; O. Gusyatiner; B. Lanz; C. Cudalbu; I. Vassallo  et al.

[41] Excitatory/inhibitory neuronal metabolic balance in mouse hippocampus upon infusion of [U-C-13(6)]glucose

A. Cherix; G. Donati; B. Lizarbe; B. Lanz; C. Poitry-Yamate  et al.

[40] The relationship between EEG and fMRI connectomes is reproducible across simultaneous EEG-fMRI studies from 1.5T to 7T

J. Wirsich; J. Jorge; G. R. Iannotti; E. A. Shamshiri; F. Grouiller  et al.

[39] Late post‐natal neurometabolic development in healthy male rats using 1 H and 31 P Magnetic Resonance Spectroscopy

V. Račkayová; D. Simicic; G. Donati; O. Braissant; R. Gruetter  et al.

[38] Brain NAD Is Associated With ATP Energy Production and Membrane Phospholipid Turnover in Humans

B. Cuenoud; O. Ipek; M. Shevlyakova; M. Beaumont; S. C. Cunnane  et al.

[37] Magnetic resonance spectroscopy in the rodent brain: Experts' consensus recommendations

B. Lanz; A. Abaei; O. Braissant; I.-Y. Choi; C. Cudalbu  et al.

[36] C-13 Dynamic Nuclear Polarization using SA-BDPA at 6.7 T and 1.1 K: Coexistence of Pure Thermal Mixing and Well-Resolved Solid Effect

A. Radaelli; H. A. Yoshihara; H. Nonaka; S. Sando; J. H. Ardenkjaer-Larsen  et al.

[35] Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine

A. Cherix; T. Larrieu; J. Grosse; J. Rodrigues; B. McEwen  et al.

[34] Glutamine-to-glutamate ratio in the nucleus accumbens predicts effort-based motivated performance in humans

A. V. I. Strasser; G. Luksys; L. Xin; M. Pessiglione; R. Gruetter  et al.

[33] Glucose transporter 2 mediates the hypoglycemia-induced increase in cerebral blood flow

H. Lei; F. Preitner; G. Labouebe; R. Gruetter; B. Thorens

[32] Nucleus accumbens neurochemistry in human anxiety: A 7 T 1H-MRS study

A. Strasser; L. Xin; R. Gruetter; C. Sandi

[31] Multi-slice passband bSSFP for human and rodent fMRI at ultra-high field

O. Reynaud; A. R. da Silva; R. Gruetter; I. O. Jelescu

[30] Methodological consensus on clinical proton MRS of the brain: Review and recommendations

M. Wilson; O. Andronesi; P. B. Barker; R. Bartha; A. Bizzi  et al.

[29] Improved off‐resonance phase behavior using a phase‐inverted adiabatic half‐passage pulse for 13 C MRS in humans at 7 T

E. Serés Roig; L. Xin; D. Gallichan; V. Mlynarik; R. Gruetter

[28] Influence of physiological noise on accelerated 2D and 3D resting state functional MRI data at 7 T

O. Reynaud; J. Jorge; R. Gruetter; J. P. Marques; W. van der Zwaag

[27] Compartmentalised energy metabolism supporting glutamatergic neurotransmission in response to increased activity in the rat cerebral cortex: A 13C MRS study in vivo at 14.1 T

S. Sonnay; J. M. Duarte; N. Just; R. Gruetter

[26] Hyperpolarized 6Li as a probe for hemoglobin oxygenation level

R. Balzan; M. Mishkovsky; Y. Simonenko; R. B. van Heeswijk; R. Gruetter  et al.

[25] Simultaneous and interleaved acquisition of NMR signals from different nuclei with a clinical MRI scanner

M. Meyerspeer; A. W. Magill; A. Kuehne; R. Gruetter; E. Moser  et al.

[24] Glutathione deficit impairs myelin maturation: relevance for white matter integrity in schizophrenia patients

A. Monin; P. Baumann; A. Griffa; L. Xin; R. Mekle  et al.

[23] Stroking or Buzzing? A Comparison of Somatosensory Touch Stimuli Using 7 Tesla fMRI

W. van der Zwaag; R. Gruetter; R. Martuzzi

[22] Physiological noise in human cerebellar fMRI

W. Van Der Zwaag; J. Jorge; D. Butticaz; R. Gruetter

[21] Correcting surface coil excitation inhomogeneities in single-shot SPEN MRI

R. Schmidt; M. Mishkovsky; J.-N. Hyacinthe; N. Kunz; R. Gruetter  et al.

[20] In Vivo Longitudinal (1)H MRS Study of Transgenic Mouse Models of Prion Disease in the Hippocampus and Cerebellum at 14.1 T

C. Cudalbu; M. Craveiro; V. Mlynárik; J. Bremer; A. Aguzzi  et al.

[19] A Modulated Closed Form solution for Quantitative Susceptibility Mapping - A thorough evaluation and comparison to iterative methods based on edge prior knowledge

D. Khabipova; Y. Wiaux; R. Gruetter; J. P. Marques

[18] Single acquisition electrical property mapping based on relative coil sensitivities: A proof-of-concept demonstration

J. P. Marques; D. K. Sodickson; O. Ipek; C. M. Collins; R. Gruetter

[17] A double-quadrature radiofrequency coil design for proton-decoupled carbon-13 magnetic resonance spectroscopy in humans at 7T

E. Serés Roig; A. W. Magill; G. Donati; M. Meyerspeer; L. Xin  et al.

[16] Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized 13C magnetic resonance

J. Bastiaansen; T. Cheng; H. Lei; R. Gruetter; A. Comment

[15] Image-Derived Input Function from the Vena Cava for 18F-FDG PET Studies in Rats and Mice

B. Lanz; C. Poitry-Yamate; R. Gruetter

[14] Definition and quantification of acute inflammatory white matter injury in the immature brain by MRI/MRS at high magnetic field

G. A. Lodygensky; N. Kunz; E. Perroud; E. Somm; V. Mlynarik  et al.

[13] In vivo brain macromolecule signals in healthy and glioblastoma mouse models: 1H magnetic resonance spectroscopy, post-processing and metabolite quantification at 14.1 T

M. Craveiro; V. Clément-Schatlo; D. Marino; R. Gruetter; C. Cudalbu

[12] Hyperpolarized 13C lactate as a substrate for in vivo metabolic studies in skeletal muscle

J. A. M. Bastiaansen; H. A. I. Yoshihara; Y. Takado; R. Gruetter; A. Comment

[11] Non-invasive diagnostic biomarkers for estimating the onset time of permanent cerebral ischemia

C. Berthet; L. Xin; L. Buscemi; C. Benakis; R. Gruetter  et al.

[10] Optimized MEGA-SPECIAL for in vivo glutamine detection in the rat brain at 14.1 T

M. Craveiro; C. Cudalbu; V. Mlynárik; R. Gruetter

[9] Improved temporal resolution for functional studies with reduced number of segments with three-dimensional echo planar imaging

M. Narsude; W. Van Der Zwaag; T. Kober; R. Gruetter; J. P. Marques

[8] Glutamatergic and GABAergic energy metabolism measured in the rat brain by 13C NMR spectroscopy at 14.1 T

J. M. N. Duarte; R. Gruetter

[7] Metabolic Flux and Compartmentation Analysis in the Brain In vivo

B. Lanz; R. Gruetter; J. M. N. Duarte

[6] Characterization of sustained BOLD activation in the rat barrel cortex and Neurochemical consequences

N. Just; L. Xin; H. Frenkel; R. Gruetter

[5] Feasibility and electromagnetic compatibility study of the ClearPEM front-end electronics for simultaneous PET-MR imaging

D. Neves; J. André; R. Bugalho; R. Gruetter; A. Magill  et al.

[4] The C57BL/6J Mouse Exhibits Sporadic Congenital Portosystemic Shunts

C. Cudalbu; V. A. Mclin; H. Lei; J. M. N. Duarte; A.-L. Rougemont  et al.

[3] Which prior knowledge? Quantification of in vivo brain (13) C MR spectra following (13) C glucose infusion using AMARES

B. Lanz; J. M. N. Duarte; N. Kunz; V. Mlynárik; R. Gruetter  et al.

[2] Single spin-echo T 2 relaxation times of cerebral metabolites at 14.1 T in the in vivo rat brain

L. Xin; G. Gambarota; C. Cudalbu; V. Mlynárik; R. Gruetter

[1] Direct mapping of 19F in 19FDG-6P in brain tissue at subcellular resolution using soft X-ray fluorescence

C. Poitry-Yamate; A. Gianoncelli; G. Kourousias; B. Kaulich; M. Lepore  et al.

Fast Quantification of Creatine Kinase Activity by Phosphorus Magnetic Resonance Fingerprinting

M. S. Widmaier / R. Gruetter; L. Xin (Dir.)

A novel fast 3D-31P-B+1 mapping method called the frequency-selective Double-AngleMethod (fDAM) has been developed. This method integrates a 3D weighted stack of spiral gradient echo acquisitions with a frequency-selective pulse to efficiently map B+1 fields based on the phosphocreatine (PCr) signal at 7 T. The protocol has been optimized using simulations and validated through both phantom experiments and skeletal muscle applications. Results showed that fDAMnot only achieves a high correlation (r = 0.94) with the classical DAM (cDAM) but also provides faster and more extensive coverage. 3D-31P-B+1 mapping of the human calf muscle was completed in just 11min using fDAM compared to 24 minutes using cDAM. Furthermore, the first full-brain 31P 3D B+1 mapping was achieved in 13 minutes using a 1 Tx/32 Rx coil. These findings show that fDAM is able to provide B+1 mapping for accurate transmit field correction within the same scanning session of other 31P-MRS/I applications. A new magnetization transfer (MT)-31P-Magnetic resonance fingerprinting (MRF) approach was developed to measure the creatine kinase (CK) metabolic reaction rate (kCK) between PCr and adenosine triphosphate (ATP) in the human brain. This method extends the MRF framework to overcome conventional challenges in 31P-Measurement, such as long scan times and high specific absorption rates. A novel nested iteration interpolation method (NIIM) was introduced to efficiently handle the growing complexity of the multiparametric dictionary matching process. The combination ofMT-31P-MRF with NIIM allows for accurate estimations of the longitudinal relaxation times T1PCr and T1ATP, ATP/PCr concentration ratios Cr, B1, off resonance foff and kCK, showing excellent agreement with existing techniques like the exchange kinetics by band inversion transfer (EBIT) method and values reported in literature. This method demonstrated a significant reduction in scan time, from 17 minutes 4 seconds using EBIT to just 4 minutes 15 seconds with MT-31P-MRF, while maintaining similar or better reproducibility (with coefficients of variation less than 12%). Further advancing this approach, the MT-31P-MRF was extended to perform 3D CreatineKinase Imaging (CKI), delivering the first whole-brain kCK maps. Whole-brain CKI were acquired on a clinical 7T MRI scanner, generating high-resolution CK ratemaps in 49minutes 30 seconds. Further, within-session reproducibility assessment showed amean coefficient of variation below 11% and suggested that scans could be completed within 25minutes. In addition to kCK mapping, CKI provided simultaneous mapping of Cr, T1PCr values, and phosphorus-based B0 maps with a spatial resolution of 7.2 × 7.2 × 20mm3. In a proof-of-concept functional study, functional CKI (fCKI) was used to demonstrate CK activation in the brain's visual cortex in response to visual stimulation, revealing a mean 17% increase in CK reaction rates in the visual cortex. This represents the first 3D CK activation map in the human brain, providing a novel functional imaging methodology to gain insights into brain functional bioenergetics

Motor Learning and Neural Plasticity in the Aging Brain: A Multimodal Neuroimaging Approach

X. Liu / R. Gruetter; L. Xin (Dir.)

Aging leads to progressive changes in brain metabolism, structure, and function, resulting in declines in motor performance. These changes include reduced cortical inhibition, impaired brain structure, and altered functional connectivity. Consequently, older adults often experience diminished motor ability, postural instability, and increased fall risk, which can negatively affect their quality of life. However, emerging evidence shows that motor learning, such as balance and strength training, can promote neuroplasticity and help mitigate age-related decline. The neural mechanisms supporting motor ability and the brain's adaptations to motor learning remain incompletely understood, particularly regarding the interplay of neurochemical, structural, and functional changes. This thesis explores the brain mechanisms underlying motor function and the neuroplastic changes associated with motor learning in older adults. The findings are based on data from a longitudinal study in which older adults were assigned to balance training, strength training, or control groups for three months. Participants underwent multimodal neuroimaging before and after the intervention. This design allowed us to assess large-scale brain networks involved in balance and examine training-induced metabolic, structural, and functional changes. To understand the neural basis of balance, we used connectome-based predictive modeling with baseline data to identify structural and functional brain networks associated with balance performance. Our results show that both structural and functional connectomes predict balance ability. Key networks include motor-subcortical, fronto-parietal, and visual circuits, emphasizing the importance of coordinated activity across these systems in supporting postural control in older adults. We then examined how balance training drives neural plasticity. Following three months of training, participants showed improved balance, enhanced functional connectivity within sensorimotor networks, and increased cortical inhibition, evidenced by elevated GABA levels in the sensorimotor cortex. These findings suggest that balance training can counteract age-related reductions in connectivity and inhibitory function, underscoring the role of the GABAergic system in neuroplastic adaptation and motor behavior. While functional changes are important for adaptive motor control, structural changes are also essential. Our third study investigated structural plasticity following balance and strength training. We found that strength training promotes white matter reorganization in key pathways and that both training types counteract age-related structural degeneration in critical brain regions. These results highlight the capacity of motor training to induce beneficial neural plasticity at multiple levels. In conclusion, this thesis provides evidence that motor learning can drive neuroplastic changes across metabolic, structural, and functional domains in the aging brain. By integrating results from various neuroimaging techniques, this work offers a comprehensive view of the neural basis of motor function and the brain's capacity for adaptation through targeted training in older adults.

New insights into rodent brain microstructure and metabolism in hepatic encephalopathy

J. J. Mosso / R. Gruetter; C. R. Cudalbu (Dir.)

Type C hepatic encephalopathy (HE) is a severe neuropsychiatric complication of chronic liver disease, for which the prognosis is poor in the absence of liver transplantation. Cirrhosis in type C HE leads to a toxic accumulation of ammonia in the blood, which will eventually travel to the brain and adversely affect its structure and function. However, the biochemical mechanisms underpinning neurological and cognitive dysfunctions are intertwined and still incompletely understood. First, it remains unclear how brain cells morphology is affected by the ammonia-induced glutamine increase and osmotic stress in HE. In the bile-duct ligated (BDL) rat model of type C HE, alterations of neurons and astrocytes' shape have been observed ex vivo by histology, but these observations were until now not replicated in vivo. Magnetic resonance (MR) spectroscopy (MRS) at ultra-high field is a powerful tool to probe metabolism in vivo, and can, with the insertion of diffusion gradients, in addition be sensitized to probe cell-specific microstructure. In this thesis, diffusion-weighted MR spectroscopy (dMRS) and imaging (dMRI) experiments were conducted at 14.1T in the developing brain of the BDL rat model of type C HE. The acquisition was optimized to measure the cerebellum, a challenging brain region due to motion and the presence of fat, but of particular vulnerability in HE. Analysed jointly through cell-specific biophysical modelling, dMRS and dMRI probed faster metabolite diffusivities and faster intra-neurite/intra-axon water diffusivity in cerebellar white and grey matter of BDL rats compared to control rats. These observations point towards an alteration of cell density and/or of neurite network complexity and reorient the debate from the restrictive hypothesis of astrocytes swelling to the wider one of multi-cellular microstructure alterations in type C HE. The dMRS acquisition was further optimized with the implementation of a new sequence, DW-SPECIAL. The latter improved the detection and subsequent estimation of the diffusion properties of strongly J-coupled metabolites such as glutamine, of particular interest in the study of HE. A post-processing denoising technique based on the Marchenko Pastur principal component analysis method (MP-PCA) was also tested on simulated, rodent and human dMRS data. MP-PCA denoising yielded both valuable and adverse features specific to the nature of the input data, an effect for which a detailed description was provided and which should be carefully considered. Second, conflicting results on brain energy metabolism alterations in type C HE have been previously reported. Positron emission tomography (PET) is an imaging modality that enables the study of glucose uptake, following the conversion of fluorodeoxyglucose (FDG) in the first steps of the glycolysis in vivo. In this thesis, a new preclinical FDG PET methodology was implemented to compute quantitative 3D maps of the regional cerebral metabolic rate of glucose (CMRglc) from a labelling steady-state PET image of the brain and an image-derived input function. A 2-fold lower CMRglc brain glucose uptake was observed in the hippocampus and cerebellum of the BDL rats. Combined with MRS, it provided for the first time local and quantitative information on both brain glucose uptake and neurometabolic profile alterations in a ratmodel of type C HE. The quantitative approach also showed its strength when comparing groups of animals with divergent physiology.

Robust gamma-aminobutyric acid and energy metabolism measurements by proton and phosphorus magnetic resonance spectroscopy and fingerprinting

S. Lim / R. Gruetter; L. Xin (Dir.)

Magnetic resonance spectroscopy (MRS) is the only technique that can detect endogenous metabolites directly and non-invasively in vivo. It allows to identify different metabolites and analyze the dynamic neurochemical processes in the brain, skeletal muscle, and other organs. In this thesis, new acquisition methods are developed in order to improve the robustness of data acquisition and to reduce the scanning duration in human at 7T. gamma-aminobutyric acid (GABA) is a primary inhibitory neurotransmitter which plays a key role to control brain activities associated with normal brain functions and the pathophysiology of disease. However, GABA signal is overlapped with more abundant metabolites, hindering its direct measurement. At ultra-high field, even with the improved SNR and spectral dispersion, it is still challenging to quantify GABA directly using a non-editing short echo time (TE) method. MEGA-sSPECIAL sequence is introduced for improved GABA measurement in the brain. The motor and medial prefrontal cortices were selected considering different GABA concentrations between the two areas. The test-retest reproducibility of the method was compared with that of short-TE measurement with the same localization. Knowledge of relaxation time is not only important for the optimization of acquisition parameters but also for the investigation of alterations in tissue microstructure integrity. Magnetic resonance fingerprinting (MRF) is a novel technique to acquire multi-parametric and quantitative data simultaneously by using varying flip angle, repetition time, and/or TE patterns. Although it has been successfully implemented in clinical studies, it has never been adapted for 31P metabolite acquisition at 7T in the human brain. Therefore, the feasibility of MRF application to T1 and T2 of 31P metabolites measurements is explored in the thesis. Phosphocreatine, Adenosine triphosphate (ATP), and creatine kinase (CK) play key roles in intracellular energy buffering and transport. The energy is needed for various processes in organisms and living cells including intracellular signaling, DNA and RNA synthesis, and muscle contraction. 31P MRS is a unique tool to measure the CK reaction rate constant (kCK) via magnetization transfer, providing important insight into energetics in the brain, muscle, and heart. However, it suffers from low SNR and long acquisition time. In order to overcome the challenges, a novel approach is suggested to acquire 3D CK exchange rate constants map using MRF. High spatial resolution with the minimum acquisition time was achieved using a stack-of-spiral trajectory. The preliminary results obtained from the skeletal muscle are presented in this thesis. In conclusion, the MEGA-sSPECIAL sequence showed more robust GABA quantification results in both of the brain regions than the short-TE method, suggesting a capability to detect small alterations in GABA levels. Fast and simultaneous measurements of T1 and T1 relaxation times of 31P metabolites were enabled by 31P MRF by shortening the acquisition time by 1.7-fold in comparison with inversion recovery and multi-TE techniques. Lastly, the feasibility of 3D kCK mapping by 31P MRF was investigated for the first time with accelerated spatial encoding, achieving 3 min 16 s of acquisition time in the skeletal muscle, paving the way for studying energy metabolism in future clinical and research studies.

Brain microstructural and functional MRI: developments and application to a rat model of Alzheimer's disease

Y. Diao / R. Gruetter; I. O. Jelescu (Dir.)

Magnetic resonance imaging (MRI) has been a valuable tool in investigating the pathological cascade of Alzheimer's disease (AD) and its progression, which are still open questions. Although some MRI-derived hallmarks in terms of functional connectivity and white matter degeneration have been revealed, the temporal involvement and interplay between them as well as other hallmarks such as amyloid load and neuronal density remain poorly understood. On the other hand, brain glucose hypometabolism is gradually taking center stage as a key player in the onset of AD, which had been described as a form of "type-3 diabetes". In this context, this thesis work presents a comprehensive study to characterize longitudinal changes in functional connectivity using resting-state functional MRI and microstructure using diffusion MRI, and to correlate them to glucose hypometabolism and neuronal density in a rat model of AD. This study is mainly focused on establishing an optimized image processing pipeline dedicated for rat fMRI data, optimizing a computational model of diffusion in white matter using a novel deep learning approach, and assessing the spatiotemporal relationships between biomarkers regarding to microstructure and function in the brain by using advanced statistical methods as well as state-of-the-art machine learning approaches in order to provide a comprehensive characterization of the pathological cascade and progression of neurodegeneration resulting from brain glucose metabolism disruption.

Deletion of Crtc1 leads to hippocampal neuroenergetic impairments associated with depressive-like behavior

A. Cherix; C. Poitry-Yamate; B. Lanz; O. Zanoletti; J. Grosse  et al.

Mood disorders (MD) are a major burden on society as their biology remains poorly understood, challenging both diagnosis and therapy. Among many observed biological dysfunctions, homeostatic dysregulation, such as metabolic syndrome (MeS), shows considerable comorbidity with MD. Recently, CREB-regulated transcription coactivator 1 (CRTC1), a regulator of brain metabolism, was proposed as a promising factor to understand this relationship. Searching for imaging biomarkers and associating them with pathophysiological mechanisms using preclinical models can provide significant insight into these complex psychiatric diseases and help the development of personalized healthcare. Here, we used neuroimaging technologies to show that deletion of Crtc1 in mice leads to an imaging fingerprint of hippocampal metabolic impairment related to depressive-like behavior. By identifying a deficiency in hippocampal glucose metabolism as the underlying molecular/physiological origin of the markers, we could assign an energy-boosting mood-stabilizing treatment, ebselen, which rescued behavior and neuroimaging markers. Finally, our results point toward the GABAergic system as a potential therapeutic target for behavioral dysfunctions related to metabolic disorders. This study provides new insights on Crtc1's and MeS's relationship to MD and establishes depression-related markers with clinical potential.

DOI : 10.1038/s41380-022-01791-5

Hyperpolarized (1-13C)Alaninamide Is a Multifunctional In Vivo Sensor of Aminopeptidase N Activity, pH, and CO2

A. Radaelli; D. Ortiz; A. Michelotti; M. Roche; R. Hata  et al.

Spin hyperpolarization enables real-time metabolic imaging of carbon-13-labeled substrates. While hyperpolarized L- (1-13C)alaninamide is a probe of the cell-surface tumor marker aminopeptidase-N (APN, CD13), its activity in vivo has not been described. Scanning the kidneys of rats infused with hyperpolarized alaninamide shows both conversion to [1-13C]alanine and several additional spectral peaks with distinct temporal dynamics. The (1-13C)alaninamide chemical shift is pH-sensitive, with a pKa of 7.9 at 37 degrees C, and the peaks correspond to at least three different compartments of pH 7.46 +/- 0.02 (1), 7.21 +/- 0.02 (2), and 6.58 +/- 0.05 (3). An additional peak was assigned to the carboxyamino adduct formed by reaction with dissolved CO2. Spectroscopic imaging showed nonuniform distribution, with the low-pH signal more concentrated in the inner medulla. Treatment with the diuretic acetazolamide resulted in significant pH shifts in compartment 1 to 7.38 +/- 0.03 (p = 0.0057) and compartment 3 to 6.80 +/- 0.05 (p = 0.0019). While the pH of compartment 1 correlates with blood pH, the pH of compartment 3 did not correspond to the pH of urine. In vitro experiments show that alaninamide readily enters blood cells and can detect intracellular pH. While carbamate formation depends on pH and pCO2, the carbamate-toalaninamide ratio did not correlate with either arterial blood pH or pCO2, suggesting that it may reflect variations in tissue pH and pCO2. This study demonstrates the feasibility of using hyperpolarized sensors to simultaneously image enzyme activity, pCO2, and pH in vivo.

DOI : 10.1021/acssensors.2c01203

Segmenting electroencephalography wires reduces radiofrequency shielding artifacts in simultaneous electroencephalography and functional magnetic resonance imaging at 7 T

T. P. Lê; R. Gruetter; J. Jorge; Ö. Ipek

Purpose: Simultaneous scalp electroencephalography and functional magnetic resonance imaging (EEG-fMRI) enable noninvasive assessment of brain function with high spatial and temporal resolution. However, at ultra-high field, the data quality of both modalities is degraded by mutual interactions. Here, we thoroughly investigated the radiofrequency (RF) shielding artifact of a state-of-the-art EEG-fMRI setup, at 7 T, and design a practical solution to limit this issue. Methods: Electromagnetic field simulations and MR measurements assessed the shielding effect of the EEG setup, more specifically the EEG wiring. The effectiveness of segmenting the wiring with resistors to reduce the transmit field disruption was evaluated on a wire-only EEG model and a simulation model of the EEG cap. Results: The EEG wiring was found to exert a dominant effect on the disruption of the transmit field, whose intensity varied periodically as a function of the wire length. Breaking the electrical continuity of the EEG wires into segments shorter than one quarter RF wavelength in air (25 cm at 7 T) reduced significantly the RF shielding artifacts. Simulations of the EEG cap with segmented wires indicated similar improvements for a moderate increase of the power deposition. Conclusion: We demonstrated that segmenting the EEG wiring into shorter lengths using commercially available nonmagnetic resistors is effective at reducing RF shielding artifacts in simultaneous EEG-fMRI. This prevents the formation of RF-induced standing waves, without substantial specific absorption rate (SAR) penalties, and thereby enables benefiting from the functional sensitivity boosts achievable at ultra-high field.

DOI : 10.1002/mrm.29298

Central nervous system and systemic oxidative stress interplay with inflammation in a bile duct ligation rat model of type C hepatic encephalopathy (vol 178, pg 295, 2022)

K. Pierzchala; D. Simicic; A. Sienkiewicz; D. Sessa; S. Mitrea  et al.

DOI : 10.1016/j.freeradbiomed.2022.01.016

[13C]bicarbonate labelled from hyperpolarized [1-13C]pyruvate is an in vivo marker of hepatic gluconeogenesis in fasted state

E. Can; J. A. M. Bastiaansen; D.-L. Couturier; R. Gruetter; H. A. I. Yoshihara  et al.

Hyperpolarized [1-13C]pyruvate enables direct in vivo assessment of real-time liver enzymatic activities by 13C magnetic resonance. However, the technique usually requires the injection of a highly supraphysiological dose of pyruvate. We herein demonstrate that liver metabolism can be measured in vivo with hyperpolarized [1-13C]pyruvate administered at two- to three-fold the basal plasma concentration. The flux through pyruvate dehydrogenase, assessed by 13C-labeling of bicarbonate in the fed condition, was found to be saturated or partially inhibited by supraphysiological doses of hyperpolarized [1-13C]pyruvate. The [13C]bicarbonate signal detected in the liver of fasted rats nearly vanished after treatment with a phosphoenolpyruvate carboxykinase (PEPCK) inhibitor, indicating that the signal originates from the flux through PEPCK. In addition, the normalized [13C]bicarbonate signal in fasted untreated animals is dose independent across a 10-fold range, highlighting that PEPCK and pyruvate carboxylase are not saturated and that hepatic gluconeogenesis can be directly probed in vivo with hyperpolarized [1-13C]pyruvate.

DOI : 10.1038/s42003-021-02978-2

Central nervous system and systemic oxidative stress interplay with inflammation in a bile duct ligation rat model of type C hepatic encephalopathy

K. Pierzchala; D. Simicic; A. Sienkiewicz; D. Sessa; S. Mitrea  et al.

The role and coexistence of oxidative stress (OS) and inflammation in type C hepatic encephalopathy (C HE) is a subject of intense debate. Under normal conditions the physiological levels of intracellular reactive oxygen species are controlled by the counteracting antioxidant response to maintain redox homeostasis. Our previous invivo 1H-MRS studies revealed the longitudinal impairment of the antioxidant system (ascorbate) in a bile-duct ligation (BDL) rat model of type C HE. Therefore, the aim of this work was to examine the course of central nervous system (CNS) OS and systemic OS, as well as to check for their co-existence with inflammation in the BDL rat model of type C HE. To this end, we implemented a multidisciplinary approach, including ex-vivo and in-vitro electron paramagnetic resonance spectroscopy (EPR) spin-trapping, which was combined with UV-Vis spectroscopy, and histological assessments. We hypothesized that OS and inflammation act synergistically in the pathophysiology of type C HE. Our findings point to an increased CNS- and systemic-OS and inflammation over the course of type C HE progression. In particular, an increase in the CNS OS was observed as early as 2-weeks post-BDL, while the systemic OS became significant at week 6 post-BDL. The CNS EPR measurements were further validated by a substantial accumulation of 8-Oxo-2 '-deoxyguanosine (Oxo-8-dG), a marker of oxidative DNA/RNA modifications on immunohistochemistry (IHC). Using IHC, we also detected increased synthesis of antioxidants, glutathione peroxidase 1 (GPX-1) and superoxide dismutases (i.e.Cu/ZnSOD (SOD1) and MnSOD (SOD2)), along with proinflammatory cytokine interleukin-6 (IL-6) in the brains of BDL rats. The presence of systemic inflammation was observed already at 2-weeks post-surgery. Thus, these results suggest that CNS OS is an early event in type C HE rat model, which seems to precede systemic OS. Finally, our results suggest that the increase in CNS OS is due to enhanced formation of intra- and extra-cellular ROS rather than due to reduced antioxidant capacity, and that OS in parallel with inflammation plays a significant role in type C HE.

DOI : 10.1016/j.freeradbiomed.2021.12.011

Evaluation of the whole auditory pathway using high-resolution and functional MRI at 7T parallel-transmit

S. Da Costa; J. Clement; R. Gruetter; O. Ipek

Purpose The aim of the present study is to show a MR procedure for the evaluation of simultaneous left and right auditory functions with functional MRI, and high-resolution acquisition of anatomical auditory pathway using parallel-transmit (pTx) methods at 7T.

DOI : 10.1371/journal.pone.0254378

Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

C. C. Zanella; A. Capozzi; H. A. Yoshihara; A. Radaelli; A. L. C. Mackowiak  et al.

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha-ketovalerate (alpha kV) and alpha-ketobutyrate (alpha kB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing alpha kV and alpha kB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of C-13-labeled substrates to the sample matrix altered the radical yield of the precursors. Using alpha kB increased the C-13-labeled glucose liquid-state polarization to 16.3% +/- 1.3% compared with 13.3% +/- 1.5% obtained with pyruvate, and 8.9% +/- 2.1% with alpha kV. For [1-C-13]butyric acid, polarization levels of 12.1% +/- 1.1% for alpha kV, 12.9% +/- 1.7% for alpha kB, 1.5% +/- 0.2% for OX063 and 18.7% +/- 0.7% for Finland trityl, were achieved. Hyperpolarized [1-C-13]butyrate metabolism in the heart revealed label incorporation into [1-C-13]acetylcarnitine, [1-C-13]acetoacetate, [1-C-13]butyrylcarnitine, [5-C-13]glutamate and [5-C-13]citrate. This study demonstrates the potential of alpha kV and alpha kB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.

DOI : 10.1002/nbm.4584

Measuring Glycolytic Activity with Hyperpolarized [2H7, U-13C6] D-Glucose in the Naive Mouse Brain under Different Anesthetic Conditions

E. Flatt; B. Lanz; Y. Pilloud; A. Capozzi; M. H. Lerche  et al.

Glucose is the primary fuel for the brain; its metabolism is linked with cerebral function. Different magnetic resonance spectroscopy (MRS) techniques are available to assess glucose metabolism, providing complementary information. Our first aim was to investigate the difference between hyperpolarized 13C-glucose MRS and non-hyperpolarized 2H-glucose MRS to interrogate cerebral glycolysis. Isoflurane anesthesia is commonly employed in preclinical MRS, but it affects cerebral hemodynamics and functional connectivity. A combination of low doses of isoflurane and medetomidine is routinely used in rodent fMRI and shows similar functional connectivity, as in awake animals. As glucose metabolism is tightly linked to neuronal activity, our second aim was to assess the impact of these two anesthetic conditions on the cerebral metabolism of glucose. Brain metabolism of hyperpolarized 13C-glucose and 2H-glucose was monitored in two groups of mice in a 9.4 T MRI system. We found that the very different duration and temporal resolution of the two techniques enable highlighting the different aspects in glucose metabolism. We demonstrate (by numerical simulations) that hyperpolarized 13C-glucose reports on de novo lactate synthesis and is sensitive to CMRGlc. We show that variations in cerebral glucose metabolism, under different anesthesia, are reflected differently in hyperpolarized and non-hyperpolarized X-nuclei glucose MRS.

DOI : 10.3390/metabo11070413

Redox Dysregulation, Myelination Deficit and Dysconnectivity in Schizophrenia: A Translational Study in First Episode Patients and Experimental Models

K. Q. Do; J.-H. Cabungcal; M. Cleusix; D. Dwir; M. Fournier  et al.

Hyperpolarized C-13-glucose magnetic resonance highlights reduced aerobic glycolysis in vivo in infiltrative glioblastoma

M. Mishkovsky; O. Gusyatiner; B. Lanz; C. Cudalbu; I. Vassallo  et al.

Glioblastoma (GBM) is the most aggressive brain tumor type in adults. GBM is heterogeneous, with a compact core lesion surrounded by an invasive tumor front. This front is highly relevant for tumor recurrence but is generally non-detectable using standard imaging techniques. Recent studies demonstrated distinct metabolic profiles of the invasive phenotype in GBM. Magnetic resonance (MR) of hyperpolarized C-13-labeled probes is a rapidly advancing field that provides real-time metabolic information. Here, we applied hyperpolarized C-13-glucose MR to mouse GBM models. Compared to controls, the amount of lactate produced from hyperpolarized glucose was higher in the compact GBM model, consistent with the accepted "Warburg effect". However, the opposite response was observed in models reflecting the invasive zone, with less lactate produced than in controls, implying a reduction in aerobic glycolysis. These striking differences could be used to map the metabolic heterogeneity in GBM and to visualize the infiltrative front of GBM.

DOI : 10.1038/s41598-021-85339-7

The relationship between EEG and fMRI connectomes is reproducible across simultaneous EEG-fMRI studies from 1.5T to 7T

J. Wirsich; J. Jorge; G. R. Iannotti; E. A. Shamshiri; F. Grouiller  et al.

Both electroencephalography (EEG) and functional Magnetic Resonance Imaging (fMRI) are non-invasive methods that show complementary aspects of human brain activity. Despite measuring different proxies of brain activity, both the measured blood-oxygenation (fMRI) and neurophysiological recordings (EEG) are indirectly coupled. The electrophysiological and BOLD signal can map the underlying functional connectivity structure at the whole brain scale at different timescales. Previous work demonstrated a moderate but significant correlation between resting-state functional connectivity of both modalities, however there is a wide range of technical setups to measure simultaneous EEG-fMRI and the reliability of those measures between different setups remains unknown. This is true notably with respect to different magnetic field strengths (low and high field) and different spatial sampling of EEG (medium to high-density electrode coverage). Here, we investigated the reproducibility of the bimodal EEG-fMRI functional connectome in the most comprehensive resting-state simultaneous EEG-fMRI dataset compiled to date including a total of 72 subjects from four different imaging centers. Data was acquired from 1.5T, 3T and 7T scanners with simultaneously recorded EEG using 64 or 256 electrodes. We demonstrate that the whole-brain monomodal connectivity reproducibly correlates across different datasets and that a moderate crossmodal correlation between EEG and fMRI connectivity of r ≈ 0.3 can be reproducibly extracted in low- and high-field scanners. The crossmodal correlation was strongest in the EEG-β frequency band but exists across all frequency bands. Both homotopic and within intrinsic connectivity network (ICN) connections contributed the most to the crossmodal relationship. This study confirms, using a considerably diverse range of recording setups, that simultaneous EEG-fMRI offers a consistent estimate of multimodal functional connectomes in healthy subjects that are dominantly linked through a functional core of ICNs across spanning across the different timescales measured by EEG and fMRI. This opens new avenues for estimating the dynamics of brain function and provides a better understanding of interactions between EEG and fMRI measures. This observed level of reproducibility also defines a baseline for the study of alterations of this coupling in pathological conditions and their role as potential clinical markers.

DOI : 10.1016/j.neuroimage.2021.117864

Development and applications of hyperpolarized 13C and 1H MR spectroscopy of cerebral metabolism at ultra-high field

E. I. Flatt / R. Gruetter (Dir.)

This thesis is composed of four studies centered on investigating cerebral metabolism using magnetic resonance spectroscopy (MRS) of hyperpolarized and non-hyperpolarized compounds at ultra-high field. In the first two chapters, we studied longitudinally the effects of different treatments for hepatic encephalopathy (HE) on the neurometabolic changes associated with chronic HE using a rat model of type C HE and a multimodal approach including 1H MRS. The effects of the antibiotic rifaximin administered at different doses as well as its combination with probiotics were studied in the established model of bile duct ligated (BDL) rats. We showed for the first time the beneficial effects of the combined rifaximin and the probiotic Vivomixx® on the neurometabolic changes in vivo and longitudinally in a rat model of type C HE. The longitudinal changes of some brain metabolites concentration (glutamine, glutamate, creatine), as well as gut bifidobacteria concentration, were significantly less pronounced in the group of rats treated with rifaximin and probiotics compared to non-treated rats. We also showed that rifaximin treatment alone had limited efficacy. When administered at human dose in rats, its effects appeared only at the early stages of the disease, whereas at a higher dose some neurometabolic changes associated with HE were attenuated but the general condition of the rats was worse. In the third study, we used dissolution dynamic nuclear polarization (DNP) to hyperpolarize [2H7, U-13C6]-D-glucose, which enabled us to monitor real-time glycolysis in the healthy mouse brain. Given that glucose metabolism is tightly linked to neuronal activity, we first investigated how the change in anesthesia impacts the cerebral metabolism of hyperpolarized glucose. Anesthetics are known to influence brain activity, and in this study we showed for the first time that switching from isoflurane anesthesia to a combination of lower doses of isoflurane and medetomidine had a high impact on cerebral glucose uptake and glycolytic flow, as reflected by the increased labelling of downstream [1-13C] lactate. The second part of this study aimed at evaluating the feasibility of quantifying cerebral glucose metabolism kinetics by evaluating two different mathematical models, where kinetic rate constants were determined by fitting the models on 13C curves by non-linear regression using the Levenberg-Marquardt algorithm. We showed that a 3-compartment model is more stable and reliable than a 4-compartment model. Finally, since lithium salts are widely used for treating bipolar disorder but its mechanism of action is still not clearly understood, we evaluated the potential of hyperpolarized 6Li injected at pharmacological doses to assess its real-time bio-distribution and pharmacokinetic in the rat brain. We demonstrated first that hyperpolarized 6Li can be detected at pharmacological concentration in the rat head with high signal-to-noise ratio (SNR). We also showed that the transport of lithium through the intact blood-brain barrier brain is a limiting factor and demonstrated a striking difference between apparent 6Li T1 values in a brain with normal or disrupted blood-brain barrier.

Late post‐natal neurometabolic development in healthy male rats using 1 H and 31 P Magnetic Resonance Spectroscopy

V. Račkayová; D. Simicic; G. Donati; O. Braissant; R. Gruetter  et al.

Brain metabolism evolves rapidly during early post‐natal development in the rat. While changes in amino acids, energy metabolites, antioxidants or metabolites involved in phospholipid metabolism have been reported in the early stages, neurometabolic changes during the later post‐natal period are less well characterized. Therefore, we aimed to assess the neurometabolic changes in male Wistar rats between post‐natal days 29 and 77 (p29‐p77) using longitudinal magnetic resonance spectroscopy (MRS) in vivo at 9.4 Tesla. 1H MRS was performed in the hippocampus between p29‐p77 at one‐week intervals (n=7) and in the cerebellum between p35‐p77 at two‐week intervals (n=7) using the SPECIAL sequence at ultra‐short echo‐time. NOE enhanced and 1H decoupled 31P MR spectra were acquired at p35, p48 and p63 (n=7) in a larger voxel covering cortex, hippocampus and part of the striatum. The hippocampus showed a decrease in taurine concentration and an increase in glutamate (with more pronounced changes until p49), seemingly a continuation of their well described changes in the early post‐natal period. A constant increase in myo‐inositol‐ and choline‐containing compounds in the hippocampus (in particular glycero‐phosphocholine as shown by 31P MRS) was measured throughout the observation period, probably related to membrane metabolism and myelination. The cerebellum showed only a significant increase in myo‐inositol between p35‐p77. In conclusion, the present study showed important changes in brain metabolites in both the hippocampus and cerebellum in the later post‐natal period (p29/p35‐p77) of male rats, something previously unreported. Based on these novel data, changes in some neurometabolites beyond p28‐35, conventionally accepted as the cut off for adulthood, should be taken into account in both experimental design and data interpretation in this animal model.

DOI : 10.1111/jnc.15294

Methods to Enhance Nuclear Magnetic Resonance Sensitivity at High Magnetic Field

C. C. Zanella / R. Gruetter (Dir.)

MR is a well-established technique routinely used in preclinical and clinical studies. This thesis focussed on the improvement of sensitivity of different methods in MR. Hyperpolarized 129Xe gas is used for studying pulmonary disease or cerebral perfusion. When hyperpolarization is conducted inside a polarizer, the achieved solid state polarization is currently limited to a few percent. One aim of this thesis was to counteract this bottleneck by investigating the effect of microwave frequency modulation for different spin systems. An increase in nuclear 129Xe spin polarization up to 14.5% was achieved compared to the polarization obtained without frequency modulation. Electron spin-lattice relaxation times T1S, as short as 5 ms, correlated directly with the gain in 129Xe nuclear polarization due to microwave frequency modulation, irrespective of the solvent or its deuteration. Simulations of the electron spin system confirmed that microwave frequency modulation could be an efficient method to considerably enhance nuclear spin polarization in short T1S matrices or in the presence of large linewidth radicals by promoting spectral diffusion. DNP matrices can also be hyperpolarized through non-persistent radicals generated by UV-irradiation. They have the advantage of quenching upon dissolution, yielding a hyperpolarized solution with no need for radical filtration. Radical precursors researched up to date either yielded low 13C polarization, contained toxic matrix components or interfered with metabolic processes. Therefore, the two novel endogenously-occurring precursors, alpha-ketovalerate (akV) and alpha-ketobutyrate (akB) were investigated. Liquid state polarization of the metabolic substrate glucose increased to 16.3% using akB compared with 13.3% obtained with pyruvate. [1-13C]butyric acid was polarized to 12.1% for akV and 12.9% for akB and used for in vivo hyperpolarized cardiac MRS. Hyperpolarized [1-13C]butyrate metabolism in the heart revealed label incorporation into a wide range of metabolites. This study demonstrated the potential of using UV-induced radicals generated in the endogenously-occurring metabolites akV and akB as polarizing agents, enabling high polarization without requiring radical filtration for radical-free hyperpolarized MRI. While MRI requires RF coils that generate high B1 homogeneity, MRS strongly depends on high sensitivity and potentially high B1 efficiency to allow for short-TE acquisitions. Two custom-made 1H coils resonating at 600 MHz, a single-channel saddle coil and an 8-leg quadrature birdcage coil, were investigated with respect to those criteria for applications at ultra-high field (14.1 T). The saddle coil yielded a 54% higher transmit field efficiency with a 20% higher SNR compared to the birdcage coil after full-FOV corrections. Using the saddle coil in glycoCEST imaging of a homogeneous phantom resulted in an MTR asymmetry coefficient of variation as small as 5.2% over an 11 mm diameter ROI. Overall, the saddle coil provided a good compromise between globally homogeneous excitation and high sensitivity. Finally, it was successfully used to map skeletal muscle glycogen content in vivo. This thesis focused on improving MR sensitivity by means of adequate custom-made RF coils as well as DNP through the implementation of a method capitalizing on electronic spin properties and the development of novel endogenous precursors for in vivo hyperpolarized MRI.

Excitatory/inhibitory neuronal metabolic balance in mouse hippocampus upon infusion of [U-C-13(6)]glucose

A. Cherix; G. Donati; B. Lizarbe; B. Lanz; C. Poitry-Yamate  et al.

Hippocampus plays a critical role in linking brain energetics and behavior typically associated to stress exposure. In this study, we aimed to simultaneously assess excitatory and inhibitory neuronal metabolism in mouse hippocampus in vivo by applying (18)FDG-PET and indirect C-13 magnetic resonance spectroscopy (H-1-[C-13]-MRS) at 14.1 T upon infusion of uniformly C-13-labeled glucose ([U-C-13(6)]Glc). Improving the spectral fitting by taking into account variable decoupling efficiencies of [U-C-13(6)]Glc and refining the compartmentalized model by including two gamma-aminobutyric acid (GABA) pools permit us to evaluate the relative contributions of glutamatergic and GABAergic metabolism to total hippocampal neuroenergetics. We report that GABAergic activity accounts for similar to 13% of total neurotransmission (V-NT) and similar to 27% of total neuronal TCA cycle (V-TCA) in mouse hippocampus suggesting a higher V-TCA/V-NT ratio for inhibitory neurons compared to excitatory neurons. Finally, our results provide new strategies and tools for bringing forward the developments and applications of C-13-MRS in specific brain regions of small animals.

DOI : 10.1177/0271678X20910535

Brain NAD Is Associated With ATP Energy Production and Membrane Phospholipid Turnover in Humans

B. Cuenoud; O. Ipek; M. Shevlyakova; M. Beaumont; S. C. Cunnane  et al.

The brain requires a large amount of energy, mostly derived from the metabolism of glucose, which decreases substantially with age and neurological diseases. While mounting evidence in model organisms illustrates the central role of brain nicotinamide adenine dinucleotide (NAD) for maintaining energy homeostasis, similar data are sparse in humans. This study explores the correlations between brain NAD, energy production and membrane phospholipid metabolism by 31-phosphorous magnetic resonance spectroscopy (P-31-MRS) across 50 healthy participants including a young (mean age 27.1-year-old) and middle-aged (mean age 56.4-year-old) group. The analysis revealed that brain NAD level and NAD(+)/NADH redox ratio were positively associated with ATP level and the rate of energy production, respectively. Moreover, a metabolic network linking NAD with membrane phospholipid metabolism, energy production, and aging was identified. An inverted trend between age and NAD level was detected. These results pave the way for the use of P-31-MRS as a powerful non-invasive tool to support the development of new therapeutic interventions targeting NAD associated phospho-metabolic pathways in brain aging and neurological diseases.

DOI : 10.3389/fnagi.2020.609517

Magnetic resonance spectroscopy in the rodent brain: Experts' consensus recommendations

B. Lanz; A. Abaei; O. Braissant; I.-Y. Choi; C. Cudalbu  et al.

In vivo MRS is a non-invasive measurement technique used not only in humans, but also in animal models using high-field magnets. MRS enables the measurement of metabolite concentrations as well as metabolic rates and their modifications in healthy animals and disease models. Such data open the way to a deeper understanding of the underlying biochemistry, related disturbances and mechanisms taking place during or prior to symptoms and tissue changes. In this work, we focus on the main preclinical(1)H,P-31 and(13)C MRS approaches to study brain metabolism in rodent models, with the aim of providing general experts' consensus recommendations (animal models, anesthesia, data acquisition protocols). An overview of the main practical differences in preclinical compared with clinical MRS studies is presented, as well as the additional biochemical information that can be obtained in animal models in terms of metabolite concentrations and metabolic flux measurements. The properties of high-field preclinical MRS and the technical limitations are also described.

DOI : 10.1002/nbm.4325

C-13 Dynamic Nuclear Polarization using SA-BDPA at 6.7 T and 1.1 K: Coexistence of Pure Thermal Mixing and Well-Resolved Solid Effect

A. Radaelli; H. A. Yoshihara; H. Nonaka; S. Sando; J. H. Ardenkjaer-Larsen  et al.

SA-BDPA is a water-soluble, narrow-line width radical previously used for dynamic nuclear polarization (DNP) signal enhancement in solid-state magic angle spinning NMR spectroscopy. Here, we report the first study using SA-BDPA under dissolution DNP conditions (6.7 T and 1.15 K). Longitudinal-detected (LOD)-electron spin resonance (ESR) and C-13 DNP measurements were performed on samples containing 8.4 M [C-13]urea dissolved in 50:50 water:glycerol (v/v) doped with either 60 or 120 mM SA-BDPA. Two distinct DNP mechanisms, both "pure" thermal mixing and a well-resolved solid effect could clearly be identified. The radical's ESR line width (30-40 MHz), broadened predominantly by dipolar coupling, excluded any contribution from the cross effect. Microwave frequency modulation increased the enhancement by DNP at the lower radical concentration but not at the higher radical concentration. These results are compared to data acquired with trityl radical AH111501, highlighting the unusual C-13 DNP properties of SA-BDPA.

DOI : 10.1021/acs.jpclett.0c01473

B(0)shimming for in vivo magnetic resonance spectroscopy: Experts' consensus recommendations

C. Juchem; C. Cudalbu; R. A. de Graaf; R. Gruetter; A. Henning  et al.

Magnetic resonance spectroscopy (MRS) and spectroscopic imaging (MRSI) allow the chemical analysis of physiological processes in vivo and provide powerful tools in the life sciences and for clinical diagnostics. Excellent homogeneity of the static B(0)magnetic field over the object of interest is essential for achieving high-quality spectral results and quantitative metabolic measurements. The experimental minimization of B(0)variation is performed in a process called B(0)shimming. In this article, we summarize the concepts of B(0)field shimming using spherical harmonic shimming techniques, specific strategies for B(0)homogenization and crucial factors to consider for implementation and use in both brain and body. In addition, experts' recommendations are provided for minimum requirements for B(0)shim hardware and evaluation criteria for the primary outcome of adequate B(0)shimming for MRS and MRSI, such as the water spectroscopic linewidth.

DOI : 10.1002/nbm.4350

Glutamine-to-glutamate ratio in the nucleus accumbens predicts effort-based motivated performance in humans

A. V. I. Strasser; G. Luksys; L. Xin; M. Pessiglione; R. Gruetter  et al.

Substantial evidence implicates the nucleus accumbens in motivated performance, but very little is known about the neurochemical underpinnings of individual differences in motivation. Here, we applied 1H magnetic resonance spectroscopy (1H-MRS) at ultra-high-field in the nucleus accumbens and inquired whether levels of glutamate (Glu), glutamine (Gln), GABA or their ratios predict interindividual differences in effort-based motivated task performance. Given the incentive value of social competition, we also examined differences in performance under self-motivated or competition settings. Our results indicate that higher accumbal Gln-to-Glu ratio predicts better overall performance and reduced effort perception. As performance is the outcome of multiple cognitive, motor and physiological processes, we applied computational modeling to estimate best-fitting individual parameters related to specific processes modeled with utility, effort and performance functions. This model-based analysis revealed that accumbal Gln-to-Glu ratio specifically relates to stamina; i.e., the capacity to maintain performance over long periods. It also indicated that competition boosts performance from task onset, particularly for low Gln-to-Glu individuals. In conclusion, our findings provide novel insights implicating accumbal Gln and Glu balance on the prediction of specific computational components of motivated performance. This approach and findings can help developing therapeutic strategies based on targeting metabolism to ameliorate deficits in effort engagement.

DOI : 10.1038/s41386-020-0760-6

Fat navigators based retrospective motion correction strategies for brain magnetic resonance imaging

F. Gretsch / R. Gruetter; D. Gallichan (Dir.)

Magnetic resonance resonance (MRI) is a widely used modality to obtain in vivo tissue information. Clinical applications are near countless, and almost all body parts can be examined using an MR scanner. As the method is non invasive, does not use ionizing radiation and provides excellent soft tissue contrast, it also appears as an excellent tool for neuroscience research. The major drawback of MRI remains the relatively long acquisition times, of the order of several minutes. During the measurement, the subject must stay still and avoid moving at all costs, as otherwise image artefacts will appear and potentially render the acquired data (partially) unusable. As higher image resolution imply longer acquisition time, probing finer anatomical details imply ultimately requires dealing with said motion. While some research goes in the way of reducing the acquisition time, it necessarily comes at the price of lower sensitivity and hence inherently diminishes the achievable gain for high- resolution imaging as the signal is weaker to start with. In this work, the focus is to try and compensate for motion during brain imaging using a navigator method. This amounts to measure not only the desired image, but also other MR based information, called navigator, at regular intervals during the scan. A modeling step then establishes a link between the navigators samples and the head position change. Incorporating the motion information into the main image reconstruction framework helps to retrospectively reduce the impact of said motion and the associated incoherences which would appear during the standard reconstruction. Brain imaging is probably the easiest case of motion correction in MRI, as the motion can readily be well approximated as rigid. The navigator methods developed and investigated in this work, called FatNavs, are based on the fat signal, which in head imaging is very sparse in space and therefore can be imaged rapidly. They also present the advantage of reduced impact on the main image water signal. Several implementation strategies were tested as, due to the versatility of MRI, all image contrasts cannot be ideally navigated using a single general implementation. Applications to inversion recovery based sequences (MP2RAGE) used a well separated navigator and image acquisition scheme. This method being routinely acquired, comparison to Moir Ì e Phase Tracking, the current gold standard for motion tracking and correction, was also performed in collaboration with Hendrik Mattern from the Magdeburg University. For gradient-echo imaging sequences (GRE), on which time-of-flight angiography and susceptibil- ity induced contrasts are based, both separate and mixed acquisition schemes were tested. Further- more, for imaging protocols using long echo time, the fluctuation of the magnetic field during the scan can also induce severe artefacts. Therefore, extension of the FatNavs to a dual-echo field-mapping version was also explored. Finally, combination of FatNavs with FID navigators, which lack spatial information but have much higher temporal resolution, was investigated for both motion and field fluctuation retrospective correction.

Impact of aerobic exercise type on blood flow, muscle energy metabolism, and mitochondrial biogenesis in experimental lower extremity artery disease

M. Pellegrin; K. Bouzourène; J.-F. Aubert; C. Bielmann; R. Gruetter  et al.

Exercise training (ET) is recommended for lower extremity artery disease (LEAD) management. However, there is still little information on the hemodynamic and metabolic adaptations by skeletal muscle with ET. We examined whether hindlimb perfusion/vascularization and muscle energy metabolism are altered differently by three types of aerobic ET. ApoE−/− mice with LEAD were assigned to one of four groups for 4 weeks: sedentary (SED), forced treadmill running (FTR), voluntary wheel running (VWR), or forced swimming (FS). Voluntary exercise capacity was improved and equally as efficient with FTR and VWR, but remained unchanged with FS. Neither ischemic hindlimb perfusion and oxygenation, nor arteriolar density and mRNA expression of arteriogenic-related genes differed between groups. 18FDG PET imaging revealed no difference in the steady-state levels of phosphorylated 18FDG in ischemic and non-ischemic hindlimb muscle between groups, nor was glycogen content or mRNA and protein expression of glucose metabolism-related genes in ischemic muscle modified. mRNA (but not protein) expression of lipid metabolism-related genes was upregulated across all exercise groups, particularly by non-ischemic muscle. Markers of mitochondrial content (mitochondrial DNA content and citrate synthase activity) as well as mRNA expression of mitochondrial biogenesis-related genes in muscle were not increased with ET. Contrary to FTR and VWR, swimming was ineffective in improving voluntary exercise capacity. The underlying hindlimb hemodynamics or muscle energy metabolism are unable to explain the benefits of running exercise.

DOI : 10.1038/s41598-020-70961-8

Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine

A. Cherix; T. Larrieu; J. Grosse; J. Rodrigues; B. McEwen  et al.

Emerging evidence suggests that hierarchical status provide vulnerability to develop stress-induced depression. Energy metabolic changes in the nucleus accumbens (NAc) were recently related to hierarchical status and vulnerability to develop depression-like behavior. Acetyl-L-carnitine (LAC), a mitochondria-boosting supplement, has shown promising antidepressant-like effects opening therapeutic opportunities for restoring energy balance in depressed patients. We investigated the metabolic impact in the NAc of antidepressant LAC treatment in chronically-stressed mice using (1)H-magnetic resonance spectroscopy ((1)H-MRS). High rank, but not low rank, mice, as assessed with the tube test, showed behavioral vulnerability to stress, supporting a higher susceptibility of high social rank mice to develop depressive-like behaviors. High rank mice also showed reduced levels of several energy-related metabolites in the NAc that were counteracted by LAC treatment. Therefore, we reveal a metabolic signature in the NAc for antidepressant-like effects of LAC in vulnerable mice characterized by restoration of stress-induced neuroenergetics alterations and lipid function.

DOI : 10.7554/eLife.50631

Glucose transporter 2 mediates the hypoglycemia-induced increase in cerebral blood flow

H. Lei; F. Preitner; G. Labouebe; R. Gruetter; B. Thorens

Glucose transporter 2 (Glut2)-positive cells are sparsely distributed in brain and play an important role in the stimulation of glucagon secretion in response to hypoglycemia. We aimed to determine if Glut2-positive cells can influence another response to hypoglycemia, i.e. increased cerebral blood flow (CBF). CBF of adult male mice devoid of Glut2, either globally (ripglut1:glut2(-)(/)(-)) or in the nervous system only (NG2KO), and their respective controls were studied under basal glycemia and insulin-induced hypoglycemia using quantitative perfusion magnetic resonance imaging at 9.4 T. The effect on CBF of optogenetic activation of hypoglycemia responsive Glut2-positive neurons of the paraventricular thalamic area was measured in mice expressing channelrhodopsin2 under the control of the Glut2 promoter. We found that in both ripglut1:glut2(-)(/)(-) mice and NG2KO mice, CBF in basal conditions was higher than in their respective controls and not further activated by hypoglycemia, as measured in the hippocampus, hypothalamus and whole brain. Conversely, optogenetic activation of Glut2-positive cells in the paraventricular thalamic nucleus induced a local increase in CBF similar to that induced by hypoglycemia. Thus, Glut2 expression in the nervous system is required for the control of CBF in response to changes in blood glucose concentrations.

DOI : 10.1177/0271678X18766743

Antibiotic Rifaximin for Treatment of Chronic Liver Disease-Induced HE: A Longitudinal In Vivo 1H-MRS Study of Brain Metabolism on BDL Rats

E. Flatt; O. Braissant; S. Mitrea; D. Sessa; R. Gruetter  et al.

Probiotics Combined With Rifaximin for the Treatment of Chronic Hepatic Encephalopathy: A Longitudinal In Vivo 1H-MRS Study of Brain Metabolism Using BDL Rats

E. Flatt; O. Braissant; S. Mitrea; D. Sessa; P. Mastromarino  et al.

Nucleus accumbens neurochemistry in human anxiety: A 7 T 1H-MRS study

A. Strasser; L. Xin; R. Gruetter; C. Sandi

Individual differences in anxiety provide a differential predisposition to develop neuropsychiatric disorders. The neurochemical underpinnings of anxiety remain elusive, particularly in deep structures, such as the nucleus accumbens (NAc) whose involvement in anxiety is being increasingly recognized. We examined the associations between the neurochemical profile of human NAc metabolites involved in neural excitation and inhibition and inter-individual variation in temperamental and situational anxiety. Twenty-seven healthy 20-30 years-old human males were phenotyped with questionnaires for state and trait anxiety (State-Trait Anxiety Inventory, STAI), social anxiety (Liebowitz Social Anxiety Scale), negative mood (Beck Depression Inventory, BDI) and fatigue (Mental and Physical State Energy and Fatigue Scales, SEF). Using proton magnetic resonance spectroscopy ((1)H-MRS) at 7 Tesla (7T), we measured metabolite levels for glutamate, glutamine, GABA and taurine in the NAc. Salivary cortisol was also measured. Strikingly, trait anxiety was negatively associated with NAc taurine content. Perceived situational stress was negatively associated with NAc GABA, while positively with the Glu/GABA ratio. No correlation was observed between NAc taurine or GABA and other phenotypic variables examined (i.e., state anxiety, social anxiety, negative mood, or cortisol), except for a negative correlation between taurine and state physical fatigue. This first 7T study of NAc neurochemistry shows relevant metabolite associations with individual variation in anxiety traits and situational stress and state anxiety measurements. The novel identified association between NAc taurine levels and trait anxiety may pave the way for clinical studies aimed at identifying new treatments for anxiety and related disorders.

DOI : 10.1016/j.euroneuro.2018.12.015

Capturing the spatiotemporal dynamics of self-generated, task-initiated thoughts with EEG and fMRI

L. Bréchet; D. Brunet; G. Birot; R. Gruetter; C. M. Michel  et al.

The temporal structure of self-generated cognition is a key attribute to the formation of a meaningful stream of consciousness. When at rest, our mind wanders from thought to thought in distinct mental states. Despite the marked importance of ongoing mental processes, it is challenging to capture and relate these states to specific cognitive contents. In this work, we employed ultra-high field functional magnetic resonance imaging (fMRI) and high-density electroencephalography (EEG) to study the ongoing thoughts of participants instructed to retrieve self-relevant past episodes for periods of 22sec. These task-initiated, participant-driven activity patterns were compared to a distinct condition where participants performed serial mental arithmetic operations, thereby shifting from self-related to self-unrelated thoughts. BOLD activity mapping revealed selective enhanced activity in temporal, parietal and occipital areas during the memory compared to the mental arithmetic condition, evincing their role in integrating the re-experienced past events into conscious representations during memory retrieval. Functional connectivity analysis showed that these regions were organized in two major subparts, previously associated to “scene-reconstruction” and “self-experience” subsystems. EEG microstate analysis allowed studying these participant-driven thoughts in the millisecond range by determining the temporal dynamics of brief periods of stable scalp potential fields. This analysis revealed selective modulation of occurrence and duration of specific microstates in the memory and in the mental arithmetic condition, respectively. EEG source analysis revealed similar spatial distributions of the sources of these microstates and the regions identified with fMRI. These findings imply a functional link between BOLD activity changes in regions related to a certain mental activity and the temporal dynamics of mentation, and support growing evidence that specific fMRI networks can be captured with EEG as repeatedly occurring brief periods of integrated coherent neuronal activity, lasting only fractions of seconds.

DOI : 10.1016/j.neuroimage.2019.03.029

Improved off‐resonance phase behavior using a phase‐inverted adiabatic half‐passage pulse for 13 C MRS in humans at 7 T

E. Serés Roig; L. Xin; D. Gallichan; V. Mlynarik; R. Gruetter

In vivo (13) C MRS at high field benefits from an improved SNR and spectral resolution especially when using surface coils in combination with adiabatic pulses, such as the adiabatic half-passage (AHP) pulse for (13) C excitation. However, the excitation profile of the AHP pulse is asymmetric relative to the carrier frequency, which could lead to asymmetric excitation of the spectral lines relative to the center of the spectrum. In this study, a pulse-acquire sequence was designed for adiabatic (13) C excitation with a symmetric bandwidth, utilizing a combination of two AHP pulses with inverted phases in alternate scans. Magnetization and phase behavior as a function of frequency offset and RF amplitude of the B1 field, as well as the steady-state transverse magnetization response to off-resonance, were simulated. Excitation properties of the combined pulse sequence were studied by (23) Na imaging and (13) C spectroscopy in vitro on a phantom and in vivo on the human calf at 7 T. Simulations demonstrated symmetric transverse magnetization and phase with respect to positive and negative frequency offsets when using two AHP pulses with inverted phases in alternate scans, thereby minimizing baseline distortion and achieving symmetric T1 weighting, as confirmed by in vitro measurements. The intensities of the lipid peaks at 15, 30, 62, 73, and 130 ppm were in agreement with those theoretically predicted using two AHP pulses with inverted phases in alternate scans. We conclude that using two phase-inverted AHP pulses improves the symmetry of the (13) C excitation profile and phase response to off-resonance effects at 7 T in comparison with using a single AHP pulse.

DOI : 10.1002/nbm.4171

A human cerebral and cerebellar 8-channel transceive RF dipole coil array at 7T

J. D. Clément; R. Gruetter; Ö. Ipek

PURPOSE: Dipole antennas that provide high transmit field penetration with large coverage, and their use in a parallel transmit setup, may be advantageous in minimizing B 1+ -field inhomogeneities at ultra-high field, i.e 7T. We have developed and evaluated an 8-channel RF dipole coil array for imaging the entire cerebral and cerebellar regions in man. METHODS: A coil array was modeled with seven dipoles: six placed covering the occipital and temporal lobes; one covering the parietal lobe; and two loops covering the frontal lobe. Center-shortened and fractionated dipoles were simulated for the array configuration and assessed with respect to B 1+ -field at maximum specific absorption rate averaged over 10 g tissue regions in human brain. The whole-brain center-shortened dipoles with frontal loops coil array was constructed and its transmit properties were assessed with respect to MR images, B 1+ -field, and homogeneity. RESULTS: In simulations, the dipole arrays showed comparable performances to cover the whole-brain. However, for ease of construction, the center-shortened dipole was favored. High spatial resolution anatomical images of the human brain with the coil array demonstrated a full coverage of the cerebral cortex and cerebellum. CONCLUSIONS: The 8-channel center-shortened dipoles and frontal loops coil array promises remarkable efficiency in highly challenging regions as the cerebellum, and phase-only RF shimming of whole-brain could greatly benefit ultra-high field magnetic resonance imaging of the human brain at 7T.

DOI : 10.1002/mrm.27476

Multi-slice passband bSSFP for human and rodent fMRI at ultra-high field

O. Reynaud; A. R. da Silva; R. Gruetter; I. O. Jelescu

Balanced steady-state free precession (bSSFP) can be used as an alternative to gradient-echo (GE) EPI for BOLD functional MRI when image distortions and signal drop-outs are severe such as at ultra-high field. However, 3D-bSSFP acquisitions have distinct drawbacks on either human or animal MR systems. On clinical scanners, 3D imaging is suboptimal for localized fMRI applications. It can also display distortions when acceleration methods such as spiral read-outs are used, and, compared to multi-slice acquisitions, suffers from increased sensitivity to motion or physiological noise which further results in blurring. On pre-clinical systems, 3D acquisitions have low temporal resolution due to limited acceleration options, while single slice often results in insufficient coverage. The aim of the present study was to implement a multi-slice bSSFP acquisition with Cartesian read-out to obtain non-distorted BOLD fMRI activation maps in the human and rat brain at ultra-high field. We show that, when using a new pseudo-steady-state, the bSSFP signal characteristics are preserved. In the human brain at 7 T, we demonstrate that both task- and resting-state fMRI can be performed with multi-slice bSSFP, with a temporal SNR that matches that of 3D-bSSFP, resulting in - at least - equal performance. In the rat brain at 14 T, we show that the multi-slice bSSFP protocol has similar sensitivity to gradient-echo EPI for task fMRI, while benefitting from much reduced distortions and drop-outs. The advantages of passband bSSFP at 14 T in comparison with GE-EPI are expected to be even more marked for mouse brain.

DOI : 10.1016/j.jmr.2019.05.010

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

M. Wilson; O. Andronesi; P. B. Barker; R. Bartha; A. Bizzi  et al.

Proton MRS (1H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good‐quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi‐adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

DOI : 10.1002/mrm.27742

High-fat diet consumption alters energy metabolism in the mouse hypothalamus

B. Lizarbe; A. Cherix; J. M. N. Duarte; J.-R. Cardinaux; R. Gruetter

BACKGROUND/OBJECTIVES: High-fat diet consumption is known to trigger an inflammatory response in the hypothalamus, which has been characterized by an initial expression of pro-inflammatory genes followed by hypothalamic astrocytosis, microgliosis, and the appearance of neuronal injury markers. The specific effects of high-fat diet on hypothalamic energy metabolism and neurotransmission are however not yet known and have not been investigated before. SUBJECTS/METHODS: We used (1)H and (13)C magnetic resonance spectroscopy (MRS) and immunofluorescence techniques to evaluate in vivo the consequences of high-saturated fat diet administration to mice, and explored the effects on hypothalamic metabolism in three mouse cohorts at different time points for up to 4 months. RESULTS: We found that high-fat diet increases significantly the hypothalamic levels of glucose (P < 0.001), osmolytes (P < 0.001), and neurotransmitters (P < 0.05) from 2 months of diet, and alters the rates of metabolic (P < 0.05) and neurotransmission fluxes (P < 0.001), and the contribution of non-glycolytic substrates to hypothalamic metabolism (P < 0.05) after 10 weeks of high-fat feeding. CONCLUSIONS/INTERPRETATION: We report changes that reveal a high-fat diet-induced alteration of hypothalamic metabolism and neurotransmission that is quantifiable by (1)H and (13)C MRS in vivo, and present the first evidence of the extension of the inflammation pathology to a localized metabolic imbalance.

DOI : 10.1038/s41366-018-0224-9

A combined 32‐channel receive‐loops/8‐channel transmit‐dipoles coil array for whole‐brain MR imaging at 7T

J. Clément; R. Gruetter; Ö. Ipek

Purpose Multichannel receive arrays provide high SNR and parallel‐imaging capabilities, while transmit‐only dipole arrays have been shown to achieve a large coverage of the whole‐brain including the cerebellum. The aim of this study was to develop and characterize the performances of a 32‐channel receive‐only loop array combined with an 8‐channel dipole coil array at 7T for the first time. Methods The 8Tx‐dipoles/32Rx‐loops coil array was characterized by the SNR, g‐factors, noise correlation matrix, accelerated image quality, and urn:x-wiley:07403194:media:mrm27808:mrm27808-math-0003 maps, and compared with a commercial 1Tx‐birdcage/32Rx‐loops array. Simulated and measured urn:x-wiley:07403194:media:mrm27808:mrm27808-math-0004 maps were shown for the 8Tx‐dipoles/32Rx‐loops coil array and compared with the 8Tx/Rx dipole array. Results The in‐house built 32‐channel receive coil demonstrated a large longitudinal coverage of the brain, particularly the upper neck area. G‐factors and accelerated MR acquisitions demonstrated robust performances up to R = 4 in 2D, and R = 8 (4 × 2) in 3D. A 83% increase in SNR was measured over the cerebellum with the in‐house built 8Tx/32Rx coil array compared to the commercial 1Tx/32Rx, while similar performances were obtained in the cerebral cortex. Conclusions The combined 32‐channel receive/8‐channel transmit coil array demonstrated high transmit‐receive performances compared to the commercial receive array at 7T, notably in the cerebellum. We conclude that in combination with parallel transmit capabilities, this coil is particularly suitable for whole‐brain MR studies at 7T.

DOI : 10.1002/mrm.27808

Feasibility of in vivo measurement of glucose metabolism in the mouse hypothalamus by (1) H-[(13) C] MRS at 14.1T

B. Lizarbe Serra; H. Lei; J. M. das Neves Duarte; B. Lanz; A. Cherix  et al.

PURPOSE: Determine the feasibility of (1) H-[(13) C] MRS in the mouse hypothalamus using a 14.1T magnet. METHODS: We optimized the design of a (1) H-[(13) C] surface coil to maximize the signal-to-noise ratio of (1) H-[(13) C] MRS in the mouse hypothalamus. With enhanced signal, (13) C accumulation in glucose metabolites was measured in a 8.7 microL voxel in the hypothalamus of 5 healthy mice during the continuous administration of [1,6-(13) C2 ]glucose. RESULTS: Accumulation of (13) C label in glucose C6 and lactate C3 was visible in the hypothalamus 11 min after glucose administration. The (13) C fractional enrichment (FE) curves of lactate C3, glutamate and glutamine C4, glutamate+glutamine C3 and C2, GABA C2, C3, and C4, and aspartate C3 were measured with a time resolution of 11 min over 190 min. FE time-courses and metabolic pool sizes were averaged to fit a novel one-compartment model of brain energy metabolism that incorporates the main features of the hypothalamus. CONCLUSION: Dynamic (1) H-[(13) C] MRS is able to measure in vivo brain metabolism in small and deep areas of the mouse brain such as the hypothalamus, and it can be used to calculate metabolic fluxes, including glutamatergic and GABAergic metabolism as well as the contribution of metabolic sources other than glucose.

DOI : 10.1002/mrm.27129

Cannabis use in early psychosis is associated with reduced glutamate levels in the prefrontal cortex

S. Rigucci; L. Xin; P. Klauser; P. S. Baumann; L. Alameda  et al.

Recent studies have shown that cannabis may disrupt glutamate (Glu) signaling depressing Glu tone in frequent users. Current evidence have also consistently reported lower Glu-levels in various brain regions, particularly in the medial prefrontal cortex (mPFC) of chronic schizophrenia patients, while findings in early psychosis (EP) are not conclusive. Since cannabis may alter Glu synaptic plasticity and its use is a known risk factor for psychosis, studies focusing on Glu signaling in EP with or without a concomitant cannabis-usage seem crucial.

DOI : 10.1007/s00213-017-4745-z

Probing cardiac metabolism by hyperpolarized 13C MR using an exclusively endogenous substrate mixture and photo-induced nonpersistent radicals

J. A. M. Bastiaansen; H. A. I. Yoshihara; A. Capozzi; J. Schwitter; R. Gruetter  et al.

Purpose To probe the cardiac metabolism of carbohydrates and short chain fatty acids simultaneously in vivo following the injection of a hyperpolarized 13C-labeled substrate mixture prepared using photo-induced nonpersistent radicals. Methods Droplets of mixed [1-13C]pyruvic and [1-13C]butyric acids were frozen into glassy beads in liquid nitrogen. Ethanol addition was investigated as a means to increase the polarization level. The beads were irradiated with ultraviolet light and the radical concentration was measured by ESR spectroscopy. Following dynamic nuclear polarization in a 7T polarizer, the beads were dissolved, and the radical-free hyperpolarized solution was rapidly transferred into an injection pump located inside a 9.4T scanner. The hyperpolarized solution was injected in healthy rats to measure cardiac metabolism in vivo. Results Ultraviolet irradiation created nonpersistent radicals in a mixture containing 13C-labeled pyruvic and butyric acids, and enabled the hyperpolarization of both substrates by dynamic nuclear polarization. Ethanol addition increased the radical concentration from 16 to 26 mM. Liquid-state 13C polarization was 3% inside the pump at the time of injection, and increased to 5% by addition of ethanol to the substrate mixture prior to ultraviolet irradiation. In the rat heart, the in vivo 13C signals from lactate, alanine, bicarbonate, and acetylcarnitine were detected following the metabolism of the injected substrate mixture. Conclusion Copolarization of two different 13C-labeled substrates and the detection of their myocardial metabolism in vivo was achieved without using persistent radicals. The absence of radicals in the solution containing the hyperpolarized 13C-substrates may simplify the translation to clinical use, as no radical filtration is required prior to injection.

DOI : 10.1002/mrm.27122

Brain metabolism during Chronic Hepatic Encephalopathy studied by in vivo 1H and 31P MRS

V. Rackayová / R. Gruetter; C. R. Cudalbu (Dir.)

Magnetic resonance spectroscopy (MRS) is a powerful tool increasingly used in biomedical research and also in clinical practice. This thesis focused on the improvements of in vivo 31P MRS and 31P magnetization transfer methods at high magnetic field (9.4T). Then, in vivo 31P MRS was combined with 1H MRS to longitudinally study neurometabolic changes during chronic hepatic encephalopathy (CHE) in adult and developing brain using a well-recognized animal model of cholestatic chronic liver disease and HE type C â bile duct ligated (BDL) rats. CHE is neuro-psychiatric disorder caused by chronic liver disease (CLD), with not well-understood molecular mechanisms, inducing important and sometimes lethal brain changes. In a next step, potential protective treatments (probiotics and high creatine diet) were tested. To the best of our knowledge such a detailed approach was not yet published. Firstly, we showed by in vivo 1H MRS in hippocampus that adult BDL rats suffered from increase in brain glutamine, decrease in metabolites involved in osmoregulation including creatine, decrease in neurotransmitters and ascorbate and we were able to show the importance of early metabolic changes compared to late changes thanks to longitudinal measurements. 31P MRS study in larger brain volume found only mild perturbation of energy metabolism such as non-significant trend in ATP decrease in adult BDL rats. Secondly, studies in young BDL rats at two developmental stages by 1H MRS in the hippocampus revealed similar changes that the one observed in adult brain during CHE. However almost all the neurometabolic changes were more profound or appeared earlier in the evolution of the disease in young BDL rats. A new surface 1H-31P double-tuned coil was built and several optimizations and development were done to improve the spectral resolution, SNR and localization, including implementation of 1H-31P NOE enhancement and 1H decoupling in static 31P MRS, optimization of localization method and also development of an optimal protocol for 31P magnetization transfer. 31P MRS study in p21 BDL rats was performed with an improved 31P MRS protocol and revealed a decreased creatine kinase rate constant 2 weeks after BDL surgery and significant changes in metabolites involved in high-phosphate metabolism, in phospholipid metabolism and also cellular redox state 6 weeks after BDL. In the last part of this thesis, some therapeutic approaches were tested. A probiotic treatment delayed some neurometabolic changes present in CHE in adult BDL rats and improved motor activity. Based on previously shown devastating effects of decreased brain creatine during brain development, we proposed the high creatine diet as a possible treatment for young BDL rats. We demonstrated that oral administration of creatine partially restored brain creatine levels and also had a positive effect on other neurometabolic changes present in CHE. In conclusion, the work done in this thesis revealed for the first time a large spectrum of neurometabolic alterations present in adult and young BDL rats suffering from CLD and CHE, together with the time evolution of these changes during the disease progression, using in vivo longitudinal 1H MRS, improved 31P MRS and 31P magnetization transfer protocols developed herein. In addition, we brought some insight on the positive effects of two innovative treatments on the progression of CHE in adult and young BDL rats.

Hyperpolarization of 2-keto[1-13C]isocaproate for in vivo studies with photo-induced radicals

F. Steffen; H. A. I. Yoshihara; M. Mishkovsky; A. Comment; R. Gruetter

Hyperpolarized 2-keto[1-13C]isocaproate (KIC) provides a means to probe brain nitrogen homeostasis and to assess molecular signatures of tumors. The dynamic nuclear polarization process requires a free-radical polarizing agent, and samples are typically doped with persistent radicals. An alternative is to use photo-induced radicals of α-keto acids that recombine upon dissolution. [1-13C]KIC hyperpolarized with photo-induced radicals could be used to measure the alterations in amino acid metabolism that are linked to neurodegenerative diseases and cancer, and the aim of the present study is to identify the main features that influence the polarization dynamic

Redox dysregulation in schizophrenia pathophysiology: add-on trial with N-acetylcysteine (NAC) in early psychosis patients

M. Fournier; P. Conus; L. Seidman; L. Xin; M. Cleusix  et al.

Altered glycogen metabolism in the brain of insulin-resistant Goto-Kakizaki rats: a 13C magnetic resonance spectroscopy study in V

J. D. N. Duarte; A. F. Soares; S. Nussbaum; R. Gruetter

Defining mitochondrial biomarkers and function using magnetic resonance spectroscopy at 14.1 Tesla in a mouse model of mood disorders

A. Cherix; G. Donati; B. Lizarbe; H. Lei; J. -R. Cardinaux  et al.

In vivo detection of hypothalamic glucose metabolism in HFD and regular fed mice

B. Lizarbe; H. Lei; A. Cherix; R. Gruetter

Influence of physiological noise on accelerated 2D and 3D resting state functional MRI data at 7 T

O. Reynaud; J. Jorge; R. Gruetter; J. P. Marques; W. van der Zwaag

Physiological noise often dominates the blood-oxygen level-dependent (BOLD) signal fluctuations in high-field functional MRI (fMRI) data. Therefore, to optimize fMRI protocols, it becomes crucial to investigate how physiological signal fluctuations impact various acquisition and reconstruction schemes at different acquisition speeds. In particular, further differences can arise between 2D and 3D fMRI acquisitions due to different encoding strategies, thereby impacting fMRI sensitivity in potentially significant ways.

DOI : 10.1002/mrm.26823

Sexual dimorphism in hepatic lipids is associated with the evolution of metabolic status in mice

A. F. Soares; J. Paz-Montoya; H. Lei; M. Moniatte; R. Gruetter

Ectopic lipid accumulation in the liver is implicated in metabolic disease in an age- and sex-dependent manner. The role of hepatic lipids has been well established within the scope of metabolic insults in mice, but has been insufficiently characterized under standard housing conditions, where age-related metabolic alterations are known to occur. We studied a total of 10 male and 10 female mice longitudinally. At 3, 7 and 11 months of age, non-invasive (1) H-magnetic resonance spectroscopy ((1) H-MRS) was used to monitor hepatic lipid content (HLC) and fatty acid composition in vivo, and glucose homeostasis was assessed with glucose and insulin challenges. At the end of the study, hepatic lipids were comprehensively characterized by nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometric analyses of liver tissue samples. In males, HLC increased from 1.4 ± 0.1% at 3 months to 2.9 ± 0.3% at 7 months (p < 0.01) and 2.7 ± 0.3% at 11 months (p < 0.05), in correlation with fasting insulin levels (p < 0.01, r = 0.51) and parameters from the insulin tolerance test (ITT; p < 0.001, r = -0.69 versus area under the curve; p < 0.01, r = -0.57 versus blood glucose drop at 1 h post-ITT; p < 0.01, r = 0.55 versus blood glucose at 3 h post-ITT). The metabolic performance of females remained the same throughout the study, and HLC was higher than that of males at 3 months (2.7 ± 0.2%, p < 0.01), but comparable at 7 months (2.2 ± 0.2%) and 11 months (2.2 ± 0.1%). Strong sexual dimorphism in bioactive lipid species, including diacylglycerols (higher in males, p < 0.0001), phosphatidylinositols (higher in females, p < 0.001) and omega-3 polyunsaturated fatty acids (higher in females, p < 0.01), was found to be in good correlation with metabolic scores at 11 months. Therefore, in mice housed under standard conditions, sex-specific composition of bioactive lipids is associated with metabolic protection in females, whose metabolic performance was independent of hepatic cytosolic lipid content.

DOI : 10.1002/nbm.3761

Coupling of the glutamate-glutamine cycle rate with both glial and neuronal oxidative metabolism in the visual cortex of the Tupaia belangeri

S. Sonnay; J. Poirot; N. Just; A. -C. Clerc; R. Gruetter  et al.

Investigating mitochondrial biomarkers and function using MRS at 14.1 Tesla in a mouse model of mood disorders

A. Cherix; G. Donati; B. Lizarbe; H. Lei; -R. Cardinaux  et al.

In vivo cerebral metabolism of glioblastoma xenografts assessed with ¹H MRS, ¹³C MRS and ¹⁸F-FDG PET

M. Lai / R. Gruetter (Dir.)

Magnetic resonance spectroscopy (MRS) is a powerful tool when studying metabolism in intact and living organs preserving cells in their natural microenvironment with minimal external interference. The unique insights offered by the observation of metabolic processes in vivo are available thanks to the non-invasiveness of the MR technique, due to the employment of non-ionizing radiations at room temperature. These conditions are ideal when studying cerebral metabolism in vivo, since the brain is a particularly complex organ where neurons and glial cells are highly interconnected and dependent from each other. Moreover, brain cells are protected by the blood-brain barrier, which regulates the delivery of nutrients and chemicals. The work of this thesis focused on the application of 1H and 13C MRS in vivo to study cerebral metabolism in mice. 18F-Fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) was used in complement to the abovementioned MR techniques to determine the cerebral metabolic rate of glucose (CMRglc) or local glucose uptake under specific physiological and pathological conditions. At first, we validated the application of 13C MRS to small volumes in the mouse brain in combination to compartmental modeling for neuro-glial metabolism, as previously applied to humans and rats in larger volumes. The second part of the thesis focused on the translation of these techniques to the study of brain metabolism during glioblastoma invasion. A highly infiltrative mouse xenograft model of glioblastoma derived from human glioma spheres was employed in our study to determine the evolution over time of the neurochemical profile detected with 1H MRS at 14.1 Tesla. 13C-labeling experiments with [1,6-13C]glucose were performed at late stage after extensive glioma invasion in the whole brain. Metabolic fluxes were determined under these conditions reflecting energetic metabolism in neurons and glioma/glial cells. Globally, infiltrating cells did not show a highly glycolytic metabolism, as often observed in cancer cells and in glioma cells close to the tumor core, suggesting that environmental stimuli induced by a central necrosis, high cellular density and poor vascularization may be crucial to the occurrence of a metabolic switch. Finally, a novel patient-derived mouse xenograft model of glioblastoma was tested and characterized longitudinally with 1H MRS at 14.1 Tesla. Glioma cells were implanted intracranially after patient tumor resection and minimal manipulation in vitro in order to minimize a selective pressure and promote a close resemblance of the xenograft with the parental tumor. This study showed that patient-derived xenografts obtained from freshly injected glioma cells offer a reliable model to study glioma invasion and related treatments while raising intriguing questions on the mechanisms of adaptation of human cells to the brain of the host.

Hyperpolarized 6Li as a probe for hemoglobin oxygenation level

R. Balzan; M. Mishkovsky; Y. Simonenko; R. B. van Heeswijk; R. Gruetter  et al.

Hyperpolarization by dissolution dynamic nuclear polarization (DNP) is a versatile technique to dramatically enhance the nuclear magnetic resonance (NMR) signal intensity of insensitive long-T1 nuclear spins such as 6 Li. The 6 Li longitudinal relaxation of lithium ions in aqueous solutions strongly depends on the concentration of paramagnetic species, even if they are present in minute amounts. We herein demonstrate that blood oxygenation can be readily detected by taking advantage of the 6 Li signal enhancement provided by dissolution DNP, together with the more than 10% decrease in 6 Li longitudinal relaxation as a consequence of the presence of paramagnetic deoxyhemoglobin.

DOI : 10.1002/cmmi.1656

Simultaneous and interleaved acquisition of NMR signals from different nuclei with a clinical MRI scanner

M. Meyerspeer; A. W. Magill; A. Kuehne; R. Gruetter; E. Moser  et al.

PURPOSE: Modification of a clinical MRI scanner to enable simultaneous or rapid interleaved acquisition of signals from two different nuclei. METHODS: A device was developed to modify the local oscillator signal fed to the receive channel(s) of an MRI console. This enables external modification of the frequency at which the receiver is sensitive and rapid switching between different frequencies. Use of the device was demonstrated with interleaved and simultaneous 31 P and 1 H spectroscopic acquisitions, and with interleaved 31 P and 1 H imaging. RESULTS: Signal amplitudes and signal-to-noise ratios were found to be unchanged for the modified system, compared with data acquired with the MRI system in the standard configuration. CONCLUSION: Interleaved and simultaneous 1 H and 31 P signal acquisition was successfully demonstrated with a clinical MRI scanner, with only minor modification of the RF architecture. While demonstrated with 31 P, the modification is applicable to any detectable nucleus without further modification, enabling a wide range of simultaneous and interleaved experiments to be performed within a clinical setting.

DOI : 10.1002/mrm.26056

Genetic Polymorphism Associated Prefrontal Glutathione and Its Coupling With Brain Glutamate and Peripheral Redox Status in Early Psychosis

L. Xin; R. Mekle; M. Fournier; P. S. Baumann; C. Ferrari  et al.

BACKGROUND: Oxidative stress and glutathione (GSH) metabolism dysregulation has been implicated in the pathophysiology of schizophrenia. GAG-trinucleotide repeat (TNR) polymorphisms in the glutamate-cysteine ligase catalytic gene (GCLC), the rate-limiting enzyme for GSH synthesis, are associated with schizophrenia. In addition, GSH may serve as a reserve pool for neuronal glutamate (Glu) through the gamma-glutamyl cycle. The aim of this study is to investigate brain [GSH] and its association with GCLC polymorphism, peripheral redox indices and brain Glu. METHODS: Magnetic resonance spectroscopy was used to measure [GSH] and [Glu] in the medial prefrontal cortex (mPFC) of 25 early-psychosis patients and 33 controls. GCLC polymorphism was genotyped, glutathione peroxidases (GPx) and glutathione reductase (GR) activities were determined in blood cells. RESULTS: Significantly lower [GSHmPFC] in GCLC high-risk genotype subjects were revealed as compared to low-risk genotype subjects independent of disease status. In male subjects, [GSHmPFC] and blood GPx activities correlate positively in controls (P = .021), but negatively in patients (P = .039). In GCLC low-risk genotypes, [GlumPFC] are lower in patients, while it is not the case for high-risk genotypes. CONCLUSIONS: GCLC high-risk genotypes are associated with low [GSHmPFC], highlighting that GCLC polymorphisms should be considered in pathology studies of cerebral GSH. Low brain GSH levels are related to low peripheral oxidation status in controls but with high oxidation status in patients, pointing to a dysregulated GSH homeostasis in early psychosis patients. GCLC polymorphisms and disease associated correlations between brain GSH and Glu levels may allow patients stratification.

DOI : 10.1093/schbul/sbw038

3D T 2-weighted imaging at 7T using dynamic kT-points on single-transmit MRI systems

F. Eggenschwiler; K. R. O'Brien; D. Gallichan; R. Gruetter; J. P. Marques

OBJECTIVES: For turbo spin echo (TSE) sequences to be useful at ultra-high field, they should ideally employ an RF pulse train compensated for the B 1 + inhomogeneity. Previously, it was shown that a single kT-point pulse designed in the small tip-angle regime can replace all the pulses of the sequence (static kT-points). This work demonstrates that the B 1 + dependence of T 2-weighted imaging can be further mitigated by designing a specific kT-point pulse for each pulse of a 3D TSE sequence (dynamic kT-points) even on single-channel transmit systems MATERIALS AND METHODS: By combining the spatially resolved extended phase graph formalism (which calculates the echo signals throughout the sequence) with a gradient descent algorithm, dynamic kT-points were optimized such that the difference between the simulated signal and a target was minimized at each echo. Dynamic kT-points were inserted into the TSE sequence to acquire in vivo images at 7T. RESULTS: The improvement provided by the dynamic kT-points over the static kT-point design and conventional hard pulses was demonstrated via simulations. Images acquired with dynamic kT-points showed systematic improvement of signal and contrast at 7T over regular TSE-especially in cerebellar and temporal lobe regions without the need of parallel transmission. CONCLUSION: Designing dynamic kT-points for a 3D TSE sequence allows the acquisition of T 2-weighted brain images on a single-transmit system at ultra-high field with reduced dropout and only mild residual effects due to the B 1 + inhomogeneity.

DOI : 10.1007/s10334-016-0545-4

Compartmentalised energy metabolism supporting glutamatergic neurotransmission in response to increased activity in the rat cerebral cortex: A 13C MRS study in vivo at 14.1 T

S. Sonnay; J. M. Duarte; N. Just; R. Gruetter

Many tissues exhibit metabolic compartmentation. In the brain, while there is no doubt on the importance of functional compartmentation between neurons and glial cells, there is still debate on the specific regulation of pathways of energy metabolism at different activity levels. Using 13C magnetic resonance spectroscopy (MRS) in vivo, we determined fluxes of energy metabolism in the rat cortex under α-chloralose anaesthesia at rest and during electrical stimulation of the paws. Compared to resting metabolism, the stimulated rat cortex exhibited increased glutamate–glutamine cycle (+67 nmol/g/min, +95%, P < 0.001) and tricarboxylic (TCA) cycle rate in both neurons (+62 nmol/g/min, +12%, P < 0.001) and astrocytes (+68 nmol/g/min, +22%, P = 0.072). A minor, non-significant modification of the flux through pyruvate carboxylase was observed during stimulation (+5 nmol/g/min, +8%). Altogether, this increase in metabolism amounted to a 15% (67 nmol/g/min, P < 0.001) increase in CMRglc(ox), i.e. the oxidative fraction of the cerebral metabolic rate of glucose. In conclusion, stimulation of the glutamate–glutamine cycle under α-chloralose anaesthesia is associated to similar enhancement of neuronal and glial oxidative metabolism.

DOI : 10.1177/0271678X16629482

Simultaneous EEG-fMRI at ultra-high field for the study of human brain function

J. P. Forjaco Jorge / R. Gruetter; P. Figueiredo (Dir.)

Scalp electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) have highly complementary domains, and their combination has been actively sought within neuroscience research. The important gains in fMRI sensitivity achieved with higher field strengths open exciting perspectives for combined EEG-fMRI; however, simultaneous acquisitions are subject to highly undesirable interactions between the two modalities, which can strongly compromise data quality and subject safety, and most of these interactions are increased at higher fields. The work described in this thesis was centered on the development of simultaneous EEG-fMRI in humans at 7T, covering aspects of subject safety, signal quality assessment, and quality improvement. Additionally, given the potential value of high-field EEG-fMRI to study the neuronal correlates of so-called negative BOLD responses, an initial fMRI study was dedicated to these phenomena. The initial fMRI study aimed to characterize positive (PBR) and negative BOLD responses (NBR) to visual checkerboard stimulation of varying contrast and duration, focusing on NBRs occurring in visual and in auditory cortical regions. Results showed that visual PBRs and both visual and auditory NBRs significantly depend on stimulus contrast and duration, suggesting a dynamic system of visual-auditory interactions, sensitive to stimulus contrast and duration. The neuronal correlates of these interactions could not be addressed in higher detail with fMRI alone, yet could potentially be clarified in future work with combined EEG-fMRI. Moving on to simultaneous EEG-fMRI implementation, the first stage comprised an assessment of potential safety concerns at 7T. The safety tests comprised numerical simulations of RF power distribution and real temperature measurements on a phantom during acquisition. Overall, no significant safety concerns were found for the setup tested. A characterization of artifacts induced on MRI data due to the presence of EEG components was then performed. With the introduction of the EEG system, functional and anatomical images exhibited general losses in spatial SNR, with a smaller loss in temporal SNR in fMRI data. B0 and B1 field mapping pointed towards RF pulse disruption as the major degradation mechanism affecting MRI data. The main part of this work focused on EEG artifacts induced by MRI. The first step focused on optimizing signal transmission between the EEG cap and amplifiers, to minimize artifact contamination at this important stage. Along this line, adequate cable shortening and bundling effectively reduced environment noise in EEG recordings. Simultaneous acquisitions were then performed on humans using the optimized setup. On average, EEG data exhibited clear alpha modulation and average visual evoked potentials (VEP), with concomitant BOLD signal changes. In the second step, a novel approach for head motion artifact detection was developed, based on a simple modification of the EEG cap, and simultaneous acquisitions were performed in volunteers undergoing visual checkerboard stimulation. After gradient artifact correction, EEG signal variance was found to be largely dominated by pulse artifacts, but contributions from spontaneous motion were still comparable to those of neuronal activity. Using a combination of pulse artifact correction, motion artifact correction and ICA denoising, strong improvements in data quality could be obtained, especially at a single-trial level.

Proton diffusion spectroscopy and modeling of brain metabolism at 14.1T

M. Dehghani Moghadam / R. Gruetter (Dir.)

As a field at the CIBM, Nuclear magnetic resonance (NMR)spectroscopy can be applied non-invasively to explore the metabolic fate of energy fuel substrates, as well as the rate at which they are consumed, using 13C and 1H nuclei. The work of this thesis encompasses both nuclei, and focuses on (1) improving the quantification and modeling of glucose-derived metabolites; and (2) characterizing diffusion-related parameters of the purportedly glial-specific energy substrate, acetate. Both aim to quantitatively explore cerebral energy metabolism, at ultra-high magnetic field, in vivo, in the healthy rat. 13C NMR spectroscopy, as a tool, enables measuring the progressive incorporation of 13C-glucose into brain glucose and then NMR detectable amino acids (glutamate and glutamine); this relies on the infusion of the 13C-labeled energy substrate. The experimentally obtained 13C labelling curves are analyzed using suitable mathematical models to provide an estimation of cerebral metabolic rates. Here, a dynamic model of time-courses of 13C multiplets arising from isotopomers was considered. So beyond the two-compartment neuronal-glial model, we took into account additional data on the dynamics of 13C isotopomers, available from the fine structure multiplets in 13C spectra of glutamate and glutamine, measured under prolonged [1-6,13C] glucose infusion. We concluded that the dynamic analyses of 13C multiplet time courses of glutamate and glutamine resulted in a higher precision for estimating the absolute values of most cerebral metabolic rates. Acetate metabolism is challenging because dynamic metabolic modeling requires prior knowledge of the transport and uptake kinetics of infused acetate. We sought this information by determining the apparent concentration and distribution volume (V_d) of cerebral acetate between the intracellular and the extracellular compartments. Experimentally, the diffusion characteristics of cerebral acetate were measured, relative to that of N-Acetylaspartate (NAA, known to be mainly intracellular) using diffusion-weighted 1H NMR spectroscopy at 14.1T, under prolonged acetate infusion. The detection of an acetate and NAA signal at large diffusion weighting provided direct experimental evidence of intracellular cerebral acetate and NAA, although a substantial fraction of acetate was extracellular. To estimate the apparent concentration of in vivo brain acetate, T1 and T2 relaxation times of acetate were measured. The longer T1 relaxation and shorter T2 relaxation times of acetate compared with NAA provided evidence of its small molecular size, and possibly different chemical environment. Our experimentally determined value of V_dled to cerebral metabolic rates of acetate (CMR acetate) of the same order reported for the glial Krebâ s cycle rate, an indication that estimates of CMR acetate are highly dependent on V_d. Finally, in order to pursue metabolic mapping of cerebral acetate uptake in the rat, in vivo, at 14.1 T, the design and construction of a combined transmit-birdcage coil and receive-quadrature pair surface coil was considered. Its performance was compared to a single birdcage coil in the transmit/receive mode. So far, the preliminary results of the 2-coil configuration are promising: homogenous excitation and a gain in sensitivity up to a distance of 5 mm are achievable. Improvements are ongoing for NMR spectroscopic and imaging applications at 14.1 T.

Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized 13C magnetic resonance

J. Bastiaansen; T. Cheng; H. Lei; R. Gruetter; A. Comment

BACKGROUND: The heart relies on continuous energy production and imbalances herein impair cardiac function directly. The tricarboxylic acid (TCA) cycle is the primary means of energy generation in the healthy myocardium, but direct noninvasive quantification of metabolic fluxes is challenging due to the low concentration of most metabolites. Hyperpolarized 13C magnetic resonance spectroscopy (MRS) provides the opportunity to measure cellular metabolism in real time in vivo. The aim of this work was to noninvasively measure myocardial TCA cycle flux (VTCA) in vivo within a single minute. METHODS AND RESULTS: Hyperpolarized [1-13C]acetate was administered at different concentrations in healthy rats. 13C incorporation into [1-13C]acetylcarnitine and the TCA cycle intermediate [5-13C]citrate was dynamically detected in vivo with a time resolution of 3s. Different kinetic models were established and evaluated to determine the metabolic fluxes by simultaneously fitting the evolution of the 13C labeling in acetate, acetylcarnitine, and citrate. VTCA was estimated to be 6.7+/-1.7mumol.g-1.min-1 (dry weight), and was best estimated with a model using only the labeling in citrate and acetylcarnitine, independent of the precursor. The TCA cycle rate was not linear with the citrate-to-acetate metabolite ratio, and could thus not be quantified using a ratiometric approach. The 13C signal evolution of citrate, i.e. citrate formation was independent of the amount of injected acetate, while the 13C signal evolution of acetylcarnitine revealed a dose dependency with the injected acetate. The 13C labeling of citrate did not correlate to that of acetylcarnitine, leading to the hypothesis that acetylcarnitine formation is not an indication of mitochondrial TCA cycle activity in the heart. CONCLUSIONS: Hyperpolarized [1-13C]acetate is a metabolic probe independent of pyruvate dehydrogenase (PDH) activity. It allows the direct estimation of VTCA in vivo, which was shown to be neither dependent on the administered acetate dose nor on the 13C labeling of acetylcarnitine. Dynamic 13C MRS coupled to the injection of hyperpolarized [1-13C]acetate can enable the measurement of metabolic changes during impaired heart function.

DOI : 10.1016/j.yjmcc.2015.08.012

A double-quadrature radiofrequency coil design for proton-decoupled carbon-13 magnetic resonance spectroscopy in humans at 7T

E. Serés Roig; A. W. Magill; G. Donati; M. Meyerspeer; L. Xin  et al.

Purpose Carbon-13 magnetic resonance spectroscopy (13C-MRS) is challenging because of the inherent low sensitivity of 13C detection and the need for radiofrequency transmission at the 1H frequency while receiving the 13C signal, the latter requiring electrical decoupling of the 13C and 1H radiofrequency channels. In this study, we added traps to the 13C coil to construct a quadrature-13C/quadrature-1H surface coil, with sufficient isolation between channels to allow simultaneous operation at both frequencies without compromise in coil performance. Methods Isolation between channels was evaluated on the bench by measuring all coupling parameters. The quadrature mode of the quadrature-13C coil was assessed using in vitro 23Na gradient echo images. The signal-to-noise ratio (SNR) was measured on the glycogen and glucose resonances by 13C-MRS in vitro, compared with that obtained with a linear-13C/quadrature-1H coil, and validated by 13C-MRS in vivo in the human calf at 7T. Results Isolation between channels was better than −30 dB. The 23Na gradient echo images indicate a region where the field is strongly circularly polarized. The quadrature coil provided an SNR enhancement over a linear coil of 1.4, in vitro and in vivo. Conclusion It is feasible to construct a double-quadrature 13C-1H surface coil for proton decoupled sensitivity enhanced 13C-NMR spectroscopy in humans at 7T.

DOI : 10.1002/mrm.25171

A Modulated Closed Form solution for Quantitative Susceptibility Mapping - A thorough evaluation and comparison to iterative methods based on edge prior knowledge

D. Khabipova; Y. Wiaux; R. Gruetter; J. P. Marques

The aim of this study is to perform a thorough comparison of quantitative susceptibility mapping (QSM) techniques and their dependence on the assumptions made. The compared methodologies were: two iterative single orientation methodologies minimizing the l2, l1TV norm of the prior knowledge of the edges of the object, one over-determined multiple orientation method (COSMOS) and anewly proposed modulated closed-form solution (MCF). The performance of these methods was compared using a numerical phantom and in-vivo high resolution (0.65 mm isotropic) brain data acquired at 7 T using a new coil combination method. For all QSM methods, the relevant regularization and prior-knowledge parameters were systematically changed in order to evaluate the optimal reconstruction in the presence and absence of a ground truth. Additionally, the QSM contrast was compared to conventional gradient recalled echo (GRE) magnitude and R2* maps obtained from the same dataset. The QSM reconstruction results of the single orientation methods show comparable performance. The MCF method has the highest correlation (corrMCF = 0.95, r2MCF = 0.97) with the state of the art method (COSMOS) with additional advantage of extreme fast computation time. The l-curve method gave the visually most satisfactory balance between reduction of streaking artifacts and over-regularization with the latter being overemphasized when the using the COSMOS susceptibility maps as ground-truth. R2* and susceptibility maps, when calculated from the same datasets, although based on distinct features of the data, have a comparable ability to distinguish deep gray matter structures.

DOI : 10.1016/j.neuroimage.2014.11.038

In Vivo Longitudinal (1)H MRS Study of Transgenic Mouse Models of Prion Disease in the Hippocampus and Cerebellum at 14.1 T

C. Cudalbu; M. Craveiro; V. Mlynárik; J. Bremer; A. Aguzzi  et al.

In vivo (1)H MR spectroscopy allows the non invasive characterization of brain metabolites and it has been used for studying brain metabolic changes in a wide range of neurodegenerative diseases. The prion diseases form a group of fatal neurodegenerative diseases, also described as transmissible spongiform encephalopathies. The mechanism by which prions elicit brain damage remains unclear and therefore different transgenic mouse models of prion disease were created. We performed an in vivo longitudinal (1)H MR spectroscopy study at 14.1 T with the aim to measure the neurochemical profile of Prnp -/- and PrPΔ32-121 mice in the hippocampus and cerebellum. Using high-field MR spectroscopy we were able to analyze in details the in vivo brain metabolites in Prnp -/- and PrPΔ32-121 mice. An increase of myo-inositol, glutamate and lactate concentrations with a decrease of N-acetylaspartate concentrations were observed providing additional information to the previous measurements.

DOI : 10.1007/s11064-015-1643-9

Single acquisition electrical property mapping based on relative coil sensitivities: A proof-of-concept demonstration

J. P. Marques; D. K. Sodickson; O. Ipek; C. M. Collins; R. Gruetter

All methods presented to date to map both conductivity and permittivity rely on multiple acquisitions to compute quantitatively the magnitude of radiofrequency transmit fields, B1+. In this work, we propose a method to compute both conductivity and permittivity based solely on relative receive coil sensitivities ( B1-) that can be obtained in one single measurement without the need to neither explicitly perform transmit/receive phase separation nor make assumptions regarding those phases.

DOI : 10.1002/mrm.25399

Brain energy metabolism measured by 13C magnetic resonance spectroscopy in vivo upon infusion of [3-13C]lactate

J. M. Duarte; F.-M. Girault; R. Gruetter

The brain uses lactate produced by glycolysis as an energy source. How lactate originated from the blood stream is used to fuel brain metabolism is not clear. The current study measures brain metabolic fluxes and estimates the amount of pyruvate that becomes labeled in glial and neuronal compartments upon infusion of [3-13 C]lactate. For that, labeling incorporation into carbons of glutamate and glutamine was measured by 13 C magnetic resonance spectroscopy at 14.1 T and analyzed with a two-compartment model of brain metabolism to estimate rates of mitochondrial oxidation, glial pyruvate carboxylation, and the glutamate-glutamine cycle as well as pyruvate fractional enrichments. Extracerebral lactate at supraphysiological levels contributes at least two-fold more to replenish the neuronal than the glial pyruvate pools. The rates of mitochondrial oxidation in neurons and glia, pyruvate carboxylase, and glutamate-glutamine cycles were similar to those estimated by administration of 13 C-enriched glucose, the main fuel of brain energy metabolism. These results are in agreement with primary utilization of exogenous lactate in neurons rather than astrocytes.

DOI : 10.1002/jnr.23531

Stroking or Buzzing? A Comparison of Somatosensory Touch Stimuli Using 7 Tesla fMRI

W. van der Zwaag; R. Gruetter; R. Martuzzi

Studying body representations in the brain helps us to understand how we humans relate to our own bodies. The in vivo mapping of the somatosensory cortex, where these representations are found, is greatly facilitated by the high spatial resolution and high sensitivity to brain activation available at ultra-high field. In this study, the use of different stimulus types for somatotopic mapping of the digits at ultra-high field, specifically manual stroking and mechanical stimulation, was compared in terms of sensitivity and specificity of the brain responses. Larger positive responses in digit regions of interest were found for manual stroking than for mechanical stimulation, both in terms of average and maximum t-value and in terms of number of voxels with significant responses to the tactile stimulation. Responses to manual stroking were higher throughout the entire post-central sulcus, but the difference was especially large on its posterior wall, i.e. in Brodmann area 2. During mechanical stimulation, cross-digit responses were more negative than during manual stroking, possibly caused by a faster habituation to the stimulus. These differences indicate that manual stroking is a highly suitable stimulus for fast somatotopic mapping procedures, especially if Brodmann area 2 is of interest.

DOI : 10.1371/journal.pone.0134610

Imaging of prolonged BOLD response in the somatosensory cortex of the rat

S. Sonnay; N. Just; J. M. N. Duarte; R. Gruetter

Blood oxygenation level-dependent (BOLD) functional MRI is a widely employed methodology in experimental and clinical neuroscience, although its nature is not fully understood. To gain insights into BOLD mechanisms and take advantage of the new functional methods, it is of interest to investigate prolonged paradigms of activation suitable for long experimental protocols and to observe any long-term modifications induced by these functional challenges. While different types of sustained stimulation paradigm have been explored in human studies, the BOLD response is typically limited to a few minutes in animal models, due to fatigue, anesthesia effects and physiological instability. In the present study, the rat forepaw was electrically stimulated for 2 h, which resulted in a prolonged and localized cortical BOLD response over that period. The stimulation paradigm, including an inter-stimulus interval (ISI) of 10 s, that is 25% of the total time, was applied at constant or variable frequency over 2 h. The steady-state level of the BOLD response was reached after 15-20 min of stimulation and was maintained until the end of the stimulation. On average, no substantial loss in activated volume was observed at the end of the stimulation, but less variability in the fraction of remaining activated volume and higher steady-state BOLD amplitude were observed when stimulation frequency was varied between 2 and 3 Hz every 5 min. We conclude that the combination of ISI and variable stimulus frequency reproducibly results in robust, prolonged and localized BOLD activation. Copyright © 2015 John Wiley & Sons, Ltd.

DOI : 10.1002/nbm.3263

Fasting-induced changes of hepatic lipid and carbohydrate stores in the absence of GLUT2

A. F. Soares; H. Lei; B. Thorens; R. Gruetter

Correcting surface coil excitation inhomogeneities in single-shot SPEN MRI

R. Schmidt; M. Mishkovsky; J.-N. Hyacinthe; N. Kunz; R. Gruetter  et al.

Given their high sensitivity and ability to limit the field of view (FOV), surface coils are often used in magnetic resonance spectroscopy (MRS) and imaging (MRI). A major downside of surface coils is their inherent radiofrequency (RF) B1 heterogeneity across the FOV, decreasing with increasing distance from the coil and giving rise to image distortions due to non-uniform spatial responses. A robust way to compensate for B1 inhomogeneities is to employ adiabatic inversion pulses, yet these are not well adapted to all imaging sequences –including to single-shot approaches like echo planar imaging (EPI). Hybrid spatiotemporal encoding (SPEN) sequences relying on frequency-swept pulses provide another ultrafast MRI alternative, that could help solve this problem thanks to their built-in heterogeneous spatial manipulations. This study explores how this intrinsic SPEN-based spatial discrimination, could be used to compensate for the B1 inhomogeneities inherent to surface coils. Experiments carried out in both phantoms and in vivo rat brains demonstrate that, by suitably modulating the amplitude of a SPEN chirp pulse that progressively excites the spins in a direction normal to the coil, it is possible to compensate for the RF transmit inhomogeneities and thus improve sensitivity and image fidelity.

DOI : 10.1016/j.jmr.2015.08.018

Glutathione deficit impairs myelin maturation: relevance for white matter integrity in schizophrenia patients

A. Monin; P. Baumann; A. Griffa; L. Xin; R. Mekle  et al.

Schizophrenia pathophysiology implies both abnormal redox control and dysconnectivity of the prefrontal cortex, partly related to oligodendrocyte and myelin impairments. As oligodendrocytes are highly vulnerable to altered redox state, we investigated the interplay between glutathione and myelin. In control subjects, multimodal brain imaging revealed a positive association between medial prefrontal glutathione levels and both white matter integrity and resting-state functional connectivity along the cingulum bundle. In early psychosis patients, only white matter integrity was correlated with glutathione levels. On the other side, in the prefrontal cortex of peripubertal mice with genetically impaired glutathione synthesis, mature oligodendrocyte numbers, as well as myelin markers, were decreased. At the molecular levels, under glutathione-deficit conditions induced by short hairpin RNA targeting the key glutathione synthesis enzyme, oligodendrocyte progenitors showed a decreased proliferation mediated by an upregulation of Fyn kinase activity, reversed by either the antioxidant N-acetylcysteine or Fyn kinase inhibitors. In addition, oligodendrocyte maturation was impaired. Interestingly, the regulation of Fyn mRNA and protein expression was also impaired in fibroblasts of patients deficient in glutathione synthesis. Thus, glutathione and redox regulation have a critical role in myelination processes and white matter maturation in the prefrontal cortex of rodent and human, a mechanism potentially disrupted in schizophrenia.

DOI : 10.1038/mp.2014.88

Physiological noise in human cerebellar fMRI

W. Van Der Zwaag; J. Jorge; D. Butticaz; R. Gruetter

Objectives To compare physiological noise contributions in cerebellar and cerebral regions of interest in high-resolution functional magnetic resonance imaging (fMRI) data acquired at 7T, to estimate the need for physiological noise removal in cerebellar fMRI. Materials and methods Signal fluctuations in high resolution (1 mm isotropic) 7T fMRI data were attributed to one of the following categories: task-induced BOLD changes, slow drift, signal changes correlated with the cardiac and respiratory cycles, signal changes related to the cardiac rate and respiratory volume per unit of time or other. R2adj values for all categories were compared across regions of interest. Results In this high-resolution data, signal fluctuations related to the phase of the cardiac cycle and cardiac rate were shown to be significant, but comparable between cerebellar and cerebral regions of interest. However, respiratory related signal fluctuations were increased in the cerebellar regions, with explained variances that were up to 80 % higher than for the primary motor cortex region. Conclusion Even at a millimetre spatial resolution, significant correlations with both cardiac and respiratory RETROICOR components were found in all healthy volunteer data. Therefore, physiological noise correction is highly likely to improve the temporal signal-to-noise ratio (SNR) for cerebellar fMRI at 7T, even at high spatial resolution.

DOI : 10.1007/s10334-015-0483-6

Impact of insulin resistance on memory performance, brain morphology and neurochemical profiles of the cortex and hippocampus

J. M. N. Duarte; R. Gruetter

Longitudinal neurochemical modifications in the aging mouse brain measured in vivo by 1H magnetic resonance spectroscopy

J. M. N. Duarte; K. Q. Do; R. Gruetter

Alterations to brain homeostasis during development are reflected in the neurochemical profile determined noninvasively by (1)H magnetic resonance spectroscopy. We determined longitudinal biochemical modifications in the cortex, hippocampus, and striatum of C57BL/6 mice aged between 3 and 24 months . The regional neurochemical profile evolution indicated that aging induces general modifications of neurotransmission processes (reduced GABA and glutamate), primary energy metabolism (altered glucose, alanine, and lactate) and turnover of lipid membranes (modification of choline-containing compounds and phosphorylethanolamine), which are all probably involved in the frequently observed age-related cognitive decline. Interestingly, the neurochemical profile was different in male and female mice, particularly in the levels of taurine that may be under the control of estrogen receptors. These neurochemical profiles constitute the basal concentrations in cortex, hippocampus, and striatum of healthy aging male and female mice.

DOI : 10.1016/j.neurobiolaging.2014.01.135

Non-invasive diagnostic biomarkers for estimating the onset time of permanent cerebral ischemia

C. Berthet; L. Xin; L. Buscemi; C. Benakis; R. Gruetter  et al.

The treatments for ischemic stroke can only be administered in a narrow time-window. However, the ischemia onset time is unknown in ~30% of stroke patients (wake-up strokes). The objective of this study was to determine whether MR spectra of ischemic brains might allow the precise estimation of cerebral ischemia onset time. We modeled ischemic stroke in male ICR-CD1 mice using a permanent middle cerebral artery filament occlusion model with laser Doppler control of the regional cerebral blood flow. Mice were then subjected to repeated MRS measurements of ipsilateral striatum at 14.1[thinsp]T. A striking initial increase in [gamma]-aminobutyric acid (GABA) and no increase in glutamine were observed. A steady decline was observed for taurine (Tau), N-acetyl-aspartate (NAA) and similarly for the sum of NAA+Tau+glutamate that mimicked an exponential function. The estimation of the time of onset of permanent ischemia within 6[thinsp]hours in a blinded experiment with mice showed an accuracy of 33[plusmn]10[thinsp]minutes. A plot of GABA, Tau, and neuronal marker concentrations against the ratio of acetate/NAA allowed precise separation of mice whose ischemia onset lay within arbitrarily chosen time-windows. We conclude that 1H-MRS has the potential to detect the clinically relevant time of onset of ischemic stroke.

DOI : 10.1038/jcbfm.2014.155

Improved temporal resolution for functional studies with reduced number of segments with three-dimensional echo planar imaging

M. Narsude; W. Van Der Zwaag; T. Kober; R. Gruetter; J. P. Marques

PURPOSE: To introduce a new k-space traversal strategy for segmented three-dimensional echo planar imaging (3D EPI) that encodes two partitions per radiofrequency excitation, effectively reducing the number excitations used to acquire a 3D EPI dataset by half. METHODS: The strategy was evaluated in the context of functional MRI applications for: image quality compared with segmented 3D EPI, temporal signal-to-noise ratio (tSNR) (the ability to detect resting state networks compared with multislice two-dimensional (2D) EPI and segmented 3D EPI, and temporal resolution (the ability to separate cardiac- and respiration-related fluctuations from the desired blood oxygen level-dependent signal of interest). RESULTS: Whole brain images with a nominal voxel size of 2 mm isotropic could be acquired with a temporal resolution under half a second using traditional parallel imaging acceleration up to 4x in the partition-encode direction and using novel data acquisition speed-up of 2x with a 32-channel coil. With 8x data acquisition speed-up in the partition-encode direction, 3D reduced excitations (RE)-EPI produced acceptable image quality without introduction of noticeable additional artifacts. Due to increased tSNR and better characterization of physiological fluctuations, the new strategy allowed detection of more resting state networks compared with multislice 2D-EPI and segmented 3D EPI. CONCLUSION: 3D RE-EPI resulted in significant increases in temporal resolution for whole brain acquisitions and in improved physiological noise characterization compared with 2D-EPI and segmented 3D EPI.

DOI : 10.1002/mrm.24975

Improving T2 -weighted imaging at high field through the use of kT -points

F. Eggenschwiler; K. R. O'Brien; R. Gruetter; J. P. Marques

PURPOSE: At high magnetic field strengths (B0 >/= 3 T), the shorter radiofrequency wavelength produces an inhomogeneous distribution of the transmit magnetic field. This can lead to variable contrast across the brain which is particularly pronounced in T2 -weighted imaging that requires multiple radiofrequency pulses. To obtain T2 -weighted images with uniform contrast throughout the whole brain at 7 T, short (2-3 ms) 3D tailored radiofrequency pulses (kT -points) were integrated into a 3D variable flip angle turbo spin echo sequence. METHODS: The excitation and refocusing "hard" pulses of a variable flip angle turbo spin echo sequence were replaced with kT -point pulses. Spatially resolved extended phase graph simulations and in vivo acquisitions at 7 T, utilizing both single channel and parallel-transmit systems, were used to test different kT -point configurations. RESULTS: Simulations indicated that an extended optimized k-space trajectory ensured a more homogeneous signal throughout images. In vivo experiments showed that high quality T2 -weighted brain images with uniform signal and contrast were obtained at 7 T by using the proposed methodology. CONCLUSION: This work demonstrates that T2 -weighted images devoid of artifacts resulting from B1 + inhomogeneity can be obtained at high field through the optimization of extended kT -point pulses.

DOI : 10.1002/mrm.24805

Optimized MEGA-SPECIAL for in vivo glutamine detection in the rat brain at 14.1 T

M. Craveiro; C. Cudalbu; V. Mlynárik; R. Gruetter

Glutamine has multiple roles in brain metabolism and its concentration can be altered in various pathological conditions. An accurate knowledge of its concentration is therefore highly desirable to monitor and study several brain disorders in vivo. However, in recent years, several MRS studies have reported conflicting glutamine concentrations in the human brain. A recent hypothesis for explaining these discrepancies is that a short T2 component of the glutamine signal may impact on its quantification at long echo times. The present study therefore aimed to investigate the impact of acquisition parameters on the quantified glutamine concentration using two different acquisition techniques, SPECIAL at ultra-short echo time and MEGA-SPECIAL at moderate echo time. For this purpose, MEGA-SPECIAL was optimized for the first time for glutamine detection. Based on the very good agreement of the glutamine concentration obtained between the two measurements, it was concluded that no impact of a short T2 component of the glutamine signal was detected.

DOI : 10.1002/nbm.3168

Experimental peripheral arterial disease: new insights into muscle glucose uptake, macrophage, and T-cell polarization during early and late stages

M. Pellegrin; K. Bouzourène; C. Poitry-Yamate; V. Mlynarik; F. Feihl  et al.

Peripheral arterial disease (PAD) is a common disease with increasing prevalence, presenting with impaired walking ability affecting patient's quality of life. PAD epidemiology is known, however, mechanisms underlying functional muscle impairment remain unclear. Using a mouse PAD model, aim of this study was to assess muscle adaptive responses during early (1 week) and late (5 weeks) disease stages. Unilateral hindlimb ischemia was induced in ApoE−/− mice by iliac artery ligation. Ischemic limb perfusion and oxygenation (Laser Doppler imaging, transcutaneous oxygen pressure assessments) significantly decreased during early and late stage compared to pre-ischemia, however, values were significantly higher during late versus early phase. Number of arterioles and arteriogenesis-linked gene expression increased at later stage. Walking ability, evaluated by forced and voluntary walking tests, remained significantly decreased both at early and late phase without any significant improvement. Muscle glucose uptake ([18F]fluorodeoxyglucose positron emission tomography) significantly increased during early ischemia decreasing at later stage. Gene expression analysis showed significant shift in muscle M1/M2 macrophages and Th1/Th2 T cells balance toward pro-inflammatory phenotype during early ischemia; later, inflammatory state returned to neutrality. Muscular M1/M2 shift inhibition by a statin prevented impaired walking ability in early ischemia. High-energy phosphate metabolism remained unchanged (31-Phosphorus magnetic resonance spectroscopy). Results show that rapid transient muscular inflammation contributes to impaired walking capacity while increased glucose uptake may be a compensatory mechanisms preserving immediate limb viability during early ischemia in a mouse PAD model. With time, increased ischemic limb perfusion and oxygenation assure muscle viability although not sufficiently to improve walking impairment. Subsequent decreased muscle glucose uptake may partly contribute to chronic walking impairment. Early inflammation inhibition and/or late muscle glucose impairment prevention are promising strategies for PAD management.

DOI : 10.1002/phy2.234

Protective effects of maternal nutritional supplementation with lactoferrin on growth and brain metabolism

E. Somm; P. Larvaron; Y. Van De Looij; A. Toulotte; A. Chatagner  et al.

Background:Intrauterine growth restriction (IUGR) is a major risk factor for both perinatal and long-term morbidity. Bovine lactoferrin (bLf) is a major milk glycoprotein considered as a pleiotropic functional nutrient. The impact of maternal supplementation with bLf on IUGR-induced sequelae, including inadequate growth and altered cerebral development, remains unknown.Methods:IUGR was induced through maternal dexamethasone infusion (100 mug/kg during last gestational week) in rats. Maternal supplementation with bLf (0.85% in food pellet) was provided during both gestation and lactation. Pup growth was monitored, and Pup brain metabolism and gene expression were studied using in vivo (1)H NMR spectroscopy, quantitative PCR, and microarray in the hippocampus at postnatal day (PND)7.Results:Maternal bLf supplementation did not change gestational weight but increased the birth body weight of control pups (4%) with no effect on the IUGR pups. Maternal bLf supplementation allowed IUGR pups to recover a normalized weight at PND21 (weaning) improving catch-up growth. Significantly altered levels of brain metabolites (gamma-aminobutyric acid, glutamate, N-acetylaspartate, and N-acetylaspartylglutamate) and transcripts (brain-derived neurotrophic factor (BDNF), divalent metal transporter 1 (DMT-1), and glutamate receptors) in IUGR pups were normalized with maternal bLf supplementation.Conclusion:Our data suggest that maternal bLf supplementation is a beneficial nutritional intervention able to revert some of the IUGR-induced sequelae, including brain hippocampal changes.

DOI : 10.1038/pr.2013.199

Hyperpolarized 13C lactate as a substrate for in vivo metabolic studies in skeletal muscle

J. A. M. Bastiaansen; H. A. I. Yoshihara; Y. Takado; R. Gruetter; A. Comment

Resting skeletal muscle has a preference for the oxidation of lipids compared to carbohydrates and a shift towards carbohydrate oxidation is observed with increasing exercise. Lactate is not only an end product in skeletal muscle but also an important metabolic intermediate for mitochondrial oxidation. Recent advances in hyperpolarized MRS allow the measurement of substrate metabolism in vivo in real time. The aim of this study was to investigate the use of hyperpolarized 13C lactate as a substrate for metabolic studies in skeletal muscle in vivo. Carbohydrate metabolism in healthy rat skeletal muscle at rest was studied in different nutritional states using hyperpolarized [1-13C]lactate, a substrate that can be injected at physiological concentrations and leaves other oxidative processes undisturbed. 13C label incorporation from lactate into bicarbonate in fed animals was observed within seconds but was absent after an overnight fast, representing inhibition of the metabolic flux through pyruvate dehydrogenase (PDH). A significant decrease in 13C labeling of alanine was observed comparing the fed and fasted group, and was attributed to a change in cellular alanine concentration and not a decrease in enzymatic flux through alanine transaminase. We conclude that hyperpolarized [1-13C]lactate can be used to study carbohydrate oxidation in resting skeletal muscle at physiological levels. The herein proposed method allows probing simultaneously both PDH activity and variations in alanine tissue concentration, which are associated with metabolic dysfunctions. A simple alteration of the nutritional state demonstrated that the observed pyruvate, alanine, and bicarbonate signals are indeed sensitive markers to probe metabolic changes in vivo.

DOI : 10.1007/s11306-014-0630-5

Genetic Association with Prefrontal Glutathione Deficit: A 3T 1H MRS Study in Early Psychosis

L. Xin; R. Mekle; C. Ferrari; P. S. Baumann; L. Alameda  et al.

Impact of hepatic lipid accumulation and composition on glucose tolerance and insulin sensitivity: a longitudinal study in male and female mice

A. F. Soares; H. Lei; R. Gruetter

Background and aims: Metabolic disruptions characterized by high hepatic lipid content (HLC) are associated with impairments in whole body glucose homeostasis. To gain insight on the role of hepatic lipids in the metabolic performance in the absence artificial metabolic stresses we measured non- invasively and longitudinally the HLC and profile in mice during adult devel - opment by Magnetic Resonance (MR) Spectroscopy in vivo . In parallel, mice were challenged with insulin and glucose tolerance tests. Materials and methods: Male (N=10) and female (N=10) C57Bl/6J mice were studied at 3 (3Mo), 7 (7Mo) and 10 months (10Mo) of age. Mice were scanned in a 14.1 T magnet with a 1 H quadrature surface coil over the abdo - men. Localized 1 H spectra were acquired from a 8 μl volume with stimulated echo acquisition mode sequence and the HLC expressed as the percent of to - tal 1 H MR signal, with corrections for spin-spin relaxation effects. Additional spectra were acquired from the same volume with suppression of the water signal to enable the detection and quantification of all the lipid protons. The lipid profile was characterized by the following indices: saturated component (SC); unsaturated fatty acyl chains (UFA); mean number of double bonds per fatty acyl chain (ndb/FA), mean number of poly-unsaturated double bonds per fatty acyl chain (PUdb/FA) and per UFA (PUdb/UFA); mean chain length (MCL). OGTTs (1.5 g/Kg) and i.p. insulin tolerance tests (ITTs) were per - formed after a 6h-fast. Plasma insulin was determined by ELISA and insulin sensitivity estimated with the quantitative insulin check index (QUICKI) as the inverse of the log 10 sum of fasting insulin (μIU/ml) and fasting glucose (mg/dl). Data are expressed as mean ± SEM. Statistical significance was ac - cepted for a P < 0.05 (one-way ANOVA with Newman-Keuls post test) and correlations assessed by the Pearson r coefficient. Results: In males, the HLC at 3Mo was 1.35 ± 0.15%, increasing to 3.06 ± 0.38% at 7Mo, not different from 2.70 ± 0.31% at 10Mo. Females had higher HLC at 3Mo (2.63 ± 0.19%) but no further changes henceforward (2.31 ± 0.20% at 7Mo; 2.36 ± 0.20% at10 Mo). In males, the SC and MCL of hepatic lipids increased with age, with a trend for decreased PUdb/FA and PUdb/ UFA with no changes in ndb/FA or UFA content. Females showed the same trends. Glycemia 3h-post ITT and 2h-post OGTT was lower in females, while QUICKI was higer. These scores were preserved until 10Mo in females. In males, glycemia 2-h post OGTT increased with age and the area above the curve (AAC) for the ITT decreased. In males, but not females, higher body weight correlated with hepatic lipid accumulation ( r = 0.7); worse ITT scores correlated with higher body weight ( r = -0.6) and HLC ( r = -0.7) and lower Pudb/UFA ( r = 0.5); worse OGTT scores correlated with higher HLC ( r = 0.4). Conclusion: In male mice, loss of insulin sensitivity correlated with weight gain, HL accumulation and lower poly-unsaturation. Glucose intolerance was specifically associated with HLC, suggesting a deleterious effect of lipids on the adaptation of hepatic metabolism to the fed state. This behaviour was not observed in females even if they showed similar HLC. In fact, the poly- unsaturation of HL in females didn’t change with HLC, suggesting a positive effect of PUFA on preserving the hepatic metabolic performance.

Image-Derived Input Function from the Vena Cava for 18F-FDG PET Studies in Rats and Mice

B. Lanz; C. Poitry-Yamate; R. Gruetter

Measurement of arterial input function is a restrictive aspect for quantitative 18F-FDG PET studies in rodents because of their small total blood volume and the related difficulties in withdrawing blood. METHODS: In the present study, we took advantage of the high spatial resolution of a recent dedicated small-animal scanner to extract the input function from the 18F-FDG PET images in Sprague-Dawley rats (n = 4) and C57BL/6 mice (n = 5), using the vena cava. In the rat experiments, the validation of the image-derived input function (IDIF) method was made using an external microvolumetric blood counter as reference for the determination of the arterial input function, the measurement of which was confirmed by additional manually obtained blood samples. Correction for tracer bolus dispersion in blood between the vena cava and the arterial tree was applied. In addition, simulation studies were undertaken to probe the impact of the different IDIF extraction approaches on the determined cerebral metabolic rate of glucose (CMRGlc). In the mice measurements, the IDIF was used to compute the CMRGlc, which was compared with previously reported values, using the Patlak approach. RESULTS: The presented IDIF from the vena cava showed a robust determination of CMRGlc using either the compartmental modeling or the Patlak approach, even without bolus dispersion correction or blood sampling, with an underestimation of CMRGlc of 7% +/- 16% as compared with the reference data. Using this approach in the mice experiments, we measured a cerebral metabolic rate in the cortex of 0.22 +/- 0.10 mumol/g/min (mean +/- SD), in good agreement with previous 18F-FDG studies in the mouse brain. In the rat experiments, dispersion correction of the IDIF and additional scaling of the IDIF using a single manual blood sample enabled an optimized determination of CMRGlc, with an underestimation of 6% +/- 7%. CONCLUSION: The vena cava time-activity curve is therefore a minimally invasive alternative for the measurement of the 18F-FDG input function in rats and mice, without the complications associated with repetitive blood sampling.

DOI : 10.2967/jnumed.113.127381

Impaired White Matter Integrity in Fornix and Anterior Commissure in a Schizophrenia Mouse Model of Redox Dysregulation

A. Corcoba; P. Steullet; J. M. N. Duarte; Y. Van De Looij; R. Gruetter  et al.

Definition and quantification of acute inflammatory white matter injury in the immature brain by MRI/MRS at high magnetic field

G. A. Lodygensky; N. Kunz; E. Perroud; E. Somm; V. Mlynarik  et al.

Background: Intrauterine growth restriction (IUGR) is a major risk factor for both perinatal and long-term morbidity. Bovine lactoferrin (bLf) is a major milk glycoprotein considered as a pleiotropic functional nutrient. The impact of maternal supplementation with bLf on IUGR-induced sequelae, including inadequate growth and altered cerebral development, remains unknown. Methods: IUGR was induced through maternal dexamethasone infusion (100 μg/kg during last gestational week) in rats. Maternal supplementation with bLf (0.85% in food pellet) was provided during both gestation and lactation. Pup growth was monitored, and Pup brain metabolism and gene expression were studied using in vivo 1H NMR spectroscopy, quantitative PCR, and microarray in the hippocampus at postnatal day (PND)7. Results: Maternal bLf supplementation did not change gestational weight but increased the birth body weight of control pups (4%) with no effect on the IUGR pups. Maternal bLf supplementation allowed IUGR pups to recover a normalized weight at PND21 (weaning) improving catch-up growth. Significantly altered levels of brain metabolites (γ-aminobutyric acid, glutamate, N-acetylaspartate, and N-acetylaspartylglutamate) and transcripts (brain-derived neurotrophic factor (BDNF), divalent metal transporter 1 (DMT-1), and glutamate receptors) in IUGR pups were normalized with maternal bLf supplementation. Conclusion: Our data suggest that maternal bLf supplementation is a beneficial nutritional intervention able to revert some of the IUGR-induced sequelae, including brain hippocampal changes.

DOI : 10.1038/pr.2013.242

High Temporal Resolution Functional Magnetic Resonance Imaging at 7 Tesla

M. Narsude / R. Gruetter (Dir.)

Speed has always been a critical consideration in magnetic resonance imaging (MRI), promising dramatic gains in imaging speed, a reduction in motion and susceptibility artifacts, and ultimately increased throughput for clinical studies. This thesis presents several new approaches to increase temporal resolution of a three dimensional (3D) echo planar imaging (EPI) sequence and explores the benefits of using high temporal resolution for both event-related brain activation and resting state networks fMRI studies. In the first approach, a new k-space traversal strategy for segmented 3D EPI is introduced which we termed as 3D reduced excitation EPI (3D RE-EPI). In this approach, two partitions are encoded per radio frequency (RF) excitation, effectively reducing the number of excitations used to acquire a 3D EPI dataset by a factor of two. With this strategy, whole-brain images with a nominal voxel size of 2mmisotropic could be acquired with a temporal resolution under half a second, using traditional parallel imaging acceleration upto 4x in the partition-encoding direction and using novel data acquisition speed-up of 2x. With 8x data acquisition speed-up in the partition-encoding direction, 3D RE-EPI produced acceptable image quality without introduction of noticeable additional artifacts. Due to increased temporal signal-to-noise ratio (tSNR) and better characterization of physiological fluctuations, the new strategy allowed detection of more resting state networks compared to standardmulti-slice two dimensional (2D) EPI and segmented 3D EPI. The approach above suffered from geometry factor (g-factor related) signal-to-noise ratio (SNR) losses, while using very high parallel imaging acceleration factor. To achieve substantial increase in the temporal resolution whilemaintaining low g-factors when high parallel imaging acceleration factors are used, the combination of segmented 3D EPI using 2D controlled aliasing with generalized autocalibrating partially parallel acquisitions (GRAPPA) was implemented in the next approach termed 3D-EPI-CAIPI. FunctionalMRI data with whole-brain coverage, a voxel size of 2 mm isotropic and a temporal resolution of 371 ms was acquired with acceptable image quality. 10-fold parallel imaging accelerated 3D-EPI-CAIPI data was shown to lower g-factor losses by as much as 10% with respect to segmented 3D EPI at 7 Tesla. Additional resting state networks were detected using 3D-EPI-CAIPI compared to a comparable standard 2D EPI acquisition. This was attributed to the improved statistics due to an increased number of volumes acquired in a given duration and because of the improved characterization of physiological signal fluctuations. Functional MRI with 400 ms temporal resolution allowed the detection of time-to-peak variations in temporal, occipital and frontal cortices haemodynamic responses due to multisensory facilitation of the order of ~200 ms. To conclude, this thesis introduces several novel techniques to increase temporal resolution of fMRI. Besides building theoretical foundations for these techniques, their applications for studying human brain functional connectivity, event-related brain activation and the ability to characterize physiological signal fluctuations at ultra-high field strength (7 Tesla) are evaluated.

In vivo brain macromolecule signals in healthy and glioblastoma mouse models: 1H magnetic resonance spectroscopy, post-processing and metabolite quantification at 14.1 T

M. Craveiro; V. Clément-Schatlo; D. Marino; R. Gruetter; C. Cudalbu

In 1H magnetic resonance spectroscopy, macromolecule signals underlay metabolite signals, the understanding of which is necessary for reliable metabolite quantification. When macromolecule signals are measured using an inversion-recovery pulse sequence, special care needs to be taken to correctly remove residual metabolite signals to obtain a pure macromolecule spectrum. Furthermore, since a single spectrum is commonly used for quantification in multiple experiments, the impact of potential macromolecule signal variability, due to regional differences or pathologies, on metabolite quantification has to be assessed. In this study, we introduced a novel method to post-process measured macromolecule signals that offers a flexible and robust way of removing residual metabolite signals. This method was applied to investigate regional differences in the mouse brain macromolecule signals that may affect metabolite quantification when not taken into account. However, since no significant differences in metabolite quantification were detected, it was concluded that a single macromolecule spectrum can be generally used for the quantification of healthy mouse brain spectra. Alternatively, the study of a mouse model of human glioma showed several alterations of the macromolecule spectrum, including, but not limited to, increased mobile lipids signals, which had to be taken into account to avoid significant metabolite quantification errors.

DOI : 10.1111/jnc.12673

Glutamatergic and GABAergic energy metabolism measured in the rat brain by 13C NMR spectroscopy at 14.1 T

J. M. N. Duarte; R. Gruetter

Energy metabolism supports both inhibitory and excitatory neurotransmission processes. This study investigated the specific contribution of astrocytic metabolism to γ-aminobutyric acid (GABA) synthesis and inhibitory GABAergic neurotransmission that remained to be ilucidated in vivo. Therefore we measured 13C incorporation into brain metabolites by dynamic 13C nuclear magnetic resonance spectroscopy at 14.1 T in rats under α-chloralose anaesthesia during infusion of [1,6-13C]glucose. The enhanced sensitivity at 14.1 T allowed to quantify incorporation of 13C into the three aliphatic carbons of GABA non-invasively. Metabolic fluxes were determined with a mathematical model of brain metabolism comprising glial, glutamatergic and GABAergic compartments. GABA synthesis rate was 0.11±0.01 μmol/g/min. GABA-glutamine cycle was 0.053±0.003 μmol/g/min and accounted for 22±1% of total neurotransmitter cycling between neurons and glia. Cerebral glucose oxidation was 0.47±0.02 μmol/g/min, of which 35±1% and 7±1% was diverted to the glutamatergic and GABAergic tricarboxylic acid cycles, respectively. The remaining fraction of glucose oxidation was in glia, where 12±1% of the TCA cycle flux was dedicated to oxidation of GABA. 16±2% of glutamine synthesis was provided to GABAergic neurons. We conclude that substantial metabolic activity occurs in GABAergic neurons and that glial metabolism supports both glutamatergic and GABAergic neurons in the living rat brain. This article is protected by copyright. All rights reserved.

DOI : 10.1111/jnc.12333

Brain metabolism in rat model of human glioma initiating cells

M. Mishkovsky; C. R. Cudalbu; A. Comment; D. Mario; I. Radovanovic  et al.

In vivo metabolic studies in skeletal and cardiac muscle using ¹³C magnetic resonance spectroscopy

J. A. M. Bastiaansen / R. Gruetter; A. Comment (Dir.)

Cardiac and skeletal muscle function relies on a continuous energy production via fatty acid metabolism and mitochondrial oxidation of pyruvate, with a fine balance between substrate delivery and utilization. Changes in metabolism are increasingly being implicated as playing an intrinsic role in many diseases, such as diabetes, cancer, and heart failure. Investigating and identifying fundamental metabolic processes are paramount to understanding pathologies. Beyond morphology and functional information, magnetic resonance can provide insights at a metabolic level using spectroscopic techniques such as 13C NMR. The low natural abundance and sensitivity of the 13C nucleus makes 13C NMR in biological systems challenging. In addressing this issue, hyperpolarized methods have emerged as a very promising tool, obtaining signal enhancements up to 10,000 fold. Spectra of hyperpolarized 13C labeled substrates and their downstream metabolic products offer insight into metabolic processes occurring in vivo within seconds after the injection. This thesis focused on the development of MR hyperpolarization methods and applications to study energy metabolism in cardiac and skeletal muscle in vivo. This ranged from development of the experimental frame work to mathematical tools to characterize the observed metabolic processes. Methods were developed to visualize 13C labeling kinetics of acetylcarnitine in vivo in resting skeletal muscle following the administration of hyperpolarized [1-13C]acetate. Two different, novel mathematical models were constructed to quantify the kinetic rate constants. Although separated by two enzymatic reactions, the conversion of acetate to acetylcarnitine was uniquely defined by the enzymatic activity of acetylCoA synthetase (ACS). A 13C MRS protocol was developed and implemented for hyperpolarized studies in the heart, which included selecting appropriate cardiac triggers to align the measurements with the cardiac phase. The 13C label propagation into acetylcarnitine and citrate could be measured in real time in the beating rat heart following the infusion of hyperpolarized [1-13C]acetate, using a newly constructed 13C RF coil which improved the detection sensitivity. The substantial spectral resolution at 9.4T and a triggered shimming and MR acquisition protocol allowed for the detection of citrate for the first time in vivo after injection of hyperpolarized [1-13C]acetate. Mathematical models were successfully extended to include mitochondrial oxidation and analytical expressions were derived to interpret the dynamic 13C labeling of citrate. Cardiac dysfunction is often associated with a shift in substrate preference, while diagnostic methods such as PET provide only information on substrate uptake. The potential of hyperpolarized 13C MRS to measure simultaneously lipid and carbohydrate oxidation was demonstrated in vivo, and the sensitivity of the method to a metabolic perturbation was assessed. Hyperpolarized [1-13C]butyrate and [1-13C]pyruvate were used as representatives of carbohydrate and lipid oxidation. Fasting led to significant changes in preference for the injected substrates. The appearance of a cohort of downstream metabolites (bicarbonate, lactate, alanine, glutamate, citrate, acetylcarnitine, β-hydroxybutyrate and acetoacetate) allowed the independent and simultaneous monitoring of myocardial oxidation of both fatty acid and carbohydrates in vivo and is a sensitive indicator of metabolic shift. Lactate is an important metabolic intermediate for mitochondrial oxidation. Carbohydrate metabolism in healthy rat skeletal muscle at rest was studied in different nutritional states using hyperpolarized [1-13C]lactate, which can be injected at physiological concentrations and leaves other oxidative processes undisturbed. A significant decrease in [1-13C]alanine and 13C bicarbonate were observed comparing both groups, attributed to a change in cellular alanine concentration and pyruvate dehydrogenase (PDH) flux. It was shown that lactate can be used to study carbohydrate oxidation in skeletal muscle at physiological levels and that the downstream metabolite signals are sensitive markers to probe metabolic changes. Since the detection of [5-13C]citrate and [5-13C]glutamate in the heart is hindered by the close proximity of the [1-13C]acetate resonance, acetylcarnitine could be an interesting substrate for hyperpolarized MR. Acetylcarnitine crosses the mitochondrial membrane easily, while skipping a few metabolic steps needed for acetate to cross the membrane. Moreover, it does not interfere with the detection of [5-13C]glutamate and [5-13C]citrate. [1-13C]Acetylcarnitine was successfully hyperpolarized and despite its short T1 it was possible to detect the formation of [5-13C]glutamate in the heart in vivo. Due to the absence of the [5-13C]citrate resonance, we hypothesized the existence of an intricate relationship between reaction, transport and relaxation rates in the choice of hyperpolarized substrates. Acetylcarnitine has relatively small poolsizes and has only been observed using hyperpolarized 13C MRS techniques. Therefore, it has not been possible to obtain reliable quantification of the metabolic turnover of acetylcarnitine, which is an essential intermediate in the metabolism of acetate. Methods were implemented to measure the oxidation of [2-13C]acetate locally with increased sensitivity at high field (14.1T), using a 1H-13C polarization transfer sequence. This allowed the observation of [2-13C]acetylcarnitine in vivo at 21.5 ppm, with a metabolic turnover time τ of 0.34 μmol/g/hr. The acetylcarnitine resonance assignment was confirmed by experiments infusing 13C labeled glucose. The measurement of the time courses of Glu C4 and C3 were also clearly observed, without lipid contamination. To conclude, this work presents novel metabolic information about skeletal and cardiac energy metabolism using developed methods in hyperpolarized 13C MRS. The use of hyperpolarized 13C substrates to measure absolute flux through metabolic pathways in vivo would not only revolutionize clinical diagnostic imaging but also serve as an important tool to better understand diseased metabolism and the metabolic effects of new drugs aimed to combat diseases.

The C57BL/6 mouse exhibits sporadic congenital portosystemic shunts

C. R. Cudalbu; V. McLin; H. Lei; J. das Neves Duarte; A.-L. Rougemont  et al.

In Vivo 1H MRS at 14.1T for the Accurate Characterization of the Lipid Profile of the Mouse Liver

A. F. Soares; H. Lei; R. Gruetter

1H MRS was employed at 14.1T to non-invasively quantify the lipid content of small samples (8-15 µl). In the mouse liver, good spectral stability was achieved by running individual scans within one breathing cycle. Ultra short TE STEAM with water suppression was used to estimate the unsaturation profile of the fatty acyl chains. This method was in good agreement with in vitro measurements in phantoms. High field is advantageous to accurately characterize the lipid profile of small samples such as the volumes selected in the liver of mice, with no need to increase the acquisition time for sensitivity gain.

Which prior knowledge? Quantification of in vivo brain (13) C MR spectra following (13) C glucose infusion using AMARES

B. Lanz; J. M. N. Duarte; N. Kunz; V. Mlynárik; R. Gruetter  et al.

The recent developments in high magnetic field (13) C magnetic resonance spectroscopy with improved localization and shimming techniques have led to important gains in sensitivity and spectral resolution of (13) C in vivo spectra in the rodent brain, enabling the separation of several (13) C isotopomers of glutamate and glutamine. In this context, the assumptions used in spectral quantification might have a significant impact on the determination of the (13) C concentrations and the related metabolic fluxes. In this study, the time domain spectral quantification algorithm AMARES (advanced method for accurate, robust and efficient spectral fitting) was applied to (13) C magnetic resonance spectroscopy spectra acquired in the rat brain at 9.4 T, following infusion of [1,6-(13) C(2) ] glucose. Using both Monte Carlo simulations and in vivo data, the goal of this work was: (1) to validate the quantification of in vivo (13) C isotopomers using AMARES; (2) to assess the impact of the prior knowledge on the quantification of in vivo (13) C isotopomers using AMARES; (3) to compare AMARES and LCModel (linear combination of model spectra) for the quantification of in vivo (13) C spectra. AMARES led to accurate and reliable (13) C spectral quantification similar to those obtained using LCModel, when the frequency shifts, J-coupling constants and phase patterns of the different (13) C isotopomers were included as prior knowledge in the analysis. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.

DOI : 10.1002/mrm.24406

In vivo brain energy metabolism in a rat model of chronic Hepatic Encephalopathy using 31P MRS

C. Cudalbu; O. Braissant; A. Jayaswal; R. Gruetter; V. A. McLin

Acetylcarnitine turnover in rat skeletal muscle measured in vivo using localized 13C NMR at 14.1 T

J. Bastiaansen; J. das Neves Duarte; A. Comment; R. Gruetter

The C57BL/6J Mouse Exhibits Sporadic Congenital Portosystemic Shunts

C. Cudalbu; V. A. Mclin; H. Lei; J. M. N. Duarte; A.-L. Rougemont  et al.

C57BL/6 mice are the most widely used strain of laboratory mice. Using <italic>in vivo</italic> proton Magnetic Resonance Spectroscopy (¹H MRS), we have repeatedly observed an abnormal neurochemical profile in the brains of both wild-type and genetically modified mice derived from the C57BL/6J strain, consisting of a several fold increase in cerebral glutamine and two fold decrease in myo-inositol. This strikingly abnormal neurochemical “phenotype” resembles that observed in chronic liver disease or portosystemic shunting and appeared to be independent of transgene, origin or chow and was not associated with liver failure. As many as 25% of animals displayed the abnormal neurochemical profile, questioning the reliability of this model for neurobiology. We conducted an independent study to determine if this neurochemical profile was associated with portosystemic shunting. Our results showed that 100% of the mice with high brain glutamine displayed portosystemic shunting by concomitant portal angiography while all mice with normal brain glutamine did not. Since portosystemic shunting is known to cause alterations in gene expression in many organs including the brain, we conclude that portosystemic shunting may be the most significant problem associated with C57BL/6J inbreeding both for its effect on the central nervous system and for its systemic repercussions.

DOI : 10.1371/journal.pone.0069782

Evolution of the Hepatic Lipid Profile of the Adult Mouse - in Vivo and in Vitro 1H MRS Assessments at 14.1T

A. F. Soares; H. Lei; R. Gruetter

The hepatic lipid content and composition were assessed in healthy mice throughout adulthood. It was found that aging and obesity contributed to increase the amount of lipids in the liver while decreasing the poly-unsaturation degree. The combination of in vivo 1H MRS assessments with in vitromeasurements on tissues extracts illustrated the important contribution of membrane lipids to the total poly-unsaturation degree of the fatty acyl chains. Changes of the unsaturation profile of cytosolic lipids can be monitored by in vivo 1H MRS, which is of interest for the study of genetic and diet-induced mice models of metabolic diseases.

Direct mapping of 19F in 19FDG-6P in brain tissue at subcellular resolution using soft X-ray fluorescence

C. Poitry-Yamate; A. Gianoncelli; G. Kourousias; B. Kaulich; M. Lepore  et al.

Low energy x-ray fluorescence (LEXRF) detection was optimized for imaging cerebral glucose metabolism by mapping the fluorine LEXRF signal of 19 F in 19 FDG, trapped as intracellular 19 F-deoxyglucose-6-phosphate ( 19 FDG-6P) at 1μm spatial resolution from 3μm thick brain slices. 19 FDG metabolism was evaluated in brain structures closely resembling the general cerebral cytoarchitecture following formalin fixation of brain slices and their inclusion in an epon matrix. 2-dimensional distribution maps of 19 FDG-6P were placed in a cytoarchitectural and morphological context by simultaneous LEXRF mapping of N and O, and scanning transmission x-ray (STXM) imaging. A disproportionately high uptake and metabolism of glucose was found in neuropil relative to intracellular domains of the cell body of hypothalamic neurons, showing directly that neurons, like glial cells, also metabolize glucose. As 19 F-deoxyglucose-6P is structurally identical to 18 F-deoxyglucose-6P, LEXRF of subcellular 19 F provides a link to in vivo 18 FDG PET, forming a novel basis for understanding the physiological mechanisms underlying the 18 FDG PET image, and the contribution of neurons and glia to the PET signal.

DOI : 10.1088/1742-6596/463/1/012003

Improved off-resonance phase behaviour using a phase-inverted adiabatic half passage pulse for 13C NMR spectroscopy in humans at 7T

E. Serés Roig; L. Xin; D. Gallichan; V. Mlynarik; R. Gruetter

Single spin-echo T 2 relaxation times of cerebral metabolites at 14.1 T in the in vivo rat brain

L. Xin; G. Gambarota; C. Cudalbu; V. Mlynárik; R. Gruetter

OBJECT: To determine the single spin-echo T 2 relaxation times of uncoupled and J-coupled metabolites in rat brain in vivo at 14.1 T and to compare these results with those previously obtained at 9.4 T. MATERIALS AND METHODS: Measurements were performed on five rats at 14.1 T using the SPECIAL sequence and TE-specific basis-sets for LCModel analysis. RESULTS AND CONCLUSION: The T 2 of singlets ranged from 98 to 148 ms and T 2 of J-coupled metabolites ranged from 72 ms (glutamate) to 97 ms (myo-inositol). When comparing the T 2s of the metabolites measured at 14.1 T with those previously measured at 9.4 T, a decreasing trend was found (p < 0.0001). We conclude that the modest shortening of T 2 at 14.1 T has a negligible impact on the sensitivity of the (1)H MRS when performed at TE shorter than 10 ms.

DOI : 10.1007/s10334-013-0378-3

Methods for Enhancing the High Field in vivo Sensitivity of Long-T1 Nuclear Spins via Dynamic Nuclear Polarization

T. Cheng / A. Comment; R. Gruetter (Dir.)

Metabolism of [1,6-C-13]glucose in glutamatergic and GABAergic compartments in vivo in the rat brain

J. Duarte; R. Gruetter

Metabolic Flux and Compartmentation Analysis in the Brain In vivo

B. Lanz; R. Gruetter; J. M. N. Duarte

Through significant developments and progresses in the last two decades, in vivo localized nuclear magnetic resonance spectroscopy (MRS) became a method of choice to probe brain metabolic pathways in a non-invasive way. Beside the measurement of the total concentration of more than 20 metabolites, 1H MRS can be used to quantify the dynamics of substrate transport across the blood brain barrier (BBB) by varying the plasma substrate level. On the other hand, 13C MRS with the infusion of 13C enriched substrates enables the characterization of brain oxidative metabolism and neurotransmission by incorporation of 13C in the different carbon positions of amino acid neurotransmitters. The quantitative determination of the biochemical reactions involved in these processes requires the use of appropriate metabolic models, whose level of details is strongly related to the amount of data accessible with in vivo MRS. In the present work, we present the different steps involved in the elaboration of a mathematical model of a given brain metabolic process and its application to the experimental data in order to extract quantitative brain metabolic rates. We review the recent advances in the localized measurement of brain glucose transport and compartmentalized brain energy metabolism, and how these reveal mechanistic details on glial support to glutamatergic and GABAergic neurons.

DOI : 10.3389/fendo.2013.00156

Feasibility and electromagnetic compatibility study of the ClearPEM front-end electronics for simultaneous PET-MR imaging

D. Neves; J. André; R. Bugalho; R. Gruetter; A. Magill  et al.

In this work we present a first feasibility study of the ClearPEM technology for simultaneous PET-MR imaging. The mutual electromagnetic interference (EMI) effects between both systems were evaluated on a 7 T magnet by characterizing the response behavior of the ClearPEM detectors and front-end electronics to pulsed RF power and switched magnetic field gradients; and by analyzing the MR system performance degradation from noise pickup into the RF receiver chain, and from magnetic susceptibility artifacts caused by PET front-end materials.

DOI : 10.1016/j.nima.2012.08.033

Characterization of sustained BOLD activation in the rat barrel cortex and Neurochemical consequences

N. Just; L. Xin; H. Frenkel; R. Gruetter

To date, only a couple of functional MR spectroscopy (fMRS) studies were conducted in rats. Due to the low temporal resolution of 1H-MRS techniques, prolonged stimulation paradigms are necessary for investigating the metabolic outcome in the rat brain during functional challenge. However, sustained activation of cortical areas is usually difficult to obtain due to neural adaptation. Anesthesia, habituation, high variability of the basal state metabolites concentrations as well as low concentrations of the metabolites of interest such as (lactate (Lac), glucose (Glc) or gamma-amminobutyric acid (GABA)) and small expected changes of metabolite concentrations need to be addressed. In the present study, the rat barrel cortex was reliably and reproducibly activated through sustained trigeminal nerve (TGN) stimulation. In addition, TGN stimulation induced significant positive changes in lactate (+1.01mumol/g, p<0.008) and glutamate (+0.92mumol/g, p<0.02) and significant negative aspartate changes (-0.63mumol/g, p<0.004) using functional 1HMRS at 9.4T in agreement with previous changes observed in human fMRS studies. Finally, for the first time, the dynamics of lactate, glucose, aspartate and glutamate concentrations during sustained somatosensory activation in rats using fMRS were assessed. These results allow demonstrating the feasibility of fMRS measurements during prolonged barrel cortex activation in rats.

DOI : 10.1016/j.neuroimage.2013.02.042

Quantification of the neurochemical profile using simulated macromolecule resonances at 3 T

B. Schaller; L. Xin; C. Cudalbu; R. Gruetter

The broad resonances underlying the entire (1) H NMR spectrum of the brain, ascribed to macromolecules, can influence metabolite quantification. At the intermediate field strength of 3 T, distinct approaches for the determination of the macromolecule signal, previously used at either 1.5 or 7 T and higher, may become equivalent. The aim of this study was to evaluate, at 3 T for healthy subjects using LCModel, the impact on the metabolite quantification of two different macromolecule approaches: (i) experimentally measured macromolecules; and (ii) mathematically estimated macromolecules. Although small, but significant, differences in metabolite quantification (up to 23% for glutamate) were noted for some metabolites, 10 metabolites were quantified reproducibly with both approaches with a Cramer-Rao lower bound below 20%, and the neurochemical profiles were therefore similar. We conclude that the mathematical approximation can provide sufficiently accurate and reproducible estimation of the macromolecule contribution to the (1) H spectrum at 3 T. Copyright © 2013 John Wiley & Sons, Ltd.

DOI : 10.1002/nbm.2896

Hepatic glucose sensing is required to preserve β cell glucose competence

P. Seyer; D. Vallois; C. Poitry-Yamate; F. Schütz; S. Metref  et al.

Liver glucose metabolism plays a central role in glucose homeostasis and may also regulate feeding and energy expenditure. Here we assessed the impact of glucose transporter 2 (Glut2) gene inactivation in adult mouse liver (LG2KO mice). Loss of Glut2 suppressed hepatic glucose uptake but not glucose output. In the fasted state, expression of carbohydrate-responsive element-binding protein (ChREBP) and its glycolytic and lipogenic target genes was abnormally elevated. Feeding, energy expenditure, and insulin sensitivity were identical in LG2KO and control mice. Glucose tolerance was initially normal after Glut2 inactivation, but LG2KO mice exhibited progressive impairment of glucose-stimulated insulin secretion even though β cell mass and insulin content remained normal. Liver transcript profiling revealed a coordinated downregulation of cholesterol biosynthesis genes in LG2KO mice that was associated with reduced hepatic cholesterol in fasted mice and reduced bile acids (BAs) in feces, with a similar trend in plasma. We showed that chronic BAs or farnesoid X receptor (FXR) agonist treatment of primary islets increases glucose-stimulated insulin secretion, an effect not seen in islets from Fxr-/- mice. Collectively, our data show that glucose sensing by the liver controls β cell glucose competence and suggest BAs as a potential mechanistic link.

DOI : 10.1172/JCI65538

MAGNETIC RESONANCE AND BIOLUMINESCENCE IMAGING MONITOR QUANTITATIVE DIFFERENCES IN PANCREATIC ISLETS AND THEIR REGENERATION AFTER BETA CELL LOSS

P. Meda; S. Lamprianou; H. Lei; L. Vinet; R. Gruetter  et al.

Development and implementation of ¹H magnetic resonance techniques for an improved in vivo assessment of models of cerebral metabolic disorders

M. Craveiro / R. Gruetter (Dir.)

Nuclear magnetic resonance (NMR) is a physical phenomenon that is widely used in the biomedical field due to its non-invasive and non-destructive properties, which make it an optimal tool for the in vivo investigation of living organs such as the brain. This thesis focused on the development of 1H magnetic resonance (MR) techniques at ultra-high magnetic field strength to improve the measurement and quantification of the 1H MR signal in the rodent brain, and to accurately assess the alterations of the metabolite and water concentrations in several cerebral metabolic disorders. In rodents, 1H MR spectroscopy (MRS) allows the assessment of the concentration of up to 20 metabolites that take part in many cerebral metabolic processes such as neurotransmission, cell energy metabolism, cell growth and osmosis. 1H MRS thus provides a unique tool to non-invasively investigate the cerebral metabolism in healthy and pathological conditions in vivo. However, it is essential for reliable quantification that systematic errors and overlap of the measured metabolite signals are minimized. To that aim, the impact of a potential additional short T2 relaxation time component, which might affect the glutamine quantification at long echo times, was assessed in this thesis. The J-difference editing technique MEGA-SPECIAL was optimized to obtain the unequivocal detection of the glutamine signal at moderate echo time. As a control, the glutamine concentration obtained with this method was then compared to short echo time 1H MR spectroscopy measurements. Since the two measurements of the glutamine concentration at short and moderate echo time did not result in significant differences, this study concluded that there is a low probability of an effect of a short T2 relaxation time component on the glutamine concentration measurement. Since a reliable quantification of 1H spectra partly relies on the accurate assessment of the overlapping macromolecule contribution to the metabolites, an optimized method for the post-processing of the measured macromolecule signal was developed to ensure an accurate assessment of its contribution. This method was applied to investigate potential regional differences in the mouse brain macromolecule signals that may affect metabolite quantification when not taken into account, as well as for the assessment of macromolecule alterations in a human-glioma mouse model. No regional macro- molecule variation was found to significantly affect the metabolite quantification in the healthy mouse brain, which supports the common use of a general macromolecule spec- trum for healthy rodent brain 1H spectra quantification. However, several alterations of the macromolecule spectrum, some of which were reported for the first time, were observed in glioma tissues, and their accurate assessment was shown to be necessary for reliable metabolite quantification. Localized 1H MR spectroscopy techniques that are designed to acquire the1H MR signal from a 3D volume can also be combined with MR imaging (MRI) to acquire information on the spatial distribution of the individual metabolites, which is often of interest when heterogeneous tissues are investigated (such as in pathological conditions). This MR spectroscopic imaging (MRSI) technique is however limited in its conventional implementation by the long measurement time that is necessary to acquire the spatial information. A fast MRSI technique, spectroscopic RARE (spRARE), was therefore implemented at high magnetic field strength in this thesis in order to benefit from the advantage of high magnetic fields in addition to the rapid spatial encoding scheme offered by spRARE. Several optimizations of the metabolite signal detection were implemented in spRARE, before the efficiency of the technique was validated at 9.4 T through a comparison of the acquired in vitro metabolite signals with numerical simulations using the density matrix formalism. The in vivo quantification of the water content is also of interest as it offers perspec- tives for investigating cerebral disease progression that is associated with brain water content alterations. In addition, when the water signal is used as an internal reference for metabolite concentration quantification in MRS, such knowledge can also contribute to a more reliable quantification. A method that is based on the multi-spin-echo MRI technique was therefore developed for the absolute quantification of the water content in the rodent brain, with the aim of assessing alterations of its content in pathological conditions. It was then applied on a bile-duct-ligation rat model of chronic hepatic encephalopathy (HE) to assess the brain water content changes associated with the disease as well as its metabolic alterations. The absence of significant brain water content changes detected in the investigated HE rat model, which was supported by post-mortem brain water content measurements using gravimetry and the dry/wet weight-ratio technique, potentially opens the way for investigating cerebral metabolic processes in chronic HE in absence of edema formation. Localized 1H MRS and conventional MRSI were also applied for the investigation of rodent models of brain disorders in order to assess the metabolic profile associated with those disorders. These studies for the first time allowed the assessment of GABA as a potential early Parkinson’s disease marker, while they also provided new insights into the role of the PPAR-β brain receptor in the early neuroprotective effects following ischemia.

In vivo enzymatic activity of acetylCoA synthetase in skeletal muscle revealed by (13)C turnover from hyperpolarized [1-(13)C]acetate to [1-(13)C]acetylcarnitine

J. Bastiaansen; T. Cheng; M. Mishkovsky; J. M. das Neves Duarte; A. Comment  et al.

BACKGROUND: Acetate metabolism in skeletal muscle is regulated by acetylCoA synthetase (ACS). The main function of ACS is to provide cells with acetylCoA, a key molecule for numerous metabolic pathways including fatty acid and cholesterol synthesis and the Krebs cycle. METHODS: Hyperpolarized [1-(13)C]acetate prepared via dissolution dynamic nuclear polarization was injected intravenously at different concentrations into rats. The (13)C magnetic resonance signals of [1-(13)C]acetate and [1-(13)C]acetylcarnitine were recorded in vivo for one minute. The kinetic rate constants related to the transformation of acetate into acetylcarnitine were deduced from the 3s time resolution measurements using two approaches, either mathematical modelling or relative metabolite ratios. RESULTS: Although separated by two biochemical transformations, a kinetic analysis of the (13)C label flow from [1-(13)C]acetate to [1-(13)C]acetylcarnitine led to a unique determination of the activity of ACS. The in vivo Michaelis constants for ACS were KM = 0.35 ± 0.13 mM and Vmax = 0.198 ± 0.030 μmol/g/min. CONCLUSIONS: The conversion rates from hyperpolarized acetate into acetylcarnitine were quantified in vivo and, although separated by two enzymatic reactions, these rates uniquely defined the activity of ACS. The conversion rates associated with ACS were obtained using two analytical approaches, both methods yielding similar results. GENERAL SIGNIFICANCE: This study demonstrates the feasibility of directly measuring ACS activity in vivo and, since the activity of ACS can be affected by various pathological states such as cancer or diabetes, the proposed method could be used to non-invasively probe metabolic signatures of ACS in diseased tissue.

DOI : 10.1016/j.bbagen.2013.03.023

Lactoferrine supplementation following Hypoxia-Ischemia in the immature rat brain: macro-, micro-structural and metabolic assessment of the neuroprotective effect using multimodal MR

Y. Van de Looij; A. Chatagner; P. Hüppi; R. Gruetter; S. Sizonenko

In vivo real time cardiac metabolism using hyperpolarized acetate

J. Bastiaansen; T. Cheng; R. Gruetter; A. Comment

Neural metabolism in vivo

Neural Metabolism In Vivo strives to offer a comprehensive and fundamental overview of cerebral metabolism by presenting leading-edge in vivo multimodal technology and its application in assessing neural activity, energy metabolism and neurotransmission in the living brain. The recent advances in our current understanding of the complex and dynamic physiological and neurochemical processes of neural function and metabolism are indebted to a variety of powerful techniques, namely nuclear magnetic resonance, positron emission tomography, radiotracer imaging, optical imaging and micro-dialysis. With the advent of these in vivo techniques, the collective and devoted efforts of neuroscientists and bioengineers made the brain readily assessable to demystify its complexity. Specific topics of this volume include energy metabolism specific to the brain from the in vivo assessment of neural activity via neurovascular coupling and glucose uptake, as well as the accompanying consumption of various cerebral energy substrates to metabolic compartmentation and interaction between neurons and astrocytes. This exciting compilation of 41 chapters across two volumes is the result of the dedicated efforts of 99 renowned experts in neurobiology.

GLUT2-/- mice are resistant to age-induced accumulation of fat in the liver – an in vivo MRS study

A. F. Soares; H. Lei; F. Preitner; B. Thorens; R. Gruetter

Introduction: Whole-body energy balance is strongly dependent on the hepatic handling of metabolic fuels, which is affected by the hepatic lipid content (HLC). With in vivo nuclear magnetic resonance spectroscopy (MRS) HLC can be measured non-invasively and longitudinally. The study of mice is relevant for the characterization of transgenic models that provide insight into disease mechanisms but comparing with bigger subjects, the reduced sample size remains a limiting factor for in vivo MRS experiments. To overcome this sensitivity issue, we performed liver MRS in vivo at high field (14.1T) in wild-type C57BL/6J mice (WT) and GLUT2-/- mice that display altered whole-body energy balance. Methods: WT and GLUT2-/- mice reexpressing GLUT1 in the pancreatic β-cells to allow for survival and normal glucose-stimulated insulin secretion were studied at 4 months and 1 year of age. Non-fasted mice under isofluorane anesthesia were scanned in the supine position with a 1H quadrature surface coil over the abdomen. MRS measurements were performed in a horizontal bore 14.1T-26 cm magnet. Multi-slice gradient echo images were acquired for anatomical identification of the liver. Localized, respiration-gated 1H-MR spectra were acquired from a 10-15 µl voxel with STEAM with and without water suppression. HLC was estimated as the T2-corrected area of 1.3 ppm-lipid resonance relative to that of the water plus 1.3 ppm-lipid. Results/Discussion: Highly sensitive MRS at 14.1T allowed to accurately quantifying HLC in short experiments in mice. Suppression of the water signal revealed fatty-acyl resonances reflecting the lipid saturation profile. At 4 months, HLC was similar between WT and GLUT2-/-. HLC increased significantly in aged WT mice but remained low in aged GLUT2-/-. Also, the body weight of aged GLUT2-/- was slightly lower than that of WT. Low HLC in aged GLUT2-/- mice probably results from a defect on intra-hepatic pathway fluxes due to GLUT2 ablation in the liver. In addition, it may reflect altered utilization of metabolic substrates due to inadequate whole-body blood glucose sensing.

In vivo longitudinal 1H Spectroscopic Imaging and Diffusion Tensor Imaging studies at 9.4T in a rat model of chronic liver failure

C. R. Cudalbu; V. McLin; O. Braissant; N. Kunz; Y. Van de Looij  et al.

Continuous arterial spin labeling of mouse cerebral blood flow using an actively-detuned two-coil system at 9.4T

H. Lei; Y. Pilloud; A. W. Magill; R. Gruetter

Among numerous magnetic resonance imaging (MRI) techniques, perfusion MRI provides insight into the passage of blood through the brain's vascular network non-invasively. Studying disease models and transgenic mice would intrinsically help understanding the underlying brain functions, cerebrovascular disease and brain disorders. This study evaluates the feasibility of performing continuous arterial spin labeling (CASL) on all cranial arteries for mapping murine cerebral blood flow at 9.4 T. We showed that with an active-detuned two-coil system, a labeling efficiency of 0.82 ± 0.03 was achieved with minimal magnetization transfer residuals in brain. The resulting cerebral blood flow of healthy mouse was 99 ± 26 mL/100g/min, in excellent agreement with other techniques. In conclusion, high magnetic fields deliver high sensitivity and allowing not only CASL but also other MR techniques, i.e. (1)H MRS and diffusion MRI etc, in studying murine brains.

A high-power RF switch for arterial spin labelling with a separate tagging coil

A. Magill; H. Lei; R. Gruetter

Intra and cross-modal negative BOLD response to contrast-varying visual stimuli

J. Jorge; P. Figueiredo; R. Gruetter; W. Van der Zwaag

The BOLD contrast is widely used to study brain function. Under externally-applied stimuli, positive BOLD responses (PBR) are generally attributed to local increases in brain activity. Negative BOLD responses (NBR) are also frequently observed, but their underlying neurovascular-coupling mechanisms are less well-understood. Here, we study the NBR to contrast-varying visual stimuli. Both intra-modal (located in regions directly associated with the stimulus) and cross-modal responses (located in functionally-distinct regions) are analyzed.

Exploring the complementarities of the MP2RAGE and the Sa2RAGE sequences - quantitative T1 mapping

J. Marques; T. Kober; F. Eggenschwiler; R. Gruetter

Mathematical Modeling of Brain Energy Metabolism, Measured with PET and MRS in Rodents

B. Lanz / R. Gruetter (Dir.)

Positron emission tomography (PET) and nuclear magnetic resonance spectroscopy (MRS) are two biomedical measurement techniques developed in the end of the XXth century, which drastically improved the amount of accessible information available in vivo. PET became popular through the most widely used tracer, fluorodeoxyglucose (FDG), which enables the measurement of the local glucose utilization and is nowadays routinely applied in clinical practice. Nuclear magnetic resonance is used in a large array of applications such as in analytical chemistry or chemical structure determination and is essentially known for its versatile medical imaging capabilities, grouped under the name of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). In vivo MRS measures concentrations of metabolites by interacting with various nuclei such as 1H, 13C or 15N, with the major asset that MRS enables the identification of the chemical position at which the detected nuclei is located in the measured molecule. When coupled with labeled substrate infusions, dynamic MRS gives the opportunity to probe specific biochemical reactions directly in vivo, which opens the way to a wide range of metabolic studies. However, the correct interpretation of the elaborate dynamic labeling data acquired either with MRS or PET tracer experiments requires the application of adapted metabolic models to derive quantitative metabolic rates characterizing the biochemical processes under study. The work of this thesis involves both PET and MRS studies of brain energy metabolism in rodents and focuses on the development of adapted metabolic models to derive reliable metabolic fluxes characterizing various brain metabolic processes, such as glucose consumption, glial and neuronal oxidative metabolism and neurotransmission. FDG-PET enables the measurement of the cerebral metabolic rate of glucose (CMRGlc) using a three-compartment metabolic model for FDG transport across the blood-brain barrier and FDG phosphorylation to FDG-6P. Although this method is well established, a main drawback of CMRGlc measurement with FDG-PET is the necessity to characterize the available FDG concentration in the plasma over the experiment duration, the so-called arterial input function. This constraint is a strong limitation for preclinical studies in rodents, due to their low blood volume. We developed therefore a new method to extract the FDG input function directly from the PET images in rats and mice, using the time-activity curve of a voxel located in the inferior vena cava. The method was validated by comparison with a dedicated external blood counter and manual blood sampling. Using this method, a CMRGlc of ~0.22 μmol/g/min was determined in the mouse cortex. Glial oxidative metabolism can be specifically assessed using glial specific substrates, in particular [1-11C] acetate. However, no existing metabolic model described the 11C tissue activity curve in terms of neuroglial energy metabolism. In order to extract quantitative metabolic fluxes characterizing the glial TCA cycle rate and glutamate/glutamine cycling, we adapted the neuroglial two-compartment modeling approach previously used in 13C MRS cerebral metabolic studies to interpret positron emission data following 11C-acetate injection in the rat. The precision and accuracy of the estimated metabolic parameters was tested and a composite glial metabolic flux Vgtg=0.136 ± 0.042 μmol/g/min and a neurotransmission flux Vnt=0.170 ± 0.103 μmol/g/min were obtained. This approach enabled a direct comparison of the metabolic fluxes measured using 11C PET with the values determined with 13C MRS. In the field of 13C MRS, we took advantage of the higher sensitivity and spectral resolution available at high magnetic field (14.1T) to measure glial and neuronal oxidative metabolism, glutamatergic neurotransmission and pyruvate carboxylation in more details and with higher precision using both [1,6-13C2] glucose and [2-13C] acetate infusions in rats. In the acetate study, the glial specific uptake of acetate enabled a quantification of glial oxidative metabolism with unrivalled precision. We could estimate for the first time separately the transmitochondrial flux Vx in the glial and neuronal compartment, a biochemical pathway which has been subject of strong controversy. The glial and neuronal Vx were determined with high precision, as calculated through Monte Carlo simulation, and their value was on the same order of magnitude than the respective glial and neuronal TCA cycle flux. The model was extended to estimate the fraction of glutamate located in the glial compartment. A glial glutamate concentration of 0.6 ± 0.1 μmol/g was found. The values of the different metabolic fluxes characterizing the neuroglial system (Vtcag=0.27, Vxg=0.17, VPC=0.087, VNT=0.15, Vtcan=0.37 and Vxn=0.46 μmol/g/min) were in very good agreement with the values found using [1,6-13C2] glucose infusion. In addition, the effect of the MRS temporal resolution, experiment duration and signal- to-noise ratio on the precision of the derived metabolic fluxes was analyzed. Previous simulation studies showed that two-compartment modeling of neuroglial metabolism using 13C glucose infusion could lead to unreliable parameter estimations. However, using polarization transfer 13C MRS at 14.1T, the carbon positions C4, C3 and C2 of glutamate and glutamine as well as the positions C3 and C2 of aspartate could be measured with high SNR. The C2 positions were used in two-compartment modeling for the first time, enabling a precise measurement of the apparent glutamatergic neurotransmission and glial metabolism, including pyruvate carboxylation, as confirmed by Monte Carlo simulation. The glial dilution at the level of acetyl-CoA, which is related to the metabolism of other substrates than glucose, could also be characterized. Brain metabolism under hyperammonemic conditions was probed with 15N MRS under labeled ammonia infusion and modeled using a modified neuroglial two- compartment model describing the dynamics of [5-15N] glutamine, [2-15N] glutamate and [2-15N] glutamine. Due to detoxification processes, the total glutamine concentration, measured with 1H MRS, was steadily increasing. This non-steady- state metabolic condition resulting from the hyperammonemic stress was taken into account in the model, resulting in the determination of the apparent neurotransmission (0.26 ± 0.030 μmol/g/min), the glutamate dehydrogenase (0.029 ± 0.002 μmol/g/min) and net glutamine accumulation (0.033 ± 0.001 μmol/g/min). Finally, compartmental modeling was applied to 13C labeling studies in the awake rat, fed during 5, 24 or 48 h with a [1-13C]-labeled glucose solution, to determine brain glycogen and NAA turnover times (τGlyc=5.3 ± 3.2 h and τNAA=15.6 ± 6.5 h), using 13C spectra of brain extracts. A group of rats followed a mild brain activation protocol over 5 hours, which resulted in a decreased glycogen turnover time (τGlyc =2.9 ± 1.2 h).

Measurements of Glycogen Metabolism in the Living Brain

R. Gruetter

To maintain brain function, the combustion of glucose for the generation of energy (ATP) is essential, yet the brain maintains fuel storage in the form of glial glycogen. Although brain glycogen levels are lower than those in the liver, they exceed the normal free glucose concentration in brain several-fold. Under normoxic conditions, when a partial glucose supply deficit is present, brain glycogen can provide fuel for extended periods of time. Brain glycogen metabolism is active, but comparatively slow, consistent with a role of an emergency fuel with neuroprotective effects. The classical analysis with biochemical means requires tissue extraction; brain glycogen metabolism can be measured directly in the living brain only by 13C NMR spectroscopy. This chapter reviews the insights gained into brain glycogen metabolism in the living brain to-date and reviews potential roles for brain glycogen in the context of neuro-glial metabolism, hypoglycemia and neuroprotection.

DOI : 10.1007/978-1-4614-1788-0_22.

The neurochemical profile quantified by in vivo(1)H NMR spectroscopy

J. M. N. Duarte; H. Lei; V. Mlynárik; R. Gruetter

Proton NMR spectroscopy is emerging from translational and preclinical neuroscience research as an important tool for evidence based diagnosis and therapy monitoring. It provides biomarkers that offer fingerprints of neurological disorders even in cases where a lesion is not yet observed in MR images. The collection of molecules used as cerebral biomarkers that are detectable by (1)H NMR spectroscopy define the so-called "neurochemical profile". The non-invasive quality of this technique makes it suitable not only for diagnostic purposes but also for therapy monitoring paralleling an eventual neuroprotection. The application of (1)H NMR spectroscopy in basic and translational neuroscience research is discussed here.

DOI : 10.1016/j.neuroimage.2011.12.038

A comparison of high-pass birdcage coils for small animal imaging at 9.4 and 14T

T. Cheng; J. Jung; A. Comment; R. Gruetter; H. Lei  et al.

In vivo enzymatic assay of carnitine acetyl transferase and acetylCoA synthetase using hyperpolarized acetate

J. Bastiaansen; T. Cheng; M. Mishkovsky; A. Comment; R. Gruetter

MRS Frontal Glutathione Levels Correlate with DSI White Matter Integrity Measure in Control Subjects but Not in Early Psychosis Patients

P. S. Baumann; A. Griffa; L. Xin; S. Crespi; C. Ferrari  et al.

N-acetylcysteine normalizes neurochemical changes in the glutathione-deficient schizophrenia mouse model during development

J. M. Das Neves Duarte; A. Kulak; M. M. Gholam-Razaee; M. Cuenod; R. Gruetter  et al.

Background: Glutathione (GSH) is the major cellular redox-regulator and antioxidant. Redox-imbalance due to genetically impaired GSH synthesis is among the risk factors for schizophrenia. Here we used a mouse model with chronic GSH deficit induced by knockout (KO) of the key GSH-synthesizing enzyme, glutamate-cysteine ligase modulatory subunit (GCLM). Methods: With high-resolution magnetic resonance spectroscopy at 14.1 T, we determined the neurochemical profile of GCLM-KO, heterozygous, and wild-type mice in anterior cortex throughout development in a longitudinal study design. Results: Chronic GSH deficit was accompanied by an elevation of glutamine (Gln), glutamate (Glu), Gln/Glu, N-acetylaspartate, myo-Inositol, lactate, and alanine. Changes were predominantly present at prepubertal ages (postnatal days 20 and 30). Treatment with N-acetylcysteine from gestation on normalized most neurochemical alterations to wild-type level. Conclusions: Changes observed in GCLM-KO anterior cortex, notably the increase in Gln, Glu, and Gln/Glu, were similar to those reported in early schizophrenia, emphasizing the link between redox imbalance and the disease and validating the model. The data also highlight the prepubertal period as a sensitive time for redox-related neurochemical changes and demonstrate beneficial effects of early N-acetylcysteine treatment. Moreover, the data demonstrate the translational value of magnetic resonance spectroscopy to study brain disease in preclinical models. © 2012 Society of Biological Psychiatry.

DOI : 10.1016/j.biopsych.2011.07.035

MP2RAGE multiple sclerosis magnetic resonance imaging at 3 T

T. Kober; C. Granziera; P. Browaeys; M. Schluep; R. Meuli  et al.

Objectives: Lesion detection and characterization in multiple sclerosis (MS) are an essential part of its clinical diagnosis and an important research field. In this pilot study, we applied the recently introduced two inversion-contrast magnetization-prepared rapid gradient echo sequence (MP2RAGE) to patients with early-stage MS. Materials and Methods: The MP2RAGE is a 3-dimensional (3D) magnetization-prepared rapid gradient echo derivative providing homogeneous T1 weighting and simultaneous T1 mapping. The MP2RAGE performance was compared with that of 2 clinical routine sequences (2D fluid-attenuated inversion recovery [FLAIR] and 3D magnetization-prepared rapid gradient echo [MP-RAGE]) and 2 state-of-the art clinical research sequences (the 3D FLAIR-SPACE [sampling perfection with application-optimized contrasts by using different flip-angle evolutions], a fluid-attenuated variable flip-angle fast spin echo technique, and the 3D double-inversion recovery SPACE). A cohort of 10 early-stage female MS patients (age, 31.6 ± 4.7 years; disease duration, 3.8 ± 1.9 years; median expanded disability status scale score, 1.75) and 10 age- and gender-matched controls were enrolled after approval of the local institutional review board was obtained. Multiple sclerosis lesions were identified and assigned to brain locations and tissue types by two experienced physicians in all 5 contrasts. Subsequently, lesions were manually delineated for comparison and statistical analysis of lesion count, volume and quantitative measures. Results and Conclusions: The results show that the 3D T1-weighted high-resolution MP2RAGE contrast provides a sensitive means for MS lesion assessment. The additional quantitative T1 relaxation time maps obtained with the MP2RAGE provide further potential diagnostic and prognostic information that could help (a) to better discriminate lesion subtypes and (b) to stage and predict the activity and the evolution of MS. Results also indicate that the T2-weighted double-inversion recovery and FLAIR-SPACE contrasts are attractive complements to the MP2RAGE for lesion detection. Copyright © 2012 by Lippincott Williams & Wilkins.

DOI : 10.1097/RLI.0b013e31824600e9

In vivo real-time metabolic studies in mice at physiological concentrations following 1-13C lactate injection

Y. Takado; M. Mishkovsky; T. Cheng; R. Gruetter; A. Comment

The real-time metabolic transformation of lactate in the mouse head was monitored following the injection of hyperpolarized 1-13C lactate at physiological doses. The results were compared with metabolic studies performed with hyperpolarized 1-13C pyruvate at similar blood concentration. From the observation that the lactate to alanine ratio was nearly identical following both the pyruvate and the lactate injections, we concluded that the substrate and its metabolites can be detected in real time at physiological concentrations after the injection of lactate.

Cerebral Glucose Transport and Homeostasis

M. J. das Neves Duarte; R. Gruetter

Cerebral glucose homeostasis is maintained by the equilibrium between glucose transport across the blood-brain-barrier (BBB), which occurs through facilitative carriers, and glucose consumption mainly initiated by hexokinase phosphorylation. Glucose concentrations can be quantified non invasively by nuclear magnetic resonance (NMR) spectroscopy and such measurements, through appropriate mathematical modelling, allow to determine the kinetics of glucose transport and metabolism. This chapter summarizes the insights gained into brain glucose transport from the measurement of the brain glucose concentration, particularly reviewing state of the art NMR methods for non invasive determination of glucose homeostasis and discussing the employment of mathematical models of glucose transport.

DOI : 10.1007/978-1-4614-1788-0_20.

The importance of priors for l2 regularization and total variation methods in quantitative susceptibility mapping

D. Khabipova; J. P. Marques; G. Puy; R. Gruetter; Y. Wiaux

Phase imaging has been demonstrated to offer a good contrast between and within brain tissues at 7T[1] with iron and myelin concentration being amongst the main modulators of the observed contrast due to their para- and dia-magnetic properties.Phase imaging suffers from a non-local contrast variation which can be overcome by calculating the underlying magnetic susceptibility maps[2].As this problem is ill-posed, many regularized methods have been proposed over the past years[2,3,4].The regularized single-orientation (RSO)[2] based on the l2 regularisation and the morphology enabled dipole inversion (MEDI)[3] method based on the l1 total variation incorporate prior knowledge of the expected edges taken from the magnitude image.In this abstract a systematic evaluation is performed of a l2 RSO method[2,5] and a l1 total variation method (TV)[3] using numerical data and different morphology priors.

In vivo Structural Imaging of the Cerebellum, the Contribution of Ultra-High Fields

J. P. Marques; R. Gruetter; W. van der Zwaag

This review covers some of the contributions to date from cerebellar imaging studies performed at ultra-high magnetic fields. A short overview of the general advantages and drawbacks of the use of such high field systems for imaging is given. One of the biggest advantages of imaging at high magnetic fields is the improved spatial resolution, achievable thanks to the increased available signal-to-noise ratio. This high spatial resolution better matches the dimensions of the cerebellar substructures, allowing a better definition of such structures in the images. The implications of the use of high field systems is discussed for several imaging sequences and image contrast mechanisms. This review covers studies which were performed in vivo in both rodents and humans, with a special focus on studies that were directed towards the observation of the different cerebellar layers.

DOI : 10.1007/s12311-010-0189-2

A high-pass birdcage coil for small animal imaging at 600MHz/14.1T

A. Magill; H. Lei; R. Gruetter

Metabolism of [1,6-13C]glucose in glutamatergic and GABAergic compartments in vivo in the rat brain

D. JMN; R. Gruetter

SA2RAGE: A new sequence for fast B(1) (+) -mapping

F. Eggenschwiler; T. Kober; A. W. Magill; R. Gruetter; J. P. Marques

At high magnetic field strengths (≥3T), the radiofrequency wavelength used in MRI is of the same order of magnitude of (or smaller than) the typical sample size, making transmit magnetic field (B 1+) inhomogeneities more prominent. Methods such as radiofrequency-shimming and transmit SENSE have been proposed to mitigate these undesirable effects. A prerequisite for such approaches is an accurate and rapid characterization of the B 1+ field in the organ of interest. In this work, a new phase-sensitive three-dimensional B 1+-mapping technique is introduced that allows the acquisition of a 64 × 64 × 8 B 1+-map in ∼20 s, yielding an accurate mapping of the relative B 1+ with a 10-fold dynamic range (0.2-2 times the nominal B 1+). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivo B 1+-shimming procedures at high magnetic fields. The proposed methodology was validated in phantom experiments and demonstrated good results in phantom and human B 1+-shimming using an 8-channel transmit-receive array. Magn Reson Med, 2011. © 2011 Wiley-Liss, Inc.

DOI : 10.1002/mrm.23145

A two-compartment mathematical model of neuroglial metabolism using [1-C-11] acetate

B. Lanz; K. Uffmann; M. T. Wyss; B. Weber; A. Buck  et al.

The purpose of this study was to develop a two-compartment metabolic model of brain metabolism to assess oxidative metabolism from [1-C-11] acetate radiotracer experiments, using an approach previously applied in C-13 magnetic resonance spectroscopy (MRS), and compared with an one-tissue compartment model previously used in brain [1-C-11] acetate studies. Compared with C-13 MRS studies, C-11 radiotracer measurements provide a single uptake curve representing the sum of all labeled metabolites, without chemical differentiation, but with higher temporal resolution. The reliability of the adjusted metabolic fluxes was analyzed with Monte-Carlo simulations using synthetic C-11 uptake curves, based on a typical arterial input function and previously published values of the neuroglial fluxes V-tca(g), V-x, V-nt, and V-tca(n) measured in dynamic C-13 MRS experiments. Assuming V-x(g)=10xV(tca)(g) and V-x(n)=V-tca(n), it was possible to assess the composite glial tricarboxylic acid (TCA) cycle flux V-gt(g) (Vgtg = V(x)(g)xV(tca)(g)/(V-x(g) + V-tca(g))) and the neurotransmission flux V-nt from C-11 tissue-activity curves obtained within 30 minutes in the rat cortex with a beta-probe after a bolus infusion of [1-C-11] acetate (n = 9), resulting in V-gt(g) = 0.136 +/- 0.042 and V-nt = 0.170 +/- 0.103 mu mol/g per minute (mean +/- s.d. of the group), in good agreement with 13C MRS measurements. Journal of Cerebral Blood Flow & Metabolism (2012) 32, 548-559; doi: 10.1038/jcbfm.2011.162; published online 30 November 2011

DOI : 10.1038/jcbfm.2011.162

Improved lipid profiling of the mouse liver by 1H-NMR spectroscopy at 14.1T in vivo

A. F. Soares; H. Lei; R. Gruetter

Purpose/Introduction: Whole-body metabolic deregulation features changes in hepatic lipid dynamics. In vivo 1H-MRS allows to noninvasively assess hepatic lipid content (HLC) and composition both in humans [1] and rodents [2]. Transgenic mice provide insight into disease mechanisms but, comparing with bigger subjects, reduced sample size and consequently increased acquisition time to compensate for sensitivity issues, remain limiting factors. We aimed to improve sensitivity and spectral resolution of the 1H-NMR spectrum of the mouse liver employing high magnetic field strength, i.e. 14.1T. Subjects and Methods: Seven C57BL/6J mice (13-20 weeks) under isofluorane anesthesia were scanned in the supine position with a 1H quadrature surface coil over the abdomen. NMR measurements were performed in a horizontal bore 14.1T-26 cm magnet. Multi-slice gradient echo images were acquired in the coronal, sagittal and axial orientations for anatomical identification of the liver. Localized, respiration-gated 1H-NMR spectra were acquired from a 9-15 μl voxel with STEAM (TM, 20 ms; TR, 6.5 s; TE, 8-35 ms; 18-25 scans). Spectra were corrected for B0 drift and phase, summed and analyzed with LCModel. T2 correction was done by mono-exponential fit of peak areas as a function of TE. HLC was estimated as the T2-corrected area of 1.3 ppm-lipid resonance relative to that of the water plus 1.3 ppm-lipid. The quantification method was validated in water-in-oil phantoms (Figure 1). Water suppression was achieved with a Gaussian-shaped pulse during TM period (84 scans). Results: Respiration-gated acquisition yielded well-defined anatomical images of the mouse liver structure (Figure 2) and very stable inter-scans signal intensity (Figure 3). STEAM-determined water T2 was 8.4 ± 0.3 ms, shorter but still comparable with previous reports of ~11 ms with PRESS at 9.4T [2] and 11.7T [3]. HLC was 1.1 ± 0.1% in young adult mice. Fatty acyl resonances were well resolved in water-suppressed spectra (Figure 4) and choline-containing compounds could be identified. Discussion/Conclusion: We report highly stable localized 1H-MRS of the mouse liver at high field. Enhanced sensitivity allowed for accurate determination of HCL from small voxels confined to hepatic tissue, in short experiment series (~10 min). The saturation profile of the fatty-acyl chains can be determined even for healthy mice with low HLC. This approach opens the possibility to study mice models with very low HLC. References: [1] Hamilton G, 2011, NMR Biomed 24, 784–790. [2] Ye Q, 2012, Mang Reson Mater Phy, in press. [3] Tang H, 2007, Proc Intl Soc Mag Reson Med 15.

High‐field diffusion tensor imaging characterization of cerebral white matter injury in LPS exposed fetal sheep

Y. Van de Looij; G. Lodygensky; J. Dean; H. Hagberg; R. Gruetter  et al.

Hyperpolarized 6Li as probe for hemoglobin oxygenation level

R. Balzan; M. Mishkovsky; Y. Solomon; R. van Heeswijk; R. Gruetter  et al.

Phosphorus MRS Study of a Murine Model of Peripheral Arterial Disease

V. Mlynárik; M. Pellegrin; C. R. Cudalbu; L. Mazzolai; R. Gruetter

Description of a mouse model of peripheral artery disease to investigate exercise training as a therapeutic strategy.

M. Pellegrin; C. Poitry-Yamate; V. Mlynarik; K. Bouzourene; R. Gruetter  et al.

In vivo longitudinal 1H Spectroscopic Imaging and Diffusion Tensor Imaging studies at 9.4T in a rat model of chronic liver failure

C. R. Cudalbu; V. McLin; O. Braissant; N. Kunz; Y. van de Looij  et al.

A new approach to short-TE full-sensitivity MRSI of human brain at 7T

L. Xin; V. Mlynarik; R. Gruetter

Localized in vivo hyperpolarization transfer sequences

M. Mishkovsky; T. Cheng; A. Comment; R. Gruetter

In vivo localized and fully adiabatic homonuclear and heteronuclear polarization transfer experiments were designed and performed in the rat brain at 9.4 T after infusion of hyperpolarized sodium [1,2-(13) C(2) ] and sodium [1-(13) C] acetate. The method presented herein leads to highly enhanced in vivo detection of short-T(1) (13) C as well as attached protons. This indirect detection scheme allows for probing additional molecular sites in hyperpolarized substrates and their metabolites and can thus lead to improved spectral resolution such as in the case of (13) C-acetate metabolism.

DOI : 10.1002/mrm.23231

Handling macromolecule signals in the quantification of the neurochemical profile

C. Cudalbu; V. Mlynárik; R. Gruetter

In vivo localized proton magnetic resonance spectroscopy (1H MRS) became a powerful and unique technique to non-invasively investigate brain metabolism of rodents and humans. The main goal of 1H MRS is the reliable quantification of concentrations of metabolites (neurochemical profile) in a well-defined region of the brain. The availability of very high magnetic field strengths combined with the possibility of acquiring spectra at very short echo time have dramatically increased the number of constituents of the neurochemical profile. The quantification of spectra measured at short echo times is complicated by the presence of macromolecule signals of particular importance at high magnetic fields. An error in the macromolecule estimation can lead to substantial errors in the obtained neurochemical profile. The purpose of the present review is to overview methods of high field 1H MRS with a focus on the metabolite quantification, in particular in handling signals of macromolecules. Three main approaches of handling signals of macromolecules are described, namely mathematical estimation of macromolecules, measurement of macromolecules in vivo, and direct acquisition of the in vivo spectrum without the contribution of macromolecules.

DOI : 10.3233/JAD-2012-120100

Brain osmolytes and brain edema in a rat model of chronic liver failure: in vivo longitudinal 1H Spectroscopic imaging and Diffusion tensor imaging studies at 9.4T

C. R. Cudalbu; O. Braissant; M. Lepore; R. Gruetter; V. McLin

Early Predictive Biomarkers for Lesion After Transient Cerebral Ischemia

C. Berthet; H. Lei; R. Gruetter; L. Hirt

Background and Purpose—Despite the improving imaging techniques, it remains challenging to predict the outcome early after transient cerebral ischemia. The aim of this study was thus to identify early metabolic biomarkers for outcome prediction. Methods—We modeled transient ischemic attacks and strokes in mice. Using high-field MR spectroscopy, we correlated early changes in the neurochemical profile of the ischemic striatum with histopathologic alterations at a later time point. Results—A significant increase in glutamine was measured between 3 hours and 8 hours after all ischemic events followed by reperfusion independently of the outcome and can thus be considered as an indicator of recent transient ischemia. On the other hand, a reduction of the score obtained by summing the concentrations of N-acetyl aspartate, glutamate, and taurine was a good predictor of an irreversible lesion as early as 3 hours after ischemia. Conclusions—We identified biomarkers of reversible and irreversible ischemic damage, which can be used in an early predictive evaluation of stroke outcome

DOI : 10.1161/STROKEAHA.110.603647

Short Erythropoietin treatment following Hypoxia-Ischemia in the immature rat brain: macro-, micro-structural and metabolic assessment using multimodal MR

Y. Van de Looij; A. Chatagner; P. Hüppi; R. Gruetter; S. Sizonenko

Development of a BOLD fMRI protocol to investigate the mouse barrel cortex at high field

K. Lopez; L. Alfredo

Advisor(s) : R. Gruetter

In vivo metabolic profiling of glioma-initiating cells using proton magnetic resonance spectroscopy at 14.1 Tesla

V. Clement-Schatlo; V. Mlynarik; C. Cudalbu; D. Marino; I. Radovanovic  et al.

High-resolution spatial mapping of changes in the neurochemical profile after focal ischemia in mice

M. Alf; H. Lei; C. Berthet; L. Hirt; R. Gruetter  et al.

Structural Magnetic Resonance Imaging : Motion Correction and Improvement Strategies

T. Kober / R. Gruetter; G. Krüger (Dir.)

Notwithstanding its great success in diagnostic routine and research, structural magnetic resonance imaging (MRI) still faces challenges. In this context, the present thesis studies new approaches to mitigate the motion susceptibility of MRI. Furthermore, it shows new possibilities to prevent inhomogeneities in T1-weighted imaging. Lastly, the performance of different MRI sequences with regard to lesion conspicuity is evaluated in a cohort of early-stage multiple sclerosis (MS) patients. Free-induction-decay (FID) navigators are first established as a novel approach to detect motion using multi-channel MR receive coils. It is demonstrated that a very short sample of the FID suffices to obtain information about the occurrence of head motion. A major advantage of FID navigators – arising from their short duration – is their negligible interference with the actual imaging procedures. This allows their incorporation in various acquisition schemes. In a first proof-of-concept demonstration, the FID navigators are employed in an anatomical imaging sequence. Subsequently, a comprehensive approach for retro- and prospective motion correction in MR diffusion imaging employing FID navigators is presented. Within the scope of T1-weighted imaging, the MP2RAGE sequence is introduced. This novel sequence enables the acquisition of homogeneous T1-weighted images at high and ultra-high fields by cancelling out the bias field which arises from inhomogeneities in the transmission and reception RF fields. In addition, it is shown that the very pure T1-weighted contrast obtained allows the derivation of quantitative T1-maps. Finally, the conspicuity of MS lesions in different routine and non-standard MR image contrasts is investigated in a cohort of ten early-stage MS patients. Thereby, the MP2RAGE sequence is directly applied in a clinical context. Together with a standard T2 measurement, its T1-mapping feature is used to provide a comprehensive picture of MR lesion characteristics in this patient group. An important finding of the study is that routine MS protocols may be considerably improved by employing new acquisition techniques. The insights gained in this study may hence contribute to optimise future clinical MS imaging protocols. To conclude, this thesis introduces different novel techniques to improve structural MRI. Besides the investigation of the theoretical aspects of the presented methods, their application is also evaluated in realistic settings.

Localized in vivo hyperpolarization transfer experiments

M. Mishkovsky; T. Cheng; A. Comment; R. Gruetter

In vivo detection of brain Krebs cycle intermediate by hyperpolarized MR

M. Mishkovsky; A. Comment; R. Gruetter

Assessing The Importance Of An Astrocyte Specific Anaplerotic Enzyme In The Central Nervous System Of Rat

S. Lengacher; C. R. Cudalbu; N. Aznavour; R. Gruetter; P. Magistretti

Evaluation of glial metabolic fluxes in vivo with [2-13C] acetate infusion in mouse brain by 1H-[13C] NMR spectroscopy

L. Xin; H. Lei; B. Lanz; R. Gruetter

Impact of the prior knowledge on the quantification of in vivo 13C spectra using two different algorithms: LCModel and AMARES

C. R. Cudalbu; B. Lanz; J. M. Duarte; N. Kunz; R. Gruetter

Compartmentalized Cerebral Metabolism of [1,6-(13)C]Glucose Determined by in vivo (13)C NMR Spectroscopy at 14.1 T

J. M. N. Duarte; B. Lanz; R. Gruetter

Cerebral metabolism is compartmentalized between neurons and glia. Although glial glycolysis is thought to largely sustain the energetic requirements of neurotransmission while oxidative metabolism takes place mainly in neurons, this hypothesis is matter of debate. The compartmentalization of cerebral metabolic fluxes can be determined by (13)C nuclear magnetic resonance (NMR) spectroscopy upon infusion of (13)C-enriched compounds, especially glucose. Rats under light α-chloralose anesthesia were infused with [1,6-(13)C]glucose and (13)C enrichment in the brain metabolites was measured by (13)C NMR spectroscopy with high sensitivity and spectral resolution at 14.1 T. This allowed determining (13)C enrichment curves of amino acid carbons with high reproducibility and to reliably estimate cerebral metabolic fluxes (mean error of 8%). We further found that TCA cycle intermediates are not required for flux determination in mathematical models of brain metabolism. Neuronal tricarboxylic acid cycle rate (V(TCA)) and neurotransmission rate (V(NT)) were 0.45 ± 0.01 and 0.11 ± 0.01 μmol/g/min, respectively. Glial V(TCA) was found to be 38 ± 3% of total cerebral oxidative metabolism, accounting for more than half of neuronal oxidative metabolism. Furthermore, glial anaplerotic pyruvate carboxylation rate (V(PC)) was 0.069 ± 0.004 μmol/g/min, i.e., 25 ± 1% of the glial TCA cycle rate. These results support a role of glial cells as active partners of neurons during synaptic transmission beyond glycolytic metabolism.

DOI : 10.3389/fnene.2011.00003

Diffusion Tensor Echo Planar Imaging Using Surface Coil Transceiver with a Semiadiabatic RF Pulse Sequence at 14.1T

Y. van de Looij; N. Kunz; P. Hueppi; R. Gruetter; S. Sizonenko

Diffusion magnetic resonance studies of the brain are typically performed using volume coils. Although in human brain this leads to a near optimal filling factor, studies of rodent brain must contend with the fact that only a fraction of the head volume can be ascribed to the brain. The use of surface coil as transceiver increases Signal-to-Noise Ratio (SNR), reduces radiofrequency power requirements and opens the possibility of parallel transmit schemes, likely to allow efficient acquisition schemes, of critical importance for reducing the long scan times implicated in diffusion tensor imaging. This study demonstrates the implementation of a semiadiabatic echo planar imaging sequence (echo time = 40 ms, four interleaves) at 14.1T using a quadrature surface coil as transceiver. It resulted in artifact free images with excellent SNR throughout the brain. Diffusion tensor derived parameters obtained within the rat brain were in excellent agreement with reported values. Magn Reson Med 65:732-737, 2011. (C) 2010 Wiley-Liss, Inc.

DOI : 10.1002/mrm.22656

Early metabolic biomarkers identifying permanent stroke in mouse brain using 1H MRS

H. Lei; C. Berthet; L. Hirt; R. Gruetter

The metabolic effect of hyperpolarized [1-13C]acetate dosage in vivo

J. Bastiaansen; T. Cheng; M. Mishkovsky; A. Comment; R. Gruetter

Phosphocreatine line with changes in localised 31P MRS of exercising muscle at 7T

M. Meyerspeer; C. Nabuurs; A. Schoisengeier; E. Unger; R. Gruetter  et al.

The aim was to investigate changes in the line width of high-energy phosphates solely originating from the intramyocellular compartment by temporally resolved localised 31P-MRS after aerobic calf muscle exercise. Semi-Laser [2] localised 31P-MRS of gastrocnemius muscle and pulse-acquire-MRS (10cm-surface coil) were performed alternatingley in three exercise bouts separated by >20min inactivity. PCr and Pi were quantified in AMARES from single acquitsitions, with TR=6s. While Pi line width is known to change due to pH, to our knowledge this is the first observation of an exercise induced line width increase of intra-myocellular PCr, which may be associated with myoglobin deoxygenation.

In vivo 1H MRS study of a mouse model for fragile-X-associated tremor/ataxia syndrome at 14.1 T

L. Xin; S. Papin; S. Jacquemont; R. Gruetter

In Vivo Assessment of Neuronal Metabolic Fluxes in Mouse Brain by 1H-[13C] NMR Spectroscopy

L. Xin; H. Lei; B. Lanz; R. Gruetter

Kinetics of hyperpolarized [1-13C]acetate metabolism in cardiac muscle

J. Bastiaansen; t. Cheng; R. Gruetter; A. Comment

Increased brain glycogen after recovery from acute hypoglycaemia suggests involvement in hypoglycaemia unawareness

J. Duarte; F. D. Morgenthaler; R. Gruetter

Proton traps for multi-nuclear RF coils: design analysis and practical implementation for 13C MRS in humans at 7T

M. Meyerspeer; R. Gruetter; A. Magill

Trapped coil designs allow the operation of lower-frequency coils in the presence of a proton coil. Adding a second capacitor to a trap in the non-proton RF coil allows control over the trap reactance at the low and high frequency (here 13C and 1H) and the frequency of the trap mode. We demonstrate the interdependence of the parameters controlled and show analytical and numerical solutions to fulfill the design constraints. Our experiments show that, using the simulations, it possible to construct an effective second order trap for 13C at 7T.

Mapping inversion efficiencies of adiabatic pulses at 7T

M. Narsude; R. F. Marques; J. Pedro; F. Eggenschwiler; R. Gruetter

Neurochemical profile in the hippocampus of aging mice as detected by in vivo 1H NMR spectroscopy at 14.1 T

J. Duarte; R. Gruetter

Brain metabolism by in vivo 13C NMR spectroscopy

J. Duarte; B. Lanz; R. Gruetter

Chronic delivery of antibody fragments using immunoisolated cell implants as a passive vaccination tool

O. Marroquin Belaunzaran; M. I. Cordero; V. Setola; S. Bianchi; C. Galli  et al.

BACKGROUND: Monoclonal antibodies and antibody fragments are powerful biotherapeutics for various debilitating diseases. However, high production costs, functional limitations such as inadequate pharmacokinetics and tissue accessibility are the current principal disadvantages for broadening their use in clinic. METHODOLOGY AND PRINCIPAL FINDINGS: We report a novel method for the long-term delivery of antibody fragments. We designed an allogenous immunoisolated implant consisting of polymer encapsulated myoblasts engineered to chronically release scFv antibodies targeted against the N-terminus of the Aβ peptide. Following a 6-month intracerebral therapy we observed a significant reduction of the production and aggregation of the Aβ peptide in the APP23 transgenic mouse model of Alzheimer's disease. In addition, functional assessment showed prevention of behavioral deficits related to anxiety and memory traits. CONCLUSIONS AND SIGNIFICANCE: The chronic local release of antibodies using immunoisolated polymer cell implants represents an alternative passive vaccination strategy in Alzheimer's disease. This novel technique could potentially benefit other diseases presently treated by local and systemic antibody administration.

DOI : 10.1371/journal.pone.0018268

Imaging glutamine synthesis rates in the hyperammonemic rat brain

C. R. Cudalbu; V. Mlynárik; B. Lanz; H. Frenkel; N. Costers  et al.

Metabolic alterations in the cortex of a mouse model with glutathione deficit – relevance to schizophrenia

D. JMN; A. Kulak; R. Gruetter; D. KQ

Metabolic alterations in the cortex of a mouse model with glutathione deficit – relevance to schizophrenia

J. Duarte; A. Kulak; R. Gruetter; D. KQ

Developmental in vivo 1H NMR spectroscopy at 14.1 T in mice with genetic redox dysregulation: an animal model with relevance to schizophrenia

J. Duarte; A. Kulak; D. KQ; R. Gruetter

Feasibility of in vivo N-15 MRS detection of hyperpolarized N-15 labeled choline in rats

C. R. Cudalbu; A. Comment; F. Kurdzesau; R. van Heeswijk; K. Uffmann  et al.

The increase of total choline in tumors has become an important biomarker in cancer diagnosis. Choline and choline metabolites can be measured in vivo and in vitro using multinuclear MRS. Recent in vivo C-13 MRS studies using labeled substrates enhanced via dynamic nuclear polarization demonstrated the tremendous potential of hyperpolarization for real-time metabolic studies. The present study demonstrates the feasibility of detecting hyperpolarized N-15 labeled choline in vivo in a rat head at 9.4 T. We furthermore report the in vitro (172 +/- 16 s) and in vivo (126 +/- 15 s) longitudinal relaxation times. We conclude that with appropriate infusion protocols it is feasible to detect hyperpolarized N-15 labeled choline in live animals.

DOI : 10.1039/c002309b

Echo-time independent signal modulations for strongly coupled systems in triple echo localization schemes: An extension of S-PRESS editing

N. Kickler; G. Gambarota; R. Mekle; R. Gruetter; R. Mulkern

The double spin-echo point resolved spectroscopy sequence (PRESS) is a widely used method and standard in clinical MR spectroscopy. Existence of important J-modulations at constant echo times, depending on the temporal delays between the rf-pulses, have been demonstrated recently for strongly coupled spin systems and were exploited for difference editing, removing singlets from the spectrum (strong-coupling PRESS, S-PRESS) A drawback of this method for in vivo applications is that large signal modulations needed for difference editing occur only at relatively long echo times. In this work we demonstrate that, by simply adding a third refocusing pulse (3S-PRESS), difference editing becomes possible at substantially shorter echo times while, as applied to citrate, more favorable lineshapes can be obtained For the example of an AB system an analytical description of the MR signal, obtained with this triple refocusing sequence (3S-PRESS), is provided (C) 2009 Elsevier Inc All rights reserved

DOI : 10.1016/j.jmr.2009.12.006

Effects of Lactoferrin on altered brain metabolism in pup rats after prenatal exposure to Dexamethasone

Y. Van de Looij; P. Larvaron; E. Somm; B. Wang; R. Gruetter  et al.

Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation

A. Comment; S. Jannin; J.-N. Hyacinthe; P. Miéville; R. Sarkar  et al.

A high throughput method was designed to produce hyperpolarized gases by combining lowtemperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to 129Xe nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.

DOI : 10.1103/PhysRevLett.105.018104

Metabolic Alterations in the Cortex of a Mouse Model with Glutathione Deficit - Relevance to Schizophrenia

J. Duarte; A. Kulak; R. Gruetter; K. Q. Do

Investigation of spatial distribution of metabolites in rat brain at elevated plasma glucose levels

V. Mlynarik; C. R. Cudalbu; H. Frenkel; N. Costers; R. Gruetter

Localized 31P saturation transfer in rat brain

V. Mlynarik; C. R. Cudalbu; Y. Pilloud; R. Gruetter

Neurochemical changes in the developing rat hippocampus during prolonged hypoglycemia

R. Rao; K. Ennis; J. D. Long; K. Ugurbil; R. Gruetter  et al.

Hypoglycemia is common during development and is associated with the risk of neurodevelopmental deficits in human infants. The effects of hypoglycemia on the developing hippocampus are poorly understood. The sequential changes in energy substrates, amino acids and phosphocreatine were measured from the hippocampus during 180 min of insulin-induced hypoglycemia (blood glucose < 2.5 mmol/L) in 14-day-old rats using in vivo1H NMR spectroscopy. Hypoglycemia resulted in neuroglycopenia (brain glucose < 0.5 μmol/g). However, the phosphocreatine/creatine (PCr/Cr) ratio was maintained in the physiological range until approximately 150 min of hypoglycemia, indicating that energy supply was sufficient to meet the energy demands. Lactate concentration decreased soon after the onset of neuroglycopenia. Beyond 60 min, glutamine and glutamate became the major energy substrates. A precipitous decrease in the PCr/Cr ratio, indicative of impending energy failure occurred only after significant depletion of these amino acids. Once glutamate and glutamine were significantly exhausted, aspartate became the final energy source. N-acetylaspartate concentration remained unaltered, suggesting preservation of neuronal/mitochondrial integrity during hypoglycemia. Correction of hypoglycemia normalized the PCr/Cr ratio and partially restored the amino acids to pre-hypoglycemia levels. Compensatory neurochemical changes maintain energy homeostasis during prolonged hypoglycemia in the developing hippocampus. © 2010 International Society for Neurochemistry.

DOI : 10.1111/j.1471-4159.2010.06797.x

Neurochemical profile of the developing mouse cortex determined by in vivo1H NMR spectroscopy at 14.1 T and the effect of recurrent anaesthesia

A. Kulak; J. M. N. Duarte; K. Q. Do; R. Gruetter

P>The neurochemical profile of the cortex develops in a region and time specific manner, which can be distorted by psychiatric and other neurological pathologies. Pre-clinical studies often involve experimental mouse models. In this study, we determined the neurochemical profile of C57BL/6 mice in a longitudinal study design to provide a reference frame for the normal developing mouse cortex. Using in vivo proton NMR spectroscopy at 14 T, we measured the concentrations of 18 metabolites in the anterior and posterior cortex on postnatal days (P) 10, 20, 30, 60 and 90. Cortical development was marked by alterations of highly concentrated metabolites, such as N-acetylaspartate, glutamate, taurine and creatine. Regional specificity was represented by early variations in the concentration of glutamine, aspartate and choline. In adult animals, regional concentration differences were found for N-acetylaspartate, creatine and myo-inositol. In this study, animals were exposed to recurrent isoflurane anaesthesia. Additional experiments showed that the latter was devoid of major effects on behaviour or cortical neurochemical profile. In conclusion, the high sensitivity and reproducibility of the measurements achieved at 14 T allowed us to identify developmental variations of cortical areas within the mouse cortex.

DOI : 10.1111/j.1471-4159.2010.07051.x

MP2RAGE, a self bias-field corrected sequence for improved segmentation and T-1-mapping at high field

J. P. Marques; T. Kober; G. Krueger; W. van der Zwaag; P.-F. Van de Moortele  et al.

The large spatial inhomogeneity in transmit B, field (B-1(+)) observable in human MR images at hi h static magnetic fields (B-0) severely impairs image quality. To overcome this effect in brain T-1-weighted images the, MPRAGE sequence was modified to generate two different images at different inversion times MP2RAGE By combining the two images in a novel fashion, it was possible to create T-1-weigthed images where the result image was free of proton density contrast, T-2* contrast, reception bias field, and, to first order transmit field inhomogeneity.

DOI : 10.1016/j.neuroimage.2009.10.002

Quantification of brain glycogen concentration and turnover through localized C-13 NMR of both the C1 and C6 resonances

R. van Heeswijk; F. D. Morgenthaler; L. Xin; R. Gruetter

We have recently shown that at isotopic steady state C-13 NMR can provide a direct measurement of glycogen concentration changes, but that the turnover of glycogen was not accessible with this protocol. The aim of the present study was to design, implement and apply a novel dual-tracer infusion protocol to simultaneously measure glycogen concentration and turnover. After reaching isotopic steady state for glycogen Cl using [1-C-13] glucose administration, [1,6-C-13(2)] glucose was infused such that isotopic steady state was maintained at the Cl position, but the C6 position reflected C-13 label incorporation. To overcome the large chemical shift displacement error between the C1 and C6 resonances of glycogen, we implemented 2D gradient based localization using the Fourier series window approach, in conjunction with time-domain analysis of the resulting FIDs using jMRUI. The glycogen concentration of 5.1 +/- 1.6 mM measured from the Cl position was in excellent agreement with concomitant biochemical determinations. Glycogen turnover measured from the rate of label incorporation into the C6 position of glycogen in the a-chloralose anesthetized rat was 0.7 mu mol/g/h. Copyright (C) 2009 John Wiley & Sons, Ltd.

DOI : 10.1002/nbm.1460

Removing air-tissue artifacts in phase images by modulating the air susceptibility

J. P. Marques; R. Gruetter

Direct in vivo measurement of glycine and the neurochemical profile in the rat medulla oblongata

L. Xin; G. Gambarota; J. M. N. Duarte; V. Mlynarik; R. Gruetter

The medulla oblongata (MO) contains a high density of glycinergic synapses and a particularly high concentration of glycine The aims of this study were to measure directly in vivo the neurochemical profile, including glycine, in MO using a spin-echo-based H-1 MRS sequence at TE = 2 8 ms and to compare it with three other brain regions (cortex, striatum and hippocampus) in the rat Glycine was quantified in MO at TE = 2 8 ms with a Cramer-Rao lower bound (CRIB) of approximately 5% As a result of the relatively low level of glycine in the other three regions, the measurement of glycine was performed at TE = 20 ms, which provides a favorable J-modulation of overlapping myo-inositol resonance The other 14 metabolites composing the neurochemical profile were quantified in vivo in MO with CRLBs below 25% Absolute concentrations of metabolites in MO, such as glutamate, glutamine, gamma-aminobutyrate, taurine and glycine, were in the range of previous in vitro quantifications in tissue extracts Compared with the other regions, MO had a three-fold higher glycine concentration, and was characterised by reduced (p < 0 001) concentrations of glutamate (-50 +/- 4%), glutamine (-54 +/- 3%) and taurine (-78 +/- 3%) This study suggests that the functional specialisation of distinct brain regions is reflected in the neurochemical profile Copyright (C) 2010 John Wiley & Sons, Ltd

DOI : 10.1002/nbm.1537

High resolution magnetic resonance imaging detects individual pancreatic islets in whole pancreas

S. Lamprianou; R. Immonen; A. Gjinovci; L. Vinet; R. Gruetter  et al.

In vivo hyperpolarized 89Y studies in a 9.4T animal scanner

M. E. Merritt; M. Mishkovsky; T. Cheng; A. Jindal; Z. Kovacs  et al.

BISEP-based, Ultra-short TE 1H–[13C] NMR Spectroscopy of the Rat Brain at 14.1 T

L. Xin; B. Lanz; H. Frenkel; R. Gruetter

In vivo localized 15N and 1H MRS in the hyperammonaemic rat brain at 9.4T

C. R. Cudalbu; B. Lanz; F. Morgenthaler; Y. Pilloud; V. Mlynarik  et al.

Simultaneous Localized in vivo 1H and 15N MRS of Glutamine Synthesis in the hyperammonaemic rat brain

C. R. Cudalbu; B. Lanz; F. D. Morgenthaler; Y. Pilloud; V. Mlynárik  et al.

On the origin of the MR image phase contrast: An in vivo MR microscopy study of the rat brain at 14.1 T

J. P. Marques; R. Maddage; V. Mlynarik; R. Gruetter

Recent studies at high magnetic fields using the phase of gradient-echo MR images have shown the ability to unveil cortical substructure in the human brain. To investigate the contrast mechanisms in phase imaging, this study extends, for the first time, phase imaging to the rodent brain. Using a 14.1 T horizontal bore animal MRI scanner for in vivo micro-imaging, images with an in-plane resolution of 33 mu m were acquired. Phase images revealed, often more clearly than the corresponding magnitude images, hippocampal fields, cortical layers (e. g. layer 4), cerebellar layers (molecular and granule cell layers) and small white matter structures present in the striatum and septal nucleus. The contrast of the phase images depended in part on the orientation of anatomical structures relative to the magnetic field, consistent with bulk susceptibility variations between tissues. This was found not only for vessels, but also for white matter structures, such as the anterior commissure, and cortical layers in the cerebellum.

DOI : 10.1016/j.neuroimage.2009.02.023

In vivo measurement of glycine with short echo-time H-1 MRS in human brain at 7 T

G. Gambarota; R. Mekle; L. Xin; M. Hergt; W. van der Zwaag  et al.

To determine whether glycine can be measured at 7 T in human brain with H-1 magnetic resonance spectroscopy (MRS).

DOI : 10.1007/s10334-008-0152-0

Evolution of the neurochemical profile in the frontal and occipital cortex of the developing mouse determined by in vivo 1H NMR spectroscopy at 14.1 T

J. Duarte; A. Frank; D. KQ; R. Gruetter

Comparison Of Single-Shot 2d EPI And Segmented 3d EVI Acquisition For fMRI at 7T

W. Van der Zwaag; T. Kober; R. F. Marques; J. Pedro; G. Glover  et al.

Mapping glucose and lactate concentrations with microliter resolution in rat brain using short-echo-time spectroscopic imaging

V. Mlynarik; C. R. Cudalbu; H. Frenkel; R. Gruetter

Investigation of high-resolution functional magnetic resonance imaging by means of surface and array radiofrequency coils at 7 T

W. van der Zwaag; J. P. Marques; M. Hergt; R. Gruetter

In this investigation, high-resolution, 1x1x1-mm(3) functional magnetic resonance imaging (fMRI) at 7 T is performed using a multichannel array head coil and a surface coil approach. Scan geometry was optimized for each coil separately to exploit the strengths of both coils. Acquisitions with the surface coil focused on partial brain coverage, while whole-brain coverage fMRI experiments were performed with the array head coil. BOLD sensitivity in the occipital lobe was found to be higher with the surface coil than with the head array, suggesting that restriction of signal detection to the area of interest may be beneficial for localized activation studies. Performing independent component analysis (ICA) decomposition of the fMRI data, we consistently detected BOLD signal changes and resting state networks. In the surface coil data, a small negative BOLD response could be detected in these resting state network areas. Also in the data acquired with the surface coil, two distinct components of the positive BOLD signal were consistently observed. These two components were tentatively assigned to tissue and venous signal changes.

DOI : 10.1016/j.mri.2009.01.013

Comparison of glutamate and glutamine quantitation in rat brain by 1H-[13C] NMR Spectroscopy at 9.4 T and 14.1 T

L. Xin; V. Mlynarik; H. Frenkel; R. Gruetter

Evolution of the neurochemical profile after transient focal cerebral ischemia in the mouse brain

H. Lei; C. Berthet; L. Hirt; R. Gruetter

Evolution of the neurochemical profile consisting of 19 metabolites after 30 mins of middle cerebral artery occlusion was longitudinally assessed at 3, 8 and 24 h in 6 to 8 mu L volumes in the striatum using localized H-1-magnetic resonance spectroscopy at 14.1 T. Profound changes were detected as early as 3 h after ischemia, which include elevated lactate levels in the presence of significant glucose concentrations, decreases in glutamate and a transient twofold glutamine increase, likely to be linked to the excitotoxic release of glutamate and conversion into glial glutamine. Interestingly, decreases in N-acetyl-aspartate (NAA), as well as in taurine, exceeded those in neuronal glutamate, suggesting that the putative neuronal marker NAA is rather a sensitive marker of neuronal viability. With further ischemia evolution, additional, more profound concentration decreases were detected, reflecting a disruption of cellular functions. We conclude that early changes in markers of energy metabolism, glutamate excitotoxicity and neuronal viability can be detected with high precision non-invasively in mice after stroke. Such investigations should lead to a better understanding and insight into the sequential early changes in the brain parenchyma after ischemia, which could be used for identifying new targets for neuroprotection.

DOI : 10.1038/jcbfm.2009.8

Grey/white matter contrast in phase images: is it susceptibility or is it not?

J. P. Marques; R. Gruetter

Probing the role of deoxyhemoglobin in white-gray matter and subcortical gray matter contrast in phase imaging

J. P. Marques; R. Maddage; R. Gruetter

Proton NMR of15N-choline metabolites enhanced by dynamic nuclear polarization

R. Sarkar; A. Comment; P. R. Vasos; S. Jannin; R. Gruetter  et al.

(Figure Presented) Chemical shifts of protons can report on metabolic transformations such as the conversion of choline to phosphocholine. To follow such processes in vivo, magnetization can be enhanced by dynamic nuclear polarization (DNP). We have hyperpolarized in this manner nitrogen-15 spins in 15N-labeled choline up to 3.3% by irradiating the 94 GHz electron spin resonance of admixed TEMPO nitroxide radicals in a magnetic field of 3.35 T during ca. 3 h at 1.2 K. The sample was subsequently transferred to a high-resolution magnet, and the enhanced polarization was converted from 15N to methyl- and methylene protons, using the small 2,3J(1H,15N) couplings in choline. The room-temperature lifetime of nitrogen polarization in choline, T 1(15N) = 200 s, could be considerably increased by partial deuteration of the molecule. This procedure enables studies of choline metabolites in vitro and in vivo using DNP-enhanced proton NMR. Copyright © 2009 American Chemical Society.

DOI : 10.1021/ja9021304

Short-echo-time 1H MRS of the mouse lacking prion protein (Prnp-/-) at 14.1T

C. R. Cudalbu; V. Mlynárik; J. Bremer; A. Aguzzi; R. Gruetter

Improved ¹H-[¹³C] and ¹H NMR methods for in vivo measurement of cerebral metabolism

L. Xin / R. Gruetter (Dir.)

Nuclear magnetic resonance (NMR) spectroscopy can be applied in vivo to measure static or dynamic biochemical information, e.g., concentrations of metabolites and metabolic fluxes, using various nuclei such as 1H, 13C, 31P and 15N. The work of this thesis involves both 1H and 13C nuclei and focuses on improving 1H NMR detection methods for measuring metabolites in vivo in rat brain. 13C NMR spectroscopy can be used to monitor the flow of 13C label from a 13C enriched substrate, such as glucose or acetate, into different NMR detectable metabolites, e.g., glutamate (Glu) and glutamine (Gln), for the quantitative study of cerebral metabolism in vivo. 1H-observed 13C-edited (1H-[13C]) NMR spectroscopy is an alternative to direct 13C NMR detection of 13C labeled metabolites, allowing a higher spatial and temporal resolution, albeit at a lower spectral resolution. In this context, a hybrid full signal intensity 1H-[13C] NMR sequence, combining a 13C editing block based on an inversion B1 insensitive spectral editing pulse (BISEP) with a spin-echo based localization (SPECIAL), was developed and implemented at ultra-high magnetic field (14.1 T) to benefit from increased sensitivity and spectral resolution at high B0. As a result, high quality 1H-[13C] NMR spectra were obtained, leading to an improved quantification of 13C labeled metabolites, which allowed the measurement of time courses of Glu C4, Gln C4, as well as, for the first time, of Glu C3 and of Gln C3, with high temporal resolution from a small acquisition volume. Although at high magnetic field spectral resolution and sensitivity are improved, spectral overlap is still present, e.g., the N-acetylaspartate (NAA) C6 is easily obscured by the intensive labeling of Glu C3 and Gln C3 (Glx C3), due to their complex coupling patterns. To improve the detection of unresolved 13C labeled metabolites, we developed an alternative 1H-[13C] NMR editing sequence termed RACED-STEAM (selective Resonances suppression by Adiabatic Carbon Editing and Decoupling single-voxel STimulated Echo Acquisition Mode), which can selectively suppress 1H resonances bound to a specific 13C to uncover resonances that are difficult to resolve in the 1H-[13C] NMR spectrum. Results showed the efficient suppression of Glx C3 and Glu C4, allowing the detection of NAA C6 and Gln C4 at 9.4 T. The application of this method to measure the time course of NAA C6 demonstrated that NAA C6 turnover can be measured at very low levels of isotopic enrichment in a small volume, within a time frame of a few hours. The proposed scheme could also be extended to lower magnetic fields provided that the 13C chemical shifts remain sufficiently resolved. Similarly, due to limited spectral resolution and sensitivity, editing techniques for the in vivo detection of metabolites using 1H NMR spectroscopy are still in development. Many in vivo 1H NMR editing sequences are usually performed at moderate to long echo times (TE). Furthermore, several 1H NMR spectroscopy studies are performed at long TE to avoid the confounding effect of macromolecular signals on metabolite quantification. In such cases, proton T2 relaxation times of metabolites have to be taken into account for proper quantification of metabolite concentrations. While T2 relaxation times of singlets have been characterized in several studies, due to the relative experimental simplicity, similar information is lacking from coupled spin resonances of cerebral metabolites. In this thesis, spectral simulations based on the density matrix formalism were initially performed to predict the response of spin systems to the pulse sequence used. T2 relaxation times of coupled spin resonances and singlet resonances of cerebral metabolites were then measured in rat brain in vivo at 9.4 T. Data analysis was performed using LCModel combined with simulated TE-specific spectra. The aforementioned spectral simulations were further used to optimize pulse sequence parameters for the detection of metabolites, such as glycine, whose resonance signal is overlapped with the more intense myo-inositol resonances. A favorable short TE (i.e., 20 ms) was sufficient to reduce the signal intensity of myo-inositol, allowing the detection of glycine in vivo in rat brain at 9.4 T. This work suggests that at high magnetic fields, glycine can be measured at a relatively short TE without additional editing efforts. Moreover, glycine is present at a particularly high concentration in the medulla oblongata (MO). Therefore, we further measured regional distribution of glycine in the hippocampus, cortex, striatum and MO of the rat brain, as well as the highly specific neurochemical profile of the MO. In conclusion, dedicated approaches for 1H NMR detection developed and validated in this thesis lead to improved dynamic measurement of 13C labeling time courses such as the time courses of 13C labeling of Glu C3 and Gln C3, as well as the spectral resolved NAA C6, the direct in vivo detection of glycine in rat brain at 9.4 T and the measurement of T2 relaxation times of numerous J-coupled metabolites for the first time.

Steady-state brain glucose transport kinetics evaluated with a four-site conformational model

J. Duarte; F. D. Morgenthaler; H. Lei; R. Gruetter

Steady-State Brain Glucose Transport Kinetics Evaluated With A Four-Site Conformational Model

J. M. Duarte; F. D. Morgenthaler; H. Lei; R. Gruetter

Biochemical evidence for a significant blood-brain barrier for glucose in hypothalamus using the reversible Michaelis-Menten model

C. L. Poitry-Yamate; H. Frenkel; H. Lei; R. Gruetter

Multimodal NMR assessment of Erythropoietin as a neuroprotective agent following Hypoxia-Ischemia on P3 pup rat brain

Y. Van de Looij; A. Chatagner; N. Kunz; P. Hüppi; R. Gruetter  et al.

A Developmental In Vivo 1 H Nmr Study In Mice With Genetic Redox Dysregulation: An Animal Model With Relevance To Schizophrenia

A. Frank; J. M. Duarte; K. Q. Do; R. Gruetter

High-sensitivity phase-contrast tomography of rat brain in phosphate buffered saline

F. Pfeiffer; C. David; O. Bunk; C. Poitry-Yamate; R. Grütter  et al.

We report advances and complementary results concerning a recently developed method for high-sensitivity grating-based x-ray phase-contrast tomography. In particular we demonstrate how the soft tissue sensitivity of the technique can be used to obtain in-vitro tomographic images of rat brain specimens. Contrary to our previous experiments with fixated specimen (chemically modified or formalin fixed), the present results on the rat's brain are closer to the in-vivo situation. The findings are particularly important from a clinical point of view, since a similar approach using three gratings can be implemented with more readily available x-ray sources, such as standard x-ray tubes. © 2009 IOP Publishing Ltd.

Caffeine consumption attenuates neurochemical modifications in the hippocampus of streptozotocin-induced diabetic rats

J. M. N. Duarte; R. A. Carvalho; R. A. Cunha; R. Gruetter

Type 1 diabetes can affect hippocampal function triggering cognitive impairment through unknown mechanisms. Caffeine consumption prevents hippocampal degeneration and memory dysfunction upon different insults and is also known to affect peripheral glucose metabolism. Thus we now characterized glucose transport and the neurochemical profile in the hippocampus of streptozotocin-induced diabetic rats using in vivo1H NMR spectroscopy and tested the effect of caffeine consumption thereupon. We found that hippocampal glucose content and transport were unaltered in diabetic rats, irrespective of caffeine consumption. However diabetic rats displayed alterations in their hippocampal neurochemical profile, which were normalized upon restoration of normoglycaemia, with the exception of myo-inositol that remained increased (36 +/- 5%, p < 0.01 compared to controls) likely reflecting osmolarity deregulation. Compared to controls, caffeine-consuming diabetic rats displayed increased hippocampal levels of myo-inositol (15 +/- 5%, p < 0.05) and taurine (23 +/- 4%, p < 0.01), supporting the ability of caffeine to control osmoregulation. Compared to controls, the hippocampus of diabetic rats displayed a reduced density of synaptic proteins syntaxin, synaptophysin and synaptosome-associated protein of 25 kDa (in average 18 +/- 1%, p < 0.05) as well increased glial fibrillary acidic protein (20 +/- 5%, p < 0.05), suggesting synaptic degeneration and astrogliosis, which were prevented by caffeine consumption. In conclusion, neurochemical alterations in the hippocampus of diabetic rats are not related to defects of glucose transport but likely reflect osmoregulatory adaptations caused by hyperglycemia. Furthermore, caffeine consumption affected this neurochemical adaptation to high glucose levels, which may contribute to its potential neuroprotective effects, namely preventing synaptic degeneration and astrogliosis.

DOI : 10.1111/j.1471-4159.2009.06349.x

New approaches to and insights into glucose, glycogen, glutamate and glutamine metabolism in brain

R. Gruetter

The rate-limiting step for glucose transport into the hypothalamus is across the blood–hypothalamus interface

C. Poitry-Yamate; H. Lei; R. Gruetter

Specialized glucosensing neurons are present in the hypothalamus, some of which neighbor the median eminence, where the blood–brain barrier has been reported leaky. A leaky blood–brain barrier implies high tissue glucose levels and obviates a role for endothelial glucose transporters in the control of hypothalamic glucose concentration, important in understanding the mechanisms of glucose sensing We therefore addressed the question of blood–brain barrier integrity at the hypothalamus for glucose transport by examining the brain tissue-to-plasma glucose ratio in the hypothalamus relative to other brain regions. We also examined glycogenolysis in hypothalamus because its occurrence is unlikely in the potential absence of a hypothalamus–blood interface. Across all regions the concentration of glucose was comparable at a given plasma glucose concentration and was a near linear function of plasma glucose. At steady-state, hypothalamic glucose concentration was similar to the extracellular hypothalamic glucose concentration reported by others. Hypothalamic glycogen fell at a rate of ∼1.5 μmol/g/h and remained present in substantial amounts. We conclude for the hypothalamus, a putative primary site of brain glucose sensing that: the rate-limiting step for glucose transport into brain cells is at the blood–hypothalamus interface, and that glycogenolysis is consistent with a substantial blood -to- intracellular glucose concentration gradient.

DOI : 10.1111/j.1471-4159.2009.05934.x

Proton MRS investigation of human glioma models in nude mice at 14.1 T

V. Mlynarik; C. R. Cudalbu; V. Clément; D. Marino; I. Radovanovic  et al.

In Vivo Detection of Hyperpolarized 15N Choline in the Rat

C. R. Cudalbu; A. Comment; K. Uffmann; R. van Heeswijk; C. Perazzolo  et al.

1H NMR T1 Relaxation Times of the Neurochemical Profiles in Rat Brain at 14.1T

C. R. Cudalbu; V. Mlynárik; L. Xin; R. Gruetter

In vivo H-1 NMR measurement of glycine in rat brain at 9.4 T at short echo time

G. Gambarota; L. Xin; C. Perazzolo; I. Kohler; V. Mlynarik  et al.

Glycine is an amino acid present in mammalian brain, where it acts as an inhibitory and excitatory neurotransmitter. The two detectable protons of glycine give rise to a singlet at 3.55 ppm that overlaps with the more intense myo-inositol resonances, and its measurement has traditionally required specific editing efforts. The aim of the current study was to reduce the signal intensity of myo-inositol relative to that of glycine by exploiting the fast signal J-evolution of the myo-inositol spin system when using a single spin-echo localization method we recently introduced. Glycine was detected at TE = 20 ms with an average Cramer-Rao lower bound (CRLB) of 8.6% +/- 1.5% in rat brain (N = 5), at 9.4 T. The concentration of glycine was determined using LCModel analysis at 1.1 +/- 0.1 mM, in good agreement with biochemical measurements previously reported. We conclude that at high magnetic fields, glycine can be measured at a relatively short echo time (TE) without additional editing efforts.

DOI : 10.1002/mrm.21695

White matter changes in lipopolysaccharide (LPS) treated fetal sheep measured by high field diffusion tensor imaging (DTI)

Y. van de Looij; J. Dean; G. Lodygensky; F. Lazeyras; I. Kjellmer  et al.

Glucose transport and neurochemical profile in the hippocampus of STZ-induced diabetic rats under hyper- and euglycaemia studied by in vivo 1H MRS at 9.4 T

D. JMN; C. RA; C. RA; R. Gruetter

Proton T-2 relaxation time of J-coupled cerebral metabolites in rat brain at 9.4T

L. Xin; G. Gambarota; V. Mlynarik; R. Gruetter

Knowledge of proton T-2 relaxation time of metabolites is essential for proper quantitation of metabolite concentrations in localized proton spectroscopy, especially at moderate to long TEs. Although the T-2 relaxation time of singlets, such as that of creatine and N-acetylaspartate, has been characterized in several studies, similar information is lacking from coupled spin resonances of cerebral metabolites. In this study, the T-2 relaxation time of coupled spin resonances and singlet resonances of cerebral metabolites was measured in rat brain in vivo at 9.4T. Spectra were acquired at 11 TEs using the SPin ECho, full Intensity Acquired Localized (SPECIAL) spectroscopy method. Data analysis was performed in the frequency domain with the LCModel software using simulated TE-specific basis sets. The T-2 relaxation times in compounds showing singlet resonances were 113 +/- 3 ms (total creatine), 178 +/- 29 ms (total choline) and 202 +/- 12 ms (N-acetylaspartate). The T-2 values of J-coupled metabolites ranged from 89 +/- 8 ms (glutamate) to 148 +/- 14 ms (myo-inositol). Copyright (C) 2007 John Wiley & Sons, Ltd.

DOI : 10.1002/nbm.1205

Magnetic Resonance Spectroscopy and Imaging at 9.4T in P4 rat pup brain following cerebral hypoxia-ischemia

Y. van de Looij; A. Chatagner; N. Kunz; P. Hüppi; S. Sizonenko  et al.

Glucose homeostasis is altered in transgenic mice with brain specific knockout of the mitochondrial enzyme glutamate dehydrogenase

F. Frigerio; S. Carobbio; V. Mlynarik; R. Gruetter; R. Maechler

Fast Bias Field Correction for 9.4 Tesla Magnetic Resonance Imaging

X. Pena; M. Bach Cuadra; N. Kunz; N. Just; R. Gruetter  et al.

In this paper the problem of intensity inhomogeneity at high magnetic field on magnetic resonance images is addressed. Specifically, rat brain images at 9.4T acquired with a surface coil are bias corrected. We propose a low- pass frequency model that takes into account not only background-object contours but also other important contours inside the image. Two pre-processing filters are proposed: first, to create a volume of interest without contours, and second, to extrapolate the image values of such masked area to the whole image. Results are assessed quantitatively and visually in comparison to standard low pass filter approach, and they show as expected better accuracy in enhancing image intensity.

High-Quality MR Spectroscopy of the Human Brain with Full Signal Intensity at Echo Times Below 6 ms on a Clinical Platform at 3T and 7T

R. Mekle; V. Mlynarik; G. Gambarota; M. Hergt; G. Krueger  et al.

Improved Spectral Dispersion in Proton MR Spectroscopy of the Neurochemical Profile in the Rat Brain at 14.1 Tesla

V. Mlynárik; C. R. Cudalbu; L. Xin; R. Gruetter

T2 relaxation of coupled spin resonances of cerebral metabolites in rat brain at 9.4 T

L. Xin; G. Gambarota; V. Mlynarik; I. Kohler; R. Gruetter

Glucose transport in the rat hippocampus measured by in vivo 1H-NMR spectroscopy

J. M. d. N. Duarte; R. Gruetter

Direct validation of in vivo localized 13C MRS measurements of brain glycogen

H. Lei; F. Morgenthaler; T. Yue; R. Gruetter

With the use of localized 13C MRS in conjunction with [1-(13)C]-D-glucose infusion, it is possible to study brain glycogen metabolism in vivo. The purpose of this study was to validate in vivo 13C MRS measurements by comparing them with results from a standard biochemical assay. To increase the [1-(13)C] glycogen concentration, 11 rats were subjected to an episode of acute hypoglycemia followed by a mild hyperglycemic recovery period during which [1-(13)C]-D-glucose was infused. The total brain [1-(13)C] glycogen of the same animal was determined from the enzymatically determined total brain glycogen content, which was fixed by focused microwave irradiation (4 kW in 1.4 s) immediately after the end of the in vivo NMR measurements. The corresponding isotopic enrichment (IE) of glycogen was measured by in vitro 1H MRS of protons bound to glucose C1-alpha. The in vivo [1-(13)C] glycogen concentration was strongly correlated to the in vitro [1-(13)C] glycogen content determined by biochemical measurement in a linear manner (R=0.79). The results are consistent with the notion that localized 13C MRS measurements closely reflect 13C glycogen content in the brain.

DOI : 10.1002/mrm.21128

Glucose and glycogen content in rat hypothalamus at euglycemia and hypoglycemia

C. Poitry-Yamate; H. Frenkel; H. Lei; R. Gruetter

P1-119 Effects of adverse prenatal conditions on rat cerebral development: a quantitative proton magnetic resonance spectroscopy (H-MRS) and histopathology analysis

E. J. Camm; N. Kunz; M. L. Aubert; R. Gruetter; S. V. Sizonenko  et al.

A novel approach towards measuring absolute brain glycogen concentration changes in vivo

F. D. Morgenthaler; S. Laus; L. Xin; R. van Heeswijk; H. Frenkel  et al.

Precision of metabolite concentrations obtained by LCModel as a function of the signal-to-noise ratio in rodent brain

V. Mlynarik; G. Gambarota; L. Xin; R. Gruetter

Design and performance of a DNP prepolarizer coupled to a rodent MRI scanner

A. Comment; B. van den Brandt; K. Uffmann; F. Kurdzesau; S. Jannin  et al.

For most of the last forty years, the techniques of Dynamic Nuclear Polarization (DNP) have been confined to particle-physics laboratories building polarized targets, but recently it has been shown that samples similar to a solid target can be transformed into room temperature liquid solutions while retaining a high nuclear polarization. This method of "hyperpolarization" is of interest in NMR/MRI/MRS. We describe a 3.35 T DNP/9.4 T MRI installation based on a continuous-flow cryostat, using a standard wide-bore low-field NMR magnet as prepolarizer magnet and a widely available radical as polarizing agent. The interfacing to a rodent scanner requires that the infusion of the polarized solution in the animal be remotely controlled, because of limited access inside the magnet bore. Physiological constraints on the infusion rate can be a serious source of polarization loss, and the discussion of efficiency is therefore limited to that of the prepolarizer itself, i.e., the spin temperatures obtained in the solid state. To put our results in context, we summarize data obtained in targets with different types of radicals, and provide a short review of the DNP mechanisms needed in their discussion. (C) 2007 Wiley Periodicals, Inc.

DOI : 10.1002/cmr.b.20099

Short-echo-time 1H MRS of the mouse lacking brain-specific glutamate-dehydrogenase

V. Mlynarik; I. Kohler; F. Frigerio; P. Maechler; R. Gruetter

Mathematical modeling of (13)C label incorporation of the TCA cycle: the concept of composite precursor function

K. Uffmann; R. Gruetter

A novel approach for the mathematical modeling of (13)C label incorporation into amino acids via the TCA cycle that eliminates the explicit calculation of the labeling of the TCA cycle intermediates is described, resulting in one differential equation per measurable time course of labeled amino acid. The equations demonstrate that both glutamate C4 and C3 labeling depend in a predictable manner on both transmitochondrial exchange rate, V(X), and TCA cycle rate, V(TCA). For example, glutamate C4 labeling alone does not provide any information on either V(X) or V(TCA) but rather a composite "flux". Interestingly, glutamate C3 simultaneously receives label not only from pyruvate C3 but also from glutamate C4, described by composite precursor functions that depend in a probabilistic way on the ratio of V(X) to V(TCA): An initial rate of labeling of glutamate C3 (or C2) being close to zero is indicative of a high V(X)/V(TCA). The derived analytical solution of these equations shows that, when the labeling of the precursor pool pyruvate reaches steady state quickly compared with the turnover rate of the measured amino acids, instantaneous labeling can be assumed for pyruvate. The derived analytical solution has acceptable errors compared with experimental uncertainty, thus obviating precise knowledge on the labeling kinetics of the precursor. In conclusion, a substantial reformulation of the modeling of label flow via the TCA cycle turnover into the amino acids is presented in the current study. This approach allows one to determine metabolic rates by fitting explicit mathematical functions to measured time courses.

DOI : 10.1002/jnr.21392

Neurochemical changes in Huntington R6/2 mouse striatum detected by in vivo 1H NMR spectroscopy

I. Tkac; J. M. Dubinsky; C. D. Keene; R. Gruetter; W. C. Low

The neurochemical profile of the striatum of R6/2 Huntington's disease mice was examined at different stages of pathogenesis using in vivo1H NMR spectroscopy at 9.4 T. Between 8 and 12 weeks, R6/2 mice exhibited distinct changes in a set of 17 quantifiable metabolites compared with littermate controls. Concentrations of creatine, glycerophosphorylcholine, glutamine and glutathione increased and N-acetylaspartate decreased at 8 weeks. By 12 weeks, concentrations of phosphocreatine, taurine, ascorbate, glutamate, and myo-inositol increased and phophorylethanolamine decreased. These metabolic changes probably reflected multiple processes, including compensatory processes to maintain homeostasis, active at different stages in the development of HD. The observed changes in concentrations suggested impairment of neurotransmission, neuronal integrity and energy demand, and increased membrane breakdown, gliosis, and osmotic and oxidative stress. Comparisons between metabolite concentrations from individual animals clearly distinguished HD transgenics from non-diseased littermates and identified possible markers of disease progression. Metabolic changes in R6/2 striata were distinctly different from those observed previously in the quinolinic acid and 3NP models of HD. Longitudinal monitoring of changes in these metabolites may provide quantifiable measures of disease progression and treatment effects in both mouse models of HD and patients. © 2007 The Authors.

DOI : 10.1111/j.1471-4159.2006.04323.x

Relaxivity of Gd-based contrast agents on X nuclei with long intrinsic relaxation times in aqueous solutions

R. van Heeswijk; S. Laus; F. D. Morgenthaler; R. Gruetter

The relaxivity of commercially available gadolinium (Gd)-based contrast agents was studied for X-nuclei resonances with long intrinsic relaxation times ranging from 6 s to several hundred seconds. Omniscan in pure 13C formic acid had a relaxivity of 2.9 mM(-1) s(-1), whereas its relaxivity on glutamate C1 and C5 in aqueous solution was approximately 0.5 mM(-1) s(-1). Both relaxivities allow the preparation of solutions with a predetermined short T1 and suggest that in vitro substantial sensitivity gains in their measurement can be achieved. 6Li has a long intrinsic relaxation time, on the order of several minutes, which was strongly affected by the contrast agents. Relaxivity ranged from approximately 0.1 mM(-1) s(-1) for Omniscan to 0.3 for Magnevist, whereas the relaxivity of Gd-DOTP was at 11 mM(-1) s(-1), which is two orders of magnitude higher. Overall, these experiments suggest that the presence of 0.1- to 10-microM contrast agents should be detectable, provided sufficient sensitivity is available, such as that afforded by hyperpolarization, recently introduced to in vivo imaging.

DOI : 10.1016/j.mri.2007.02.015

Sensitivity of single-voxel 1H-MRS in investigating the metabolism of the activated human visual cortex at 7 T

S. Mangia; I. Tkáč; R. Gruetter; P.-F. Van De Moortele; F. Giove  et al.

Proton magnetic resonance spectroscopy (1H-MRS) has been used in a number of studies to noninvasively assess the temporal changes of lactate in the activated human brain. However, the results have not been consistent. The aim of the present study was to test the sensitivity of 1H-MRS during functional experiments at the highest magnetic field currently available for human studies (7 T). Stability and reproducibility of the measurements were evaluated from LCModel analysis of time series of spectra measured during a visual stimulation paradigm and by examination of the difference between spectra obtained at rest and during activation. The sensitivity threshold to detect concentration changes was 0.2 μmol/g for most of the quantified metabolites. The possible variations of metabolite concentrations during visual stimulation were within the same range (±0.2 μmol/g). In addition, the influence of a small line-narrowing effect due to the blood oxygenation level-dependent (BOLD) T2* changes on the estimated concentrations was simulated. Quantification of metabolites was, in general, not affected beyond 1% by line-width changes within 0.5 Hz. © 2006 Elsevier Inc. All rights reserved.

DOI : 10.1016/j.mri.2005.12.023

Localized short-echo-time proton MR spectroscopy with full signal-intensity acquisition

V. Mlynárik; G. Gambarota; H. Frenkel; R. Gruetter

We developed a short-echo-time (TE) sequence for proton localized spectroscopy by combining a 1D add-subtract scheme with a doubly slice-selective spin-echo (SE) sequence. The sequence preserves the full magnetization available from the selected volume of interest (VOI). By reducing the number of radiofrequency (RF) pulses acting on transverse magnetization, we were able to minimize the TE to the level that is achievable with the stimulated echo acquisition mode (STEAM) technique, and also gained a twofold increase in sensitivity. The use of an adiabatic pulse in the add-subtract localization improved the efficiency of excitation in spatially inhomogeneous RF fields, which are frequently encountered at high magnetic fields. The localization performance and sensitivity gains of this method, which is termed SPin ECho, full Intensity Acquired Localized (SPECIAL) spectroscopy, were demonstrated in vivo in rat brains. In conjunction with spectroscopic imaging, a 2-microl spatial resolution was accomplished with a signal-to-noise ratio (SNR) above 30, which is usually sufficient for reliable quantification of a large number of metabolites (neurochemical profile).

DOI : 10.1002/mrm.21043

Detection of an antioxidant profile in the human brain in vivo via double editing with MEGA-PRESS

M. Terpstra; M. Marjanska; P.-G. Henry; I. Tkáč; R. Gruetter

Vitamin C (ascorbate) and glutathione (GSH) are the two most concentrated non-enzymatic antioxidants in the human brain. Double editing with (DEW) MEGA-PRESS at 4T was designed in this study to measure both antioxidants in the same amount of time previously required to measure one. In the occipital lobe of four human subjects, resolved ascorbate (Asc) and GSH resonances were detected repeatedly and simultaneously using DEW MEGA-PRESS. The Asc and GSH concentrations measured using LCModel analysis of DEW MEGA-PRESS spectra were 0.8 ± 0.1 and 1.0 ± 0.1 μmol/g (mean ± SD), with average Cramer-Rao lower bounds (CRLB) of 10% and 7%, respectively. Aside from the effects of J-modulation at a common echo time (TE), double editing did not compromise sensitivity. To determine the extent to which the oxidized forms of Asc and GSH contribute to DEW MEGA-PRESS spectra in vivo, chemical shifts and coupling constants for dehydroascorbate (DHA) and oxidized glutathione (GSSG) were measured at physiologic pH and temperature. DHA does not contribute to the 3.73 ppm DEW MEGA-PRESS Asc resonance. GSSG contributions to the DEW MEGA-PRESS GSH resonance (3.0 ppm) are negligible under physiologic conditions, and would be evidenced by a distinct GSSG resonance (3.3 ppm) at exceptionally high concentrations. © 2006 Wiley-Liss, Inc.

DOI : 10.1002/mrm.21086

Pre-labeling: a novel approach towards measuring absolute glycogen concentration changes with NMR

F. D. Morgenthaler; S. Laus; H. Lei; R. Gruetter

In-vivo NMR spectroscopy of the Brain at High Fields

R. Gruetter; P.-G. Henry; H. Lei; S. Mangia; G. Oz  et al.

DOI : 10.1007/978-0-387-49648-1_12.

Brain glucose concentrations in healthy humans subjected to recurrent hypoglycemia

A. B. Criego; I. Tkac; A. Kumar; W. Thomas; R. Gruetter  et al.

Mechanisms responsible for hypoglycemia unawareness remain unknown. Previously, we found that patients with type 1 diabetes and hypoglycemia unawareness had increased brain glucose concentrations as measured by (1)H-magnetic resonance spectroscopy (MRS) compared with controls measured under the same metabolic condition, suggesting that an alteration in brain glucose transport and/or metabolism may play a role in the pathogenesis of hypoglycemia unawareness. To determine whether the brain glucose concentration is altered in normal subjects subjected to recurrent hypoglycemia, we compared the brain glucose concentrations measured in healthy subjects after three episodes of hypoglycemia to blunt the counterregulatory response over 24 hr and compared this value with that measured at a time remote from the antecedent hypoglycemia protocol. Sixteen subjects (9 M/7 F, age 36 +/- 10 years, mean +/- SD) underwent three hypoglycemic clamps for 30 min at 8 AM (0 hr), 5 PM (9 hr), and 7 AM (24 hr). After the third hypoglycemic clamp, subjects underwent a hyperglycemic clamp during which brain glucose concentration was measured by MRS at 4 T. Brain glucose concentration after repeated hypoglycemia was not different from the brain glucose concentration measured in the same subjects during a control study (5.1 +/- 0.8 vs. 4.5 +/- 0.5 mumol/g wet weight, respectively, P = 0.05). These observations suggest that brain glucose transport or metabolism is not altered following short episodes of recurrent hypoglycemia in healthy human volunteers.

DOI : 10.1002/jnr.20654

Uncovering hidden in vivo resonances using editing based on localized TOCSY

M. Marjanska; P.-G. Henry; P. J. Bolan; B. Vaughan; E. R. Seaquist  et al.

A novel single-shot spectral editing technique for in vivo proton NMR is proposed to recover resonances of low-concentration metabolites obscured by very strong resonances. With this new method, editing is performed by transferring transverse magnetization to J-coupled spins from selected coupling partners using a homonuclear Hartmann-Hahn polarization transfer with adiabatic pulses. The current implementation uses 1D-TOCSY with single-voxel localization based on LASER to recover the H1 proton of beta-glucose at 4.63 ppm from under water and the lactate methyl resonances from beneath a strong lipid signal. The method can be extended to further spin systems where conventional editing methods are difficult to perform.

DOI : 10.1002/mrm.20425

Evaluation of brain mitochondrial glutamate and alpha-ketoglutarate transport under physiologic conditions

D. A. Berkich; Y. Xu; K. F. LaNoue; R. Gruetter; S. M. Hutson

Some models of brain energy metabolism used to interpret in vivo (13)C nuclear magnetic resonance spectroscopic data assume that intramitochondrial alpha-ketoglutarate is in rapid isotopic equilibrium with total brain glutamate, most of which is cytosolic. If so, the kinetics of changes in (13)C-glutamate can be used to predict citric acid cycle flux. For this to be a valid assumption, the brain mitochondrial transporters of glutamate and alpha-ketoglutarate must operate under physiologic conditions at rates much faster than that of the citric acid cycle. To test the assumption, we incubated brain mitochondria under physiologic conditions, metabolizing both pyruvate and glutamate and measured rates of glutamate, aspartate, and alpha-ketoglutarate transport. Under the conditions employed (66% of maximal O(2) consumption), the rate of synthesis of intramitochondrial alpha-ketoglutarate was 142 nmol/min.mg and the combined initial rate of alpha-ketoglutarate plus glutamate efflux from the mitochondria was 95 nmol/min.mg. It thus seems that much of the alpha-ketoglutarate synthesized within the mitochondria proceeds around the citric acid cycle without equilibrating with cytosolic glutamate. Unless the two pools are in such rapid exchange that they maintain the same percent (13)C enrichment at all points, (13)C enrichment of glutamate alone cannot be used to determine tricarboxylic acid cycle flux. The alpha-ketoglutarate pool is far smaller than the glutamate pool and will therefore approach steady state faster than will glutamate at the metabolite transport rates measured.

DOI : 10.1002/jnr.20325

Highly resolved in vivo 1H NMR spectroscopy of the mouse brain at 9.4 T

I. Tkác; P.-G. Henry; P. Andersen; C. D. Keene; W. C. Low  et al.

An efficient shim system and an optimized localization sequence were used to measure in vivo 1H NMR spectra from cerebral cortex, hippocampus, striatum, and cerebellum of C57BL/6 mice at 9.4 T. The combination of automatic first- and second-order shimming (FASTMAP) with strong custom-designed second-order shim coils (shim strength up to 0.04 mT/cm2) was crucial to achieve high spectral resolution (water line width of 11-14 Hz). Requirements for second-order shim strengths to compensate field inhomogeneities in the mouse brain at 9.4 T were assessed. The achieved spectral quality (resolution, S/N, water suppression, localization performance) allowed reliable quantification of 16 brain metabolites (LCModel analysis) from 5-10-microL brain volumes. Significant regional differences (up to 2-fold, P < 0.05) were found for all quantified metabolites but Asp, Glc, and Gln. In contrast, 1H NMR spectra measured from the striatum of C57BL/6, CBA, and CBA/BL6 mice revealed only small (<13%, P < 0.05) interstrain differences in Gln, Glu, Ins, Lac, NAAG, and PE. It is concluded that 1H NMR spectroscopy at 9.4 T can provide precise biochemical information from distinct regions of the mouse brain noninvasively that can be used for monitoring of disease progression and treatment as well as phenotyping in transgenic mice models.

DOI : 10.1002/mrm.20184

Neuroglial metabolism in the awake rat brain: CO2 fixation increases with brain activity

G. Oz; D. A. Berkich; P.-G. Henry; Y. Xu; K. LaNoue  et al.

Glial cells are thought to supply energy for neurotransmission by increasing nonoxidative glycolysis; however, oxidative metabolism in glia may also contribute to increased brain activity. To study glial contribution to cerebral energy metabolism in the unanesthetized state, we measured neuronal and glial metabolic fluxes in the awake rat brain by using a double isotopic-labeling technique and a two-compartment mathematical model of neurotransmitter metabolism. Rats (n = 23) were infused simultaneously with 14C-bicarbonate and [1-13C]glucose for up to 1 hr. The 14C and 13C labeling of glutamate, glutamine, and aspartate was measured at five time points in tissue extracts using scintillation counting and 13C nuclear magnetic resonance of the chromatographically separated amino acids. The isotopic 13C enrichment of glutamate and glutamine was different, suggesting significant rates of glial metabolism compared with the glutamate-glutamine cycle. Modeling the 13C-labeling time courses alone and with 14C confirmed significant glial TCA cycle activity (V(PDH)((g)), approximately 0.5 micromol x gm(-1) x min(-1)) relative to the glutamate-glutamine cycle (V(NT)) (approximately 0.5-0.6 micromol x gm(-1) x min(-1)). The glial TCA cycle rate was approximately 30% of total TCA cycle activity. A high pyruvate carboxylase rate (V(PC), approximately 0.14-0.18 micromol x gm(-1) x min(-1)) contributed to the glial TCA cycle flux. This anaplerotic rate in the awake rat brain was severalfold higher than under deep pentobarbital anesthesia, measured previously in our laboratory using the same 13C-labeling technique. We postulate that the high rate of anaplerosis in awake brain is linked to brain activity by maintaining glial glutamine concentrations during increased neurotransmission.

DOI : 10.1523/JNEUROSCI.3564-04.2004

Whole-brain glutamate metabolism evaluated by steady-state kinetics using a double-isotope procedure: effects of gabapentin

Y. Xu; G. Oz; K. F. LaNoue; C. J. Keiger; D. A. Berkich  et al.

Cerebral rates of anaplerosis are known to be significant, yet the rates measured in vivo have been debated. In order to track glutamate metabolism in brain glutamatergic neurons and brain glia, for the first time unrestrained awake rats were continuously infused with a combination of H14CO3- and [1 - 13C]glucose in over 50 infusions ranging from 5 to 60 min. In whole-brain extracts from these animals, the appearance of 14C in brain glutamate and glutamine and appearance of 13C in the C-4 position of glutamate and glutamine were measured as a function of time. The rate of total neuronal glutamate turnover, the anaplerotic rate of synthesis of glutamine and glutamate from H14CO3-, flux through the glutamate/glutamine cycle, and a minimum estimate of whole-brain anaplerosis was obtained. The rate of synthesis of 14C-glutamate from H14CO3- was 1.29 +/- 0.11 nmoles/min/mg protein, whereas the rate of synthesis of 14C-glutamine was 1.48 +/- 0.10 nmoles/min/mg protein compared to total glutamate turnover of 9.39 +/- 0.73 nmoles/min/mg protein. From the turnover rate of glutamine, an upper limit for flux through the glutamate/glutamine cycle was estimated at 4.6 nmoles/min/mg protein. Synthesis of glutamine from H14CO3- was substantial, amounting to 32% of the glutamate/glutamine cycle. These rates were not significantly affected by a single injection of 100 mg/kg of the antiepileptic drug gabapentin. In contrast, acute administration of gabapentin significantly lowered incorporation of H14CO3- into glutamate and glutamine in excised rat retinas, suggesting metabolic effects of gabapentin may require chronic treatment and/or are restricted to brain areas enriched in target enzymes such as the cytosolic branched chain aminotransferase. We conclude that the brain has a high anaplerotic activity and that the combination of two tracers with different precursors affords unique insights into the compartmentation of cerebral metabolism.

DOI : 10.1111/j.1471-4159.2004.02576.x

1H NMR detection of vitamin C in human brain in vivo

M. Terpstra; R. Gruetter

Vitamin C (ascorbate) is well established as an essential nutrient that functions as an antioxidant. Since it is present in the human brain at detectable concentrations, this study was designed to detect and quantify ascorbate in the human brain in vivo using 1H NMR spectroscopy (MRS). Ascorbate was consistently detected in all five study subjects, and was measured using MEGA-PRESS difference editing. The in vivo resonance pattern was consistent with that of ascorbate based on position, line width, peak pattern, and relative intensity. Metabolites with a potential for coediting were assessed using phantom solutions. The putative resonances of myo-inositol, lactate, glycerophosphocholine, phosphocholine, and phosphoethanolamine were detected at positions distinct from those of ascorbate. This study represents the first in vivo detection of vitamin C in the human brain using 1H MRS. A concentration of 1.3 +/- 0.3 micromol/g (mean +/- SD, N = 4) was estimated.

DOI : 10.1002/mrm.10715

Dynamic or inert metabolism? Turnover of N-acetyl aspartate and glutathione from D-[1-13C]glucose in the rat brain in vivo

I.-Y. Choi; R. Gruetter

The rate of (13)C-label incorporation into both aspartyl (NAA C3) and acetyl (NAA C6) groups of N-acetyl aspartate (NAA) was simultaneously measured in the rat brain in vivo for up to 19 h of [1-(13)C]glucose infusion (n = 8). Label incorporation was detected in NAA C6 approximately 1.5 h earlier than in NAA C3 because of the delayed labeling of the precursor of NAA C3, aspartate, compared to that of NAA C6, glucose. The time courses of NAA were fitted using a mathematical model assuming synthesis of NAA in one kinetic compartment with the respective precursor pools of aspartate and acetyl coenzyme A (acetyl-CoA). The turnover rates of NAA C6 and C3 were 0.7 +/- 0.1 and 0.6 +/- 0.1 micromol/(g h) with the time constants 14 +/- 2 and 13 +/- 2 h, respectively, with an estimated pool size of 8 micromol/g. The results suggest that complete label turnover of NAA from glucose occurs in approximately 70 h. Several hours after starting the glucose infusion, label incorporation into glutathione (GSH) was also detected. The turnover rate of GSH was 0.06 +/- 0.02 micromol/(g h) with a time constant of 13 +/- 2 h. The estimated pool size of GSH was 0.8 micromol/g, comparable to the cortical glutathione concentration. We conclude that NAA and GSH are completely turned over and that the metabolism is extremely slow (< 0.05% of the glucose metabolic rate).

DOI : 10.1111/j.1471-4159.2004.02716.x

Developmental and regional changes in the neurochemical profile of the rat brain determined by in vivo 1H NMR spectroscopy

I. Tkác; R. Rao; M. K. Georgieff; R. Gruetter

Sixteen metabolites were quantified from 11-24 micro l volumes in three different brain regions (hippocampus, striatum, and cerebral cortex) during postnatal development. Rat pups from the same litter were repeatedly measured on postnatal days 7, 10, 14, 21, and 28 using a completely noninvasive and longitudinal study design. Metabolite quantification was based on ultra-short echo-time (1)H NMR spectroscopy at 9.4 T and LCModel processing. Most of the brain metabolites were quantified with Cramer-Rao lower bounds (CRLB) less than 20%, which corresponded to an estimated concentration error <0.2 micro mol/g. Taurine and total creatine were quantified with CRLB < or = 5% from all 114 processed spectra. The resulting high reliability and reproducibility revealed significant regional and age-related changes in metabolite concentrations. The most sensitive markers for developmental and regional variations between hippocampus, striatum, and cerebral cortex were N-acetylaspartate, myo-inositol, taurine, glutamate, and choline compounds. Absolute values of metabolite concentrations were in very good agreement with previously published in vitro results based on chromatographic measurements of brain extracts. The current data may serve as a reference for studies focused on developmental defects and pathologies using neonatal rat models.

DOI : 10.1002/mrm.10497

Toward dynamic isotopomer analysis in the rat brain in vivo: automatic quantitation of 13C NMR spectra using LCModel

P.-G. Henry; G. Oz; S. Provencher; R. Gruetter

The LCModel method was adapted to analyze localized in vivo (13)C NMR spectra obtained from the rat brain in vivo at 9.4 T. Prior knowledge of chemical-shifts, J-coupling constants and J-evolution was included in the analysis. Up to 50 different isotopomer signals corresponding to 10 metabolites were quantified simultaneously in 400 microl volumes in the rat brain in vivo during infusion of [1,6-(13)C(2)]glucose. The analysis remained accurate even at low signal-to-noise ratio of the order of 3:1. The relative distribution of isotopomers in glutamate, glutamine and aspartate determined in vivo in 22 min was in excellent agreement with that measured in brain extracts. Quantitation of time series of (13)C spectra yielded time courses of total (13)C label incorporation into up to 16 carbon positions, as well as time courses of individual isotopomer signals, with a temporal resolution as low as 5 min (dynamic isotopomer analysis). The possibility of measuring in vivo a wealth of information that was hitherto accessible only in extracts is likely to expand the scope of metabolic studies in the intact brain.

DOI : 10.1002/nbm.840

Measurement of reduced glutathione (GSH) in human brain using LCModel analysis of difference-edited spectra

M. Terpstra; P.-G. Henry; R. Gruetter

The concentration of reduced glutathione (GSH), an antioxidant, may be altered in various brain diseases. MEGA-PRESS was used to edit for the (1)H NMR signal from GSH in the occipital lobe of 12 normal humans. In all studies, GSH was clearly detected with a spectral pattern consistent with spectra acquired from a phantom containing GSH. Retention of singlet resonances in the subspectra, a key advantage of this difference-editing technique, provided an unambiguous reference for the offset and phase of the edited signal. Linear combination model (LCModel) analysis provided an unbiased means for quantifying signal contribution from edited metabolites. GSH concentration was estimated from the in vivo spectra as 1.3 +/- 0.2 micro mol/g (mean +/- SD, n = 12).

DOI : 10.1002/mrm.10499

Direct in vivo measurement of human cerebral GABA concentration using MEGA-editing at 7 Tesla

M. Terpstra; K. Ugurbil; R. Gruetter

Spectral editing of the GABA spin system is hampered by coediting of macromolecule (MM) coherences. To reduce contamination arising from MMs in spectra edited for GABA, the highest field strength currently available for human experimentation (7 Tesla) and MEGA-based editing were used. Despite judicious choice of experimental parameters, MM contamination was found to arise from field drifts. When the MM contribution was accounted for, [GABA] = 0.75 +/- 0.14 micromol/g (mean +/- SD, N = 16) relative to 8 micromol/g creatine (Cr), whereas without accounting for the MM signal [GABA*] = 0.88 +/- 0.23 micromol/g (mean +/- SD, N = 16). Incorporating the direct experimental assessment of MM contamination to the edited GABA signal substantially reduced the variance of the measurement, resulting in concentrations that were in excellent agreement with previous (13)C labeling experiments.

DOI : 10.1002/mrm.10146

Effect of deep pentobarbital anesthesia on neurotransmitter metabolism in vivo: on the correlation of total glucose consumption with glutamatergic action

I.-Y. Choi; H. Lei; R. Gruetter

The effect of deep barbiturate anesthesia on brain glucose transport, TCA cycle flux, and aspartate, glutamate, and glutamine metabolism was assessed in the rat brain using 13C nuclear magnetic resonance spectroscopy at 9.4 T in conjunction with [1-13C] glucose infusions. Brain glucose concentrations were elevated, consistent with a twofold reduced cerebral metabolic rate for glucose (CMRglc) compared with light alpha-chloralose anesthesia. Using a mathematical model of neurotransmitter metabolism, several metabolic reaction rates were extracted from the rate of label incorporation. Total oxidative glucose metabolism, CMRglc(ox), was 0.33 +/- 0.03 micromol x g(-1) x min(-1). The neuronal TCA cycle rate was similar to that in the glia, 0.35 +/- 0.03 micromol x g(-1) x min(-1) and 0.26 +/- 0.06 micromol x g(-1) x min(-1), respectively, suggesting that neuronal energy metabolism was mainly affected. The rate of pyruvate carboxylation was 0.03 +/- 0.01 micromol x g(-1) x min(-1). The exchange rate between cytosolic glutamate and mitochondrial 2-oxoglutarate, Vx, was equal to the rate of neuronal pyruvate dehydrogenase flux. This indicates that Vx is coupled to CMRglc(ox), implying that the malate-aspartate shuttle is the major mechanism that facilitates label exchange across the inner mitochondrial membrane. The apparent rate of glutamatergic neurotransmission, V(NT), was 0.04 +/- 0.01 micromol x g x min, consistent with strong reductions in electrical activity. However, the rates of cerebral oxidative glucose metabolism and glutamatergic neurotransmission, CMRglc(ox)/V(NT), did not correlate with a 1:1 stoichiometry.

DOI : 10.1097/00004647-200211000-00008

Brain glycogen: an insulin-sensitive carbohydrate store

E. R. Seaquist; R. Gruetter

In vivo 13C NMR studies of compartmentalized cerebral carbohydrate metabolism

R. Gruetter

Localized 13C nuclear magnetic resonance (NMR) spectroscopy provides a unique window for studying cerebral carbohydrate metabolism through, e.g. the completely non-invasive measurement of cerebral glucose and glycogen metabolism. In addition, label incorporation into amino acid neurotransmitters such as glutamate (Glu), GABA and aspartate can be measured providing information on Krebs cycle flux and oxidative metabolism. Given the compartmentation of key enzymes such as pyruvate carboxylase and glutamine synthetase, the detection of label incorporation into glutamine indicated that neuronal and glial metabolism can be measured in vivo. The purpose of this paper is to provide a critical overview of these recent advances into measuring compartmentation of brain energy metabolism using localized in vivo 13C NMR spectroscopy. The studies reviewed herein showed that anaplerosis is significant in brain, as is oxidative ATP generation in glia and the rate of glial glutamine synthesis attributed to the replenishment of the neuronal Glu pool and that brain glycogen metabolism is slow under resting conditions. This new modality promises to provide a new investigative tool to study aspects of normal and diseased brain hitherto unaccessible, such as the interplay between glutamatergic action, glucose and glycogen metabolism during brain activation, and the derangements thereof in patients with hepatic encephalopathy, neurodegenerative diseases and diabetes.

DOI : 10.1016/S0197-0186(02)00034-7

In vivo 1H NMR spectroscopy of the human brain at 7 T

I. Tkac; P. Andersen; G. Adriany; H. Merkle; K. Uurbil  et al.

In vivo 1H NMR spectra from the human brain were measured at 7 T. Ultrashort echo-time STEAM was used to minimize J-modulation and signal attenuation caused by the shorter T2 of metabolites. Precise adjustment of higher-order shims, which was achieved with FASTMAP, was crucial to benefit from this high magnetic field. Sensitivity improvements were evident from single-shot spectra and from the direct detection of glucose at 5.23 ppm in 8-ml volumes. The linewidth of the creatine methyl resonance was at best 9 Hz. In spite of the increased linewidth of singlet resonances at 7 T, the ability to resolve overlapping multiplets of J-coupled spin systems, such as glutamine and glutamate, was substantially increased. Characteristic spectral patterns of metabolites, e.g., myo-inositol and taurine, were discernible in the in vivo spectra, which facilitated an unambiguous signal assignment. © 2001 Wiley-Liss, Inc.

DOI : 10.1002/mrm.1213

The Effect of Insulin on in Vivo Cerebral Glucose Concentrations and Rates of Glucose Transport/Metabolism in Humans

E. R. Seaquist; G. S. Damberg; I. Tkac; R. Gruetter

The continuous delivery of glucose to the brain is critically important to the maintenance of normal metabolic function. However, elucidation of the hormonal regulation of in vivo cerebral glucose metabolism in humans has been limited by the lack of direct, noninvasive methods with which to measure brain glucose. In this study, we sought to directly examine the effect of insulin on glucose concentrations and rates of glucose transport/metabolism in human brain using 1H-magnetic resonance spectroscopy at 4 Tesla. Seven subjects participated in paired hyperglycemic (16.3 ± 0.3 mmol/1) clamp studies performed with and without insulin. Brain glucose remained constant throughout (5.3 ± 0.3 μmol/g wet wt when serum insulin = 16 ± 7 pmol/1 vs. 5.5 ± 0.3 μmol/g wet wt when serum insulin = 668 ± 81 pmol/1, P = NS). Glucose concentrations in gray matter-rich occipital cortex and white matter-rich periventricular tissue were then simultaneously measured in clamps, where plasma glucose ranged from 4.4 to 24.5 mmol/1 and insulin was infused at 0.5 mU · kg-1 · min-1. The relationship between plasma and brain glucose was linear in both regions. Reversible Michaelis-Menten kinetics fit these data best, and no differences were found in the kinetic constants calculated for each region. These data support the hypothesis that the majority of cerebral glucose uptake/metabolism is an insulin-independent process in humans.

Single-shot, three-dimensional 'non-echo' localization method for in vivo NMR spectroscopy

I.-Y. Choi; I. Tkac; R. Gruetter

Metabolite signals with short T1 or T2 are difficult to localize with full sensitivity. This limitation was overcome with the development and implementation of a single-shot, complete three-dimensional 'non-echo' localization method with reduced sensitivity to spatial B1 variation, which is suitable for measuring signals with very short T1 or T2, e.g., the 13C NMR signals of glycogen. The proposed method is based on a T1-optimized outer volume suppression scheme using pulses of the hyperbolic secant type applied at different power levels, which is robust over a fivefold range of T1. Strong lipid, muscle glycogen, and glucose signals originating outside the rat brain were suppressed. Signals of glycogen, aspartate, glutathione, GABA C4, N-acetyl aspartate as well as the C3 and C4 signals of glutamate and glutamine with resolved homonuclear 13C-13C coupling were fully resolved in vivo at 9.4 Tesla using higher-order shimming. The method can be extended to other nuclei and to localized MRS of humans. (C) 2000 Wiley-Liss, Inc.

DOI : 10.1002/1522-2594(200009)44:3<387::AID-MRM8>3.0.CO;2-3

Noninvasive measurements of [1-13C]glycogen concentrations and metabolism in rat brain in vivo

I.-Y. Choi; I. Tkáč; K. Uǧurbil; R. Gruetter

Using a specific 13C NMR localization method, 13C label incorporation into the glycogen C1 resonance was measured while infusing [1- 13C]glucose in intact rats. The maximal concentration of [1-13C]glycogen was 5.1 ± 0.6 μmol g-1 (mean ± SE, n = 8). During the first 60 rain of acute hyperglycemia, the rate of 13C label incorporation (synthase flux) was 2.3 ± 0.7 μmol g-1 h-1 (mean ± SE, n = 9 rats), which was higher (p < 0.01) than the rate of 0.49 ± 0.14 μmol g-1 h-1 measured ≥2 h later. To assess whether the incorporation of 13C label was due to turnover or net synthesis, the infusion was continued in seven rats with unlabeled glucose. The rate of 13C label decline (phosphorylase flux) was lower (0.33 = 0.10 μmol g-1 h-1) than the initial rate of label incorporation (p < 0.01) and appeared to be independent of the duration of the preceding infusion of [1-13C]glucose (p > 0.05 for correlation). The results implied that net glycogen synthesis of ~3 μmol g-1 had occurred, similar to previous reports. When infusing unlabeled glucose before [1-13C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 ± 0.08 μmol g-1 h-1. The results suggest that steady-state glycogen turnover rates during hyperglycemia are ~1% of glucose consumption.

DOI : 10.1046/j.1471-4159.1999.0731300.x

Localized Eddy Current Compensation Using Quantitative Field Mapping

M. Terpstra; P. M. Andersen; R. Gruetter

Eddy current effects induced by switched gradients in proximal conducting structures are traditionally reduced by applying preemphasis currents whose amplitudes and decay characteristics must be set to offset the eddy current fields. We present an expeditious, localized, and quantitative method for mapping and adjusting the parameters for eddy current compensation. Mapping is based on analysis of projections as used in the fast automatic shimming technique by mapping along projections (FASTMAP). Adjustment methods are demonstrated in high-field horizontal bore systems. The proposed localized eddy current mapping technique may also be used for localized measurements in situations where asymmetric conducting structures may cause nonlinear eddy current fields, such as in interventional MRI and open magnet designs. © 1998 Academic Press.

DOI : 10.1006/jmre.1997.1353

Resolution Improvements in in Vivo 1H NMR Spectra with Increased Magnetic Field Strength

R. Gruetter; S. A. Weisdorf; V. Rajanayagan; M. Terpstra; H. Merkle  et al.

The measurement of cerebral metabolites using highly homologous localization techniques and similar shimming methods was performed in the human brain at 1.5 and 4 T as well as in the dog and rat brain at 9.4 T. In rat brain, improved resolution was achieved by shimming all first- and second-order shim coils using a fully adiabatic FASTMAP sequence. The spectra showed a clear improvement in spectral resolution for all metabolite resonances with increased field strength. Changes in cerebral glutamine content were clearly observed at 4 T compared to 1.5 T in patients with hepatic encephalopathy. At 9.4 T, glutamine H4 at 2.46 ppm was fully resolved from glutamate H4 at 2.37 ppm, as was the potential resonance from γ-amino-butyric acid at 2.30 ppm and N-acetyl-aspartyl-glutamate at 2.05 ppm. Singlet linewidths were found to be as low as 6 Hz (0.015 ppm) at 9.4 T, indicating a substantial decrease in ppm linewidth with field strength. Furthermore, the methylene peak of creatine was partially resolved from phosphocreatine, indicating a close to 1:1 relationship in gray matter. We conclude that increasing the magnetic field strength increases spectral resolution also for 1H NMR, which can lead to more than linear sensitivity gains. © 1998 Academic Press.

DOI : 10.1006/jmre.1998.1542

Simultaneous in vivo spectral editing and water suppression

M. Mescher; H. Merkle; J. Kirsch; M. Garwood; R. Gruetter

Water suppression is typically performed in vivo by exciting the longitudinal magnetization in combination with dephasing, or by using frequency-selective coherence generation. MEGA, a frequency-selective refocusing technique, can be placed into any pulse sequence element designed to generate a Hahn spin-echo or stimulated echo, to dephase transverse water coherences with minimal spectral distortions. Water suppression performance was verified in vivo using stimulated echo acquisition mode (STEAM) localization, which provided water suppression comparable with that achieved with four selective pulses in 3,1-DRYSTEAM. The advantage of the proposed method was exploited for editing J-coupled resonances. Using a double-banded pulse that selectively inverts a J-coupling partner and simultaneously suppresses water, efficient metabolite editing was achieved in the point resolved spectroscopy (PRESS) and STEAM sequences in which MEGA was incorporated. To illustrate the efficiency of the method, the detection of γ-aminobutyric acid (GABA) was demonstrated, with minimal contributions from macromolecules and overlying singlet peaks at 4 T. The estimated occipital GABA concentration was consistent with previous reports, suggesting that editing for GABA is efficient when based on MEGA at high field strengths.

DOI : 10.1002/(SICI)1099-1492(199810)11:6<266::AID-NBM530>3.0.CO;2-J

Steady-state cerebral glucose concentrations and transport in the human brain

R. Gruetter; K. Ugurbil; E. R. Seaquist

Understanding the mechanism of brain glucose transport across the blood- brain barrier is of importance to understanding brain energy metabolism. The specific kinetics of glucose transport nave been generally described using standard Michaelis-Menten kinetics. These models predict that the steady- state glucose concentration approaches an upper limit in the human brain when the plasma glucose level is well above the Michaelis-Menten constant for half-maximal transport, K(t). In experiments where steady-state plasma glucose content was varied from 4 to 30 mM, the brain glucose level was a linear function of plasma glucose concentration. At plasma concentrations nearing 30 mM, the brain glucose level approached 9 mM, which was significantly higher than predicted from the previously reported K(t) of ~4 mM (p < 0.05). The high brain glucose concentration measured in the human brain suggests that ablumenal brain glucose may compete with lumenal glucose for transport. We developed a model based on a reversible Michaelis-Menten kinetic formulation of unidirectional transport rates. Fitting this model to brain glucose level as a function of plasma glucose level gave a substantially lower K(t) of 0.6 ± 2.0 mM, which was consistent with the previously reported millimolar K(m) of GLUT-1 in erythrocyte model systems. Previously reported and reanalyzed quantification provided consistent kinetic parameters. We conclude that cerebral glucose transport is most consistently described when using reversible Michaelis-Menten kinetics.

DOI : 10.1046/j.1471-4159.1998.70010397.x

In vivo magnetic resonance spectroscopy of human brain: the biophysical basis of dementia

B. D. Ross; S. Bluml; R. Cowan; E. Danielsen; N. Farrow  et al.

Nuclear magnetic resonance spectroscopy (MRS) in low and medium magnetic fields yields well-resolved natural abundance proton and decoupled phosphorus spectra from small (1-10 cc) volumes of brain in vivo in minutes. With this tool, neurochemical research has advanced through identification and non-invasive assay of specific neuronal--(N-acetylaspartate), glial (myo-inositol)--markers, energetics and osmolytes, and neurotransmitters (glutamate, GABA). From these simple measurements, several dozen disease states are recognized, including birth injury, and white matter and Alzheimer disease. Addition of stable isotopes of carbon (in man) or nitrogen (in experimental animals) has provided in vivo assays of enzyme flux through glucose transport, glycolysis, TCA-cycle, and the glutamine-glutamate-GABA system. Finally, a number of xenobiotics are recognized with heteronuclear NMR techniques. Together, these tools are having a major impact on neuroscience and clinical medicine. Through diagnosis and therapeutic monitoring, a new generation of in vivo metabolite imaging is expected with the advent of conforming RF coils and higher field NMR systems.

DOI : 10.1016/S0301-4622(97)00032-X

Identification of a high concentration of scyllo-inositol in the brain of a healthy human subject using 1H- and 13C-NMR

E. R. Seaquist; R. Gruetter

The peak at 3.35 ppm in the 1H-NMR spectrum characteristic for scyllo- inositol may be a marker for cerebral pathology, although it has a well- known constant concentration relative to myo-inositol. Such a peak was observed with an intensity at least 300% above normal in the brain of a healthy volunteer. The scyllo-inositol signal was assigned based on the detection of a corresponding peak at 74.5 ppm in the 13C-NMR spectrum and on the demonstration of singlet characteristics of the proton signal. The presence of substantial brain concentrations of scyllo-inositol suggests that scyllo-inositol metabolism may be regulated independently from myo-inositol and that such concentrations are compatible with normal health.

DOI : 10.1002/mrm.1910390220

Broadband decoupled, 1H-localized 13C MRS of the human brain at 4 Tesla

R. Gruetter; G. Adriany; H. Merkle; P. M. Andersen

Broadband proton decoupling of the entire 13C spectrum was possible within power absorption guidelines and resulted in the detection of narrow (as low as 2-3 Hz), natural abundance signals from metabolites such as myo- inositol, glutamate, N-acetyl-aspartate, and glutamine from 72 cm3 volumes in the human brain. To overcome the chemical shift displacement error, three- dimensional localization on the 1H z magnetization was combined with polarization transfer. Efficiency of the heteronuclear localization method was demonstrated by the elimination of all scalp lipid resonances. A signal- to-noise ratio of 5:1 for 0.07 mM [13C] was achieved in 12 min, which is approximately a fivefold improvement over the sensitivity reported at 2.1 Tesla.

DOI : 10.1002/mrm.1910360503

Observation of resolved glucose signals in 1H NMR spectra of the human brain at 4 tesla

R. Gruetter; M. Garwood; K. Ugurbil; E. R. Seaquist

Measurement of the resonances of glucose between 3.2 and 3.9 ppm in 1H NMR spectra from the human brain is difficult due to spectral overlap with peaks from more concentrated metabolites. The H1 resonance of α-D-glucose at 5.23 ppm is resolved from other metabolite peaks, but potentially overlaps with the intense water signal at 4.72 ppm. This paper demonstrates that the increased resolution at 4 Tesla permits to suppress the water signal sufficiently to reliably detect glucose directly at 5.23 ppm by 1H MRS and the estimated peak intensity is consistent with previous 13C NMR quantification.

DOI : 10.1002/mrm.1910360102

Localized 13C NMR spectroscopy in the human brain of amino acid labeling from D-[1-13C]glucose

R. Gruetter; E. J. Novotny; S. D. Boulware; G. F. Mason; D. L. Rothman  et al.

Cerebral metabolism of D[1-13C]glucose was studied with localized 13C NMR spectroscopy during intravenous infusion of enriched [1-13C]glucose in four healthy subjects. The use of three-dimensional localization resulted in the complete elimination of triacylglycerol resonance that originated in scalp and subcutaneous fat. The sensitivity and resolution were sufficient to allow 4 min of time-resolved observation of label incorporation into the C3 and C4 resonances of glutamate and C4 of glutamine, as well as C3 of aspartate with lower time resolution. [4-13C]Glutamate labeled rapidly reaching close to maximum labeling at 60 min. The label flow into [3- 13C]glutamate clearly lagged behind that of [4-13C]glutamate and peaked at t = 110-140 min. Multiplets due to homonuclear 13C-13C coupling between the C3 and C4 peaks of the glutamate molecule were observed in vivo. Isotopomer analysis of spectra acquired between 120 and 180 min yielded a 13C isotopic fraction at C4 glutamate of 27 ± 2% (n = 4), which was slightly less than one-half the enrichment of the C1 position of plasma glucose (63 ± 1%), p < 0.05. By comparison with an external standard the total amount of [4-13C]glutamate was directly quantified to be 2.4 ± 0.1 μmol/ml-brain. Together with the isotopomer data this gave a calculated brain glutamate concentration of 9.1 ± 0.7 μmol/ml, which agrees with previous estimates of total brain glutamate concentrations. The agreement suggests that essentially all of the brain glutamate is derived from glucose in healthy human brain.

DOI : 10.1046/j.1471-4159.1994.63041377.x

Determination of saturation factors in 31P NMR spectra of the developing human brain

R. Gruetter; C. Fusch; E. Martin; C. Boesch

In order to assess the influence of longitudinal relaxation on previously reported variations in 31P NMR signals during brain development, we used an accelerated two-point technique to determine T1 at 2.35 Tesla in 8 min. Comparison between 10 normal neonates (age range 37-46 weeks postconception) and 10 healthy infants (age range 80-157 weeks postconception) indicated that T1 does not vary substantially during the first year of life, except in the phosphomonoester (PME) region of the spectra. T1 of total PME decreases with age which we could explain by its variable multicomponent nature: The signal from (unresolved) components at the downfield shoulder of PME (attributed mostly to phosphorylethanolamine at 6.72 ppm) with a T1 of at least 6.4 s decreases with age relative to contributions from other phosphomonoester compounds resonating predominantly at the upfield side of the peak (approximately 6.3 ppm), with T1 below 2.9 s. Because the T1 heterogeneity of PME may depend on its relative composition, quantitative 31P NMR spectroscopy may require an assessment of the influence of longitudinal relaxation on the signal amplitudes in each measurement.

DOI : 10.1002/mrm.1910290104

Automatic, localized in vivo adjustment of all first- and second-order shim coils

R. Gruetter

The implementation of a "fast, automatic shimming technique by mapping along projections" (FASTMAP) on a 2.1-Tesla whole-body system is described. The method provides a localized adjustment of all first- (X, Y, Z) and second-order (Z2, ZX, ZY, X2-Y2, 2XY) coils in 2 min. The time savings are achieved by mapping the magnetic field along six projections rather than from whole-imaging data sets. An analysis of noise error propagation suggests that in 64-ml volumes the residual linewidths obtained with the method are negligible when the signal-to-rms-noise ratio is above 30. The initial application of the method to localized 13C, 31P, and 1H spectroscopy of the human brain resulted in linewidths of approximately 2 Hz for 13C (144-ml volume), approximately 2.5 Hz for 31P (36-ml volume) and approximately 4 Hz for 1H (36-ml volume) with symmetric and reproducible lineshapes.

DOI : 10.1002/mrm.1910290613

Localized 13C NMR spectroscopy of myo-inositol in the human brain in vivo

R. Gruetter; D. L. Rothman; E. J. Novotny; R. G. Shulman

Natural abundance 13C NMR spectra obtained from 144-cm3 volumes in the human brain contained well-resolved resonances of myo-inositol after 60 min of data accumulation. A mean concentration of 7.2 +/- 0.5 mumol/g (+/- SE, n = 7) was calculated from the comparison with phantoms. 13C NMR spectroscopy thus provides a complementary role in the quantitation of metabolites also observed in the crowded 1H spectrum.

DOI : 10.1002/mrm.1910250121

Temporal and spatial analysis of fields generated by eddy currents in superconducting magnets: Optimization of corrections and quantitative characterization of magnet/gradient systems

C. Boesch; R. Gruetter; E. Martin

We propose methods for the spatial and temporal characterization of time-dependent magnetic fields generated by eddy currents after switching gradients. For an on-line determination of the temporal variations of the fields, we extract two terms from the unresolved signal of an extended sample, describing the time evolution of a frequency shift γΔB(z)(t) and of a decay constant k(t). This procedure allows us to optimize interactively the multiexponential pre-emphasis as well as any spectral volume selection method with respect to eddy currents. Additionally, we suggest an imaging sequence which allows us to determine the spatial distribution of eddy current fields at a chosen time-point after any gradient sequence to be tested. Expansion of these eddy currents fields into spherical harmonic functions proves the existence of a higher order terms, which cannot be corrected by a standard pre-emphasis device, where time constants and amplitudes are adjusted on the X, Y, Z, and Z0 coils. The proposed numerical analysis gives a tool to characterize any magnet/gradient system quantitatively with respect to eddy current performance.

DOI : 10.1002/mrm.1910200209

13C NMR visibility of rabbit muscle glycogen in vivo

R. Gruetter; T. A. Prolla; R. G. Shulman

The integrated 13C NMR intensity of the glycogen C1 resonance was measured in skeletal muscle (biceps femoris region) of nine rabbits under in vivo conditions. Concurrent chemical determinations of glycogen content showed that the in vivo signal was 1.02 ± 0.06 the intensity of analytical samples, where glycogen is known to be ~100% visible.

DOI : 10.1002/mrm.1910200216

A simple design for a double-tunable probe head for imaging and spectroscopy at high fields

R. Gruetter; C. Boesch; M. Muri; E. Martin; K. Wuthrich