NMR measurement of brain oxidative metabolism in monkeys using 13C-labeled glucose without a 13C radiofrequency channel

Reproducibility of 3D MRSI for imaging human brain glucose metabolism using direct (2H) and indirect (1H) detection of deuterium labeled compounds at 7T and clinical 3T

INTRODUCTION

Deuterium metabolic imaging (DMI) and quantitative exchange label turnover (QELT) are novel MR spectroscopy techniques for non-invasive imaging of human brain glucose and neurotransmitter metabolism with high clinical potential. Following oral or intravenous administration of non-ionizing [6,6'-2H2]-glucose, its uptake and synthesis of downstream metabolites can be mapped via direct or indirect detection of deuterium resonances using 2H MRSI (DMI) and 1H MRSI (QELT), respectively. The purpose of this study was to compare the dynamics of spatially resolved brain glucose metabolism, i.e., estimated concentration enrichment of deuterium labeled Glx (glutamate+glutamine) and Glc (glucose) acquired repeatedly in the same cohort of subjects using DMI at 7T and QELT at clinical 3T.

METHODS

Five volunteers (4 m/1f) were scanned in repeated sessions for 60 min after overnight fasting and 0.8 g/kg oral [6,6'-2H2]-glucose administration using time-resolved 3D 2H FID-MRSI with elliptical phase encoding at 7T and 3D 1H FID-MRSI with a non-Cartesian concentric ring trajectory readout at clinical 3T.

RESULTS

One hour after oral tracer administration regionally averaged deuterium labeled Glx4 concentrations and the dynamics were not significantly different over all participants between 7T 2H DMI and 3T 1H QELT data for GM (1.29±0.15 vs. 1.38±0.26 mM, p=0.65 & 21±3 vs. 26±3 µM/min, p=0.22) and WM (1.10±0.13 vs. 0.91±0.24 mM, p=0.34 & 19±2 vs. 17±3 µM/min, p=0.48). Also, the observed time constants of dynamic Glc6 data in GM (24±14 vs. 19±7 min, p=0.65) and WM (28±19 vs. 18±9 min, p=0.43) dominated regions showed no significant differences. Between individual 2H and 1H data points a weak to moderate negative correlation was observed for Glx4 concentrations in GM (r=-0.52, p<0.001), and WM (r=-0.3, p<0.001) dominated regions, while a strong negative correlation was observed for Glc6 data GM (r=-0.61, p<0.001) and WM (r=-0.70, p<0.001).

CONCLUSION

This study demonstrates that indirect detection of deuterium labeled compounds using 1H QELT MRSI at widely available clinical 3T without additional hardware is able to reproduce absolute concentration estimates of downstream glucose metabolites and the dynamics of glucose uptake compared to 2H DMI data acquired at 7T. This suggests significant potential for widespread application in clinical settings especially in environments with limited access to ultra-high field scanners and dedicated RF hardware.

Noninvasive 3-Dimensional 1 H-Magnetic Resonance Spectroscopic Imaging of Human Brain Glucose and Neurotransmitter Metabolism Using Deuterium Labeling at 3T : Feasibility and Interscanner Reproducibility

OBJECTIVES

Noninvasive, affordable, and reliable mapping of brain glucose metabolism is of critical interest for clinical research and routine application as metabolic impairment is linked to numerous pathologies, for example, cancer, dementia, and depression. A novel approach to map glucose metabolism noninvasively in the human brain has been presented recently on ultrahigh-field magnetic resonance (MR) scanners (≥7T) using indirect detection of deuterium-labeled glucose and downstream metabolites such as glutamate, glutamine, and lactate. The aim of this study was to demonstrate the feasibility to noninvasively detect deuterium-labeled downstream glucose metabolites indirectly in the human brain via 3-dimensional (3D) proton ( 1 H) MR spectroscopic imaging on a clinical 3T MR scanner without additional hardware.

MATERIALS AND METHODS

This prospective, institutional review board-approved study was performed in 7 healthy volunteers (mean age, 31 ± 4 years, 5 men/2 women) after obtaining written informed consent. After overnight fasting and oral deuterium-labeled glucose administration, 3D metabolic maps were acquired every ∼4 minutes with ∼0.24 mL isotropic spatial resolution using real-time motion-, shim-, and frequency-corrected echo-less 3D 1 H-MR spectroscopic Imaging on a clinical routine 3T MR system. To test the interscanner reproducibility of the method, subjects were remeasured on a similar 3T MR system. Time courses were analyzed using linear regression and nonparametric statistical tests. Deuterium-labeled glucose and downstream metabolites were detected indirectly via their respective signal decrease in dynamic 1 H MR spectra due to exchange of labeled and unlabeled molecules.

RESULTS

Sixty-five minutes after deuterium-labeled glucose administration, glutamate + glutamine (Glx) signal intensities decreased in gray/white matter (GM/WM) by -1.63 ± 0.3/-1.0 ± 0.3 mM (-13% ± 3%, P = 0.02/-11% ± 3%, P = 0.02), respectively. A moderate to strong negative correlation between Glx and time was observed in GM/WM ( r = -0.64, P < 0.001/ r = -0.54, P < 0.001), with 60% ± 18% ( P = 0.02) steeper slopes in GM versus WM, indicating faster metabolic activity. Other nonlabeled metabolites showed no significant changes. Excellent intrasubject repeatability was observed across scanners for static results at the beginning of the measurement (coefficient of variation 4% ± 4%), whereas differences were observed in individual Glx dynamics, presumably owing to physiological variation of glucose metabolism.

CONCLUSION

Our approach translates deuterium metabolic imaging to widely available clinical routine MR scanners without specialized hardware, offering a safe, affordable, and versatile (other substances than glucose can be labeled) approach for noninvasive imaging of glucose and neurotransmitter metabolism in the human brain.

Reproducibility of 3D MRSI for imaging human brain glucose metabolism using direct ( 2 H) and indirect ( 1 H) detection of deuterium labeled compounds at 7T and clinical 3T

Introduction

Deuterium metabolic imaging (DMI) and quantitative exchange label turnover (QELT) are novel MR spectroscopy techniques for non-invasive imaging of human brain glucose and neurotransmitter metabolism with high clinical potential. Following oral or intravenous administration of non-ionizing [6,6'- 2 H 2 ]-glucose, its uptake and synthesis of downstream metabolites can be mapped via direct or indirect detection of deuterium resonances using 2 H MRSI (DMI) and 1 H MRSI (QELT), respectively. The purpose of this study was to compare the dynamics of spatially resolved brain glucose metabolism, i.e., estimated concentration enrichment of deuterium labeled Glx (glutamate+glutamine) and Glc (glucose) acquired repeatedly in the same cohort of subjects using DMI at 7T and QELT at clinical 3T.

Methods

Five volunteers (4m/1f) were scanned in repeated sessions for 60 min after overnight fasting and 0.8g/kg oral [6,6'- 2 H 2 ]-glucose administration using time-resolved 3D 2 H FID-MRSI with elliptical phase encoding at 7T and 3D 1 H FID-MRSI with a non-Cartesian concentric ring trajectory readout at clinical 3T.

Results

One hour after oral tracer administration regionally averaged deuterium labeled Glx 4 concentrations and the dynamics were not significantly different over all participants between 7T 2 H DMI and 3T 1 H QELT data for GM (1.29±0.15 vs. 1.38±0.26 mM, p=0.65 & 21±3 vs. 26±3 µM/min, p=0.22) and WM (1.10±0.13 vs. 0.91±0.24 mM, p=0.34 & 19±2 vs. 17±3 µM/min, p=0.48). Also, the observed time constants of dynamic Glc 6 data in GM (24±14 vs. 19±7 min, p=0.65) and WM (28±19 vs. 18±9 min, p=0.43) dominated regions showed no significant differences. Between individual 2 H and 1 H data points a weak to moderate negative correlation was observed for Glx 4 concentrations in GM (r=-0.52, p<0.001), and WM (r=-0.3, p<0.001) dominated regions, while a strong negative correlation was observed for Glc 6 data GM (r=- 0.61, p<0.001) and WM (r=-0.70, p<0.001).

Conclusion

This study demonstrates that indirect detection of deuterium labeled compounds using 1 H QELT MRSI at widely available clinical 3T without additional hardware is able to reproduce absolute concentration estimates of downstream glucose metabolites and the dynamics of glucose uptake compared to 2 H DMI data acquired at 7T. This suggests significant potential for widespread application in clinical settings especially in environments with limited access to ultra-high field scanners and dedicated RF hardware.

Mapping of glutamate metabolism using 1H FID-MRSI after oral administration of [1-13C]Glc at 9.4 T

Glutamate is the major excitatory transmitter in the brain and malfunction of the related metabolism is associated with various neurological diseases and disorders. The observation of labeling changes in the spectra after the administration of a 13C labelled tracer is a common tool to gain better insights into the function of the metabolic system. But so far, only a very few studies presenting the labeling effects in more than two voxels to show the spatial dependence of metabolism. In the present work, the labeling effects were measured in a transversal plane in the human brain using ultra-short TE and TR 1H FID-MRSI. The measurement set-up was most simple: The [1-13C]Glc was administered orally instead of intravenous and the spectra were measured with a pure 1H technique without the need of a 13C channel (as Boumezbeur et al. demonstrated in 2004). Thus, metabolic maps and enrichment curves could be obtained for more metabolites and in more voxels than ever before in human brain. Labeling changes could be observed in [4-13C]glutamate, [3-13C]glutamate+glutamine, [2-13C]glutamate+glutamine, [4-13C]glutamine, and [3-13C]aspartate with a high temporal (3.6 min) and spatial resolution (32 × 32 grid with nominal voxel size of 0.33 µL) in five volunteers.

Measurement of glucose metabolism in the occipital lobe and frontal cortex after oral administration of [1-13C]glucose at 9.4 T

For the first time, labeling effects after oral intake of [1-13C]glucose are observed in the human brain with pure 1H detection at 9.4 T. Spectral time series were acquired using a short-TE 1H MRS MC-semiLASER (Metabolite Cycling semi Localization by Adiabatic SElective Refocusing) sequence in two voxels of 5.4 mL in the frontal cortex and the occipital lobe. High-quality time-courses of [4-13C]glutamate, [4-13C]glutamine, [3-13C]glutamate + glutamine, [2-13C] glutamate+glutamine and [3-13C]aspartate for individual volunteers and additionally, group-averaged time-courses of labeled and non-labeled brain glucose could be obtained. Using a one-compartment model, mean metabolic rates were calculated for each voxel position: The mean rate of the TCA-cycle (Vtca) value was determined to be 1.36 and 0.93 μmol min-1 g-1, the mean rate of glutamine synthesis (Vgln) was calculated to be 0.23 and 0.45 μmol min-1 g-1, the mean exchange rate between cytosolic amino acids and mitochondrial Krebs cycle intermediates (Vx) rate was found to be 0.57 and 1.21 μmol min-1 g-1 for the occipital lobe and the frontal cortex, respectively. These values were in agreement with previously reported data. Altogether, it can be shown that this most simple technique combining oral administration of [1-13C]Glc with pure 1H MRS acquisition is suitable to measure metabolic rates.

A Novel Biomarker of Neuronal Glutamate Metabolism in Nonhuman Primates Using Localized 1H-Magnetic Resonance Spectroscopy: Development and Effects of BNC375, an α7 Nicotinic Acetylcholine Receptor Positive Allosteric Modulator

BACKGROUND

The development of treatments for cognitive deficits associated with central nervous system disorders is currently a significant medical need. Despite the great need for such therapeutics, a significant challenge in the drug development process is the paucity of robust biomarkers to assess target modulation and guide clinical decisions. We developed a novel, translatable biomarker of neuronal glutamate metabolism, the ¹³C-glutamate+glutamine (Glx) H3:H4 labeling ratio, in nonhuman primates using localized ¹H-magnetic resonance spectroscopy combined with ¹³C-glucose infusions.

METHODS

We began with numerical simulations in an established model of brain glutamate metabolism, showing that the ¹³C-Glx H3:H4 ratio should be a sensitive biomarker of neuronal tricarboxylic acid cycle activity, a key measure of overall neuronal metabolism. We showed that this biomarker can be measured reliably using a standard ¹H-magnetic resonance spectroscopy method (point-resolved spectroscopy sequence/echo time = 20 ms), obviating the need for specialized hardware and pulse sequences typically used with ¹³C-magnetic resonance spectroscopy, thus improving overall clinical translatability. Finally, we used this biomarker in 8 male rhesus macaques before and after administration of the compound BNC375, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor that enhances glutamate signaling ex vivo and elicits procognitive effects in preclinical species.

RESULTS

The ¹³C-Glx H3:H4 ratios in the monkeys showed that BNC375 increases neuronal metabolism in nonhuman primates in vivo, detectable on an individual basis.

CONCLUSIONS

This study demonstrates that the ratio of ¹³C-Glx H3:H4 labeling is a biomarker that may provide an objective readout of compounds affecting glutamatergic neurotransmission and could improve decision making for the development of therapeutic agents.

Detection of 13C labeling of glutamate and glutamine in human brain by proton magnetic resonance spectroscopy

A proton magnetic resonance spectroscopy (MRS) technique was used to measure ¹³C enrichments of glutamate and glutamine in a 3.5 × 1.8 × 2 cm³ voxel placed in the dorsal anterior cingulate cortex of five healthy participants after oral administration of [U-¹³C]glucose. Strong pseudo singlets of glutamate and glutamine were induced to enhance the signal strength of glutamate and glutamine. This study demonstrated that ¹³C labeling of glutamate and glutamine can be measured with the high sensitivity and spatial resolution of ¹H MRS using a proton-only MRS technique with standard commercial hardware. Furthermore, it is feasible to measure ¹³C labeling of glutamate and glutamine in limbic structures, which play major roles in behavioral and emotional responses and whose abnormalities are involved in many neuropsychiatric disorders.

PRESS timings for resolving 13 C4 -glutamate 1 H signal at 9.4 T: Demonstration in rat with uniformly labelled 13 C-glucose

MRS of ¹³ C₄ -labelled glutamate (¹³ C₄ -Glu) during an infusion of a carbon-13 (¹³ C)-labelled substrate, such as uniformly labelled glucose ([U-¹³ C₆ ]-Glc), provides a measure of Glc metabolism. The presented work provides a single-shot indirect ¹³ C detection technique to quantify the approximately 2.51 ppm ¹³ C₄ -Glu satellite proton (¹ H) peak at 9.4 T. The methodology is an optimized point-resolved spectroscopy (PRESS) sequence that minimizes signal contamination from the strongly coupled protons of N-acetylaspartate (NAA), which resonate at approximately 2.49 ppm. J-coupling evolution of protons was characterized numerically and verified experimentally. A (TE₁ , TE₂ ) combination of (20 ms, 106 ms) was found to be suitable for minimizing NAA signal in the 2.51 ppm ¹ H ¹³ C₄ -Glu spectral region, while retaining the ¹³ C₄ -Glu ¹ H satellite peak. The efficacy of the technique was verified on phantom solutions and on two rat brains in vivo during an infusion of [U-¹³ C₆ ]-Glc. LCModel was employed for analysis of the in vivo spectra to quantify the 2.51 ppm ¹ H ¹³ C₄ -Glu signal to obtain Glu C4 fractional enrichment time courses during the infusions. Cramér-Rao lower bounds of about 8% were obtained for the 2.51 ppm ¹³ C₄ -Glu ¹ H satellite peak with the optimal TE combination.

In Vivo Multidimensional Brain Imaging in Huntington's Disease Animal Models

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an abnormal expansion of a CAG repeat located in the gene encoding for huntingtin protein. This mutation induces the expression of a polyglutamine stretch in the mutated protein resulting in the modification of various biological properties of the wild-type protein and the progressive appearance of motor, cognitive, and psychiatric disorders that are typically associated to this condition. Although the exact neuropathological mechanisms of degeneration are still not fully understood, HD pathology is characterized by severe neuronal losses in various brain regions including the basal ganglia and many cortical areas. Early signs of astrogliosis may precede actual neuronal degeneration. Early metabolic impairment at least in part associated with mitochondrial complex II deficiency may play a key role in huntingtin-induced mechanisms of neurodegeneration. Clinical trials are actively prepared including various gene-silencing approaches aiming at decreasing mutated huntingtin production. However, with the lack of a specific imaging biomarker capable of visualizing mutated huntingtin or huntingtin aggregates, there is a need for surrogate markers of huntingtin neurodegeneration. MRI and caudate nucleus atrophy is one of the most sensitive imaging biomarkers of HD. As such it can be used as a means to study disease progression and potential halting of the neurodegenerative process by therapeutic intervention, but this marker relies on actual neuronal loss which is a somewhat a late event in the pathology. As a means to develop, characterize and evaluate new, potentially earlier biomarkers of HD pathology we have recently embarked on a series of NMR developments looking for brain imaging techniques that allow for noninvasive longitudinal evaluation/characterization of functional alterations in animal models of HD. This chapter describes an assemblage of innovative NMR methods that have proved useful in detecting pathological cell dysfunctions in various preclinical models of HD.

Glutamate metabolism in temporal lobe epilepsy as revealed by dynamic proton MRS following the infusion of [U13-C] glucose

Focal metabolic dysfunction commonly observed in temporal lobe epilepsy (TLE), and is associated with the development of medical intractability and neurocognitive deficits. It has not been established if this dysfunction is due to cell loss or biochemical dysfunction in metabolic pathways. To explore this question, dynamic ¹H MRS following an infusion of [U¹³- C] glucose was performed to measure glutamate (Glu) metabolism. Subjects (n=6) showed reduced Glu levels (p<0.01) in the ipsilateral mesial temporal lobe (MTL) compared with controls (n=4). However, the rate of ¹³C incorporation into Glu did not differ between those with epilepsy and controls (p=0.77). This suggests that reduced Glu concentrations in the region of the seizure focus are not due to disruptions in metabolic pathways, but may instead be due to neuronal loss or simplification.

A Flux Balance of Glucose Metabolism Clarifies the Requirements of the Warburg Effect

The Warburg effect, or aerobic glycolysis, is marked by the increased metabolism of glucose to lactate in the presence of oxygen. Despite its widespread prevalence in physiology and cancer biology, the causes and consequences remain incompletely understood. Here, we show that a simple balance of interacting fluxes in glycolysis creates constraints that impose the necessary conditions for glycolytic flux to generate lactate as opposed to entering into the mitochondria. These conditions are determined by cellular redox and energy demands. By analyzing the constraints and sampling the feasible region of the model, we further study how cell proliferation rate and mitochondria-associated NADH oxidizing and ATP producing fluxes are interlinked. Together this analysis illustrates the simplicity of the origins of the Warburg effect by identifying the flux distributions that are necessary for its instantiation.

Experimental strategies for in vivo13C NMR spectroscopy

In vivo carbon-13 (¹³C) MRS opens unique insights into the metabolism of intact organisms, and has led to major advancements in the understanding of cellular metabolism under normal and pathological conditions in various organs such as skeletal muscles, the heart, the liver and the brain. However, the technique comes at the expense of significant experimental difficulties. In this review we focus on the experimental aspects of non-hyperpolarized ¹³C MRS in vivo. Some of the enrichment strategies which have been proposed so far are described; the various MRS acquisition paradigms to measure ¹³C labeling are then presented. Finally, practical aspects of ¹³C spectral quantification are discussed.

(13)C NMR spectroscopy applications to brain energy metabolism

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

pH as a biomarker of neurodegeneration in Huntington's disease: a translational rodent-human MRS study

Early diagnosis and follow-up of neurodegenerative diseases are often hampered by the lack of reliable biomarkers. Neuroimaging techniques like magnetic resonance spectroscopy (MRS) offer promising tools to detect biochemical alterations at early stages of degeneration. Intracellular pH, which can be measured noninvasively by (31)P-MRS, has shown variations in several brain diseases. Our purpose has been to evaluate the potential of MRS-measured pH as a relevant biomarker of early degeneration in Huntington's disease (HD). We used a translational approach starting with a preclinical validation of our hypothesis before adapting the method to HD patients. (31)P-MRS-derived cerebral pH was first measured in rodents during chronic intoxication with 3-nitropropionic acid (3NP). A significant pH increase was observed early into the intoxication protocol (pH=7.17±0.02 after 3 days) as compared with preintoxication (pH=7.08±0.03). Furthermore, pH changes correlated with the 3NP-induced inhibition of succinate dehydrogenase and preceded striatum lesions. Using a similar MRS approach implemented on a clinical MRI, we then showed that cerebral pH was significantly higher in HD patients (n=7) than in healthy controls (n=6) (7.05±0.03 versus 7.02±0.01, respectively, P=0.026). Altogether, both preclinical and human data strongly argue in favor of MRS-measured pH being a promising biomarker for diagnosis and follow-up of HD.

State of the art direct 13C and indirect 1H-[13C] NMR spectroscopy in vivo. A practical guide

Carbon-13 NMR spectroscopy in combination with (13)C-labeled substrate infusion is a powerful technique for measuring a large number of metabolic fluxes noninvasively in vivo. It has been used to quantify glycogen synthesis rates, establish quantitative relationships between energy metabolism and neurotransmission, and evaluate the importance of different substrates. Measurements can, in principle, be performed through direct (13)C NMR detection or via indirect (1)H-[(13)C] NMR detection of the protons attached to (13)C nuclei. The choice of detection scheme and pulse sequence depends on the magnetic field strength, whereas substrate selection depends on metabolic pathways. (13)C NMR spectroscopy remains a challenging technique that requires several nonstandard hardware modifications, infusion of (13)C-labeled substrates, and sophisticated processing and metabolic modeling. In this study, the various aspects of direct (13)C and indirect (1)H-[(13)C] NMR are reviewed with the aim of providing a practical guide.

Simplified (13)C metabolic modeling for simplified measurements of cerebral TCA cycle rate in vivo

(13)C NMR spectroscopy is a unique tool to measure the cerebral tricarboxylic acid (TCA) cycle rate in vivo. The measurement relies on metabolic modeling of glutamate C3 and C4 enrichment time courses during a (13)C-glucose intravenous infusion. Usual metabolic models require the plasma glucose and (13)C-glucose time courses as input functions, as well as the knowledge of Michaelis-Menten kinetics parameters governing passage through the blood-brain barrier. It is shown in the present work that, when using an infusion protocol yielding a rapidly stable plasma glucose fractional enrichment, metabolic modeling can be simplified in such a manner that this additional information on input function and glucose transport is no longer required, significantly simplifying the measurement of cerebral TCA cycle rate in vivo.

Multimodal neuroimaging provides a highly consistent picture of energy metabolism, validating 31P MRS for measuring brain ATP synthesis

Neuroimaging methods have considerably developed over the last decades and offer various noninvasive approaches for measuring cerebral metabolic fluxes connected to energy metabolism, including PET and magnetic resonance spectroscopy (MRS). Among these methods, (31)P MRS has the particularity and advantage to directly measure cerebral ATP synthesis without injection of labeled precursor. However, this approach is methodologically challenging, and further validation studies are required to establish (31)P MRS as a robust method to measure brain energy synthesis. In the present study, we performed a multimodal imaging study based on the combination of 3 neuroimaging techniques, which allowed us to obtain an integrated picture of brain energy metabolism and, at the same time, to validate the saturation transfer (31)P MRS method as a quantitative measurement of brain ATP synthesis. A total of 29 imaging sessions were conducted to measure glucose consumption (CMRglc), TCA cycle flux (V(TCA)), and the rate of ATP synthesis (V(ATP)) in primate monkeys by using (18)F-FDG PET scan, indirect (13)C MRS, and saturation transfer (31)P MRS, respectively. These 3 complementary measurements were performed within the exact same area of the brain under identical physiological conditions, leading to: CMRglc = 0.27 +/- 0.07 micromol x g(-1) x min(-1), V(TCA) = 0.63 +/- 0.12 micromol x g(-1) x min(-1), and V(ATP) = 7.8 +/- 2.3 micromol x g(-1) x min(-1). The consistency of these 3 fluxes with literature and, more interestingly, one with each other, demonstrates the robustness of saturation transfer (31)P MRS for directly evaluating ATP synthesis in the living brain.

Clinical NOE 13C MRS for neuropsychiatric disorders of the frontal lobe

In this communication, a scheme is described whereby in vivo (13)C MRS can safely be performed in the frontal lobe, a human brain region hitherto precluded on grounds of SAR, but important in being the seat of impaired cognitive function in many neuropsychiatric and developmental disorders. By combining two well known features of (13)C NMR-the use of low power NOE and the focus on (13)C carbon atoms which are only minimally coupled to protons, we are able to overcome the obstacle of SAR and develop means of monitoring the (13)C fluxes of critically important metabolic pathways in frontal brain structures of normal volunteers and patients. Using a combination of low-power WALTZ decoupling, variants of random noise for nuclear overhauser effect enhancement it was possible to reduce power deposition to 20% of the advised maximum specific absorption rate (SAR). In model solutions (13)C signal enhancement achieved with this scheme were comparable to that obtained with WALTZ-4. In human brain, the low power procedure effectively determined glutamine, glutamate and bicarbonate in the posterior parietal brain after [1-(13)C] glucose infusion. The same (13)C enriched metabolites were defined in frontal brain of human volunteers after administration of [1-(13)C] acetate, a recognized probe of glial metabolism. Time courses of incorporation of (13)C into cerebral glutamate, glutamine and bicarbonate were constructed. The results suggest efficacy for measurement of in vivo cerebral metabolic rates of the glutamate-glutamine and tricarboxylic acid cycles in 20 min MR scans in previously inaccessible brain regions in humans at 1.5 T. We predict these will be clinically useful biomarkers in many human neuropsychiatric and genetic conditions.

Diffusion-weighted NMR spectroscopy allows probing of 13C labeling of glutamate inside distinct metabolic compartments in the brain

In the present work, diffusion-weighted (DW)-NMR spectroscopy of glutamate was performed during a (13)C-labeled glucose infusion in monkey brain (six experiments). It is shown that glutamate (13)C labeling occurs significantly faster at higher diffusion weightings-slightly for glutamate in position C4, and more markedly for glutamate in position C3. This demonstrates the existence of different diffusion compartments for glutamate, associated with different metabolic rates. Metabolic modeling of (13)C enrichment time-courses suggests that these compartments might be gray and white matter, each having a specific oxidative metabolism rate possibly paralleled by a specific glutamate diffusion coefficient.

Measuring N-acetylaspartate synthesis in vivo using proton magnetic resonance spectroscopy

N-Acetylaspartate (NAA) is an important marker of neuronal function and viability that can be measured using magnetic resonance spectroscopy (MRS). In this paper, we proposed a method to measure NAA synthesis using proton MRS with infusion of uniformly (13)C-labeled glucose, and demonstrated its feasibility in an in vivo study of the rat brain. The rate of (13)C-label incorporation into the acetyl group of NAA was measured using a localized, long echo-time proton MRS method. Signals from the (13)C satellites of the main NAA methyl protons at 2.02 ppm were continuously monitored for 10h. Quantification of the data based on a linear kinetic model showed that NAA synthesis rate in isoflurane-anesthetized rats was 0.19+/-0.02 micromol/g/h (mean+/-standard deviation, n=12).

Novel strategy for cerebral 13C MRS using very low RF power for proton decoupling

One of the major difficulties of in vivo 13C MRS is the need to decouple the large, one-bond, 1H-13C scalar couplings in order to obtain useful signal-to-noise ratios (SNRs) and spectral resolution at magnetic field strengths that are accessible to clinical studies. In this report a new strategy for in vivo cerebral 13C MRS is proposed. We realized that the turnover kinetics of glutamate (Glu) C5 from exogenous [2-(13)C]glucose (Glc) is identical to that of Glu C4 from exogenous [1-(13)C]Glc. The carboxylic/amide carbons are only coupled to protons via very weak long-range 1H-13C scalar couplings. As such, they can be effectively decoupled at very low RF power. Therefore, decoupling of the large 1H-13C scalar couplings can be avoided by the use of [2-(13)C]Glc. An additional advantage of this strategy is the lack of contamination from subcutaneous lipids because there are no overlapping fat signals in the vicinity of the Glu C5 and glutamine (Gln) C5 peaks. The feasibility of this strategy was demonstrated using 13C MRS on rhesus monkey brains at 4.7T.

Proton-observed carbon-edited NMR spectroscopy in strongly coupled second-order spin systems

Proton-observed carbon-edited (POCE) NMR spectroscopy is commonly used to measure 13C labeling with higher sensitivity compared to direct 13C NMR spectroscopy, at the expense of spectral resolution. For weakly coupled first-order spin systems, the multiplet signal at a specific proton chemical shift in POCE spectra directly reflects 13C enrichment of the carbon attached to this proton. The present study demonstrates that this is not necessarily the case for strongly coupled second-order spin systems. In such cases NMR signals can be detected in the POCE spectra even at chemical shifts corresponding to protons bound to 12C. This effect is demonstrated theoretically with density matrix calculations and simulations, and experimentally with measured POCE spectra of [3-13C]glutamate.

Investigating brain metabolism at high fields using localized 13C NMR spectroscopy without 1H decoupling

Most in vivo 13C NMR spectroscopy studies in the brain have been performed using 1H decoupling during acquisition. Decoupling imposes significant constraints on the experimental setup (particularly for human studies at high magnetic field) in order to stay within safety limits for power deposition. We show here that incorporation of the 13C label from 13C-labeled glucose into brain amino acids can be monitored accurately using localized 13C NMR spectroscopy without the application of 1H decoupling. Using LCModel quantification with prior knowledge of one-bond and multiple-bond J(CH) coupling constants, the uncertainty on metabolites concentrations was only 35% to 91% higher (depending on the carbon resonance of interest) in undecoupled spectra compared to decoupled spectra in the rat brain at 9.4 Tesla. Although less sensitive, 13C NMR without decoupling dramatically reduces experimental constraints on coil setup and pulse sequence design required to keep power deposition within safety guidelines. This opens the prospect of safely measuring 13C NMR spectra in humans at varied brain locations (not only the occipital lobe) and at very high magnetic fields above 4 Tesla.

In vivo 13C NMR spectroscopy and metabolic modeling in the brain: a practical perspective

In vivo 13C NMR spectroscopy has the unique capability to measure metabolic fluxes noninvasively in the brain. Quantitative measurements of metabolic fluxes require analysis of the 13C labeling time courses obtained experimentally with a metabolic model. The present work reviews the ingredients necessary for a dynamic metabolic modeling study, with particular emphasis on practical issues.

Glycolysis versus TCA cycle in the primate brain as measured by combining 18F-FDG PET and 13C-NMR

The glycolytic flux (cerebral metabolic rate of glucose CMRglc) and the TCA cycle flux (VTCA) were measured in the same monkeys by 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) and 13C NMR spectroscopy, respectively. Registration of nuclear magnetic resonance (NMR) and PET data were used for comparison of CMRglc and VTCA in the exact same area of the brain. Both fluxes were in good agreement with literature values (CMRglc=0.23+/-0.03 micromol/g min, VTCA=0.53+/-0.13 micromol/g min). The resulting [CMRglc/VTCA] ratio was 0.46+/-0.12 (n=5, mean+/-s.d.), not significantly different from the 0.5 expected when glucose is the sole fuel that is completely oxidized. Our results provide a cross-validation of both techniques. Comparison of CMRglc with VTCA is in agreement with a metabolic coupling between the TCA cycle and glycolysis under normal physiologic conditions.

Effect of strong homonuclear proton coupling on localized13C detection using PRESS

The effect of strong homonuclear proton coupling on (13)C incorporation measurements by either indirect or direct means was investigated (and illustrated with glutamate) both numerically and experimentally at 3.0 T. In particular, two sequences were considered, each using a proton PRESS sequence for localization. The indirect (13)C detection method incorporated the POCE (proton observe carbon edited) technique onto PRESS, and for direct (13)C detection a DEPT (distortionless enhancement by polarization transfer) sequence was appended to the PRESS localization. Both analysis and experiment demonstrate that when strong homonuclear coupling of protons is additional to heteronuclear coupling with (13)C spins, the (13)C measures derived from either the indirect PRESS-POCE sequence or the direct-but-enhanced PRESS-DEPT sequence are significantly modified. Specifically, the MR lineshapes of both (13)C-bonded and nonbonded protons are changed during (13)C incorporation, giving rise, for example, to a potential cross-contamination of < or =30% between glutamate (13)C(3) and (13)C(4) measures from the PRESS-POCE indirect method. During direct-but-enhanced detection, the DEPT enhancement is reduced for glutamate (13)C(2), (13)C(3), and (13)C(4) but not equally, and the reduction is further exacerbated by proton PRESS localization, which gives rise to enhancements that are strong functions of PRESS TE(1) and TE(2).

In vivo detection of cortical GABA turnover from intravenously infused [1-13C]D-glucose

In this study [2-(13)C] gamma-aminobutyric acid (GABA) was spectrally resolved in vivo and detected simultaneously with [4-(13)C]glutamate (Glu) and [4-(13)C]glutamine (Gln) in the proton spectra obtained from a localized 40 microL voxel in rat neocortex with the use of an adiabatic (1)H-observed, (13)C-edited (POCE) spectroscopy method and an 89-mm-bore vertical 11.7 Tesla microimager. The time-resolved kinetics of (13)C label incorporation from intravenously infused [1-(13)C]glucose into [4-(13)C]Glu, [4-(13)C]Gln, and [2-(13)C]GABA were measured after acute administration of gabaculine, a potent and specific inhibitor of GABA-transaminase. In contrast to previous observations of a rapid turnover of [2-(13)C]GABA from [1-(13)C]glucose in intact rat brain, the rate of (13)C incorporation from [1-(13)C]glucose into [2-(13)C]GABA in the gabaculine-treated rats was found to be significantly reduced as a result of the blockade of the GABA shunt.

High-field localized 1H NMR spectroscopy in the anesthetized and in the awake monkey

Localized cerebral in vivo 1H NMR spectroscopy (MRS) was performed in the anesthetized as well as the awake monkey using a novel vertical 7 T/60 cm MR system. The increased sensitivity and spectral dispersion gained at high field enabled the quantification of up to 16 metabolites in 0.1- to 1-ml volumes. Quantification was accomplished by using simulations of 18 metabolite spectra and a macromolecule (MM) background spectrum consisting of 12 components. Major cerebral metabolites (concentrations >3 mM) such as glutamate (Glu), N-acetylaspartate (NAA), creatine (Cr)/phosphocreatine (PCr) and myo-inositol (Ins) were identified with an error below 3%; most other metabolites were quantified with errors in the order of 10%. Metabolite ratios were 1.39:1 for total NAA, 1.38:1 for glutamate (Glu)/glutamine (Gln) and 0.09:1 for cholines (Cho) relative to total Cr. Taurine (Tau) was detectable at concentrations lower than 1 mM, while lactate (Lac) remained below the detection limit. The spectral dispersion was sufficient to separate metabolites of similar spectral patterns, such as Gln and Glu, N-acetylaspartylglutamate (NAAG) and NAA, and PCr-Cr. MRS in the awake monkey required the development and refinement of acquisition and correction strategies to minimize magnetic susceptibility artifacts induced by respiration and movement of the mouth or body. Periods with major motion artifacts were rejected, while a frequency/phase correction was performed on the remaining single spectra before averaging. In resting periods, both spectral amplitude and line width, that is, the voxel shim, were unaffected permitting reliable measurements. The corrected spectra obtained from the awake monkey afforded the reliable detection of 6-10 cerebral metabolites of 1-ml volumes.