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

Toward structure and metabolism of glycogen C1-C6 in humans at 7 T by localized 13C MRS using low-power bilevel broadband 1H decoupling

This work aims to develop and implement a pulse-acquire sequence for three-dimensional (3D) single-voxel localized 13C MRS in humans at 7 T, in conjunction with bilevel broadband 1H decoupling, and to test its feasibility in vitro and in vivo in human calf muscle with emphasis on the detection of glycogen C1-C6. A localization scheme suitable for measuring fast-relaxing 13C signals in humans at 7 T was developed and implemented using the outer volume suppression (OVS) and one-dimensional image selected in vivo spectroscopy (ISIS-1D) schemes, similar to that which was previously reported in humans at 4 T. The 3D 13C localization scheme was followed by uniform 13C adiabatic excitation, all complemented with an option for bilevel broadband 1H decoupling to improve both 13C sensitivity and spectral resolution at 7 T. The performance of the pulse-acquire sequence was investigated in vitro on phantoms and in vivo in the human calf muscle of three healthy volunteers, while measuring glycogen C1-C6. In addition, T1 and T2 of glycogen C1-C6 were measured in vitro at 7 T, as well as T1 of glycogen C1 in vivo. The glycerol C2 and C1,3 lipid resonances were efficiently suppressed in vitro at 7 T using the OVS and ISIS-1D schemes, allowing distinct detection of glycogen C2-C6. While some glycerol remained in calf muscle in vivo, the intense lipid at 130 ppm was efficiently suppressed. The 13C sensitivity and spectral resolution of glycogen C1-C6 in vitro and glycogen C1 in vivo were improved at 7 T using bilevel broadband 1H decoupling. The T1 and T2 of glycogen C1-C6 in vitro at 7 T were consistent compared with those at 8.5 T, while the T1 of glycogen C1 in vivo at 7 T resulted similar to that in vitro. Localized 13C MRS is feasible in human calf muscle in vivo at 7 T, and this will allow further extension of this method for 13C MRS measurements such as in the brain.

In vivo Human MR Spectroscopy Using a Clinical Scanner: Development, Applications, and Future Prospects

MR spectroscopy (MRS) is a unique and useful method for noninvasively evaluating biochemical metabolism in human organs and tissues, but its clinical dissemination has been slow and often limited to specialized institutions or hospitals with experts in MRS technology. The number of 3-T clinical MR scanners is now increasing, representing a major opportunity to promote the use of clinical MRS. In this review, we summarize the theoretical background and basic knowledge required to understand the results obtained with MRS and introduce the general consensus on the clinical utility of proton MRS in routine clinical practice. In addition, we present updates to the consensus guidelines on proton MRS published by the members of a working committee of the Japan Society of Magnetic Resonance in Medicine in 2013. Recent research into multinuclear MRS equipped in clinical MR scanners is explained with an eye toward future development. This article seeks to provide an overview of the current status of clinical MRS and to promote the understanding of when it can be useful. In the coming years, MRS-mediated biochemical evaluation is expected to become available for even routine clinical practice.

Fast in vivo 23 Na imaging and T 2 ∗ mapping using accelerated 2D-FID UTE magnetic resonance spectroscopic imaging at 3 T: Proof of concept and reliability study

PURPOSE

To implement an accelerated MR-acquisition method allowing to map T 2 ∗ relaxation and absolute concentration of sodium within skeletal muscles at 3T.

METHODS

A fast-UTE-2D density-weighted concentric-ring-trajectory ²³ Na-MRSI technique was used to acquire 64 time points of FID with a spectral bandwidth of 312.5 Hz with an in-plane resolution of 2.5 × 2.5 mm² in ~15 min. The fast-relaxing ²³ Na signal was localized with a single-shot, inversion-recovery-based, non-echo (SIRENE) outer volume suppression (OVS) method. The sequence was verified using simulation and phantom studies before implementing it in human calf muscles. To evaluate the 2D-SIRENE-MRSI (UTE = 0.55 ms) imaging performance, it was compared to a 3D-MRI (UTE = 0.3 ms) sequence. Both data sets were acquired within 2 same-day sessions to assess repeatability. The T 2 ∗ values were fitted voxel-by-voxel using a biexponential model for the 2D-MRSI data. Finally, intra-subject coefficients of variation (CV) were estimated.

RESULTS

The MRSI-FID data allowed us to map the fast and slow components of T 2 ∗ in the calf muscles. The spatial distributions of ²³ Na concentration for both MRSI and 3D-MRI acquisitions were significantly correlated (P < .001). The test-retest analysis rendered high repeatability for MRSI with a CV of 5%. The mean T 2 Fast ∗ in muscles was 0.7 ± 0.1 ms (contribution fraction = 37%), whereas T 2 Slow ∗ was 13.2 ± 0.2 ms (63%). The mean absolute muscle ²³ Na concentration calculated from the T 2 ∗ -corrected data was 28.6 ± 3.3 mM.

CONCLUSION

The proposed MRSI technique is a reliable technique to map sodium's absolute concentration and T 2 ∗ within a clinically acceptable scan time at 3T.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Prospective frequency correction using outer volume suppression-localized navigator for MR spectroscopy and spectroscopic imaging

PURPOSE

New frequency correction methods are required to achieve the accurate measurement of frequency drifts in MRS and MRSI. We present a prospective frequency correction method with outer volume suppression (OVS)-based localization and selective water excitation for effective frequency correction with better SNR improvement compared to other techniques.

METHODS

An OVS-localized navigator was developed to prospectively correct frequency drifts during MRS and MRSI measurements. The performance of the navigator was tested on the human brain and a solution phantom for frequency drifts induced by head motion or gradient heating by a preceding DWI experiment at 3T.

RESULTS

The OVS-localized navigator could accurately track motion-induced frequency drifts with an RMS error of 0.5 Hz. The SNR of MRS signals was not affected by use of the OVS-localized navigator when compared with and without the navigator (P > 0.05). The frequency drifts induced by DWI experiments were 5.1 ± 0.3 Hz/min during MRSI measurements on humans, resulting in increased spectral linewidth, significant bias in metabolite concentrations, and significantly increased Cramér-Rao lower bounds (P < 0.05). After prospective frequency corrections, the quality of MRSI was recovered to the level of those without any DWI-induced frequency drifts, judged by the spectral linewidth, metabolite concentrations, and Cramér-Rao lower bounds.

CONCLUSION

The OVS-localized navigator demonstrated effective prospective frequency corrections for large frequency drifts (5 Hz/min) without introducing any saturation-induced SNR loss. These benefits can be particularly beneficial for the acquisition of MRS signals with long T₁ and/or short TR, and spectral editing.

Magnetic Resonance Spectroscopy Studies of Mouse Models of Cancer

Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) enables the detection of metabolites, amino acids, and lipids, among other biomolecules, in tumors of live mouse models of cancer. Tumor-bearing mice are anesthetized by breathing isoflurane in a magnetic resonance (MR) scanner dedicated to small animal MR. Here we describe the overall setup and steps for measuring ¹H and ³¹P MRS and ¹H MRSI of orthotopic breast tumor models in mice with surface coils. This protocol can be adapted to the use of volume coils to measure ¹H and ³¹P MRS(I) of tumor models that grow inside the body. We address issues of animal handling, setting up the measurement, measurement options, and data analysis.

Comparison of direct 13C and indirect 1H-[13C] MR detection methods for the study of dynamic metabolic turnover in the human brain

A wide range of direct ¹³C and indirect ¹H-[¹³C] MR detection methods exist to probe dynamic metabolic pathways in the human brain. Choosing an optimal detection method is difficult as sequence-specific features regarding spatial localization, broadband decoupling, spectral resolution, power requirements and sensitivity complicate a straightforward comparison. Here we combine density matrix simulations with experimentally determined values for intrinsic ¹H and ¹³C sensitivity, T₁ and T₂ relaxation and transmit efficiency to allow selection of an optimal ¹³C MR detection method for a given application and magnetic field. The indirect proton-observed, carbon-edited (POCE) detection method provides the highest accuracy at reasonable RF power deposition both at 4T and 7T. The various polarization transfer methods all have comparable performances, but may become infeasible at 7T due to the high RF power deposition. 2D MR methods have limited value for the metabolites considered (primarily glutamate, glutamine and γ-amino butyric acid (GABA)), but may prove valuable when additional information can be extracted, such as isotopomers or lipid composition. While providing the lowest accuracy, the detection of non-protonated carbons is the simplest to implement with the lowest RF power deposition. The magnetic field homogeneity is one of the most important parameters affecting the detection accuracy for all metabolites and all acquisition methods.

In vivo N-15 MRS study of glutamate metabolism in the rat brain

In vivo ¹⁵N MRS has made a unique contribution to kinetic studies of the individual pathways that control glutamate flux in the rat brain. This review covers the following topics: (1) the advantages and limitations of in vivo ¹⁵N MRS and its indirect detection through coupled ¹H; (2) kinetic methods; (3) major findings from our and other laboratories in the areas: (a) the uptake of the neurotransmitter glutamate from the extracellular fluid into glia; (b) the metabolism of glutamate to glutamine; (c) glutamine transport to the extracellular fluid; (d) hydrolysis of neuronal glutamine to glutamate; and (e) contribution of transamination from leucine to replenish the glutamate nitrogen. In vivo glutamine synthetase activities measured at several levels of hyperammonemia showed that this enzyme becomes saturated at blood ammonia concentration >0.9 μmol/g, and causes the elevation of brain ammonia. Implications of the results for the cause of hyperammonemic encephalopathy are discussed. Leucine provides >25% of glutamate nitrogen. An intriguing possibility that supplementing leucine may restore cognitive function after brain injury is discussed. Finally, some characteristics of ¹⁵N MRS that may facilitate the future application of this technique to the study of the human brain at 4 or 7 T are described.

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.

1H/31P polarization transfer at 9.4 Tesla for improved specificity of detecting phosphomonoesters and phosphodiesters in breast tumor models

PURPOSE

To assess the ability of a polarization transfer (PT) magnetic resonance spectroscopy (MRS) technique to improve the detection of the individual phospholipid metabolites phosphocholine (PC), phosphoethanolamine (PE), glycerophosphocholine (GPC), and glycerophosphoethanolamine (GPE) in vivo in breast tumor xenografts.

MATERIALS AND METHODS

The adiabatic version of refocused insensitive nuclei enhanced by polarization transfer (BINEPT) MRS was tested at 9.4 Tesla in phantoms and animal models. BINEPT and pulse-acquire (PA) 31P MRS was acquired consecutively from the same orthotopic MCF-7 (n = 10) and MDA-MB-231 (n = 10) breast tumor xenografts. After in vivo MRS measurements, animals were euthanized, tumors were extracted and high resolution (HR)-MRS was performed. Signal to noise ratios (SNRs) and metabolite ratios were compared for BINEPT and PA MRS, and were also measured and compared with that from HR-MRS.

RESULTS

BINEPT exclusively detected metabolites with 1H-31P coupling such as PC, PE, GPC, and GPE, thereby creating a significantly improved, flat baseline because overlapping resonances from immobile and partly mobile phospholipids were removed without loss of sensitivity. GPE and GPC were more accurately detected by BINEPT in vivo, which enabled a reliable quantification of metabolite ratios such as PE/GPE and PC/GPC, which are important markers of tumor aggressiveness and treatment response.

CONCLUSION

BINEPT is advantageous over PA for detecting and quantifying the individual phospholipid metabolites PC, PE, GPC, and GPE in vivo at high magnetic field strength. As BINEPT can be used clinically, alterations in these phospholipid metabolites can be assessed in vivo for cancer diagnosis and treatment monitoring.

Silencing of the glycerophosphocholine phosphodiesterase GDPD5 alters the phospholipid metabolite profile in a breast cancer model in vivo as monitored by (31) P MRS

Abnormal choline phospholipid metabolism is an emerging hallmark of cancer, which is implicated in carcinogenesis and tumor progression. The malignant metabolic phenotype is characterized by high levels of phosphocholine (PC) and relatively low levels of glycerophosphocholine (GPC) in aggressive breast cancer cells. Phosphorus ((31) P) MRS is able to non-invasively detect these water-soluble metabolites of choline as well as ethanolamine phospholipid metabolism. Here we have investigated the effects of stably silencing glycerophosphoester diesterase domain containing 5 (GDPD5), which is an enzyme with glycerophosphocholine phosphodiesterase activity, in MDA-MB-231 breast cancer cells and orthotopic tumor xenografts. Tumors in which GDPD5 was stably silenced with GDPD5-specific shRNA contained increased levels of GPC and phosphoethanolamine (PE) compared with control tumors.

Glutamate and glutamine: a review of in vivo MRS in the human brain

Our understanding of the roles that the amino acids glutamate (Glu) and glutamine (Gln) play in the mammalian central nervous system has increased rapidly in recent times. Many conditions are known to exhibit a disturbance in Glu-Gln equilibrium, and the exact relationships between these changed conditions and these amino acids are not fully understood. This has led to increased interest in Glu/Gln quantitation in the human brain in an array of conditions (e.g. mental illness, tumor, neuro-degeneration) as well as in normal brain function. Accordingly, this review has been undertaken to describe the increasing number of in vivo techniques available to study Glu and Gln separately, or pooled as 'Glx'. The present MRS methods used to assess Glu and Gln vary in approach, complexity, and outcome, thus the focus of this review is on a description of MRS acquisition approaches, and an indication of relative utility of each technique rather than brain pathologies associated with Glu and/or Gln perturbation. Consequently, this review focuses particularly on (1) one-dimensional (1)H MRS, (2) two-dimensional (1)H MRS, and (3) one-dimensional (13)C MRS techniques.

Cerebral glutamine metabolism under hyperammonemia determined in vivo by localized (1)H and (15)N NMR spectroscopy

Brain glutamine synthetase (GS) is an integral part of the glutamate-glutamine cycle and occurs in the glial compartment. In vivo Magnetic Resonance Spectroscopy (MRS) allows noninvasive measurements of the concentrations and synthesis rates of metabolites. (15)N MRS is an alternative approach to (13)C MRS. Incorporation of labeled (15)N from ammonia in cerebral glutamine allows to measure several metabolic reactions related to nitrogen metabolism, including the glutamate-glutamine cycle. To measure (15)N incorporation into the position 5N of glutamine and position 2N of glutamate and glutamine, we developed a novel (15)N pulse sequence to simultaneously detect, for the first time, [5-(15)N]Gln and [2-(15)N]Gln+Glu in vivo in the rat brain. In addition, we also measured for the first time in the same experiment localized (1)H spectra for a direct measurement of the net glutamine accumulation. Mathematical modeling of (1)H and (15)N MRS data allowed to reduce the number of assumptions and provided reliable determination of GS (0.30±0.050 μmol/g per minute), apparent neurotransmission (0.26±0.030 μmol/g per minute), glutamate dehydrogenase (0.029±0.002 μmol/g per minute), and net glutamine accumulation (0.033±0.001 μmol/g per minute). These results showed an increase of GS and net glutamine accumulation under hyperammonemia, supporting the concept of their implication in cerebral ammonia detoxification.

Quantitative 31P magnetic resonance spectroscopy of the human breast at 7 T

This study presents quantified levels of phosphorylated metabolites in glandular tissue of human breast using (31)P magnetic resonance spectroscopy at 7 T. We used a homebuilt (1)H/(31)P radiofrequency coil to obtain artifact-free (31)P MR spectra of glandular tissue of healthy females by deploying whole breast free induction decay (FID) detection with adiabatic excitation and outer volume suppression. Using progressive saturation, the estimated apparent T(1) relaxation time of (31)P spins of phosphocholine and phosphoethanolamine was 4.4 and 5.7 s, respectively. Quantitative measures for phosphocholine and phosphoethanolamine levels in glandular tissue were established based on MR imaging. We used a 3D (1)H image of the breast to segment the glandular tissue; this was matched to a 3D (31)P image of the B1- field of the (31)P coil to correct for differences in glandular tissue volume and B(1) inhomogeneity of the (31)P coil. The (31)P MR spectra were calibrated using a phantom with known concentration. Average levels of phosphocholine and phosphoethanolamine in 11 volunteers were 0.84 ± 0.21 mM and 1.18 ± 0.41 mM, respectively. In addition, data of three patients with breast cancer showed higher levels of phosphocholine and phosphoethanolamine compared with healthy volunteers. This may indicate a potential role for the use of (31)P magnetic resonance spectroscopy for characterization, prognosis, and treatment monitoring in breast cancer.

Localized in vivo hyperpolarization transfer sequences

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.

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.

Combination of two fat saturation pulses improves detectability of glucose signals in carbon-13 MR spectroscopy

In order to improve the fat suppression performance of in vivo (13)C-MRS operating at 3.0 Tesla, a phantom model study was conducted using a combination of two fat suppression techniques; a set of pulses for frequency (chemical shift) selective suppression (CHESS), and spatial saturation (SAT). By optimizing the slab thickness for SAT and the irradiation bandwidth for CHESS, the signals of the -(13)CH(3) peak at 49 ppm and the -(13)CH(2)- peak at 26 ppm simulating fat components were suppressed to 5% and 19%, respectively. Combination of these two fat suppression pulses achieved a 53% increase of the height ratio of the glucose C1β peak compared with the sum of all other peaks, indicating better sensitivity for glucose signal detection. This method will be applicable for in vivo (13)C-MRS by additional adjustment with the in vivo relaxation times of the metabolites.

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

We have recently shown that at isotopic steady state (13)C 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 C1 using [1-(13)C] glucose administration, [1,6-(13)C(2)] glucose was infused such that isotopic steady state was maintained at the C1 position, but the C6 position reflected (13)C 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 C1 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 alpha-chloralose anesthetized rat was 0.7 micromol/g/h.

In vivo 13C magnetic resonance spectroscopy of human brain on a clinical 3 T scanner using [2-13C]glucose infusion and low-power stochastic decoupling

This study presents the detection of [2-(13)C]glucose metabolism in the carboxylic/amide region in the human brain, and demonstrates that the cerebral metabolism of [2-(13)C]glucose can be studied in human subjects in the presence of severe hardware constraints of widely available 3 T clinical scanners and with low-power stochastic decoupling. In the carboxylic/amide region of human brain, the primary products of (13)C label incorporation from [2-(13)C]glucose into glutamate, glutamine, aspartate, gamma-aminobutyric acid, and N-acetylaspartate were detected. Unlike the commonly used alkanyl region where lipid signals spread over a broad frequency range, the carboxylic carbon signal of lipids was found to be confined to a narrow range centered at 172.5 ppm and present no spectral interference in the absence of lipid suppression. Comparison using phantoms shows that stochastic decoupling is far superior to the commonly used WALTZ sequence at very low decoupling power at 3 T. It was found that glutamine C1 and C5 can be decoupled using stochastic decoupling at 2.2 W, although glutamine protons span a frequency range of approximately 700 Hz. Detailed specific absorption rate analysis was also performed using finite difference time domain numerical simulation.

Slice-selective FID acquisition, localized by outer volume suppression (FIDLOVS) for (1)H-MRSI of the human brain at 7 T with minimal signal loss

In comparison to 1.5 and 3 T, MR spectroscopic imaging at 7 T benefits from signal-to-noise ratio (SNR) gain and increased spectral resolution and should enable mapping of a large number of metabolites at high spatial resolutions. However, to take full advantage of the ultra-high field strength, severe technical challenges, e.g. related to very short T(2) relaxation times and strict limitations on the maximum achievable B(1) field strength, have to be resolved. The latter results in a considerable decrease in bandwidth for conventional amplitude modulated radio frequency pulses (RF-pulses) and thus to an undesirably large chemical-shift displacement artefact. Frequency-modulated RF-pulses can overcome this problem; but to achieve a sufficient bandwidth, long pulse durations are required that lead to undesirably long echo-times in the presence of short T(2) relaxation times. In this work, a new magnetic resonance spectroscopic imaging (MRSI) localization scheme (free induction decay acquisition localized by outer volume suppression, FIDLOVS) is introduced that enables MRSI data acquisition with minimal SNR loss due to T(2) relaxation and thus for the first time mapping of an extended neurochemical profile in the human brain at 7 T. To overcome the contradictory problems of short T(2) relaxation times and long pulse durations, the free induction decay (FID) is directly acquired after slice-selective excitation. Localization in the second and third dimension and skull lipid suppression are based on a T(1)- and B(1)-insensitive outer volume suppression (OVS) sequence. Broadband frequency-modulated excitation and saturation pulses enable a minimization of the chemical-shift displacement artefact in the presence of strict limits on the maximum B(1) field strength. The variable power RF pulses with optimized relaxation delays (VAPOR) water suppression scheme, which is interleaved with OVS pulses, eliminates modulation side bands and strong baseline distortions. Third order shimming is based on the accelerated projection-based automatic shimming routine (FASTERMAP) algorithm. The striking SNR and spectral resolution enable unambiguous quantification and mapping of 12 metabolites including glutamate (Glu), glutamine (Gln), N-acetyl-aspartatyl-glutamate (NAAG), gamma-aminobutyric acid (GABA) and glutathione (GSH). The high SNR is also the basis for highly spatially resolved metabolite mapping.

Non-invasive quantification of brain glycogen absolute concentration

The only currently available method to measure brain glycogen in vivo is 13C NMR spectroscopy. Incorporation of 13C-labeled glucose (Glc) is necessary to allow glycogen measurement, but might be affected by turnover changes. Our aim was to measure glycogen absolute concentration in the rat brain by eliminating label turnover as variable. The approach is based on establishing an increased, constant 13C isotopic enrichment (IE). 13C-Glc infusion is then performed at the IE of brain glycogen. As glycogen IE cannot be assessed in vivo, we validated that it can be inferred from that of N-acetyl-aspartate IE in vivo: After [1-13C]-Glc ingestion, glycogen IE was 2.2 +/- 0.1 fold that of N-acetyl-aspartate (n = 11, R(2) = 0.77). After subsequent Glc infusion, glycogen IE equaled brain Glc IE (n = 6, paired t-test, p = 0.37), implying isotopic steady-state achievement and complete turnover of the glycogen molecule. Glycogen concentration measured in vivo by 13C NMR (mean +/- SD: 5.8 +/- 0.7 micromol/g) was in excellent agreement with that in vitro (6.4 +/- 0.6 micromol/g, n = 5). When insulin was administered, the stability of glycogen concentration was analogous to previous biochemical measurements implying that glycogen turnover is activated by insulin. We conclude that the entire glycogen molecule is turned over and that insulin activates glycogen turnover.

Chemical-shift artifact reduction in Hadamard-encoded MR spectroscopic imaging at high (3T and 7T) magnetic fields

Proton MR spectroscopic imaging (MRSI) at higher magnetic fields (B(0)) suffers metabolite localization errors from different chemical-shift displacements (CSDs) if spatially-selective excitation is used. This phenomenon is exacerbated by the decreasing radiofrequency (RF) field strength, B(1), at higher B(0)s, precluding its suppression with stronger gradients. To address this, two new methods are proposed: 1) segmenting the volume-of-interest (VOI) into several slabs, allowing proportionally stronger slice-select gradients; and 2) sequentially cascading rather than superposing the components of the Hadamard selective pulses used for reasons of better point-spread function (PSF) to localize the few slices within each slab. This can reduce the peak B(1) to that of a single slice. Combining these approaches permits us to increase the selective gradient four- to eightfold per given B(1), to 12 or 18mT/m for 4- or 2-cm VOIs. This "brute force" approach reduces the CSD to under 0.05 cm/ppm at 7T, or less than half that at 3T.

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

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.

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

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.

Effect of chronic hypoglycaemia on glucose concentration and glycogen content in rat brain: A localized 13C NMR study

While chronic hypoglycaemia has been reported to increase unidirectional glucose transport across the blood-brain barrier (BBB) and to increase GLUT1 expression at the endothelium, the effect on steady-state brain d-glucose and brain glycogen content is currently unknown. Brain glucose and glycogen concentrations were directly measured in vivo using localized 13C magnetic resonance spectroscopy (MRS) following 12-14 days of hypoglycaemia. Brain glucose content was significantly increased by 48%, which is consistent with an increase in the maximal glucose transport rate, Tmax, by 58% compared with the sham-treated animals. The localized 13C NMR measurements of brain glucose were directly validated by comparison with biochemically determined brain glucose content after rapid focused microwave fixation (1.4 s at 4 kW). Both in vivo MRS and biochemical measurements implied that brain glycogen content was not affected by chronic hypoglycaemia, consistent with brain glucose being a major factor controlling brain glycogen content. We conclude that the increased glucose transporter expression in chronic hypoglycaemia leads to increased brain glucose content at a given level of glycaemia. Such increased brain glucose concentrations can result in a lowered glycaemic threshold of counter-regulation observed in chronic hypoglycaemia.

In vivo detection of gray and white matter differences in GABA concentration in the human brain

A novel selective multiple quantum filtering-based chemical shift imaging method was developed for acquiring GABA images in the human brain at 3 T. This method allows a concomitant acquisition of an interleaved total creatine image with the same spatial resolution. Using T(1)-based image segmentation and a nonlinear least square regression analysis of GABA-to-total creatine concentration ratios in frontal and parietal lobes of healthy adult volunteers as a function of the tissue gray matter fraction, the mean GABA concentration in gray and white matter was determined to be 1.30+/-0.36 micromol/g and 0.16+/-0.16 micromol/g (mean+/-SD, n=13), respectively. It is expected that this method will become a useful tool for studying GABAergic function in the human brain in vivo.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Neuronal-glial glucose oxidation and glutamatergic-GABAergic function

Prior 13C magnetic resonance spectroscopy (MRS) experiments, which simultaneously measured in vivo rates of total glutamate-glutamine cycling (V(cyc(tot))) and neuronal glucose oxidation (CMR(glc(ox), N)), revealed a linear relationship between these fluxes above isoelectricity, with a slope of approximately 1. In vitro glial culture studies examining glutamate uptake indicated that glutamate, which is cotransported with Na+, stimulated glial uptake of glucose and release of lactate. These in vivo and in vitro results were consolidated into a model: recycling of one molecule of neurotransmitter between glia and neurons was associated with oxidation of one glucose molecule in neurons; however, the glucose was taken up only by glia and all the lactate (pyruvate) generated by glial glycolysis was transferred to neurons for oxidation. The model was consistent with the 1:1 relationship between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) measured by 13C MRS. However, the model could not specify the energetics of glia and gamma-amino butyric acid (GABA) neurons because quantitative values for these pathways were not available. Here, we review recent 13C and 14C tracer studies that enable us to include these fluxes in a more comprehensive model. The revised model shows that glia produce at least 8% of total oxidative ATP and GABAergic neurons generate approximately 18% of total oxidative ATP in neurons. Neurons produce at least 88% of total oxidative ATP, and take up approximately 26% of the total glucose oxidized. Glial lactate (pyruvate) still makes the major contribution to neuronal oxidation, but approximately 30% less than predicted by the prior model. The relationship observed between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) is determined by glial glycolytic ATP as before. Quantitative aspects of the model, which can be tested by experimentation, are discussed.

Biochemical quantification of total brain glycogen concentration in rats under different glycemic states

All (13)C NMR studies of brain glycogen to date relied on observing the incorporation of (13)C label into glycogen, and thus interpretation was potentially affected by changes in (13)C label turnover rates. The goal of this study was to quantify total brain glycogen concentration under conditions of hypoglycemia or normoglycemia using biochemical methods. Rats were sacrificed using a focused microwave fixation device. The results showed that metabolism of brain glycogen was Glc- and insulin-sensitive and that insulin-induced hypoglycemia promoted a gradual glycogenolysis. Moreover, we show that there are very mild effects of isoflurane and alpha-chloralose anesthesia on brain glycogen concentration. Altogether these results show that total brain glycogen serves as a substantial source of glucosyl units during insulin-induced moderate hypoglycemia and therefore may be neuroprotective. Finally we also conclude that previous interpretation of (13)C NMR spectroscopy data accurately reflected the changes in total brain glycogen content.

Sensitivity-enhanced13C MR spectroscopy of the human brain at 3 Tesla

A new coil design for sensitivity-enhanced 13C MR spectroscopy (MRS) of the human brain is presented. The design includes a quadrature transmit/receive head coil optimized for 13C MR sensitivity. Loss-less blocking circuits inside the coil conductors allow this coil to be used inside a homogeneous circularly polarized 1H B1 field for 1H decoupled 13C MRS. A quadrature 1H birdcage coil optimized for minimal local RF heating makes broadband 1H decoupling in the entire human brain possible at 3 Tesla while remaining well within international safety guidelines for RF absorption. Apart from a substantial increase in sensitivity compared to conventional small linear coils, the quadrature 13C coil combined with the quadrature 1H birdcage coil allows efficient cross polarization (CP) in the brain, resulting in an additional 3.5-fold sensitivity improvement compared to direct 13C measurements without nuclear Overhauser enhancement (NOE) or polarization transfer. Combined with the gain in power efficiency, this setup allows broadband 1H to 13C CP over large areas of the brain. Clear 13C resonances from glutamate (Glu), glutamine (Gln), aspartate (Asp), lactate (Lac), and gamma-aminobutyrate (GABA) carbon spins in the human brain demonstrate the quality of 13C MR spectra obtained in vivo with this coil setup.

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

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).

1H-localized broadband 13C NMR spectroscopy of the rat brain in vivo at 9.4 T

Localized (13)C NMR spectra were obtained from the rat brain in vivo over a broad spectral range (15-100 ppm) with minimal chemical-shift displacement error (<10%) using semi-adiabatic distortionless enhancement by polarization transfer (DEPT) combined with (1)H localization. A new gradient dephasing scheme was employed to eliminate unwanted coherences generated by DEPT when using surface coils with highly inhomogeneous B(1) fields. Excellent sensitivity was evident from the simultaneous detection of natural abundance signals for N-acetylaspartate, myo-inositol, and glutamate in the rat brain in vivo at 9.4 T. After infusion of (13)C-labeled glucose, up to 18 (13)C resonances were simultaneously measured in the rat brain, including glutamate C2, C3, C4, glutamine C2, C3, C4, aspartate C2, C3, glucose C1, C6, N-acetyl-aspartate C2, C3, C6, as well as GABA C2, lactate C3, and alanine C3. (13)C-(13)C multiplets corresponding to multiply labeled compounds were clearly observed, suggesting that extensive isotopomer analysis is possible in vivo. This unprecedented amount of information will be useful for metabolic modeling studies aimed at understanding brain energy metabolism and neurotransmission in the rodent brain.

Single-shot two-echo technique for simultaneous measurement of GABA and creatine in the human brain in vivo

A single-shot, two-echo method for the simultaneous detection of multiple-quantum (MQ)-filtered gamma-aminobutyric acid (GABA) and creatine (Cr) was developed and demonstrated in the human brain in vivo at 3 Tesla. The simultaneously measured Cr singlet served as a navigator for the spectral phase of GABA and any frequency shift during measurements due to drift in the static magnetic field (B(0)) or subject movement, as well as an internal concentration reference. In addition, the use of a double-band frequency-selective MQ filter for C(3) and C(4) methylene protons of GABA provided a very robust measurement of GABA, with excellent suppression of overlapping metabolites such as Cr and glutathione (GSH) in each single scan. Contamination from overlapping macromolecules was also demonstrated to be negligible with this method. The GABA-to-Cr ratio was 0.09 +/- 0.03 (mean +/- SD, N = 17) and the estimated concentration of GABA in the frontoparietal region of the human brain in vivo was 0.66 +/- 0.19 micromol/g (mean +/- SD, N = 17) with the internal reference method, and 0.69 +/- 0.18 micromol/g (mean +/- SD, N = 17) with the external reference method. The observed pattern of GABA doublet was consistent among all subjects, with a frequency separation of approximately 13 Hz.

Glycogen: The forgotten cerebral energy store

The brain contains a significant amount of glycogen that is an order of magnitude smaller than that in muscle, but several-fold higher than the cerebral glucose content. Although the precise role of brain glycogen to date is unknown, it seems affected by focal activation, neurotransmitters, and overall electrical activity and hormones. Based on its relatively low concentration, the role of brain glycogen as a significant energy store has been discounted. This work reviews recent experimental evidence that brain glycogen is an important reserve of glucose equivalents: (1) glial glycogen can provide the majority of the glucose supply deficit during hypoglycemia for more than 100 min, consistent with the proposal that glial lactate is a fuel for neurons; (2) glycogen concentrations may be as high as 10 micromol/g, substantially higher than was thought previously; (3) glucose cycling in and out of glycogen amounts to approximately 1% of the cerebral metabolic rate of glucose (CMRglc) in human and rat brain, amounting to an effective stability of glycogen in the resting awake brain during euglycemia and hyperglycemia, (4) brain glycogen metabolism/concentrations are insulin/glucose sensitive; and (5) after a single episode of hypoglycemia, brain glycogen levels rebound to levels that exceed the pre-hypoglycemic concentrations (supercompensation). This experimental evidence supports the proposal that brain glycogen may be involved in the development of diabetes complications, specifically impaired glucose sensing (hypoglycemia unawareness) observed clinically in some diabetes patients under insulin treatment. It is proposed further that brain glycogen becomes important in any metabolic state where supply transiently cannot meet demand, such conditions that could occur during prolonged focal activation, sleep deprivation, seizures, and mild hypoxia.

In vivo 13C NMR assessment of brain glycogen concentration and turnover in the awake rat

Brain glycogen metabolism was recently observed in vivo and found to be very slow in the lightly alpha-chloralose anesthetized rat [J. Neurochem. 73 (1999) 1300]. Based on that slow turnover, the total glycogen content in the awake rat brain and its turnover time were assessed after administering 13C-labeled glucose for 48 h. Label incorporation into glycogen, glucose, amino acid, and N-acetyl-aspartate (NAA) resonances was observed. The amount of 13C label incorporated into glycogen was variable and did not correlate with that in glutamate (r=-0.1, P>0.86). However, the amount of 13C label incorporated into glycogen was very similar to that in NAA (r=0.93), implying similar turnover times between brain glycogen and NAA (approximately 10 h). Absolute quantification of the total concentration of brain glycogen in the awake, normoglycemic rat yielded 3.3+/-0.8 micromol/g (n=6, mean+/-S.D.).

Direct, noninvasive measurement of brain glycogen metabolism in humans

The concentration and metabolism of the primary carbohydrate store in the brain, glycogen, is unknown in the conscious human brain. This study reports the first direct detection and measurement of glycogen metabolism in the human brain, which was achieved using localized 13C NMR spectroscopy. To enhance the NMR signal, the isotopic enrichment of the glucosyl moieties was increased by administration of 80 g of 99% enriched [1-13C]glucose in four subjects. 3 h after the start of the label administration, the 13C NMR signal of brain glycogen C1 was detected (0.36+/-0.07 micromol/g, mean+/-S.D., n=4). Based on the rate of 13C label incorporation into glycogen and the isotopic enrichment of plasma glucose, the flux through glycogen synthase was estimated at 0.17+/-0.05 micromol/(gh). This study establishes that brain glycogen can be measured in humans and indicates that its metabolism is very slow in the conscious human. The noninvasive detection of human brain glycogen opens the prospect of understanding the role and function of this important energy reserve under various physiological and pathophysiological conditions.

In vivo NMR studies of neurodegenerative diseases in transgenic and rodent models

In vivo magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide unique quality to attain neurochemical, physiological, anatomical, and functional information non-invasively. These techniques have been increasingly applied to biomedical research and clinical usage in diagnosis and prognosis of diseases. The ability of MRS to detect early yet subtle changes of neurochemicals in vivo permits the use of this technology for the study of cerebral metabolism in physiological and pathological conditions. Recent advances in MR technology have further extended its use to assess the etiology and progression of neurodegeneration. This review focuses on the current technical advances and the applications of MRS and MRI in the study of neurodegenerative disease animal models including amyotrophic lateral sclerosis, Alzheimer's, Huntington's, and Parkinson's diseases. Enhanced MR measurable neurochemical parameters in vivo are described in regard to their importance in neurodegenerative disorders and their investigation into the metabolic alterations accompanying the pathogenesis of neurodegeneration.

Effect of hypoglycemia on brain glycogen metabolism in vivo

The brain contains a small but significant amount of glycogen, which has long been considered to play an insignificant role in the brain. In this study, brain glycogen metabolism was measured using (13)C NMR spectroscopy at 9.4 T. Brain glycogen metabolism was modulated by hyperinsulinemia resulting in a net accumulation. The role of glycogen in maintaining brain function is unknown; one possibility is that it may serve as an endogenous glucose reservoir to protect the brain against severe hypoglycemia. To address this possibility, rats were subjected to insulin-induced moderate hypoglycemia and when the level of brain glucose approached zero, brain glycogen content began to decrease gradually, demonstrating utilization of this glucose reservoir. The brain glycogen signal never became undetectable, however, even during 2 hr of hypoglycemia. When plasma and brain glucose concentrations were restored, glycogen increased and the concentration exceeded the pre-hypoglycemic level by several-fold. The data suggest that brain glycogen can provide fuel for extended periods of time when glucose supply is inadequate. Furthermore, brain glycogen can rebound (super-compensate) after a single episode of hypoglycemia. We postulate that brain glycogen serves as an energy store during hypoglycemia and that it may participate in the creation of reduced physiological responses to hypoglycemia that are involved in a symptom often observed in patients with diabetes, hypoglycemia unawareness.

Elucidation of the role of fructose 2,6-bisphosphate in the regulation of glucose fluxes in mice using in vivo (13)C NMR measurements of hepatic carbohydrate metabolism

Fructose 2,6-bisphosphate (Fru-2,6-P2) plays an important role in the regulation of major carbohydrate fluxes as both allosteric activator and inhibitor of target enzymes. To examine the role of Fru-2,6-P2 in the regulation of hepatic carbohydrate metabolism in vivo, Fru-2,6-P2 levels were elevated in ADM mice with adenovirus-mediated overexpression of a double mutant bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (n = 6), in comparison to normal control mice (control, n = 6). The rates of hepatic glycogen synthesis in the ADM and control mouse liver in vivo were measured using new advances in 13C NMR including 3D localization in conjunction with [1-13C]glucose infusion. In addition to glycogen C1, the C6 and C2-C5 signals were measured simultaneously for the first time in vivo, which provide the basis for the estimation of direct and indirect synthesis of glycogen in the liver. The rate of label incorporation into glycogen C1 was not different between the control and ADM group, whereas the rate of label incorporation into glycogen C6 signals was in the ADM group 5.6 +/- 0.5 micro mol.g-1.h-1, which was higher than that of the control group of 3.7 +/- 0.5 micro mol.g-1.h-1 (P < 0.02). The rates of net glycogen synthesis, determined by the glycogen C2-C5 signal changes, were twofold higher in the ADM group (P = 0.04). The results provide direct in vivo evidence that the effects of elevated Fru-2,6-P2 levels in the liver include increased glycogen storage through indirect synthesis of glycogen. These observations provide a key to understanding the mechanisms by which elevated hepatic Fru-2,6-P2 levels promote reduced hepatic glucose production and lower blood glucose in diabetes mellitus.

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

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.

In vivo 13C NMR studies of compartmentalized cerebral carbohydrate metabolism

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.

The first in vivo observation of (13)C-(15)N coupling in mammalian brain

[5-(13)C,(15)N]Glutamine, with (1)J((13)C-(15)N) of 16 Hz, was observed in vivo in the brain of spontaneously breathing rats by (13)C MRS at 4.7 T. The brain [5-(13)C]glutamine peak consisted of the doublet from [5-(13)C,(15)N]glutamine and the center [5-(13)C,(14)N]glutamine peak, resulting in an apparent triplet with a separation of 8 Hz. The time course of formation of brain [5-(13)C,(15)N]glutamine was monitored in vivo with a time resolution of 20-35 min. This [5-(13)C,(15)N]glutamine was formed by glial uptake of released neurotransmitter [5-(13)C]glutamate and its reaction with (15)NH(3) catalyzed by the glia-specific glutamine synthetase. The neurotransmitter glutamate C5 was selectively (13)C-enriched by intravenous [2,5-(13)C]glucose infusion to (13)C-label whole-brain glutamate C5, followed by [(12)C]glucose infusion to chase (13)C from the small and rapidly turning-over glial glutamate pool, leaving (13)C mainly in the neurotransmitter [5-(13)C]glutamate pool, which is sequestered in vesicles until release. Hence, the observed [5-(13)C,(15)N]glutamine arises from a coupling between (13)C of neuronal origin and (15)N of glial origin. Measurement of the rate of brain [5-(13)C,(15)N]glutamine formation provides a novel noninvasive method of studying the kinetics of neurotransmitter uptake into glia in vivo, a process that is crucial for protecting the brain from glutamate excitotoxicity.

In vivo measurements of brain glucose transport using the reversible Michaelis-Menten model and simultaneous measurements of cerebral blood flow changes during hypoglycemia

Glucose is the major substrate that sustains normal brain function. When the brain glucose concentration approaches zero, glucose transport across the blood-brain barrier becomes rate limiting for metabolism during, for example, increased metabolic activity and hypoglycemia. Steady-state brain glucose concentrations in alpha-chloralose anesthetized rats were measured noninvasively as a function of plasma glucose. The relation between brain and plasma glucose was linear at 4.5 to 30 mmol/L plasma glucose, which is consistent with the reversible Michaelis-Menten model. When the model was fitted to the brain glucose measurements, the apparent Michaelis-Menten constant, Kt, was 3.3 +/- 1.0 mmol/L, and the ratio of the maximal transport rate relative to CMRglc, Tmax/CMRglc, was 2.7 +/- 0.1. This Kt is comparable to the authors' previous human data, suggesting that glucose transport kinetics in humans and rats are similar. Cerebral blood flow (CBF) was simultaneously assessed and constant above 2 mmol/L plasma glucose at 73 +/- 6 mL 100 g(-1) min(-1). Extrapolation of the reversible Michaelis-Menten model to hypoglycemia correctly predicted the plasma glucose concentration (2.1 +/- 0.6 mmol/L) at which brain glucose concentrations approached zero. At this point, CBF increased sharply by 57% +/- 22%, suggesting that brain glucose concentration is the signal that triggers defense mechanisms aimed at improving glucose delivery to the brain during hypoglycemia.