Detection of glutathione in the human brain in vivo by means of double quantum coherence filtering

In Vivo Electron Paramagnetic Resonance Molecular Profiling of Tumor Microenvironment upon Tumor Progression to Malignancy in an Animal Model of Breast Cancer

PURPOSE

Hypoxia and acidosis are recognized tumor microenvironment (TME) biomarkers of cancer progression. Alterations in cancer redox status and metabolism are also associated with elevated levels of intracellular glutathione (GSH) and interstitial inorganic phosphate (Pi). This study aims to evaluate the capability of these biomarkers to discriminate between stages and inform on a switch to malignancy.

PROCEDURES

These studies were performed using MMTV-PyMT( +) female transgenic mice that spontaneously develop breast cancer and emulate human tumor staging. In vivo assessment of oxygen concentration (pO2), extracellular acidity (pHe), Pi, and GSH was performed using L-band electron paramagnetic resonance spectroscopy and multifunctional trityl and GSH-sensitive nitroxide probes.

RESULTS

Profiling of the TME showed significant deviation of measured biomarkers upon tumor progression from pre-malignancy (pre-S4) to the malignant stage (S4). For the combined marker, HOP: (pHe × pO2)/Pi, a value > 186 indicated that the tumors were pre-malignant in 85% of the mammary glands analyzed, and when < 186, they were malignant 42% of the time. For GSH, a value < 3 mM indicated that the tumors were pre-malignant 74% of the time, and when > 3 mM, they were malignant 80% of the time. The only marker that markedly deviated as early as stage 1 (S1) from its value in pre-S1 was elevated Pi, followed by a decrease of pHe and pO2 and increase in GSH at later stages.

CONCLUSION

Molecular TME profiling informs on alteration of tumor redox and metabolism during tumor staging. Early elevation of interstitial Pi at S1 may reflect tumor metabolic alterations that demand elevated phosphorus supply in accordance with the high rate growth hypothesis. These metabolic changes are supported by the following decrease of pHe due to a high tumor reliance on glycolysis and increase of intracellular GSH, a major intracellular redox buffer. The appreciable decrease in TME pO2 was observed only at malignant S4, apparently as a consequence of tumor mass growth and corresponding decrease in perfusion efficacy and increase in oxygen consumption as the tumor cells proliferate.

KALPANA: Advanced Spectroscopic Signal Processing Platform for Improved Accuracy to Aid in Early Diagnosis of Brain Disorders in Clinical Setting

Magnetic resonance spectroscopy (MRS) plays a substantial role in the non-invasive detection of brain neurochemicals, antioxidants, and neurotransmitters. Quantitative monitoring of these neurochemicals and neurotransmitters in the brain has a profound application for the understanding of brain disorders. Significant progress in the MR scanner as well as MR pulse sequence development to detect in vivo neurochemicals has been accomplished. The processing of MR signal from these low abundant neurochemicals/neurotransmitters should be very robust and sensitive in order to provide distinctive observations of disease-related neurochemical alterations and their absolute quantitation to aid in early clinical diagnosis. We highlight the diversity in currently available MRS processing tools, and recently introduced, KALPANA, a promising package integrating the end-to-end processing as well as robust quantitation of neurochemicals in a user-friendly approach through a graphical user interface. This further necessitates the futuristic need for advanced MRS processing pipeline and the respective readout that can help in early diagnosis and prognosis of diseases in the clinical environment.

Spectral editing in 1 H magnetic resonance spectroscopy: Experts' consensus recommendations

Spectral editing in in vivo ¹ H-MRS provides an effective means to measure low-concentration metabolite signals that cannot be reliably measured by conventional MRS techniques due to signal overlap, for example, γ-aminobutyric acid, glutathione and D-2-hydroxyglutarate. Spectral editing strategies utilize known J-coupling relationships within the metabolite of interest to discriminate their resonances from overlying signals. This consensus recommendation paper provides a brief overview of commonly used homonuclear editing techniques and considerations for data acquisition, processing and quantification. Also, we have listed the experts' recommendations for minimum requirements to achieve adequate spectral editing and reliable quantification. These include selecting the right editing sequence, dealing with frequency drift, handling unwanted coedited resonances, spectral fitting of edited spectra, setting up multicenter clinical trials and recommending sequence parameters to be reported in publications.

1H magnetic resonance spectroscopy of 2H-to-1H exchange quantifies the dynamics of cellular metabolism in vivo

The quantitative mapping of the in vivo dynamics of cellular metabolism via non-invasive imaging contributes to the understanding of the initiation and progression of diseases associated with dysregulated metabolic processes. Current methods for imaging cellular metabolism are limited by low sensitivities, by costs, or by the use of specialized hardware. Here, we introduce a method that captures the turnover of cellular metabolites by quantifying signal reductions in proton magnetic resonance spectroscopy (MRS) resulting from the replacement of ¹H with ²H. The method, which we termed quantitative exchanged-label turnover MRS, only requires deuterium-labelled glucose and standard MRI scanners, and with a single acquisition provides steady-state information and metabolic rates for several metabolites. We used the method to monitor glutamate, glutamine, γ-aminobutyric acid and lactate in the brains of normal and glioma-bearing rats following the administration of ²H₂-labelled glucose and ²H₃-labelled acetate. Quantitative exchanged-label turnover MRS should broaden the applications of routine ¹H MRS.

Retrospective motion compensation for edited MR spectroscopic imaging

Edited magnetic resonance spectroscopic imaging (MRSI) is capable of mapping the distribution of low concentration metabolites such as gamma-aminobutyric acid (GABA) or and glutathione (GSH), but is prone to subtraction artifacts due to head motion or other instabilities. In this study, a retrospective motion compensation algorithm for edited MRSI is proposed. The algorithm identifies movement-affected signals by comparing residual water and lipid peaks between different transients recorded at the same point in k-space, and either phase corrects, replaces or removes affected spectra prior to spatial Fourier transformation. The method was tested on macromolecule-unsuppressed GABA-edited spin-echo MR spectroscopic imaging data acquired from 8 healthy adults scanned at 3T. Relative to non-motion compensated data sets, the motion compensated data had significantly less subtraction artifacts across subjects. The residual choline (Cho) peak in the spectrum (which is well resolved from as a different chemical shift from GABA and is completely absent in a spectrum without subtraction artifact) was used as a metric of motion artifact severity. The normalized Cho area was 5.14 times lower with motion compensation than without motion compensation. A 'removal-only' version of the technique is also shown to be promising in removing motion-corrupted artifacts in a GSH-edited MRSI acquisition acquired in 1 healthy subject. This study introduces a motion compensation technique and demonstrates that retrospective compensation in k-space is possible and significantly reduces the amount of subtraction artifacts in the resulting edited spectra.

Quantitation of in vivo brain glutathione conformers in cingulate cortex among age-matched control, MCI, and AD patients using MEGA-PRESS

Oxidative stress (OS) plays an important role in Alzheimer's disease (AD) and glutathione (GSH) mitigates this effect by maintaining redox-imbalance and free-radical neutralization. Quantified brain GSH concentration provides distinct information about OS among age-matched normal control (NC), mild cognitive impairment (MCI) and AD patients. We report alterations of in vivo GSH conformers, along with the choline, creatine, and N-acetylaspartate levels in the cingulate cortex (CC) containing anterior (ACC) and posterior (PCC) regions of 64 (27 NC, 19 MCI, and 18 AD) participants using MEscher-GArwood-Point-RESolved spectroscopy sequence. Result indicated, tissue corrected GSH depletion in PCC among MCI (p = .001) and AD (p = .028) and in ACC among MCI (p = .194) and AD (p = .025) as compared to NC. Effects of the group, region, and group × region on GSH with age and gender as covariates were analyzed using a generalized linear model with Bonferroni correction for multiple comparisons. A significant effect of group with GSH depletion in AD and MCI was observed as compared to NC. Receiver operator characteristic (ROC) analysis of GSH level in CC differentiated between MCI and NC groups with an accuracy of 82.8% and 73.5% between AD and NC groups. Multivariate ROC analysis for the combined effect of the GSH alteration in both ACC and PCC regions provided improved diagnostic accuracy of 86.6% for NC to MCI conversion and 76.4% for NC to AD conversion. We conclude that only closed GSH conformer depletion in the ACC and PCC regions is critical and constitute a potential biomarker for AD.

MEGA-PRESS and PRESS measure oxidation of glutathione in a phantom

PURPOSE

Investigate the possibility of measuring changes in glutathione (GSH) concentration using the MRS PRESS and MEGA-PRESS sequences by tracking the natural oxidation of GSH, and to examine the accuracy of the two methods.

METHODS

122 GSH edited MEGA-PRESS and PRESS acquisitions were acquired on a "braino" based phantom +3.0 mM GSH during a period of 11 days. All spectra were analyzed in LCModel. (The MEGA-PRESS data were first preprocessed in Matlab). Degradation curves were modeled. A one year follow-up on the same phantom and measurements from a similar phantom without GSH and one pure GSH phantom were also included.

RESULTS

Both MEGA-PRESS and PRESS showed degradation of the measured GSH signal. Modeling the exponential decay of the GSH signal in MEGA-PRESS and PRESS gave for t = 0; 2.9 i.u. for MEGA-PRESS and 2.3 i.u. for PRESS. As t increased, the GSH concentration converged to zero for MEGA-PRESS but not for PRESS (0.7 i.u.). GSH for the one year follow up were 0.0 i.u. for MEGA-PRESS and 0.6 i.u. for PRESS. Similar phantom without GSH yielded 0.0 i.u. for both MEGA-PRESS and PRESS.

CONCLUSION

It is possible to measure changes in GSH concentration in a phantom using both PRESS and MEGA-PRESS techniques, however the PRESS spectrum appears to include oxidized GSH (GSSG). In addition, GSH edited MEGA-PRESS measurement gives more precise values at lower GSH concentrations.

Intact metabolite spectrum mining by deep learning in proton magnetic resonance spectroscopy of the brain

PURPOSE

To develop a robust method for brain metabolite quantification in proton magnetic resonance spectroscopy (¹ H-MRS) using a convolutional neural network (CNN) that maps in vivo brain spectra that are typically degraded by low SNR, line broadening, and spectral baseline into noise-free, line-narrowed, baseline-removed intact metabolite spectra.

METHODS

A CNN was trained (n = 40 000) and tested (n = 5000) on simulated brain spectra with wide ranges of SNR (6.90-20.74) and linewidth (10-20 Hz). The CNN was further tested on in vivo spectra (n = 40) from five healthy volunteers with substantially different SNR, and the results were compared with those from the LCModel analysis. A Student t test was performed for the comparison.

RESULTS

Using the proposed method the mean-absolute-percent-errors (MAPEs) in the estimated metabolite concentrations were 12.49% ± 4.35% for aspartate, creatine (Cr), γ-aminobutyric acid (GABA), glucose, glutamine, glutamate, glutathione (GSH), myo-Inositol (mI), N-acetylaspartate, phosphocreatine (PCr), phosphorylethanolamine, and taurine over the whole simulated spectra in the test set. The metabolite concentrations estimated from in vivo spectra were close to the reported ranges for the proposed method and the LCModel analysis except mI, GSH, and especially Cr/PCr for the LCModel analysis, and phosphorylcholine to glycerophosphorylcholine ratio (PC/GPC) for both methods. The metabolite concentrations estimated across the in vivo spectra with different SNR were less variable with the proposed method (~10% or less) than with the LCModel analysis.

CONCLUSION

The robust performance of the proposed method against low SNR may allow a subminute ¹ H-MRS of human brain, which is an important technical development for clinical studies.

Non-invasive mapping of glutathione levels in mouse brains by in vivo electron paramagnetic resonance (EPR) imaging: Applied to a kindling mouse model

Glutathione (GSH) is an important antioxidant that can protect cells under oxidative stress. Thus, a non-invasive method to measure and map the distribution of GSH in live animals is needed. To image the distribution of GSH levels in specific brain regions, a new method using electron paramagnetic resonance (EPR) imaging with a nitroxide imaging probe was developed. Pixel-based mapping of brain GSH levels was successfully obtained by using the linear relationship between reduction rates for nitroxides in brains, measured by an in vivo EPR imager, and brain GSH levels, measured by an in vitro biochemical assay. The newly developed method was applied to a kindling mouse model induced with pentylenetetrazole (PTZ) to visualize changes in GSH levels in specific brain regions after seizure. The obtained map of brain GSH levels clearly indicated decreased GSH levels around the hippocampal region compared to control mice.

Measuring Cerebral and Cerebellar Glutathione in Children Using 1H MEGA-PRESS MRS

BACKGROUND AND PURPOSE

Glutathione is an important antioxidant in the human brain and therefore of interest in neurodegenerative disorders. The purpose of this study was to investigate the feasibility of measuring glutathione in healthy nonsedated children by using the ¹H Mescher-Garwood point-resolved spectroscopy (MEGA-PRESS) sequence at 3T and to compare glutathione levels between the medial parietal gray matter and the cerebellum.

MATERIALS AND METHODS

Glutathione was measured using MEGA-PRESS MRS (TR = 1.8 seconds, TE = 131 ms) in the parietal gray matter (35 × 25 × 20 mm³) of 6 healthy children (10.0 ± 2.4 years of age; range, 7-14 years; 3 males) and in the cerebellum of 11 healthy children (12.0 ± 2.7 years of age; range, 7-16 years; 6 males). A postprocessing pipeline was developed to account for frequency and phase variations in the edited ON and nonedited OFF spectra. Metabolites were quantified with LCModel and reported both as ratios and water-scaled values. Glutathione was quantified in the ON-OFF spectra, whereas total NAA, total Cho, total Cr, mIns, Glx, and taurine were quantified in the OFF spectra.

RESULTS

We found significantly higher glutathione, total Cho, total Cr, mIns, and taurine in the cerebellum (P < .01). Glx and total NAA were significantly higher in the parietal gray matter (P < .01). There was no significant difference in glutathione/total Cr (P = .93) between parietal gray matter and cerebellum.

CONCLUSIONS

We demonstrated that glutathione measurement in nonsedated children is feasible. We found significantly higher glutathione in the cerebellum compared with the parietal gray matter. Metabolite differences between the parietal gray matter and cerebellum agree with published MRS data in adults.

A pilot study to assess the effect of acute exercise on brain glutathione

The brain is highly susceptible to oxidative stress due to its high metabolic demand. Increased oxidative stress and depletion of glutathione (GSH) are observed with aging and many neurological diseases. Exercise training has the potential to reduce oxidative stress in the brain. In this study, nine healthy sedentary males (aged 25 ± 4 years) undertook a bout of continuous moderate intensity exercise and a high-intensity interval (HII) exercise bout on separate days. GSH concentration in the anterior cingulate was assessed by magnetic resonance spectroscopy (MRS) in four participants, before and after exercise. This was a pilot study to evaluate the ability of the MRS method to detect exercise-induced changes in brain GSH in humans for the first time. MRS is a non-invasive method based on nuclear magnetic resonance, which enables the quantification of metabolites, such as GSH, in the human brain in vivo. To add context to brain GSH data, other markers of oxidative stress were also assessed in the periphery (in blood) at three time points [pre-, immediately post-, and post (∼1 hour)-exercise]. Moderate exercise caused a significant decrease in brain GSH from 2.12 ± 0.64 mM/kg to 1.26 ± 0.36 mM/kg (p = .04). Blood GSH levels increased immediately post-HII exercise, 580 ± 101 µM to 692 ± 102 µM (n = 9, p = .006). The findings from this study show that brain GSH is altered in response to acute moderate exercise, suggesting that exercise may stimulate an adaptive response in the brain. Due to the challenges in MRS methodology, this pilot study should be followed up with a larger exercise intervention trial.

Edited 1 H magnetic resonance spectroscopy in vivo: Methods and metabolites

The Proton magnetic resonance (1 H-MRS) spectrum contains information about the concentration of tissue metabolites within a predefined region of interest (a voxel). The conventional spectrum in some cases obscures information about less abundant metabolites due to limited separation and complex splitting of the metabolite peaks. One method to detect these metabolites is to reduce the complexity of the spectrum using editing. This review provides an overview of the one-dimensional editing methods available to interrogate these obscured metabolite peaks. These methods include sequence optimizations, echo-time averaging, J-difference editing methods (single BASING, dual BASING, and MEGA-PRESS), constant-time PRESS, and multiple quantum filtering. It then provides an overview of the brain metabolites whose detection can benefit from one or more of these editing approaches, including ascorbic acid, γ-aminobutyric acid, lactate, aspartate, N-acetyl aspartyl glutamate, 2-hydroxyglutarate, glutathione, glutamate, glycine, and serine. Magn Reson Med 77:1377-1389, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Glutathione in the human brain: Review of its roles and measurement by magnetic resonance spectroscopy

We review the transport, synthesis and catabolism of glutathione in the brain as well as its compartmentation and biochemistry in different brain cells. The major reactions involving glutathione are reviewed and the factors limiting its availability in brain cells are discussed. We also describe and critique current methods for measuring glutathione in the human brain using magnetic resonance spectroscopy, and review the literature on glutathione measurements in healthy brains and in neurological, psychiatric, neurodegenerative and neurodevelopmental conditions In summary: Healthy human brain glutathione concentration is ∼1-2 mM, but it varies by brain region, with evidence of gender differences and age effects; in neurological disease glutathione appears reduced in multiple sclerosis, motor neurone disease and epilepsy, while being increased in meningiomas; in psychiatric disease the picture is complex and confounded by methodological differences, regional effects, length of disease and drug-treatment. Both increases and decreases in glutathione have been reported in depression and schizophrenia. In Alzheimer's disease and mild cognitive impairment there is evidence for a decrease in glutathione compared to age-matched healthy controls. Improved methods to measure glutathione in vivo will provide better precision in glutathione determination and help resolve the complex biochemistry of this molecule in health and disease.

Quantification of glutathione in the human brain by MR spectroscopy at 3 Tesla: Comparison of PRESS and MEGA-PRESS

PURPOSE

Glutathione (GSH) is an important intracellular antioxidant in the brain. A number of studies report its measurement by localized ¹ H spectroscopy using PRESS and STEAM. This study evaluates the reliability and accuracy of GSH measurements from PRESS at 3 Tesla (T) and compares the results to those obtained with MEGA-PRESS.

METHODS

Phantoms containing brain metabolites, identical except for variable GSH concentration between 0 and 24 mM, were scanned using PRESS (echo time (TE) = 35 ms) and MEGA-PRESS (optimized TE = 130 ms) at 3 T. Spectra of the anterior cingulate cortex and occipital cortex in seven healthy volunteers were also acquired.

RESULTS

Phantom GSH concentrations from 0 to 3mM were unreliably quantified using PRESS, although at 4 mM and above there was a linear relationship between measured and true concentrations (R²  = 0.99). Using MEGA-PRESS, there was no signal detected at 0 mM GSH, plus a linear relationship (R²  = 0.99) over the full range from 0-24 mM. In brain, concentrations calculated from MEGA-PRESS and PRESS were significantly different in occipital cortex (P < 0.001). Moreover, only MEGA-PRESS reported significant differences in GSH between the two brain regions (P = 0.003).

CONCLUSION

Due to uncertainties in GSH quantification raised by the study, the authors conclude that physiological concentrations (<4 mM) of GSH cannot be reliably quantified from PRESS (TE = 35 ms) spectra at 3 T. Magn Reson Med 78:1257-1266, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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

BACKGROUND

Oxidative stress and glutathione (GSH) metabolism dysregulation has been implicated in the pathophysiology of schizophrenia. GAG-trinucleotide repeat (TNR) polymorphisms in the glutamate-cysteine ligase catalytic gene (GCLC), the rate-limiting enzyme for GSH synthesis, are associated with schizophrenia. In addition, GSH may serve as a reserve pool for neuronal glutamate (Glu) through the γ-glutamyl cycle. The aim of this study is to investigate brain [GSH] and its association with GCLC polymorphism, peripheral redox indices and brain Glu.

METHODS

Magnetic resonance spectroscopy was used to measure [GSH] and [Glu] in the medial prefrontal cortex (mPFC) of 25 early-psychosis patients and 33 controls. GCLC polymorphism was genotyped, glutathione peroxidases (GPx) and glutathione reductase (GR) activities were determined in blood cells.

RESULTS

Significantly lower [GSHmPFC] in GCLC high-risk genotype subjects were revealed as compared to low-risk genotype subjects independent of disease status. In male subjects, [GSHmPFC] and blood GPx activities correlate positively in controls (P = .021), but negatively in patients (P = .039). In GCLC low-risk genotypes, [GlumPFC] are lower in patients, while it is not the case for high-risk genotypes.

CONCLUSIONS

GCLC high-risk genotypes are associated with low [GSHmPFC], highlighting that GCLC polymorphisms should be considered in pathology studies of cerebral GSH. Low brain GSH levels are related to low peripheral oxidation status in controls but with high oxidation status in patients, pointing to a dysregulated GSH homeostasis in early psychosis patients. GCLC polymorphisms and disease associated correlations between brain GSH and Glu levels may allow patients stratification.

Echo time optimization for J-difference editing of glutathione at 3T

PURPOSE

To investigate the echo time (TE) dependence of J-difference editing of glutathione and to determine the optimal TE for in vivo measurements at 3T.

METHODS

Spatially resolved density-matrix simulations and phantom experiments were performed at a range of TEs to establish the spatial and TE modulation of glutathione signals in editing-on, editing-off, and difference spectra at 3T. In vivo data were acquired in five healthy subjects to compare a TE of 68 ms and a TE of 120 ms. At the longer TE, high-bandwidth, frequency-modulated, slice-selective refocusing pulses were also compared with conventional amplitude-modulated pulses.

RESULTS

Simulations and relaxation-corrected phantom experiments suggest that the maximum edited signal occurs at TE 160 ms, ignoring transverse relaxation. Considering in vivo T₂ relaxation times of 67-89 ms, the optimal in vivo TE is estimated to be 120 ms. In vivo measurements showed that this TE yielded 15% more signal than TE 68 ms. A further gain of 57% resulted from using improved slice-selective refocusing pulses.

CONCLUSION

J-difference editing of glutathione using TE 120 ms delivers increased signal due to improved editing efficiency that more than offsets T₂ losses. The additional TE also allows for use of improved slice-selective refocusing pulses, which results in additional signal gains. Magn Reson Med 77:498-504, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Neurobiology of schizophrenia onset

The clinical symptoms and cognitive and functional deficits of schizophrenia typically begin to gradually emerge during late adolescence and early adulthood. Recent findings suggest that disturbances of a specific subset of inhibitory neurons that contain the calcium-binding protein parvalbumin (PV), which may regulate the course of postnatal developmental experience-dependent synaptic plasticity in the cerebral cortex, including the prefrontal cortex (PFC), may be involved in the pathogenesis of the onset of this illness. Specifically, converging lines of evidence suggest that oxidative stress, extracellular matrix (ECM) deficit and impaired glutamatergic innervation may contribute to the functional impairment of PV neurons, which may then lead to aberrant developmental synaptic pruning of pyramidal cell circuits during adolescence in the PFC. In addition to promoting the functional integrity of PV neurons, maturation of ECM may also play an instrumental role in the termination of developmental PFC synaptic pruning; thus, ECM deficit can directly lead to excessive loss of synapses by prolonging the course of pruning. Together, these mechanisms may contribute to the onset of schizophrenia by compromising the integrity, stability, and fidelity of PFC connectional architecture that is necessary for reliable and predictable information processing. As such, further characterization of these mechanisms will have implications for the conceptualization of rational strategies for the diagnosis, early intervention, and prevention of this debilitating disorder.

Oxidative stress and schizophrenia: recent breakthroughs from an old story

PURPOSE OF REVIEW

Oxidative stress has become an exciting area of schizophrenia research, and provides ample opportunities and hope for a better understanding of its pathophysiology, which may lead to novel treatment strategies. This review describes how recent methodological advances have allowed the study of oxidative stress to tackle fundamental questions and have provided several conceptual breakthroughs to the field.

RECENT FINDINGS

Recent human studies support the notion that intrinsic susceptibility to oxidative stress may underlie the pathophysiology of schizophrenia. More than one animal model that may be relevant to study the biology of schizophrenia also shows sign of oxidative stress in the brain.

SUMMARY

These advances have made this topic of paramount importance to the understanding of schizophrenia and will play a role in advancing the treatment options. This review covers topics from the classic biochemical studies of human biospecimens to the use of magnetic resonance spectroscopy and novel mouse models, and focuses on highlighting the promising areas of research.

Selective spectral modulation of strongly coupled spins with an echo top refocusing pulse in PRESS sequences

The double spin echo is the basis of the point resolved spectroscopy (PRESS) sequence. In this study we sought to investigate the effects of a broadband 180° pulse - incorporated in the PRESS sequence at the location of the first echo (gPRESS) - on the citrate resonances, chosen as a model of strongly coupled spin system. A significant signal modulation generated by the additional 180° pulse was predicted with simulations and observed experimentally in the citrate resonances. No effects were observed on the singlet resonance of glycine and the weakly coupled resonances of lactate. The phenomenon observed in gPRESS was attributed to the off-diagonal Hamiltonian elements responsible for a coherence transfer occurring throughout the evolution periods. The results of this study show that it is necessary to assess the effects of broadband 180° pulses on strongly coupled spin systems, since these pulses can selectively modify the spectral shape of strongly coupled resonances.

Doubly selective multiple quantum chemical shift imaging and T(1) relaxation time measurement of glutathione (GSH) in the human brain in vivo

Mapping of a major antioxidant, glutathione (GSH), was achieved in the human brain in vivo using a doubly-selective multiple quantum filtering based chemical shift imaging (CSI) of GSH at 3 T. Both in vivo and phantom tests in CSI and single voxel measurements were consistent with excellent suppression of overlapping signals from creatine, γ-Amino butyric acid (GABA) and macromolecules. GSH concentration in the fronto-parietal region was 1.20 ± 0.16 µmol/g (mean ± SD, n = 7). The longitudinal relaxation time (T(1) ) of GSH in the human brain was 397 ± 44 ms (mean ± SD, n = 5), which was substantially shorter than that of other metabolites. This GSH-CSI method permits us to address regional differences of GSH in the human brain under conditions where oxidative stress has been implicated, including multiple sclerosis, aging and neurodegenerative diseases.

In vivo assessment of cancerous tumors using boron doped diamond microelectrode

The in vitro and in vivo electrochemical detection of the reduced form of glutathione (L-γ-glutamyl-L-cysteinyl-glycine, GSH) using boron doped diamond (BDD) microelectrode for potential application in the assessment of cancerous tumors is presented. Accurate calibration curve for the determination of GSH could be obtained by the in vitro electrochemical measurements. Additionally, it was shown that it was possible to separate the detection of GSH from the oxidized form of glutathione (GSSG) using chronoamperometry measurements. In vivo GSH detection measurements have been performed in human cancer cells inoculated in immunodeficient mice. These measurements have shown that the difference of GSH level between cancerous and normal tissues can be detected. Moreover, GSH detection measurements carried out before and after X-ray irradiation have proved that it is possible to assess in vivo the decrease in GSH concentration in the tumor after a specific treatment.

Enhancement of spectral editing efficacy of multiple quantum filters in in vivo proton magnetic resonance spectroscopy

The performance of multiple quantum filters (MQFs) can be disappointing when the background signal also arises from coupled spins. Moreover, at 3.0 T and even higher fields the majority of the spin systems of key brain metabolites fall into the strong-coupling regime. In this manuscript we address comprehensively, the importance of the phase of the multiple quantum coherence-generating pulse (MQ-pulse) in the design of MQFs, using both product operator and numerical analysis, in both zero and double quantum filter designs. The theoretical analyses were experimentally validated with the examples of myo-inositol editing and the separation of glutamate from glutamine. The results demonstrate that the phase of the MQ-pulse per se provides an additional spectral discrimination mechanism based on the degree of coupling beyond the conventional level-of-coherence approach of MQFs. To obtain the best spectral discrimination of strongly-coupled spin systems, therefore, the phase of the MQ-pulse must be included in the portfolio of the sequence parameters to be optimized.

Differential redox potential between the human cytosolic and mitochondrial branched-chain aminotransferase

The human branched-chain aminotransferase (hBCAT) isoenzymes are CXXC motif redox sensitive homodimers central to glutamate metabolism in the central nervous system. These proteins respond differently to oxidation by H(2)O(2), NO, and S-glutathionylation, suggesting that the redox potential is distinct between isoenzymes. Using various reduced to oxidized glutathione ratios (GSH:GSSG) to alter the redox environment, we demonstrate that hBCATc (cytosolic) has an overall redox potential that is 30 mV lower than hBCATm (mitochondrial). Furthermore, the CXXC motif of hBCATc was estimated to be 80 mV lower, suggesting that hBCATm is more oxidizing in nature. Western blot analysis revealed close correlations between hBCAT S-glutathionylation and the redox status of the assay environment, offering the hBCAT isoenzymes as novel biomarkers for cytosolic and mitochondrial oxidative stress.

Measuring T2 in vivo with J-difference editing: application to GABA at 3 Tesla

PURPOSE

To develop an experimental approach for determining in vivo transverse relaxation rates (T(2)) of metabolites that are detected by spectral editing without using simulations, and to demonstrate this approach to measure the T(2) of γ-aminobutyric acid (GABA).

MATERIALS AND METHODS

The proposed method first determines the TE-dependence of the edited signals using measurements in a pure phantom solution (10 mM γ-aminobutyric acid; GABA); the phantom T(2) is also determined. Once the editing echo time (TE)-modulation pattern is known, it can then be used to determine T(2) in vivo. The method was applied to measure GABA T(2) in the occipital lobe of five healthy adult subjects at 3T, using a J-difference editing method. Unwanted macromolecular contributions to the GABA signal were also measured.

RESULTS

The in vivo T(2) of edited GABA signal was 88 ± 12 ms; this preliminary result is somewhat shorter than other metabolite T(2) values in the literature at this field strength.

CONCLUSION

Spectral editing methods are now widely used to detect low concentration metabolites, such as GABA, but to date no edited acquisition methods have been proposed for the measurement of transverse relaxation times (T(2)). The method described has been successfully applied to measuring the T(2) of GABA.

Magnetic resonance spectroscopy study of the antioxidant defense system in schizophrenia

Accumulating evidence suggests that oxidative stress associated with impaired metabolism of the antioxidant glutathione (GSH) plays a key role in the pathophysiology of schizophrenia. Magnetic resonance spectroscopy (MRS) is one of the brain-imaging techniques that can quantitatively measure bioactive substances such as GSH in the intact human brain. Four different measurement sequences including double quantum coherence (DQC) filtering, MEscher-GArwood Point-RESolved Spectroscopy (MEGA-PRESS), Stimulated Echo Acquisition Mode (STEAM), and PRESS have been used to evaluate the (1)H-MRS measurement of GSH in the brains of patients with schizophrenia. Although the results of these studies were somewhat diverse, a negative correlation between brain GSH levels and the severity of negative symptoms in schizophrenia patients suggests that increasing the brain GSH levels might be beneficial for schizophrenia patients with negative symptoms. Moreover, a recent double-blind, placebo-controlled study demonstrated that add-on of N-acetyl-l-cysteine (NAC), a precursor of GSH, to antipsychotics improved the negative symptoms and reduced the side effects (akathisia) in patients with chronic schizophrenia. MRS study of the antioxidant defense system in schizophrenia still remains in the infantile stage; future studies are needed to examine the brain GSH level before and after NAC treatment, and thereby to provide direct evidence of the induced production of GSH in the living brain.

J-refocused coherence transfer spectroscopic imaging at 7 T in human brain

Short echo spectroscopy is commonly used to minimize signal modulation due to J-evolution of the cerebral amino acids. However, short echo acquisitions suffer from high sensitivity to macromolecules which make accurate baseline determination difficult. In this report, we describe implementation at 7 T of a double echo J-refocused coherence transfer sequence at echo time (TE) of 34 msec to minimize J-modulation of amino acids while also decreasing interfering macromolecule signals. Simulation of the pulse sequence at 7 T shows excellent resolution of glutamate, glutamine, and N-acetyl aspartate. B(1) sufficiency at 7 T for the double echo acquisition is achieved using a transceiver array with radiofrequency (RF) shimming. Using an alternate RF distribution to minimize receiver phase cancellation in the transceiver, accurate phase determination for the coherence transfer is achieved with rapid single scan calibration. This method is demonstrated in spectroscopic imaging mode with n = 5 healthy volunteers resulting in metabolite values consistent with literature and in a patient with epilepsy.

Improvement of resolution for brain coupled metabolites by optimized (1)H MRS at 7T

Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE-optimized point-resolved spectroscopy (PRESS) at 7 T is reported. Sub-TE dependences of the multiplets of Glu, Gln, GSH, γ-aminobutyric acid (GABA) and N-acetylaspartate (NAA) at 2.2-2.6 ppm were investigated with density matrix simulations, incorporating three-dimensional volume localization. The numerical simulations indicated that the C4-proton multiplets can be completely separated with (TE(1), TE(2)) = (37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5 ppm signal. Phantom experiments reproduced the signal yield and lineshape from simulations within experimental errors. In vivo tests of optimized PRESS were conducted on the prefrontal cortex of six healthy volunteers. In spectral fitting by LCModel, Cramér-Rao lower bounds (CRLBs) of Glu, Gln and GSH were 2 ± 1, 5 ± 1 and 6 ± 2 (mean ± SD), respectively. To evaluate the performance of the optimized PRESS method under identical experimental conditions, stimulated-echo spectra were acquired with (TE, TM) = (14, 37) and (74, 68) ms. The CRLB of Glu was similar between PRESS and short-TE stimulated-echo acquisition mode (STEAM), but the CRLBs of Gln and GSH were lower in PRESS than in both STEAM acquisitions.

(1)H MRS detection of glycine residue of reduced glutathione in vivo

Glutathione (GSH) is a powerful antioxidant found inside different kinds of cells, including those of the central nervous system. Detection of GSH in the human brain using (1)H MR spectroscopy is hindered by low concentration and spectral overlap with other metabolites. Previous MRS methods focused mainly on the detection of the cysteine residue (GSH-Cys) via editing schemes. This study focuses on the detection of the glycine residue (GSH-Gly), which is overlapped by glutamate and glutamine (Glx) under physiological pH and temperature. The first goal of the study was to obtain the spectral parameters for characterization of the GSH-Gly signal under physiological conditions. The second goal was to investigate a new method of separating GSH-Gly from Glx in vivo. The characterization of the signal was carried out by utilization of numerical simulations as well as experiments over a wide range of magnetic fields (4.0-14T). The proposed separation scheme utilizes J-difference editing to quantify the Glx contribution to separate it from the GSH-Gly signal. The presented method retains 100% of the GSH-Gly signal. The overall increase in signal to noise ratio of the targeted resonance is calculated to yield a significant SNR improvement compared to previously used methods that target GSH-Cys residue. This allows shorter acquisition times for in vivo human clinical studies.

Contamination of single-voxel multiple quantum filters by external water signals arising from intermolecular multiple quantum coherences

Multiple-quantum filtered pulse sequences simplify overlapping metabolite spectra by the elimination of peaks from uncoupled spin species, most notably from methyl groups and water, and the minimization of unwanted coupled-spin peaks. However, it is shown in this study that a significant contaminant water signal can pass through this family of filters in the form of intermolecular multiple-quantum coherences. An imaging evaluation of a single-voxel multiple quantum filter experiment confirms that the water contamination is excited from outside of the voxel of interest, thus having an increased potential for broad spectral contamination. Phantom and in vivo experiments at 3.0 T are used to illustrate, first, significant water contamination of a single-voxel double quantum filter experiment optimized for the observation of glutamate, and second, the elimination of the unwanted water signal with conventional phase cycling and optimized filter gradient orientations.

Measurement of glutathione in normal volunteers and stroke patients at 3T using J-difference spectroscopy with minimized subtraction errors

PURPOSE

To develop and optimize a (1)H magnetic resonance spectroscopy (MRS) method for measuring brain glutathione (GSH) levels.

MATERIALS AND METHODS

Phantom experiments and density operator simulations were performed to determine the optimal TE for measuring GSH at 3T using J-difference spectral editing. In vivo data collected from 11 normal volunteers (43 measurements) and five stroke patients (10 measurements) were processed using a new spectral alignment method (adaptive spectral registration).

RESULTS

In phantom experiments and density operator simulations where relaxation effects were ignored, close to maximum GSH signal (2.95 ppm) was obtained at TE approximately 131 msec with minimum N-acetyl-aspartate (NAA) signal interference. Using adaptive spectral registration, GSH levels in healthy volunteers were found to be 1.20 +/- 0.14 mM (mean +/- standard deviation [SD]). GSH levels in stroke patients were found to be 1.19 +/- 0.24 mM in lesion and 1.25 +/- 0.19 mM in contralateral normal tissue. In comparison, the SDs were significantly larger when only the NAA singlet (2.01 ppm) was used as a navigator for spectral alignment.

CONCLUSION

Spectral editing using J-differences is a reliable method for measuring GSH levels in volunteers and stroke patients.

Measurement of glutathione in human brain at 3T using an improved double quantum filter in vivo

A single voxel proton NMR double quantum filter (DQF) for measurement of glutathione (GSH) in human brain at 3T is reported. Yield enhancement for the CH(2) resonances of the cysteine moiety at 2.95ppm has been achieved by means of dual encoding. After the preparation of double quantum and zero quantum coherences (DQC and ZQC) at equal magnitude, the first DQC encoding was followed by interchange of DQC and ZQC, and another DQC encoding. The multi-quantum coherences were fully utilized to generate a GSH target signal at approximately 2.95ppm. The optimal echo time and the editing efficiency were obtained with numerical analysis of the filtering performance and phantom measurements. The dual-DQC encoding method provided GSH yield greater by a factor of 2.1 than single-DQC encoding for identical slice-selective RF pulses in phantom tests. Using the phantom relaxation times and the ratio of edited GSH to N-acetylaspartate (NAA) 2.0-ppm peak areas, the concentration of GSH in the medial parietal cortex of the healthy human brain in vivo was estimated to be 1.0+/-0.3mM (mean+/-SD, n=7), with reference to NAA at 10mM.

Glutathione precursor, N-acetyl-cysteine, improves mismatch negativity in schizophrenia patients

In schizophrenia patients, glutathione dysregulation at the gene, protein and functional levels, leads to N-methyl-D-aspartate (NMDA) receptor hypofunction. These patients also exhibit deficits in auditory sensory processing that manifests as impaired mismatch negativity (MMN), which is an auditory evoked potential (AEP) component related to NMDA receptor function. N-acetyl-cysteine (NAC), a glutathione precursor, was administered to patients to determine whether increased levels of brain glutathione would improve MMN and by extension NMDA function. A randomized, double-blind, cross-over protocol was conducted, entailing the administration of NAC (2 g/day) for 60 days and then placebo for another 60 days (or vice versa). 128-channel AEPs were recorded during a frequency oddball discrimination task at protocol onset, at the point of cross-over, and at the end of the study. At the onset of the protocol, the MMN of patients was significantly impaired compared to sex- and age- matched healthy controls (p=0.003), without any evidence of concomitant P300 component deficits. Treatment with NAC significantly improved MMN generation compared with placebo (p=0.025) without any measurable effects on the P300 component. MMN improvement was observed in the absence of robust changes in assessments of clinical severity, though the latter was observed in a larger and more prolonged clinical study. This pattern suggests that MMN enhancement may precede changes to indices of clinical severity, highlighting the possible utility AEPs as a biomarker of treatment efficacy. The improvement of this functional marker may indicate an important pathway towards new therapeutic strategies that target glutathione dysregulation in schizophrenia.

Proton echo-planar spectroscopic imaging of J-coupled resonances in human brain at 3 and 4 Tesla

In this multicenter study, 2D spatial mapping of J-coupled resonances at 3T and 4T was performed using short-TE (15 ms) proton echo-planar spectroscopic imaging (PEPSI). Water-suppressed (WS) data were acquired in 8.5 min with 1-cm(3) spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. Constrained spectral fitting in reference to a non-WS (NWS) scan was performed with LCModel using correction for relaxation attenuation and partial-volume effects. The concentrations of total choline (tCho), creatine + phosphocreatine (Cr+PCr), glutamate (Glu), glutamate + glutamine (Glu+Gln), myo-inositol (Ins), NAA, NAA+NAAG, and two macromolecular resonances at 0.9 and 2.0 ppm were mapped with mean Cramer-Rao lower bounds (CRLBs) between 6% and 18% and approximately 150-cm(3) sensitive volumes. Aspartate, GABA, glutamine (Gln), glutathione (GSH), phosphoethanolamine (PE), and macromolecules (MMs) at 1.2 ppm were also mapped, although with larger mean CRLBs between 30% and 44%. The CRLBs at 4T were 19% lower on average as compared to 3T, consistent with a higher signal-to-noise ratio (SNR) and increased spectral resolution. Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter (GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies.

Incorporating homonuclear polarization transfer into PRESS for proton spectral editing: illustration with lactate and glutathione

A proton spectral editing pulse sequence for the detection of metabolites with spin systems that involve weak coupling is presented. The sequence is based on homonuclear polarization transfer incorporated into the standard PRESS (Point RESolved Spectroscopy) sequence, which is a volume-selective double spin echo method, to enable spatial localization. All peaks in the region of interest are initially suppressed whether they are peaks from the target metabolite or from contaminating background. The target signal is then restored by polarization transfer from a proton that has a resonance outside the suppressed region and to which the target spins are weakly coupled. This is achieved by the application of a 90 degrees hard pulse with phase orthogonal to that of the PRESS excitation pulse at the location of the first echo in PRESS and by optimizing the two PRESS timings, TE(1) and TE(2), for most efficient yield. Background signal not coupled to any protons outside the initially saturated region remains suppressed. The advantage of this sequence compared to multiple quantum filters is that signal from singlet peaks outside the suppressed area are preserved and can thus be used as a reference. The efficacy of the sequence was verified experimentally on phantom solutions of lactate and glutathione at 3.0 T. For the AX(3) spin system of lactate, the sequence timings were optimized by product operator calculations whereas for the ABX spin system of the cysteinyl group of glutathione numerical calculations were performed for sequence timing optimization.

Correlation between serum brain-derived neurotrophic factor level and an in vivo marker of cortical integrity

BACKGROUND

Brain-derived neurotrophic factor (BDNF) signaling at synapses improves synaptic strengthening associated with learning and memory. In the present study we hypothesized that serum BDNF concentration is associated with in vivo level of cerebral N-acetylaspartate (NAA), a well established marker of neuronal integrity.

METHODS

In 36 healthy subjects BDNF serum concentration and absolute concentration of NAA together with other metabolites were measured by proton magnetic resonance spectroscopy (1H-MRS) in regions with high BDNF levels (anterior cingulate cortex [ACC], left hippocampus). Relationship between BDNF concentration and brain metabolites was studied in linear regression analysis with BDNF concentration as dependent variable and metabolite concentrations, age, and gender as predictor variables.

RESULTS

The BDNF serum concentrations were positively associated with the concentrations of NAA (T = 2.193, p = .037) and total choline (T = 1.997, p = .055; trend) but not total creatine or glutamate in the ACC. No significant association was observed between BDNF serum concentration and absolute metabolite concentrations in the hippocampus.

CONCLUSIONS

The preliminary data might indicate that BDNF serum concentration reflects some aspects of neuronal plasticity as indicated by its association with NAA level in the cerebral cortex. The results would be in line with the notion that BDNF plays a central role in the regulation of neuronal survival and differentiation in the human brain.

Novel method for measurement of glutathione kinetics in neonates using liquid chromatography coupled to isotope ratio mass spectrometry

A novel analytical method using liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) was developed for measuring the fractional synthesis rate (FSR) of glutathione (GSH) in neonates after infusion of [1-(13)C]-glycine as a tracer. After transformation of GSH into GSSG, its dimeric form, the intra-erythrocytic concentration and (13)C-isotopic enrichment of GSH were determined using 200 microL of blood. The results showed that, using LC/IRMS, the concentration (range of micromol/mL) was reliably measured using norvaline as internal standard with precision better than 0.1 micromol/mL. In addition, the (13)C-isotopic enrichment measured in the same run gave reliable values with excellent precision (with standard deviation (sd) lower than 0.3 per thousand) and accuracy (measured between 0 and 2 Atom % Excess (APE)). The inter-assay repeatability of delta(13)C of norvaline used as internal standard with in vivo samples was assessed at -26.07 +/- 0.28 per thousand with coefficient of variance (CV) at 1.1%. The FSR calculated either with GSH or GSSG showed similar results with slightly higher values for GSSG (41.6 +/- 4.7 and 46.5 +/- 4.4, respectively). The slightly lower FSR of GSH is probably due to interfering compounds in the biological matrix. Successfully used in a clinical study, this rapid and reliable method opens up a variety of kinetic studies with relatively low administration of tracer infusates, reducing the total cost of the study design. The small volume of blood needed enables studies even in extremely small subjects, such as premature infants, as reported in this study.

New double quantum coherence filter for localized detection of glutathione in vivo

In this work, a new double quantum filter for glutathione (GSH) editing is introduced, combined with the point-resolved spectroscopy localization sequence (PRESS), and demonstrated in vivo. Compared to the conventional double quantum coherence filter, the new filter has two major advantages. First, it eliminates the need for calibration scans for optimizing the signal yield, making it more efficient and convenient for routine use. Second, it removes the influence of water saturation pulses on the GSH yield, further improving its accuracy. With this method, GSH concentrations in the left and right parietal lobes of five healthy volunteers were determined to be 0.91+/-0.16 mM and 0.89+/-0.16 mM, respectively, in agreement with previous studies.

Contribution of reduced and oxidized glutathione to signals detected by magnetic resonance spectroscopy as indicators of local brain redox state

The reduced form of glutathione (GSH; gamma-glutamyl cysteinyl glycine) is supposedly the most powerful reducing battery in the central nervous system against oxidative stress. We evaluated the contribution of GSH and GSSG to MEGA-PRESS (a frequency-selective refocusing technique) signals assessed by magnetic resonance spectroscopy (MRS). GSH gave a single positive signal (2.95 ppm) by the MEGA-PRESS. In contrast, GSSG gave a multiplet of reversed signals (3.03, 3.23, and 3.34 ppm). A phantom solution mimicking the normal in vivo condition (GSH:GSSG=100:1) gave a single positive peak. Even when the ratio was changed to 10:1, corresponding to toxic oxidative stress, GSH was prominent and GSSG signals were minimal. Thus, GSSG signals could be negligible. In the phantom solution (creatine:GSH:aspartate:gamma-aminobutyric acid=7:3:1:1), the creatine signal overshadowed the other signals. Through the MEGA-PRESS, a single peak of GSH stood out over other signals. In vivo, the brains of healthy volunteers gave similar signals as the in vitro phantom solution, indicating that the signal originated from GSH. The estimated concentration of GSH in the human brain was 1.9+/-0.37 mM (mean+/-S.D., n=4). In conclusion, MEGA-PRESS allowed us to assess GSH levels in vivo non-invasively.

Validation of glutathione quantitation from STEAM spectra against edited 1H NMR spectroscopy at 4T: application to schizophrenia

OBJECTIVE

Quantitation of glutathione (GSH) in the human brain in vivo using short echo time 1H NMR spectroscopy is challenging because GSH resonances are not easily resolved. The main objective of this study was to validate such quantitation in a clinically relevant population using the resolved GSH resonances provided by edited spectroscopy. A secondary objective was to compare several of the neurochemical concentrations quantified along with GSH using LCModel analysis of short echo time spectra in schizophrenia versus control.

MATERIALS AND METHODS

GSH was quantified at 4T from short echo STEAM spectra and MEGA-PRESS edited spectra from identical volumes of interest (anterior cingulate) in ten volunteers. Neurochemical profiles were quantified in nine controls and 13 medicated schizophrenic patients.

RESULTS

GSH concentrations as quantified using STEAM, 1.6 +/- 0.4 micromol/g (mean +/- SD, n = 10), were within error of those quantified using edited spectra, 1.4 +/- 0.4 micromol/g, and were not different (p = 0.4). None of the neurochemical measurements reached sufficient statistical power to detect differences smaller than 10% in schizophrenia versus control. As such, no differences were observed.

CONCLUSIONS

Human brain GSH concentrations can be quantified in a clinical setting using short-echo time STEAM spectra at 4T.

In vivo magnetic resonance spectroscopy in cancer

Magnetic resonance spectroscopy (MRS) has been used for more than two decades to interrogate metabolite distributions in living cells and tissues. Techniques have been developed that allow multiple spectra to be obtained simultaneously with individual volume elements as small as 1 uL of tissue (i.e., 1 x 1 x 1 mm(3)). The most common modern applications of in vivo MRS use endogenous signals from (1)H, (31)P, or (23)Na. Important contributions have also been made using exogenous compounds containing (19)F, (13)C, or (17)O. MRS has been used to investigate cardiac and skeletal muscle energetics, neurobiology, and cancer. This review focuses on the latter applications, with specific reference to the measurement of tissue choline, which has proven to be a tumor biomarker that is significantly affected by anticancer therapies.

Selective homonuclear Hartmann-Hahn transfer method for in vivo spectral editing in the human brain

A novel selective homonuclear Hartmann-Hahn transfer method for in vivo spectral editing is proposed and applied to measurements of gamma-aminobutyric acid (GABA) in the human brain at 3 T. The proposed method utilizes a new concept for in vivo spectral editing, the spectral selectivity of which is not based on a conventional editing pulse but based on the stringent requirement of the doubly selective Hartmann-Hahn match. The sensitivity and spectral selectivity of GABA detection achieved by this doubly selective Hartmann-Hahn match scheme was superior to that achievable by conventional in vivo spectral editing techniques providing both sensitivity enhancement and excellent suppression of overlapping resonances in a single shot. Since double-quantum filtering gradients were not employed, singlets such as the NAA methyl group at 2.02 ppm and the creatine methylene group at 3.92 ppm were detected simultaneously. These singlets may serve as navigators for the spectral phase of GABA and for frequency shifts during measurements. The estimated concentration of GABA in the frontoparietal region of the human brain in vivo was 0.7 +/- 0.2 mumol/g (mean +/- SD, n = 12).

Detection of the inhibitory neurotransmitter GABA in macrophages by magnetic resonance spectroscopy

Macrophages are key components of the inflammatory response to tissue injury, but their activities can exacerbate neuropathology. High-resolution magnetic resonance spectroscopy was used to identify metabolite levels in perchloric acid extracts of cultured cells of the RAW 264.7 murine macrophage line under resting and lipopolysaccharide-activated conditions. Over 25 metabolites were identified including gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter not previously reported to be present in macrophages. The presence of GABA was also demonstrated in extracts of human peripheral blood monocyte-derived macrophages. This finding suggests that there may be communication between damaged central nervous system (CNS) tissue and recruited macrophages and resident microglia, which could help orchestrate the immune response. On activation, lactate, glutamine, glutamate, and taurine levels were elevated significantly, and GABA and alanine were reduced significantly. Strong resonances from glutathione, evident in the macrophage two-dimensional 1H spectrum, suggest that this may have potential as a noninvasive marker of macrophages recruited to the CNS, as it is only present at low levels in normal brain. Alternatively, a specific combination of spectroscopic changes, such as lactate, alanine, glutathione, and polyamines, may prove to be the most accurate means of detecting macrophage recruitment to the CNS.

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 single-shot three-dimensionally localized multiple quantum spectroscopy of GABA in the human brain with improved spectral selectivity

A single-shot multiple quantum filtering method is developed that uses two double-band frequency selective pulses for enhanced spectral selectivity in combination with a slice-selective 90 degrees, a slice-selective universal rotator 90 degrees, and a spectral-spatial pulse composed of two slice-selective universal rotator 45 degrees pulses for single-shot three-dimensional localization. The use of this selective multiple quantum filtering method for C(3) and C(4) methylene protons of GABA resulted in improved spectral selectivity for GABA and effective suppression of overlapping signals such as creatine and glutathione in each single scan, providing reliable measurements of the GABA doublet in all subjects. The concentration of GABA was measured to be 0.7 +/- 0.2 micromol/g (means +/- SD, n = 15) in the fronto-parietal region of the human brain in vivo.

Spurious signals in DQF spectroscopy: two-shot stimulated echoes

The most widely used technique for double-quantum filtered (DQF) single-voxel spectroscopy (SVS) is based on a symmetric PRESS sequence with two additional spatially unselective pi/2 pulses, one of which is usually frequency selective. The actual filtering, rejecting signals from all uncoupled resonances, can be done by suitable phase cycling of the rf pulses in successive shots, but in practice gradient filtering is always used. Under usual conditions the sequence repetition time is comparable to the spin-lattice relaxation time, and a stimulated echo is formed by five out of the ten rf pulses in two consecutive shots. This echo is not filtered out by the gradients, and additional phase cycling is needed to eliminate it. Its spatial origin is the full transverse slice selected by the last pulse of the PRESS sequence. The SVS shimming procedure may create an important field variation in this slice (outside the volume of interest VOI). Water singlet signals therefore appear in a band of frequencies other than 4.7 ppm, and remain unaffected by water suppression pulses. In practice phase-alternation schemes can reduce these spurious signals by several orders of magnitude, but even then they may mask the weak metabolite signals of interest. We describe a strategy to minimize these spurious signals and propose a 16-step phase cycling scheme that attenuates the stimulated echo in every two-step subcycle.

Strategy for the spectral filtering of myo-inositol and other strongly coupled spins

A multiple quantum filter strategy is presented for spectrally discriminating metabolites with strongly coupled spins from those whose spins are either uncoupled or weakly coupled. The strategy also includes a means for selectively suppressing the background multiplets of metabolites that also have strongly coupled spins. As a demonstration of its efficacy at 3.0 T, the strategy is shown to enhance by a factor of approximately 5 the signal-to-background ratio of the myo-inositol band at 3.6 ppm relative to that in response to a PRESS sequence with the same sequence timings. This is done by eliminating the uncoupled resonance of glycine and the weakly coupled multiplets of glutamate and glutamine, and by selectively suppressing the strongly coupled taurine multiplet 3-fold. The macromolecular background was effectively removed through its transverse decay over 105 ms. The associated cost of gaining the signal to background enhancement is a drop in signal yield by a factor of 0.75 relative to PRESS at the same timings. The myo-inositol signal to noise ratio was nevertheless maintained by the filter at approximately 12.