Simultaneous detection of resolved glutamate, glutamine, and gamma-aminobutyric acid at 4 T

Glutamate Chemical Exchange Saturation Transfer (GluCEST) Magnetic Resonance Imaging of Rat Brain With Acute Carbon Monoxide Poisoning

OBJECTIVES

To evaluate the diagnostic and prognostic values of glutamate chemical exchange saturation transfer (GluCEST) magnetic resonance imaging as a quantitative method for pathogenetic research and clinical application of carbon monoxide (CO) poisoning-induced encephalopathy combined with the proton magnetic resonance spectroscopy (¹H-MRS) and the related histopathological and behavioral changes.

METHODS

A total of 63 Sprague-Dawley rats were randomly divided into four groups. Group A (n = 12) was used for animal modeling verification; Group B (n = 15) was used for magnetic resonance molecular imaging, Group C (n = 15) was used for animal behavior experiments, and Group D (n = 21) was used for histopathological examination. All the above quantitative results were analyzed by statistics.

RESULTS

The peak value of carboxyhemoglobin saturation in the blood after modeling was 7.3-fold higher than before and lasted at least 2.5 h. The GluCEST values of the parietal lobe, hippocampus, and thalamus were significantly higher than the base values in CO poisoning rats (p < 0.05) and the ¹H-MRS showed significant differences in the parietal lobe and hippocampus. In the Morris water maze tests, the average latency and distance were significantly prolonged in poisoned rats (p < 0.05), and the cumulative time was shorter and negatively correlated with GluCEST.

CONCLUSION

The GluCEST imaging non-invasively reflects the changes of glutamate in the brain in vivo with higher sensitivity and spatial resolution than ¹H-MRS. Our study implies that GluCEST imaging may be used as a new imaging method for providing a pathogenetic and prognostic assessment of CO-associated encephalopathy.

Diminished reward responsiveness is associated with lower reward network GluCEST: an ultra-high field glutamate imaging study

Low reward responsiveness (RR) is associated with poor psychological well-being, psychiatric disorder risk, and psychotropic treatment resistance. Functional MRI studies have reported decreased activity within the brain's reward network in individuals with RR deficits, however the neurochemistry underlying network hypofunction in those with low RR remains unclear. This study employed ultra-high field glutamate chemical exchange saturation transfer (GluCEST) imaging to investigate the hypothesis that glutamatergic deficits within the reward network contribute to low RR. GluCEST images were acquired at 7.0 T from 45 participants (ages 15-29, 30 females) including 15 healthy individuals, 11 with depression, and 19 with psychosis spectrum symptoms. The GluCEST contrast, a measure sensitive to local glutamate concentration, was quantified in a meta-analytically defined reward network comprised of cortical, subcortical, and brainstem regions. Associations between brain GluCEST contrast and Behavioral Activation System Scale RR scores were assessed using multiple linear regressions. Analyses revealed that reward network GluCEST contrast was positively and selectively associated with RR, but not other clinical features. Follow-up investigations identified that this association was driven by the subcortical reward network and network areas that encode the salience of valenced stimuli. We observed no association between RR and the GluCEST contrast within non-reward cortex. This study thus provides new evidence that reward network glutamate levels contribute to individual differences in RR. Decreased reward network excitatory neurotransmission or metabolism may be mechanisms driving reward network hypofunction and RR deficits. These findings provide a framework for understanding the efficacy of glutamate-modulating psychotropics such as ketamine for treating anhedonia.

Fitness Level Influences White Matter Microstructure in Postmenopausal Women

Aerobic exercise has both neuroprotective and neurorehabilitative benefits. However, the underlying mechanisms are not fully understood and need to be investigated, especially in postmenopausal women, who are at increased risk of age-related disorders such as Alzheimer’s disease and stroke. To advance our understanding of the potential neurological benefits of aerobic exercise in aging women, we examined anatomical and functional responses that may differentiate women of varying cardiorespiratory fitness using neuroimaging and neurophysiology. A total of 35 healthy postmenopausal women were recruited (59 ± 3 years) and cardiorespiratory fitness estimated (22–70 mL/kg/min). Transcranial magnetic stimulation was used to assess -aminobutyric acid (GABA) and glutamate (Glu) receptor function in the primary motor cortex (M1), and magnetic resonance spectroscopy (MRS) was used to quantify GABA and Glu concentrations in M1. Magnetic resonance imaging was used to assess mean cortical thickness (MCT) of sensorimotor and frontal regions, while the microstructure of sensorimotor and other white matter tracts was evaluated through diffusion tensor imaging. Regression analysis revealed that higher fitness levels were associated with improved microstructure in pre-motor and sensory tracts, and the hippocampal cingulum. Fitness level was not associated with MCT, MRS, or neurophysiology measures. These data indicate that, in postmenopausal women, higher cardiorespiratory fitness is linked with preserved selective white matter microstructure, particularly in areas that influence sensorimotor control and memory.

Towards a theory of functional magnetic resonance spectroscopy (fMRS): A meta-analysis and discussion of using MRS to measure changes in neurotransmitters in real time

Proton magnetic resonance spectroscopy is a powerful tool to investigate neurochemistry and physiology in vivo. Recently researchers have started to use MRS to measure neurotransmitter changes related to neural activity, so called functional MRS (fMRS). Particular interest has been placed on measuring glutamate changes associated with neural function, but differences are reported in the size of changes seen. This review discusses fMRS, and includes meta-analyses of the relative size of glutamate changes seen in fMRS, and the impact experimental design and stimulus paradigm may have. On average glutamate was found to increase by 6.97% (±1.739%) in response to neural activation. However, factors of experimental design may have a large impact on the size of these changes. For example an increase of 4.749% (±1.45%) is seen in block studies compared to an increase of 13.429% (±3.59) in studies using event related paradigms. The stimulus being investigated also seems to play a role with prolonged visual stimuli showing a small mean increase in glutamate of 2.318% (±1.227%) while at the other extreme, pain stimuli show a mean stimulation effect of 14.458% (±3.736%). These differences are discussed with regards to possible physiologic interpretations, as well experimental design implications.

In vivo detection and automatic analysis of GABA in the mouse brain with MEGA-PRESS at 9.4 T

The goals of this study were to develop an acquisition protocol and the analysis tools for Meshcher-Garwood point-resolved spectroscopy (MEGA-PRESS) in mouse brain at 9.4 T, to allow the in vivo detection of γ-aminobutyric acid (GABA) and to examine whether isoflurane alters GABA levels in the thalamus during anesthesia. We implemented the MEGA-PRESS sequence on a Bruker 94/20 system with ParaVision 6.0.1, and magnetic resonance spectra were acquired from nine male wild-type C57BL/6 J mice at the thalamus. Four individual scans were obtained for each mouse in a 2-h time course whilst the mouse was anesthetized with isoflurane. We developed an automated analysis program with improved correction for frequency and phase drift compared with the standard creatine (Cr) fitting-based method and provided automatic quantification. During MEGA-PRESS acquisition, a single voxel with a size of 5 × 3 × 3 mm³ was placed at the thalamus to evaluate GABA to Cr (GABA/Cr) ratios during anesthesia. Detection and quantitative analysis of thalamic GABA levels were successfully achieved. We noticed a significant decrease in GABA/Cr during the 2-h anesthesia (by linear regression analysis: slope < 0, p < 0.0001). In summary, our findings demonstrate that MEGA-PRESS is a feasible technique to measure in vivo GABA levels in the mouse brain at 9.4 T.

Improved resolution of glutamate, glutamine and γ-aminobutyric acid with optimized point-resolved spectroscopy sequence timings for their simultaneous quantification at 9.4 T

Glutamine (Gln), glutamate (Glu) and γ-aminobutyric acid (GABA) are relevant brain metabolites that can be measured with magnetic resonance spectroscopy (MRS). This work optimizes the point-resolved spectroscopy (PRESS) sequence echo times, TE₁ and TE₂ , for improved simultaneous quantification of the three metabolites at 9.4 T. Quantification was based on the proton resonances of Gln, Glu and GABA at ≈2.45, ≈2.35 and ≈2.28 ppm, respectively. Glu exhibits overlap with both Gln and GABA; in addition, the Gln peak is contaminated by signal from the strongly coupled protons of N-acetylaspartate (NAA), which resonate at about 2.49 ppm. J-coupling evolution of the protons was characterized numerically and verified experimentally. A {TE₁ , TE₂ } combination of {106 ms, 16 ms} minimized the NAA signal in the Gln spectral region, whilst retaining Gln, Glu and GABA peaks. The efficacy of the technique was verified on phantom solutions and on rat brain in vivo. LCModel was employed to analyze the in vivo spectra. The average T₂ -corrected Gln, Glu and GABA concentrations were found to be 3.39, 11.43 and 2.20 mM, respectively, assuming a total creatine concentration of 8.5 mM. LCModel Cramér-Rao lower bounds (CRLBs) for Gln, Glu and GABA were in the ranges 14-17%, 4-6% and 16-19%, respectively. The optimal TE resulted in concentrations for Gln and GABA that agreed more closely with literature concentrations compared with concentrations obtained from short-TE spectra acquired with a {TE₁ , TE₂ } combination of {12 ms, 9 ms}. LCModel estimations were also evaluated with short-TE PRESS and with the optimized long TE of {106 ms, 16 ms}, using phantom solutions of known metabolite concentrations. It was shown that concentrations estimated with LCModel can be inaccurate when combined with short-TE PRESS, where there is peak overlap, even when low (<20%) CRLBs are reported.

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.

Imaging increased glutamate in children with Sturge-Weber syndrome: Association with epilepsy severity

BACKGROUND

Sturge-Weber syndrome (SWS) is strongly associated with epilepsy. Brain tissue studies have suggested that epileptic activity in SWS is driven by glutamatergic synaptic activity. Here, we used proton magnetic resonance spectroscopic imaging (MRSI) to test if glutamate (GLU) concentrations are increased in the affected hemisphere and if such increases are associated with severity of epilepsy in children with SWS. We also studied the metabolic correlates of MRSI abnormalities, using glucose positron emission tomography (PET) imaging.

METHODS

3T MRI and glucose PET were performed in 10 children (age: 7-78 months) with unilateral SWS and a history of epilepsy. MRSI data were acquired from the affected (ipsilateral) and non-affected (contralateral) hemispheres. GLU, N-acetyl-aspartate (NAA) and creatine (Cr) were quantified in multiple voxels; GLU/Cr and NAA/Cr ratios were calculated and compared to seizure frequency as well as glucose PET findings.

RESULTS

The highest GLU/Cr ratios were found in the affected hemisphere in all children except one with severe atrophy. The maximum ipsilateral/contralateral GLU/Cr ratios ranged between 1.0 and 2.5 (mean: 1.6). Mean ipsilateral/contralateral GLU/Cr ratios were highest in the youngest children and showed a strong positive correlation with clinical seizure frequency scores assessed at the time of the scan (r=0.88, p=0.001) and also at follow-up (up to 1 year, r=0.80, p=0.009). GLU increases in the affected hemisphere coincided with areas showing current or previous increases of glucose metabolism on PET in 5 children. NAA/Cr ratios showed no association with clinical seizure frequency.

CONCLUSIONS

Increased glutamate concentrations in the affected hemisphere, measured by MRSI, are common in young children with unilateral SWS and are associated with frequent seizures. The findings lend support to the role of excess glutamate in SWS-associated epilepsy.

Two-dimensional J-resolved LASER and semi-LASER spectroscopy of human brain

PURPOSE

Two-dimensional J-resolved localized and semi-localized by adiabatic selective refocusing (LASER and semi-LASER) spectroscopy, named "J-resolved LASER" and "J-resolved semi-LASER", were introduced to suppress chemical shift artifacts, additional J-refocused artifactual peaks from spatially dependent J-coupling evolution, and sensitivity to radiofrequency (RF) field inhomogeneity.

METHODS

Three pairs of adiabatic pulses were employed for voxel localization in J-resolved LASER and two pairs in J-resolved semi-LASER. The first half of t1 period was inserted between the last pair of adiabatic pulses, which was proposed in this work to obtain two-dimensional adiabatic J-resolved spectra of human brain for the first time. Phantom and human experiments were performed to demonstrate their feasibility and advantages over conventional J-resolved spectroscopy (JPRESS).

RESULTS

Compared to JPRESS, J-resolved LASER or J-resolved semi-LASER exhibited significant suppression of chemical shift artifacts and additional J-refocused peaks from spatially dependent J-coupling evolution, and demonstrated insensitivity to the change of RF frequency offset over large bandwidth.

CONCLUSION

Experiments on phantoms and human brains verified the feasibility and strengths of two-dimensional adiabatic J-resolved spectroscopy at 3T. This technique is expected to advance the application of in vivo two-dimensional MR spectroscopy at 3T and higher field strengths for more reliable and accurate quantification of metabolites.

Impact of family history of alcoholism on glutamine/glutamate ratio in anterior cingulate cortex in substance-naïve adolescents

Neuroimaging studies of individuals with family histories of alcoholism provide evidence suggesting neurobiological risk factors for alcoholism. Youth family history positive (FH+) for alcoholism exhibit increased impulsivity compared to family history negative (FH-) peers in conjunction with altered functional activation in prefrontal cortex, including anterior cingulate cortex (ACC). This study examined glutamate (Glu) and glutamine (Gln), amino acids vital to protein synthesis, cellular metabolism and neurotransmission, acquired from ACC and parieto-occipital cortex (POC) using magnetic resonance spectroscopy (MRS) at 4T. Participants were 28 adolescents (13 male, 12-14 yrs) and 31 emerging adults (16 male, 18-25 yrs), stratified into FH- and FH+ groups. Significantly higher ACC Gln/Glu was observed in emerging adults versus adolescents in FH- but not FH+ groups. In FH- adolescents, higher impulsivity was significantly associated with higher ACC Gln/Glu. In FH+ emerging adults, higher impulsivity was negatively associated with ACC Gln/Glu. No differences or associations were observed for POC. These findings provide preliminary evidence that family history of alcoholism is associated with a neurochemical profile that may influence normative age differences in glutamatergic metabolites and their association with impulse control, which together could confer greater genetic risk of addiction later in life.

Nuclear spin singlet states as a contrast mechanism for NMR spectroscopy

Nuclear magnetic resonance (NMR) spectra of complex chemical mixtures often contain unresolved or hidden spectral components, especially when strong background signals overlap weaker peaks. In this article we demonstrate a quantum filter utilizing nuclear spin singlet states, which allows undesired NMR spectral background to be removed and target spectral peaks to be uncovered. The quantum filter is implemented by creating a nuclear spin singlet state with spin quantum numbers j = 0, mz  = 0 in a target molecule, applying a continuous RF field to both preserve the singlet state and saturate the magnetization of undesired molecules and then mapping the target molecule singlet state back into an NMR observable state so that its spectrum can be read out unambiguously. The preparation of the target singlet state can be carefully controlled with pulse sequence parameters, so that spectral contrast can be achieved between molecules with very similar structures. We name this NMR contrast mechanism 'Suppression of Undesired Chemicals using Contrast-Enhancing Singlet States' (SUCCESS) and we demonstrate it in vitro for three target molecules relevant to neuroscience: aspartate, threonine and glutamine.

Current practice in the use of MEGA-PRESS spectroscopy for the detection of GABA

There is increasing interest in the use of edited proton magnetic resonance spectroscopy for the detection of GABA in the human brain. At a recent meeting held at Cardiff University, a number of spectroscopy groups met to discuss the acquisition, analysis and interpretation of GABA-edited MR spectra. This paper aims to set out the issues discussed at this meeting, reporting areas of consensus around parameters and procedures in the field and highlighting those areas where differences remain. It is hoped that this paper can fulfill two needs, providing a summary of the current 'state-of-the-art' in the field of GABA-edited MRS at 3T using MEGA-PRESS and a basic guide to help researchers new to the field to avoid some of the pitfalls inherent in the acquisition and processing of edited MRS for GABA.

Phase-adjusted echo time (PATE)-averaging 1 H MRS: application for improved glutamine quantification at 2.89 T

(1) H MRS investigations have reported altered glutamatergic neurotransmission in a variety of psychiatric disorders. The unraveling of glutamate from glutamine resonances is crucial for the interpretation of these observations, although this remains a challenge at clinical static magnetic field strengths. Glutamate resolution can be improved through an approach known as echo time (TE) averaging, which involves the acquisition and subsequent averaging of multiple TE steps. The process of TE averaging retains the central component of the glutamate methylene multiplet at 2.35 ppm, with the simultaneous attenuation of overlapping phase-modulated coupled resonances of glutamine and N-acetylaspartate. We have developed a novel post-processing approach, termed phase-adjusted echo time (PATE) averaging, for the retrieval of glutamine signals from a TE-averaged (1) H MRS dataset. The method works by the application of an optimal TE-specific phase term, which is derived from spectral simulation, prior to averaging over TE space. The simulation procedures and preliminary in vivo spectra acquired from the human frontal lobe at 2.89 T are presented. Three metabolite normalization schemes were developed to evaluate the frontal lobe test-retest reliability for glutamine measurement in six subjects, and the resulting values were comparable with previous reports for within-subject (9-14%) and inter-subject (14-20%) measures. Using the acquisition parameters and TE range described, glutamine quantification is possible in approximately 10 min. The post-processing methods described can also be applied retrospectively to extract glutamine and glutamate levels from previously acquired TE-averaged (1) H MRS datasets.

Magnetic resonance spectroscopy metabolite profiles predict survival in paediatric brain tumours

Background

Brain tumours cause the highest mortality and morbidity rate of all childhood tumour groups and new methods are required to improve clinical management. ¹H magnetic resonance spectroscopy (MRS) allows non-invasive concentration measurements of small molecules present in tumour tissue, providing clinically useful imaging biomarkers. The primary aim of this study was to investigate whether MRS detectable molecules can predict the survival of paediatric brain tumour patients.

Patients and methods

Short echo time (30 ms) single voxel ¹H MRS was performed on children attending Birmingham Children’s Hospital with a suspected brain tumour and 115 patients were included in the survival analysis. Patients were followed-up for a median period of 35 months and Cox-Regression was used to establish the prognostic value of individual MRS detectable molecules. A multivariate model of survival was also investigated to improve prognostic power.

Results

Lipids and scyllo-inositol predicted poor survival whilst glutamine and N-acetyl aspartate predicted improved survival (p < 0.05). A multivariate model of survival based on three MRS biomarkers predicted survival with a similar accuracy to histologic grading (p < 5e–5). A negative correlation between lipids and glutamine was found, suggesting a functional link between these molecules.

Conclusions

MRS detectable biomolecules have been identified that predict survival of paediatric brain tumour patients across a range of tumour types. The evaluation of these biomarkers in large prospective studies of specific tumour types should be undertaken. The correlation between lipids and glutamine provides new insight into paediatric brain tumour metabolism that may present novel targets for therapy.

Glutamatergic effects of divalproex in adolescents with mania: a proton magnetic resonance spectroscopy study

OBJECTIVES

This study used proton magnetic resonance spectroscopy ((1)H MRS) to evaluate the in vivo effects of extended-release divalproex sodium on the glutamatergic system in adolescents with bipolar disorder, and to identify baseline neurochemical predictors of clinical remission.

METHOD

Adolescents with bipolar disorder who were experiencing a manic or mixed episode (N = 25) were treated with open-label, extended-release divalproex (serum levels 85-125 μg/mL) and underwent (1)H MRS scanning at baseline (before treatment) and on days 7 and 28. Healthy comparison subjects (n = 15) also underwent (1)H MRS scanning at the same time points. Glutamate (Glu) and glutamate+glutamine (Glx) concentrations were measured in three voxels: anterior cingulate cortex (ACC), left ventrolateral prefrontal cortex (LVLPFC), and right ventrolateral prefrontal cortex (RVLPFC), and were compared between bipolar and healthy subjects. Within the bipolar subjects, Glu and Glx concentrations at baseline and each time point were also compared between remitters and nonremitters after divalproex treatment.

RESULTS

At baseline, no differences in Glu or Glx concentrations between bipolar and healthy subjects were observed. Group (HC vs. BP) by time effects revealed an interaction for Glu in the ACC, and change over time effects for Glx were noted in the ACC in patients with bipolar disorder (increase from day 0 to day 7 and then a decrease from day 7 to day 28) but not in HC. Remitters had significantly lower baseline Glx concentrations in LVLPFC, and in remitters the change in LVLPFC Glu correlated with the change in YMRS score.

CONCLUSIONS

Successful treatment of mania with divalproex may be predicted by lower baseline concentrations of Glx in the LVLPFC. In addition, in remitters, the degree of symptomatic improvement is related to the change in Glu concentrations in this region, suggesting that divalproex may work via modulation of the prefrontal glutamatergic system in youth with bipolar disorder.

Robust discrimination of glioblastomas from metastatic brain tumors on the basis of single-voxel (1)H MRS

This article investigates methods for the accurate and robust differentiation of metastases from glioblastomas on the basis of single-voxel (1)H MRS information. Single-voxel (1)H MR spectra from a total of 109 patients (78 glioblastomas and 31 metastases) from the multicenter, international INTERPRET database, plus a test set of 40 patients (30 glioblastomas and 10 metastases) from three different centers in the Barcelona (Spain) metropolitan area, were analyzed using a robust method for feature (spectral frequency) selection coupled with a linear-in-the-parameters single-layer perceptron classifier. For the test set, a parsimonious selection of five frequencies yielded an area under the receiver operating characteristic curve of 0.86, and an area under the convex hull of the receiver operating characteristic curve of 0.91. Moreover, these accurate results for the discrimination between glioblastomas and metastases were obtained using a small number of frequencies that are amenable to metabolic interpretation, which should ease their use as diagnostic markers. Importantly, the prediction can be expressed as a simple formula based on a linear combination of these frequencies. As a result, new cases could be straightforwardly predicted by integrating this formula into a computer-based medical decision support system. This work also shows that the combination of spectra acquired at different TEs (short TE, 20-32 ms; long TE, 135-144 ms) is key to the successful discrimination between glioblastomas and metastases from single-voxel (1)H MRS.

Correlating brain blood oxygenation level dependent (BOLD) fractal dimension mapping with magnetic resonance spectroscopy (MRS) in Alzheimer's disease

OBJECTIVES

To correlate temporal fractal structure of resting state blood oxygen level dependent (rsBOLD) functional magnetic resonance imaging (fMRI) with in vivo proton magnetic resonance spectroscopy ((1)H-MRS), in Alzheimer's disease (AD) and healthy age-matched normal controls (NC).

MATERIALS AND METHODS

High temporal resolution (4 Hz) rsBOLD signal and single voxel (left putamen) magnetic resonance spectroscopy data was acquired in 33 AD patients and 13 NC. The rsBOLD data was analyzed using two types of fractal dimension (FD) analysis based on relative dispersion and frequency power spectrum. Comparisons in FD were performed between AD and NC, and FD measures were correlated with (1)H-MRS findings.

RESULTS

Temporal fractal analysis of rsBOLD, was able to differentiate AD from NC subjects (P = 0.03). Low FD correlated with markers of AD severity including decreased concentrations of N-acetyl aspartate (R = 0.44, P = 0.015) and increased myoinositol (mI) (R = -0.45, P = 0.012).

CONCLUSION

Based on these results we suggest fractal analysis of rsBOLD could provide an early marker of AD.

Efficient spectral editing at 7 T: GABA detection with MEGA-sLASER

At high field (7 T) spectral editing of γ-aminobutyric acid with MEGA-point-resolved spectroscopy is inefficient due to the large chemical shift displacement error. In this article, a new pulse sequence is designed which has minimal chemical shift displacement error to perform an efficient spectral editing of the γ-aminobutyric acid 3.0 ppm resonance at 7 T. The sequence consists of the conventional MEGA editing pulses and a semi-localized by adiabatic selective refocusing sequence. Phantom and in vivo measurements demonstrated an efficient detection of γ-aminobutyric acid. Using ECG triggering, excellent in vivo performance of the MEGA-semi-localized by adiabatic selective refocusing (MEGA-sLASER) provided well-resolved γ-aminobutyric acid signals in 27 mL volumes in the human brain at an echo time of 74 ms within a relatively short acquisition time (5 min). Furthermore, the high efficiency of the MEGA-sLASER was demonstrated by acquiring small volumes (8 mL) at an echo time of 74 ms, as well as long echo time measurements (222 ms in 27 mL volume).

Applications of multi-nuclear magnetic resonance spectroscopy at 7T

AIM: To discuss the advantages of ultra-high field (7T) for ¹H and ¹³C magnetic resonance spectroscopy (MRS) studies of metabolism.

METHODS: Measurements of brain metabolites were made at both 3 and 7T using ¹H MRS. Measurements of glycogen and lipids in muscle were measured using ¹³C and ¹H MRS respectively.

RESULTS: In the brain, increased signal-to-noise ratio (SNR) and dispersion allows spectral separation of the amino-acids glutamate, glutamine and γ-aminobutyric acid (GABA), without the need for sophisticated editing sequences. Improved quantification of these metabolites is demonstrated at 7T relative to 3T. SNR was 36% higher, and measurement repeatability (% coefficients of variation) was 4%, 10% and 10% at 7T, vs 8%, 29% and 21% at 3T for glutamate, glutamine and GABA respectively. Measurements at 7T were used to compare metabolite levels in the anterior cingulate cortex (ACC) and insula. Creatine and glutamate levels were found to be significantly higher in the insula compared to the ACC (P < 0.05). In muscle, the increased SNR and spectral resolution at 7T enables interleaved studies of glycogen (¹³C) and intra-myocellular lipid (IMCL) and extra-myocellular lipid (EMCL) (¹H) following exercise and re-feeding. Glycogen levels were significantly decreased following exercise (-28% at 50% VO₂ max; -58% at 75% VO₂ max). Interestingly, levels of glycogen in the hamstrings followed those in the quadriceps, despite reduce exercise loading. No changes in IMCL and EMCL were found in the study.

CONCLUSION: The demonstrated improvements in brain and muscle MRS measurements at 7T will increase the potential for use in investigating human metabolism and changes due to pathologies.

Comparison of spectral fitting methods for overlapping J-coupled metabolite resonances

There is increasing interest in the use of two-dimensional J-resolved spectroscopic acquisition (multiecho) methods for in vivo proton magnetic resonance spectroscopy due to the improved discrimination of overlapping J-coupled multiplet resonances that is provided. Of particular interest is the potential for discrimination of the overlapping resonances of glutamate and glutamine. In this study, a new time-domain parametric spectral model that makes use of all available data is described for fitting the complete two-dimensional multiecho data, and the performance of this method was compared with fitting of one-dimensional spectra obtained following averaging multiecho data (echo time-averaged) and single-echo time PRESS (Point Resolved Spectroscopy) acquired spectra. These methods were compared using data obtained from a phantom containing typical brain metabolites and a human brain. Results indicate that improved performance and accuracy is obtained for the two-dimensional acquisition and spectral fitting model.

Metabolomic profiling with NMR discriminates between biphosphonate and doxorubicin effects on B16 melanoma cells

The metabolomic profiles of B16 melanoma cells were investigated in vitro with high resolution-magic angle spinning proton magnetic resonance spectroscopy and OPLS multivariate statistical analyse. We compared the profiles for untreated melanoma B16-F10 cells and Ca(2+) chelating EGTA, doxorubicin or BP7033 bisphosphonate treated cells. The two last molecules are known to induce anti-proliferative effects by different mechanisms of action in cells. Untreated and EGTA treated cells had similar profiles and were considered together as control cells. Several spectral regions could discriminate control from doxorubicin as well as BP7033 treated cells. Doxorubicin and BP7033 displayed distinct metabolic profiles. Important changes in neutral lipids and inositol were related to doxorubicin activity whereas BP7033 affected essentially phospholipids and alanine/lactate metabolism. These results provide new putative targets for both drugs. Metabolomics by NMR is shown here to be a good tool for the investigation of the mechanisms of action of drugs in pre-clinical studies.

Field strength dependence of PRESS timings for simultaneous detection of glutamate and glutamine from 1.5 to 7T

An optimization of the PRESS sequence for magnetic resonance spectroscopy is presented to simultaneously detect the important brain metabolites of glutamate (Glu) and glutamine (Gln) at field strengths of 1.5, 3, 4.7, and 7T. Standard, clinical examinations typically use short echo times which in general are not ideal for the separation of Glu and Gln. The optimization procedure is based on numerical product operator simulations to produce yield and overlap measurements for all possible practical choices of PRESS inter-echo timings. The simulations illustrate the substantial modulations in Glu and Gln with field strength. At all field strengths, the optimized timings demonstrate a significant reduction in overlap compared to short echo PRESS, while maintaining a high metabolite signal, with Glu and Gln yields >90% when excluding T2 relaxation losses. Minimal overlap was attained at 7T (0.3% Gln contamination in the Glu signal), and 4.7T (1.2%). The optimized timings were applied in vivo on healthy volunteers at field strengths of 1.5 and 4.7T.

Metabolic correlatives of brain activity in a FOS epilepsy patient

The correlation and the interactions between neuronal activity and underlying metabolic dynamics are still a matter of debate, especially in pathological conditions. This study reports findings obtained on a subject suffering from fixation-off sensitivity (FOS) epilepsy, exploited as a model system of triggerable anomalous electrical activity. Functional Magnetic Resonance Spectroscopy was used to investigate the metabolic response to visual spike-inducing stimuli in a single voxel placed in the temporo-occipital lobe of a FOS epilepsy patient. MRS measurements were additionally performed on a control group of five healthy volunteers. The FOS patient also underwent an EEG session with the same stimulus paradigm. Uniquely in the FOS patient, glutamate and glutamine concentration increased during the first 10 min of stimulation and then returned to baseline. On the other hand, FOS-induced epileptic activity (spiking) endured throughout all the stimulation epoch. The observed metabolic dynamics may be likely linked to a complex interplay between alterations of the metabolic pathways of glutamate and modulation of the neuronal activity.

Difference spectroscopy using PRESS asymmetry: application to glutamate, glutamine, and myo-inositol

A simple, clinically viable technique utilizing PRESS and strong coupling properties is presented for discrimination of coupled brain metabolites. The method relies on signal variation due to alteration of inter-echo timings (PRESS asymmetry) while maintaining a constant total echo time. Spin response of singlets and weakly coupled spins is unchanged due to PRESS asymmetry, allowing difference spectroscopy to detect unobstructed strongly coupled resonances. No changes to the standard PRESS sequence are required except variation of inter-echo timings. The procedure is illustrated for the separate detection of glutamate from glutamine and the detection of myo-inositol in simulation, phantom, and in vivo experiments at 4.7 T. The subtraction yields calculated from the simulation were 53% for glutamate and 75% for myo-inositol, and a resultant contribution of 96% glutamate to the total glutamate/glutamine multiplet in the 2.04-2.14 ppm range. To extend the treatment to other field strengths and metabolites, an analytical approximation based on a strongly coupled AB system was used to model individual spin groups. Subtraction spectroscopy yields for different combinations of coupling parameters were calculated for the detection of various strongly coupled metabolites at common clinical field strengths. The approximation also predicts adequate glutamate/glutamine discrimination at 3.0 T using the difference spectroscopy method.

Quantification of J-resolved proton spectra in two-dimensions with LCModel using GAMMA-simulated basis sets at 4 Tesla

A two-dimensional, J-resolved magnetic resonance spectroscopic extraction approach was developed employing GAMMA-simulated, LCModel basis-sets. In this approach, a two-dimensional J-resolved (2D-JPRESS) dataset was resolved into a series of one-dimensional spectra where each spectrum was modeled and fitted with its theoretically customized LCModel template. Metabolite levels were derived from the total integral across the J-series of spectra for each metabolite. Phantoms containing physiologic concentrations of the major brain chemicals were used for validation. Varying concentrations of glutamate and glutamine were evaluated at and around their accepted in vivo concentrations in order to compare the accuracy and precision of our method with 30 ms PRESS. We also assessed 2D-JPRESS and 30 ms PRESS in vivo, in a single voxel within the parieto-occipital cortex by scanning ten healthy volunteers once and a single healthy volunteer over nine repeated measures. Phantom studies demonstrated that serial fitting of 2D-JPRESS spectra with simulated LCModel basis sets provided accurate concentration estimates for common metabolites including glutamate and glutamine. Our in vivo results using 2D-JPRESS suggested superior reproducibility in measuring glutamine and glutamate relative to 30 ms PRESS. These novel methods have clear implications for clinical and research studies seeking to understand neurochemical dysfunction.

Comparative reliability of proton spectroscopy techniques designed to improve detection of J-coupled metabolites

Improved detection of J-coupled neurometabolites through the use of modified proton magnetic resonance spectroscopy (1H-MRS) techniques has recently been reported. TE-averaged point-resolved spectroscopy (PRESS) uses the J modulation effects by averaging FIDs with differing echo times to improve detection of glutamate, while standard PRESS detection of glutamate can be improved by using an appropriate single echo determined from J-modulation simulations. In the present study, the reliabilities of TE-averaged PRESS, standard PRESS with TE = 40 ms, and standard PRESS with TE = 30 ms in detecting metabolite levels in the cingulate gyrus of the human brain at 3T were compared in six subjects. TE-averaged PRESS measures showed a mean variability of 9% for N-acetyl aspartate, choline, and creatine, compared with < 4% for the 30- and 40-ms PRESS techniques. The coefficients of variation for glutamate were 10%, 7%, and 5% for TE-averaged, 30-ms, and 40-ms PRESS, respectively. PRESS with a TE of 40 ms also demonstrated improved reliability for GABA and glutamine concentrations. These results show that with the appropriate selection of echo time standard PRESS can be a reliable (1)H-MRS technique for the measurement of J-coupled neurometabolites in the human brain and, moreover, compares favorably with at least one J-edited technique.

MRI and MRS of human brain tumors

The purpose of this chapter is to provide an introduction to magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) of human brain tumors, including the primary applications and basic terminology involved. Readers who wish to know more about this broad subject should seek out the referenced books (1. Tofts (2003) Quantitative MRI of the brain. Measuring changes caused by disease. Wiley; Bradley and Stark (1999) 2. Magnetic resonance imaging, 3rd Edition. Mosby Inc; Brown and Semelka (2003) 3. MRI basic principles and applications, 3rd Edition. Wiley-Liss) or reviews (4. Top Magn Reson Imaging 17:127-36, 2006; 5. JMRI 24:709-724, 2006; 6. Am J Neuroradiol 27:1404-1411, 2006).MRI is the most popular means of diagnosing human brain tumors. The inherent difference in the magnetic resonance (MR) properties of water between normal tissues and tumors results in contrast differences on the image that provide the basis for distinguishing tumors from normal tissues. In contrast to MRI, which provides spatial maps or images using water signals of the tissues, proton MRS detects signals of tissue metabolites. MRS can complement MRI because the observed MRS peaks can be linked to inherent differences in biochemical profiles between normal tissues and tumors.The goal of MRI and MRS is to characterize brain tumors, including tumor core, edge, edema, volume, types, and grade. The commonly used brain tumor MRI protocol includes T2-weighted images and T1-weighted images taken both before and after the injection of a contrast agent (typically gadolinium: Gd). The commonly used MRS technique is either point-resolved spectroscopy (PRESS) or stimulated echo acquisition mode (STEAM).

Brain-mapping techniques for evaluating poststroke recovery and rehabilitation: a review

Brain-mapping techniques have proven to be vital in understanding the molecular, cellular, and functional mechanisms of recovery after stroke. This article briefly summarizes the current molecular and functional concepts of stroke recovery and addresses how various neuroimaging techniques can be used to observe these changes. The authors provide an overview of various techniques including diffusion-tensor imaging (DTI), magnetic resonance spectroscopy (MRS), ligand-based positron emission tomography (PET), single-photon emission computed tomography (SPECT), regional cerebral blood flow (rCBF) and regional metabolic rate of glucose (rCMRglc) PET and SPECT, functional magnetic resonance imaging (fMRI), near infrared spectroscopy (NIRS), electroencephalography (EEG), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS). Discussion in the context of poststroke recovery research informs about the applications and limitations of the techniques in the area of rehabilitation research. The authors also provide suggestions on using these techniques in tandem to more thoroughly address the outstanding questions in the field.

Spectral simplification for resolved glutamate and glutamine measurement using a standard STEAM sequence with optimized timing parameters at 3, 4, 4.7, 7, and 9.4T

The C4 multiplet proton resonances of glutamate (Glu) around 2.35 ppm and glutamine (Gln) around 2.45 ppm usually overlap in MR spectra, particularly at low- and mid-field strengths (1.5-4.7T). A spectral simplification approach is introduced that provides unobstructed Glu and Gln measurement using a standard STEAM localization sequence with optimized interpulse timings. The underlying idea is to exploit the dependence of response of a coupled spin system on the echo time (TE) and mixing time (TM) to find an optimum timing set (TE, TM), at which the outer-wings of C4 "pseudo-triplet" proton resonances of Glu and Gln are significantly suppressed while the central peaks are maintained. The spectral overlap is thus resolved as the overlap exists exclusively at the outer-wings and the central peaks are readily separated due to the approximate 0.1-ppm difference in chemical shift. Density matrix simulation for Glu, Gln, and other overlapping metabolites at 2.3-2.5 ppm was conducted to predict the optimum timing sets. The simulated, phantom, and in vivo results demonstrated that the C4 multiplet proton resonances of Glu and Gln can be resolved for unobstructed detection at 3T, 4T, and 4.7T. For simplicity, only simulated data are illustrated at 7T and 9.4T.