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.     

Unedited in vivo detection and quantification of γ‐aminobutyric acid in the occipital cortex using short‐TE MRS at 3 T

Short‐TE MRS has been proposed recently as a method for the in vivo detection and quantification of γ‐aminobutyric acid (GABA) in the human brain at 3 T. In this study, we investigated the accuracy and reproducibility of short‐TE MRS measurements of GABA at 3 T using both simulations and experiments. LCModel analysis was performed on a large number of simulated spectra with known metabolite input concentrations. Simulated spectra were generated using a range of spectral linewidths and signal‐to‐noise ratios to investigate the effect of varying experimental conditions, and analyses were performed using two different baseline models to investigate the effect of an inaccurate baseline model on GABA quantification. The results of these analyses indicated that, under experimental conditions corresponding to those typically observed in the occipital cortex, GABA concentration estimates are reproducible (mean reproducibility error, <20%), even when an incorrect baseline model is used. However, simulations indicate that the accuracy of GABA concentration estimates depends strongly on the experimental conditions (linewidth and signal‐to‐noise ratio). In addition to simulations, in vivo GABA measurements were performed using both spectral editing and short‐TE MRS in the occipital cortex of 14 healthy volunteers. Short‐TE MRS measurements of GABA exhibited a significant positive correlation with edited GABA measurements (R = 0.58, p < 0.05), suggesting that short‐TE measurements of GABA correspond well with measurements made using spectral editing techniques. Finally, within‐session reproducibility was assessed in the same 14 subjects using four consecutive short‐TE GABA measurements in the occipital cortex. Across all subjects, the average coefficient of variation of these four GABA measurements was 8.7 ± 4.9%. This study demonstrates that, under some experimental conditions, short‐TE MRS can be employed for the reproducible detection of GABA at 3 T, but that the technique should be used with caution, as the results are dependent on the experimental conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Gamma‐aminobutyric acid B Receptor Immunoreactivity in the Mouse Adrenal Medulla

The present study examined gamma‐aminobutyric acid B (GABA_B) receptor, GABA, choline acetyltransferase (ChAT), and neuronal nitric oxide synthase (nNOS) immunoreactivities in the mouse adrenal medulla. GABA_B receptor immunoreactivity was seen in numerous chromaffin cells and in a few ganglion cells of the adrenal medulla. By using a formaldehyde‐induced fluorescence (FIF) method, GABA_B receptor immunoreactivity was observed in numerous adrenaline (A) cells, but not in noradrenaline (NA) cells showing blue‐white fluorescence. This suggests that GABA_B receptors may be present in the A cells and be related to the secretory activity of A cells but not NA cells in the mouse adrenal medulla. GABA_B receptor immunoreactive ganglion cells were shown to be nNOS immunopositive by using a double immunostaining method. Weak GABA immunoreactivity was visible in some chromaffin cells and in the numerous nerve fibers of the medulla. By using the FIF method, weak GABA‐immunoreactive chromaffin cells were shown to be in the NA cells showing blue‐white fluorescence. GABA‐immunoreactive nerve fibers were in dense contact in A cells, but not NA cells. GABA‐immunoreactive nerve fibers closely contacted a few ganglion cells. Numerous GABA‐immunoreactive nerve fibers in the medulla showed ChAT immunoreactive. This result suggests that GABA and acetylcholine may be released from the same nerve fibers and may have a secretory effect on the A cells of the medulla. Anat Rec, 296:971–978, 2013. © 2013 Wiley Periodicals, Inc.     

Reproducibility and effect of tissue composition on cerebellar γ‐aminobutyric acid (GABA) MRS in an elderly population

MRS provides a valuable tool for the non‐invasive detection of brain γ‐aminobutyric acid (GABA) in vivo. GABAergic dysfunction has been observed in the aging cerebellum. The study of cerebellar GABA changes is of considerable interest in understanding certain age‐related motor disorders. However, little is known about the reproducibility of GABA MRS in an aged population. Therefore, this study aimed to explore the feasibility and reproducibility of GABA MRS in the aged cerebellum at 3.0 T and to examine the effect of differing tissue composition on GABA measurements. MRI and ¹H MRS examinations were performed on 10 healthy elderly volunteers (mean age, 75.2 ± 6.5 years) using a 3.0‐T Siemens Tim Trio scanner. Among them, five subjects were scanned twice to assess the short‐term reproducibility. The MEGA‐PRESS (Mescher–Garwood point‐resolved spectroscopy) J‐editing sequence was used for GABA detection in two volumes of interest (VOIs) in the left and right cerebellar dentate. MRS data processing and quantification were performed with LCModel 6.3‐0L using two separate basis sets, generated from density matrix simulations using published values for chemical shifts and J couplings. Raw metabolite levels from LCModel outputs were corrected for cerebrospinal fluid contamination and relaxation. GABA‐edited spectra yielded robust and stable GABA measurements with averaged intra‐individual coefficients of variation for corrected GABA+ between 4.0 ± 2.8% and 13.4 ± 6.3%, and inter‐individual coefficients of variation between 12.6% and 24.2%. In addition, there was a significant correlation between GABA+ obtained with the two LCModel basis sets. Overall, our results demonstrated the feasibility and reproducibility of cerebellar GABA‐edited MRS at 3.0 T in an elderly population. This information might be helpful for studies using this technique to study GABA changes in normal or diseased aging brain, e.g. for power calculations and the interpretation of longitudinal observations. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantification of GABA,glutamate and glutamine in a single measurement at 3 T using GABA‐edited MEGA‐PRESS

γ‐Aminobutyric acid (GABA) and glutamate (Glu), major neurotransmitters in the brain, are recycled through glutamine (Gln). All three metabolites can be measured by magnetic resonance spectroscopy in vivo, although GABA measurement at 3 T requires an extra editing acquisition, such as Mescher–Garwood point‐resolved spectroscopy (MEGA‐PRESS). In a GABA‐edited MEGA‐PRESS spectrum, Glu and Gln co‐edit with GABA, providing the possibility to measure all three in one acquisition. In this study, we investigated the reliability of the composite Glu + Gln (Glx) peak estimation and the possibility of Glu and Gln separation in GABA‐edited MEGA‐PRESS spectra. The data acquired in vivo were used to develop a quality assessment framework which identified MEGA‐PRESS spectra in which Glu and Gln could be estimated reliably. Phantoms containing Glu, Gln, GABA and N‐acetylaspartate (NAA) at different concentrations were scanned using GABA‐edited MEGA‐PRESS at 3 T. Fifty‐six sets of spectra in five brain regions were acquired from 36 healthy volunteers. Based on the Glu/Gln ratio, data were classified as either within or outside the physiological range. A peak‐by‐peak quality assessment was performed on all data to investigate whether quality metrics can discriminate between these two classes of spectra. The quality metrics were as follows: the GABA signal‐to‐noise ratio, the NAA linewidth and the Glx Cramer–Rao lower bound (CRLB). The Glu and Gln concentrations were estimated with precision across all phantoms with a linear relationship between the measured and true concentrations: R¹ = 0.95 for Glu and R¹ = 0.91 for Gln. A quality assessment framework was set based on the criteria necessary for a good GABA‐edited MEGA‐PRESS spectrum. Simultaneous criteria of NAA linewidth <8 Hz and Glx CRLB <16% were defined as optimum features for reliable Glu and Gln quantification. Glu and Gln can be reliably quantified from GABA‐edited MEGA‐PRESS acquisitions. However, this reliability should be controlled using the quality assessment methods suggested in this work.     

The in vivo J‐difference editing MEGA‐PRESS technique for the detection of n‐3 fatty acids

In this study, we present a method for the detection of n‐3 fatty acid (n‐3 FA) signals using MRS in adipose tissue in vivo. This method (called oMEGA‐PRESS) is based on the selective detection of the CH₃ signal of n‐3 FA using the MEGA‐PRESS (MEshcher–GArwood Point‐RESolved Spectroscopy) J‐difference editing technique. We optimized the envelope shape and frequency of spectral editing pulses to minimize the spurious co‐editing and incomplete subtraction of the CH₃ signal of other FAs, which normally obscure the n‐3 FA CH₃ signal in MR spectra acquired using standard PRESS techniques. The post‐processing of the individual data scans with the phase and frequency correction before data subtraction and averaging was implemented to further improve the quality of in vivo spectra. The technique was optimized in vitro on lipid phantoms using various concentrations of n‐3 FA and examined in vivo at 3 T on 15 healthy volunteers. The proportion of n‐3 FA estimated by the oMEGA‐PRESS method in phantoms showed a highly significant linear correlation with the n‐3 FA content determined by gas chromatography. The signal attributed to n‐3 FA was observed in all subjects. Comparisons with the standard PRESS technique revealed an enhanced identification of the n‐3 FA signal using oMEGA‐PRESS. The presented method may be useful for the non‐invasive quantification of n‐3 FA in adipose tissue, and could aid in obtaining a better understanding of various aspects of n‐3 FA metabolism. Copyright © 2014 John Wiley & Sons, Ltd.     

Measurement of regional variation of GABA in the human brain by optimized point‐resolved spectroscopy at 7 T in vivo

The ¹H resonances of γ‐aminobutyric acid (GABA) in the human brain in vivo are extensively overlapped with the neighboring abundant resonances of other metabolites and remain indiscernible in short‐TE MRS at 7 T. Here we report that the GABA resonance at 2.28 ppm can be fully resolved by means of echo time optimization of a point‐resolved spectroscopy (PRESS) scheme. Following numerical simulations and phantom validation, the subecho times of PRESS were optimized at (TE, TE₂) = (31, 61) ms for detection of GABA, glutamate (Glu), glutamine (Gln), and glutathione (GSH). The in vivo feasibility of the method was tested in several brain regions in nine healthy subjects. Spectra were acquired from the medial prefrontal, left frontal, medial occipital, and left occipital brain and analyzed with LCModel. Following the gray and white matter (GM and WM) segmentation of T₁‐weighted images, linear regression of metabolite estimates was performed against the fractional GM contents. The GABA concentration was estimated to be about seven times higher in GM than in WM. GABA was overall higher in frontal than in occipital brain. Glu was about twice as high in GM as in WM in both frontal and occipital brain. Gln was significantly different between frontal GM and WM while being similar between occipital GM and WM. GSH did not show significant dependence on tissue content. The signals from N‐acetylaspartylglutamate were clearly resolved, giving the concentration more than 10 times higher in WM than in GM. Our data indicate that the PRESS TE = 92 ms method provides an effective means for measuring GABA and several challenging J‐coupled spin metabolites in human brain at 7 T. Copyright © 2014 John Wiley & Sons, Ltd.     

In silico GABA+ MEGA‐PRESS: Effects of signal‐to‐noise ratio and linewidth on modeling the 3 ppm GABA+ resonance

To investigate the GABA+ modeling accuracy of MEGA‐PRESS GABA+‐edited MRS data with various spectral quality scenarios, the influence of varying signal‐to‐noise ratio (SNR) and linewidth on the model estimates was quantified. MEGA‐PRESS data from 46 volunteers were averaged to generate a template MEGA‐PRESS spectrum, which was modeled and quantified to generate a GABA+ level ground truth. This spectrum was then manipulated by adding 427 combinations of varying artificial noise levels and line broadening, mimicking variations in GABA+ SNR and B₀ homogeneity. GABA+ modeling and quantification was performed with 100 simulated spectra per condition using automated routines in both Gannet 3.0 and Tarquin. The GABA+ estimation error was calculated as the relative deviation to the quantified GABA+ ground truth levels to assess the accuracy of GABA+ modeling. Finally, the accordance between the simulations and different in vivo scenarios was assessed. The GABA+ estimation error was smaller than 5% for all GABA+ SNR values with creatine linewidths lower than 9.7 Hz in Gannet 3.0 or unequal 10.6 Hz in Tarquin. The standard deviation of the GABA+ amplitude over 100 spectra per condition varied between 3.1 and 17% (Gannet 3.0) and between 1 and 11% (Tarquin) over the in vivo relevant GABA+ SNR range between 2.6 and 3.5. GABA+ edited studies might be realized for voxels with low GABA+ SNR at the cost of higher group‐level variance. The accuracy of GABA+ modeling had no relation to commonly used quality metrics. The Tarquin algorithm was found to be more robust against linewidth changes than the fitting algorithm in Gannet.     

Simultaneous detection of valine and lactate using MEGA‐PRESS editing in pyogenic brain abscess

Valine and lactate have been recognized as important metabolic markers to diagnose brain abscess by means of MRS. However, in vivo unambiguous detection and quantification is hampered by macromolecular contamination. In this work, MEGA‐PRESS difference editing of valine and lactate is proposed. The method is validated in vitro and applied for quantitative in vivo experiments in one healthy subject and two brain abscess patients. It is demonstrated that with this technique the overlapping lipid signal can be reduced by more than an order of magnitude and thus the robustness of valine and lactate detection in vivo can be enhanced. Quantification of the two abscess MEGA‐PRESS spectra yielded valine/lactate concentration ratios of 0.10 and 0.27. These ratios agreed with the concentration ratios determined from concomitantly acquired short‐T_E PRESS data and were in line with literature values. The quantification accuracy of lactate (as measured with Cramér‐Rao lower bounds in LCModel processing) was better for MEGA‐PRESS than for short‐T_E PRESS in all acquired in vivo datasets. The Cramér‐Rao lower bounds of valine were only better for MEGA‐PRESS in one of the two abscess cases, while in the other case coediting of isoleucine confounded the quantification in the MEGA‐PRESS analysis. MEGA‐PRESS and short‐T_E PRESS should be combined for unambiguous quantification of amino acids in abscess measurements. Simultaneous valine/lactate MEGA‐PRESS editing might benefit the distinction of brain abscesses from tumors, and further categorization of bacteria with reasonable sensitivity and specificity.     

Noninvasive quantification of ascorbate and glutathione concentration in the elderly human brain

In this study, ascorbate (Asc) and glutathione (GSH) concentrations were quantified noninvasively using double-edited (1)H MRS at 4 T in the occipital cortex of healthy young [age (mean ± standard deviation) = 20.4 ± 1.4 years] and elderly (age = 76.6 ± 6.1 years) human subjects. Elderly subjects had a lower GSH concentration than younger subjects (p < 0.05). The Asc concentration was not significantly associated with age. Furthermore, the lactate (Lac) concentration was higher in elderly than young subjects. Lower GSH and higher Lac concentrations are indications of defective protection against oxidative damage and impaired mitochondrial respiration. The extent to which the observed concentration differences could be associated with physiological differences and methodological artifacts is discussed. In conclusion, GSH and Asc concentrations were compared noninvasively for the first time in young vs elderly subjects.     

Accurate determination of brain metabolite concentrations using ERETIC as external reference

Magnetic Resonance Spectroscopy (MRS) can provide in vivo metabolite concentrations in standard concentration units if a reliable reference signal is available. For ¹H MRS in the human brain, typically the signal from the tissue water is used as the (internal) reference signal. However, a concentration determination based on the tissue water signal most often requires a reliable estimate of the water concentration present in the investigated tissue. Especially in clinically interesting cases, this estimation might be difficult. To avoid assumptions about the water in the investigated tissue, the Electric REference To access In vivo Concentrations (ERETIC) method has been proposed. In this approach, the metabolite signal is compared with a reference signal acquired in a phantom and potential coil‐loading differences are corrected using a synthetic reference signal. The aim of this study, conducted with a transceiver quadrature head coil, was to increase the accuracy of the ERETIC method by correcting the influence of spatial B₁ inhomogeneities and to simplify the quantification with ERETIC by incorporating an automatic phase correction for the ERETIC signal. Transmit field ( ) differences are minimized with a volume‐selective power optimization, whereas reception sensitivity changes are corrected using contrast‐minimized images of the brain and by adapting the voxel location in the phantom measurement closely to the position measured in vivo. By applying the proposed B₁ correction scheme, the mean metabolite concentrations determined with ERETIC in 21 healthy subjects at three different positions agree with concentrations derived with the tissue water signal as reference. In addition, brain water concentrations determined with ERETIC were in agreement with estimations derived using tissue segmentation and literature values for relative water densities. Based on the results, the ERETIC method presented here is a valid tool to derive in vivo metabolite concentration, with potential advantages compared with internal water referencing in diseased tissue.     

A comparative study of short‐ and long‐TE 1H MRS at 3 T for in vivo detection of 2‐hydroxyglutarate in brain tumors

2‐Hydroxyglutarate (2HG) is produced in gliomas with mutations of isocitrate dehydrogenase (IDH) 1 and 2. The ¹H resonances of the J‐coupled spins of 2HG are extensively overlapped with signals from other metabolites. Here, we report a comparative study at 3 T of the utility of the point‐resolved spectroscopy sequence with a standard short TE (35 ms) and a long TE (97 ms), which had been theoretically designed for the detection of the 2HG 2.25‐ppm resonance. The performance of the methods is evaluated using data from phantoms, seven healthy volunteers and 22 subjects with IDH‐mutated gliomas. The results indicate that TE = 97 ms provides higher detectability of 2HG than TE = 35 ms, and that this improved capability is gained when data are analyzed with basis spectra that include the effects of the volume localizing radiofrequency and gradient pulses. Copyright © 2013 John Wiley & Sons, Ltd.     

The Effect of Sevoflurane Inhalation on Gabaergic Neurons Activation: Observation on the GAD67‐GFP Knock‐In Mouse

The mechanisms underlying volatile anesthesia agents are not well elucidated. Emerging researches have focused on the participation of γ‐aminobutyric acid (GABA) neurons but there still lacks morphological evidence. To elucidate the possible activation of GABAergic neurons by sevoflurane inhalation in morphology, Fos (as neuronal activity marker) and GABA neurons double labeling were observed on the brain of glutamic acid decarboxylase (GAD) 67‐GFP knock‐in mice after sevoflurane inhalation. Twenty GAD67‐GFP knock‐in mice were divided into three groups: S1 group: incomplete anesthesia state induced by sevoflurane; S2 group: complete anesthesia state induced by sevoflurane; control(C) group. Sevoflurane induced a significant increase of Fos expression in the dorsomedial hypothalamic nucleus (DM), periaqueductal grey (PAG), hippocampus (CA1, DG), paraventricular thalamic nucleus (PV), lateral septal nucleus (LS), and cingulate cortex (Cg1 and Cg2) in S1 group compared to C group, and increase of Fos expression in S2 group compared to S1 group. In S2 group, Fos was only expressed in the medial amygdaloid nucleus (MeA), Edinger–Westphal (E–W) nucleus, arcuate hypothalamic nucleus (Arc) and the ventral part of paraventricular hypothalamic nucleus (PaV). Double immunofluroscent staining indicated that in LS, almost all Fos werepresent in GABAergic neurons. In CA1, DG, DM, cg1, cg2, and PAG, Fos was expressed as well, but only few were present in GABAergic neurons. Fos expression was very high in thalamus, but no coexistence were found as noGABAergic neuron was detected in this area. Our results provided morphological evidence that GABAergic transmission in specific brain areas may participate in the sevoflurane‐induced anesthesia. Anat Rec, 2010. © 2010 Wiley‐Liss, Inc.     

GABA‐A receptor β3 and α5 subunit gene cluster on chromosome 15q11–q13 and bipolar disorder: A genetic association study

There is accumulated evidence that the genes coding for the receptor of gamma aminobutyric acid (GABA), the most important inhibitory neurotransmitter in the CNS, may be involved in the pathogenesis of affective disorders. In a previous study, we have found a genetic association between the GABA‐A receptor α5 subunit gene locus (GABRA5) on chromosome 15q11–of 13 and bipolar affective disorder. The aim of the present study was to examine the same subjects to see if there exists a genetic association between bipolar affective disorder and the GABA receptor β3 subunit gene (GABRB3), which is located within 100 kb from GABRA5. The sample consisted of 48 bipolar patients compared to 44 controls (blood donors). All subjects were Greek, unrelated, and personally interviewed. Diagnosis was based on DSM‐IV and ICD‐10 criteria. The marker used was a dinucleotide (CA) repeat polymorphism with 12 alleles 179 to 201 bp long; genotyping was successful in all patients and 43 controls. The distribution of GABRB3 genotypes among the controls did not deviate significantly from the Hardy‐Weinberg equilibrium. No differences in allelic frequencies between bipolar patients and controls were found for GABRB3, while this locus and GABRA5 did not seem to be in significant linkage disequilibrium. In conclusion, the GABRB3 CA‐repeat polymorphism we investigated does not present the observed association between bipolar affective illness and GABRA5. This could be due to higher mutation rate in the GABRB3 CA‐repeat polymorphism, but it might also signify that GABRA5 is the gene actually associated with the disease. © 2001 Wiley‐Liss, Inc.     

Simultaneous observation of glutamate, gamma-aminobutyric acid, and glutamine in human brain at 4.7 T using localized two-dimensional constant-time correlation spectroscopy

Localized two-dimensional constant-time correlation spectroscopy (CT-COSY) was used to resolve glutamate (Glu), gamma-aminobutyric acid (GABA), and glutamine (Gln) in the human brain at 4.7 T. In this method, three-dimensional localization was achieved using three radio frequency pulses of the CT-COSY module for slice selection. As this sequence could decouple JHH along the F1 direction, peak resolution of metabolites was improved even on a magnitude-mode display. In experiments on a phantom containing N-acetylaspartate, creatine, Glu, Gln, and GABA with a constant time delay (Tct) of 110 ms, cross peaks of Glu, Gln, and GABA were obtained on a spectrum processed with standard sine-bell windows, which emphasize sine-dependent signals along the t2 direction. In contrast, diagonal peaks of Glu C4H at 2.35 ppm, GABA C2H at 2.28 ppm, and Gln C4H at 2.44 ppm were resolved on a spectrum processed with Gaussian windows, which emphasize cosine-dependent signals along t2. Human brain spectra were obtained from a 27 mL voxel within the parieto-occipital region using a volume transverse electromagnetic (TEM) coil for both transmission and reception. Tct was 110 ms; the total scan time was 30 min. Diagonal peaks of Glu C4H, GABA C2H, and Gln C4H were also resolved on the spectrum processed with Gaussian windows. These results show that the localized two-dimensional CT-COSY method featuring 1H decoupling along the F1 direction could resolve Glu, GABA, and Gln signals in the human brain.     

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.     

3D spatially encoded and accelerated TE‐averaged echo planar spectroscopic imaging in healthy human brain

Several different pathologies, including many neurodegenerative disorders, affect the energy metabolism of the brain. Glutamate, a neurotransmitter in the brain, can be used as a biomarker to monitor these metabolic processes. One method that is capable of quantifying glutamate concentration reliably in several regions of the brain is TE‐averaged ¹H spectroscopic imaging. However, this type of method requires the acquisition of multiple TE lines, resulting in long scan durations. The goal of this experiment was to use non‐uniform sampling, compressed sensing reconstruction and an echo planar readout gradient to reduce the scan time by a factor of eight to acquire TE‐averaged spectra in three spatial dimensions. Simulation of glutamate and glutamine showed that the 2.2–2.4 ppm spectral region contained 95% glutamate signal using the TE‐averaged method. Peak integration of this spectral range and home‐developed, prior‐knowledge‐based fitting were used for quantitation. Gray matter brain phantom measurements were acquired on a Siemens 3 T Trio scanner. Non‐uniform sampling was applied retrospectively to these phantom measurements and quantitative results of glutamate with respect to creatine 3.0 (Glu/Cr) ratios showed a coefficient of variance of 16% for peak integration and 9% for peak fitting using eight‐fold acceleration. In vivo scans of the human brain were acquired as well and five different brain regions were quantified using the prior‐knowledge‐based algorithm. Glu/Cr ratios from these regions agreed with previously reported results in the literature. The method described here, called accelerated TE‐averaged echo planar spectroscopic imaging (TEA‐EPSI), is a significant methodological advancement and may be a useful tool for categorizing glutamate changes in pathologies where affected brain regions are not known a priori. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatial variability and reproducibility of GABA‐edited MEGA‐LASER 3D‐MRSI in the brain at 3 T

The reproducibility of gamma‐aminobutyric acid (GABA) quantification results, obtained with MRSI, was determined on a 3 T MR scanner in healthy adults. In this study, a spiral‐encoded, GABA‐edited, MEGA‐LASER MRSI sequence with real‐time motion–scanner‐instability corrections was applied for robust 3D mapping of neurotransmitters in the brain. In particular, the GABA⁺ (i.e. GABA plus macromolecule contamination) and Glx (i.e. glutamate plus glutamine contamination) signal was measured. This sequence enables 3D‐MRSI with about 3 cm³ nominal resolution in about 20 min. Since reliable quantification of GABA is challenging, the spatial distribution of the inter‐subject and intra‐subject variability of GABA⁺ and Glx levels was studied via test–retest assessment in 14 healthy volunteers (seven men–seven women). For both inter‐subject and intra‐subject repeated measurement sessions a low coefficient of variation (CV) and a high intraclass correlation coefficient (ICC) were found for GABA⁺ and Glx ratios across all evaluated voxels (intra?/inter‐subject: GABA⁺ ratios, CV ~ 8%–ICC > 0.75; Glx ratios, CV ~ 6%–ICC > 0.70). The same was found in selected brain regions for Glx ratios versus GABA⁺ ratios (CV varied from about 5% versus about 8% in occipital and parietal regions, to about 8% versus about 10% in the frontal area, thalamus, and basal ganglia). These results provide evidence that 3D mapping of GABA⁺ and Glx using the described methodology provides high reproducibility for application in clinical and neuroscientific studies.