In vivo and ex vivo evidence for ketamine-induced hyperglutamatergic activity in the cerebral cortex of the rat: Potential relevance to schizophrenia

Subanesthetic doses of ketamine, a noncompetitive N‐methyl‐D ‐aspartate (NMDA) receptor antagonist, impair prefrontal cortex (PFC) function in the rat and produce symptoms in humans similar to those observed in patients with schizophrenia. In the present study, in vivo ¹H‐MRS and ex vivo ¹H high‐resolution magic angle spinning (HR‐MAS) spectroscopy was used to examine the brain metabolism of rats treated with subanesthetic doses of ketamine (30 mg/kg) for 6 days. A single voxel localization sequence (PRESS, TR/TE = 4000/20 ms and NEX = 512) was used to acquire the spectra in a 30‐µl voxel positioned in the cerebral cortex (including mainly PFC) of the rats (ketamine group: n = 12; saline group: n = 12) anesthetized with isoflurane. After the in vivo ¹H‐MRS acquisition, the animals were sacrificed and the cerebral cortex tissues were extracted (ketamine group: n = 7; saline group: n = 7) for ex vivo ¹H HR‐MAS spectroscopy (CPMG sequence, 2.0‐s presaturation delay, 2.0‐s acquisition time, 128 transients and 4‐ms inter‐pulse delay) using a 500‐MHz NMR spectrometer. All proton metabolites were quantified using the LCModel. For the in vivo spectra, there was a significant increase in glutamate concentration in the cerebral cortex of the ketamine group compared with the controls (p < 0.05). For the ex vivo HR‐MAS spectra, there was a significant increase in the glutamate/total creatine ratio, and a decrease in the glutamine/total creatine and glutamine/glutamate ratios in the cerebral cortex tissue of the ketamine group compared with the controls. The results of the present study demonstrated that administration of subanesthetic doses of ketamine in the rat may exert at least part of their effect in the cerebral cortex by activation of glutamatergic neurotransmission. Copyright © 2011 John Wiley & Sons, Ltd.     

Regional metabolic differences in rat prefrontal cortex measured with in vivo 1H‐MRS correlate with regional histochemical differences

Many neurological/psychiatric disorders are associated with metabolic abnormalities in the brain observable with in vivo proton MRS (¹H‐MRS). The underlying molecular/cellular mechanisms and functional correlations of such metabolic alterations, however, are yet to be understood fully. The rodent prefrontal cortex (PFC) is comprised of multiple sub‐regions with distinctive cytoarchitecture and functions, providing a good model system to study the correlations among cerebral metabolism, regional cytoarchitecture and connectivity. In this study, the metabolic profiles in two voxels containing mainly the medial PFC (mPFC) and posterior part of the cingulate cortex (pCG), respectively, were measured with single‐voxel in vivo ¹H‐MRS in adult male rats. The levels of glutamine synthetase and glutamatergic synaptic proteins, including vesicular glutamate transporter 1, vesicular glutamate transporter 2 (VGLUT2) and post‐synaptic density protein 95 (PSD95), as well as the density of astrocytes, in these two regions were also compared semi‐quantitatively. It was shown that, relative to the pCG voxel, the mPFC voxel had significantly higher N‐acetyl aspartate, glutamate (Glu), glutamine (Gln), Glx (Glu + Gln), myo‐inositol and taurine levels. The VGLUT2/PSD95 levels and astrocyte density were also higher in the mPFC voxel than in the pCG voxel. Taken together, these results indicated that regional metabolic variations in the PFC of the adult male rat may reflect regional differences in the density of astrocytes and glutamatergic terminals associated with subcortical projections. The study provided a link between the Glu concentration measured with localized in vivo ¹H‐MRS and regional glutamatergic activities/connections in the rat PFC.     

Metabolic changes detected in vivo by 1H MRS in the MPTP‐intoxicated mouse

We used in vivo proton (¹H) Magnetic Resonance Spectroscopy (MRS) to measure the levels of the main excitatory amino acid, glutamate (Glu) and also glutamine (Gln) and GABA in the striatum and cerebral cortex in the MPTP‐intoxicated mouse, a model of dopaminergic denervation, before and after dopamine (DA) replacement. The study was performed at 9.4T on control mice (n = 8) and MPTP‐intoxicated mice (n = 8). In vivo spectra were acquired in a voxel (8 µL) centered in the striatum, and in the cortex (4.6 µL). Three days after basal MRS acquisitions new spectra were acquired in the striatum and cortex, after levodopa (200 mg.kg^?1). Glu, Gln and GABA concentrations obtained in the basal state were significantly increased in the striatum of MPTP‐lesioned mice (Glu: 20.2 ± 0.8 vs 11.4 ± 0.9 mM, p < 0.001; Gln: 5.4 ± 1.6 vs 2.0 ± 0.6 mM, p < 0.05; GABA: 3.6 ± 0.8 vs 1.6 ± 0.2 mM, p < 0.05). Levodopa lowered metabolites concentrations in the striatum of MPTP‐lesioned mice (Glu: 20.2 ± 0.8 vs 11.2 ± 0.4 mM (+ Ldopa), p < 0.001; Gln: 5.4 ± 1.6 vs 1.6 ± 0.4 mM (+ Ldopa), p < 0.05; GABA: 3.6 ± 0.8 vs 1.7 ± 0.4 mM (+ Ldopa), p < 0.01). Metabolite levels in the striatum of MPTP‐intoxicated mice + levodopa were not significantly different from those in the striatum of controls. No change was found in the cortex after DA denervation and after DA replacement between the two animals groups. These results strongly support a predominant change in striatal Glu synaptic activity in the cortico‐striatal pathway. Acute levodopa administration reverses the increase of metabolites in the striatum. Copyright © 2010 John Wiley & Sons, Ltd.     

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. Copyright © 2009 John Wiley & Sons, Ltd.     

Reproducibility of 7-T brain spectroscopy using an ultrashort echo time STimulated Echo Acquisition Mode sequence and automated voxel repositioning

Establishing the reproducibility of brain MRS is important for clinical studies so that researchers can evaluate changes in metabolites due to treatment or the course of a disease and better understand the brain in healthy and disordered states. Prior 7-T MRS reproducibility studies using the stimulated echo acquisition mode (STEAM) sequence have focused on the anterior cingulate cortex or posterior cingulate cortex and precuneus. The purpose of this study was to evaluate the reproducibility of metabolite measurements in the dorsolateral prefrontal cortex (DLPFC) using an ultrashort echo time (TE) STEAM sequence and automated voxel repositioning. Spectra were acquired during two scan sessions from nine subjects using the AutoAlign method for voxel repositioning. Reproducibility was evaluated with coefficients of variation (CVs) and percentage differences. The mean intrasubject CVs were less than 6% for the major metabolites glutamate, N-acetylaspartate, total creatine, total choline, and myo-inositol. The mean CVs were less than 20% for the smaller signals of GABA, glutamine, glutathione, and taurine. These results indicate that 7-T MRS using a STEAM sequence with ultrashort TE and automated voxel repositioning provides excellent reproducibility of metabolites in the DLPFC.     

Single‐voxel 1H spectroscopy in the human hippocampus at 3 T using the LASER sequence: characterization of neurochemical profile and reproducibility

The hippocampus is crucial for long‐term episodic memory and learning. It undergoes structural change in aging and is sensitive to neurodegenerative and psychiatric diseases. MRS studies have seldom been performed in the hippocampus due to technical challenges. The reproducibility of MRS in the hippocampus has not been evaluated at 3 T. The purpose of the present study was to quantify the concentration of metabolites in a small voxel placed in the hippocampus and evaluate the reproducibility of the quantification. Spectra were measured in a 2.4 mL voxel placed in the left hippocampus covering the body and most of the tail of the structure in 10 healthy subjects across three different sessions and quantified using LCModel. High‐quality spectra were obtained, which allowed a reliable quantification of 10 metabolites including glutamate and glutamine. Reproducibility of MRS was evaluated with coefficient of variation, standard errors of measurement, and intraclass correlation coefficients. All of these measures showed improvement with increased number of averages. Changes of less than 5% in concentration of N‐acetylaspartate, choline‐containing compounds, and total creatine and of less than 10% in concentration of myo‐inositol and the sum of glutamate and glutamine can be confidently detected between two measurements in a group of 20 subjects. A reliable and reproducible neurochemical profile of the human hippocampus was obtained using MRS at 3 T in a small hippocampal volume. Copyright © 2015 John Wiley & Sons, Ltd.     

In vivo 1H MRS study in microlitre voxels in the hippocampus of a mouse model of Down syndrome at 11.7 T

In this article, we report in vivo ¹H MRS performed in 1.8‐μL voxels in a mouse model of Down syndrome (DS). To characterise the excitation–inhibition imbalance observed in DS, metabolite concentrations in the hippocampi of adult Ts65Dn mice, which recapitulate features of DS, were compared with those of their euploid littermates at a voxel 42‐fold smaller than in a previously published study. Quantification of the metabolites was performed using a linear combination model. We detected 16 metabolites in the right and left hippocampi. Principal component analysis revealed that the absolute concentrations of the 16 detected metabolites could differentiate between Ts65Dn and euploid hippocampi. Although measurements in the left and right hippocampi were highly correlated, the concentration of individual metabolites was sometimes significantly different in the left and right structures. Thus, bilateral values from Ts65Dn and euploid mice were further compared with Hotelling's test. The level of glutamine was found to be significantly lower, whereas myo‐inositol was significantly higher, in the hippocampi of Ts65Dn relative to euploid mice. However, γ‐aminobutyric acid (GABA) and glutamate levels remained similar between the groups. Thus, the excitation–inhibition imbalance described in DS does not appear to be related to a radical change in the levels of either GABA or glutamate in the hippocampus. In conclusion, microliter MRS appears to be a valuable tool to detect changes associated with DS, which may be useful in investigating whether differences can be rescued after pharmacological treatments or supplementation with glutamine. Copyright © 2014 John Wiley & Sons, Ltd.     

In vivo MRS and histochemistry of status epilepticus‐induced hippocampal pathology in a juvenile model of temporal lobe epilepsy

Childhood status epilepticus (SE) initiates an epileptogenic process that leads to spontaneous seizures and hippocampal pathology characterized by neuronal loss, gliosis and an imbalance between excitatory and inhibitory neurotransmission. It remains unclear whether these changes are a cause or consequence of chronic epilepsy. In this study, in vivo MRS was used in a post‐SE juvenile rat model of temporal lobe epilepsy (TLE) to establish the temporal evolution of hippocampal injury and neurotransmitter imbalance. SE was induced in P21 rats by injection of lithium and pilocarpine. Four and eight weeks after SE, in vivo ¹H and γ‐aminobutyric acid (GABA)‐edited MRS of the hippocampus was performed in combination with dedicated ex vivo immunohistochemistry for the interpretation and validation of MRS findings. MRS showed a 12% decrease (p < 0.0001) in N‐acetylaspartate and a 15% increase (p = 0.0226) in choline‐containing compound concentrations, indicating neuronal death and gliosis, respectively. These results were confirmed by FluoroJade and vimentin staining. Furthermore, severe and progressive decreases in GABA (?41%, p < 0.001) and glutamate (Glu) (?17%, p < 0.001) were found. The specific severity of GABAergic cell death was confirmed by parvalbumin immunoreactivity (?68%, p < 0.001). Unexpectedly, we found changes in glutamine (Gln), the metabolic precursor of both GABA and Glu. Gln increased at 4 weeks (+36%, p < 0.001), but returned to control levels at 8 weeks. This decrease was consistent with the simultaneous decrease in glutamine synthase immunoreactivity (?32%, p = 0.037). In vivo MRS showed gliosis and (predominantly GABAergic) neuronal loss. In addition, an increase in Gln was detected, accompanied by a decrease in glutamine synthase immunoreactivity. This may reflect glutamine synthase downregulation in order to normalize Gln levels. These changes occurred before spontaneous recurrent seizures were present but, by creating a pre‐epileptic state, may play a role in epileptogenesis. MRS can be applied in a clinical setting and may be used as a noninvasive tool to monitor the development of TLE. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct in vivo measurement of glycine and the neurochemical profile in the rat medulla oblongata

The medulla oblongata (MO) contains a high density of glycinergic synapses and a particularly high concentration of glycine. The aims of this study were to measure directly in vivo the neurochemical profile, including glycine, in MO using a spin‐echo‐based ¹H MRS sequence at TE = 2.8 ms and to compare it with three other brain regions (cortex, striatum and hippocampus) in the rat. Glycine was quantified in MO at TE = 2.8 ms with a Cramér–Rao lower bound (CRLB) of approximately 5%. As a result of the relatively low level of glycine in the other three regions, the measurement of glycine was performed at TE = 20 ms, which provides a favorable J‐modulation of overlapping myo‐inositol resonance. The other 14 metabolites composing the neurochemical profile were quantified in vivo in MO with CRLBs below 25%. Absolute concentrations of metabolites in MO, such as glutamate, glutamine, γ‐aminobutyrate, taurine and glycine, were in the range of previous in vitro quantifications in tissue extracts. Compared with the other regions, MO had a three‐fold higher glycine concentration, and was characterised by reduced (p < 0.001) concentrations of glutamate (?50 ± 4%), glutamine (–54 ± 3%) and taurine (?78 ± 3%). This study suggests that the functional specialisation of distinct brain regions is reflected in the neurochemical profile. Copyright © 2010 John Wiley & Sons, Ltd.     

Expanded neurochemical profile in the early stage of Huntington disease using proton magnetic resonance spectroscopy

The striatum is a well‐known region affected in Huntington disease (HD). However, other regions, including the visual cortex, are implicated. We have identified previously an abnormal energy response in the visual cortex of patients at an early stage of HD using ³¹P magnetic resonance spectroscopy (³¹P MRS). We therefore sought to further characterize these metabolic alterations with ¹H MRS using a well‐validated semi‐localized by adiabatic selective refocusing (semi‐LASER) sequence that allows the measurement of an expanded number of neurometabolites. Ten early affected patients [Unified Huntington Disease Rating Scale (UHDRS), total motor score = 13.6 ± 10.8] and 10 healthy volunteers of similar age and body mass index (BMI) were recruited for the study. We performed ¹H MRS in the striatum – the region that is primarily affected in HD – and the visual cortex. The protocol allowed a reliable quantification of 10 metabolites in the visual cortex and eight in the striatum, compared with three to five metabolites in previous ¹H MRS studies performed in HD. We identified higher total creatine (p < 0.05) in the visual cortex and lower glutamate (p < 0.001) and total creatine (p < 0.05) in the striatum of patients with HD compared with controls. Less abundant neurometabolites [glutamine, γ‐aminobutyric acid (GABA), glutathione, aspartate] showed similar concentrations in both groups. The protocol allowed the measurement of several additional metabolites compared with standard vendor protocols. Our study points to early changes in metabolites involved in energy metabolism in the visual cortex and striatum of patients with HD. Decreased striatal glutamate could reflect early neuronal dysfunction or impaired glutamatergic neurotransmission.     

Longitudinal (1) H MRS assessment of the thalamus in a Coriaria lactone-induced rhesus monkey status epilepticus model

Neurophysiological, biochemical and anatomical evidence implicates the thalamus as playing a role in epileptic seizures. Until recently, however, longitudinal characterization of in vivo thalamus dynamics had not been reported. In this study, we investigated the metabolism in the thalamus to identify the changes that occur following Coriaria lactone (CL)‐induced status epilepticus (SE) and to observe whether the epileptiform discharges could present a difference between the left and right thalami. Five rhesus monkeys underwent whole‐brain MRI and single‐voxel MRS on a Siemens Trio Tim 3‐T MR scanner with a 12‐channel head coil. Spectra were processed using LCModel. Scans were performed in five animals before SE and at 1, 7, 21 and 42 days after the onset of SE. Statistical analysis of the data obtained demonstrated no significant difference in the bilateral thalamus of healthy macaques. Our MRS data showed symmetrical distributions of N‐acetylaspartate in the right and left thalami after SE (p = 0.003). In addition, this longitudinal study demonstrated elevated glutamate/glutamine (p < 0.05) and reduced myo‐inositol (p < 0.05) in the bilateral thalamus 1 day after SE, and all metabolites approached their baseline levels by the fifth scan. Our results demonstrate that metabolic changes occur in the thalamus during CL‐induced SE in rhesus monkeys. The various metabolic changes may indicate that the left thalamus is more vulnerable to epileptic strike. Copyright © 2012 John Wiley & Sons, Ltd.     

Does MPTP intoxication in mice induce metabolite changes in the nucleus accumbens? A 1H nuclear MRS study

Using in vivo ¹H NMR spectroscopy in a mouse model of Parkinson's disease, we previously showed that glutamate concentrations in the dorsal striatum were highest after dopamine denervation associated with an increase in gamma‐aminobutyric acid (GABA) and (Gln) glutamine levels. The aim of this study was to determine whether the changes previously observed in the motor part of the striatum were reproduced in a ventral part of the striatum, the nucleus accumbens (NAc). This study was carried out on controls and 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐intoxicated mice. In vivo spectra were acquired for a voxel (8 μL) in the dorsal striatum, and in the NAc (1.56 μL). NMR acquisitions were first performed 10 days after the last MPTP injection in a basal condition [after saline intraperitoneal (i.p.) injection] and then in the same animal the week after basal NMR acquisitions, after acute levodopa administration (200 mg kg^–1, i.p.). Immunohistochemistry was used to determine the levels of (Glu) glutamate, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD) isoform 67 in these two structures. The Glu, Gln and GABA concentrations obtained in the basal state were higher in the NAc of MPTP‐intoxicated mice which have the higher dopamine denervation in the ventral tegmental area (VTA) and in the dorsal striatum. Levodopa decreased the levels of these metabolites in MPTP‐intoxicated mice to levels similar to those in controls. In parallel, immunohistochemical staining showed that glutamate, GS and GAD67 immunoreactivity increased in the dorsal striatum of MPTP‐intoxicated mice and in the NAc for animals with a severe dopamine denervation in VTA. These findings strongly supported a hyperactivity of the glutamatergic cortico‐striatal pathway and changes in glial activity when the dopaminergic denervation in the VTA and substantia nigra pars compacta (SNc) was severe. Copyright © 2012 John Wiley & Sons, Ltd.     

Proton magnetic resonance spectroscopy of brain metabolic shifts induced by acute administration of 2‐deoxy‐d‐glucose and lipopolysaccharides

In vivo proton magnetic resonance spectroscopy (¹H MRS) of outbred stock ICR male mice (originating from the Institute of Cancer Research) was used to study the brain (hippocampus) metabolic response to the pro‐inflammatory stimulus and to the acute deficiency of the available energy, which was confirmed by measuring the maximum oxygen consumption. Inhibition of glycolysis by means of an injection with 2‐deoxy‐d ‐glucose (2DG) reduced the levels of gamma‐aminobutyric acid (GABA, p < 0.05, in comparison with control, least significant difference (LSD) test), N‐acetylaspartate (NAA, p < 0.05, LSD test) and choline compounds, and at the same time increased the levels of glutamate and glutamine. An opposite effect was found after injection with bacterial lipopolysaccharide (LPS) – a very common pro‐inflammatory inducer. An increase in the amounts of GABA, NAA and choline compounds in the brain occurred in mice treated with LPS. Different metabolic responses to the energy deficiency and the pro‐inflammatory stimuli can explain the contradictory results of the brain ¹H MRS studies under neurodegenerative pathology, which is accompanied by both mitochondrial dysfunction and inflammation. The prevalence of the excitatory metabolites such as glutamate and glutamine in 2DG treated mice is in good agreement with excitation observed during temporary reduction of the available energy under acute hypoxia or starvation. In turn, LPS, as an inducer of the sickness behavior, which was manifested as depression, sleepiness, loss of appetite etc., shifts the brain metabolic pattern toward the prevalence of the inhibitory neurotransmitter GABA. Copyright © 2014 John Wiley & Sons, Ltd.     

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

¹H 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 ¹H 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 ¹H MRS datasets. Copyright © 2012 John Wiley & Sons, Ltd.     

A quantitative comparison of metabolite signals as detected by in vivo MRS with ex vivo 1H HR‐MAS for childhood brain tumours

¹H MRS provides a powerful method for investigating tumour metabolism by allowing the measurement of metabolites in vivo. Recently, the technique of ¹H high‐resolution magic angle spinning (HR‐MAS) has been shown to produce high‐quality data, allowing the accurate measurement of many metabolites present in unprocessed biopsy tissue. The purpose of this study was to evaluate the agreement between the techniques of in vivo MRS and ex vivo HR‐MAS for investigating childhood brain tumours. Short‐TE (30 ms), single‐voxel, in vivo MRS was performed on 16 paediatric patients with brain tumours at 1.5 T. A frozen biopsy sample was available for each patient. HR‐MAS was performed on the biopsy samples, and metabolite quantities were determined from the MRS and HR‐MAS data using the LCModel? and TARQUIN algorithms, respectively. Linear regression was performed on the metabolite quantities to asses the agreement between MRS and HR‐MAS. Eight of the 12 metabolite quantities were found to correlate significantly (P < 0.05). The four worst correlating metabolites were aspartate, scyllo‐inositol, glycerophosphocholine and N‐acetylaspartate, and, except for glycerophosphocholine, this error was reflected in their higher Cramer–Rao lower bounds (CRLBs), suggesting that low signal‐to‐noise was the greatest source of error for these metabolites. Glycerophosphocholine had a lower CRLB implying that interference with phosphocholine and choline was the most significant source of error. The generally good agreement observed between the two techniques suggests that both MRS and HR‐MAS can be used to reliably estimate metabolite quantities in brain tumour tissue and that tumour heterogeneity and metabolite degradation do not have an important effect on the HR‐MAS metabolite profile for the tumours investigated. HR‐MAS can be used to improve the analysis and understanding of MRS data. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectral fitting using basis set modified by measured B0 field distribution

This study sought to demonstrate and evaluate a novel spectral fitting method to improve quantification accuracy in the presence of large magnetic field distortion, especially with high fields. MRS experiments were performed using a point‐resolved spectroscopy (PRESS)‐type sequence at 7 T. A double‐echo gradient echo (GRE) sequence was used to acquire B₀ maps following MRS experiments. The basis set was modified based on the measured B₀ distribution within the MRS voxel. Quantification results were obtained after fitting the measured MRS data using the modified basis set. The proposed method was validated using numerical Monte Carlo simulations, phantom measurements, and comparison of occipital lobe MRS measurements under homogeneous and inhomogeneous magnetic field conditions. In vivo results acquired from voxels placed in thalamus and prefrontal cortex regions close to the frontal sinus agreed well with published values. Instead of noise‐amplifying complex division, the proposed method treats field variations as part of the signal model, thereby avoiding inherent statistical bias associated with regularization. Simulations and experiments showed that the proposed approach reliably quantified results in the presence of relatively large magnetic field distortion. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Reproducibility of prefrontal γ-aminobutyric acid measurements with J-edited spectroscopy

γ‐Aminobutyric acid (GABA) is the chief inhibitory neurotransmitter of the human brain, and GABA‐ergic dysfunction has been implicated in a variety of neuropsychiatric disorders. Recent MRS techniques have allowed the quantification of GABA concentrations in vivo, and could therefore provide biologically relevant information. Few reports have formally characterized the reproducibility of these techniques, and differences in field strength, acquisition and processing parameters may result in large differences in measured GABA values. Here, we used a J‐edited, single‐voxel spectroscopy method of measurement of GABA + macromolecules (GABA + ) in the anterior cingulate cortex (ACC) and right frontal white matter (rFWM) at 3 T. We measured the coefficient of variation within subjects (CVw) and intra‐class correlation coefficients on two repeated scans obtained from 10 healthy volunteers with processing procedures developed in‐house for the quantification of GABA + and other major metabolites. In addition, by segmenting the spectroscopic voxel into cerebrospinal fluid, gray matter and white matter, and employing a linear regression technique to extrapolate metabolite values to pure gray and white matter, we determined metabolite differences between gray and white matter in ACC and rFWM. CVw values for GABA + /creatine, GABA + /H₂O, GABA + , creatine, partially co‐edited glutamate + glutamine (Glx)/creatine, partially co‐edited Glx and N‐acetylaspartic acid (NAA)/creatine were all below 12% in both ACC and rFWM. After extrapolation to pure gray and pure white matter, CVw values for all metabolites were below 16%. We found metabolite ratios between gray and white matter for GABA + /creatine, GABA + , creatine, partially co‐edited Glx and NAA/creatine to be 0.88 ± 0.21 (standard deviation), 1.52 ± 0.32, 1.77 ± 0.4, 2.69 ± 0.74 and 0.70 ± 0.05, respectively. This study validates a reproducible method for the quantification of brain metabolites, and provides information on gray/white matter differences that may be important in the interpretation of results in clinical populations. Published in 2011 by John Wiley & Sons, Ltd.     

Brain γ‐aminobutyric acid (GABA) detection in vivo with the J‐editing 1H MRS technique: a comprehensive methodological evaluation of sensitivity enhancement,macromolecule contamination and test–retest reliability

Abnormalities in brain γ‐aminobutyric acid (GABA) have been implicated in various neuropsychiatric and neurological disorders. However, in vivo GABA detection by ¹H MRS presents significant challenges arising from the low brain concentration, overlap by much stronger resonances and contamination by mobile macromolecule (MM) signals. This study addresses these impediments to reliable brain GABA detection with the J‐editing difference technique on a 3‐T MR system in healthy human subjects by: (i) assessing the sensitivity gains attainable with an eight‐channel phased‐array head coil; (ii) determining the magnitude and anatomic variation of the contamination of GABA by MM; and (iii) estimating the test–retest reliability of the measurement of GABA with this method. Sensitivity gains and test–retest reliability were examined in the dorsolateral prefrontal cortex (DLPFC), whereas MM levels were compared across three cortical regions: DLPFC, the medial prefrontal cortex (MPFC) and the occipital cortex (OCC). A three‐fold higher GABA detection sensitivity was attained with the eight‐channel head coil compared with the standard single‐channel head coil in DLPFC. Despite significant anatomical variation in GABA + MM and MM across the three brain regions (p < 0.05), the contribution of MM to GABA + MM was relatively stable across the three voxels, ranging from 41% to 49%, a non‐significant regional variation (p = 0.58). The test–retest reliability of GABA measurement, expressed as either the ratio to voxel tissue water (W) or to total creatine, was found to be very high for both the single‐channel coil and the eight‐channel phased‐array coil. For the eight‐channel coil, for example, Pearson's correlation coefficient of test vs. retest for GABA/W was 0.98 (R² = 0.96, p = 0.0007), the percentage coefficient of variation (CV) was 1.25% and the intraclass correlation coefficient (ICC) was 0.98. Similar reliability was also found for the co‐edited resonance of combined glutamate and glutamine (Glx) for both coils. Copyright © 2016 John Wiley & Sons, Ltd.     

Multi‐center reproducibility of neurochemical profiles in the human brain at 7 T

The purpose of this work was to harmonize data acquisition and post‐processing of single voxel proton MRS (¹H‐MRS) at 7 T, and to determine metabolite concentrations and the accuracy and reproducibility of metabolite levels in the adult human brain. This study was performed in compliance with local institutional human ethics committees. The same seven subjects were each examined twice using four different 7 T MR systems from two different vendors using an identical semi‐localization by adiabatic selective refocusing spectroscopy sequence. Neurochemical profiles were obtained from the posterior cingulate cortex (gray matter, GM) and the corona radiata (white matter, WM). Spectra were analyzed with LCModel, and sources of variation in concentrations (‘subject’, ‘institute’ and ‘random’) were identified with a variance component analysis. Concentrations of 10–11 metabolites, which were corrected for T₁, T₂, magnetization transfer effects and partial volume effects, were obtained with mean Cramér–Rao lower bounds below 20%. Data variances and mean concentrations in GM and WM were comparable for all institutions. The primary source of variance for glutamate, myo‐inositol, scyllo‐inositol, total creatine and total choline was between subjects. Variance sources for all other metabolites were associated with within‐subject and system noise, except for total N‐acetylaspartate, glutamine and glutathione, which were related to differences in signal‐to‐noise ratio and in shimming performance between vendors. After multi‐center harmonization of acquisition and post‐processing protocols, metabolite concentrations and the sizes and sources of their variations were established for neurochemical profiles in the healthy brain at 7 T, which can be used as guidance in future studies quantifying metabolite and neurotransmitter concentrations with ¹H‐MRS at ultra‐high magnetic field. Copyright © 2015 John Wiley & Sons, Ltd.     

Detection of metabolite changes in response to a varying visual stimulation paradigm using short‐TE 1H MRS at 7 T

The two‐fold benefit of ¹H magnetic resonance spectroscopy (MRS) at high B₀ fields – enhanced sensitivity and increased spectral dispersion – has been used previously to study dynamic changes in metabolite concentrations in the human brain in response to visual stimulation. In these studies, a strong visual on/off stimulus was combined with MRS data acquisition in a voxel location in the occipital cortex determined by an initial functional magnetic resonance imaging experiment. However, 1) to exclude the possibility of systemic effects (heartbeat, blood flow, etc.), which tend to be different for on/off conditions, a modified stimulation condition not affecting the target voxel needs to be employed, and 2) to assess important neurotransmitters of low concentration, in particular γ‐aminobutyric acid (GABA), it may be advantageous to analyze steady‐state, rather than dynamic, conditions. Thus, the aim of this study was to use short‐TE ¹H MRS methodology at 7 T to detect differences in steady‐state metabolite levels in response to a varying stimulation paradigm in the human visual cortex. The two different stimulation conditions were termed voxel and control activation. Localized MR spectra were acquired using the SPECIAL (spin‐echo full‐intensity acquired localized) sequence. Data were analyzed using LCModel. Fifteen individual metabolites were reliably quantified. On comparison of steady‐state concentrations for voxel versus control activation, a decrease in GABA of 0.05 mmol/L (5%) and an increase in lactate of 0.04 mmol/L (7%) were found to be the only significant effects. The observed reduction in GABA can be interpreted as reduced neuronal inhibition during voxel activation, whereas the increase in lactate hints at an intensification of anaerobic glycolysis. Differences from previous studies, notably the absence of any changes in glutamate, are attributed to the modified experimental conditions. This study demonstrates that the use of advanced ¹H MRS methodology at 7 T allows the detection of subtle changes in metabolite concentrations involved in neuronal activation and inhibition.