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

Relationship between body mass index and hippocampal glutamate/glutamine in bipolar disorder

BACKGROUND

We previously reported that patients with early-stage bipolar disorder, but not healthy comparison controls, had body mass index (BMI)-related volume reductions in limbic brain areas, suggesting that the structural brain changes characteristic of bipolar disorder were more pronounced with increased weight.

AIMS

To determine whether the most consistently reported neurochemical abnormality in bipolar disorder, increased glutamate/glutamine (Glx), was also more prominent with higher BMI.

METHOD

We used single-voxel proton magnetic resonance spectroscopy to measure hippocampal Glx in 51 patients with first-episode mania (mean BMI = 24.1) and 28 healthy controls (mean BMI = 23.3).

RESULTS

In patients, but not healthy controls, linear regression demonstrated that higher BMI predicted greater Glx. Factorial ANCOVA showed a significant BMI × diagnosis interaction, confirming a distinct effect of weight on Glx in patients.

CONCLUSIONS

Together with our volumetric studies, these results suggest that higher BMI is associated with more pronounced structural and neurochemical limbic brain changes in bipolar disorder, even in early-stage patients with low obesity rates.

Frontal P3 event-related potential is related to brain glutamine/glutamate ratio measured in vivo

BACKGROUND

The auditory P3 event-related potential (ERP) is thought to index cognitive processing relevant to attention and working memory processes. Drug challenge studies suggest that glutamate neurotransmission plays an important role in modulating P3 ERP. However, while direct links between glutamate activity and P3 ERP response in humans are suspected, mechanistic details remain largely unknown. We investigated here the relationships between P3 ERP and indices of glutamatergic processing measured in vivo with proton magnetic resonance spectroscopy ((1)H MRS). We hypothesized that a higher index of glutamatergic processing (glutamine/glutamate ratio; abbreviated Gln/Glu) in the anterior cingulate (ACC) and in the parietal-occipital (POC) cortices would associate with larger frontal P3a and parietal P3b amplitudes, respectively.

METHODS

Frontal P3a (Fz) and parietal P3b (Pz) were collected from 32 healthy participants who performed an auditory oddball task. Resting glutamate (Glu), glutamine (Gln), and Gln/Glu (an index of glutamatergic processing) measures were obtained on a 4T MR scanner using J-resolved MR spectroscopy. Linear regression and partial correlations were used for statistical analysis.

RESULTS

Significant positive correlations were found between frontal P3a amplitude and ACC Gln/Glu ratio (partial R=0.57; P=0.001) and between frontal P3a amplitude and ACC Gln concentration (partial R=0.43; P=0.02). Relationships between parietal P3b and the glutamate indices in the POC were not significant.

CONCLUSIONS

These results indicate a specific connection between an index of glutamate neurotransmitter function in ACC and frontal P3 ERP, providing a novel insight into the neurochemistry underlying scalp recorded EEG response. Abnormalities in glutamate neurotransmission have been observed in schizophrenia and other psychiatric conditions and may underlie illness related deficits of P3 ERP.

In vivo (1)H MRSI of glycine in brain tumors at 3T

PURPOSE

MR spectroscopic imaging (SI) of glycine (Gly) in the human brain is challenging due to the interference of the abundant neighboring J-coupled resonances. Our aim is to accomplish reliable imaging of Gly in healthy brain and brain tumors using an optimized MR sequence scheme at 3 tesla.

METHODS

Two-dimensional (1)H SI was performed with a point-resolved spectroscopy scheme. An echo time of 160 ms was used for separation between Gly and myo-inositol signals. Data were collected from eight healthy volunteers and 14 subjects with gliomas. Spectra were analyzed with the linear combination model using numerically calculated basis spectra. Metabolite concentrations were estimated with reference to creatine in white matter (WM) regions at 6.4 molar concentrations (mM).

RESULTS

From a linear regression analysis with respect to the fractional gray matter (GM) content, the Gly concentrations in pure GM and WM in healthy brains were estimated to be 1.1 and 0.3 mM, respectively. Gly was significantly elevated in tumors. The tumor-to-contralateral Gly concentration ratio was more extensive with higher grades, showing ∼ 10-fold elevation of Gly in glioblastomas.

CONCLUSION

The Gly level is significantly different between GM and WM in healthy brains. Our data indicate that SI of Gly may provide a biomarker of brain tumor malignancy.

Alterations of GABA and glutamate-glutamine levels in premenstrual dysphoric disorder: a 3T proton magnetic resonance spectroscopy study

Increasing evidence has suggested that the GABAergic neurotransmitter system is involved in the pathogenesis of premenstrual dysphoric disorder (PMDD). We used proton magnetic resonance spectroscopy ((1)H MRS) to investigate whether PMDD is associated with alterations in brain GABA levels. Levels of glutamate-glutamine (Glx) were also explored. Participants comprised 22 women with PMDD and 22 age-matched healthy controls who underwent 3T (1)H MRS during the late luteal phase of the menstrual cycle. GABA+ and Glx levels were quantified in the anterior cingulate cortex/medial prefrontal cortex (ACC/mPFC) and the left basal ganglia (ltBG). Water-scaled GABA+ concentrations and GABA+/tCr ratios were significantly lower in both the ACC/mPFC and ltBG regions of PMDD women than in healthy controls. Glx/tCr ratios were significantly higher in the ACC/mPFC region of PMDD women than healthy controls. Our preliminary findings provide the first report of abnormal levels of GABA+ and Glx in mood-related brain regions of women with PMDD, indicating that dysregulation of the amino acid neurotransmitter system may be an important neurobiological mechanism in the pathogenesis of PMDD.

Bimodal effect of lithium plasma levels on hippocampal glutamate concentrations in bipolar II depression: a pilot study

BACKGROUND

The hippocampus has been highly implicated in the pathophysiology of bipolar disorder (BD). Nevertheless, no study has longitudinally evaluated hippocampal metabolite levels in bipolar depression under treatment with lithium.

METHODS

Nineteen medication-free BD patients (78.9% treatment-naïve and 73.7% with BD type II) presenting an acute depressive episode and 17 healthy controls were studied. Patients were treated for 6 weeks with lithium in an open-label trial. N-acetyl aspartate (NAA), creatine, choline, myo-Inositol, and glutamate levels were assessed in the left hippocampus before (week 0) and after (week 6) lithium treatment using 3T proton magnetic resonance spectroscopy (1H-MRS). The metabolite concentrations were estimated using internal water as reference and voxel segmentation for partial volume correction.

RESULTS

At baseline, acutely depressed BD patients and healthy controls exhibited similar hippocampal metabolites concentrations, with no changes after 6 weeks of lithium monotherapy. A significant correlation between antidepressant efficacy and increases in NAA concentration over time was observed. Also, there was a significant positive correlation between the changes in glutamate concentrations over follow-up and plasma lithium levels at endpoint. Mixed effects model analysis revealed a bimodal effect of lithium plasma levels in hippocampal glutamate concentrations: levels of 0.2 to 0.49 mmol/L (n=9) were associated with a decrease in glutamate concentrations, whereas the subgroup of BD subjects with "standard" lithium levels (≥ 0.50 mmol/L; n = 10) showed an overall increase in glutamate concentrations over time.

CONCLUSIONS

These preliminary results suggest that lithium has a bimodal action in hippocampal glutamate concentration depending on the plasma levels.

Statistical mapping of metabolites in the medial wall of the brain: a proton echo planar spectroscopic imaging study

With magnetic resonance spectroscopic imaging (MRSI), it is possible to simultaneously map distributions of several brain metabolites with relatively good spatial resolution in a short time. Although other functional imaging modalities have taken advantage of population-based inferences using spatially extended statistics, this approach remains little utilized for MRSI. In this study, statistical nonparametric mapping (SnPM) was applied to two-dimensional MRSI data from the medial walls of the human brain to assess the effect of normal aging on metabolite concentrations. The effects of different preprocessing steps on these results were then explored. Short echo time MRSI of left and right medial walls was acquired in conjunction with absolute quantification of total choline, total creatine (tCr), glutamate and glutamine, myo-inositol, and N-acetyl-aspartate. Individual images were spatially warped to a common anatomical frame of reference. Age effects were assessed within SnPM as were the effects of voxel subsampling, variance smoothing, and spatial smoothing. The main findings were: (1) regions in the bilateral dorsal anterior cingulate and in the left posterior cingulate exhibited higher tCr concentrations with age; (2) voxel subsampling but not spatial smoothing enhanced the cluster-level statistical sensitivity; and (3) variance smoothing was of little benefit in this study. Our study shows that spatially extended statistics can yield information about regional-specific changes in metabolite concentrations obtained by short echo time MRSI. This opens up the possibility for systematic comparisons of metabolites in the medial wall of the brain.

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

The (1)H 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.

Optimized MEGA-SPECIAL for in vivo glutamine detection in the rat brain at 14.1 T

Glutamine has multiple roles in brain metabolism and its concentration can be altered in various pathological conditions. An accurate knowledge of its concentration is therefore highly desirable to monitor and study several brain disorders in vivo. However, in recent years, several MRS studies have reported conflicting glutamine concentrations in the human brain. A recent hypothesis for explaining these discrepancies is that a short T2 component of the glutamine signal may impact on its quantification at long echo times. The present study therefore aimed to investigate the impact of acquisition parameters on the quantified glutamine concentration using two different acquisition techniques, SPECIAL at ultra-short echo time and MEGA-SPECIAL at moderate echo time. For this purpose, MEGA-SPECIAL was optimized for the first time for glutamine detection. Based on the very good agreement of the glutamine concentration obtained between the two measurements, it was concluded that no impact of a short T2 component of the glutamine signal was detected.

Thalamic glutamate decreases with cigarette smoking

RATIONALE

Findings from animal studies and human PET imaging indicate that nicotine and cigarette smoking affect glutamate (Glu) and related neurochemical markers in the brain and imply that smoking reduces extracellular Glu. As Glu release is mediated by nicotinic acetylcholine receptors (nAChRs), which are present at high concentrations in the thalamus, we examined the effects of smoking on thalamic Glu.

OBJECTIVE

To determine the effects of tobacco smoking on thalamic glutamate levels.

METHODS

Thalamic Glu levels were measured in vivo in 18 smokers and 16 nonsmokers using proton magnetic resonance spectroscopic imaging ((1)H MRSI) at 1.5 T.

RESULTS

Mean Glu levels did not differ significantly between the subject groups. However, within smokers, Glu levels were negatively correlated with self-reports of both cigarettes/day over the last 30 days (r = -0.64, p = 0.006) and pack-years of smoking (r = -0.66, p = 0.005).

CONCLUSIONS

Consistent with expectations based on preclinical studies, within smokers, cigarettes/day and pack-years are associated with reduced Glu in thalamus, a brain region rich in nAchRs. These results encourage work on candidate glutamatergic therapies for smoking cessation and suggest a noninvasive metric for their action in the brain.

Proton magnetic resonance spectroscopy of prefrontal white matter in psychotropic naïve children and adolescents with obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) has a typical onset during childhood or adolescence. Although recent in-vivo proton magnetic resonance spectroscopy ((1)H-MRS) studies report gray matter metabolite abnormalities in children and adolescents with OCD, there are no existing (1)H-MRS studies that measure white matter (WM) metabolite levels in this population. In the present study, we measured metabolite levels in the left and right prefrontal WM (LPFWM and RPFWM, respectively) of psychotropic-naïve children and adolescents with OCD (LPFWM: N=15, mean age 13.3±2.4 years; right RPFWM: N=14, mean age 13.0±2.3 years) and healthy controls (LPFWM: N=17, mean age 11.8±2.7 years; RPFWM: N=18, mean age 12.2±2.8 years). Spectra were acquired using a 3T single voxel PRESS sequence (1.5×2.0×2.0cm(3)). When age and sex effects were controlled, OCD patients had higher levels of RPFWM choline and N-acetyl-aspartate (NAA). In addition, RPFWM levels of NAA, creatine and myo-inositol were positively and significantly correlated with severity of OCD symptoms. In summary, this is the first published study of WM metabolite levels in children and adolescents with OCD. Our preliminary findings lend further support to the previous findings of WM abnormalities in OCD.

Comparison of real-time water proton spectroscopy and echo-planar imaging sensitivity to the BOLD effect at 3 T and at 7 T

Gradient-echo echo-planar imaging (GE EPI) is the most commonly used approach to assess localized blood oxygen level dependent (BOLD) signal changes in real-time. Alternatively, real-time spin-echo single-voxel spectroscopy (SE SVS) has recently been introduced for spatially specific BOLD neurofeedback at 3 T and at 7 T. However, currently it is not known how neurofeedback based on real-time SE SVS compares to real-time GE EPI-based. We therefore compared both methods at high (3 T) and at ultra-high (7 T) magnetic field strengths. We evaluated standard quality measures of both methods for signals originating from the motor cortex, the visual cortex, and for a neurofeedback condition. At 3 T, the data quality of the real-time SE SVS and GE EPI R2* estimates were comparable. At 7 T, the data quality of the real-time GE EPI acquisitions was superior compared to those of the real-time SE SVS. Despite the somehow lower data quality of real-time SE SVS compared to GE EPI at 7 T, SE SVS acquisitions might still be an interesting alternative. Real-time SE SVS allows for a direct and subject-specific T2* estimation and thus for a physiologically more plausible neurofeedback signal.

Comparison of sagittal and transverse echo planar spectroscopic imaging on the quantification of brain metabolites

PURPOSE

We quantitatively compared sagittal and transverse echo planar spectroscopic imaging (EPSI) on the quantification of metabolite concentrations with consideration of tissue variation. A quantification strategy is proposed to collect the necessary information for quantification of concentrations in a minimized acquisition time.

METHODS

Six transverse and six sagittal EPSI data were collected on healthy volunteers. Metabolite concentrations of N-acetyl-aspartate (NAA), total creatine (tCr), total choline (tCho), myo-inositol (mI), and glutamate and glutamine complex (Glx) were quantified using water scaling with partial volume and relaxation correction. Linear regression analysis was performed to extract concentrations in gray matter (GM) and white matter (WM). The inter- and intrasubject coefficients of variance (CV) were estimated.

RESULTS

Concentrations and fitting errors of sagittal and transverse EPSI were at same level. GM to WM contrast of concentrations was found in NAA, tCr, and tCho. The intersubject CVs revealed greater variability in the sagittal EPSI than in the transverse EPSI. The intrasubject CVs of the transverse EPSI were below 5% for NAA, tCr, and tCho.

CONCLUSION

We showed that quantified concentrations of sagittal and transverse EPSI after partial volume correction are comparable and reproducible. The proposed quantification strategy can be conveniently adapted into various MRI protocols.

A subspace approach to high-resolution spectroscopic imaging

PURPOSE

To accelerate spectroscopic imaging using sparse sampling of (k,t)-space and subspace (or low-rank) modeling to enable high-resolution metabolic imaging with good signal-to-noise ratio.

METHODS

The proposed method, called SPectroscopic Imaging by exploiting spatiospectral CorrElation, exploits a unique property known as partial separability of spectroscopic signals. This property indicates that high-dimensional spectroscopic signals reside in a very low-dimensional subspace and enables special data acquisition and image reconstruction strategies to be used to obtain high-resolution spatiospectral distributions with good signal-to-noise ratio. More specifically, a hybrid chemical shift imaging/echo-planar spectroscopic imaging pulse sequence is proposed for sparse sampling of (k,t)-space, and a low-rank model-based algorithm is proposed for subspace estimation and image reconstruction from sparse data with the capability to incorporate prior information and field inhomogeneity correction.

RESULTS

The performance of the proposed method has been evaluated using both computer simulations and phantom studies, which produced very encouraging results. For two-dimensional spectroscopic imaging experiments on a metabolite phantom, a factor of 10 acceleration was achieved with a minimal loss in signal-to-noise ratio compared to the long chemical shift imaging experiments and with a significant gain in signal-to-noise ratio compared to the accelerated echo-planar spectroscopic imaging experiments.

CONCLUSION

The proposed method, SPectroscopic Imaging by exploiting spatiospectral CorrElation, is able to significantly accelerate spectroscopic imaging experiments, making high-resolution metabolic imaging possible.

High-resolution echo-planar spectroscopic imaging of the human calf

Background

This study exploits the speed benefits of echo-planar spectroscopic imaging (EPSI) to acquire lipid spectra of skeletal muscle. The main purpose was to develop a high-resolution EPSI technique for clinical MR scanner, to visualise the bulk magnetic susceptibility (BMS) shifts of extra-myocellular lipid (EMCL) spectral lines, and to investigate the feasibility of this method for the assessment of intra-myocellular (IMCL) lipids.

Methods

The study group consisted of six healthy volunteers. A two dimensional EPSI sequence with point-resolved spectroscopy (PRESS) spatial localization was implemented on a 3T clinical MR scanner. Measurements were performed by means of 64×64 spatial matrix and nominal voxel size 3×3×15 mm³. The total net measurement time was 3 min 12 sec for non-water-suppressed (1 acquisition) and 12 min 48 sec for water-suppressed scans (4 acquisitions).

Results

Spectra of the human calf had a very good signal-to-noise ratio and linewidths sufficient to differentiate IMCL resonances from EMCL. The use of a large spatial matrix reduces inter-voxel signal contamination of the strong EMCL signals. Small voxels enabled visualisation of the methylene EMCL spectral line splitting and their BMS shifts up to 0.5 ppm relative to the correspondent IMCL line. The mean soleus muscle IMCL content of our six volunteers was 0.30±0.10 vol% (range 0.18–0.46) or 3.6±1.2 mmol/kg wet weight (range: 2.1–5.4).

Conclusion

This study demonstrates that high-spatial resolution PRESS EPSI of the muscle lipids is feasible on standard clinical scanners.

Myoinositol and glutamate complex neurometabolite abnormality after mild traumatic brain injury

OBJECTIVE

To obtain quantitative neurometabolite measurements, specifically myoinositol (mI) and glutamate plus glutamine (Glx), markers of glial and neuronal excitation, in deep gray matter structures after mild traumatic brain injury (mTBI) using proton magnetic resonance spectroscopy ((1)H-MRS) and to compare these measurements against normal healthy control subjects.

METHODS

This study approved by the institutional review board is Health Insurance Portability and Accountability Act compliant. T1-weighted MRI and multi-voxel (1)H-MRS imaging were acquired at 3 tesla from 26 patients with mTBI an average of 22 days postinjury and from 13 age-matched healthy controls. Two-way analysis of variance was used to compare patients and controls for mean N-acetylaspartate, choline, creatine (Cr), Glx, and mI levels as well as the respective ratios to Cr within the caudate, globus pallidus, putamen, and thalamus.

RESULTS

Quantitative putaminal mI was higher in patients with mTBI compared with controls (p = 0.02). Quantitative neurometabolite ratios of putaminal mI and Glx relative to Cr, mI/Cr, and Glx/Cr were also higher among patients with mTBI compared with controls (p = 0.01 and 0.02, respectively). No other differences in neurometabolite levels or ratios were observed in any other brain region evaluated.

CONCLUSION

Increased putaminal mI, mI/Cr, and Glx/Cr in patients after mTBI compared with control subjects supports the notion of a complex glial and excitatory response to injury without concomitant neuronal loss, evidenced by preserved N-acetylaspartate levels in this region.

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

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

Structure-specific glial response in a macaque model of neuroAIDS: multivoxel proton magnetic resonance spectroscopic imaging at 3 Tesla

OBJECTIVE

As ~40% of persons with HIV also suffer neurocognitive decline, we sought to assess metabolic dysfunction in the brains of simian immunodeficiency virus (SIV)-infected rhesus macaques, an advanced animal model, in structures involved in cognitive function. We test the hypothesis that SIV-infection produces proton-magnetic resonance spectroscopic imaging (H-MRSI)-observed decline in the neuronal marker, N-acetylaspartate (NAA), and elevations in the glial marker, myo-inositol (mI), and associated creatine (Cr) and choline (Cho) in these structures.

DESIGN

Pre- and 4-6 weeks post-SIV infection (with CD8 T-lymphocyte depletion) was monitored with T2-weighted quantitative MRI and 16×16×4 multivoxel H-MRSI (TE/TR = 33/1400 ms) in the brains of five rhesus macaques.

METHODS

Exploiting the high-resolution H-MRSI grid, we obtained absolute, cerebrospinal fluid partial volume-corrected NAA, Cr, Cho and mI concentrations from centrum semiovale, caudate nucleus, putamen, thalamus and hippocampus regions.

RESULTS

Pre- to post-infection mean Cr increased in the thalamus: 7.2±0.4 to 8.0±0.8 mmol/l (+11%, P<0.05); mI increased in the centrum semiovale: 5.1±0.8 to 6.6±0.8 mmol/l, caudate: 5.7±0.7 to 7.3±0.5 mmol/l, thalamus: 6.8±0.8 to 8.5±0.8 mmol/l and hippocampus: 7.7±1.2 to 9.9±0.4 mmol/l (+29%, +27%, +24% and +29%, all P<0.05). NAA and Cho changes were not significant.

CONCLUSION

SIV-infection appears to cause brain injury indirectly, through glial activation, while the deep gray matter structures' neuronal cell bodies are relatively spared. Treatment regimens to reduce gliosis may, therefore, prevent neuronal damage and its associated neurocognitive impairment.

Myelin and axon abnormalities in schizophrenia measured with magnetic resonance imaging techniques

BACKGROUND

In schizophrenia (SZ), disturbances in integration of activity among brain regions seem to be as important as abnormal activity of any single region. Brain regions are connected through white matter (WM) tracts, and diffusion tensor imaging has provided compelling evidence for WM abnormalities in SZ. However, diffusion tensor imaging alone cannot currently pinpoint the biological basis of these abnormalities.

METHODS

In this study, we combined a myelin-specific and an axon-specific magnetic resonance imaging approach to examine potentially distinct abnormalities of WM components in SZ. Magnetization transfer ratio (MTR) provides information on myelin content, whereas diffusion tensor spectroscopy provides information on metabolite diffusion within axons. We collected data from a 1 × 3 × 3 cm voxel within the right prefrontal cortex WM at 4 Tesla and studied 23 patients with SZ and 22 age- and sex-matched healthy control participants.

RESULTS

The MTR was significantly reduced in SZ, suggesting reduced myelin content. By contrast, the apparent diffusion coefficient of N-acetylaspartate (NAA) was significantly elevated, suggesting intra-axonal abnormalities. Greater abnormality of both MTR and the apparent diffusion coefficient of NAA correlated with more adverse outcomes in the patient group.

CONCLUSIONS

The results suggest that WM abnormalities in SZ include both abnormal myelination and abnormal NAA diffusion within axons. These processes might be associated with abnormal signal transduction and abnormal information processing in SZ.

High-speed real-time resting-state FMRI using multi-slab echo-volumar imaging

We recently demonstrated that ultra-high-speed real-time fMRI using multi-slab echo-volumar imaging (MEVI) significantly increases sensitivity for mapping task-related activation and resting-state networks (RSNs) compared to echo-planar imaging (Posse et al., 2012). In the present study we characterize the sensitivity of MEVI for mapping RSN connectivity dynamics, comparing independent component analysis (ICA) and a novel seed-based connectivity analysis (SBCA) that combines sliding-window correlation analysis with meta-statistics. This SBCA approach is shown to minimize the effects of confounds, such as movement, and CSF and white matter signal changes, and enables real-time monitoring of RSN dynamics at time scales of tens of seconds. We demonstrate highly sensitive mapping of eloquent cortex in the vicinity of brain tumors and arterio-venous malformations, and detection of abnormal resting-state connectivity in epilepsy. In patients with motor impairment, resting-state fMRI provided focal localization of sensorimotor cortex compared with more diffuse activation in task-based fMRI. The fast acquisition speed of MEVI enabled segregation of cardiac-related signal pulsation using ICA, which revealed distinct regional differences in pulsation amplitude and waveform, elevated signal pulsation in patients with arterio-venous malformations and a trend toward reduced pulsatility in gray matter of patients compared with healthy controls. Mapping cardiac pulsation in cortical gray matter may carry important functional information that distinguishes healthy from diseased tissue vasculature. This novel fMRI methodology is particularly promising for mapping eloquent cortex in patients with neurological disease, having variable degree of cooperation in task-based fMRI. In conclusion, ultra-high-real-time speed fMRI enhances the sensitivity of mapping the dynamics of resting-state connectivity and cerebro-vascular pulsatility for clinical and neuroscience research applications.

Real-time automated spectral assessment of the BOLD response for neurofeedback at 3 and 7T

Echo-planar imaging is the dominant functional MRI data acquisition scheme for evaluating the BOLD signal. To date, it remains the only approach providing neurofeedback from spatially localized brain activity. Real-time functional single-voxel proton spectroscopy (fSVPS) may be an alternative for spatially specific BOLD neurofeedback at 7T because it allows for a precise estimation of the local T2* signal, EPI-specific artifacts may be avoided, and the signal contrast may increase. In order to explore and optimize this alternative neurofeedback approach, we tested fully automated real-time fSVPS spectral estimation procedures to approximate T2* BOLD signal changes from the unsuppressed water peak, i.e. lorentzian non-linear complex spectral fit (LNLCSF) in frequency and frequency-time domain. The proposed approaches do not require additional spectroscopic localizers in contrast to conventional T2* approximation based on linear regression of the free induction decay (FID). For methods comparison, we evaluated quality measures for signals from the motor and the visual cortex as well as a real-time feedback condition at high (3T) and at ultra-high (7T) magnetic field strengths. Using these methods, we achieved reliable and fast water peak spectral parameter estimations. At 7T, we observed an absolute increase of spectra line narrowing due to the BOLD effect, but quality measures did not improve due to artifactual line broadening. Overall, the automated fSVPS approach can be used to assess dynamic spectral changes in real-time, and to provide localized T2* neurofeedback at 3 and 7T.

Computerized MRS voxel registration and partial volume effects in single voxel 1H-MRS

Partial volume effects in proton magnetic resonance spectroscopy in the brain have been studied previously in terms of proper water concentration calculations, but there is a lack of disclosure in terms of voxel placement techniques that would affect the calculations. The purpose of this study is to facilitate a fully automated MRS voxel registration method which is time efficient, accurate, and can be extended to all imaging modalities. A total of thirteen healthy adults underwent single voxel 1H-MRS scans in 3.0T MRI scanners. Transposition of a MRS voxel onto an anatomical scan is derived along with a full calculation of water concentration with a correction term to account for the partial volume effects. Five metabolites (tNAA, Glx, tCr, mI, and tCho) known to yield high reliability are studied. Pearson's correlation analyses between tissue volume fractions and metabolite concentrations were statistically significant in parietal (tCr, Glx, and tNAA) lobe and occipital lobe (tNAA). MRS voxel overlaps quantified by dice metric over repeated visits yielded 60%~70% and coefficients of variance in metabolites concentration were 4%~10%. These findings reiterate an importance of considering the partial volume effects when tissue water is used as an internal concentration reference so as to avoid misinterpreting a morphometric difference as a metabolic difference.

Brain MR spectroscopic abnormalities in "MRI-negative" tuberous sclerosis complex patients

Since approximately 5-10% of the ~50,000 tuberous sclerosis complex (TSC) patients in the US are "MRI-negative," our goal was to test the hypothesis that they nevertheless exhibit metabolic abnormalities. To test this, we used proton MR spectroscopy to obtain and compare gray and white matter (GM and WM) levels of the neuronal marker, N-acetylaspartate (NAA), the glial marker, myo-inositol (mI), and its associated creatine (Cr), and choline (Cho) between two "MRI-negative" female TSC patients (ages 5 and 43 years) and their matched controls. The NAA, Cr, Cho and mI concentrations, 9.8, 6.3, 1.4, and 5.7 mM, in the pediatric control were similar to those of the patients, whereas the adult patient revealed a 17% WM NAA decrease and 16% WM Cho increase from their published means for healthy adults - both outside their respective 90% prediction intervals. These findings suggest that longer disease duration and/or TSC2 gene mutation may cause axonal dysfunction and demyelination.

Probing myelin and axon abnormalities separately in psychiatric disorders using MRI techniques

In this manuscript we present novel MRI approaches to dissecting axon vs. myelin abnormalities in psychiatric disorders. Existing DTI approaches are not able to provide specific information on these subcellular elements but novel approaches are beginning to do so. We review two approaches (magnetization transfer ratio—MTR; and diffusion tensor spectroscopy—DTS) and the theoretical framework for interpreting data derived from these approaches. Work is ongoing to collect data that will answer some relevant questions using these techniques in schizophrenia and related conditions.

Global gray and white matter metabolic changes after simian immunodeficiency virus infection in CD8-depleted rhesus macaques: proton MRS imaging at 3 T

To test the hypotheses that global decreased neuro-axonal integrity reflected by decreased N-acetylaspartate (NAA) and increased glial activation reflected by an elevation in its marker, the myo-inositol (mI), present in a CD8-depleted rhesus macaque model of HIV-associated neurocognitive disorders. To this end, we performed quantitative MRI and 16 × 16 × 4 multivoxel proton MRS imaging (TE/TR = 33/1400 ms) in five macaques pre- and 4-6 weeks post-simian immunodeficiency virus infection. Absolute NAA, creatine, choline (Cho), and mI concentrations, gray and white matter (GM and WM) and cerebrospinal fluid fractions were obtained. Global GM and WM concentrations were estimated from 224 voxels (at 0.125 cm(3) spatial resolution over ~35% of the brain) using linear regression. Pre- to post-infection global WM NAA declined 8%: 6.6 ± 0.4 to 6.0 ± 0.5 mM (p = 0.05); GM Cho declined 20%: 1.3 ± 0.2 to 1.0 ± 0.1 mM (p < 0.003); global mI increased 11%: 5.7 ± 0.4 to 6.5 ± 0.5 mM (p < 0.03). Global GM and WM brain volume fraction changes were statistically insignificant. These metabolic changes are consistent with global WM (axonal) injury and glial activation, and suggest a possible GM host immune response.

Diffuse axonal injury in mild traumatic brain injury: a 3D multivoxel proton MR spectroscopy study

Since mild traumatic brain injury (mTBI) often leads to neurological symptoms even without clinical MRI findings, our goal was to test whether diffuse axonal injury is quantifiable with multivoxel proton MR spectroscopic imaging ((1)H-MRSI). T1- and T2-weighted MRI images and three-dimensional (1)H-MRSI (480 voxels over 360 cm(3), about 30 % of the brain) were acquired at 3 T from 26 mTBI patients (mean Glasgow Coma Scale score 14.7, 18-56 years old, 3-55 days after injury) and 13 healthy matched contemporaries as controls. The N-acetylaspartate (NAA), choline (Cho), creatine (Cr) and myo-inositol (mI) concentrations and gray-matter/white-matter (GM/WM) and cerebrospinal fluid fractions were obtained in each voxel. Global GM and WM absolute metabolic concentrations were estimated using linear regression, and patients were compared with controls using two-way analysis of variance. In patients, mean NAA, Cr, Cho and mI concentrations in GM (8.4 ± 0.7, 6.9 ± 0.6, 1.3 ± 0.2, 5.5 ± 0.6 mM) and Cr, Cho and mI in WM (4.8 ± 0.5, 1.4 ± 0.2, 4.6 ± 0.7 mM) were not different from the values in controls. The NAA concentrations in WM, however, were significantly lower in patients than in controls (7.2 ± 0.8 vs. 7.7 ± 0.6 mM, p = 0.0125). The Cho and Cr levels in WM of patients were positively correlated with time since mTBI. This (1)H-MRSI approach allowed us to ascertain that early mTBI sequelae are (1) diffuse (not merely local), (2) neuronal (not glial), and (3) in the global WM (not GM). These findings support the hypothesis that, similar to more severe head trauma, mTBI also results in diffuse axonal injury, but that dysfunction rather than cell death dominates shortly after injury.

The role of gray and white matter segmentation in quantitative proton MR spectroscopic imaging

Since the brain's gray matter (GM) and white matter (WM) metabolite concentrations differ, their partial volumes can vary the voxel's ¹H MR spectroscopy (¹H-MRS) signal, reducing sensitivity to changes. While single-voxel ¹H-MRS cannot differentiate between WM and GM signals, partial volume correction is feasible by MR spectroscopic imaging (MRSI) using segmentation of the MRI acquired for VOI placement. To determine the magnitude of this effect on metabolic quantification, we segmented a 1-mm³ resolution MRI into GM, WM and CSF masks that were co-registered with the MRSI grid to yield their partial volumes in approximately every 1 cm³ spectroscopic voxel. Each voxel then provided one equation with two unknowns: its i- metabolite's GM and WM concentrations C(i) (GM) , C(i) (WM) . With the voxels' GM and WM volumes as independent coefficients, the over-determined system of equations was solved for the global averaged C(i) (GM) and C(i) (WM) . Trading off local concentration differences offers three advantages: (i) higher sensitivity due to combined data from many voxels; (ii) improved specificity to WM versus GM changes; and (iii) reduced susceptibility to partial volume effects. These improvements made no additional demands on the protocol, measurement time or hardware. Applying this approach to 18 volunteered 3D MRSI sets of 480 voxels each yielded N-acetylaspartate, creatine, choline and myo-inositol C(i) (GM) concentrations of 8.5 ± 0.7, 6.9 ± 0.6, 1.2 ± 0.2, 5.3 ± 0.6 mM, respectively, and C(i) (WM) concentrations of 7.7 ± 0.6, 4.9 ± 0.5, 1.4 ± 0.1 and 4.4 ± 0.6mM, respectively. We showed that unaccounted voxel WM or GM partial volume can vary absolute quantification by 5-10% (more for ratios), which can often double the sample size required to establish statistical significance.

Quantitative mapping of total choline in healthy human breast using proton echo planar spectroscopic imaging (PEPSI) at 3 Tesla

PURPOSE

To quantitatively measure tCho levels in healthy breasts using Proton-Echo-Planar-Spectroscopic-Imaging (PEPSI).

MATERIALS AND METHODS

The two-dimensional mapping of tCho at 3 Tesla across an entire breast slice using PEPSI and a hybrid spectral quantification method based on LCModel fitting and integration of tCho using the fitted spectrum were developed. This method was validated in 19 healthy females and compared with single voxel spectroscopy (SVS) and with PRESS prelocalized conventional Magnetic Resonance Spectroscopic Imaging (MRSI) using identical voxel size (8 cc) and similar scan times (∼7 min).

RESULTS

A tCho peak with a signal to noise ratio larger than 2 was detected in 10 subjects using both PEPSI and SVS. The average tCho concentration in these subjects was 0.45 ± 0.2 mmol/kg using PEPSI and 0.48 ± 0.3 mmol/kg using SVS. Comparable results were obtained in two subjects using conventional MRSI. High lipid content in the spectra of nine tCho negative subjects was associated with spectral line broadening of more than 26 Hz, which made tCho detection impossible. Conventional MRSI with PRESS prelocalization in glandular tissue in two of these subjects yielded tCho concentrations comparable to PEPSI.

CONCLUSION

The detection sensitivity of PEPSI is comparable to SVS and conventional PRESS-MRSI. PEPSI can be potentially used in the evaluation of tCho in breast cancer. A tCho threshold concentration value of ∼0.7 mmol/kg might be used to differentiate between cancerous and healthy (or benign) breast tissues based on this work and previous studies.

Automated prescription of oblique brain 3D magnetic resonance spectroscopic imaging

Two major difficulties encountered in implementing Magnetic Resonance Spectroscopic Imaging (MRSI) in a clinical setting are limited coverage and difficulty in prescription. The goal of this project was to automate completely the process of 3D PRESS MRSI prescription, including placement of the selection box, saturation bands and shim volume, while maximizing the coverage of the brain. The automated prescription technique included acquisition of an anatomical MRI image, optimization of the oblique selection box parameters, optimization of the placement of outer-volume suppression saturation bands, and loading of the calculated parameters into a customized 3D MRSI pulse sequence. To validate the technique and compare its performance with existing protocols, 3D MRSI data were acquired from six exams from three healthy volunteers. To assess the performance of the automated 3D MRSI prescription for patients with brain tumors, the data were collected from 16 exams from 8 subjects with gliomas. This technique demonstrated robust coverage of the tumor, high consistency of prescription and very good data quality within the T2 lesion.

Detection of "oncometabolite" 2-hydroxyglutarate by magnetic resonance analysis as a biomarker of IDH1/2 mutations in glioma

Somatic mutations in isocitrate dehydrogenase (IDH)1 and 2 have been identified in a subset of gliomas, rendering these tumors with elevated levels of "oncometabolite," D-2-hydroxyglutarate (2HG). Herein, we report that 2HG can be precisely detected by magnetic resonance (MR) in human glioma specimens and used as a reliable biomarker to identify this subset of tumors. Specifically, we developed a two-dimensional correlation spectroscopy resonance method to reveal the distinctive cross-peak pattern of 2HG in the complex metabolite nuclear MR spectra of brain tumor tissues. This study demonstrates the feasibility, specificity, and selectivity of using MR detection and quantification of 2HG for the diagnosis and classification of IDH1/2 mutation-positive brain tumors. It further opens up the possibility of developing analogous non-invasive MR-based imaging and spectroscopy studies directly in humans in the neuro-oncology clinic.

Localized 1H NMR spectroscopy in different regions of human brain in vivo at 7 T: T2 relaxation times and concentrations of cerebral metabolites

At the high field strength of 7 T, in vivo spectra of the human brain with exceptional spectral quality sufficient to quantify 16 metabolites have been obtained previously only in the occipital lobe. However, neurochemical abnormalities associated with many brain disorders are expected to occur in brain structures other than the occipital lobe. The purpose of the present study was to obtain high-quality spectra from various brain regions at 7 T and to quantify the concentrations of different metabolites. To obtain concentrations of metabolites within four different regions of the brain, such as the occipital lobe, motor cortex, basal ganglia and cerebellum, the T(2) relaxation times of the singlets and J-coupled metabolites in these regions were measured for the first time at 7 T. Our results demonstrate that high-quality, quantifiable spectra can be obtained in regions other than the occipital lobe at 7 T utilizing a 16-channel transceiver coil and B(1)(+) shimming.

Quantitative proton magnetic resonance spectroscopy and spectroscopic imaging of the brain: a didactic review

This article presents background information related to methodology for estimating brain metabolite concentration from magnetic resonance spectroscopy (MRS) and magnetic resonance spectroscopic imaging measurements of living human brain tissue. It reviews progress related to this methodology, with emphasis placed on progress reported during the past 10 years. It is written for a target audience composed of radiologists and magnetic resonance imaging technologists. It describes in general terms the relationship between MRS signal amplitude and concentration. It then presents an overview of the many practical problems associated with deriving concentration solely from absolute measured signal amplitudes and demonstrates how a various signal calibration approaches can be successfully used. The concept of integrated signal amplitude is presented with examples that are helpful for qualitative reading of MRS data as well as for understanding the methodology used for quantitative measurements. The problems associated with the accurate measurement of individual signal amplitudes in brain spectra having overlapping signals from other metabolites and overlapping nuisance signals from water and lipid are presented. Current approaches to obtaining accurate amplitude estimates with least-squares fitting software are summarized.

Multivoxel proton MR spectroscopy used to distinguish anterior cingulate metabolic abnormalities in patients with schizophrenia

PURPOSE

To test the hypothesis that anterior cingulate cortex (ACC) subregions in patients with schizophrenia are metabolically different from those in healthy control subjects.

MATERIALS AND METHODS

This institutional review board-approved study was HIPAA compliant, and all participants provided written informed consent. Twenty-two patients with schizophrenia (13 male, nine female; 39.4 years ± 10.6 [standard deviation]) and 11 age- and sex-matched control subjects (seven male, four female; 35.5 years ± 10.7) underwent magnetic resonance (MR) imaging and three-dimensional 3-T voxel proton MR spectroscopy to measure absolute rostral and caudal ACC N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) concentrations. Exact Mann-Whitney test was used to compare patient data with control data, paired-sample Wilcoxon signed rank test was used to compare subregions within groups, and receiver operating characteristic curve analysis was used to assess sensitivity and specificity in diagnosis of schizophrenia.

RESULTS

There were no significant metabolic differences between patients and control subjects or between ACC subregions in control subjects. In patients, rostral ACC NAA and Cr concentrations were significantly lower than those in caudal ACC (6.2 mM ± 1.3 vs 7.1 mM ± 1.3, P < .01; 5.7 mmol/L ± 1.4 vs 6.3 mmol/L ± 1.6, P < .01; respectively); however, this did not hold true for Cho concentrations (1.7 mmol/L ± 0.5 vs 1.8 mmol/L ± 0.5). For individual differences between caudal and rostral measurements, only NAA in patients was different from that in control subjects (0.9 mmol/L ± 1.3 vs -0.1 mmol/L ± 0.5, P < .01), enabling prediction of schizophrenia with 68% sensitivity and 91% specificity, for a difference of more than 0.4.

CONCLUSION

Significant differences between caudal and rostral NAA concentration are found in ACC of patients with schizophrenia but not in ACC of healthy control subjects, indicating that neuronal density or integrity differences between ACC subregions may be characteristic of the disease.

Relationship between genetic variation in the glutaminase gene GLS1 and brain glutamine/glutamate ratio measured in vivo

BACKGROUND

Abnormalities in glutamatergic neurotransmission are implicated in several psychiatric disorders, but in vivo neurochemical studies of the glutamate (Glu) system have been hampered by a lack of adequate probes. By contrast, glutamine (Gln) and Glu can be quantified separately in proton magnetic resonance spectroscopy studies in vivo. Accumulating evidence suggests that the Gln/Glu ratio is a putative index of glutamatergic neurotransmission but interpretation of changes in the Gln/Glu ratio depends on the conditions of the system, including ammonia levels.

METHODS

Here, we explored whether variation in GLS1 (the gene encoding the brain isoform of glutaminase, which catalyzes Gln-to-Glu conversion) is associated with Gln/Glu measured in vivo in two brain regions (anterior cingulate cortex, parieto-occipital cortex).

RESULTS

A specific haplotype of four single nucleotide polymorphisms within GLS1 was significantly associated with Gln/Glu in the parieto-occipital cortex in an magnetic resonance spectroscopy-genetics dataset optimized for Gln/Glu detection (n = 42). This finding was replicated in a second magnetic resonance spectroscopy dataset that was optimized for γ-aminobutyric acid detection where Gln and Glu measurements could still be extracted (n = 40).

CONCLUSIONS

These findings suggest that genetic variation in a key component of glutamatergic machinery is associated with a putative in vivo index of glutamatergic neurotransmission. Thus, GLS1 genotype might provide insight into normal brain function and into the pathophysiology of many psychiatric conditions where glutamatergic neurotransmission has been implicated. It might also serve as a biomarker for predicting response to existing and novel therapeutic interventions in psychiatry that target glutamatergic neurotransmission.

Longitudinal inter- and intra-individual human brain metabolic quantification over 3 years with proton MR spectroscopy at 3 T

The longitudinal repeatability of proton MR spectroscopy ((1) H-MRS) in the healthy human brain at high fields over long periods is not established. Therefore, we assessed the inter- and intra-subject repeatability of (1) H-MRS in an approach suited for diffuse pathologies in 10 individuals, at 3T, annually for 3 years. Spectra from 480 voxels over 360 cm(3) (∼30%) of the brain, were individually phased, frequency-aligned, and summed into one average spectrum. This dramatically increases metabolites' signal-to-noise-ratios while maintaining narrow linewidths that improve quantification precision. The resulting concentrations of the N-acetylaspartate, creatine, choline, and myo-inositol are: 8.9 ± 0.8, 5.9 ± 0.6, 1.4 ± 0.1, and 4.5 ± 0.5 mM (mean ± standard-deviation). the inter-subject coefficients of variation are 8.7%, 10.2%, 10.7%, and 11.8%; and the longitudinal (intra-subject) coefficients of variation are lower still: 6.6%, 6.8%, 6.8%, and 10%, much better than the 35%, 44%, 55%, and 62% intra-voxel coefficients of variation. The biological and nonbiological components of the summed spectra coefficients of variation had similar contributions to the overall variance.

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.

Atlas-based automated positioning of outer volume suppression slices in short-echo time 3D MR spectroscopic imaging of the human brain

Spatial suppression of peripheral lipid-containing regions in volumetric MR spectroscopic imaging of the human brain requires placing large numbers of outer volume suppression (OVS) slices, which is time-consuming, prone to operator error and may introduce subject-dependent variability in volume coverage. We developed a novel, computationally efficient atlas-based approach for automated positioning of up to 16 OVS slices and the MR spectroscopic imaging slab. Standardized positions in Montreal Neurological Institute atlas space were established offline using a recently developed iterative optimization procedure. During the scanning session, positions in subject space were computed using affine transformation of standardized positions in Montreal Neurological Institute space. Offline analysis using magnetization prepared rapid gradient echo scans from 11 subjects demonstrated reliable OVS placement, comparable with but faster than iterative placement in subject space. This atlas-based method was further validated in 14 subjects using 3D short-echo time proton-echo-planar-spectroscopic-imaging at 3 T. Comparison of manual and automatic placement using 8 OVS slices demonstrated consistent MR spectroscopic imaging volume selection and comparable spectral quality with similar degree of lipid suppression and number of usable voxels. Automated positioning of 16 OVS slices enabled larger volume coverage, while maintaining similar spectral quality and lipid suppression. Atlas-based automatic prescription of short echo time MR spectroscopic imaging is expected to be advantageous for longitudinal and cross-sectional studies.

Proton Magnetic Resonance Spectroscopy in adult cancer patients with delirium

Delirium is associated with a host of negative outcomes, including increased risk of mortality, longer hospital stay, and poor long-term cognitive function. The pathophysiology of delirium is not well understood. Cancer patients undergoing a bone marrow transplant (BMT) are at high risk for developing delirium and Proton Magnetic Resonance Spectroscopy ((1)H MRS) could lead to better understanding of the delirium process. Fourteen BMT patients and 10 controls completed (1)H MRS, positioned above the corpus callosum, shortly after delirium onset or at study end if no delirium occurred. In the BMT-delirium group, statistically significantly elevated tCho/tCr was found in contrast to the BMT-no delirium group. The BMT-delirium group also showed statistically significantly lesser NAA/tCho compared with both controls and the BMT-no delirium group. Elevated choline and reduced NAA indicate inflammatory processes and white matter damage as well as neuronal metabolic impairment. Further research is needed to separate the choline peaks, as well as more detailed collection of medication regimens to determine whether a higher choline concentration is a function of the delirium process or cancer treatment effects.

Glutamate as a marker of cognitive function in schizophrenia: a proton spectroscopic imaging study at 4 Tesla

BACKGROUND

Cognitive deficits in schizophrenia may be related to glutamatergic dysfunction, but in vivo measurement of glutamate metabolism has been challenging. We examined the relationship between glutamate metabolism and cognitive function in schizophrenia.

METHODS

Thirty subjects with DSM-IV schizophrenia and 28 healthy volunteers were studied using 4 Tesla proton echo planar spectroscopic imaging. Glutamate plus glutamine (Glx), N-acetylaspartate compounds, and Inositol concentrations in gray and white matter and broad neuropsychological function were assessed in all subjects.

RESULTS

Glutamate plus glutamine was positively correlated with overall cognitive performance in the schizophrenia group (p = .0006), accounting for about 36% of the variance. No correlation was found in control subjects. Group-averaged Glx levels were similar in schizophrenia and control subjects. N-acetylaspartate compounds were reduced in cortical gray matter in the younger schizophrenia subjects (age < 30; p = .04) compared with age-matched control subjects. Inositol was increased in cortical gray (p = .002) and white matter (p = .02) in the older schizophrenia subjects (age > 30) compared with age-matched control subjects.

CONCLUSIONS

Although not reduced in schizophrenia as a group, lower Glx levels correlates with impaired cognition in the illness. This suggests heterogeneity in mechanisms that regulate glutamate function in schizophrenia. Patients with reduced glutamatergic reserves may be rendered into a more severe hypoglutamatergic state with cognitive consequences. Reduced cortical gray matter N-acetylaspartate compound concentration early in the illness with normalization in older subjects is consistent with a process of early dendritic retraction with subsequent increased neuronal packing. Later in the illness, Inositol elevation suggests glial involvement.

Automatic placement of outer volume suppression slices in MR spectroscopic imaging of the human brain

Spatial suppression of peripheral regions (outer volume suppression) is used in MR spectroscopic imaging to reduce contamination from strong lipid and water signals. The manual placement of outer volume suppression slices requires significant operator interaction, which is time consuming and introduces variability in volume coverage. Placing a large number of outer volume saturation bands for volumetric MR spectroscopic imaging studies is particularly challenging and time consuming and becomes unmanageable as the number of suppression bands increases. In this study, a method is presented that automatically segments a high-resolution MR image in order to identify the peripheral lipid-containing regions. This method computes an optimized placement of suppression bands in three dimensions and is based on the maximization of a criterion function. This criterion function maximizes coverage of peripheral lipid-containing areas and minimizes suppression of cortical brain regions and regions outside of the head. Computer simulation demonstrates automatic placement of 16 suppression slices to form a convex hull that covers peripheral lipid-containing regions above the base of the brain. In vivo metabolite mapping obtained with short echo time proton-echo-planar spectroscopic imaging shows that the automatic method yields a placement of suppression slices that is very similar to that of a skilled human operator in terms of lipid suppression and usable brain voxels.

Robust reconstruction of MRSI data using a sparse spectral model and high resolution MRI priors

We introduce a novel algorithm to address the challenges in magnetic resonance (MR) spectroscopic imaging. In contrast to classical sequential data processing schemes, the proposed method combines the reconstruction and postprocessing steps into a unified algorithm. This integrated approach enables us to inject a range of prior information into the data processing scheme, thus constraining the reconstructions. We use high resolution, 3-D estimate of the magnetic field inhomogeneity map to generate an accurate forward model, while a high resolution estimate of the fat/water boundary is used to minimize spectral leakage artifacts. We parameterize the spectrum at each voxel as a sparse linear combination of spikes and polynomials to capture the metabolite and baseline components, respectively. The constrained model makes the problem better conditioned in regions with significant field inhomogeneity, thus enabling the recovery even in regions with high field map variations. To exploit the high resolution MR information, we formulate the problem as an anatomically constrained total variation optimization scheme on a grid with the same spacing as the magnetic resonance imaging data. We analyze the performance of the proposed scheme using phantom and human subjects. Quantitative and qualitative comparisons indicate a significant improvement in spectral quality and lower leakage artifacts.

Improving 1H MRSI measurement of cerebral lactate for clinical applications

Accurate measurement of cerebral lactate is critical to the understanding of brain function for psychiatric disorders such as panic disorder and bipolar disorder as well as mitochondrial dysfunction. Proton magnetic spectroscopic imaging (MRSI) techniques can be used to study lactate in vivo; however, accurate measurement of cerebral lactate, which is normally at low basal abundance, can be challenging. In this study, regional lactate measurements obtained with two different MRSI analytic approaches were evaluated using proton echo-planar spectroscopic imaging (PEPSI) data from 18 healthy adults participating in an in vivo sodium lactate infusion study. The results demonstrate that averaging data within a region of interest (ROI) before spectral fitting with LCModel results in significantly improved lactate measurement as compared to averaging chemical concentrations derived from the fitting of individual voxels in the ROI. Simulation results that confirm this finding are also presented. This study additionally outlines an atlas-based approach for the systematic computation of regional distributions of chemical concentrations in large MRSI data sets.

Proton MRS in twin pairs discordant for schizophrenia

Proton magnetic resonance spectroscopy ((1)H MRS) neurometabolite abnormalities have been detected widely in subjects with and at risk for schizophrenia. We hypothesized that such abnormalities would be present both in patients with schizophrenia and in their unaffected twin siblings. We acquired magnetic resonance spectra (TR/TE=3000/30 ms) at voxels in the mesial prefrontal gray matter, left prefrontal white matter and left hippocampus in 14 twin pairs discordant for schizophrenia (2 monozygotic, 12 dizygotic), 13 healthy twin pairs (4 monozygotic, 9 dizygotic) and 1 additional unaffected co-twin of a schizophrenia proband. In the mesial prefrontal gray matter voxel, N-acetylaspartate (NAA), creatine+phosphocreatine (Cr), glycerophosphocholine+phosphocholine (Cho) and myo-inositol (mI) did not differ significantly between patients with schizophrenia, their unaffected co-twins or healthy controls. However, glutamate (Glu) was significantly lower in patients with schizophrenia (31%, percent difference) and unaffected co-twins (21%) than in healthy controls (collapsed across twin pairs). In the left hippocampus voxel, levels of NAA (23%), Cr (22%) and Cho (36%) were higher in schizophrenia patients compared with controls. Hippocampal NAA (25%), Cr (22%) and Cho (37%) were also significantly higher in patients than in their unaffected co-twins. Region-to-region differences in metabolite levels were also notable within all three diagnosis groups. These findings suggest that (1)H MRS neurometabolite abnormalities are present not only in patients with schizophrenia, but also in their unaffected co-twins. Thus, reduced mesial prefrontal cortical Glu and elevated hippocampal NAA, Cr and Cho may represent trait markers of schizophrenia risk and, when exacerbated, state markers of schizophrenia itself.

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.

In vivo 1H NMR spectroscopy of the human brain at high magnetic fields: metabolite quantification at 4T vs. 7T

A comprehensive comparative study of metabolite quantification from the human brain was performed on the same 10 subjects at 4T and 7T using MR scanners with identical consoles, the same type of RF coils, and identical pulse sequences and data analysis. Signal-to-noise ratio (SNR) was increased by a factor of 2 at 7T relative to 4T in a volume of interest selected in the occipital cortex using half-volume quadrature radio frequency (RF) coils. Spectral linewidth was increased by 50% at 7T, which resulted in a 14% increase in spectral resolution at 7T relative to 4T. Seventeen brain metabolites were reliably quantified at both field strengths. Metabolite quantification at 7T was less sensitive to reduced SNR than at 4T. The precision of metabolite quantification and detectability of weakly represented metabolites were substantially increased at 7T relative to 4T. Because of the increased spectral resolution at 7T, only one-half of the SNR of a 4T spectrum was required to obtain the same quantification precision. The Cramér-Rao lower bounds (CRLB), a measure of quantification precision, of several metabolites were lower at both field strengths than the intersubject variation in metabolite concentrations, which resulted in a strong correlation between metabolite concentrations of individual subjects measured at 4T and 7T.

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

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

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.