Radhika Srinivasan

Quantitative In Vivo Magnetic Resonance Imaging of Multiple Sclerosis at 7 Tesla with Sensitivity to Iron

Magnetic resonance imaging at 7 Tesla produces high‐resolution gradient‐echo phase images of patients with multiple sclerosis (MS) that quantify the local field shifts from iron in the basal ganglia and lesions. Phase imaging is easily integrated into clinical examinations because it is a postprocessing technique and does not require additional scanning. The purpose of this study was to quantify local field shifts in MS and to investigate their relation to disease duration and disability status.

Measurement of brain glutamate using TE-averaged PRESS at 3T.

A method is introduced that provides improved in vivo spectroscopic measurements of glutamate (Glu), glutamine (Gln), choline (Cho), creatine (Cre), N‐acetyl compounds (NAtot, NAA + NAAG), and the inositols (mI and sI). It was found that at 3T, TE averaging, the f1 = 0 slice of a 2D J‐resolved spectrum, yielded unobstructed signals for Glu, Glu + Gln (Glx), mI, NAtot, Cre, and Cho. The C4 protons of Glu at 2.35 ppm, and the C2 protons of Glx at 3.75 ppm were well resolved and yielded reliable measures of Glu/Gln stasis. Apparent T1/T2 values were obtained from the raw data, and metabolite tissue levels were determined relative to a readily available standard. A repeatibility error of <5%, and a coefficient of variation (CV) of <10% were observed for brain Glu levels in a study of six normal volunteers. Magn Reson Med 51:435–440, 2004.

Genotype–Phenotype correlations in multiple sclerosis: HLA genes influence disease severity inferred by 1HMR spectroscopy and MRI measures

Genetic susceptibility to multiple sclerosis (MS) is associated with the human leukocyte antigen (HLA) DRB1*1501 allele. Here we show a clear association between DRB1*1501 carrier status and four domains of disease severity in an investigation of genotype-phenotype associations in 505 robust, clinically well characterized MS patients evaluated cross-sectionally: (i) a reduction in the N-acetyl-aspartate (NAA) concentration within normal appearing white matter (NAWM) via (1)HMR spectroscopy (P = 0.025), (ii) an increase in the volume of white matter (WM) lesions utilizing conventional anatomical MRI techniques (1,127 mm(3); P = 0.031), (iii) a reduction in normalized brain parenchymal volume (nBPV) (P = 0.023), and (iv) impairments in cognitive function as measured by the Paced Auditory Serial Addition Test (PASAT-3) performance (Mean Z Score: DRB1*1501+: 0.110 versus DRB1*1501-: 0.048; P = 0.004). In addition, DRB1*1501+ patients had significantly more women (74% versus 63%; P = 0.009) and a younger mean age at disease onset (32.4 years versus 34.3 years; P = 0.025). Our findings suggest that DRB1*1501 increases disease severity in MS by facilitating the development of more T2-foci, thereby increasing the potential for irreversible axonal compromise and subsequent neuronal degeneration, as suggested by the reduction of NAA concentrations in NAWM, ultimately leading to a decline in brain volume. These structural aberrations may explain the significant differences in cognitive performance observed between DRB1*1501 groups. The overall goal of a deep phenotypic approach to MS is to develop an array of meaningful biomarkers to monitor the course of the disease, predict future disease behaviour, determine when treatment is necessary, and perhaps to more effectively recommend an available therapeutic intervention.

Magnetic Resonance of 2-Hydroxyglutarate in IDH1-Mutated Low-Grade Gliomas

2-Hydroxyglutarate can be detected ex vivo in biopsy tissue from IDH1-mutant low-grade gliomas with proton high-resolution magic angle spinning NMR spectroscopy. Gliomas Make the Grade Tumors of the central nervous systems can be classified, or “graded,” on a scale of I to IV, according to their capacity to proliferate and invade surrounding tissue (with I being benign). Although determining the grade of brain tumor malignancy is important for doctors to predict survival and prescribe treatment, it cannot sufficiently explain the variation in clinical outcome. Some have attempted to classify brain tumors on the basis of acquired mutations, which has provided further insight into the characteristic diversity observed in survival. For example, one mutation in the gene isocitrate dehydrogenase (IDH) has demonstrated prolonged life expectancy for patients with low-grade brain tumors. Now, Elkhaled, Jalbert, and colleagues have shown that accumulation of a metabolite resulting from this mutation can be detected using magnetic resonance imaging techniques. Under normal conditions, the IDH enzyme converts the metabolite isocitrate to α-ketoglutarate. When IDH is mutated, its enzyme product further converts α-ketoglutarate to an otherwise scarce metabolite, 2-hydroxyglutarate (2HG). Because improved patient outcome has been associated with IDH mutations, the accumulation of 2HG might therefore be able to predict favorable genotypes. Elkhaled, Jalbert, and colleagues used an imaging method based on proton high-resolution magic angle spinning (1H HR-MAS) nuclear magnetic resonance (NMR) spectroscopy to determine whether the presence of 2HG was detectable. A total of 104 tissue (biopsy) samples from 52 patients with recurrent grade II gliomas (some of which had converted to grades III or IV) were evaluated for the presence of 2HG and the IDH1 mutation. 2HG proved to be detectable by spectroscopic analysis and showed approximately 86% concordance with the status of IDH1 mutation, as determined by antibody staining and genetic sequencing. Furthermore, 2HG abundance was shown to be similar across all brain tumor grades when normalized by cellularity, suggesting that its relative production per cell remains the same even after lesions have converted to higher histologic grades. This finding bears considerable significance for clinical evaluation of the malignancy grade and extent of tumor lesions. Finally, 2HG levels were determined to negatively correlate with normal vascularity. Given the current hypotheses in the literature regarding the influence of 2HG on vascular endothelial growth factor (VEGF), this result may be of interest for designing anti-angiogenic strategies for treating tumors with IDH mutations. This ex vivo study—along with its in vivo companion by Andronesi et al.—shows that the use of magnetic resonance imaging technology, already routine in the clinic, could significantly improve the management of brain tumors. Recent studies have indicated that a significant survival advantage is conferred to patients with gliomas whose lesions harbor mutations in the genes isocitrate dehydrogenase 1 and 2 (IDH1/2). IDH1/2 mutations result in aberrant enzymatic production of the potential oncometabolite d-2-hydroxyglutarate (2HG). Here, we report on the ex vivo detection of 2HG in IDH1-mutated tissue samples from patients with recurrent low-grade gliomas using the nuclear magnetic resonance technique of proton high-resolution magic angle spinning spectroscopy. Relative 2HG levels from pathologically confirmed mutant IDH1 tissues correlated with levels of other ex vivo metabolites and histopathology parameters associated with increases in mitotic activity, relative tumor content, and cellularity. Ex vivo spectroscopic measurements of choline-containing species and in vivo magnetic resonance measurements of diffusion parameters were also correlated with 2HG levels. These data provide extensive characterization of mutant IDH1 lesions while confirming the potential diagnostic value of 2HG as a surrogate marker of patient survival. Such information may augment the ability of clinicians to monitor therapeutic response and provide criteria for stratifying patients to specific treatment regimens.

MR spectroscopic imaging of glutathione in the white and gray matter at 7 T with an application to multiple sclerosis.

Detection of glutathione (GSH) is technically challenging at clinical field strengths of 1.5 or 3 T due to its low concentration in the human brain coupled with the fact that conventional single-echo acquisitions, typically used for magnetic resonance (MR) spectroscopy acquisitions, cannot be used to resolve GSH given its overlap with other resonances. In this study, an MR spectral editing scheme was used to generate an unobstructed detection of GSH at 7 T. This technique was used to obtain normative white (WM) and gray matter (GM) GSH concentrations over a two-dimensional region. Results indicated that GSH was significantly higher (P<.001) in GM relative to WM in normal subjects. This finding is consistent with previous radionuclide experiments and histochemical staining and validates this 7 T MR spectroscopy technique. To our knowledge, this is the first study to report normative differences in WM and GM glutathione concentrations in the human brain. Glutathione is a biomarker for oxidative status and this non-invasive in vivo measurement of GSH was used to explore its sensitivity to oxidative state in multiple sclerosis (MS) patients. There was a significant reduction (P<.001) of GSH between the GM in MS patients and normal controls. No statistically significant GSH differences were found between the WM in controls and MS patients. Reduced GSH was also observed in a MS WM lesion. This preliminary investigation demonstrates the potential of this marker to probe oxidative state in MS.

High-resolution phased-array MRI of the human brain at 7 tesla: initial experience in multiple sclerosis patients.

Recent advancement for magnetic resonance imaging (MRI) involves the incorporation of higher‐field strengths. Although imagers with higher magnetic field strengths were developed and tested in research labs, the direct application to patient MR studies have been extremely limited. Imaging at 7 Tesla (7T) affords advantages in signal‐to‐noise ratio and image contrast and resolution; however, these benefits can only be realized if the correct coils exist to capture the images. The objective of this study was to develop optimized high‐resolution 7T MRI techniques using high sensitivity, specialized phased‐array coils, for improved gray matter (GM) and white matter differentiation, in an effort to improve visualization of multiple sclerosis (MS) lesions in vivo. Twenty‐three subjects were enrolled in this preliminary study, 17 with clinically definite MS (11 females, 6 males; mean age 43.4 years; range 22‐64 years) and 6 healthy controls (2 females, 4 males; mean age 39.0 years; range 27‐67 years). MR imaging of MS patients at 7T was demonstrated to be safe, well tolerated, and provided high‐resolution anatomical images allowing visualization of structural abnormalities localized near or within the cortical layers. Clear involvement of the GM was observed with improved morphological detail in comparison to imaging at lower‐field strength.

Genetic variation influences glutamate concentrations in brains of patients with multiple sclerosis

Glutamate is the main excitatory neurotransmitter in the mammalian brain. Appropriate transmission of nerve impulses through glutamatergic synapses is required throughout the brain and forms the basis of many processes including learning and memory. However, abnormally high levels of extracellular brain glutamate can lead to neuroaxonal cell death. We have previously reported elevated glutamate levels in the brains of patients suffering from multiple sclerosis. Here two complementary analyses to assess the extent of genomic control over glutamate levels were used. First, a genome-wide association analysis in 382 patients with multiple sclerosis using brain glutamate concentration as a quantitative trait was conducted. In a second approach, a protein interaction network was used to find associated genes within the same pathway. The top associated marker was rs794185 (P < 6.44 x 10(-7)), a non-coding single nucleotide polymorphism within the gene sulphatase modifying factor 1. Our pathway approach identified a module composed of 70 genes with high relevance to glutamate biology. Individuals carrying a higher number of associated alleles from genes in this module showed the highest levels of glutamate. These individuals also showed greater decreases in N-acetylaspartate and in brain volume over 1 year of follow-up. Patients were then stratified by the amount of annual brain volume loss and the same approach was performed in the high (n = 250) and low (n = 132) neurodegeneration groups. The association with rs794185 was highly significant in the group with high neurodegeneration. Further, results from the network-based pathway analysis remained largely unchanged even after stratification. Results from these analyses indicated that variance in the activity of neurochemical pathways implicated in neurodegeneration is explained, at least in part, by the inheritance of common genetic polymorphisms. Spectroscopy-based imaging provides a novel quantitative endophenotype for genetic association studies directed towards identifying new factors that contribute to the heterogeneity of clinical expression of multiple sclerosis.

TE-Averaged two-dimensional proton spectroscopic imaging of glutamate at 3 T

Glutamate and glutamine are important neurochemicals in the central nervous system and the neurotoxic properties of excess glutamate have been associated with several neurodegenerative diseases. The TE-Averaged PRESS technique has been shown by our group to detect an unobstructed glutamate signal at 3 T that is resolved from glutamine and NAA at 2.35 ppm. TE-Averaged PRESS therefore provides an unambiguous measurement of glutamate as well as other metabolites such as NAA, choline, creatine, and myo-inositol. In this study, we extend the single voxel TE-Averaged PRESS technique for two-dimensional (2D) spectroscopic imaging (TE-Averaged MRSI) to generate 2D glutamate maps. To facilitate TE-Averaged MRSI within a reasonable time, a fast encoding trajectory was used. This enabled rapid acquisition of TE-Averaged spectral arrays with good spectral bandwidth (977 Hz) and resolution (approximately 2 Hz). MRSI data arrays of 10 x 16 were acquired with 1.8 cm3 spatial resolution over a approximately 110 cm3 volume in a scan time of approximately 21 min. Two-dimensional metabolite maps were obtained with good SNR and clear differentiation in glutamate levels was observed between gray and white matter with significantly higher glutamate in gray matter relative to white matter as anticipated.

Ex vivo MR spectroscopic measure differentiates tumor from treatment effects in GBM

The motivation of this study was to address the urgent clinical problem related to the inability of magnetic resonance (MR) imaging measures to differentiate tumor progression from treatment effects in patients with glioblastoma multiforme (GBM). While contrast enhancement on MR imaging (MRI) is routinely used for assessment of tumor burden, therapy response, and progression-free survival in GBM, it is well known that changes in enhancement following treatment are nonspecific to tumor. To address this issue, the objective of this study was to investigate whether MR spectroscopy can provide improved biomarker surrogates for tumor following treatment. High-resolution metabolic profiles of tissue samples obtained from patients with GBM were directly correlated with their pathological assessment to determine metabolic markers that correspond to pathological indications of tumor or treatment effects. Acquisition of tissue samples with image guidance enabled the association of ex vivo biochemical and pathological properties of the tissue samples with in vivo MR anatomical and structural properties derived from presurgical MR images. Using this approach, we found that metabolic concentration levels of [Myo-inositol/total choline (MCI)] in tissue samples are able to differentiate tumor from nontumor and treatment-induced reactive astrocytosis with high significance (P < .001) in newly diagnosed and recurrent GBM. The MCI index has a sensitivity of 93% to tumor in recurrent GBM and delineates the contribution of cellularity that originates from tumor and astrocytic proliferation following treatment. Low levels of MCI for tumor were associated with a reduced apparent diffusion coefficient and elevated choline-N-acetyl-aspartate index derived from in vivo MR images.

Comparison of T(1) and T(2) metabolite relaxation times in glioma and normal brain at 3T.

To measure T1 and T2 relaxation times of metabolites in glioma patients at 3T and to investigate how these values influence the observed metabolite levels.