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

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

Improvement of resolution for brain coupled metabolites by optimized (1)H MRS at 7T

Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE‐optimized point‐resolved spectroscopy (PRESS) at 7 T is reported. Sub‐TE dependences of the multiplets of Glu, Gln, GSH, γ‐aminobutyric acid (GABA) and N‐acetylaspartate (NAA) at 2.2–2.6 ppm were investigated with density matrix simulations, incorporating three‐dimensional volume localization. The numerical simulations indicated that the C4‐proton multiplets can be completely separated with (TE₁, TE₂) = (37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5 ppm signal. Phantom experiments reproduced the signal yield and lineshape from simulations within experimental errors. In vivo tests of optimized PRESS were conducted on the prefrontal cortex of six healthy volunteers. In spectral fitting by LCModel, Cramér–Rao lower bounds (CRLBs) of Glu, Gln and GSH were 2 ± 1, 5 ± 1 and 6 ± 2 (mean ± SD), respectively. To evaluate the performance of the optimized PRESS method under identical experimental conditions, stimulated‐echo spectra were acquired with (TE, TM) = (14, 37) and (74, 68) ms. The CRLB of Glu was similar between PRESS and short‐TE stimulated‐echo acquisition mode (STEAM), but the CRLBs of Gln and GSH were lower in PRESS than in both STEAM acquisitions. Copyright © 2010 John Wiley & Sons, Ltd.     

Quantification of glutamate and glutamine using constant‐time point‐resolved spectroscopy at 3 T

Separate quantification of glutamate (Glu) and glutamine (Gln) using conventional MRS on clinical scanners is challenging. In previous work, constant‐time point‐resolved spectroscopy (CT‐PRESS) was optimized at 3 T to detect Glu, but did not resolve Gln. To quantify Glu and Gln, a time‐domain basis set was constructed taking into account metabolite T₂ relaxation times and dephasing from B₀ inhomogeneity. Metabolite concentrations were estimated by fitting the basis one‐dimensional CT‐PRESS diagonal magnitude spectra to the measured spectrum. This method was first validated using seven custom‐built phantoms containing variable metabolite concentrations, and then applied to in vivo data acquired in rats exposed to vaporized ethanol and controls. Separate metabolite quantification revealed increased Gln after 16 weeks and increased Glu after 24 weeks of vaporized ethanol exposure in ethanol‐treated compared with control rats. Without separate quantification, the signal from the combined resonances of Glu and Gln (Glx) showed an increase at both 16 and 24 weeks in ethanol‐exposed rats, precluding the determination of the independent and differential contribution of each metabolite at each time. Copyright © 2012 John Wiley & Sons, Ltd.     

Neurometabolic and structural alterations in rat brain due to acute hypobaric hypoxia: in vivo 1H MRS at 7 T

In response to hypobaric hypoxia (HH), which occurs at high altitude, the brain undergoes deleterious changes at the structural and metabolite level. In vivo T₂ weighted imaging (T2WI) and ¹H‐MRS was performed to understand the structural and metabolic changes in the hippocampus region of rat brain. Data were acquired pre‐exposure (baseline controls), immediately after exposure and subsequently at the first, fourth, seventh and 14th days post exposure at normoxia. T₂ weighted images of rat brain showed hyperintensity in the CA2/CA3 region of the hippocampus 7 d after acute HH, which persisted till 14 d, probably indicating structural changes in the hippocampus. ¹H‐MRS results showed no change in metabolite level immediately after acute HH exposure, but on the first day of normoxia the myo‐inositol level was significantly decreased, possibly due to altered astrocyte metabolism. Metabolic alterations showing an increase in choline and decrease in glutamate on the fourth day of normoxia may be seen as a process of demyelination and loss of glutamate pool respectively. On the seventh and 14th days of normoxia, decreases in N‐acetylaspartate, creatine and glutamine + glutamate were observed, which might be due to decreased viability of glutamatergic neurons. In vivo ¹H‐MRS demonstrated early neurometabolic changes prior to probable structural changes post acute HH exposure. The extension of these studies will help in early risk assessment, developing intervention and strategies for combating HH related changes. Copyright © 2014 John Wiley & Sons, Ltd.     

PRESS timings for resolving 13C4‐glutamate 1H signal at 9.4 T: Demonstration in rat with uniformly labelled 13C‐glucose

MRS of ¹³C₄‐labelled glutamate (¹³C₄‐Glu) during an infusion of a carbon‐13 (¹³C)‐labelled substrate, such as uniformly labelled glucose ([U‐¹³C₆]‐Glc), provides a measure of Glc metabolism. The presented work provides a single‐shot indirect ¹³C detection technique to quantify the approximately 2.51 ppm ¹³C₄‐Glu satellite proton (¹H) peak at 9.4 T. The methodology is an optimized point‐resolved spectroscopy (PRESS) sequence that minimizes signal contamination from the strongly coupled protons of N‐acetylaspartate (NAA), which resonate at approximately 2.49 ppm. J‐coupling evolution of protons was characterized numerically and verified experimentally. A (TE₁, TE₂) combination of (20 ms, 106 ms) was found to be suitable for minimizing NAA signal in the 2.51 ppm ¹H ¹³C₄‐Glu spectral region, while retaining the ¹³C₄‐Glu ¹H satellite peak. The efficacy of the technique was verified on phantom solutions and on two rat brains in vivo during an infusion of [U‐¹³C₆]‐Glc. LCModel was employed for analysis of the in vivo spectra to quantify the 2.51 ppm ¹H ¹³C₄‐Glu signal to obtain Glu C4 fractional enrichment time courses during the infusions. Cramér‐Rao lower bounds of about 8% were obtained for the 2.51 ppm ¹³C₄‐Glu ¹H satellite peak with the optimal TE combination.     

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

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

1H MRS characterization of neurochemical profiles in orthotopic mouse models of human brain tumors

Glioblastoma (GBM), the most common primary brain tumor, is resistant to currently available treatments. The development of mouse models of human GBM has provided a tool for studying mechanisms involved in tumor initiation and growth as well as a platform for preclinical investigation of new drugs. In this study we used ¹H MR spectroscopy to study the neurochemical profile of a human orthotopic tumor (HOT) mouse model of human GBM. The goal of this study was to evaluate differences in metabolite concentrations in the GBM HOT mice when compared with normal mouse brain in order to determine if MRS could reliably differentiate tumor from normal brain. A T_E =19 ms PRESS sequence at 9.4 T was used for measuring metabolite levels in 12 GBM mice and 8 healthy mice. Levels for 12 metabolites and for lipids/macromolecules at 0.9 ppm and at 1.3 ppm were reliably detected in all mouse spectra. The tumors had significantly lower concentrations of total creatine, GABA, glutamate, total N‐acetylaspartate, aspartate, lipids/macromolecules at 0.9 ppm, and lipids/macromolecules at 1.3 ppm than did the brains of normal mice. The concentrations of glycine and lactate, however, were significantly higher in tumors than in normal brain. Copyright © 2014 John Wiley & Sons, Ltd.     

J‐refocused 1H PRESS DEPT for localized 13C MR Spectroscopy

Proton point‐resolved spectroscopy (PRESS) localization has been combined with distortionless enhanced polarization transfer (DEPT) in multinuclear MRS to overcome the signal contamination problem in image‐selected in vivo spectroscopy (ISIS)‐combined DEPT, especially for lipid detection. However, homonuclear proton scalar couplings reduce the DEPT enhancement by modifying the spin coherence distribution under J modulation during proton PRESS localization. Herein, a J‐refocused proton PRESS‐localized DEPT sequence is presented to obtain simultaneously enhanced and localized signals from a large number of metabolites by in vivo ¹³C MRS. The suppression of J modulation during PRESS and the substantial recovery of signal enhancement by J‐refocused PRESS‐localized DEPT were demonstrated theoretically by product operator formalism, numerically by the spin density matrix simulations for different scalar coupling conditions, and experimentally with a glutamate phantom at various TEs, as well as a colza oil phantom. The application of the sequence for localized detection of saturated and unsaturated fatty acids in the calf bone marrow and skeletal muscle of healthy subjects yielded high signal enhancements simultaneously obtained for all components. Copyright © 2013 John Wiley & Sons, Ltd.     

Proton MRS of large multiple sclerosis lesions reveals subtle changes in metabolite T(1) and area

The T₁ values of metabolites were measured in eight subjects with clinically definite multiple sclerosis (MS) having at least one large brain lesion (2.6 ± 0.7 mL) and in eight age‐ and sex‐matched healthy controls. MRS examinations were conducted at 1.5 T using point‐resolved spectroscopy (PRESS) (TE = 30 ms, TR = 530, 750, 1200, 1500, 3500, 5000 ms). Spectra were acquired from a voxel placed in the largest lesion in the subject with MS, and in a corresponding voxel (same size and region) in normal white matter (NWM) in the matched control, and were fitted using LCModel. As there are regional variations in metabolite and water T₁ and metabolite signal areas, careful placement of the control voxel was necessary to measure subtle differences between the lesions and NWM. The T₁ and T₁‐corrected signal areas of creatine were the same in MS lesions as in controls. The T₁ values of choline were significantly shorter in MS lesions located in occipital and parietal, but not in frontal, white matter. N‐Acetylaspartate (NAA) and myoinositol T₁ values in MS lesions were similar to those in NWM; however, the area of myoinositol correlated directly with lesion water T₁, and the area of NAA correlated inversely with lesion water T₁. MR spectra acquired at short TR require T₁ correction of choline for accurate quantification. Careful voxel placement in controls to match lesion location in subjects with MS enables a clearer view of the subtle changes in lesions. Copyright © 2010 John Wiley & Sons, Ltd.     

Short-TE localised 1H MRS of the human brain at 3 T: quantification of the metabolite signals using two approaches to account for macromolecular signal contributions

The goal of this study was to validate metabolite quantification at short TE, with particular focus on how to best account for the macromolecular signal contribution. A robust, short-TE PRESS protocol is presented, which allows reliable quantification, in vivo, of metabolite signals at 3 T in human brain. Water suppression was adapted to the experimental conditions at 3 T. Metabolite signal from the parietal white matter was quantified in the time domain using QUEST (jMRUI). The increased macromolecular signal contribution at short TE was dealt with by two approaches, based on either metabolite nulling or initial signal truncation. A detailed comparison of the two approaches was made. The first used a metabolite-nulled signal, measured either individually or averaged over different subjects. The second used the total signal, metabolites and macromolecules, from a single scan. The two approaches gave similar quantification results in terms of metabolite concentrations, but differed in their precision and the number of metabolites quantified reliably. With an average metabolite-nulled baseline, a set of seven metabolites could be reliably quantified in parietal white matter under these experimental conditions: N-acetylaspartate, myo-inositol, glucose, glutamate, glutathione, creatine and choline. When initial signal truncation was used, glucose was removed from this set. The short TE (10-11 ms) facilitated quantification of glutamate. The reliable quantification of N-acetylaspartyl glutamate at 3 T proved very difficult.     

A model for the analysis of competitive relaxation effects of manganese and iron in vivo

Manganese (Mn) and iron (Fe) are both paramagnetic species that can affect magnetic resonance relaxation rates. They also share common transport systems in vivo and thus in experimental models of metal exposure their effects on relaxation rates may interact in a complex fashion. Here we present a novel model to interpret the combined effects of Mn and Fe on MRI relaxation rates. To achieve varying levels of both metals, adult rats were separated into four groups; a control group and three groups treated with weekly intravenous injections of 3 mg Mn/kg body for 14 weeks. The three treated groups were fed either a normal diet, Fe deficient or Fe enriched diet. All rats were scanned using MRI at the 14th week to measure regional water relaxation rates. Rat brains were removed at the end of the study (14th week) and dissected into regions for measurement of Mn and Fe by atomic absorption spectroscopy. For the normal diet groups, R₁ was strongly correlated with tissue Mn concentrations. However, the slopes of the linear regression fits varied significantly among different brain regions, and a simple linear model failed to explain the changes in relaxation rate when both Mn and Fe contents changed. We propose a competition model, which is based on the ability of Mn and Fe to compete in vivo for common binding sites. The combined effect of Mn and Fe on the relaxation rates is complicated and additional studies will be necessary to explain how MRI signals are affected when the levels of both metals are varied. Copyright © 2009 John Wiley & Sons, Ltd.     

Functional analysis of human parotid gland in vivo using the (1)H MRS MT effect

Magnetization transfer (MT) was measured in the parotid gland in vivo by (1)H MR spectroscopy in 10 adult volunteers. A comparison was made of stimulated (excess saliva) and resting parotid gland (SPG and RPG, respectively). Following irradiation at an MT pulse of 150 Hz downfield from the water proton signal, signal reductions in SPG and RPG were 83.8 +/- 4.7 and 91.4 +/- 5.7%, respectively. The larger reduction for SPG indicates that an increase in the amount of water in gland cells for the production of more parotid saliva may lead to greater affinity between the protons adjacent to macromolecules and free water which contributes to the MT effect. Activity in the parotid gland correlates with the effect. This method is useful for diagnosing disorders of parotid gland secretion.     

Age related changes in brain metabolites observed by 1H MRS in APP/PS1 mice

Translational biomarkers in Alzheimer's disease based on non-invasive in vivo methods are highly warranted. ¹H magnetic resonance spectroscopy (MRS) is non-invasive and applicable in vivo in both humans and experimental animals. In vivo ¹H MRS and 3D MRI were performed on brains of double transgenic (tg) mice expressing a double mutant human β-amyloid precursor protein APP(K670N,M671L) and human mutated presenilin gene PS1M146L, and wild-type (wt) littermates at 2.5, 6.5 and 9 months of age using a 9.4 T magnet. For quantification, LCModel™ was used, and the data were analyzed using multivariate data analysis (MVDA). MVDA evidenced a significant separation, which became more pronounced with age, between tg and wt mice at all time points. While myo-inositol and guanidoacetate were important for group separation in young mice, N-acetylaspartate, glutamate and macrolipids were important for separation of aged tg and wt mice. Volume segmentation revealed that brain and hippocampus were readily smaller in tg as compared to wt mice at the age of 2.5 months. Amyloid plaques were seen in 6.5 and 9 months, but not in 2.5 months old animals.In conclusion, differences in brain metabolites could be accurately depicted in tg and wt mice in vivo by combining MRS with MVDA. First differences in metabolite content were readily seen at 2.5 months, when volume defects in tg mice were present, but no amyloid plaques.     

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

This article investigates methods for the accurate and robust differentiation of metastases from glioblastomas on the basis of single‐voxel ¹H MRS information. Single‐voxel ¹H MR spectra from a total of 109 patients (78 glioblastomas and 31 metastases) from the multicenter, international INTERPRET database, plus a test set of 40 patients (30 glioblastomas and 10 metastases) from three different centers in the Barcelona (Spain) metropolitan area, were analyzed using a robust method for feature (spectral frequency) selection coupled with a linear‐in‐the‐parameters single‐layer perceptron classifier. For the test set, a parsimonious selection of five frequencies yielded an area under the receiver operating characteristic curve of 0.86, and an area under the convex hull of the receiver operating characteristic curve of 0.91. Moreover, these accurate results for the discrimination between glioblastomas and metastases were obtained using a small number of frequencies that are amenable to metabolic interpretation, which should ease their use as diagnostic markers. Importantly, the prediction can be expressed as a simple formula based on a linear combination of these frequencies. As a result, new cases could be straightforwardly predicted by integrating this formula into a computer‐based medical decision support system. This work also shows that the combination of spectra acquired at different TEs (short TE, 20–32 ms; long TE, 135–144 ms) is key to the successful discrimination between glioblastomas and metastases from single‐voxel ¹H MRS. Copyright © 2011 John Wiley & Sons, Ltd.     

An investigation of human brain tumour lipids by high-resolution magic angle spinning 1H MRS and histological analysis

NMR-visible lipid signals detected in vivo by 1H MRS are associated with tumour aggression and believed to arise from cytoplasmic lipid droplets. High-resolution magic angle spinning (HRMAS) 1H MRS and Nile Red staining were performed on human brain tumour biopsy specimens to investigate how NMR-visible lipid signals relate to viable cells and levels of necrosis across different grades of glioma. Presaturation spectra were acquired from 24 adult human astrocytoma biopsy samples of grades II (8), III (2) and IV (14) using HRMAS 1H MRS and quantified using LCModel to determine lipid concentrations. Each biopsy sample was then refrozen, cryostat sectioned, and stained with Nile Red, to determine the number of lipid droplets and droplet size distribution, and with Haematoxylin and Eosin, to determine cell density and percentage necrosis. A strong correlation (R=0.92, P<0.0001) was found between the number of Nile Red-stained droplets and the approximately 1.3 ppm lipid proton concentration by 1H MRS. Droplet sizes ranged from 1 to 10 microm in diameter, and the size distribution was constant independent of tumour grade. In the non-necrotic biopsy samples, the number of lipid droplets correlated with cell density, whereas in the necrotic samples, there were greater numbers of droplets that showed a positive correlation with percentage necrosis. The correlation between 1H MRS lipid signals and number of Nile Red-stained droplets, and the presence of lipid droplets in the non-necrotic biopsy specimens provide good evidence that the in vivo NMR-visible lipid signals are cytoplasmic in origin and that formation of lipid droplets precedes necrosis.     

Proton T1 relaxation times of cerebral metabolites differ within and between regions of normal human brain

Saturation recovery spectra (STEAM) were acquired at 1.5 T with 7 TRs ranging from 530 to 5000 ms and a constant TE of 30 ms in voxels (7.2 ml) located in occipital grey, parietal white and frontal white matter (10 subjects each location). Spectra were also acquired at 7, 21 and 37 degrees C from separate 100 mm solutions of inositol (Ins), choline-containing compounds (Cho), N-acetyl-aspartate (NAA) and creatine. Simulations of T(1) fits with 2, 3 and 7 TRs demonstrated that at typical SNR there is potential for both inaccurate and biased results. In vivo, different metabolites had significantly different T(1)s within the same brain volume. The same order from shortest to longest T(1) (Ins, Cho, NAA, creatine) was found for all three brain regions. The order (Ins, NAA, creatine, Cho) was found in the metabolite solutions and was consistent with a simple model in which T(1) is inversely proportional to molecular weight. For all individual metabolites, T(1) increased from occipital grey to parietal white to frontal white matter. This study demonstrates that, in spectra acquired with TR near 1 s, T(1) weightings are substantially different for metabolites within a single tissue and also for the same metabolites in different tissues.     

Absolute metabolite concentrations calibrated using the total water signal in brain 1H MRS

Magnetic resonance spectroscopy (MRS) has been coupled with a multi‐echo imaging sequence to determine the relaxation corrected signal areas of the metabolites and the tissue water. Stimulated echo acquisition mode (STEAM) spectra (TE/TM/TR 30/13.7/5000 ms) acquired from gray and white matter voxels in 43 healthy volunteers were fit using LCModel. Corresponding water signals, measured using a multi‐echo T₂ imaging sequence, were fit with a Non‐Negative Least Squares algorithm. Using this approach the water area could be T₁ and T₂ corrected for all three water compartments: cerebrospinal fluid (CSF), intra‐ and extra‐cellular water, and myelin water. The image‐based water measurement is an improvement over spectroscopy methods because it can be more sensitive to water changes in diseased tissue. Metabolite areas were also corrected for relaxation losses. In occipital gray matter, the concentrations of Cho, Cr, and N‐acetyl aspartate (NAA) were 1.27 (0.06), 8.9 (0.3), and 9.3 (0.3) mmol/L tissue, respectively and in parietal white matter they were 1.90 (0.05), 7.9 (0.2), and 9.8 (0.2) mmol/L tissue. The Cho and Cr concentrations were different in occipital gray compared to parietal white matter (p < 0.0001 and <0.005, respectively). Copyright © 2008 John Wiley & Sons, Ltd.     

Characterization of the response of taurine protons to PRESS at 9.4 T for Resolving choline and Determining taurine T2

Point‐resolved spectroscopy (PRESS), characterized by two TEs (TE₁ and TE₂), can be employed to perform animal magnetic resonance spectroscopy (MRS) studies at 9.4 T. Taurine (Tau) and choline (Cho) are relevant metabolites that can be measured by MRS. In this work, the response of the J‐coupled protons of Tau as a function of PRESS TE₁ and TE₂ was characterized at 9.4 T to achieve two objectives. The first was to determine two TE₁ and TE₂ combinations that could be used to obtain T₂‐corrected measures of Tau (3.42 ppm) that were minimally influenced by J coupling. The second was to exploit the Tau J coupling to find a timing combination that minimized the 3.25‐ppm Tau signal to enable the Cho (3.22 ppm) resonance to be resolved from the overlapping Tau signal. The response of Tau protons was investigated both numerically and experimentally. It was numerically determined that the timings {TE₁, TE₂} = {17 ms, 10 ms} and {TE₁, TE₂} = {80 ms, 70 ms} yielded similar 3.42‐ppm Tau resonance areas (5% difference), rendering them suitable for Tau T₂ determination. {TE₁, TE₂} = {25 ms, 50 ms} was found to yield minimal 3.25‐ppm Tau signal, reducing its interference with Cho. The efficacy of the timings was demonstrated on phantom solutions and in vivo in four Sprague Dawley rats. LCModel was employed to analyse the in vivo spectra and Tau T₂ values were estimated by fitting the Tau peak areas obtained with {TE₁, TE₂} = {17 ms, 10 ms} and {TE₁, TE₂} = {80 ms, 70 ms} to a monoexponentially decaying function. An average Tau T₂ of 106 ms (standard deviation, 12 ms) was obtained. LCModel analysis of rat spectra obtained with {TE₁, TE₂} = {25 ms, 50 ms} demonstrated negligible levels of Tau signal, compared with that obtained with short TE.