Improvement of resolution for brain coupled metabolites by optimized 1H MRS at 7 T (original) (raw)
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Proton echo planar spectroscopic imaging (PEPSI) is a fast magnetic resonance spectroscopic imaging (MRSI) technique that allows mapping spatial metabolite distributions in the brain. Although the medial wall of the cortex is involved in a wide range of pathological conditions, previous MRSI studies have not focused on this region. To decide the magnitude of metabolic changes to be considered significant in this region, the reproducibility of the method needs to be established. The study aims were to establish the short-and long-term reproducibility of metabolites in the right medial wall and to compare regional differences using a constant short-echo time (TE30) and TE averaging (TEavg) optimized to yield glutamatergic information. 2D sagittal PEPSI was implemented at 3 T using a 32 channel head coil. Acquisitions were repeated immediately and after approximately 2 weeks to assess the coefficients of variation (COV). COVs were obtained from eight regions-of-interest (ROIs) of varying size and location. TE30 resulted in better spectral quality and similar or lower quantitation uncertainty for all metabolites except glutamate (Glu). When Glu and glutamine (Gln) were quantified together (Glx) reduced quantitation uncertainty and increased reproducibility was observed for TE30. TEavg resulted in lowered quantitation uncertainty for Glu but in less reliable quantification of several other metabolites. TEavg did not result in a systematically improved short-or long-term reproducibility for Glu. The ROI volume was a major factor influencing reproducibility. For both short-and long-term repetitions, the Glu COVs obtained with TEavg were 5-8% for the large ROIs, 12-17% for the medium sized ROIs and 16-26% for the smaller cingulate ROIs. COVs obtained with TE30 for the less specific Glx were 3-5%, 8-10% and 10-15%. COVs for N-acetyl aspartate, creatine and choline using TE30 with long-term repetition were between 2-10%. Our results show that the cost of more specific glutamatergic information (Glu versus Glx) is the requirement of an increased effect size especially with increasing anatomical specificity. This comes in addition to the loss of sensitivity for other metabolites. Encouraging results were obtained with TE30 compared to other previously reported MRSI studies. The protocols implemented here are reliable and may be used to study disease progression and intervention mechanisms.
TE-averaged two-dimensional proton spectroscopic imaging of glutamate at 3 T
2006
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 twodimensional (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 (¨2 Hz). MRSI data arrays of 10 Â 16 were acquired with 1.8 cm 3 spatial resolution over a¨110 cm 3 volume in a scan time of¨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. D
NMR in Biomedicine, 2013
We present a method for the robust and accurate estimation of brain metabolite transverse relaxation times (T 2 ) from multiple spin-echo data acquired with a single-shot Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic sequence. Each acquired echo consists of a small number of complex time-domain data points. The amplitudes of the spectral components in each echo are calculated by solving a set of linear equations in which previously estimated frequencies and linewidths serve as prior information. These priors are obtained from a short MRS experiment in which a large number of time-domain data points are acquired, and are subsequently estimated using linear prediction with singular value decomposition (LPSVD) processing. We show that this process can be used to accurately and rapidly measure the T 2 values for the main singlet resonances in single-volume MRS measurements in the brain. The proposed method can be generalized to any set of MRS experiments comprising repeated measurements of amplitude changes, e.g. as a function of an experimental parameter, such as TE, inversion time or diffusion weighting.
T 2 measurement of J-coupled metabolites in the human brain at 3T
Proton T 2 relaxation times of metabolites in the human brain were measured using point resolved spectroscopy at 3T in vivo. Four echo times (54, 112, 246 and 374ms) were selected from numerical and phantom analyses for effective detection of the glutamate multiplet at~2.35ppm. In vivo data were obtained from medial and left occipital cortices of five healthy volunteers. The cortices contained predominantly gray and white matter, respectively. Spectra were analyzed with LCModel software using volume-localized calculated spectra of brain metabolites. The estimate of the signal strength vs. TE was fitted to a monoexponential function for estimation of apparent T 2 (T 2 † ). T 2 † was estimated to be similar between the brain regions for creatine, choline, glutamate and myo-inositol, but significantly different for N-acetylaspartate singlet and multiplet. T 2 † s of glutamate and myo-inositol were measured as 181AE16 and 197AE14ms (meanAESD, N=5) for medial occipital cortices, and 180AE12 and 196AE17ms for left occipital cortices, respectively.
In vivo high-resolution localized 1 H MR spectroscopy in the awake rat brain at 7 T
Magnetic Resonance in Medicine, 2013
In vivo localized high-resolution 1 H MR spectroscopy was performed in multiple brain regions without the use of anesthetic or paralytic agents in awake head-restrained rats that were previously trained in a simulated MRI environment using a 7 Tesla MR system. Spectra were obtained using a short echo time single-voxel point-resolved spectroscopy technique with voxel size ranging from 27-32.4 mm 3 in the regions of anterior cingulate cortex, somatosensory cortex, hippocampus, and thalamus. Quantifiable spectra, without the need for any additional post-processing to correct for possible motion were reliably detected including the metabolites of interest such as γaminobutyric acid, glutamine, glutamate, myo-inositol, N-acetylaspartate, taurine, glycerophosphorylcholine/phosphorylcholine, creatine/phosphocreatine, and N-acetylaspartate/Nacetylaspartylglutamate. The spectral quality was comparable to spectra from anesthetized animals with sufficient spectral dispersion to separate metabolites such as glutamine and glutamate. Results from this study suggest that reliable information on major metabolites can be obtained without the confounding effects of anesthesia or paralytic agents in rodents.
Line scan imaging of brain metabolites with CPMG sequences at 1.5 tesla
Journal of Magnetic Resonance Imaging, 1996
A line scan Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic imaging sequence has been implemented on a standard 1.5 T clinical scanner to map metabolite signals at multiple echo times from voxels along selected tissue columns through the brain. The CPMG multiecho spectroscopic image data sets are used to estimate brain metabolite T2 decay parameters in a group of healthy volunteers and in one tumor patient. Inherent trade-offs between T2 decay, spectral resolution, and echo spacing prove to be important limiting factors. In particular, separate quantitation of choline and creatine resonances at 1.5 T was not achieved in the present implementation. However, the ability to collect data sets suitable for T2 decay analyses of combined choline and creatine resonances and N-acetyl aspartate resonances in under 10 minutes may prove of clinical utility in the study of brain pathology. Index terms: Spectroscopic imagingbrain metabolites-T2 dcray-CPMG * quantitation
Whole-brain quantitative mapping of metabolites using short echo three-dimensional proton MRSI
Journal of Magnetic Resonance Imaging, 2014
Purpose-To improve the extent over which whole brain quantitative 3D-MRSI maps can be obtained and be used to explore brain metabolism in a population of healthy volunteers. Materials and Methods-Two short TE (20 ms) acquisitions of 3D Echo Planar Spectroscopic Imaging at two orientations, one in the anterior commissure-posterior commissure (AC-PC) plane and the second tilted in the AC-PC +15° plane were obtained at 3T in a group of ten healthy volunteers. B 1 + , B 1 − , and B 0 correction procedures and normalization of metabolite signals with quantitative water proton density measurements were performed. A combination of the two spatially normalized 3D-MRSI, using a weighted mean based on the pixel wise standard deviation metabolic maps of each orientation obtained from the whole group, provided metabolite maps for each subject allowing regional metabolic profiles of all parcels of the automated anatomical labeling (AAL) atlas to be obtained. Results-The combined metabolite maps derived from the two acquisitions reduced the regional inter-subject variance. The numbers of AAL regions showing NAA SD/Mean ratios lower than 30% increased from 17 in the AC-PC orientation and 41 in the AC-PC+15° orientation, to a value of 76 regions out of 116 for the combined NAA maps. Quantitatively, regional differences in absolute metabolite concentrations (mM) over the whole brain were depicted such as in the GM of frontal lobes (c NAA =10.03+1.71, c Cho =1.78±0.55, c Cr =7.29±1.69; c mIns =5.30±2.67) and in cerebellum (c NAA =5.28±1.77, c Cho =1.60±0.41, c Cr =6.95±2.15; c mIns =3.60±0.74). Conclusion-A double-angulation acquisition enables improved metabolic characterization over a wide volume of the brain.
Magnetic Resonance Imaging, 1998
The precision of cerebral proton magnetic resonance spectroscopy (MRS) measurements is critical both in the clinical setting and for research purposes. Marshall et al. have recently concluded that ''disappointing in vivo repeatability. . .is likely to limit'' the ability of MRS to detect modest changes. We present here a comprehensive study of the precision of short-and long-term metabolite peak area ratios and water referenced metabolite peak areas for long echo time point resolved spectroscopy (PRESS) spectra (repetition time (TR) ؍ 2000 ms, echo time (TE) ؍ 136 ms) acquired from the occipital lobes of normal volunteers and a phantom using a conventional whole body 1.5 T MR system and conventional acquisition and analysis protocols. Short-term in vitro precision determined by five repeat scans on five occasions was excellent as measured by a mean coefficient of variation (NAA/Cho ؍ 1.3%, NAA/Cr ؉ PCr ؍ 1.0%, Cho/Cr ؉ PCr ؍ 1.6%, NAA/H 2 O ؍ 0.5%, Cho/H 2 O ؍ 1.2%, Cr ؉ PCr/H 2 O ؍ 0.8%). Long term in vitro precision using 100 spectra acquired over 2 years was also very good (NAA/Cho ؍ 2.7%, NAA/Cr ؉ PCr ؍ 1.4%, Cho/Cr ؉ PCr ؍ 2.2%, NAA/H 2 O ؍ 1.5%, Cho/H 2 O ؍ 2.4%, Cr ؉ PCr/H 2 O ؍ 1.5%). Short-term in vivo precision determined by five repeat scans in a single scanning session on eight subjects was also excellent (NAA/Cho ؍ 5.2%, NAA/Cr ؉ PCr ؍ 3.0%, Cho/Cr ؉ PCr ؍ 6.6%, NAA/H 2 O ؍ 1.4%, Cho/H 2 O ؍ 4.9%, Cr ؉ PCr/H 2 O ؍ 2.7%) and only worsened slightly for long-term in vivo precision determined by five repeat scans on eight subjects over 3 months (NAA/Cho ؍ 5.2%, NAA/Cr ؉ PCr ؍ 4.8%, Cho/Cr ؉ PCr ؍ 7.7%, NAA/H 2 O ؍ 2.5%, Cho/H 2 O ؍ 6.4%, Cr ؉ PCr/H 2 O ؍ 3.8%). We attribute the excellent precision reported here to the use of highly automated techniques for voxel shimming, water suppression and peak area measurements. These results allow us to repudiate Marshall's assertion regarding disappointing repeatability of in vivo MRS.