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要旨

INTRODUCTION Glutamate is the main excitatory neurotransmitter in the CNS and plays together with glutamine an important role in brain physiology. A spatially resolved in vivo analysis of glutamate separate from glutamine is therefore of particular neuroscientific interest. The feasibility of glutamate MR spectroscopic imaging (MRSI) is demonstrated in the ventricle region of a macaque monkey, where largely different glutamate concentrations are expected for brain tissue vs. the ventricles filled with CSF. In addition, first results of a high resolution MRSI study are shown from the occipital lobe focused on NAA in gray vs. white matter. METHODS The monkey setup and MR system (vertical Bruker 7T/60cm) has been described elsewhere [1,2]. The anatomical scout images (Fig.1, MSME, TE=100ms, MRSI FOV assigned as yellow frame) as well as the glutamate MRSI data where measured with a 80mm surface coil [3]. The water line width was ~13Hz in the selected 28x28x4mm3 axial slice through the ventricles. For MRSI, a STEAM localization was used with a conventional 8x8 phase encoding scheme, leading to a nominal in-plane resolution of 3.5x3.5mm 2 (TE/TM/TR=10/10/4000ms, NA=35). Mild Gaussian apodization and spatial zero-filling up to 16x16 was applied before Fourier transformation. Quantification was done voxelwise with LCModel, assuming 10mM total creatine in brain matter. RESULTS DISCUSSION In a typical brain matter MRSI voxel, the spectral separation of the glutamate and glutamine multiplets at 2.35 and 2.45ppm is obvious (Fig.2). Image smoothing of a reduced FOV of 21x24.5mm2 led to the glutamate map in figure 3. For brain tissue, Cramer-Rao bounds were in the range of 6-10 and metabolite peak widths were 7-11Hz. The ventricle anatomy is assigned by a yellow contour that fits well with the expected low glutamate concentrations. In most MRSI studies, insufficient sensitivity and/or long TE values do not permit the quantification of glutamate+glutamine. Typically glutamate and glutamine can not be separated due to limited spectral dispersion/resolution. In this study, utilizing high magnetic field, we were able to separate glutamate and glutamine and to measure a pure MRSI glutamate map in the primate. For cortical brain regions in the vicinity of the skull surface, significant sensitivity enhancements could be achieved by use of a combination coil setup (30mm receive). This resulted in better spatial resolution - much smaller MRSI slice geometries could be addressed (Fig4a, yellow frame in axial slice). Figure 4b shows the center cantle of a 16x2x16mm3 ’coronal’ MRSI slice (TE/TM/TR=6/10/4000ms, encoding matrix 13x13, nominal in-plane resolution 1.1mm). A single spectroscopic image for water was acquired in 11min, nine averages for a NAA image required 101min (Fig.4c: water, Fig.4d: NAA), which demonstrate nicely a pattern of gray vs. white mater. These first results demonstrate that brain metabolites previously difficult to measure are accessible by MRSI methods and that MRSI can meet the requirements regarding the spatial resolution that are defined by brain structures in the millimeter range.