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Abstract:
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.
