Region and volume dependencies in spectral linewidth assessed by 1H 2D MR chemical shift imaging in the monkey brain at 7T

High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadenedin vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral linewidth are therefore of critical importance. In the present study, 1H 2D chemical shift imaging (CSI) at short echo times was performed in the macaque monkey brain at 7 T. Large brain coverage was obtained at high spatial resolution with voxel sizes down to 50 uL being able to quantify up to nine metabolites in vivo with good reliability. Measured linewidths of water decreased from 14.2 to 7.6 Hz with voxel volumes of 3.14 mL to 50 uL (at increased spatial resolution). The linewidth distribution of the metabolites (7.6 +/- 1.6 Hz, ranging from 5.5 to 10 Hz) was considerably smaller compared to that of water (10.6 +/- 2.4 Hz), and was also smaller than reported in 1H MRS at 7 T in the human brain. Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral linewidths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral linewidth - directly correlated with improved spectral quality - can only be achieved when voxel volumes are significantly reduced. Our linewidth optimization was sufficient to permit clear glutamate-glutamine separation, yielding distinct glutamate maps for brain areas including regions of greatly different glutamate concentration (e.g. ventricles vs. surrounding tissue)