Effect of linewidth on estimation of metabolic concentration when using water lineshape spectral model fitting for single voxel proton spectroscopy at 7 T.

Good B0 field homogeneity is considered an essential requirement to obtain high-quality MRS data. Many commonly used spectral fitting methods assume that all metabolite signals have Lorentzian or Gaussian shapes. However, B0 inhomogeneity can both broaden the linewidth and modify the lineshape. In this study, it is hypothesized that a realistic metabolite fitting model, which accounts for B0 homogeneity on the basis of the water lineshape, will improve the accuracy of estimation of metabolite concentrations. In-vivo water suppressed/unsuppressed single voxel spectroscopy signals were acquired under three different B0 field homogeneity regimes. Individual realistic basis sets were created for each acquisition. Frequency-domain spectral fitting with LCModel was used to quantify the metabolite concentrations with fitting uncertainties given in terms of the Cramer-Rao lower bound. The quantification results obtained using the water lineshape basis set yielded similar concentrations independent of linewidth and showed a larger fitting error as the linewidth increased. The conventional approach, however quantifies metabolite concentrations with greater variations despite showing a supposedly improved fitting quality. The water lineshape basis set achieved single voxel spectroscopy accuracy that is less sensitive to the linewidth compared to the conventional spectral fitting method for the range of linewidths used in this study, but the precision deteriorated with worsening B0 field inhomogeneity. The beneficial effect was ascribed to a reduction in the number of degrees of freedom when using the water lineshape to generate the basis set.

Implications of the magnetic susceptibility difference between grey and white matter for single-voxel proton spectroscopy at 7 T.

Magnetic susceptibility differences between grey matter (GM) and white matter (WM) can potentially affect lineshapes and chemical shifts in single-voxel spectroscopy. This study aimed to investigate the consequences and potential utility of these effects. Spectroscopy voxels were segmented into GM, WM, and cerebrospinal fluid based on T1-weighted images. GM and WM lineshapes were computed using multi-echo gradient-echo images to measure the frequency distribution. Twenty 7 Tesla single voxel spectra with corresponding T1-weighted images were acquired from the frontal and parietal lobes from five healthy human volunteers. Consistent frequency shifts (mean [±SD] 4.9 ±â€¯2.0 Hz) and linewidth differences (2.4 ±â€¯1.5 Hz) between the two tissue types were observed. Directly visible metabolites (creatine, choline, and myo-inositol) exhibited frequency shifts and linewidth differences that were consistent with a linear-weighted summation of their expected GM and WM distribution ratios. The magnetic susceptibility difference between GM and WM had a detectable effect on single-voxel proton spectra, which results in both frequency shifts and lineshape broadening. This effect can be used to estimate the relative metabolic distribution in the GM and WM for directly observable metabolites. Fractional distributions estimated with this method demonstrated good agreement with literature values for the selected metabolites.