Map-based B₀ shimming for single voxel brain spectroscopy at 7T

Area of Science:

• Neuroimaging
• Magnetic Resonance Spectroscopy (MRS)
• Biophysics

Background:

• B0 shimming is essential for in vivo brain spectroscopy, but the necessity of advanced shim methods for single voxel spectroscopy (SVS) remains debated.
• The extent of higher-order magnetic field inhomogeneities and the efficacy of corresponding shims in small SVS voxels are controversial.

Purpose of the Study:

• To evaluate the effectiveness of advanced B0 shimming techniques, including higher-order shims and map-based methods, in improving SVS data quality.
• To compare map-based shimming (Bolero) with projection-based shimming (FAST(EST)MAP) and assess the impact of additional shim hardware.

Main Methods:

• Acquired SVS data from the rostral prefrontal cortex (rPFC) and hippocampus (Hc) in healthy subjects at 7T.
• Compared shimming strategies: FAST(EST)MAP vs. Bolero (first- and second-order shims), and Bolero with added third- and fourth-order shims.
• Utilized stimulated echo acquisition mode (STEAM) and LCModel for spectral analysis, with tissue water for concentration reference.

Main Results:

• In the rPFC, advanced shimming reduced residual inhomogeneity (B0 distribution variance) from 9.8 ± 4.5 Hz to 4.0 ± 0.8 Hz.
• In the Hc, residual inhomogeneity decreased from 8.6 ± 1.9 Hz to 4.6 ± 0.9 Hz with advanced shimming.
• Reduced spectral linewidth (full width at half maximum) correlated with lower Cramer-Rao lower bounds and improved metabolite concentration accuracy.

Conclusions:

• Higher-order and map-based B0 shimming methods demonstrably enhance the accuracy and consistency of 7T SVS data.
• Improved B0 homogeneity leads to more reliable quantification of brain metabolites, supporting their use in neuroscientific research.