Role of very high order and degree B0 shimming for spectroscopic imaging of the human brain at 7 tesla

With the advent of ultrahigh field systems (7T), significant improvements in spectroscopic imaging (SI) studies of the human brain have been anticipated. These gains are dependent upon the achievable B0 homogeneity, both globally (sB?0Global, over the entire regions of interest or slice) and locally (sB?0Global, influencing the linewidth of individual SI voxels within the regions of interest). Typically the B0 homogeneity is adjusted using shim coils with spatial distributions modeled on spherical harmonics which can be characterized by a degree (radial dependence) and order (azimuthal symmetry). However, the role of very high order and degree shimming (e.g., 3rd and 4th degree) in MRSI studies has been controversial. Measurements of sB?0Global and sB?0Local were determined from B0 field maps of 64 x 64 resolution. In a 10 mm thick slice taken through the region of the subcortical nuclei, we find that in comparison to 1st2nd degree shims, use of 1st3rd and 1st4th degree shims reduces sB?0Global by 29% and 55%, respectively. Using a SI voxel size of similar to 1cc with an estimate of sB?0Local from 3 x 3 x 3 B0 map pixels in this subcortical region, the number of pixels with sB?0Local of less than 5 Hz increased from 24 to 59% with 1st3rd and 1st4th over 1st2nd degree shims, respectively. Magn Reson Med, 2012. (c) 2011 Wiley Periodicals, Inc.