Role of very high order and degree B 0 shimming for spectroscopic imaging of the human brain at 7 tesla

Abstract 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 B 0 homogeneity, both globally (σ B , over the entire regions of interest or slice) and loc...

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Published in:Magnetic resonance in medicine 2012-10, Vol.68 (4), p.1007-1017
Main Authors: Pan, Jullie W., Lo, Kai‐Ming, Hetherington, Hoby P.
Format: Article
Language:English
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Summary:Abstract 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 B 0 homogeneity, both globally (σ B , over the entire regions of interest or slice) and locally (σ B , influencing the linewidth of individual SI voxels within the regions of interest). Typically the B 0 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 σ B and σ B were determined from B 0 field maps of 64 × 64 resolution. In a 10 mm thick slice taken through the region of the subcortical nuclei, we find that in comparison to 1st–2nd degree shims, use of 1st–3rd and 1st–4th degree shims reduces σ B by 29% and 55%, respectively. Using a SI voxel size of ∼1cc with an estimate of σ B from 3 × 3 × 3 B 0 map pixels in this subcortical region, the number of pixels with σ B of less than 5 Hz increased from 24 to 59% with 1st–3rd and 1st–4th over 1st–2nd degree shims, respectively. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
ISSN:0740-3194
1522-2594
DOI:10.1002/mrm.24122