Modeling real shim fields for very high degree (and order) B₀ shimming of the human brain at 9.4 T

Purpose To describe the process of calibrating a B 0 shim system using high‐degree (or high order) spherical harmonic models of the measured shim fields, to provide a method that considers amplitude dependency of these models, and to show the advantage of very high‐degree B 0 shimming for whole‐brain and single‐slice applications at 9.4 Tesla (T). Methods An insert shim with up to fourth and partial fifth/sixth degree (order) spherical harmonics was used with a Siemens 9.4T scanner. Each shim field was measured and modeled as input for the shimming algorithm. Optimal shim currents can therefore be calculated in a single iteration. A range of shim currents was used in the modeling to account for possible amplitude nonlinearities. The modeled shim fields were used to compare different degrees of whole‐brain B 0 shimming on healthy subjects. Results The ideal shim fields did not correctly shim the subject brains. However, using the modeled shim fields improved the B 0 homogeneity from 55.1 (second degree) to 44.68 Hz (partial fifth/sixth degree) on the whole brains of 9 healthy volunteers, with a total applied current of 0.77 and 6.8 A, respectively. Conclusions The necessity of calibrating the shim system was shown. Better B 0 homogeneity drastically reduces signal dropout and distortions for echo‐planar imaging, and significantly improves the linewidths of MR spectroscopy imaging. Magn Reson Med 79:529–540, 2018.