Modeling Real Shim Fields for Very High Degree (and Order) B Shimming of the Human Brain at 9.4 T

Purpose: To describe the process of calibrating a B 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 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 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 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 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. © 2017 International Society for Magnetic Resonance in Medicine.