Dynamic shimming of low-order spherical harmonics using a shim coil array in conjunction with a field camera.

Motivation: Dynamic cancellation of field variations in the brain resulting from scanner drift and body movement, improves the quality of image data, particularly at high field. Goal(s): To evaluate the feasibility of using a shim-coil array in conjunction with a field camera to cancel time-varying field perturbations. Approach: Field probe measurements were used to update shim array currents, based on a low-order spherical harmonic decomposition and accounting for the fields generated at the probes by the shim coils. Results: Controlled field changes produced by the scanner and an external field perturber could be cancelled in a spherical phantom and human head using this approach. Impact: The combination of a field camera and separately controlled shim array allows cancellation of time-varying fields up to high order, largely independent of the scanner. This approach can be used to improve the quality of high field images and spectra.