Improving brain B0 shimming using an easy and accessible multi-coil shim array at ultra-high field

Object

Improve shimming capabilities of ultra-high field systems, with addition of an accessible low-complexity B₀ shim array for head MRI at 7 T.

Materials and methods

An eight channel B₀ shim coil array was designed as a tradeoff between shimming improvement and construction complexity, to provide an easy to use shim array that can be employed with the standard 7 T head coil. The array was interfaced using an open-source eight-channel shim amplifier rack. Improvements in field homogeneity for whole-brain and slice-based shimming were compared to standard second-order shimming, and to more complex higher order dynamic shimming and shim arrays with 32 and 48 channels.

Results

The eight-channel shim array provided 12% improvement in whole brain static shimming and provided 33% improvement when using slice-based shimming. With this, the eight-channel array performed similar to third-order dynamic shimming (without the need for higher order eddy current compensation). More complex shim arrays with 32 and 48 channels performed better, but require a dedicated RF coil.

Discussion

The designed eight-channel shim array provides a low-complexity and low-cost approach for improving B₀ field shimming on an ultra-high field system. In both static and dynamic shimming, it provides improved B₀ homogeneity over standard shimming.