RF shimming methods for optimizing B1+ homogeneity and SAR reduction for a high channel density transmit array at 7T

Motivation: Balancing B1+ homogeneity and power efficiency with SAR reduction remains a challenge at 7T. RF shimming of multi-channel transmit coils can address that challenge. Goal(s): To develop and assess an RF shimming strategy for 7T RF coils with high transmit channel density to obtain low levels of SAR and extended B1+ coverage. Approach: A multi-ROI, histogram loss based shimming strategy is used to optimize B1+. A SAR-shimming strategy follows to reduce SAR while maintaining similar B1+. Results: A histogram based B1+ shimming algorithm enables homogeneous, whole-brain B1+. SAR-shimming is able to reduce SAR while maintaining similar B1+ to the initial conditions. Impact: Multi-ROI RF shimming using histogram loss is successful at optimizing B1+ throughout the brain. By performing B1+-restrictive RF shimming with SAR-heavy cost functions, the electromagnetic fields of a multichannel transmit array are manipulated to maintain B1+ conditions while reducing SAR.