Safety and imaging performance of two-channel RF shimming for fetal MRI at 3T

Purpose: This study investigates whether two-channel radiofrequency (RF) shimming can improve imaging without increasing specific absorption rate (SAR) for fetal MRI at 3T.

Methods: Transmit field ( $B_1^{+}$ ) average and variation in the fetus was simulated in seven numerical pregnant body models. Safety was quantified by maternal and fetal peak local SAR and fetal average SAR. The shim parameter space was divided into improved $B_1^{+}$ (magnitude and homogeneity) and improved SAR regions, and an overlap where RF shimming improved both classes of metrics compared with birdcage mode was assessed. Additionally, the effect of fetal position, tissue detail, and dielectric properties on transmit field and SAR was studied.

Results: A region of subject-specific RF shim parameter space improving both $B_1^{+}$ and SAR metrics was found for five of the seven models. Optimizing only $B_1^{+}$ metrics improved $B_1^{+}$ efficiency across models by 15% on average and 28% for the best-case model. $B_1^{+}$ variation improved by 26% on average and 49% for the best case. However, for these shim settings, fetal SAR increased by up to 106%. The overlap region, where both $B_1^{+}$ and SAR metrics improve, showed an average $B_1^{+}$ efficiency improvement of 6% on average across models and 19% for the best-case model. $B_1^{+}$ variation improved by 13% on average and 40% for the best case. RFS could also decrease maternal/fetal SAR by up to 49%/58%.

Conclusion: RF shimming can improve imaging compared with birdcage mode without increasing fetal and maternal SAR when a patient-specific SAR model is incorporated into the shimming procedure.