Abstract: Objective To study the efficacy of B1 field shimming technique and evaluate the specific absorption rate (SAR) in
ultra-high field magnetic resonance imaging (MRI) for MRI-guided high-intensity focused ultrasound (HIFU). Methods An
electromagnetic model of the female pelvis with a temperature gradient was established. B1 field homogeneity and local SAR
were simulated and calculated using regular and optimized B1 shimming coefficients. Results The maximum local SAR
reached 10.24 W/kg, which exceeded the safe threshold of 10 W/kg, as calculated using regular B1 shimming coefficients in the
normal model. Using the optimized B1 shimming coefficients, the maximum local SAR of the tissue was 9.65 W/kg, which was
below the safe threshold. Conclusion The temperature distributions in the body generated by ultrasound energy need to be
considered in ultra-high field MRI-guided HIFU surgery. The proposed optimized B1 shimming strategy based on the
temperature gradient can be used to control the local SAR levels.