Local B₁⁺ shimming for prostate imaging with transceiver arrays at 7T based on subject-dependent transmit phase measurements

High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B₁ (B ₁⁺) shimming to minimize B₁⁺ losses resulting from destructive interferences. B₁⁺ shimming was performed by altering the input phase of individual RF channels based on relative B₁⁺ phase maps rapidly obtained in vivo for each channel of an eight-element stripline coil. The relative transmit phases needed to maximize B₁⁺ coherence within a limited region around the prostate greatly differed from those dictated by coil geometry and were highly subject-dependent. A set of transmit phases determined by B ₁⁺ shimming provided a gain in transmit efficiency of 4.2 ± 2.7 in the prostate when compared to the standard transmit phases determined by coil geometry. This increased efficiency resulted in large reductions in required RF power for a given flip angle in the prostate which, when accounted for in modeling studies, resulted in significant reductions of local specific absorption rates. Additionally, B₁⁺ shimming decreased B₁⁺ nonuniformity within the prostate from (24 ± 9%) to (5 ± 4%). This study demonstrates the tremendous impact of fast local B₁⁺ phase shimming on ultrahigh magnetic field body imaging.