LOCAL B1 SHIMMING FOR PROSTATE IMAGING//TRANSCEIVER ARRAYS AT 7 TESLA

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. High-quality prostate images were obtained with transceiver arrays at 7T after performing subject-dependent local transmit B1 (B1 ) 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 B1 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.