Local B 1 + shimming for prostate imaging with transceiver arrays at 7T based on subject‐dependent transmit phase measurements

• Published in: Magnetic resonance in medicine Vol. 59; no. 2; pp. 396 - 409
• Main Authors: Metzger, Gregory J., Snyder, Carl, Akgun, Can, Vaughan, Tommy, Ugurbil, Kamil, Van de Moortele, Pierre‐Francois
• Format: Journal Article
• Language: English
• Published: 01.02.2008
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.21476

High‐quality prostate images were obtained with transceiver arrays at 7T after performing subject‐dependent local transmit B 1 ( B 1 + ) shimming to minimize B 1 + losses resulting from destructive interferences. B 1 + shimming was performed by altering the input phase of individual RF channels based on relative B 1 + phase maps rapidly obtained in vivo for each channel of an eight‐element stripline coil. The relative transmit phases needed to maximize B 1 + 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, B 1 + shimming decreased B 1 + nonuniformity within the prostate from (24 ± 9%) to (5 ± 4%). This study demonstrates the tremendous impact of fast local B 1 + phase shimming on ultrahigh magnetic field body imaging. Magn Reson Med 59:396–409, 2008. © 2008 Wiley‐Liss, Inc.