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Local B 1 + shimming for prostate imaging with transceiver arrays at 7T based on subject‐dependent transmit phase measurements
Author(s) -
Metzger Gregory J.,
Snyder Carl,
Akgun Can,
Vaughan Tommy,
Ugurbil Kamil,
Van de Moortele PierreFrancois
Publication year - 2008
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.21476
Subject(s) - prostate , radiofrequency coil , electromagnetic coil , flip angle , imaging phantom , transceiver , physics , nuclear magnetic resonance , materials science , nuclear medicine , chemistry , magnetic resonance imaging , optics , medicine , optoelectronics , cmos , radiology , quantum mechanics , cancer
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.