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

Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA.
Magnetic Resonance in Medicine (Impact Factor: 3.4). 03/2008; 59(2):396-409. DOI: 10.1002/mrm.21476
Source: PubMed

ABSTRACT 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.

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