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 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.
Metzger
Gregory J
GJ
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA. gmetzger@umn.edu
Snyder
Carl
C
Akgun
Can
C
Vaughan
Tommy
T
Ugurbil
Kamil
K
Van de Moortele
Pierre-Francois
PF
D000328:Adult; D004867:Equipment Design; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008297:Male; D011467:Prostate / Q000033:anatomy & histology*