Metzger Gregory J, Snyder Carl, Akgun Can, Vaughan Tommy, Ugurbil Kamil, Van de Moortele Pierre-Francois
Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA.
Magn Reson Med. 2008 Feb;59(2):396-409. 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.
在7T条件下，使用收发阵列获取高质量前列腺图像。此前进行了基于个体的局部发射B(1)（B(1) (+)）匀场，以尽量减少由相消干涉导致的B(1) (+)损耗。通过基于在体内快速获取的每个八元带状线线圈通道的相对B(1) (+)相位图来改变各个射频通道的输入相位，从而进行B(1) (+)匀场。在前列腺周围有限区域内使B(1) (+)相干性最大化所需的相对发射相位与由线圈几何形状决定的相位有很大不同，并且高度依赖个体。与由线圈几何形状确定的标准发射相位相比，通过B(1) (+)匀场确定的一组发射相位在前列腺中提供了4.2±2.7的发射效率增益。这种效率的提高导致在前列腺中对于给定翻转角所需射频功率大幅降低，在建模研究中考虑这一点时，会使局部比吸收率显著降低。此外，B(1) (+)匀场将前列腺内的B(1) (+)不均匀性从(24±9%)降低到了(5±4%)。本研究证明了快速局部B(1) (+)相位匀场对超高磁场人体成像的巨大影响。
