Robert Steiner MRI Unit, Imaging Sciences Department, MRC Clinical Sciences Center, Hammersmith Hospital, Imperial College London, London, United Kingdom.
Magn Reson Med. 2012 Nov;68(5):1481-94. doi: 10.1002/mrm.24153. Epub 2012 Jan 13.
A spatially resolved extended phase graph (SR-EPG) framework is proposed for prediction of echo amplitudes in the presence of spatially variable radio frequency (RF) fields. The method may be used to examine any regularly repeating pulse sequence and provides a design framework for parallel transmission (PTx) systems; in this work signal homogeneity in static pseudo-steady state (SPSS) turbo spin echo (TSE) imaging was investigated. Building on SR-EPG calculations with PTx, a dynamic RF-shimming approach is proposed in which, RF pulse amplitudes and phases are optimized on a per channel and per pulse basis to yield the desired signal response for all echoes. Results show significant improvements over "static" RF shimming (in which the relative amplitude/phase of the PTx channels are fixed for all pulses). SPSS-TSE imaging using dynamic RF shimming resulted in excellent image quality, both in phantoms and in vivo, and confirmed SR-EPG predictions.
提出了一种用于预测空间变化射频 (RF) 场中回波幅度的空间分辨扩展相位图 (SR-EPG) 框架。该方法可用于检查任何规则重复的脉冲序列，并为并行传输 (PTx) 系统提供设计框架；在这项工作中，研究了静态伪稳态 (SPSS) 涡轮自旋回波 (TSE) 成像中的信号均匀性。基于 PTx 的 SR-EPG 计算，提出了一种动态 RF 调谐方法，其中，根据每个通道和每个脉冲优化 RF 脉冲幅度和相位，以产生所有回波的期望信号响应。结果表明，与“静态”RF 调谐（其中 PTx 通道的相对幅度/相位对于所有脉冲都是固定的）相比，有显著的改进。使用动态 RF 调谐的 SPSS-TSE 成像在体模和体内均产生了出色的图像质量，并证实了 SR-EPG 的预测。
