Department of Experimental Physics 5, University of Würzburg, Würzburg, Germany.
Magn Reson Med. 2012 Aug;68(2):529-36. doi: 10.1002/mrm.23259. Epub 2011 Nov 30.
Fast and accurate B(1)(+) mapping is possible using phase-based Bloch-Siegert (BS) methods. Importantly, the off-resonant pulses needed for BS B(1)(+) mapping methods can easily be implemented in multiple MR sequences. BS-based B(1)(+) mapping has thus been introduced for gradient echo (BS-FLASH), spin-echo (BS-SE), and Carr, Purcell, Meiboom, Gill (CPMG)-based multi-SE and turbo-SE sequences. When using SE and multi-SE/turbo-SE-based BS sequences, however, the high intrinsic specific absorption rates must be considered in clinical situations. This study introduces a fast BS B(1)(+) mapping method based on a SE-BURST sequence (BS-SE-BURST). With SE-BURST sequences, multiple low-magnitude excitation pulses are applied prior to the refocusing pulse. Thus, multiple and different phase-encoded echoes can be acquired per excitation cycle. Compared with a SE sequence, this excitation strategy results in a similar signal-to-noise ratio (SNR) per unit time but with reduced specific absorption rate. The proposed BS-SE-BURST sequence was implemented on a conventional 3 T whole body MRI scanner and applied successfully.
使用基于相位的 Bloch-Siegert (BS) 方法可以实现快速准确的 B(1)(+)映射。重要的是，BS B(1)(+)映射方法所需的离频脉冲可以很容易地在多个 MR 序列中实现。因此，BS 基 B(1)(+)映射已被引入梯度回波 (BS-FLASH)、自旋回波 (BS-SE) 和基于 Carr, Purcell, Meiboom, Gill (CPMG) 的多 SE 和 turbo-SE 序列。然而，在使用 SE 和多 SE/turbo-SE 基 BS 序列时，必须在临床情况下考虑到高固有比吸收率。本研究介绍了一种基于 SE-BURST 序列 (BS-SE-BURST) 的快速 BS B(1)(+)映射方法。在 SE-BURST 序列中，在聚焦脉冲之前施加多个低幅度激发脉冲。因此，每个激发周期可以采集多个不同的相位编码回波。与 SE 序列相比，这种激发策略在单位时间内具有相似的信噪比 (SNR)，但比吸收率降低。所提出的 BS-SE-BURST 序列在常规的 3 T 全身 MRI 扫描仪上实现并成功应用。
