Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania 19104, USA.
Magn Reson Med. 2010 Mar;63(3):719-27. doi: 10.1002/mrm.22213.
Spin-echo-based pulse sequences are desirable for the application of high-resolution imaging of trabecular bone but tend to involve high-power deposition. Increased availability of ultrahigh field scanners has opened new possibilities for imaging with increased signal-to-noise ratio (SNR) efficiency, but many pulse sequences that are standard at 1.5 and 3 T exceed specific absorption rate limits at 7 T. A modified, reduced specific absorption rate, three-dimensional, fast spin-echo pulse sequence optimized specifically for in vivo trabecular bone imaging at 7 T is introduced. The sequence involves a slab-selective excitation pulse, low-power nonselective refocusing pulses, and phase cycling to cancel undesired out-of-slab signal. In vivo images of the distal tibia were acquired using the technique at 1.5, 3, and 7 T field strengths, and SNR was found to increase at least linearly using receive coils of identical geometry. Signal dependence on the choice of refocusing flip angles in the echo train was analyzed experimentally and theoretically by combining the signal from hundreds of coherence pathways, and it is shown that a significant specific absorption rate reduction can be achieved with negligible SNR loss.
基于自旋回波的脉冲序列在实现高分辨率的小梁骨成像方面很有优势，但往往涉及高功率沉积。超高场扫描仪的可用性增加为提高信噪比（SNR）效率的成像开辟了新的可能性，但许多在 1.5T 和 3T 标准的脉冲序列在 7T 时超过了特定吸收率限制。本文介绍了一种经过改良的、降低特定吸收率的三维快速自旋回波脉冲序列，专门针对 7T 下的小梁骨进行体内成像进行了优化。该序列涉及一个板状选择性激发脉冲、低功率非选择性重聚焦脉冲以及相循环以消除不需要的板外信号。该技术在 1.5T、3T 和 7T 场强下采集了远端胫骨的体内图像，发现使用相同几何形状的接收线圈，SNR 至少呈线性增加。通过结合数百个相干路径的信号，对回波链中重聚焦翻转角的选择对信号的影响进行了实验和理论分析，结果表明可以在几乎不损失 SNR 的情况下显著降低特定吸收率。
