Huang Susie Y, Witzel Thomas, Wald Lawrence L
Department of Radiology, Athinoula A Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129, USA.
Magn Reson Med. 2008 Nov;60(5):1112-21. doi: 10.1002/mrm.21702.
Control of the longitudinal magnetization in fast gradient-echo (GRE) sequences is an important factor in enabling the high efficiency of balanced steady-state free precession (bSSFP) sequences. We introduce a new method for accelerating the return of the longitudinal magnetization to the +z-axis that is independent of externally applied RF pulses and shows improved off-resonance performance. The accelerated radiation damping for increased spin equilibrium (ARISE) method uses an external feedback circuit to strengthen the radiation damping (RD) field. The enhanced RD field rotates the magnetization back to the +z-axis at a rate faster than T(1) relaxation. The method is characterized in GRE phantom imaging at 3T as a function of feedback gain, phase, and duration, and compared with results from numerical simulations of the Bloch equations incorporating RD. A short period of feedback (10 ms) during a refocused interval of a crushed GRE sequence allowed greater than 99% recovery of the longitudinal magnetization when very little T(2) relaxation had time to occur. An appropriate application might be to improve navigated sequences. Unlike conventional flip-back schemes, the ARISE "flip-back" is generated by the spins themselves, thereby offering a potentially useful building block for enhancing GRE sequences.
在快速梯度回波（GRE）序列中控制纵向磁化是实现高效平衡稳态自由进动（bSSFP）序列的一个重要因素。我们引入了一种新方法，用于加速纵向磁化回到 +z 轴，该方法独立于外部施加的射频脉冲，并且具有改善的失谐性能。用于增强自旋平衡的加速辐射阻尼（ARISE）方法使用外部反馈电路来增强辐射阻尼（RD）场。增强后的RD场以比T(1)弛豫更快的速率将磁化旋转回到 +z 轴。该方法在3T场强下通过GRE体模成像进行表征，作为反馈增益、相位和持续时间的函数，并与包含RD的布洛赫方程数值模拟结果进行比较。在破坏的GRE序列的重聚焦间隔期间进行短时间反馈（10毫秒），当几乎没有时间发生T(2)弛豫时，纵向磁化恢复率大于99%。一个合适的应用可能是改进导航序列。与传统的翻转回方案不同，ARISE“翻转回”是由自旋自身产生的，从而为增强GRE序列提供了一个潜在有用的构建模块。
