Bio-Molecular Imaging Center, Department of Radiology, University of Washington, Seattle, Washington 98109, USA.
Magn Reson Med. 2010 Jun;63(6):1610-26. doi: 10.1002/mrm.22394.
Variable flip angle T(1) mapping and actual flip-angle imaging B(1) mapping are widely used quantitative MRI methods employing radiofrequency spoiled gradient-echo pulse sequences. Incomplete elimination of the transverse magnetization in these sequences has been found to be a critical source of T(1) and B(1) measurement errors. In this study, comprehensive theoretical analysis of spoiling-related errors in variable flip angle and actual flip-angle imaging methods was performed using the combined isochromat summation and diffusion propagator model and validated by phantom experiments. The key theoretical conclusion is that correct interpretation of spoiling phenomena in fast gradient-echo sequences requires accurate consideration of the diffusion effect. A general strategy for improvement of T(1) and B(1) measurement accuracy was proposed based on the strong spoiling regimen, where diffusion-modulated spatial averaging of isochromats becomes a dominant factor determining magnetization evolution. Practical implementation of strongly spoiled variable flip angle and actual flip-angle imaging techniques requires sufficiently large spoiling gradient areas (A(G)) in combination with optimal radiofrequency phase increments (phi(0)). Optimal regimens providing <2% relative T(1) and B(1) measurement errors in a variety of tissues were theoretically derived for prospective in vivo variable flip angle (pulse repetition time = 15-20 ms, A(G) = 280-450 mT.ms/m, phi(0) = 169 degrees) and actual flip-angle imaging (pulse repetition time(1)/pulse repetition time(2) = 20/100 ms, A(G1)/A(G2) = 450/2250 mT.ms/m, phi(0) = 39 degrees) applications based on 25 mT/m maximal available gradient strength.
可变翻转角 T(1) 映射和实际翻转角成像 B(1) 映射是广泛使用的定量 MRI 方法，采用射频扰相梯度回波脉冲序列。在这些序列中，横向磁化的不完全消除已被发现是 T(1) 和 B(1) 测量误差的一个关键来源。在这项研究中，使用联合等色求和和扩散传播子模型对可变翻转角和实际翻转角成像方法中的扰相相关误差进行了全面的理论分析，并通过体模实验进行了验证。关键的理论结论是，正确解释快速梯度回波序列中的扰相现象需要准确考虑扩散效应。基于强扰相方案，提出了一种提高 T(1) 和 B(1) 测量精度的一般策略，其中等色的扩散调制空间平均成为决定磁化演变的主要因素。强扰相可变翻转角和实际翻转角成像技术的实际实现需要足够大的扰相梯度区域 (A(G)) 与最佳射频相位增量 (phi(0)) 相结合。对于各种组织，理论上推导了提供 <2%相对 T(1) 和 B(1) 测量误差的优化方案，用于前瞻性体内可变翻转角 (脉冲重复时间 = 15-20ms，A(G) = 280-450mT.ms/m，phi(0) = 169 度) 和实际翻转角成像 (脉冲重复时间(1)/脉冲重复时间(2) = 20/100ms，A(G1)/A(G2) = 450/2250mT.ms/m，phi(0) = 39 度)，基于 25mT/m 的最大可用梯度强度。
