Williams C F, Redpath T W
Department of Bio-Medical Physics and Bio-Engineering, Aberdeen University, Foresterhill, Scotland.
Magn Reson Med. 1999 Jan;41(1):63-71. doi: 10.1002/(sici)1522-2594(199901)41:1<63::aid-mrm10>3.0.co;2-f.
Snapshot FLASH preceded by a preparation sequence is used to measure NMR parameters. Some preparation information is lost during snapshot FLASH acquisition as the magnetization evolves towards the steady state. This problem is often addressed by assuming that image intensity is proportional to the central k-space line, which may be sampled immediately after preparation using a centric-reordered scheme. Although some limitations of this assumption have been discussed, they are often ignored in parameter measurement. In this study, magnetization evolution effects in snapshot FLASH were investigated by computer simulation and phantom experiments in the context of T2 measurement. It was found that the standard analysis may lead to significant error in parameter measurement. Some improvement may be obtained using a more sophisticated analysis to take account of magnetization evolution. However, at the expense of a 50% reduction in signal intensity, accurate values may be obtained by a novel method that uses crusher gradients to eliminate evolved magnetization.
在准备序列之后的快速成像（Snapshot FLASH）用于测量核磁共振参数。在快速成像（Snapshot FLASH）采集过程中，由于磁化强度向稳态演化，一些准备信息会丢失。这个问题通常通过假设图像强度与中心k空间线成正比来解决，使用中心重排方案可在准备后立即对其进行采样。尽管已经讨论了该假设的一些局限性，但在参数测量中它们常常被忽略。在本研究中，在T2测量的背景下，通过计算机模拟和体模实验研究了快速成像（Snapshot FLASH）中的磁化强度演化效应。结果发现，标准分析可能会导致参数测量中的显著误差。使用更复杂的分析来考虑磁化强度演化可能会有所改进。然而，以信号强度降低50%为代价，通过使用去耦梯度消除演化后的磁化强度的新方法可以获得准确值。
