Weigel Matthias, Helms Gunther, Hennig Juergen
Department of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany.
Magn Reson Med. 2010 Jan;63(1):230-4. doi: 10.1002/mrm.22145.
Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. Since the strength of magnetization transfer effects is related to the radiofrequency power and therefore specific absorption rate applied, magnetization transfer induced signal attenuations are investigated for a variety of TSE sequences with low constant and variable flip angles. Noticeable differences between the sequences have been observed. In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho-TSE and smooth transitions between pseudo steady states-TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer-induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence.
磁化传递效应是多层涡轮自旋回波序列（TSE）/快速自旋回波序列中对比度的主要来源。一般来说，低重聚焦翻转角在这类MRI序列中已很常见，尤其是为了减轻比吸收率问题。由于磁化传递效应的强度与射频功率相关，因此也与所施加的比吸收率相关，所以针对各种具有低恒定和可变翻转角的TSE序列，研究了磁化传递引起的信号衰减。已观察到各序列之间存在显著差异。特别是，对于低翻转角的TSE序列，如高回波TSE和伪稳态TSE之间的平滑过渡，观察到的信号衰减较少，从而导致对比度对层数的依赖性降低。结果表明，磁化传递引起的信号衰减强度可以通过一个基于给定TSE序列均方翻转角的物理框架来理解和描述。
