University Duisburg-Essen, Essen, Germany.
Magn Reson Med. 2012 Apr;67(4):1033-41. doi: 10.1002/mrm.23081. Epub 2011 Aug 19.
As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time-interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B 1+ modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B 1+ homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T(2) , and T(2) *-weighted imaging and, if a specified homogeneity criterion is met, in T(1) -weighted imaging as well.
随着 MRI 中的磁场强度和工作频率增加，射频波长接近人体头部/身体的大小，从而导致波效应，导致信号减少和丢失。特别是，在 7T 及更高场强下进行全身成像极具挑战性。最近，提出了一种称为模式的时间交错采集的采集方案，以解决高场 MRI 中的非均匀性问题。其基本前提是使用静态射频调谐在交错采集中激发两个（或更多）不同的 B1+模式，其中两个模式的互补射频模式可用于改善整体信号均匀性。在这项工作中，详细研究了模式的时间交错采集对图像对比度以及时间平均比吸收率的影响。与传统的射频调谐相比，模式的时间交错采集在 B1+均匀性方面具有优势，同时比吸收率效率非常高。如果满足指定的均匀性标准，则模式的时间交错采集可以在 PD、T2 和 T2*加权成像中实现整个视野的几乎均匀的高场成像，以及 T1 加权成像中。
