Williamson D S, Mulken R V, Jakab P D, Jolesz F A
Department of Radiology (D.S.S., P.D.J., F.A.J., Boston, MA 02115, USA.
Magn Reson Med. 1996 Apr;35(4):506-13. doi: 10.1002/mrm.1910350409.
It is well known that when compared to conventional spin-echo (CSE) imaging for equivalent effective echo times, fast spin-echo (FSE) imaging experiments yield higher signal intensities for coupled spin systems, such as that for lipid. One hypothesis put forth for this phenomenon is the removal of scalar coupling-based echo amplitude modulation by the FSE pi pulse train. This would result in the maintenance of signal intensity in the late echoes, with an overall increase in image signal when the multiecho train data is combined to form the image data. It will be shown that in images and spectra obtained from the final echo of a Carr-Purcell-Meiboom-Gill (CPMG) pi pulse train, an increase in signal in coupled spin systems occurs, when compared to conventional single-echo images and spectra at identical echo times. One- and two-dimensional spectroscopy experiments confirm that it is the generation of an isotropic mixing Hamiltonian by the pi pulse train in FSE that is responsible for the increased signal in images of a simple AX system and of corn oil, a model for human fat. This relative increase in signal is due to the maintenance of in-phase magnetization in the coupled spin systems by this Hamiltonian. In CSE, the weak coupling Hamiltonian allows development of antiphase coherences which, in the presence of the line broadening due to the imaging gradients, result in signal loss.
众所周知，在等效有效回波时间下，与传统自旋回波（CSE）成像相比，快速自旋回波（FSE）成像实验对于耦合自旋系统（如脂质）会产生更高的信号强度。针对这一现象提出的一种假设是，FSE π脉冲序列消除了基于标量耦合的回波幅度调制。这将导致晚期回波中信号强度得以维持，当多回波序列数据组合形成图像数据时，图像信号会整体增加。结果表明，在由Carr-Purcell-Meiboom-Gill（CPMG）π脉冲序列的最终回波获得的图像和光谱中，与相同回波时间下的传统单回波图像和光谱相比，耦合自旋系统中的信号会增加。一维和二维光谱实验证实，FSE中π脉冲序列产生的各向同性混合哈密顿量是导致简单AX系统和玉米油（人体脂肪模型）图像中信号增加的原因。这种信号的相对增加是由于该哈密顿量维持了耦合自旋系统中的同相磁化。在CSE中，弱耦合哈密顿量会导致反相相干的发展，在成像梯度引起的线展宽存在的情况下，会导致信号损失。
