EFFECTS OF SELECTIVE PULSES ON THE MEASUREMENT OF T2 AND APPARENT DIFFUSION IN MULTIECHO MRI
EFFECTS OF SELECTIVE PULSES ON THE MEASUREMENT OF T2 AND APPARENT DIFFUSION IN MULTIECHO MRI
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DOI：
10.1002/mrm.1910040204
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发表时间：
1987-02-01
影响因子：
3.3
通讯作者：
GORE, JC
中科院分区：
文献类型：
--
作者：
MAJUMDAR, S;GORE, JC
通过计算机仿真计算了空间选择性非均匀射频脉冲对多回波序列表观T2的影响。结果表明，在成像实验中，选择性脉冲的使用会产生T2估计误差，因为切片的区域有效地关闭了共振，因此接收自旋旋转比理想情况下要少。不完全旋转180度的横向磁化分量。重聚焦脉冲产生纵向分量，这些分量后来返回到横向平面，并增加随后的回波。这些所谓的“幽灵分量”会影响估计的T2的精确值，但即使完全去除，结果估计也太短了。表观T2受张动角非均匀性大小的影响，矛盾的是，随着回波数量的增加，表观T2并不总是更准确。通过增加材料薄板上的切片选择梯度引起的表观T2的减少重复了先前归因于扩散的效应，并提供了不依赖自旋运动的另一种解释。
The effects of using spatially selective and nonuniform radiofrequency pulses on the apparent T2 of a train of multiple echoes have been calculated by computer simulation. It is demonstrated that the use of selective pulses produces errors in estimates of T2 in imaging experiments because regions of the slice are effectively off resonance and therefore receive spin rotations less than in an ideal case. Components of the transverse magnetization that are incompletely rotated by 180.degree. refocusing pulses produce longitudinal components that are later returned to the transverse plane and add to subsequent echoes. These so-called ghost components influence the precise value of T2 estimated, but even when completely removed the resultant estimate is too short. The apparent T2 is influenced by the magnitude of the nutation angle inhomogeneity across the slice and paradoxically is not always more accurate with larger numbers of echoes. The reduction of apparent T2 caused by increasing the slice-selection gradient across a thin slab of material replicates the effects previously ascribed to diffusion and affords an alternative explanation for these without recourse to motion of the spins.