Howseman A M, Grootoonk S, Porter D A, Ramdeen J, Holmes A P, Turner R
Wellcome Department of Cognitive Neurology, Institute of Neurology, London, WC1N 3BG, United Kingdom.
Neuroimage. 1999 Apr;9(4):363-76. doi: 10.1006/nimg.1998.0418.
Multislice echo-planar imaging (EPI) is a commonly used technique for fMRI studies. Brain activation images acquired using fMRI are sensitive to T2* changes, reflecting the level of blood oxygenation (BOLD contrast), and may also contain an element of T1 contrast which detects blood flow changes in large vessels. If slice inflow (T1) effects are significant in multislice EPI, then as the order in which the slices are acquired is changed, differences in the activation maps are predicted. However, in experiments presented here using visual stimulation, the data demonstrate that highly consistent results can be achieved for repetition times (TR) of 6.0, 3.0, and 1.5 s. This suggests that, for whole-brain multislice EPI, fMRI activation is dominated by T2*, BOLD contrast. The thickness of the imaging slice is also an important parameter in these studies, having implications for spatial resolution, sensitivity, and acquisition time. In separate visual cortex experiments the effect on the values of the fMRI Z scores and the number of activated voxels is investigated as a function of slice thickness (from 1 to 8 mm). The maximum Z scores in the data are similar for all slice thicknesses and, after resampling to allow a direct comparison to be made, the volume of visual cortex detected as significantly activated increases with slice thickness.
多层回波平面成像（EPI）是功能磁共振成像（fMRI）研究中常用的技术。使用fMRI获取的脑激活图像对T2变化敏感，反映了血液氧合水平（血氧水平依赖对比，BOLD对比），并且可能还包含检测大血管中血流变化的T1对比成分。如果在多层EPI中层面流入（T1）效应显著，那么随着层面采集顺序的改变，激活图会出现差异。然而，在此处使用视觉刺激的实验中，数据表明对于6.0、3.0和1.5秒的重复时间（TR）可以获得高度一致的结果。这表明，对于全脑多层EPI，fMRI激活主要由T2、BOLD对比主导。成像层面的厚度也是这些研究中的一个重要参数，对空间分辨率、灵敏度和采集时间都有影响。在单独的视觉皮层实验中，研究了作为层面厚度（从1到8毫米）函数的fMRI Z分数值和激活体素数量的影响。所有层面厚度的数据中最大Z分数相似，在重新采样以进行直接比较后，检测为显著激活的视觉皮层体积随层面厚度增加。
