Power Jonathan D, Plitt Mark, Kundu Prantik, Bandettini Peter A, Martin Alex
National Institute of Mental Health, National Institute of Health, Bethesda, Maryland, United States of America.
Department of Radiology, Mount Sinai Hospital, New York City, New York, United States of America.
PLoS One. 2017 Sep 7;12(9):e0182939. doi: 10.1371/journal.pone.0182939. eCollection 2017.
Head motion can be estimated at any point of fMRI image processing. Processing steps involving temporal interpolation (e.g., slice time correction or outlier replacement) often precede motion estimation in the literature. From first principles it can be anticipated that temporal interpolation will alter head motion in a scan. Here we demonstrate this effect and its consequences in five large fMRI datasets. Estimated head motion was reduced by 10-50% or more following temporal interpolation, and reductions were often visible to the naked eye. Such reductions make the data seem to be of improved quality. Such reductions also degrade the sensitivity of analyses aimed at detecting motion-related artifact and can cause a dataset with artifact to falsely appear artifact-free. These reduced motion estimates will be particularly problematic for studies needing estimates of motion in time, such as studies of dynamics. Based on these findings, it is sensible to obtain motion estimates prior to any image processing (regardless of subsequent processing steps and the actual timing of motion correction procedures, which need not be changed). We also find that outlier replacement procedures change signals almost entirely during times of motion and therefore have notable similarities to motion-targeting censoring strategies (which withhold or replace signals entirely during times of motion).
在功能磁共振成像（fMRI）图像处理的任何阶段都可以估计头部运动。在文献中，涉及时间插值的处理步骤（例如，切片时间校正或异常值替换）通常先于运动估计。从基本原理可以预期，时间插值会改变扫描过程中的头部运动。在此，我们在五个大型fMRI数据集中展示了这种效应及其后果。在进行时间插值后，估计的头部运动减少了10% - 50%或更多，而且这种减少通常肉眼可见。这种减少使得数据看起来质量有所提高。然而，这种减少也会降低旨在检测与运动相关伪影的分析的灵敏度，并且可能导致一个存在伪影的数据集错误地显示为无伪影。对于需要及时估计运动的研究，如动力学研究，这些降低的运动估计将特别成问题。基于这些发现，在进行任何图像处理之前获取运动估计是明智的（无论后续的处理步骤以及运动校正程序的实际时间安排如何，这些都无需改变）。我们还发现，异常值替换程序几乎完全在运动期间改变信号，因此与运动靶向审查策略（在运动期间完全保留或替换信号）有显著的相似之处。
