Andersson J L, Hutton C, Ashburner J, Turner R, Friston K
The Wellcome Department of Cognitive Neurology, London, United Kingdom.
Neuroimage. 2001 May;13(5):903-19. doi: 10.1006/nimg.2001.0746.
Even after realignment there is residual movement-related variance present in fMRI time-series, causing loss of sensitivity and, potentially, also specificity. One cause is the differential deformation of the sampling matrix, by field inhomogeneities, at different object positions, i.e., a movement-by-inhomogeneity interaction. This has been addressed previously by using empirical field measurements. In the present paper we suggest a forward model of how data is affected by an inhomogeneous field at different object positions. From this model we derive a method to solve the inverse problem of estimating the field inhomogeneities and their derivatives with respect to object position, directly from the EPI data and estimated realignment parameters. The field is modeled as a linear combination of cosine basis fields, which facilitates a fast way of implementing the necessary matrix operations. Simulations suggest that the solution is tractable and that the fields are estimable given the deformed images and knowledge of the relative positions at which they have been acquired. An experiment on a subject performing voluntary movements in the scanner yielded plausible estimates of the deformation fields and their application to "unwarp" the time series significantly reduced movement-related variance.
即使在重新对齐之后，功能磁共振成像（fMRI）时间序列中仍存在与运动相关的残余方差，这会导致灵敏度下降，甚至可能导致特异性下降。一个原因是采样矩阵在不同物体位置因场不均匀性而产生的差异变形，即运动与不均匀性的相互作用。以前通过使用经验场测量来解决这个问题。在本文中，我们提出了一个正向模型，说明数据在不同物体位置如何受到不均匀场的影响。从这个模型中，我们推导出一种方法，直接从回波平面成像（EPI）数据和估计的重新对齐参数中，解决估计场不均匀性及其相对于物体位置的导数的逆问题。场被建模为余弦基场的线性组合，这便于实现必要矩阵运算的快速方法。模拟表明，该解决方案是可行的，并且在给定变形图像以及获取它们时的相对位置信息的情况下，可以估计场。对一名在扫描仪中进行自主运动的受试者进行的实验，得出了变形场的合理估计值，并且将其应用于“去扭曲”时间序列，显著降低了与运动相关的方差。
