Epi Distortion Correction by Constrained Nonlinear Coregistration Improves Group Fmri

Introduction Because of its short volume acquisition time, echo planar imaging (EPI) has remained the method of choice for most fMRI studies. A well-known problem with EPI are geometric distortions caused by magnetic susceptibility inhomogeneities in the field of view. Especially in the frontal part of the brain, this can cause activation to appear at an offset along the phase encoding (PE) direction from its true location. Additionally, as the distortions tend to vary in severity between subjects and interfere with affine coregistration, activation at the group level can suffer from the resultant registration errors. We show that unwarping the EPI images to match a distortion-free anatomical image using a method similar to [1] can reduce these problems and improve the sensitivity of the statistical analysis.