Geometric Distortions in Diffusion MRI
In this chapter we describe the mechanisms by which an off-resonance field causes distortions in an echo planar imaging (EPI) image. We describe the two types of off-resonance fields that affect diffusion-weighted EPI images: susceptibility-induced fields and eddy current-induced fields. We present an overview of the literature methods for how to detect and correct for these fields. These methods fall into two broad categories: those that attempt to measure the field through additional or modified sequences and those that use image-processing techniques to try to deduce them from the distorted images. We end with a short description of methods recently developed at the Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB) for how to obtain undistorted data. © 2014 Elsevier Inc. All rights reserved.