Correcting susceptibility-induced distortion in diffusion-weighted MRI using constrained nonrigid registration

Echo Planar Imaging (EPI) is the standard pulse sequence used in fast diffusion-weighted magnetic resonance imaging (MRI), but is sensitive to susceptibility-induced inhomogeneities in the main B₀ magnetic field. In diffusion MRI of the human head, this leads to geometric distortion of the brain in reconstructed diffusion images and a resulting lack of correspondence with the high-resolution MRI scans that are used to define the subject anatomy. In this study, we propose and test an approach to estimate and correct this distortion using a non-linear registration framework based on mutual-information. We use an anatomical image as the registration-template and constrain the registration using spatial regularization and physics-based information about the characteristics of the distortion, without requiring any additional data collection. Results are shown for simulated and experimental data. The proposed method aligns diffusion images to the anatomical image with an error of 1-3 mm in most brain regions.