It is known that along the phase encoding direction the effect of magnetic field inhomogeneity causes significant spatial
distortions in fast functional MRI Echo Planar Imaging (EPI). In this work, our previously developed distortion
correction technique via a non-rigid registration of EPI to anatomical MRI is improved by adding information from field
maps to achieve a more accurate and efficient registration. Local deformation models are used in regions of distortion
artifacts instead of using a global non-rigid transformation. The use of local deformations not only enhances the
efficiency of the non-rigid registration by reducing the number of deformation model parameters, but also provides
constraints to avoid physically incorrect deformations in undistorted regions. The accuracy and reliability of the non-rigid
registration technique is improved by using an additional high-resolution gradient echo EPI scan. In-vivo validation
is performed by comparing the similarity of the low-resolution EPI to various structural MRI scans before and after
applying the computed deformation models. Visual inspection of the images, as well as Mutual Information (MI) and
Normalized Cross Correlation (NCC) comparisons, reveal improvements within the sub-voxel range in the moderately
distorted areas but not in the signal loss regions.
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