Physics-based constraints for correction of geometric distortions in gradient echo EP images via nonrigid registration

Abstract

Geometric distortion is a well-recognized problem in echo planar (EP) images. One strategy for the correction of these distortions is to register an EP image to a reference image, such as a high resolution anatomical MR image in which geometric distortion is minimal. Non-rigid registration methods, which warp images locally, have been used for this purpose. While a physics-based distortion model for spin-echo (SE) EP image has been developed and used as a constraint in nonrigid registration algorithms, such a model for gradient-echo (GE) EP image has not been investigated. Here, we propose to use a physics-based model for GE EP image that incorporates a term that takes dephasing into consideration. To evaluate this technique, we generate a distortion-free EP image using an MR simulator we have developed. We then distort the image and modify its intensity values using a real field map and an analytical expression that includes dephasing. The geometric distortion computed from the field map is used as the ground truth to which the deformation fields obtained with our method is compared. We show that including the dephasing term improves the results.