Nonrigid registration based method for correction of distortions in echo planar images
Echo Planar Imaging (EPI) is a magnetic resonance imaging (MRI) technique permitting very rapid data acquisition. This makes EPI a widely used fast imaging technique in many applications, such as functional magnetic resonance imaging (fMRI), diffusion weighted imaging (DWI), and dynamic imaging with contrast agent enhancement. However, a well-known problem with EPI is that it is more sensitive to image distortions than conventional MRI due to the encoding scheme that is used. In this wok we developed methods based on nonrigid registration to correct in EPI images the geometric and intensity distortions caused by the inhomogeneity in the main magnetic field. Our methods are founded on the physics of EPI. First, we incorporate along with a standard Jacobian correction factor a new rephasing factor into our nonrigid registration algorithm to account for signal loss due to dephasing in Gradient Echo EPI images such that not only geometric distortion but also intensity distortion and attenuation in Gradient Echo (GE) EPI images can be corrected after the registration process. Second, we introduce a spatially varying scale mechanism into our registration algorithm to adapt the local scale properties of the deformation field to match these with the characteristics of the actual geometric distortions in EPI images. Third, we proposed in addition a hybrid method that combines both a field map and nonrigid registration for correction of distortions in EPI images. Finally, we also compared three distortion correction methods to study their advantages and disadvantages, which provides insights into the selection of field-map or registration methods for distortion correction of EPI images under certain circumstances. Experiments are performed on both simulated and real images.