Combining prospective motion correction and distortion correction for EPI: towards a comprehensive correction of motion and susceptibility-induced artifacts

Abstract

State-of-the-art MR techniques that rely on echo planar imaging (EPI), such as real-time fMRI, are limited in their applicability by both subject motion and B(0) field inhomogeneities. The goal of this work is to demonstrate that in principle it is possible to accurately predict the B(0) field inhomogeneities that occur during echo planar imaging in the presence of large scale head motion and apply this knowledge for distortion correction. In this work, prospective motion correction is combined with a field-prediction method and a method for correcting geometric distortions in EPI. To validate the methods, echo planar images were acquired of a custom-made phantom rotated to different angles relative to the B(0) field. For each orientation, field maps were acquired for comparison with the field predictions. The calculated field maps are very similar to the measured field maps for all orientations used in the experiments. The root mean squared error (RMSE) of the difference maps was between 15 to 20 Hz. The quality of distortion correction using calculated field maps is comparable to distortion correction done with measured field maps. The results suggest that distortion-free echo planar imaging of moving objects may be feasible if prospective motion correction is combined with a field inhomogeneity estimation approach.