Highly accelerated PSF-mapping for EPI distortion correction with improved fidelity

This study presents an improved point-spread-function (PSF) mapping-based distortion correction method and accelerated PSF acquisition for distortion correction in EPI without loss of quality or reliability compared to full encoding. To correct geometric distortions accurately, the PSF in the EPI phase-encoding coordinates (EPI-PSF) was measured and used as a kernel for distortion correction. FOV reduction was applied in the PSF mapping dimension for highly accelerated PSF acquisition. A novel approach for fold-over artifact correction in this reduced dimension is introduced. Conventional gradient-echo EPI and corresponding full PSF reference data were acquired in phantoms and in human brain at 7 T. The distortion corrected EPI data with the proposed acceleration were compared to result with full encoding. Previously published interpolation methods based on shift maps, non-uniform Fourier transformation and a b-spline interpolation were compared with the proposed method. The results demonstrate that the proposed method corrects geometric distortions in EPI with high accuracy and quality despite the high acceleration. In contrast to partial parallel imaging acceleration, no noise enhancement is introduced. The proposed EPI-PSF-based distortion correction improves correction of EPI and accelerates PSF reference data acquisition and computation.