Correction of Shearing Distortions in Echo-Planar Imaging

In this study, the existence of linear geometric distortions due to readout (RO) train-induced background gradients was investigated under B₀ homogeneity; this paper was aimed at deriving and implementing a simple method for correcting such linear geometric distortions without carrying out the reference measurements of field errors within the echo-planar imaging (EPI) acquisition window. The background gradient in the EPI acquisition window in the RO direction was estimated from the autocorrelation phase, which was calculated from the complex-valued RO 1-D inverse-Fourier-transformed k-space (x-ky space). All the correction procedures were performed in the complex domain in the hybrid x-ky space after acquiring the data. No prior adjustments or additional measurements are required, and this correction algorithm can be applied to most EPI techniques. The echo-planar images of several phantoms and a human brain were acquired with one 1.5-T and two 3-T MRI systems to validate the effectiveness of the correction method. The severe global spatial shearing distortion caused by the linear background RO gradient in the images was corrected, and improvements were observed in all cases. The results indicate that distortions caused by the background RO gradient under B₀ homogeneity can be corrected by online processing, even in high-resolution echo-planar images such as 128 × 128 pixel imaging.