Improved correction of B0 inhomogeneity-induced distortions in diffusion-weighted images of the prostate

BackgroundConventional distortion correction techniques include the Reversed Polarity Gradient (RPG) method and FSL-topup, which estimate tissue displacement from EPI images of opposite phase-encoding polarity, and scale image intensity by the Jacobian of the estimated displacement. PurposeTo demonstrate that Jacobian intensity correction (JIC) can cause misleading improvement of EPI image distortion. We propose an alternative distortion correction approach (multi-b RPG; mRPG) that eliminates the JIC factor by normalizing opposite-polarity EPI images across multiple b-values. Study typeRetrospective. Population163 prostate cancer patients without metallic implants. Fieldstrength/Sequence3T diffusion-weighted sequence with EPI readout, using multiple b-values. AssessmentMaps of spatial shift (distortion) were estimated from opposite-polarity EPI volumes using RPG, topup, and mRPG. The estimated spatial shifts from each method were then applied to correct the b=0s/mm2 images (both with and without JIC) and ADC maps (for which JIC is meaningless). Distortion was quantified by the Pearson correlation between opposite-polarity volumes. The distribution of correlation coefficients across all patients was examined for b=0s/mm2 images and ADC maps, before and after distortion correction by each method. The mean, median, and 10th percentile were reported for each distribution. Statistical testsWilcoxon signed-rank tests (=0.05) were used to assess whether correlation increased significantly after distortion correction by each method, and whether mRPG yielded a larger increase versus RPG or topup. ResultsMedian improvement in the correlation between b=0s/mm2 volumes was significantly smaller without JIC (p<0.001): 0.04 vs 0.16 (RPG), 0.06 vs 0.18 (topup). mRPG yielded significantly larger improvements compared to RPG or topup (p<0.001). b=0s/mm2: 0.09 vs 0.04 (RPG) and 0.06 (topup). ADC: 0.09 vs 0.02 (RPG) and 0.03 (topup). Data conclusionDisparity in the distortion-correction performance of conventional methods with and without JIC suggests underestimation of tissue displacement. mRPG shows improved correction of distortion artifacts compared to conventional methods.