Improved correction ofB₀inhomogeneity-induced distortions in diffusion-weighted images of the prostate

Background:Conventional 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.Purpose:To demonstrate that Jacobian intensity correction (JIC) can cause misleading improvement of EPI image distortion. We propose an alternative distortion correction approach (multi-bRPG; mRPG) that eliminates the JIC factor by normalizing opposite-polarity EPI images across multipleb-values.Study type:Retrospective.Population:163 prostate cancer patients without metallic implants.Fieldstrength/Sequence:3T diffusion-weighted sequence with EPI readout, using multipleb-values.Assessment:Maps 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 theb=0s/mm2images (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 forb=0s/mm2images and ADC maps, before and after distortion correction by each method. The mean, median, and 10th percentile were reported for each distribution.Statistical tests:Wilcoxon signed-rank tests (α=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.Results:Median improvement in the correlation betweenb=0s/mm2volumes 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 conclusion:Disparity 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.