Improved correction of B0 inhomogeneity-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- b RPG; mRPG) that eliminates the JIC factor by normalizing opposite-polarity EPI images across multiple b -values. Study type: Retrospective. Population: 163 prostate cancer patients without metallic implants. Fieldstrength/Sequence: 3T diffusion-weighted sequence with EPI readout, using multiple b -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 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 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 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 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. ### Competing Interest Statement MEH reports honoraria from Multimodal Imaging Services Corporation and research funding from GE Healthcare. RRP has an equity interest in CorTechs Labs and Curemetrix, serves on the scientific advisory board of Imagine Scientific, and has received consulting fees from Bayer and research funding from GE Healthcare. AMD is a founder of and holds equity in CorTechs Labs, Inc, and serves on its scientific advisory board; he also is a member of the scientific advisory board of Human Longevity, Inc, and receives funding through research agreements with GE Healthcare. TMS reports honoraria from CorTechs Labs, Varian Medical Systems, WebMD, GE Healthcare, and Janssen; has an equity interest in CorTechs Labs, Inc, and serves on its scientific advisory board; and has received in-kind research support from GE Healthcare via a research agreement with the University of California, San Diego. These companies might potentially benefit from the research results. The terms of these arrangements have been reviewed and approved by the University of California, San Diego in accordance with its conflict-of-interest policies. ### Funding Statement This work was supported, in part, by the National Institutes of Health (NIH/NIBIB K08 EB026503), the Department of Defense (DOD/CDMRP PC220278), the American Society for Radiation Oncology, and the Prostate Cancer Foundation. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The Institutional Review Board of the University of California San Diego gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the corresponding author