Evaluation of EPI-Based Distortion Correction Techniques for Cardiac Diffusion Tensor Imaging

Cardiac diffusion tensor imaging (cDTI) is susceptible to image distortion while using an echo planar imaging (EPI) readouts. FSL TOPUP and TORTOISE DR-BUDDI are image distortion correction techniques that have been implemented to correct EPI distortion in neurological applications. We sought to establish which EPI-based distortion correction technique is most suitable for cDTI. Using a free-breathing second-order moment-compensated spin-echo technique, cDTI was acquired in healthy volunteers (N = 10) using both blip-up (BU) and blip-down (BD) EPI readouts. These datasets were then distortion corrected using the TOPUP and DR-BUDDI software packages. BU, BD, TOPUP, and DR-BUDDI images were then characterized by (1) geometric fidelity using the Dice Similarity coefficient (DSC), epicardial average Hausdorff distance (AHD ${}_{\mathrm{epi}}$ ), and endocardial average Hausdorff distance (AHD ${}_{\text{endo}}$ ); (2) quantitative parametric maps: mean diffusivity (MD), fractional anisotropy (FA), and helix angle pitch (HAP); and (3) 95% confidence intervals in uncertainty measurements for MD ( $\mathrm{dMD}$ ), FA ( $\mathrm{dFA}$ ), and the primary eigenvector ( $d{\overrightarrow{e}}_1$ ). Distortion correction displayed improved geometric fidelity for both median DSC (BU = 0.77, BD = 0.81, DR-BUDDI = 0.85, and TOPUP = 0.86) and median AHD ${}_{\mathrm{epi}}$ (BU = 2.19 mm, BD = 1.94 mm, DR-BUDDI = 1.46 mm, and TOPUP = 1.33 mm). Decreases in median FA were observed for distortion correction data (BU = 0.30, BD = 0.28, DR-BUDDI = 0.27, and TOPUP = 0.26), while median MD fell between conventional BU and BD encoding (BU = 1.58 μm²/ms, BD = 1.70 μm²/ms, DR-BUDDI = 1.65 μm²/ms, and TOPUP = 1.64 μm²/ms). There was little to no change in median HAP between distorted and distortion-corrected data (BU = -0.41, BD = -0.40, DR-BUDDI = -0.41, and TOPUP = -0.36). Distortion correction increased the median for $\mathrm{dMD}$ (BU = 0.32, BD = 0.32, DR-BUDDI = 0.36, and TOPUP = 0.41), $\mathrm{dFA}$ (BU = 0.18, BD = 0.17, DR-BUDDI = 0.24, and TOPUP = 0.23), and $d{\overrightarrow{e}}_1$ (BU = 22°, BD = 22°, DR-BUDDI = 44°, and TOPUP = 41°). EPI distortion correction techniques improved geometrical characterization (DSC and ${\mathrm{AHD}}_{\mathrm{epi}}$ ), but decreased precision in measuring uncertainty ( $\mathrm{dMD}$ , $\mathrm{dFA}$ , and $d{\overrightarrow{e}}_1$ ) when compared to conventional (distorted) BU and BD acquisitions. When comparing the two distortion correction techniques, TOPUP provides improved results for cDTI applications.