The Bermuda Triangle of d- and f-MRI sailors - software for susceptibility distortions (SDCFlows)

Echo-Planar Imaging (EPI) allows very fast acquisition of whole-brain data, which enables standard functional & diffusion MRI (f/dMRI). However, EPI is notably sensitive to variations in the base B0 field. Small deviations in parts-per-million from the nominal B0 caused by steps in magnetic susceptibility (tissue interfaces) introduce misplacements in the registered location of voxels of up to some cm in standard settings along the phase-encoding direction (PE), apparent as local geometrical distortions of the imaged specimen. In humans, the susceptibility distortion (SD) is prominent starting at the petrous bone and extending towards the ear canals, defining a sort of triangle where signal vanishes (Fig. 1). SD is well-known, but existing solutions require mapping B0 deviations and are sensitive to several imaging parameters. In practice, addressing SDs is error-prone and often overlooked. Here, we introduce SDCFlows (SD Correction Flows), an open-source utility that leverages BIDS1 and several existing software tools to provide standardized, best-effort SD correction.