All post‐processing was performed using Software Library (FSL).56 The DW images were corrected for eddy current distortions separately for each combination of δ and Δ using FSL's E d d y module (www.fmrib.ox.ac.uk/fsl/eddy) with its default settings. The images were then co‐registered using FSL's F n i r t package. As the 48 shells were acquired across a wide range of TEs, over two days, we chose to proceed in two steps. First, within each quarter of the dataset (different day, different δ) we registered all the b=0 images together. We then applied these transformations to their intermediary DW images, using a trilinear resampling interpolation. The second stage involved co‐registering the four different quarters. To help the co‐registration, especially between the two days images that required some through‐plane adjustment as well, we omitted areas of considerable eddy‐current distortions by reducing the number of slices from 20 to 5 (i.e. leaving two images either side of the mid‐sagittal plane) and reducing the in‐plane image size to 75×80.