I have a question about this problem: Compared with dhollander, there is a bump in the center of the image of msmt_5tt, which results from underestimating the b=0 signal?
I think the ‘bump’ you are referring to here is that in the top-right image, i.e. the WM response function for the maximal b-value? In which case: This isn’t to do with diffusivities, or indeed the b=0 image at all, but manifests when the DWI signal from multiple single-fibre WM voxels are rotated and combined together to produce a single WM response function. We used to see this sort of effect with much more prevalence when using the old FA > 0.7 single-fibre selection heuristic. It’s probably a combination of using voxels that are not ‘filled’ with WM fibres, voxels that do contain some small crossing-fibre bundles, that sort of thing.
Both dwi2response msmt_5tt and dwi2response dhollander in fact use dwi2response tournier internally to go from a WM mask to a WM single-fibre mask. So there must be something about the conservative-ness of dwi2response dhollander’s initial WM mask that helps to prevent using voxels in the WM response function estimation that dwi2response msmt_5tt used but perhaps shouldn’t have.
Additional point: As of Release Candidate 1, dwi2response algorithms use a new mechanism for performing the combination of single-fibre DWI signals, which explicitly forbids the formation of precisely this type of ‘bump’.
Rob