@cbajada, for future reference: the dwi2response dhollander is more accurate in general for this; there’s a mechanism in there (some sort of outlier rejection, one could say) that excludes high diffusivity values from the mask used for the tournier algorithm. Even though the single-fibre WM voxels are quite expected to come from that area, it’s still worthwhile to have some filtered out that risk coming too close to the ventricles. They may look like nice single-fibre candidates, but for multi-tissue CSD, it’s also not unimportant that they aren’t contaminated (too) much by CSF.
But for the HCP data in general, the CSF response is at risk being a bit mis-estimated by the msmt_5tt algorithm. See this quite lengthy post for more info: Dwi2response dhollander questions