Hi Heidi - TRACULA does not use deterministic constraints like pre-defined ROIs to reconstruct the tracts. For each point along a tract (and not just the end points), it has a prior probability (that it has computed from the atlas subjects) that a tract goes through each of the aparc+aseg labels, or goes next to (in the L, R, A, P, etc direction) each of the aparc+aseg labels.

Best,
a.y