Hi--
The paper for the tractography algorithm used in 3dTrackID and 3dProbTrackID is now finally published:
Taylor PA, Cho K-H, Lin C-P, Biswal BB (2012) Improving DTI Tractography by including Diagonal Tract Propagation. PLoS ONE 7(9): e43415. doi:10.1371/journal.pone.0043415
(should be available at: [
www.plosone.org]).
It describes the motivations and method of FACTID and also comparisons with a number of existing codes. I hope it's useful, and, of course, I'm happy to answer any questions about it.
Cheers,
pt