Individual steps of the pair-wise registration pipeline are performed via external applications - some of them being 3D Slicer modules. Starting with two input DTI images, scalar FA maps are generated via dtiprocess. Registration is then performed between these FA maps, via BRAINSFit/BRAINSDemonWarp or ANTS -Advanced Normalization Tools-, which provide different registration schemes: rigid, affine, BSpline, diffeomorphic, logDemons. The final deformation is then applied to the source DTI image via ResampleDTIlogEuclidean.
DTI-Reg is available in a 3D Slicer (http://www.slicer.org) extension called DTIAtlasBuilder