Diffusion tensor MRI tractography. (A) The 3 × 3 Diffusion tensor: The off-diagonal elements of the tensor are equal (Dxy = Dyx, Dxz = Dzx, Dyz = Dzy). (B) Diagonalization of the tensor produces three eigenvectors (λ1, λ2, λ3). The principal eigenvector (λ1) is the largest of these and describes the direction of diffusion along the long axis of muscle fibers (white axis). The principal eigenvector in a voxel, however, is a measure of the average direction of diffusion in that voxel and cannot resolve the individual directions of more than one fiber population in the voxel. (C) The principal eigenvectors in adjacent voxels can be linked to form streamlines/tractograms. DTI-tractograms, however, are less robust than those produced by DSI because only one average vector and seed point (the principal eigenvector) is present per voxel. Panel C, adapted with permission from Helm et al .