A new approach to structural integrity assessment based on axial and radial diffusivities

This study describes a method for performing diffusivity measures along and across a specific direction, derived from white matter in healthy controls. The diffusion tensor (DT) assigns a principal eigenvector (v₁) and eigenvalue (axial diffusivity, d_ax) to each voxel. The average of the second and third eigenvalues is the radial diffusivity, d_rad•v₁ may be affected by pathology, therefore when comparing d _ax and d_rad in patients one has to consider the direction of the measurement and underlying anatomy. Here we created a representative super-DT dataset, DT r_ref, whose eigenvector, v_1,ref, was considered the most likely direction of diffusivity per voxel. We defined the projected axial diffusivity, d_p-ax, as the projection of individual DTs along v_1,ref and the projected radial diffusivity, d_p-rad, as the average of the projections along the second and third eigenvectors of DT_ref. The projected diffusivities are promising new parameters for studying white matter pathology.