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(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.