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.