diffusion tensor imaging measurement radial diffusivity measurement PMID:41039478 PMID:40717819 radial diffusivity A quantitative parameter derived from diffusion tensor imaging that measures water molecule diffusion perpendicular to the principal direction of the diffusion tensor. Radial diffusivity (RD) is calculated as the average of the second and third eigenvalues ((λ2 + λ3)/2) of the diffusion tensor and provides information about myelin integrity and tissue microstructure. This measurement can be applied to various biological tissues including brain white matter, skeletal muscle, kidney, and other organs. transverse diffusivity perpendicular diffusivity RD AI agent