radial diffusivity measurement (Experimental Factor Ontology identifier EFO_0920014) is 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. Also known as: perpendicular diffusivity, radial diffusivity, transverse diffusivity.