diffusion tensor imaging measurement axial diffusivity measurement longitudinal diffusivity parallel diffusivity A quantitative parameter derived from diffusion tensor imaging that measures water molecule diffusion parallel to the principal direction of the diffusion tensor. Axial diffusivity (AD or AxD) represents the largest eigenvalue (λ1) of the diffusion tensor and provides information about axonal integrity and tissue microstructure. This measurement can be applied to various biological tissues including brain white matter, skeletal muscle, kidney, and other organs. AD axial diffusivity PMID:40717819 AxD AI agent PMID:41039478