mean diffusivity measurement (EFO_0920012, an Experimental Factor Ontology id) is a quantitative parameter derived from diffusion tensor imaging that measures the average magnitude of water molecule diffusion across all spatial directions within a tissue. Mean diffusivity (MD) is calculated as the mean of the three eigenvalues of the diffusion tensor and provides information about tissue microstructure, cellularity, and integrity. This measurement can be applied to various biological tissues including brain, kidney, skeletal muscle, and other organs. Also known as: average diffusivity, mean diffusivity.