Apparent Fiber Density (AFD) is a metric derived from diffusion MRI (dMRI) data, particularly from high-angular resolution diffusion imaging (HARDI) or diffusion spectrum imaging (DSI). It quantifies the density of axons within a specific region of interest or white matter pathway in the brain. AFD is calculated based on the orientation distribution functions (ODFs) obtained from dMRI data. It reflects the density of axons within the imaging voxel and provides information about the microstructural integrity of white matter tracts.