Diffusion and tissue microstructure

Diffusion of molecules in brain and other tissues is important in a wide range of biological processes, such as the delivery of drugs, and for measurements, such as diffusion-weighted magnetic resonance imaging. The time-dependent diffusion coefficient D(t) of mobile molecules confined in pores or cells carries information about the confining geometry. At early times, D(t) gives, irrespective of details, the pore surface to volume ratio (S/Vp), and the cell-wall permeability ?. At long times, D(t) reaches a limiting value D0/?, where tortuosity ? is a characteristic of the geometry of the medium.