I need to model the perfusion of contrast agent in the blood in the leg's district. I need to know, about the dispersion ( Taylor dispersion) of contrast agent, when i can assume that the dispersion of contrast agent in blood is in regime of pure advection, when is pure axial diffusion and when advection and radial diffusion dominate over axial diffusion instead. So i have to distinguish constrast agent behavior on the basis of this 3 regimes. Does Anybody know if there is a difference for small and large vessel or something like this?