In diffusion based MRI we have a set of gradients which is applied based on which the diffusion behavior in the brain is captured. So I read that there is a loss in the gradient signal in the direction of which diffusion of water is taking place. So signal loss in a particular signal indicates that fiber might be present in that direction. But what exactly leads to the signal loss in the direction of water diffusion ? Also what happens to a diffusion signal in areas where movement of water is restricted?