Diffusion is the random motion of molecules through a medium. The freedom water molecules have to diffuse depends on their local environment. For example, water within cells is much more restricted than extracellular water. Since diffusion characteristics can change with disease, it is useful to be able to probe this phenomenon. In MRI the signal can be made sensitive to diffusion by the addition of large gradient pulses after the RF excitation pulse, which first encode and then decode the spatial position of the water molecules a short time later. The spatial position of water molecules that diffuse rapidly along the direction of the applied gradient is not properly decoded, leading to dephasing and thus signal loss. Conversely, water molecules that are in restricted environments cannot diffuse very far and therefore the signal strength is not greatly affected by the diffusion gradients. Comparison of images with and without diffusion weighting allows the calculation of an apparent diffusion coefficient (ADC) in each voxel. A high ADC corresponds to larger net displacement of the water molecules (i.e. relatively unrestricted) and therefore low signal strength in the diffusion-weighted image.