Describes the fact that the resonant frequency of protons varies with their molecular environment. The chemical shift most important in clinical MR imaging is sthat between protons in fat and water. As a result of the chemical shift, the protons of fat and water which coexist in the same voxel may be alternately in phase, i.e. their transverse magnetization vectors add together, or out of phase (opposed phase), i.e. their magnetization vectors point in opposite directions. This phenomenon can be exploited to differeniate fatty tissue (signal drop on image acquired while fat and water are out of phase) from other tissue (no signal drop on out-of-phase image).