MRI signal void due to in‐plane motion is all‐or‐none

The process of MRI signal attenuation due to in‐plane intra‐voxel velocity inhomogeneity is described. Given rigid rotation or linear shear, velocity phase‐sensitivity will induce a phase distribution that varies linearly with position, which is exactly equivalent to the effect of a spatial phase encoding gradient pulse. It follows that the effect of such motion on the raw MRI signal is to displace it a fixed distance in k‐space. Attenuation becomes marked when the center of the spinecho reaches an edge of k ‐space, which happens when intra‐voxlel phase shifts reach π radianlvoxel. Because spin echoes are typically peaked sharply at center, this attenuation usually is abrupt. Analytic and numerical simulations of linear and nonlinear velocity fields confirm abrupt MRI attenuation where phase dispersion exceeds π radianlvoxel. Examples of this phenomenon include the abrupt loss of blood signal adjacent the vessel wall in laminar flow, abrupt loss of subendocardial signal in early diastole, and sudden disappearance due to rotation of a kidney during a measurement of diffusion.