Magnetic resonance signals have long been known to be sensitive to motion. One of the principal sources of motion sensitivity is the fact that when the local magnetization of a material is altered, it carries the altered magnetization with it when it moves (within the limits of the relaxation times). It has been known to measure blood flow by locally altering the magnetization of blood and then detecting the passage of this "tagged" blood downstream. Analogous techniques have been proposed for measurement of blood flow with magnetic resonance imaging (MRI) by detecting the passage of blood with locally saturated or inverted magnetization through the region being imaged. Other techniques have been proposed for imaging of myocardial motion within the plane of the image by using selective excitation to produce a pattern of lines of altered magnetization whose motion may be followed.