1. Apparatus comprising: an image acquisition mechanism that generates a plurality of images of an imaging target in a time series during a scan; an image processor that processes the images; a computer which detects motion of the target during the scan by comparing an actual location of at least one point of reference in each individual processed image with an expected location of the at least one point of reference derived from a set of the processed images; and a mechanism for prompting a responsive action if motion is detected.