Methods for reducing motion induced artifacts in a projection imaging system

A medical imaging system has a source of X-rays and a detector which produces an electrical signal in response to X-rays. A signal generated by the detector, during an acquisition interval, is used to construct an image of a medical patient, which may contain artifacts due to motion of patient. A signal is produced indicative of the motion, which has a quiescent period of minimum movement. A first portion of the signal is selected such that if image acquisition commenced at the end of the first portion, the acquisition interval would occur during a quiescent period. This first portion is initially used as a reference signal portion. Then subsequent portions of the signal are compared to the reference signal portion to produce a descriptor of the degree of similarity. An image acquisition is commenced when the descriptor indicates a given degree of similarity that is above a defined threshold. Periodically a subsequent portion of the signal is combined with the previous reference signal portion to create an updated reference signal portion which thereafter is used in the comparison step.