1. A method for retrospective internal gating, said method comprising: scanning a patient to obtain projection data; generating images from the projection data; obtaining a signal representing information regarding a physiological cycle of motion of an object within the patient; associating a plurality of sample points of the physiological cycle with each of the generated images, each plurality of sample points associated with phases of the physiological cycle; calculating a phase of each of the generated images based on an average of the phases of a respective plurality of sample points associated with each of the generated images; specifying at least one target phase in the physiological cycle of motion by calculating an average delta phase that is an average of phase differences between consecutive images of the generated images; and sorting the generated images based on the at least one target phase to generate at least one set of images.