1. A method, comprising: automatically triggering, by a processing device, image acquisition of a plurality of pretreatment images of a target, wherein the plurality of pretreatment images are internal to a patient, wherein acquisition of each of the plurality of pretreatment images is triggered at a different time; and generating, by the processing device, a correlation model distinguishing between inspiration and expiration movements that maps the movements of an external marker to a target location of the target using the plurality of pretreatment images.