A method of determining respiratory phase of a living body from a sequence of digitized fluoroscopic images of a living-body region exhibiting respiratory displacement, the method employing programmable computing apparatus and comprising the steps of: (a) in each living-body-region image in the sequence, defining one or more zones with each image having identical image-to-image zone locations, sizes, and shapes； (b) for each image, computing an average pixel intensity for each zone to form a sequence thereof for each zone； (c) for each zone, modifying the average pixel intensities by (i) computing the mean value of the sequence of average pixel intensities for such zone, and (ii) subtracting the mean from each average pixel intensity in the zone； (d) for each zone sequence, computing a figure-of-merit； (e) selecting the zone having the highest figure-of-merit； and (f) using the sequence of pixel intensities of the selected zone to determine respiratory phase.
