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 employs programmable computing apparatus and includes the steps of: (1) 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； (2) for each image, computing an average pixel intensity for each zone to form a sequence thereof for each zone； (3) for each zone, modifying the average pixel intensities by (a) computing the mean value of the sequence of average pixel intensities for such zone, (b) subtracting the mean from each average pixel intensity in the zone, and (c) summing the absolute values of the modified average pixel intensities to form a zone-sequence sum A； (4) for each zone, computing absolute-value first differences for each sequential pair of average pixel intensities and summing the differences to form a zone-sequence first-difference sum B； (5) selecting the zone having the highest ratio A/B； and (6) using the sequence of modified average pixel intensities of the selected zone to determine respiratory phase.
