A method for defining edge boundaries of an image formed on a flat-panel radiographic detector. The method includes obtaining digital image pixel values from the radiographic detector; obtaining dimensional information about a collimator in the radiographic signal path; analyzing the image to detect one or more collimator blade edges; and processing the digital image pixel values at least once to detect remaining collimator blade edges. The method accumulates image pixel values along each of a plurality of lines in a first direction parallel to a first vector that extends across the image, to obtain a first profile value for each line in said first direction, thereby forming an ordered set of first profile values. The method then identifies first and second threshold values in the set of first profile values, wherein the paired first and second threshold values indicate an edge boundary corresponding to a collimator blade projection along the first direction.