1. Apparatus for calibration of radiographic images, the images comprising a spherical calibration marker appearing therein, the apparatus implemented on an electronic processor and comprising: a) an image receiver, configured to receive a radiographic image with said spherical calibration marker appearing therein; and b) a marker finder, associated with said image receiver and configured to automatically find an image of the spherical calibration marker in the received image by: edge detecting the image, forming a binary image from the edge detection, the binary image being formed by defining pixels therein either as a first value indicating belonging to an edge or to a second value indicating not belonging to an edge; pixels in said binary image corresponding to pixels detected as edges being set to said first value, and testing respective pixels of said binary image to determine whether a circle of said pixels of said first value belonging to an edge is described around a respective pixel in said binary image, and if so, setting said edge as a detected circle for calibration, wherein said testing comprises: stepwise around said pixels belonging to said edge selecting pairs of pixels and a presumed center pixel and following said selecting, pairwise testing said pairs of pixels for having a radius about said presumed center pixel, setting a key and a counter for each combination of center and radius and applying a respective counter for pixel pairs that share a same radius and center, and then, using said keys and counters, selecting as the detected circle that circle on which lies the largest number of said pairs of pixels which share a same center and radius; and c) a calibrator associated with the marker finder, for using said detected described circle and respective pixel as a centered circle and calibrating said image.