1. A method for correcting motion artefacts in captured projection images of a bone of a subject caused by movement of the bone relative to the co-ordinate system of an imaging system between capture of the images by the imaging system, comprising the steps of: obtaining a first image data set representing a first captured projection image of the bone for which the imaging system had a first geometry in the co-ordinate system; obtaining a second image data set representing a second captured projection image of the same bone for which the imaging system had a second geometry in the co-ordinate system, and in which the second captured projection image is captured from a direction different than the direction used to capture the first captured projection image; identifying the position of at least a first anatomical landmark point of the bone in the first captured projection image from the first image data set; projecting the position of the at least first anatomical landmark point of the bone into the second captured projection image; determining a transformation required to map the position of the first anatomical landmark point of the bone from the second image data set onto the projected position of the at least first anatomical landmark point of the bone, wherein the so determined transformation allows the true position of the bone relative to the imaging system when the second image was captured to be determined without changing the captured projection images themselves; and using the positions in the captured images of orthogonal incidence of an X-ray beam of the imaging system on an imaging plane to determine the positions of the anatomical landmark in the first and second images.