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 of the bone by the imaging system, comprising: (i) 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; (ii) 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 from a different direction to the first captured projection image; (iii) identifying the position (414, 416) of at least a first anatomical landmark point of the bone in the first captured projection image from the first image data set; (iv) projecting the position of the first anatomical landmark point back as a ray in the 3D space of the co-ordinate system and defining maximum and minimum positions along the ray enclosing the image volume; (v) projecting the maximum and minimum positions of the at least first anatomical landmark point of the bone into the second captured projection image to define a line in the second captured projection image; (vi) determining the orthogonal distance between the line and the position of the same anatomical landmark point of the bone from the second image data set; and (vii) determining a transformation required to map the position of the same 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 by minimising the orthogonal distance, 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. (i) 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; (ii) 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 from a different direction to the first captured projection image; (iii) identifying the position (414, 416) of at least a first anatomical landmark point of the bone in the first captured projection image from the first image data set; (iv) projecting the position of the first anatomical landmark point back as a ray in the 3D space of the co-ordinate system and defining maximum and minimum positions along the ray enclosing the image volume; (v) projecting the maximum and minimum positions of the at least first anatomical landmark point of the bone into the second captured projection image to define a line in the second captured projection image; (vi) determining the orthogonal distance between the line and the position of the same anatomical landmark point of the bone from the second image data set; and (vii) determining a transformation required to map the position of the same 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 by minimising the orthogonal distance, 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.