A method for calibrating a radiographic system such as a CT system is disclosed. A calibration object, such as a phantom comprising a plurality of strongly X-ray attenuating spherical markers (18, figure 2) arranged within a weakly X-ray attenuating supporting material (16, figure 2) is utilised, these could be for example ball bearings made of tungsten carbide or the like encased in low-density foam such as carbon or silicon carbide foam. In an embodiment the phantom 12 is placed on a turntable 10 on a platform between the source 2 and detector 4. A plurality of radiographic images of the calibration object are then obtained as the turntable rotates. The markers carve out elliptical paths in the images, a circle detection method is used to determine the position of the markers in the images and derive the elliptical paths. The ellipses are then analysed to determine geometrical parameters of the system. In one aspect, the parameters are determined using a nominal value assigned for the distance between the source and the axis of rotation. The principle point (52, figure 14) and the central slice (32, figure 14) can be determined from the analysis. The parameters can therefore be determined without prior knowledge of the geometry of the calibration object.
