The invention relates to a method for detecting a phantom, comprising the steps of: arranging a phantom with respect to an object, acquiring at least one image of said object by means of an x-ray apparatus, such that the image contains projections of the object and projections of at least three first calibration fiducials of the phantom, detecting the projections of the at least three first calibration fiducials in said at least one image, and establishing a correspondence between the 2D image coordinates of said projections of the at least three first calibration fiducials and the 3D coordinates of said at least three first calibration fiducials in a local coordinate system of the phantom for computing the projection matrix at least up to a scale factor.
