1. A Method for detecting a phantom, comprising the steps of: arranging a phantom with respect to an object, the phantom comprising a plurality of first calibration fiducials (M i 1 , M i 2 , M i 3 ) in a first plane, acquiring at least one image of said object by means of an x-ray apparatus, such that the image contains projections of the object as well as projections (IM i 1 , IM i 2 , IM i 3 ) of at least three first calibration fiducials (M i 1 , M i 2 , M 1 3 ), detecting the projections (IM i 1 , IM i 2 , IM i 3 ) of the at least three first calibration fiducials (M i 1 , M i 2 , M i 3 ) in said at least one image, and establishing a correspondence between the 2D image coordinates (I x , I y ) of said projections (IM i 1 , IM i 2 , IM i 3 ) of the at least three first calibration fiducials (M i 1 , M i 2 , M i 3 ) and the 3D coordinates (x, y, z) of said at least three first calibration fiducials (M i 1 , M i 2 , M i 3 ) in a local coordinate system of the phantom for computing the projection matrix (P) or at least an element of the projection matrix (P) at least up to a scale factor (α), which projection matrix (P) relates said 2D image coordinates (I x , I y ) to said corresponding 3D coordinates (x, y, z) in a local coordinate system of the phantom according to α[I x , I y , 1] T =P[x,y,z,1] T .