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 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, detecting the projections of the at least three first calibration fiducials in said at least one image, and establishing a correspondence between 2D image coordinates (I x , I y ) of said projections of the at least three first calibration fiducials and 3D coordinates (x, y, z) of said at least three first calibration fiducials in a local coordinate system of the phantom for computing a 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 , wherein the at least three first calibration fiducials (M i 1 , M i 2 , M i 3 ) corresponding to said detected projections are successively arranged on an associated line of said first plane, wherein the ratio r i =|M i 1 M i 2 |/|M i 2 M i 3 | is different from 1.