A calibration method for eliminating substantially circular ring artifacts (RA 1, RA 2, RA 3, RA 4, RA 5) to occur in a 3D image reconstruction based on a set of radiographic 2D projection images to be acquired from a number of distinct projection directions by a not perfectly isocentric 3D rotational X-ray scanner system equipped with at least one X-ray tube and at least one associated X-ray detector diametrically arranged with respect to said X-ray tube, said calibration method comprising the steps of: scanning a calibration phantom from each of said projection directions when moving each of said X-ray tube and X-ray detector along a not perfectly circular trajectory ( T F, T CD) around the phantom; for each projection direction, calculating the 3D position ( P F) of the at least one X-ray tube's focal spot and the 3D center position ( P CD) of the associated at least one X-ray detector from the acquired 2D projection images of the phantom; determining, from a set of geometrical calibration data which is obtained as a result of a 3D calibration procedure executed on the basis of the calculated 3D position data, the 3D coordinates of the effective center of rotation ( M RC, P Iso eff or P Iso eff ʹ) about which the not perfectly isocentric 3D rotational X-ray scanner system rotates from a set of geometrical calibration data which is obtained as a result of a 3D calibration procedure executed on the basis of the calculated 3D position data; wherein said 3D calibration procedure is based on a circular regression algorithm which calculates the center position ( M RC, P Iso eff) and the radius ( r RC) of a regression circle best fitted to a set of points obtained by drawing a straight connection line from the X-ray focus to the detector center for each projection direction and dividing each such line according to a division ratio (χ n), wherein the center position ( M RC) of the regression circle is interpreted as the effective center of rotation P Iso eff; reconstructing an object of interest from the acquired set of radiographic 2D projection images using the determined coordinates of the effective center of rotation to correct substantially circular ring artifacts corresponding to the effective center of rotation determined using said set of 2D projection images. scanning a calibration phantom from each of said projection directions when moving each of said X-ray tube and X-ray detector along a not perfectly circular trajectory ( T F, T CD) around the phantom; for each projection direction, calculating the 3D position ( P F) of the at least one X-ray tube's focal spot and the 3D center position ( P CD) of the associated at least one X-ray detector from the acquired 2D projection images of the phantom; determining, from a set of geometrical calibration data which is obtained as a result of a 3D calibration procedure executed on the basis of the calculated 3D position data, the 3D coordinates of the effective center of rotation ( M RC, P Iso eff or P Iso eff ʹ) about which the not perfectly isocentric 3D rotational X-ray scanner system rotates from a set of geometrical calibration data which is obtained as a result of a 3D calibration procedure executed on the basis of the calculated 3D position data;