A method is provided for determining, in a fourth-generation computed tomography (CT) scanner, a positional offset of a center of a ring of fixed energy-discriminating detectors with respect to an iso-center of a third-generation X-ray source/detector system. The method includes obtaining a plurality of offset images, each offset image being obtained from a scan executed with the ring of fixed energy-discriminating detectors positioned in a known offset location with respect to the iso-center; performing a current scan executed with the ring positioned in an unknown offset location with respect to the iso-center, to obtain a current image; calculating, for each offset image, a corresponding error value between the offset image and the current image; and determining the positional offset of the center of the ring to be an offset location corresponding to an offset image having the smallest error value.