1. A method for determining, for 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 comprising: 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 of the third-generation X-ray source/detector system; performing a current scan executed with the ring of fixed energy-discriminating detectors positioned in an unknown offset location with respect to the iso-center of the third-generation X-ray source/detector system, to obtain a current image; calculating, for each offset image, a corresponding error value between the offset image and the current image to obtain a plurality of error values; determining a smallest error value of the plurality of error values; and determining the positional offset of the center of the ring of fixed energy-discriminating detectors to be an offset location corresponding to an offset image, of the plurality of offset images, having the determined smallest error value.