A method for calibration of a CT comprises sequentially positioning a phantom having a non-circular cross section and a length commensurate with the extent of a detector at a plurality of positions between an X ray source and the detector array or sequentially positioning a plurality of generally similar phantoms or sequentially positioning a same phantom at a plurality of positions between the X ray source and detector array of the CT scanner; acquiring calibration attenuation data for X rays that have been attenuated by traversing the phantom positioned at each of the plurality of positions; and calculating calibration corrections for CT scanner scan data from the calibration attenuation data.