Abstract: In an improved CT scanner calibration method and apparatus, a radiation beam, for example an x-ray beam, is directed at a plurality of radiation detectors. The beam is attenuated by a phantom of known attenuation characteristics. The detectors generate measured attenuation data from the attenuated beam. The measured attenuation data is back-projected to generate a measured phantom image. The measured phantom image is converted to an ideal phantom image. The ideal phantom image is forward projected to generate ideal attenuation data. Calibration values for each detector channel in the scanner are generated by cross-referencing the ideal and measured attenuation data. In a preferred embodiment, this cross-referenced data is compressed and stored in a look-up table for referencing during later interrogation of subjects.