A rotating-patient computed tomography scanning system in which the detector offset is measured in real time for each scan. At least one x-ray--absorbent reference structure is disposed in the field of view, so that a projection of the reference structure is present in every view of the patient. The position of the reference structure can be expressed in terms of the detector offset for each view of the patient. The detector offset for a given image can therefore be determined at the time the image data is obtained. The offset value is used in the data obtained from the current scan to reconstruct images that are free of artifacts that are caused by errors in the detector offset.