Radiography

Claim 1

Medical radiographic apparatus including means defining a patient position to be occupied by a cross-sectional slice of the body of a patient, a source of X-radiation mounted to project said radiation through said patient position, means for moving said source angularly around said patient position so as to irradiate said patient position from a plurality of different angular locations, detector means for detecting the radiation emergent from the patient position along a plurality of substantially linear beam paths from each of said locations, said detector means including a plurality of detector devices and means to provide electrical output signals indicative of the radiation detected by said detector devices, processing means for utilising said output signals to evaluate a variable characteristic, with respect to said X-radiation, at each of a number of regions distributed over said slice, means for comparing output signals, derived from first and second detectors and relating to substantially the same beam path, to obtain a comparison signal indicative of differences between the performances of the detectors and means for averaging said comparison signal with other comparison signals relating to the same two detectors and obtained in relation to other beam paths viewed by both detectors to obtain an averaged comparison signals, and means for utilising said averaged comparison signal to reduce the effects of said differences in performance upon the accuracy of said evaluation.