Abstract: Computerized tomographic X-ray apparatuses direct X-rays, usually in a fan-shaped distribution, through the body of a patient. In some examples the fan distribution may subtend angles of 50.degree. or more at the source. It has been found that such tubes, particularly of the rotating anode type, tend to have X-ray emission from regions surrounding the focal spot and it is not convenient to use anode shields to eliminate it. This off-focus radiation can present problems, especially when attenuators are used to equalize absorbing paths through the patient, since the off-focus radiation can sometimes be less attenuated than the main beam. It is proposed to correct the signals produced by the X-ray detectors, for off-focus radiation. A profile of off-focus or halo radiation is produced and the attenuation imposed on it is estimated.