Abstract: In a radiographic apparatus for deriving a representation of the variation of absortion of radiation in a slice of the body of a patient, data are derived which represent the intensity of radiation transmitted through the body. Detectors arranged to receive the transmitted radiation also receive further radiation, scattered within the body, which can cause errors in the final representation. The invention interposes occulting means between the radiation source and some detectors to obtain data representing only scatter to correct the transmission readings of the other detectors. The same detectors may serve both purposes at different times.