Abstract: A transverse section tomography system has an ultra-high-speed data processing unit for performing back projection and updating. An X-ray scanner directs X-ray beams through a planar section of a subject from a sequence of orientations and positions. The scanner includes a movably supported radiation detector for detecting the intensity of the beams of radiation after they pass through the subject. The detector generates a series of scan data signals representative of the detected intensities. The data processing unit is coupled to the scanner and operates on the scan data signals to produce a set of filtered scan data signals representative of the contribution of each of the scan data signals towards reconstructing an image of the planar section of the subject.The data processing unit includes a scan storage section for retrievably storing the filtered scan signals in scan storage locations corresponding to predetermined beam orientations.