Abstract: In a computerized tomographic apparatus in which the requisite absorption data are derived by rotating a source of fanned radiation around a body to be examined and detecting radiation emergent from a cross-sectional slice of the body at many times during the rotation the data, when assembled into sets relating to parallel beam paths through the slice, tends to relate to beam paths which are non-uniformly spaced across the slice. Some techniques for processing such data to produce a representation of the variation of absorption of said radiation over the slice should preferably have said data presented thereto in the form of sets relating to substantially uniformly spaced, parallel beam paths. The application of this invention provides considerably more data then is actually required for the processing so that data relating to beam paths of the required numbers and dispositions can be derived therefrom.