Abstract: Computerized axial tomographic apparatus provides sets of "edge readings" which relate to the absorption suffered by penetrative radiation on traversing respective paths through a part of interest in a body, the various paths being coplanar. These edge readings are processed to permit the evaluation of the absorption coefficients at a plurality of locations distributed over the part of the body in the irradiated plane. In accordance with the invention the necessity for such processing to involve large scale multiplication is overcome by employing a processing technique which utilizes Walsh functions.