A test body is provided for determining an operating characteristic of a tomographic apparatus of the type that scans a human body member with an X-ray beam along an edge of a slice through the member, accumulates data in electrical form which is representative of the variations in the intensity of X-ray transmission through the body during a plurality of scans and reconstructs a cross sectional image of the slice from the accumulated data. The body comprises an energy absorption means which is arranged in a predetermined configuration and is adapted for absorbing electromagnetic energy from an impinging X-ray beam. The body also includes support means for positioning the energy absorbing means between a scanning X-ray beam and a detector of the apparatus.