[en] An arrangement is provided for sensing and indicating if a cross section of a patient extends beyond an image reconstruction circle during examination. The arrangement is positioned within a generally vertical gantry having a generally cylindrical opening for receiving the patient. The arrangement includes a plurality of light emitting sources disposed within the gantry and which are closely adjacent the reconstruction circle. Each light emitting source projects a light beam along a line which is parallel with the plane of the reconstruction circle and tangent to a cylinder having a diameter and central axis in agreement with the reconstruction circle. The light beams extend substantially across the opening in the gantry and generally inscribe the reconstruction circle. A plurality of photodetectors are disposed within the gantry and aligned to receive each of the light beams. The photodetectors are interconnected to the system by means for indicating an interruption of any of the light beams between any of the sources and detectors