One or two gamma rays:
To obtain a clear picture with a gamma camera or a PET scan, a radioactive substance is inserted into the body and attached to the organ in question. This insertion is often performed intravenously. Once the substance has been absorbed by the subject, a sensitive detection system is used to detect and analyse the emitted gamma rays. With a gamma camera, lead tubes are used to select the direction of photon emission. In PE, two photons simultaneously emitted back-to-back are detected : detectors are grouped in pairs, 180 degrees apart.