Positron emission tomography (PET) is based on the detection in coincidence of the two 511-keV annihilation radiations that originate from β + -emitting sources, such as the patient containing β + -emitting radioactivity. Positrons are annihilated in body tissue and produce two 511-keV annihilation photons that are emitted in opposite directions (180°). Two photons are detected in an electronic time interval, called “coincidence time window”, by two detectors connected in coincidence. Conversion of 511-keV photons to light photons in the detector, formation of a pulse by the PM tube, and pulse-height analysis follow the same principles as in conventional gamma cameras. Detectors are arranged in the array of several rings to have the organ of interest in the field of view. Data collected over 360° simultaneously around the body axis of the patient are used to reconstruct the image of the activity distribution in the slice of interest. Because the two opposite photons are detected in a straight line, no collimator is needed to limit the field of view, and the technique is called the electronic collimation.