Fig. 1.

Annihilation coincidence detection (ACD). When a positron is emitted by a nuclear transformation, it scatters through matter losing energy and annihilates with an electron, resulting in two 511-keV photons that are emitted in nearly opposite directions (left). When two interactions are simultaneously detected within a ring of detectors surrounding the patient (right), it is assumed that annihilation occurred on the line connecting the interactions (line of response, LOR). ACD acts as a collimator for the positron emission tomography (PET) scanner (electronic collimation) by determining the path of the detected photons.