What is coincidence detection in PET?

Coincidence detection is the method by which PET scanners identify and record pairs of gamma photons produced during positron annihilation. When two 511 keV photons are detected simultaneously (within a very short time window) by opposing detectors, the system assumes they originated from a single annihilation event.

Coincidence detection identifies pairs of simultaneously detected 511 keV photons and uses them to localise positron annihilation events in PET imaging.

Rather than using a physical collimator to determine the direction of incoming photons, PET uses electronic timing information. If two photons are detected within the predefined coincidence timing window, the system draws a line between the two detectors. This line, called the line of response (LOR), represents the possible location of the annihilation event.

By collecting many such coincident events from multiple angles, PET systems reconstruct a three-dimensional image of tracer distribution.

Understanding the physics

Following positron annihilation, two gamma photons of 511 keV are emitted in nearly opposite directions. These photons travel outward through the body and may be detected by scintillation crystals arranged in a ring around the patient.

PET detectors continuously monitor for photon interactions. When two detectors on opposite sides of the ring register photons within a very short time interval (typically a few nanoseconds) the system assumes that both photons arose from the same annihilation event.

This narrow interval is known as the coincidence timing window. If two photons are detected within this window, they are classified as a true coincidence event, and a line is drawn between the two detectors. The annihilation event must have occurred somewhere along that line.

Not all detected coincidences are true events. Some may be:

Random coincidences, where two unrelated photons are detected within the timing window.
Scatter coincidences, where one or both photons have been deflected before detection.

Modern PET systems apply correction algorithms to reduce the impact of these events.

Because PET relies on electronic collimation rather than physical lead collimators, it achieves much higher sensitivity than SPECT imaging.

Where this matters clinically

Coincidence detection allows PET to localise tracer uptake with high sensitivity and improved contrast resolution. The accuracy of timing detection influences image quality, noise, and spatial resolution. Advances such as time-of-flight (TOF) PET refine this process further by estimating where along the line of response the annihilation occurred.