19.3. True Coincidences

Table of Contents

Same annihilation event

In PET, a true coincidence is defined by its origin. Two detected photons form a true coincidence if they both come from the same positron annihilation and each photon reaches the detector without undergoing any significant interaction that alters its path or energy.

In GATE, you usually see coincidences after the digitizer has processed detector hits into singles and then paired them in time. From a physics point of view, a true coincidence corresponds to two singles that both originate from the two back‑to‑back 511 keV photons produced by one positron annihilation. The positron is emitted by a radionuclide, slows down, then annihilates with an electron, and this single annihilation produces the photon pair.

To decide whether a recorded coincidence is true, you conceptually follow the history of each detected photon back to the annihilation point. If both photons share the same annihilation event, they are candidates for a true coincidence. In ideal conditions, they travel directly to the detectors and deposit their full energy there. In practice, GATE can store event identifiers and parent information inside hits or coincidences, which can be used to trace each detected photon back to a common annihilation.

For educational studies and performance evaluation, it is useful to compare different coincidence classes. A true coincidence preserves a clear geometric relationship between the annihilation position and the two detector elements that fired. This direct link is what allows PET to reconstruct lines of response and ultimately the activity distribution. When coincidences involve scattering, additional annihilations, or unrelated decays, this one‑to‑one relationship is lost and the event is no longer considered true.

19.3. True Coincidences

Same annihilation event

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