How many types of events are detected as a result of annihilation photons?

When positron emission tomography (PET) imaging occurs, an annihilation event typically results from the interaction between a positron emitted from a radiotracer and an electron in the surrounding tissue. This interaction leads to the annihilation of both particles, producing two photons that are emitted in opposite directions, each carrying energy corresponding to the mass-energy equivalence of the electron and positron.

These two key photons represent what is commonly referred to as a "pair" of annihilation photons. However, when considering the different types of events that can be detected by the PET scanner, there are several categories to examine beyond the basic annihilation event.

Firstly, you have the direct detection of the two photons resulting from each annihilation event; these are the main signals the PET system is designed to detect. Secondly, there are additional types of events related to factors such as scatter (where a photon interacts with matter and is redirected), random coincidences (where photons from unrelated annihilation events are detected within the same time window), and decay events that can occur in various interactions.

Thus, within the context of detection events that a PET scanner observes, including annihilations and the complexities introduced by scattering and additional sources of detected signal, there are indeed several types of