In general, an event (positron annihilation/photon emissions) is regarded as valid and is registered by the coincidence detection circuit if the following criteria are satisfied [26, 53]:
• two photons are detected within a predefined coincidence window,
• the LoR formed between the two photons is within a valid acceptance angle of the tomograph, and
• the energy deposited in the crystal of the detector by both photons is within the selected energy window.
Such coincident events are often referred to as prompt events.
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