PET Coincidence Detection of 511 keV Photons

D. Two 511 keV photons emitted in exactly opposite directions (180°) — detected simultaneously (coincidence detection)

PET imaging is based on positron-electron annihilation physics. When a positron (β+) is emitted and travels a short distance (range), it collides with an electron. This annihilation produces TWO 511 keV gamma photons emitted in EXACTLY OPPOSITE DIRECTIONS (180° apart). PET scanners detect these two photons simultaneously (within a coincidence timing window of ~3–10 ns) using opposing detector rings — a process called coincidence detection. This coincidence detection provides electronic collimation (no physical collimator needed), far superior sensitivity compared to SPECT. The 511 keV photon energy corresponds to the rest-mass energy of an electron (0.511 MeV = m_e c²).