Infographics / Nuclear Medicine

PET coincidence detection & annihilation geometry

A positron from beta-plus decay travels a short range in tissue and annihilates with an electron, producing two 511 keV photons emitted about 180 degrees apart. A circumferential ring of scintillation detectors registers the two photons within a short coincidence timing window; the pair of struck detectors defines a line of response. A lower panel contrasts true coincidences (both photons from one event, line of response passes through it), scatter coincidences (one photon is Compton-scattered so the line of response is mispositioned), and random coincidences (single photons from two separate events fall in the window, giving a false line of response).

PET coincidence detection and the line of response A positron from beta-plus decay travels a short range in tissue and annihilates with an electron, producing two 511 keV photons emitted about 180 degrees apart. A circumferential ring of scintillation detectors registers the two photons within a short coincidence timing window; the pair of struck detectors defines a line of response. A lower panel contrasts true coincidences (both photons from one event, line of response passes through it), scatter coincidences (one photon is Compton-scattered so the line of response is mispositioned), and random coincidences (single photons from two separate events fall in the window, giving a false line of response). Positron annihilation gives a 511 keV photon pair that defines a line of response annihilation ≈180° positron (β⁺) range 511 keV photons line of response (LOR) Key points Positron annihilates with an electron → two 511 keV photons, ≈180° apart (non-collinearity ≈0.5°). Detected within a coincidence timing window (~few ns) → line of response. Ring of dense fast scintillators (LSO/ LYSO); no mechanical collimator. One photon gives no direction — two coincident photons are needed to localise the source. Coincidence types photon / true LOR false LOR True Both 511 keV photons from one event → LOR through it. Scatter scatter One photon Compton-scatters → LOR mispositioned. Random Single photons from two events in the window → false LOR.
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