A tracer that emits positrons is taken up by the tissue under study. Each positron meets an electron and annihilates, producing two gamma photons of 0.511 MeV that fly off in exactly opposite directions. A ring of detectors records the pair in coincidence; the line joining them passes through the annihilation point. Thousands of these lines cross where the tracer collected, building the image.
MissionWatch the lines of response pile up: every one passes through the tracer, so they cross exactly where it sits.Streak 0Best 0
The two gamma photons from a positron-electron annihilation travel:
each gamma photon energy0.511 MeV (= electron rest energy)
photon directions180° apart (back to back)
coincidence lines recorded0
Two detectors firing at the same instant define a line of response through the tracer; many lines reconstruct where it concentrated.