Feed the body a tracer that emits positrons. Each positron meets an electron and the pair vanishes into two gamma photons flying apart in a straight line. Catch both, and you know the line they came from; catch millions, and you have an image of where the tracer went.
A positron-emitting tracer is taken up by the tissue under study. Each positron annihilates with an electron, producing two gamma photons of 0.511 MeV that travel in opposite directions. A ring of detectors records the pair in coincidence; the line joining the two detectors that fire passes through the annihilation site, and many such lines reconstruct an image of the tracer's distribution.
Each annihilation sends two gamma photons to opposite detectors; the line joining them runs through the tracer. Let the events pile up and the lines of response all cross exactly where the tracer concentrated, reconstructing its position. That is how PET maps function, not just structure.
Four linked ideas.
The 0.511 MeV photon energy is exactly the rest energy of an electron, m₋c²; two are produced so the total comes from the electron and the positron. The photons go in opposite directions for momentum conservation, which is what makes the line of response work. PET shows physiological function (e.g. metabolic activity), complementing the structural detail of CT or MRI.
Four quick checks on PET scanning. Each correct answer earns XP and lights this skill on your star map.
The tracer used in a PET scan is a substance that emits:
When a positron annihilates with an electron, the result is:
Each gamma photon from the annihilation has an energy of about:
Two detectors on the ring registering photons at the same instant define:
Confirm the 0.511 MeV yourself: m₋c² = 9.11 × 10⁻³¹ × (3.0 × 10⁸)² = 8.2 × 10⁻¹⁴ J, which is 0.51 MeV. Each annihilation gives two such photons. The short half-life of PET tracers limits patient dose but means they must be produced close to the scanner.
This skill is now lit on your star map. Keep the chain going.