Page "Positron emission tomography" Paragraph 13

As the radioisotope undergoes positron emission decay ( also known as positive beta decay ), it emits a positron, an antiparticle of the electron with opposite charge.

The emitted positron travels in tissue for a short distance ( typically less than 1 mm, but dependent on the isotope ), during which time it loses kinetic energy, until it decelerates to a point where it can interact with an electron.

The encounter annihilates both electron and positron, producing a pair of annihilation ( gamma ) photons moving in approximately opposite directions.

These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes ( Si APD ).

The technique depends on simultaneous or coincident detection of the pair of photons moving in approximately opposite direction ( it would be exactly opposite in their center of mass frame, but the scanner has no way to know this, and so has a built-in slight direction-error tolerance ).

Photons that do not arrive in temporal " pairs " ( i. e. within a timing-window of a few nanoseconds ) are ignored.