Welcome to the eTraining PET Essentials

PET Physics:
Positron Annihilation                                                   

A concise history of PETCoincidence Events

Positrona positively charged particle having the same mass and magnitude of charge as the electron and constituting the antiparticle of the electron-emitting radioisotopes have an excess of protonsan elementary particle that is identical with the nucleus of the hydrogen atom, that along with neutrons is a constituent of all other atomic nuclei, that carries a positive charge numerically equal to the charge of an electron, and that has a mass of 1.673 x 10(-24) gram. This unstable state ends once the excess positron (β+) is emitted. It travels a certain distance (1-3 mm) before it undergoes an annihilation with an electron (β-) creating a pair of collinear gamma rays.

This spatial uncertainty in the annihilation localization sets the limit to the detection precision of the scanner (i.e. implicitly to the PET spatial resolution).
Proton-rich nuclei may reduce their excess positive nuclear charge by decaying through the emission of a positron. Depending of their energy, the emmitted positrons travel inside the body in a range of few millimeters. While travelling in tissue, a positron loses energy in collisions with atomic electrons. It eventually annihilates with an atomic electron, resulting in the emission of two 511 keVgeneral unit of energy of moving particles, equal to the kinetic energy acquired by an electron losing one volt of potential, equal to 1.602*10-19 J; abbrev: eV. 1keV= 1000 eV photonsa particle with zero rest mass consisting of a quantum of electromagnetic radiation, which travels at the speed of light leaving in opposite directions.
Two detectors in electronic time coincidence can be used to sense the two annihilation photons. The line that joins the two detectors defines a LORabbreviation for Line Of Response, the virtual line connecting the two opposite detectors registering the coincidence event.
Details on spatial resolution. Find more about electronic collimation and energy resolution.