Practice question

Positron emission tomography physics begins with unstable neutron-deficient isotopes that emit a positron, antimatter counterpart of electron. Positron loses kinetic energy quickly traveling 1-2 mm in tissue, then annihilates with a nearby electron. Mass converts entirely into electromagnetic energy producing two 511 keV gamma photons propagating approximately 180 degrees apart, conserving momentum. Coincidence detection circuitry identifies simultaneous photon arrivals in opposite detectors, defining line of response for reconstruction. This positron annihilation principle underlies metabolic imaging, distinct from electron detection, X-ray absorption or radio-wave resonance.