What physical event occurs when an emitted positron encounters a local electron in tissue during a PET scan?
Answer and explanation
Correct answer: D. Annihilation into two antiparallel 511 keV gamma photons at 180 degrees
- Step 1: The positron (anti-electron, e+) travels 1-2 mm in tissue before encountering an electron (e-). Step 2. Mutual matter-antimatter annihilation converts their rest masses completely into two 511 keV photons flying apart at 180 degrees (E = m_e c^2 = 511 keV). Answer: Annihilation of both particles, producing two 511 keV gamma photons traveling in opposite directions (180 degrees). Why not the others: Collisions annihilate rest masses completely; they do not form neutrons; alpha particles are helium nuclei.
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