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Comprehensive and Detailed Explanation: When a positron (β+) loses its kinetic energy, it combines with an electron; the combined mass is converted into two 511 keV annihilation photons that are emitted in nearly opposite directions (180° apart). This collinear emission is the physical basis of coincidence detection in PET imaging. A single 511 keV photon is not produced. Beta-minus decay emits an electron and an antineutrino. Characteristic x-rays result from electron-shell transitions. Understanding annihilation radiation is essential for PET system design, time-of-flight imaging, scatter and random coincidence rejection, and radiation safety around PET patients. The 511 keV photons require higher-energy shielding than Tc-99m photons and produce different survey-meter responses. Technologists must also recognize that annihilation radiation can appear as background in SPECT energy windows if a PET patient is nearby.