What is the imaging principle of PET?
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.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.