= Solution
TODO: Biology deferred at the user's request: biological tracer behavior and clinical PET applications.
The generic physics of <positron emission tomography> uses <electron-positron annihilation>. A slowly moving electron-positron pair predominantly produces two nearly opposite <photons> with <energy> close to $m_ec^2=511\,\mathrm{keV}$ each. A ring of detectors records approximately simultaneous interactions; a coincidence identifies a <coincidence line of response> along which an annihilation could have occurred. Finite <Positron> travel before annihilation and nonzero pair <momentum> limit how directly this line identifies the original emission location.
Many such measurements constrain an emission distribution through a <tomography> model. Attenuation, scattered coincidences, unrelated random coincidences, finite detector resolution and dead time alter the observed counts and require <calibration> or modeling. With useful time-of-flight information, the arrival-time difference further localizes <position> along the line: a timing uncertainty $\delta t$ corresponds to a <position> scale $c\delta t/2$. The coincidence data therefore provide spatial constraints, rather than each event directly supplying a complete three-dimensional image point.
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