= Solution
After injection, the <radiopharmaceutical> circulates and accumulates according to its biological targeting and clearance. <Technetium-99m> undergoes an isomeric <nuclear transition>, producing approximately $140\,\mathrm{keV}$ <gamma rays>; its roughly six-hour <half-life> permits useful counting with limited persistence. Some <photons> undergo absorption or <Compton scattering> in the patient. A lead <collimator> in the <gamma camera> accepts a narrow range of directions so the accepted event retains positional information.
An accepted <gamma ray> deposits energy, commonly in a NaI(Tl) <scintillator>, by <photoelectric effect> or <Compton scattering>. The resulting visible <particle scintillation> light is collected by <photomultipliers>. Their relative pulse sizes determine an event position and their summed pulse estimates its <energy>. A <pulse-height analyser> accepts an energy window around the <photopeak> to reject many scattered events. Position and uniformity corrections are applied; accepted events increment a two-dimensional pixel histogram. After acquisition the computer displays this count map as a planar projection of tracer activity. Finite counting statistics, <attenuation>, <scattering> and the <collimator> limit the image's quantitative accuracy and <spatial resolution>.<Bio-medical Applications of Jefferson Lab's Nuclear Physics Detector Technology by Drew Weisenberger>[Gamma-camera detection, technetium-99m photon energy and half-life]{ref}
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