= Solution
<Single-photon emission computed tomography> detects individual <gamma rays> from a tracer. Mechanically collimated <gamma cameras> acquire projections from multiple angles for <tomography>. The <collimator> discards most emitted <photons>, creating a strong sensitivity–<spatial resolution> trade-off.
<Positron emission tomography> uses a <Positron>-emitting tracer. The emitted <Positron> travels a short distance before <electron-positron annihilation> produces an approximately back-to-back photon pair. Coincident detection in a detector ring defines a <line of response>, giving electronic collimation and greater detection efficiency; <time-of-flight PET> further constrains position along that line. The <positron range>, non-collinearity, detector size, scatter and accidental coincidences limit <spatial resolution> and quantification. In both hybrids, <computed tomography> supplies anatomy and an <attenuation correction> map, with attention to registration and respiratory motion.<Nuclear medicine by NIBIB>[Sections describing SPECT, PET and hybrid CT imaging]{ref}
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