Practice question

Gamma-ray based tomographic methods differ fundamentally by detection scheme and collimation. Single photon emission computed tomography uses isotopes like technetium-99m, iodine-123 or xenon-133 decaying by emission of single gamma photons randomly oriented. Lead collimators absorb oblique photons, permitting only perpendicular photons to reach scintillation crystal, enabling three-dimensional reconstruction of cerebral perfusion or dopamine transporter distribution. Positron emission tomography requires coincident detection of paired annihilation photons, computed tomography generates external X-rays, MRI uses radiofrequency from protons, so singly detected gamma imaging characterizes SPECT uniquely.