Among the common x-ray interactions, which one is least likely to occur during standard imaging procedures?

• A. Photoelectric Absorption
• B. Compton Scatter
• C. Coherent Scatter
• D. Pair Production

Answer: (C)

In diagnostic X-ray imaging, photon energies are typically in the tens of keV to around 100 keV. The interaction that is least likely in this energy range is pair production because it requires photon energies above 1.022 MeV. Since standard imaging photons don’t reach that threshold, pair production effectively does not occur, making it the least likely interaction in this setting.

Coherent (elastic) scattering can occur at these energies, though its probability is small and diminishes with rising energy. Photoelectric absorption and Compton scattering are the dominant interactions in diagnostic imaging: photoelectric absorption contributes to image contrast and is more likely at lower energies and with higher-Z materials, while Compton scattering is prevalent at typical energies and across most tissues, producing scatter that affects image quality.