Which of the following is the least common x-ray interaction?

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

Answer: (C)

In diagnostic x-ray interactions, which processes occur and how often they happen depend on photon energy. The most important interactions in this energy range are photoelectric absorption and Compton scattering, which contribute most to image formation and patient dose. Coherent scattering is an elastic process, meaning the photon changes direction but retains almost all its energy, and its probability is extremely small at diagnostic energies. Pair production, on the other hand, only becomes possible when photon energy exceeds 1.022 MeV and is still relatively rare in typical imaging beams.

Because coherent scattering has the smallest likelihood among the listed interactions in the energies used for medical radiography, it is the least common.