Which interaction is not a significant contributor to attenuation at typical diagnostic x-ray energies?

• A. Photoelectric absorption
• B. Pair production
• C. Compton scattering
• D. Coherent scatter

Answer: (B)

In diagnostic x-ray imaging, attenuation is mainly due to interactions that remove photons from the beam or deflect them out of the image path. Pair production requires photon energy above 1.022 MeV, and at the typical diagnostic energies (hundreds of keV), its probability is extremely small, so it doesn’t meaningfully contribute to attenuation. The processes that do dominate attenuation in this range are photoelectric absorption, which is highly dependent on atomic number and drives contrast especially in high-Z materials, and Compton scattering, which depends on electron density and is the primary source of scattered radiation at these energies. Coherent (Rayleigh) scattering also occurs but contributes only a smaller amount to attenuation. Therefore, the interaction not significantly contributing at diagnostic energies is pair production.