Which X-ray interaction with matter predominates in soft tissue above 80 kVp?

• A. Classical
• B. Compton
• C. Photoelectric
• D. Pair production

Answer: (B)

The main idea is how X-ray interactions in soft tissue change with photon energy. In soft tissue, which is low-Z, the photoelectric effect dominates at lower energies, but its probability falls off quickly as energy increases (roughly with E^3) and is very small at diagnostic energies. As the energy climbs above about 80 kVp, Compton scattering becomes the predominant interaction because it depends mainly on electron density rather than atomic number and remains relatively likely across this energy range. Pair production, which requires photons with energy above 1.022 MeV, is not significant at diagnostic energies, so it doesn’t contribute much here. Classical (Rayleigh) scattering is also minimal in this range. Therefore, at X-ray energies above 80 kVp in soft tissue, Compton scattering is the dominant interaction.