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

• A. classical
• B. photoelectric
• C. Compton
• D. pair production

Answer: (C)

Compton scattering dominates in soft tissue at diagnostic X-ray energies around 80 keV and higher. In this interaction, the photon collides with a loosely bound outer electron, transferring some energy to the electron and emerging with reduced energy and a change in direction. The likelihood of photoelectric absorption falls off rapidly with increasing energy, especially in soft tissue with low atomic number, so it becomes less significant as energy rises. Pair production requires photons with at least 1.022 MeV, which is far above diagnostic energies, and coherent (classical) scattering is negligible at these energies. Hence, Compton scattering is the primary interaction in soft tissue at these voltages.