Which interaction(s) contribute to attenuation of diagnostic x-rays within the body?

• A. Photoelectric and Compton
• B. Photoelectric interaction
• C. Compton interaction
• D. Pair production

Answer: (A)

Attenuation of diagnostic x-rays in the body mainly comes from two interactions that remove photons from the primary beam: photoelectric absorption and Compton scattering. In photoelectric absorption, the photon gives all its energy to an inner-shell electron and is fully absorbed, with the electron ejected. This process is highly dependent on the tissue's atomic number and the photon energy, so higher‑Z structures like bone produce more attenuation through this mechanism, contributing to image contrast. In Compton scattering, the photon collides with a loosely bound outer electron and is deflected, losing energy in the process; the photon is typically removed from the primary beam, reducing its intensity and adding scatter that can affect image quality. Compton scattering tends to dominate attenuation in soft tissues at typical diagnostic energies. Pair production requires photons with energy above 1.02 MeV and is negligible in diagnostic radiography, so it does not contribute to attenuation there. Therefore, the combination of photoelectric absorption and Compton scattering best accounts for attenuation in diagnostic x-ray imaging.