Scatter radiation is mainly produced by which interaction in the patient?

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

Answer: (C)

In the diagnostic X-ray range, scatter radiation in the patient mainly comes from Compton scattering. Here, a photon collides with a loosely bound outer-shell electron, loses some energy, and changes direction. This process occurs more often than other interactions at typical clinical energies (roughly 60–120 kVp), so most scattered photons exiting the patient—and contributing to image fog and occupational exposure—are the result of Compton interactions.

Photoelectric absorption, by contrast, removes photons from the beam without producing scatter. Coherent (Rayleigh) scattering does occur but is relatively minor at diagnostic energies and small angles, so it doesn’t contribute much to the observable scatter. Pair production would require photon energies above 1.022 MeV, which are not used in standard imaging, so it isn’t a factor here.