Which mechanism is most responsible for scatter that contributes to image formation at diagnostic energies?

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

Answer: (A)

In diagnostic radiography, the scattered photons that most influence image formation come from Compton scattering. Here, the incoming X-ray photon interacts with a loosely bound outer electron, ejects the electron, and is deflected with lower energy. The scattered photon travels in a new direction and can reach the detector, adding noise and reducing contrast, which is why Compton scattering dominates scatter at clinical energies. Pair production requires energies above about 1 MeV and produces a photon-electron pair rather than scatter. The photoelectric effect involves absorption of the photon by ejecting a bound electron, contributing to image contrast through attenuation rather than scatter. Coherent scattering is elastic (no energy loss) and occurs with low probability at diagnostic energies, contributing little to image formation.