Which of the following photon-tissue interactions does not occur in diagnostic radiography?

• A. Photoelectric interaction
• B. Compton interaction
• C. Coherent scatter
• D. Pair production

Answer: (D)

The key idea is energy thresholds for photon-tissue interactions. Pair production can occur only when the photon has enough energy to create an electron-positron pair, which requires at least 1.022 MeV (twice the electron rest mass energy). Diagnostic radiography uses X-ray energies roughly in the 20–150 keV range, far below that threshold, so there isn’t enough energy to produce a pair. As a result, pair production does not take place in diagnostic imaging.

The other interactions do occur in this energy range: the photoelectric effect happens when photons are absorbed by atoms, with absorption more likely in higher-Z materials and at lower energies, contributing to image contrast. Compton scattering involves inelastic scattering off electrons and is common at diagnostic energies, producing scatter that influences image quality and patient dose. Coherent (Rayleigh) scatter can occur at very low energies, with negligible energy transfer and a minor role in image formation.