What are the three main photon interaction mechanisms in the energy range used for medical imaging?

• A. Photoelectric effect, Bremsstrahlung, and Compton scattering.
• B. Photoelectric effect, Compton scattering, and pair production.
• C. Bremsstrahlung, Cherenkov radiation, and Raman scattering.
• D. Coherent scattering, Photoelectric effect, and Bremsstrahlung.

Answer: (B)

In diagnostic imaging energies, the processes that shape how photons interact with matter are primarily absorption via the photoelectric effect and scattering via Compton interactions, with coherent (Rayleigh) scattering also present. The photoelectric effect is more probable at lower energies and in higher-Z materials, so it contributes to strong image contrast by absorbing photons, especially in bone versus soft tissue. Compton scattering becomes the dominant interaction at intermediate energies and is a major source of scattered photons that can blur images and reduce contrast. Coherent scattering occurs without energy transfer but is generally less impactful than the other two at diagnostic energies, adding a small background component.

Pair production, on the other hand, needs photon energy above 1.022 MeV to create an electron-positron pair, and its probability grows with energy only beyond the diagnostic range. Therefore it is not considered a main interaction mechanism for typical medical imaging energies, though it becomes more relevant at higher energies used in other applications.