Which interaction is more likely to occur at low energy photons in bone and is responsible for high contrast in radiographs?

• A. High energy photons and high atomic number materials
• B. Low energy photons and high atomic number materials
• C. Low energy photons and low atomic number materials
• D. High energy photons and low atomic number materials

Answer: (B)

At low photon energies, the photoelectric interaction is the dominant way photons interact with matter, and its likelihood rises steeply with atomic number. Bone contains elements like calcium and phosphorus with higher effective atomic numbers than surrounding soft tissues, so low-energy photons are much more likely to be absorbed by bone than by soft tissue. This strong, Z-dependent absorption causes a large difference in attenuation between bone and surrounding tissue, producing the high radiographic contrast you see. As photon energy increases, Compton scattering becomes more common and contrast between structures decreases, so the low-energy, high-Z combination is what drives the high-contrast appearance in radiographs.