Which photon-tissue interaction creates image contrast by differential absorption?

Which photon-tissue interaction creates image contrast by differential absorption?

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

Answer: (C)

Image contrast in X-ray imaging comes from how differently tissues absorb photons. The photoelectric effect is the interaction that creates this absorption-based contrast because its likelihood depends strongly on the tissue’s atomic number and density as well as the photon energy. At diagnostic energies, lower-energy photons are more readily absorbed by higher-Z structures, so bone absorbs more photons than soft tissue. This greater absorption reduces transmission through bone, making it appear radiopaque (white) on the image and producing clear contrast between bone and surrounding tissue. Other interactions don’t establish the same kind of absorption-based contrast. Compton scattering mainly scatters photons, contributing to image fog and reduced sharpness rather than creating tissue-dependent absorption differences. Coherent scattering is a low-probability process at diagnostic energies and doesn’t drive the contrast. Pair production requires energies well above typical diagnostic X-ray ranges, so it doesn’t govern the contrast seen in standard radiographs.

Image contrast in X-ray imaging comes from how differently tissues absorb photons. The photoelectric effect is the interaction that creates this absorption-based contrast because its likelihood depends strongly on the tissue’s atomic number and density as well as the photon energy. At diagnostic energies, lower-energy photons are more readily absorbed by higher-Z structures, so bone absorbs more photons than soft tissue. This greater absorption reduces transmission through bone, making it appear radiopaque (white) on the image and producing clear contrast between bone and surrounding tissue.

Other interactions don’t establish the same kind of absorption-based contrast. Compton scattering mainly scatters photons, contributing to image fog and reduced sharpness rather than creating tissue-dependent absorption differences. Coherent scattering is a low-probability process at diagnostic energies and doesn’t drive the contrast. Pair production requires energies well above typical diagnostic X-ray ranges, so it doesn’t govern the contrast seen in standard radiographs.