Absorption of x-ray photons in a patient’s body is primarily from which interaction?

• A. Bremsstrahlung
• B. Photoelectric
• C. Compton
• D. Characteristic

Answer: (B)

The main mechanism by which x-ray photons are absorbed in body tissues is the photoelectric effect. In this process, the photon is completely absorbed and ejects a bound electron from an inner shell of an atom. This absorption dominates at diagnostic energies, especially in higher- Z tissues like bone, because the probability of photoelectric absorption increases rapidly with atomic number and decreases with photon energy. This is what drives much of the image contrast between bone and soft tissue—the photons that undergo photoelectric absorption are removed from the beam, depositing energy in the tissue.

Bremsstrahlung and characteristic radiation are about X-ray production in the tube or secondary photons produced within the body, not the primary absorption mechanism in tissue. Compton scattering, while common at diagnostic energies, is a scattering interaction that changes the photon’s direction (and some energy) rather than fully absorbing it, so it contributes to scatter radiation rather than the absorption that reduces beam intensity.