In diagnostic radiography, which interaction is most associated with complete absorption of photons by tissue atoms?

In diagnostic radiography, which interaction is most associated with complete absorption of photons by tissue atoms?

• A. Photoelectric effect
• B. Compton scatter
• C. Bremsstrahlung
• D. Coherent scattering

Answer: (A)

The event described is the photoelectric effect, where a photon is completely absorbed by a tissue atom. In this process an inner-shell electron absorbs all of the photon’s energy (above the binding energy) and is ejected from the atom. The atom ends up with a vacancy and energy is deposited locally, which can lead to characteristic X-ray emission or Auger electrons afterward. This complete absorption is more likely at lower photon energies and in atoms with higher atomic number, which is why bone (high Z) tends to show up more prominently in diagnostic radiographs. In contrast, Compton scatter involves only partial energy transfer—the photon is deflected and retains energy and travels away with reduced energy, so it is not fully absorbed. Coherent scattering is an elastic process with negligible energy transfer at diagnostic energies, and bremsstrahlung refers to radiation produced by decelerating electrons, not a photon-tissue absorption event.

The event described is the photoelectric effect, where a photon is completely absorbed by a tissue atom. In this process an inner-shell electron absorbs all of the photon’s energy (above the binding energy) and is ejected from the atom. The atom ends up with a vacancy and energy is deposited locally, which can lead to characteristic X-ray emission or Auger electrons afterward. This complete absorption is more likely at lower photon energies and in atoms with higher atomic number, which is why bone (high Z) tends to show up more prominently in diagnostic radiographs.

In contrast, Compton scatter involves only partial energy transfer—the photon is deflected and retains energy and travels away with reduced energy, so it is not fully absorbed. Coherent scattering is an elastic process with negligible energy transfer at diagnostic energies, and bremsstrahlung refers to radiation produced by decelerating electrons, not a photon-tissue absorption event.