Which interaction between x-ray photons and matter involves the greatest patient dose?

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

Answer: (C)

The photoelectric effect is the interaction between x-ray photons and matter that results in the greatest patient dose. This interaction occurs when an x-ray photon is completely absorbed by an inner-shell electron of an atom. As a result, the energy of the photon is transferred to the electron, causing it to be ejected from the atom. This leads to ionization and creates vacancy in the inner shell, which can result in the emission of characteristic radiation when outer-shell electrons transition to fill the vacancy.

Because the photoelectric effect involves the complete absorption of the photon, it contributes significantly to the dose the patient receives. Additionally, the likelihood of the photoelectric effect occurring increases with higher atomic number tissues (like bone vs. soft tissue) and lower photon energies, making it particularly relevant in diagnostic radiography where the goal is to create images of different body tissues.

In contrast, interactions such as Compton scatter, coherent scatter, and pair production do not result in absorption of the photon. Compton scatter can lead to some energy being lost to the patient, but part of the photon continues, which does not contribute as much to patient dose. Coherent scatter involves low-energy photons and essentially does not ionize the tissue, causing minimal dose. Pair production only occurs