Approximately what percentage of incident photons emerges from the patient to form the image?

• A. 20
• B. 80
• C. 95
• D. 5

Answer: (D)

The main idea is that the x‑ray beam is heavily attenuated as it passes through the body, so only a small portion actually reaches the image receptor to form the image. Inside the patient, most photons are removed from the beam by interactions with tissue: photoelectric absorption removes them from the beam entirely, and Compton scattering deflects them away from the image path. The photons that do emerge in the direction of the detector are mainly those that pass through without interaction, and this transmitted fraction is quite small.

For typical diagnostic imaging, about five percent of the incident photons emerge from the patient to contribute to the image. This fraction can vary with how thick or dense the part is and the energy of the beam: thicker or denser parts and lower energy reduce transmission, while higher energy (higher kVp) increases it, though the transmitted portion generally remains a minority.