Energy distribution of an X-ray beam is determined by which factors?

• A. Screen speed and receptor color.
• B. Focal spot size and patient age.
• C. kVp, inherent and added filtration, target material and angle.
• D. Room temperature and humidity.

Answer: (C)

Energy distribution, or the X-ray spectrum, is shaped by how the electrons are slowed and how the emitted photons exit the tube. The tube voltage sets the maximum photon energy and the overall shape of the spectrum—the higher the kVp, the more high-energy photons you get. Filtration removes the lower-energy photons, so inherent filtration (like the tube window and oil) plus any added filtration (aluminum, filters) harden the beam by shifting the spectrum toward higher energies.

The target material matters because it determines how photons are produced. Higher-Z targets produce more bremsstrahlung and also introduce characteristic radiation at energies specific to that material’s electron binding energies. Those characteristic lines appear on top of the continuous spectrum and change the distribution of photon energies you obtain.

The target angle affects the anode heel effect, which changes the balance of photons exiting toward different parts of the beam. This alters the energy distribution across the field because photons emerging from thicker portions of the target can be more attenuated, shifting the spectrum slightly in different directions.

Other options don’t change photon energies—the screen speed, receptor color, room temperature, and humidity don’t influence the X-ray energy distribution.