Which statement describes the effect of aluminum filtration on the energy spectrum of an unfiltered x-ray beam?

• A. Aluminum filtration preferentially absorbs low-energy photons, shifting the spectrum to higher energies
• B. Aluminum filtration absorbs high-energy photons
• C. Aluminum filtration leaves the spectrum unchanged
• D. Aluminum filtration increases low-energy photon production

Answer: (A)

Aluminum filtration removes the soft, low-energy photons from the beam, causing the spectrum to become harder. Low-energy photons are absorbed more readily by aluminum due to the higher probability of photoelectric absorption at those energies. As the filter thickness increases, these low-energy photons are preferentially attenuated, while higher-energy photons pass through with less attenuation. The result is a shift of the average photon energy upward (the spectrum moves toward higher energies) and a reduction in overall photon fluence. This hardening improves image quality by reducing patient dose from useless low-energy photons while preserving the more penetrating photons that contribute to image formation.