Aluminum filtration primarily absorbs photons with which energy and what effect does this have on the spectrum?

• A. Higher-energy photons are absorbed, narrowing the spectrum
• B. All photons are absorbed equally
• C. Lower-energy photons are absorbed, shifting the spectrum to higher energies
• D. Medium-energy photons are absorbed, with no spectral shift

Answer: (C)

Aluminum filtration works by removing the lower-energy portion of the x-ray spectrum. Low-energy photons are more easily absorbed by the filter material because the attenuation is higher at these energies, so the filter preferentially attenuates them while higher-energy photons pass through with less attenuation. The result is spectral hardening: the beam loses low-energy photons and the average energy of the transmitted photons increases. This is why the correct description is that lower-energy photons are absorbed, shifting the spectrum toward higher energies. Higher-energy photons are not preferentially absorbed, photons are not attenuated equally across the spectrum, and there is a spectral shift, not none.