The NRCP minimum filtration for machines above 70 kVp is 2.5 mm aluminum equivalence. Which statement correctly identifies the material used for this filtration?

• A. Lead equivalence (Pb Eq.)
• B. Aluminum equivalence (Al Eq.)
• C. Lead and Aluminum equivalence combined
• D. No filtration required

Answer: (B)

Filtration in diagnostic X‑ray beams is described using aluminum equivalence, not a specific metal. Saying that machines above 70 kVp require 2.5 mm aluminum equivalence means the beam must be filtered so that its attenuation matches what 2.5 mm of aluminum would do across the spectrum. Aluminum is used as the reference material because it provides a practical, consistent standard for beam quality at diagnostic energies, allowing you to compare filtration regardless of the actual filtration material used. In practice, the filter can be aluminum or any material arranged to produce the same attenuation as 2.5 mm of aluminum. Lead would over-attenuate for this standard, and no filtration would fail the requirement, while a combination would only meet the standard if it results in the same 2.5 mm Al equivalent.