The thickness of added filtration within general use x-ray tube assemblies is approximately

• A. 0.2 mm aluminum equivalence (Al eq.)
• B. 2 mm aluminum equivalence (Al eq.)
• C. 0.2 mm lead equivalence (Pb eq.)
• D. 2 mm lead equivalence (Pb eq.)

Answer: (B)

Added filtration is described in aluminum-equivalent thickness because aluminum is the standard reference material used to compare how a beam is hardened. In general-use x-ray tube assemblies, the typical added filtration is about 2 mm of aluminum equivalent. This level removes many of the low-energy photons that would deliver dose without improving image quality, while still allowing enough higher-energy photons to maintain image contrast and penetration. Lead is used for shielding, not as the standard filtration in clinical tubes, and 0.2 mm of aluminum equivalent would be far too little to meaningfully harden the beam.