How much total aluminum equivalent filtration is required for a fixed x-ray unit operating over 70 kilovolts peak (kVp)?

• A. 2.5 mm.
• B. 1.5 mm.
• C. 1.0 mm.
• D. 2.0 mm.

Answer: (A)

Filtration standards are set to limit patient dose by removing low-energy photons from the beam, and the required amount depends on the beam’s kVp. When a fixed x-ray unit operates at more than 70 kVp, the minimum total filtration must be 2.5 mm aluminum equivalent. This total accounts for inherent filtration inside the tube plus added filtration outside the tube; with typical inherent filtration around 0.5 mm Al, you’d add about 2.0 mm Al to meet the 2.5 mm Al total. Totals below this value would not provide the required protection, so 2.5 mm Al is the correct minimum.