What is the minimum filtration equivalence for x-ray machines operating above 70 kVp according to NCRP Report 102?

• A. 1.0 mm Al eq
• B. 2.5 mm Al eq
• C. 0.5 mm Al eq
• D. 3.0 mm Al eq

Answer: (B)

For high-kVp X-ray beams, it’s essential to remove the lowest-energy photons that contribute more to patient dose than to image quality. This is done by filtration, and there is a regulatory standard that sets a minimum amount of filtration to ensure the beam is properly hardened.

NCRP Report 102 specifies that for X-ray machines operating above 70 kVp, the total filtration must be at least 2.5 mm aluminum equivalent. This means the combination of inherent filtration (from the tube housing and window) plus any added filtration must reduce the beam as if 2.5 mm of aluminum were in the path. This level helps minimize patient skin dose while preserving sufficient beam quality for diagnostic imaging.

The other values listed would not meet that minimum requirement (they are too low to provide the same protective effect), while a value like 3.0 mm Al eq would exceed the minimum but isn’t the standard minimum; the question asks for the minimum filtration.