What is the recommended HVL for x-ray tubes operating above 120 kVp?

• A. 0.5 mm Al
• B. 1.2 mm Al
• C. 4.1 mm Al
• D. 2.5 mm Al

Answer: (C)

Filtration and beam quality are assessed by the half-value layer in aluminum. For x-ray tubes operating above 120 kVp, the recommended HVL is 4.1 mm Al.

As kVp increases, the emitted X-ray spectrum contains more high-energy photons, but there’s still a significant fraction of lower-energy photons that contribute to patient dose without improving image quality. Increasing filtration raises the HVL, removing these low-energy photons and producing a more uniform, higher-energy beam. The 4.1 mm Al value is the standard minimum filtration specified for beams above 120 kVp to balance dose and image quality. Values like 0.5, 1.2, or 2.5 mm Al would provide inadequate filtration at these voltages, leading to higher patient dose and poorer beam quality.