The half-value layer (HVL) measures beam quality; for tubes operating above 120 kVp, the HVL is about which value?

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

Answer: (C)

Beam quality is characterized by the half-value layer, the thickness of material needed to cut the beam’s intensity in half. As you increase the tube voltage, the X-ray beam becomes more penetrating, so a larger HVL is required to achieve the same attenuation. For tubes operating above 120 kVp, the HVL is about 4.1 mm of aluminum, reflecting that higher-energy photons are less readily attenuated by aluminum. The other values correspond to less penetrating beams at lower kVp or with less filtration, whereas the standard reference at high kVp is roughly 4.1 mm Al.