HVl units are typically expressed in which of the following?

• A. Meters of lead
• B. Millimeters of aluminum
• C. Inches of tungsten
• D. Grams per square centimeter

Answer: (B)

Half-value layer is the thickness of material needed to cut the x‑ray beam intensity in half. In diagnostic radiology, this filtration is expressed in millimeters of aluminum equivalent because aluminum is the standard reference material with well-characterized attenuation for the diagnostic energy range. Saying the HVL as millimeters of aluminum provides a consistent way to compare beam quality and filtration across systems. Other units like meters of lead, inches of tungsten, or grams per square centimeter aren’t the conventional way to express HVL in this context, since they don’t offer the same standardized reference for beam attenuation in diagnostic beams.