The HU for Water is

The HU for Water is

• A. -1000
• B. 1000
• C. 2000
• D. 0

Answer: (D)

In CT imaging, Hounsfield units are defined by how much a material attenuates X-rays compared to water. The CT number is calculated as HU = 1000 × (μ − μ_water)/μ_water, where μ is the tissue’s linear attenuation coefficient and μ_water is that of water. Since water has the same attenuation as the reference (μ = μ_water), its HU is 0. This baseline lets you interpret other materials: air is about -1000 HU, while dense bone is around +1000 HU or higher. Therefore, the HU value for water is 0.

In CT imaging, Hounsfield units are defined by how much a material attenuates X-rays compared to water. The CT number is calculated as HU = 1000 × (μ − μ_water)/μ_water, where μ is the tissue’s linear attenuation coefficient and μ_water is that of water. Since water has the same attenuation as the reference (μ = μ_water), its HU is 0. This baseline lets you interpret other materials: air is about -1000 HU, while dense bone is around +1000 HU or higher. Therefore, the HU value for water is 0.