The CT number accuracy tolerance for water is which value?

• A. ±2 Hu
• B. ±7 Hu
• C. ±1 Hu
• D. ±5 Hu

Answer: (D)

CT numbers quantify how much a voxel attenuates X-rays relative to water, with water defined at 0 HU. The accuracy tolerance describes how close the measured water value should stay to zero under routine scanning conditions. In practice, about ±5 HU is used as the standard tolerance. This range accounts for typical day-to-day variations from scanner calibration drift, different kVp settings, reconstruction kernels, noise levels, and partial-volume effects when measuring a water phantom. It provides a practical balance between precision and achievable reproducibility across different scanners and QA setups.

Smaller tolerances, like ±1 or ±2 HU, are often too stringent for routine QA because of the factors above, while a looser tolerance like ±7 HU is less protective of CT number accuracy. Therefore, ±5 HU best fits common clinical QA expectations for water CT-number accuracy.