What is the CT number accuracy tolerance for water?

• A. ±1 Hu
• B. ±3 Hu
• C. ±4 Hu
• D. ±5 Hu

Answer: (D)

CT number accuracy for water is about how closely a scanner must report the reference value of 0 HU across typical scanning conditions. Water is the baseline because its CT number is defined as 0; the tolerance shows how much deviation from that zero is acceptable due to normal variations in beam energy, reconstruction algorithms, slice thickness, and phantom placement. The standard tolerance is ±5 HU, meaning a measured water value should fall within -5 to +5 HU. This range accommodates inevitable scanner variability while still keeping CT-number–based calculations reliable. Smaller tolerances like ±1, ±3, or ±4 HU are not usually achievable consistently across different scanners and protocols, so ±5 HU is the practical, accepted limit.