Which of the following describes the CT number accuracy tolerance for water-equivalent materials?

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

Answer: (D)

In CT QA, you evaluate how close the measured CT numbers are to the expected values. Water is defined as 0 HU, and materials that are designed to be water-equivalent should also appear near 0 HU. Because scanners vary a bit in calibration, reconstruction, and setup, a small allowance is allowed. The standard tolerance for water-equivalent materials is ±5 HU, meaning measurements between -5 and +5 HU are acceptable. Tolerances like ±1, ±3, or ±4 HU are too stringent given typical system variability, so they aren’t used as the standard. If a measured value falls outside ±5 HU, it indicates a drift or discrepancy that could affect accurate electron density mapping for planning.