Which tolerance is used to specify CT number accuracy for water in imaging QA?

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

Answer: (A)

In CT QA, you verify CT number accuracy by measuring a water phantom and checking that its CT number sits at 0 HU. The tolerance defines how far from 0 HU is still considered acceptable under normal operating conditions.

A tolerance of ±5 HU is chosen because it accounts for the typical variability introduced by kVp settings, beam hardening, reconstruction algorithms, and small differences in phantom placement or ROI measurement, while still being tight enough to detect meaningful calibration drift. It strikes a practical balance: not so strict that normal fluctuations cause false failures, but not so loose that real calibration issues slip by unnoticed.

Smaller tolerances like ±1 or ±2 HU would be overly sensitive to everyday variability and might trigger frequent unnecessary checks. A larger tolerance like ±7 HU would be too lenient and could let significant calibration drift go unchecked. So the ±5 HU range best reflects the level of precision achievable in routine imaging QA for water CT number accuracy.