HU
Hounsfield Unit — the CT image pixel value, normalized to water (0 HU) and air (−1000 HU). Named for Godfrey Hounsfield, who built the first commercial CT in 1972. Reference values:
- Air: −1000 HU
- Lung: −500 to −900 HU
- Fat: −50 to −150 HU
- Water: 0 HU
- Soft tissue: +20 to +70 HU
- Hemorrhage (acute): +60 to +100 HU
- Bone (cortical): +1000 to +2000 HU
- Metal: > +3000 HU (clipped on display)
Why it matters to buyers: Window / level presets use HU ranges (e.g., brain window: 80W / 40L; bone window: 2000W / 600L). Specific clinical measurements — coronary calcium scoring, liver lesion characterization, dual-energy material decomposition — depend on accurate HU calibration. Calibration drift produces systematically biased numbers and silently invalid clinical measurements.
Why it matters to engineers: HU calibration drifts over time; daily water-phantom QC verifies it. CT numbers out of spec → recalibrate before scanning patients. Routine recalibration is part of the daily / monthly QA cadence on every clinical CT.
Related
- CT
- First Clinical CT (1972) — Hounsfield's original work
- Dental CBCT (uses CT-number scale)
- CT Trauma
- ACR Accreditation
- Biomed Engineer