HU

Hounsfield Unit — the CT image pixel value normalized to water (0 HU) and air (−1000 HU). Bone is typically +1000 HU or higher.

Why it matters to buyers: Window / level presets use HU ranges. Specific clinical measurements (calcium scoring, liver lesions) depend on accurate HU calibration.

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.