Hounsfieldenheder

Hounsfield units, denoted as HU and occasionally referred to as Hounsfieldenheder in Danish sources, are the quantitative scale used in computed tomography (CT) imaging to express radiodensity of tissues. The scale is calibrated such that distilled water at 20 °C has a value of 0 HU, while air measures –1000 HU. The conversion formula is derived from the X‑ray attenuation coefficients of a material relative to water and is expressed as HU = (μ – μ_water)/(μ_water) × 1000, where μ represents the linear attenuation coefficient.

The system was introduced by Sir Godfrey Hounsfield, one of the pioneers of CT, in the late 1970s. It enables clinicians to differentiate tissues and pathological changes based on their density. Typical ranges include: soft tissue –80 HU to +200 HU, bone > 700 HU, and hemorrhage generally ≥ 30 HU. Fat appears at –100 HU, whereas calcified plaques can exceed 1000 HU.

In practice, Hounsfieldenheder assist in the assessment of lung parenchyma, detection of solid tumors, grading of liver fibrosis, and planning of radiotherapy. Measurement variability is influenced by scanner calibration, patient factor, and reconstruction parameters; therefore, standardized protocols are essential for reliable interpretation. Despite its widespread adoption, the HU scale remains a relative, not absolute, measurement, and cross‑modality correlation requires cautious interpretation.