What Are Hounsfield Units?
When a radiologist looks at a CT image, every shade of grey corresponds to a precise numerical value called a Hounsfield Unit (HU) — named after Sir Godfrey Hounsfield, co-inventor of the CT scanner and a Nobel Prize laureate. The HU scale provides a standardised, universal measurement of radiodensity, allowing clinicians worldwide to communicate about tissue characteristics with precision.
The scale is anchored at two fixed reference points: distilled water = 0 HU and air = −1000 HU. Everything else in the human body falls somewhere along this continuum.
The Hounsfield Unit Scale at a Glance
| Tissue / Material | Approximate HU Range | Appearance on CT |
|---|---|---|
| Air | −1000 | Black |
| Lung (aerated) | −900 to −500 | Dark black |
| Fat | −120 to −60 | Dark grey |
| Water / Simple fluid | 0 | Mid grey |
| Soft tissue / Muscle | +20 to +80 | Light grey |
| Blood (acute) | +50 to +80 | Bright grey |
| Calcification | +100 to +400 | Bright white |
| Cortical bone | +400 to +1000+ | Brilliant white |
Why Does Density Matter for Diagnosis?
The HU value of a structure can immediately narrow down what it is made of. A cyst filled with simple fluid will read close to 0 HU, while one containing protein, blood, or fat will deviate significantly. This difference guides the radiologist's differential diagnosis without any additional testing.
- Distinguishing haemorrhage from infarction: Acute blood in the brain typically reads +50 to +80 HU (hyperdense), whereas a fresh ischaemic stroke appears lower density than normal brain parenchyma.
- Characterising adrenal nodules: An adrenal adenoma usually contains fat, producing values below +10 HU on an unenhanced scan — a useful benign indicator.
- Evaluating renal stones: Different stone compositions (uric acid, calcium oxalate, struvite) produce distinct HU ranges, informing treatment choices.
Window Width and Window Level
Because the human eye cannot distinguish thousands of grey shades simultaneously, radiologists apply windowing to focus on a specific HU range. Two key parameters control this:
- Window Level (WL): The central HU value of the display range. Set this to the tissue of interest — e.g., WL +40 for soft tissue.
- Window Width (WW): The range of HU values shown. A narrow window (e.g., WW 80) increases contrast between similar tissues; a wide window (e.g., WW 400) shows a broader density range.
Common preset windows include brain window (WL +35, WW 80), lung window (WL −600, WW 1500), and bone window (WL +400, WW 1800). The same raw CT data can be viewed in any window after acquisition.
Hypo-, Iso-, and Hyperdense: The Language of CT Reports
Radiologists describe structures relative to a reference tissue:
- Hypodense: Lower HU than the reference — appears darker (e.g., oedema, cysts, fat).
- Isodense: Similar HU to the reference — blends in, making lesion detection challenging.
- Hyperdense: Higher HU than the reference — appears brighter (e.g., acute blood, calcification, iodinated contrast).
Practical Tips for Interpreting CT Density
- Always check the HU value using the region-of-interest (ROI) tool on the workstation rather than relying solely on visual impression.
- Compare both sides of the body and always look at the same structure in different windows.
- Remember that beam hardening and partial volume artefacts can artificially alter apparent HU values near dense structures.
Conclusion
Hounsfield units are the foundation of CT image interpretation. Mastering this scale — and understanding how windowing shapes what you see — transforms a confusing grey image into a map of tissue composition. For students and clinicians alike, this knowledge is essential to reading CT scans confidently and accurately.