CT: windows, Hounsfield units and contrast phases
- Modality
A CT scanner spins an X-ray tube around the body and a computer builds thin slices, giving every tiny square a density number.
What it is
Computed tomography (CT) passes a thin fan of X-rays through the patient while the tube and the detectors rotate around the table, which moves through the gantry. A computer turns the many measurements into cross-sectional slices that can be rebuilt in any plane (axial, coronal, sagittal) or in 3D. Each pixel carries a number in Hounsfield units (HU), a scale anchored at water = 0 and air = −1000; dense bone reaches +1000 or more. A screen can show only a few dozen shades of grey, so the reader picks a window: a width (how many HU are spread across the grey scale) and a level (the HU at the middle grey). The same data looks completely different in a lung, soft-tissue or bone window. Intravenous iodinated contrast raises the density of blood and of tissues that take it up, and the moment the images are taken after the injection (the phase) decides what is seen: arterial for arteries and bleeding, portal venous for most abdominal organs, delayed for the urinary tract.
How the image is made
- The X-ray tube and a ring of detectors rotate while the table moves (helical scanning); one rotation takes well under a second.
- The computer reconstructs slices from how much each ray was attenuated; thin slices show fine detail, thicker slices look smoother.
- Each pixel gets a Hounsfield value: air −1000, fat about −100, water 0, soft tissue about +20 to +60, bone several hundred to over +1000.
- The window width sets the contrast (narrow = more contrast); the level sets the centre. Values above the window are white, below it black.
- Common windows: lung (wide, low level), soft tissue or mediastinum (narrow, level near +40), bone (very wide, high level), brain (very narrow).
- Contrast phases: non-contrast; arterial (seconds after injection, timed to the aorta); portal venous (about a minute); delayed (several minutes or later).
What it is good for
- Fast cross-sectional anatomy in minutes: trauma, acute abdomen, stroke, suspected pulmonary embolism and aortic disease.
- Non-contrast CT is enough, and better, for fresh blood in the head, urinary stones, calcification and baseline density.
- CT angiography (arterial phase) shows emboli, dissection, aneurysm and active bleeding; portal venous phase suits most abdominal questions.
- Density numbers help characterise lesions: fat (e.g. in an angiomyolipoma or dermoid), simple fluid, calcium or enhancement.
| Tissue | Approximate HU | How it looks (soft-tissue window) |
|---|---|---|
| Air | −1000 | Black |
| Aerated lung | about −700 to −900 | Black (use the lung window to see it) |
| Fat | about −50 to −100 | Dark grey to black |
| Water, simple fluid | 0 to about +20 | Dark grey |
| Soft tissue, muscle | about +20 to +60 | Mid grey |
| Fresh clotted blood | about +50 to +80 | Slightly brighter than brain or muscle |
| Contrast-filled artery | often +200 or more | White |
| Cortical bone | several hundred to over +1000 | White |
Limitations and risks
- Ionising radiation: a chest CT is roughly two years of natural background radiation, on the order of a hundred chest X-rays; justify it.
- Iodinated contrast can cause allergic-like reactions and contrast-associated kidney injury; extravasation into the arm can occur.
- Beam hardening and metal cause dark and bright streaks (posterior fossa, hip prostheses, dental fillings) that can hide or mimic disease.
- Partial volume averaging: a small structure inside a thick slice takes the average density of its neighbours and can be misread.
- Motion (breathing, heartbeat) blurs images; very large patients give noisy images; the wrong phase can miss the finding.
Next step
- Write the clinical question on the request so the radiologist chooses the protocol, the phase and whether contrast is needed.
- Before contrast: ask about previous contrast reactions and check kidney function and pregnancy status as your hospital protocol requires.
- Previous moderate or severe contrast reaction: discuss with the radiologist whether to use another study or a premedication protocol.
- Suspected acute intracranial bleed or kidney stone: request non-contrast CT; suspected embolus, dissection or active bleeding: CT angiography.
- Unsure which study or phase answers your question: call the radiologist before booking, not after the scan.
Test yourself
Think of the answer, then tap the question to see it.
What are the Hounsfield values of water and air?
Water is 0 HU and air is −1000 HU; fat is negative (about −50 to −100) and bone is strongly positive.
What does narrowing the window width do to the CT image?
It increases contrast: a small range of HU is spread over the whole grey scale, so subtle soft-tissue differences stand out.
Which contrast phase is used to look for pulmonary embolism or aortic dissection?
The arterial phase (CT angiography), timed so that contrast is densest in the pulmonary arteries or the aorta.
Why is a suspected kidney stone scanned without intravenous contrast?
Stones are dense on their own; contrast in the urinary tract would hide them and adds risk without helping.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Computed Tomography Instrumentation and Physics (StatPearls (NCBI Bookshelf), 2026); Hounsfield Unit (StatPearls (NCBI Bookshelf), 2023); Intravenous Contrast Media in Medical Imaging (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.