[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Neuroradiology](https://durgical.com/en/radiology/areas/neuro/) › Reading a head CT step by step

# Reading a head CT step by step

- Reading

A head CT is a fast X-ray scan of the skull and brain. Reading it in the same order every time finds bleeding, swelling and fractures.

Updated Nov 13, 2026

## Key points

- Non-contrast head CT first: fast and very sensitive for fresh blood.
- Search order: blood, cisterns, brain, ventricles, bone («Blood Can Be Very Bad»).
- Use brain, subdural and bone windows every time.
- Blood fades from bright to isodense to dark over weeks.
- Effaced basal cisterns or midline shift is an emergency.

## What it is

Non-contrast CT of the head is the first study for most acute neurological problems: trauma, sudden headache, suspected stroke, sudden confusion or coma. It takes a few minutes, is available in almost every emergency department and shows fresh blood very well. Each tissue has its own grey: bone is white, fresh blood is bright (hyperdense), brain is mid-grey with grey matter slightly brighter than white matter, CSF is dark and air is black. A fixed search order, such as «Blood Can Be Very Bad» (blood, cisterns, brain, ventricles, bone), stops the eye from settling on the first finding and missing the second. The images are read in at least two windows: a brain window for the parenchyma and a bone window for fractures, plus a wider subdural window for thin collections next to the skull.

## What to check

- Fresh blood is bright; it fades to the same grey as brain (isodense) over roughly one to three weeks and becomes dark (hypodense) when chronic.
- The basal cisterns around the midbrain should be open and dark; effaced cisterns warn of herniation.
- Grey-white differentiation should be crisp; its loss is an early sign of ischaemia or oedema.
- Midline structures (septum pellucidum, third ventricle, falx) should be central; measure any shift.
- Dilated temporal horns are an early sign of hydrocephalus.
- Normal calcifications (pineal gland, choroid plexus, basal ganglia, falx) are symmetric and should not be read as blood.

## Reading it step by step (and why)

- Check name, date, clinical question and whether contrast was given — a contrast scan can make vessels look like blood.
- Blood: look for bright collections outside the brain (lentiform, crescent) and inside it — fresh blood is the commonest emergency.
- Look along the falx, the tentorium and under the skull in the subdural window — thin collections hide there.
- Cisterns: check the basal, suprasellar and quadrigeminal cisterns — blood means subarachnoid bleeding; effacement means mass effect.
- Brain: compare left and right for grey-white loss, dark areas, masses and midline shift — asymmetry is the clue to most lesions.
- Look at the sulci: effaced sulci on one side suggest swelling or an isodense subdural — it may be the only sign.
- Ventricles: judge size, symmetry and any blood or fluid level — big ventricles with small sulci suggest hydrocephalus.
- Bone: switch to the bone window and follow the calvarium, skull base, orbits and face — fractures change the risk of bleeding.
- Check sinuses and mastoids for fluid and look for air inside the skull — in trauma they suggest a skull-base fracture.
- Finish with the scalp: a soft-tissue swelling marks the impact; look for a contrecoup injury on the opposite side.

## Common mistakes

- Stopping at the first finding: a large bleed often comes with a second one, a fracture or herniation.
- Missing a thin subdural along the tentorium or falx because only the brain window was viewed.
- Calling a subacute subdural normal: it is the same grey as brain; look for effaced sulci and a displaced grey-white junction.
- Reading a normal early stroke CT as reassuring: in the first hours CT is often normal.
- Taking streak artefact in the posterior fossa or normal calcification for blood.
- In anaemia, blood is less bright and can be missed.

## Next step

- Any acute blood, midline shift, effaced basal cisterns or hydrocephalus: call the senior and neurosurgery now and repeat the neuro exam.
- Suspected stroke within the treatment window: CT angiography in the same visit and call the stroke team.
- Thunderclap headache with a normal CT done more than about six hours after onset: lumbar puncture or CT angiography as local protocol says.
- Symptoms not explained by CT (posterior fossa, small infarct, axonal injury): ask for MRI.
- Unsure about a finding: ask the radiologist before discharge; do not rely on an unreported scan.

## Memory aid

«Blood Can Be Very Bad»: Blood, Cisterns, Brain, Ventricles, Bone.

## Test yourself

Think of the answer, then tap the question to see it.

## What does fresh blood look like on non-contrast head CT?

Bright (hyperdense) compared with brain; it becomes isodense over about one to three weeks and dark when chronic.

## What does the mnemonic «Blood Can Be Very Bad» stand for?

Blood, cisterns, brain, ventricles and bone: the order to search a head CT.

## Why should a head CT also be viewed in a subdural and a bone window?

The subdural window shows thin collections against the skull, and the bone window shows fractures that the brain window hides.

## Which sign suggests a subacute subdural haematoma that is the same grey as brain?

Effaced sulci on one side, a grey-white junction pushed away from the skull and midline shift.

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## Elsewhere in DurgiPedia

- [Level of consciousness and the Glasgow Coma Scale](https://durgical.com/en/clinical-exam/level-of-consciousness-and-the-glasgow-coma-scale/)
- [Pupils: light reflex and abnormal pupils (Marcus Gunn, Horner, Argyll Robertson)](https://durgical.com/en/clinical-exam/pupils-light-reflex-and-abnormal-pupils/)
- [Intracranial hypertension syndrome](https://durgical.com/en/clinical-exam/intracranial-hypertension-syndrome/)

## Connections

## Related 8

- [Bleeds on head CT: epidural, subdural, subarachnoid and intraparenchymal](https://durgical.com/en/radiology/bleeds-on-head-ct-epidural-subdural-subarachnoid-and-intraparenchymal/)
- [Imaging in acute stroke: CT first, then CT angiography](https://durgical.com/en/radiology/imaging-in-acute-stroke-ct-first-then-ct-angiography/)
- [Head trauma: who needs a CT (Canadian CT head rule)](https://durgical.com/en/radiology/head-trauma-who-needs-a-ct/)
- [Mass effect, oedema and herniation on CT](https://durgical.com/en/radiology/mass-effect-oedema-and-herniation-on-ct/)
- [Hydrocephalus on imaging](https://durgical.com/en/radiology/hydrocephalus-on-imaging/)
- [Brain MRI: when it beats CT](https://durgical.com/en/radiology/brain-mri-when-it-beats-ct/)
- [CT: windows, Hounsfield units and contrast phases](https://durgical.com/en/radiology/ct-windows-hounsfield-units-and-contrast-phases/)
- [The five densities on an X-ray](https://durgical.com/en/radiology/the-five-densities-on-an-x-ray/)

## Related conditions 3

- [Traumatic brain injury](https://durgical.com/en/conditions/traumatic-brain-injury/)
- [Intracerebral haemorrhage](https://durgical.com/en/conditions/intracerebral-haemorrhage/)
- [Subarachnoid haemorrhage](https://durgical.com/en/conditions/subarachnoid-haemorrhage/)

[← All of radiology](https://durgical.com/en/radiology/)

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: [Intracranial Hemorrhage Overview](https://www.ncbi.nlm.nih.gov/books/NBK470242/) (StatPearls (NCBI Bookshelf), 2024); [Traumatic Brain Injury](https://www.ncbi.nlm.nih.gov/books/NBK557861/) (StatPearls (NCBI Bookshelf), 2025); [Stroke Imaging](https://www.ncbi.nlm.nih.gov/books/NBK546635/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/reading-a-head-ct-step-by-step/