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Imaging in acute stroke: CT first, then CT angiography

  • Finding

In a sudden stroke, a fast brain scan shows whether it is a bleed or a blocked artery, and which treatment can still help.

Updated

What it is

In suspected acute stroke, imaging answers three questions in order. First, is there bleeding? Non-contrast CT excludes haemorrhage and some mimics, and is required before thrombolysis. Second, is a large artery blocked? CT angiography from the aortic arch to the vertex shows occlusion of the internal carotid, the first segment of the middle cerebral artery or the basilar artery, which makes the patient a candidate for mechanical thrombectomy. Third, how much brain is already lost and how much can still be saved? Early CT signs (scored with ASPECTS), CT perfusion or MRI diffusion separate the irreversible core from the at-risk penumbra. CT is often normal in the first hours of an ischaemic stroke, so a normal scan never excludes it. Imaging must be fast: time from arrival to scan is a quality measure.

What it looks like

  • Hyperdense artery sign: the clot makes a segment of the middle cerebral artery look bright (the «dot sign» when seen end-on in the sylvian fissure).
  • Loss of grey-white differentiation: the cortex and the lentiform nucleus blur into the white matter.
  • Insular ribbon sign: the bright insular cortex disappears in middle cerebral artery infarction.
  • Sulcal effacement and subtle swelling of the affected territory.
  • Hours to days later, the infarct becomes a clear dark area that follows an arterial territory and involves both cortex and white matter.
  • On MRI, an acute infarct is bright on diffusion (DWI) and dark on the ADC map within minutes of onset.

Why it looks that way

  • Ischaemic cells take up water (cytotoxic oedema), which lowers CT density and restricts water movement on diffusion MRI.
  • A fresh clot is dense, so the blocked artery looks brighter than the other side.
  • ASPECTS divides the middle cerebral artery territory into ten regions and subtracts one for each with early change: ten is normal, a low score means a large infarct.
  • CT perfusion maps blood flow and transit time: low blood volume marks the core; delayed flow with preserved volume marks the penumbra.

Differential and pitfalls

  • A normal CT does not exclude ischaemic stroke, especially in the first hours and in the posterior fossa.
  • Mimics: hypoglycaemia (check glucose first), seizure with post-ictal weakness, migraine, tumour, subdural haematoma, abscess.
  • A bright artery can be a false positive with high haematocrit or vessel-wall calcification: compare both sides.
  • Old infarcts are dark as CSF with volume loss; acute ones are less dark and swollen.
  • Haemorrhagic transformation can appear in the days after an infarct, with or without treatment.
  • Large middle cerebral or cerebellar infarcts swell over the first days and can herniate or compress the fourth ventricle.

Next step

  • Activate the stroke code: note the time last known well, check glucose and send the patient for CT plus CT angiography in one trip.
  • Do not delay CT angiography for a creatinine result in suspected large-vessel occlusion unless kidney disease is known.
  • Bleed excluded and within about 4.5 hours of last known well: the stroke team decides on thrombolysis.
  • Large-vessel occlusion: thrombectomy is considered up to six hours, and in selected patients up to 24 hours with perfusion or MRI showing salvageable brain.
  • Unknown onset (wake-up stroke) or posterior symptoms with normal CT: ask for MRI with diffusion and FLAIR.
  • After treatment or if the patient worsens: repeat CT to look for bleeding or swelling.

Test yourself

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

What is the main purpose of the first non-contrast CT in suspected acute stroke?

To exclude haemorrhage (and some mimics) before reperfusion treatment; it is often normal in early ischaemic stroke.

What is the hyperdense middle cerebral artery sign?

A fresh clot that makes a segment of the middle cerebral artery look brighter than the opposite side on non-contrast CT.

What does a low ASPECTS score mean?

Early ischaemic change in many regions of the middle cerebral artery territory, that is, a large infarct core and less benefit from treatment.

Which imaging test is most sensitive for an acute ischaemic infarct?

MRI diffusion-weighted imaging: the infarct is bright on DWI and dark on the ADC map within minutes.

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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: Stroke Imaging (StatPearls (NCBI Bookshelf), 2026); Acute Stroke: Evaluation and Management (StatPearls (NCBI Bookshelf), 2026); Ischemic Stroke (StatPearls (NCBI Bookshelf), 2026).

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