What Does a Stroke Look Like on CT?

What is a stroke?

The brain requires a continuous supply of oxygen and glucose through the bloodstream.

If this supply is suddenly interrupted, brain cells can become injured.

The symptoms depend on which part of the brain is affected.

Possible stroke symptoms include sudden:

• —facial weakness
• —arm or leg weakness
• —numbness
• —difficulty speaking
• —difficulty understanding speech
• —visual loss
• —imbalance
• —coordination problems
• —other focal neurological symptoms.

The FAST acronym is commonly used:

• —F — Face weakness
• —A — Arm weakness
• —S — Speech difficulty
• —T — Time to call 999

Stroke is a medical emergency.

Why do we scan the brain so quickly?

The first questions in acute stroke imaging include:

• —Is there bleeding?
• —Is this likely to be an ischaemic stroke?
• —Is a major artery blocked?
• —Is there already a large area of irreversible brain injury?
• —Is the patient potentially suitable for clot-dissolving treatment or thrombectomy?
• —Could something else be causing the symptoms?

Imaging therefore helps both diagnosis and treatment selection.^[2,3]

What is a non-contrast CT brain?

The first scan is often a non-contrast CT brain.

No intravenous contrast is required for this initial part.

The scan can usually be acquired very rapidly.

Its most important early role is identifying or excluding intracranial haemorrhage and major alternative abnormalities.

Current NICE guidance recommends immediate non-enhanced CT in several groups with suspected acute stroke, including people who may be candidates for thrombolysis or thrombectomy.^[1]

What does bleeding look like on CT?

Fresh blood within the brain usually appears bright or hyperdense on a non-contrast CT scan.

This often makes acute intracranial haemorrhage easier to detect than very early ischaemic change.

A radiologist considers:

• —where the bleeding is
• —how large it is
• —whether blood has entered the ventricles
• —whether there is swelling
• —whether the brain is being compressed or shifted
• —whether there is hydrocephalus
• —whether imaging suggests an underlying cause.

CT angiography may sometimes be added to investigate blood vessels or look for features associated with continuing bleeding.^[4]

What does an ischaemic stroke look like on CT?

An ischaemic stroke evolves over time.

In the earliest stages, abnormalities can be very subtle.

Possible early signs include:

• —Loss of grey-white differentiation — normally, grey matter and white matter have slightly different appearances on CT. Early ischaemia can make this distinction less clear.
• —Loss of the insular ribbon — the cortex of the insula may become less well defined in a middle cerebral artery stroke.
• —Obscuration of the basal ganglia — structures such as the lentiform nucleus may become less distinct.
• —Sulcal effacement — swelling can cause the normal grooves on the surface of the brain to become less visible.
• —Hyperdense artery sign — occasionally, clot within an artery appears unusually dense on the non-contrast scan. For example, thrombus within the middle cerebral artery can produce a hyperdense MCA sign.

Why can the first CT look normal?

It takes time for some of the structural tissue changes caused by ischaemia to become visible on conventional CT.

A patient can therefore have a clinically significant acute ischaemic stroke with little or no obvious abnormality on a very early non-contrast CT.

This is a crucial concept.

The purpose of the first CT is not simply to look for a big dark area of stroke.

It also rapidly excludes haemorrhage and helps determine what additional imaging and treatment are needed.^[2,3]

What happens to the appearance over time?

As an ischaemic infarction evolves, the affected brain tissue typically becomes increasingly low density or darker on CT.

There may also be increasing:

• —swelling
• —sulcal effacement
• —loss of grey-white differentiation
• —local mass effect.

Large infarctions can produce significant brain swelling.

What is ASPECTS?

ASPECTS stands for: Alberta Stroke Program Early CT Score.

It is a structured way of assessing early ischaemic change within the territory of the middle cerebral artery.

The brain is divided into defined regions and points are subtracted when early ischaemic change is present.

ASPECTS can help clinicians communicate the extent of visible early infarction and contribute to treatment decisions in selected patients.

It is a professional imaging tool rather than something patients should attempt to calculate from screenshots of their scan.

What is CT angiography?

CT angiography — CTA — uses intravenous contrast to image the arteries supplying the brain.

This answers a different question from the initial non-contrast CT:

Is there an important artery blocked?

CTA may assess vessels from the neck through to the brain.

Radiologists look for occlusions involving arteries such as:

• —internal carotid artery
• —middle cerebral artery
• —anterior cerebral artery
• —basilar artery
• —posterior cerebral artery.

Identifying a large-vessel occlusion is particularly important because some patients may benefit from mechanical thrombectomy.[1–3]

What is thrombectomy?

Mechanical thrombectomy is a procedure in which specialists pass devices through the arterial system to physically remove a clot from a blocked brain artery.

It has transformed treatment for appropriately selected patients with large-vessel ischaemic stroke.

Imaging is essential in determining whether the patient may benefit.

What is CT perfusion?

CT perfusion examines how blood is flowing through the brain.

Computer-generated maps can estimate areas where:

• —blood flow is severely reduced and tissue may already be irreversibly damaged
• —blood supply is impaired but tissue may still be salvageable.

The potentially salvageable region is often referred to as the ischaemic penumbra.

NICE recommends adding CT perfusion or an MR equivalent when thrombectomy may be indicated beyond 6 hours from symptom onset.^[1]

Perfusion maps are sophisticated estimates and should not be interpreted as absolute measurements of dead versus living brain tissue in isolation.

They must be considered alongside:

• —symptoms
• —non-contrast CT
• —CT angiography
• —time from symptom onset
• —collateral circulation
• —the wider clinical picture.

What is the difference between CT and MRI in stroke?

MRI — particularly diffusion-weighted imaging — can be very sensitive for acute ischaemia.

However, CT has major practical advantages in emergency stroke pathways:

• —very fast
• —widely available
• —excellent for detecting acute haemorrhage
• —can immediately be followed by CTA
• —can include perfusion imaging
• —generally easier to perform in acutely unwell patients.

The best imaging pathway depends on the hospital and clinical situation.

Can CT tell exactly when a stroke happened?

Not precisely.

Certain appearances suggest different stages of infarction, but imaging is interpreted alongside the clinical history.

Knowing the last time the person was known to be well remains extremely important.

Can CT show a TIA?

A TIA — transient ischaemic attack — may not produce a visible abnormality on CT.

NICE specifically advises against routine CT brain imaging for suspected TIA unless there is concern for an alternative diagnosis that CT could detect.^[1]

After specialist assessment, MRI may be considered in selected patients.

Does every stroke look the same?

No.

Stroke appearance depends on:

• —which artery is affected
• —clot location
• —collateral circulation
• —duration of reduced blood supply
• —whether blood flow returns
• —patient-specific factors.

A small brainstem infarct looks very different from a large middle cerebral artery infarction.

What else can mimic a stroke?

Not everybody with sudden neurological symptoms ultimately has a stroke.

Possible mimics include:

• —seizure
• —migraine
• —low blood glucose
• —tumour
• —infection
• —functional neurological disorder
• —other neurological conditions.

Imaging helps clinicians distinguish these possibilities, although the diagnosis always requires integration with clinical assessment.^[5]

Ischaemic versus haemorrhagic stroke on CT

A simplified way to think about the initial scan is:

Haemorrhagic stroke

Fresh blood is often clearly bright on CT.

Ischaemic stroke

The scan may initially be normal or subtly abnormal, with affected tissue becoming more clearly low density over time.

This is a useful teaching concept but not a method for self-diagnosis.

Stroke CT interpretation can be highly nuanced.

Why does speed matter?

The phrase:

“Time is brain”

reflects the fact that prolonged interruption of blood supply can result in progressive brain injury.

Rapid imaging allows stroke teams to move quickly toward appropriate treatment.

This is why somebody with new FAST-positive symptoms should not:

• —wait to see whether they improve
• —drive themselves to hospital
• —arrange a routine GP appointment
• —wait for a private scan.

They should seek emergency medical attention.

What happens after the scan?

The pathway depends on what the imaging and clinical assessment show.

Treatment may include:

• —intravenous thrombolysis
• —mechanical thrombectomy
• —antiplatelet treatment
• —management of haemorrhage
• —blood-pressure management
• —neurosurgical treatment in selected cases
• —specialist stroke-unit care.

Not every treatment is appropriate for every stroke.

The key message

CT does not simply answer “stroke or no stroke”.

Modern stroke imaging can help determine:

• —Is there blood?
• —Is there early brain injury?
• —Is an artery blocked?
• —Is there brain tissue that may still be saved?

These answers can directly influence emergency treatment.