Doctor’s Passport

Find your next topic

Explore the current textbook

CardiologyRespiratoryNeurology
Available drafts · Clinical review pending
Membership
Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Textbook/Clinical imaging and interpretation/CT and MRI in acute stroke
Full textbook

CT and MRI in acute stroke

Select and interpret CT, angiographic, perfusion and MRI techniques in acute stroke so haemorrhage is excluded quickly, arterial occlusion is found and treatment is not delayed by unnecessary imaging.

26 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
Saved on this device
!
Stroke imaging is a treatment clock

A disabling focal deficit can reflect treatable arterial occlusion even when the first non-contrast CT looks normal, while unexpected haemorrhage changes the whole pathway.

Action: Activate the stroke team, establish last known well and glucose, perform immediate brain imaging when NICE criteria apply, and add CTA promptly if thrombectomy is being considered.

Open the sections you need. The overview is shown first.
01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Each modality answers a different question. Non-contrast CT asks whether there is blood or a large established lesion. CTA asks whether an artery is occluded and shows cervical and intracranial anatomy. Perfusion estimates severely injured core and tissue at risk. MRI adds diffusion, susceptibility, FLAIR and vascular sequences that can detect small infarcts and selected mimics.

Early CT signs include focal grey–white loss, insular-ribbon obscuration, lentiform-nucleus blurring, sulcal effacement and a hyperdense artery. Their absence does not rule out stroke, and their presence does not independently decide treatment. An ASPECTS score communicates extent in the anterior circulation but should be checked against clinical severity, image quality and specialist criteria.

MRI diffusion restriction reflects cytotoxic oedema and can appear within minutes, while ADC confirms reduced diffusion. FLAIR and susceptibility help with timing and blood products. Very small, early or posterior circulation infarcts can still be DWI-negative; a mismatch between a strong syndrome and imaging should prompt review, repeat imaging or an alternative sequence rather than diagnostic closure.

Key points

  • Non-contrast CT is the usual first hyperacute test because it detects haemorrhage quickly and shows large established infarction, although early ischaemic change may be absent.
  • Perform brain imaging immediately when reperfusion is considered or when anticoagulation, bleeding tendency, depressed consciousness, progressive symptoms, papilloedema, fever with neck stiffness or severe onset headache applies.
  • When none of the immediate criteria applies, NICE still recommends scanning suspected acute stroke as soon as possible and within 24 hours.
  • After initial non-contrast CT, add CTA promptly for a potential thrombectomy candidate; CTA defines occlusion and access anatomy but does not measure viable tissue directly.
  • CT perfusion or MR perfusion can support selected extended-window decisions, but thresholds are software- and protocol-dependent and never replace the clinical and vascular picture.
  • Diffusion MRI is highly sensitive for small or posterior-fossa infarction, yet treatment should not wait for MRI when CT-based reperfusion decisions can already be made.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Anterior circulation syndromeRed flag

Look for contralateral face-arm weakness, aphasia, neglect, gaze deviation or visual-field loss, then inspect the matching arterial territory and proximal vessels.

Posterior circulation syndromeRed flag

Diplopia, dysarthria, dysphagia, crossed motor-sensory signs, limb or truncal ataxia and reduced consciousness may localise behind the CT-sensitive supratentorial compartment.

Early CT change

Record subtle hypoattenuation, grey–white loss, sulcal effacement and dense vessels without equating an absent sign with absent ischaemia.

Haemorrhage exclusionRed flag

Identify parenchymal, subarachnoid, intraventricular or extra-axial blood because each redirects reperfusion, vascular investigation and specialist management.

Stroke mimic clues

Seizure, hypoglycaemia, migraine, functional symptoms, infection and mass lesions can mimic stroke; imaging narrows the differential but does not replace bedside assessment.

Technical adequacy

Review motion, coverage, contrast opacification, collateral phase and reconstruction. An uninterpretable vascular segment must be reported as limited rather than normal.

Red flags requiring action

  • Sudden aphasia, neglect, visual-field loss, gaze deviation or unilateral weakness requires immediate stroke assessment even if symptoms fluctuate or partially improve.
  • Reduced consciousness, severe headache at onset, neck stiffness, papilloedema, anticoagulant use or bleeding tendency increases the need for immediate non-contrast CT.
  • Brainstem and cerebellar signs can accompany a normal early CT; dysarthria, diplopia, crossed findings or severe truncal ataxia need posterior-circulation evaluation.
  • Neurological deterioration after initial imaging may represent infarct expansion, oedema, haemorrhagic transformation, re-occlusion or another diagnosis and mandates urgent reassessment.
  • A technically inadequate CTA, poor bolus or motion artefact cannot safely exclude large-vessel occlusion when the clinical syndrome remains convincing.
03Method and interpretationA systematic approach to the test and its findings.
Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

  1. 01
    Non-contrast CT headFirst step
    Why
    Exclude intracranial haemorrhage and assess early infarction or a competing structural diagnosis.
    Interpretation and limitations
    Normal appearances are common early. Extensive established low attenuation may alter treatment assessment, but reperfusion decisions use the whole clinical and imaging profile.
  2. 02
    CT angiography head and neck
    Why
    Locate a treatable arterial occlusion and define cervical, intracranial and access anatomy.
    Interpretation and limitations
    Check bolus timing, motion, tandem lesions and distal branches. A single-phase study may underestimate collateral flow, and an equivocal study needs expert review.
  3. 03
    CT perfusion
    Why
    Estimate core and hypoperfused tissue in selected late-window or uncertain-onset pathways.
    Interpretation and limitations
    Motion, chronic infarction, seizure, delay and software choice can distort maps. Inspect raw curves and source images before accepting automated volumes.
  4. 04
    MRI diffusion and ADC
    Why
    Confirm small, early or posterior-fossa infarction and clarify selected stroke mimics.
    Interpretation and limitations
    True restricted diffusion is bright on DWI with low ADC, but seizure, abscess and other lesions can restrict. A negative early DWI does not universally exclude stroke.
  5. 05
    MRI FLAIR and susceptibility
    Why
    Assess established parenchymal change, oedema, haemorrhage and selected unknown-onset mismatch.
    Interpretation and limitations
    FLAIR may lag diffusion early, while susceptibility is sensitive to blood products and vessels but can bloom and exaggerate size.
  6. 06
    Repeat brain and vascular imaging
    Why
    Investigate deterioration, re-occlusion, haemorrhagic transformation or a discordant initial study.
    Interpretation and limitations
    Choose CT for rapid blood and mass-effect assessment or MRI for unresolved small infarction; repeat because the question changed, not by schedule alone.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked reperfusion pathwayDisabling deficit inside the treatment windowFirst stepA patient arrives soon after onset with aphasia and hemiparesis.
  1. 1Activate the stroke pathway, document last known well, deficit severity, glucose and relevant anticoagulants while stabilising physiology.
  2. 2Perform immediate non-contrast CT and proceed directly to CTA if thrombectomy is being considered.
  3. 3Use perfusion or MRI only when required by the selected time-window pathway and when acquisition will not introduce avoidable delay.
  4. 4Communicate occlusion, infarct extent, haemorrhage and technical limitations directly to the treating stroke team.
02Posterior-fossa pathwayPersistent brainstem signs despite negative CTThe first CT is normal but focal posterior circulation findings continue.
  1. 1Keep stroke as a working diagnosis and obtain appropriate CTA rather than reclassifying the syndrome from the CT alone.
  2. 2Arrange diffusion MRI when it can resolve small posterior-fossa infarction and review image quality with neuroradiology.
  3. 3AlternativeIf initial MRI is negative but convincing deficits persist, continue observation and specialist-led repeat or alternative imaging.
03Deterioration pathwayWorsening after initial stroke imagingConsciousness or focal deficit worsens during acute stroke care.
  1. 1Repeat ABC assessment, glucose, pupils and neurological score while alerting stroke and critical-care clinicians.
  2. 2Obtain urgent non-contrast CT to assess haemorrhage, oedema, hydrocephalus or another complication, adding vascular imaging when re-occlusion is plausible.
  3. 3EscalationCompare all studies side by side and base escalation on clinical trajectory plus the new finding rather than an isolated automated score.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
  • Record deficit and consciousness serially with the same scale, while documenting sedation, seizure and metabolic factors that can alter examination.
  • Track acquisition, report and treatment decision times because unnecessary imaging transitions can consume clinically meaningful minutes.
  • Reassess after reperfusion treatment for sudden headache, vomiting, hypertension or neurological decline and obtain urgent CT when haemorrhage is suspected.
  • Check the final neuroradiology report against provisional treatment decisions, especially when distal occlusion or subtle posterior-fossa infarction was initially uncertain.
  • Document responsibility for repeat imaging and follow-up of incidental vascular or structural findings discovered during the stroke work-up.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

CT and MRI answer different questions

Greater lesion sensitivity does not automatically make MRI the better first test when CT can answer the immediate treatment decision faster.

ASPECTS is communication

A semiquantitative score helps describe early anterior-circulation change but cannot replace lesion location, clinical severity and treatment criteria.

Perfusion is model-dependent

Automated core and penumbra volumes vary with thresholds, delay correction and artefact; source data and the vascular study remain indispensable.

DWI can be initially negative

Small brainstem strokes and very early lesions are recognised exceptions, so persistent focal signs justify continued specialist assessment.

Haemorrhage can evolve

A scan that safely supported initial treatment does not exclude later haemorrhagic transformation when symptoms or physiology change.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Waiting for visible CT hypoattenuation before activating reperfusion assessment in a patient with a disabling acute deficit.

  2. 02

    Omitting CTA in a plausible thrombectomy candidate because the non-contrast images appear nearly normal.

  3. 03

    Treating automated perfusion numbers as ground truth despite motion, poor bolus or chronic infarction.

  4. 04

    Using normal early CT to dismiss a posterior-circulation syndrome without vascular imaging or diffusion MRI.

  5. 05

    Allowing a technically attractive MRI sequence to delay a decision already supported by faster CT-based imaging.

Practice

Two practice questions

Question 1 of 20 correct
Clinical imaging and interpretationOriginal SBA

Imaging before thrombectomy assessment

A 72-year-old presents 90 minutes after sudden aphasia and dense right hemiparesis. What imaging sequence best supports immediate reperfusion assessment without avoidable delay?

Sources and review status4 sources · checked 13 Sept 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.

  • NICE NG128 stroke and TIA recommendationsPublished May 2019; acute stroke and ICH recommendations, including imaging sequence, read in the current body on 13 September 2026. Applies to people over 16 and does not define paediatric pathways. Applied specifically to CT and MRI in acute stroke.
  • NICE NG228 aneurysmal subarachnoid haemorrhage recommendationsPublished 23 November 2022; recommendations 1.1.1–1.1.23 read for CT, timing-dependent negative scans, lumbar puncture, spectrophotometry and specialist transfer. The six-hour route assumes accurate onset, adequate imaging and radiologist reporting. Applied specifically to CT and MRI in acute stroke.
  • ESO/EANS 2025 guideline on spontaneous intracerebral haemorrhagePublished 22 May 2025; diagnostic-cause imaging, prognostic-score, blood-pressure, haemostatic, hydrocephalus and surgery sections read. European adult spontaneous ICH guidance; many recommendations are weak and do not govern traumatic haemorrhage. Applied specifically to CT and MRI in acute stroke.
  • NICE NG232 head injury recommendationsPublished 18 May 2023; recommendations on acute assessment, CT criteria, neurosurgical discussion and neurological deterioration read 13 September 2026. This source governs traumatic head injury across ages and was not generalised to spontaneous disease. Applied specifically to CT and MRI in acute stroke.
Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom