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Textbook/Clinical imaging and interpretation/Non-contrast CT in acute neurological disease
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Non-contrast CT in acute neurological disease

Use non-contrast head CT safely in acute neurological presentations, recognise major patterns and limitations, and continue urgent diagnostic pathways when a negative scan cannot exclude the suspected disease.

26 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
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Imaging must not interrupt neurological resuscitation

Falling consciousness, new focal deficit, seizure, unequal pupils or a sudden severe headache can represent haemorrhage, large-vessel stroke, hydrocephalus or herniation even before CT is available.

Action: Stabilise airway, breathing and circulation, record glucose and neurological findings, activate the relevant stroke, trauma or neurosurgical pathway, and obtain urgent non-contrast CT when transport is safe.

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

CT assigns a number to x-ray attenuation and displays it through chosen window settings. Air is dark, CSF is low attenuation, brain is intermediate, acute clot is commonly bright, and calcium or bone is very bright. Those relative appearances change with acquisition, reconstruction, windowing, haemoglobin and time, so comparison with anatomy and prior images is safer than a single density label.

The initial question is often exclusionary: is there blood, mass effect, hydrocephalus or a large established infarct that changes immediate treatment? A scan may answer that question while remaining unable to exclude small infarction, cerebral venous thrombosis, encephalitis, meningitis or subtle posterior-fossa disease. The report and clinical plan should state what remains unresolved.

Pattern recognition must lead to action. Extra-axial blood, basal cisternal blood, asymmetric low attenuation, compressed ventricles or obliterated cisterns demand different vascular and specialist pathways. Image interpretation should be integrated with onset time, trauma, anticoagulants, fever, malignancy, pregnancy and examination rather than used as a stand-alone diagnosis.

Key points

  • Non-contrast CT is fast and excellent for acute haemorrhage, hydrocephalus, skull injury and substantial mass effect; it is less sensitive for early ischaemia and some posterior-fossa lesions.
  • Review systematically: technical adequacy, extra-axial spaces, cisterns and sulci, ventricles, parenchymal symmetry and grey–white differentiation, posterior fossa, skull and extracranial tissues.
  • Fresh blood is usually hyperattenuating, but anaemia, very small volume, lesion age and artefact can alter conspicuity; describe compartment and mass effect before inferring cause.
  • Loss of the insular ribbon, obscured lentiform nucleus, sulcal effacement and a dense artery can support early ischaemia, yet a normal scan never ends a time-critical stroke pathway.
  • For suspected SAH, a radiologist-reported negative CT within six hours may avoid routine LP; after six hours, consider LP and wait at least 12 hours from onset for spectrophotometry.
  • Repeat or escalate imaging when neurology deteriorates, the first study is technically limited, or the modality cannot answer the persisting high-risk clinical question.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Acute haemorrhageRed flag

Search sulci, basal cisterns, fissures, ventricles, brain parenchyma and extra-axial spaces. State side, compartment, volume impression, ventricular extension and associated oedema or shift.

Early ischaemiaRed flag

Compare hemispheres for grey–white loss, focal hypoattenuation, sulcal effacement and a dense artery. Small or very early infarcts may remain invisible, especially in the posterior fossa.

Mass effectRed flag

Assess sulci, ventricles, basal cisterns, midline position and herniation corridors. Millimetres of shift alone do not determine clinical severity; lesion location and trajectory matter.

Hydrocephalus

Judge ventricular size against age and prior imaging, then look for transependymal CSF seepage and an obstructing lesion. Acute ventricular enlargement with declining consciousness is an emergency.

Technical limitation

Check motion, coverage, reconstruction thickness and window settings. Beam hardening near the skull base can conceal posterior-fossa haemorrhage or infarction and should be named explicitly.

Negative scan boundaryRed flag

Define the disease, timing and image quality before calling CT reassuring. A negative result is meaningful only for the question and population in which its performance is known.

Red flags requiring action

  • A fall in GCS, progressive focal deficit, new seizure or pupil asymmetry requires immediate reassessment and urgent imaging or repeat imaging despite an earlier reassuring scan.
  • Thunderclap headache with neck stiffness, vomiting, collapse or exertional onset remains concerning for subarachnoid haemorrhage even with a normal examination.
  • Anticoagulant exposure, bleeding tendency and recent trauma lower the threshold for urgent CT but do not identify the lesion or replace drug-specific haemostasis planning.
  • A normal early CT does not exclude acute ischaemia; for suspected SAH, a negative radiologist-reported CT within six hours may avoid routine LP, while a later negative scan can require LP at least 12 hours after onset.
  • Posterior-fossa and skull-base disease may be obscured by beam-hardening artefact; persistent brainstem or cerebellar signs need a more sensitive pathway.
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
    Identify acute blood, major mass effect, hydrocephalus, fracture and established infarction rapidly.
    Interpretation and limitations
    A negative study does not universally exclude early infarction or SAH. Review time from onset, technical adequacy, haemoglobin, artefact and the suspected compartment.
  2. 02
    CT angiography
    Why
    Identify arterial occlusion, aneurysm or vascular lesion when the presentation and initial CT require vascular definition.
    Interpretation and limitations
    CTA adds contrast and timing dependence. It follows the relevant stroke, SAH or ICH pathway and should not be ordered as a generic substitute for clinical formulation.
  3. 03
    CT perfusion
    Why
    Estimate infarct core and hypoperfused tissue in selected reperfusion pathways.
    Interpretation and limitations
    Maps depend on acquisition and software thresholds and can be distorted by motion, delay or poor cardiac output; interpret with symptoms, non-contrast CT and CTA.
  4. 04
    MRI brain with diffusion sequences
    Why
    Detect small, early or posterior-fossa infarction and characterise lesions unresolved by CT.
    Interpretation and limitations
    Diffusion restriction is sensitive but not perfectly specific, and occasional very early strokes are DWI-negative. MRI must not create avoidable reperfusion delay.
  5. 05
    Lumbar puncture after negative CT for suspected SAH
    Why
    Detect bilirubin when later CT has not resolved a high-probability SAH presentation.
    Interpretation and limitations
    Consider after a negative scan performed more than six hours from onset; wait at least 12 hours, confirm procedural safety and use spectrophotometry.
  6. 06
    Repeat CT head
    Why
    Detect evolution after neurological deterioration or reassess a study limited by motion or timing.
    Interpretation and limitations
    Repeat urgently for a new deficit, GCS decline or other defined change. Routine repetition without a clinical question adds radiation without resolving uncertainty.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked acute pathwayThunderclap headache with a negative scanFirst stepAn adult presents after sudden maximal-at-onset headache and initial non-contrast CT shows no blood.
  1. 1Confirm exact onset, neurological state, scan timing and technical adequacy, while maintaining urgent SAH assessment.
  2. 2If CT was obtained and radiologist-reported within six hours, use the NG228 pathway rather than adding routine LP automatically.
  3. 3If imaging occurred after six hours and suspicion persists, consider LP when safe and at least 12 hours after onset, with bilirubin spectrophotometry.
  4. 4EscalationEscalate to specialist and vascular imaging routes when blood or an aneurysmal pattern is demonstrated.
02Stroke pathwayAcute focal deficit with no haemorrhageNon-contrast CT shows no acute blood in a patient with a disabling focal deficit.
  1. 1Keep the stroke call active and assess reperfusion eligibility from last-known-well time, deficit, premorbid state and contraindications.
  2. 2Obtain CTA promptly when thrombectomy is a possibility and use perfusion or MRI only where the pathway requires it.
  3. 3Do not wait for a mature low-attenuation infarct before making time-critical treatment decisions.
03Deterioration pathwayEarlier reassuring CT but worsening neurologyConsciousness or focal signs worsen after an initially normal or minor CT finding.
  1. 1Repeat ABC assessment, glucose, pupils, GCS and limb findings and call senior neurological support immediately.
  2. 2Re-review the first images and report for timing, posterior-fossa coverage and other stated limits.
  3. 3AlternativeArrange urgent repeat or alternative imaging based on the suspected complication while treating life-threatening physiology.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
  • Timestamp GCS components, pupils, limb power, speech and vital signs so a change can be distinguished from sedation, fatigue or documentation variation.
  • After a negative or limited study, document the residual differential and the explicit trigger for CTA, MRI, LP or repeat CT.
  • Track contrast exposure, renal context and allergies when CTA or perfusion is added, but do not delay life-saving emergency imaging for routine laboratory results.
  • Ensure urgent and unexpected findings are communicated directly to the responsible clinical team with acknowledgement and an agreed next action.
  • Review final reports and addenda against treatment already given; a provisional interpretation should not become the unexamined endpoint.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

Windows alter visibility

Brain, subdural and bone windows emphasise different attenuation ranges; a systematic review uses each to avoid missing thin extra-axial blood or fracture.

Density evolves

Clot generally becomes less conspicuous over time, and profound anaemia can reduce the brightness of acute blood, so pattern and context remain essential.

Posterior fossa is difficult

Dense skull-base bone creates artefact and small brainstem infarcts may be occult; MRI is often the problem-solving modality when signs persist.

Negative means question-specific

A scan can exclude a large haemorrhage yet fail to exclude a small infarct, venous thrombosis, meningitis or later-presenting subarachnoid haemorrhage.

Comparison adds trajectory

Prior imaging helps separate chronic ventriculomegaly, calcification and old infarction from new oedema, haemorrhage or developing hydrocephalus.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Calling an early normal CT a rule-out test for acute ischaemic stroke and allowing the reperfusion pathway to stop.

  2. 02

    Applying the six-hour SAH rule without a secure onset time, adequate scan or radiologist interpretation.

  3. 03

    Ignoring posterior-fossa artefact when the clinical syndrome localises to cerebellum or brainstem.

  4. 04

    Describing blood without its compartment, ventricular extension, mass effect or likely need for vascular imaging.

  5. 05

    Repeating CT by habit while failing to define the clinical change or the unresolved question.

Practice

Two practice questions

Question 1 of 20 correct
Clinical imaging and interpretationOriginal SBA

Negative CT after late thunderclap

An adult presents 10 hours after a sudden severe thunderclap headache. A technically adequate non-contrast CT is reported as normal, but clinical suspicion of subarachnoid haemorrhage remains. What is the best next diagnostic step?

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 Non-contrast CT in acute neurological disease.
  • 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 Non-contrast CT in acute neurological disease.
  • 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 Non-contrast CT in acute neurological disease.
  • ENLS Intracranial Hypertension and Herniation Protocol version 6.0Version 6.0, updated September 2024; communication, diagnosis, Tier Zero and Tier One sections read. Multi-aetiology professional consensus for the first hours of an ICP crisis, not a UK national or disease-specific outcomes guideline. Applied specifically to Non-contrast CT in acute neurological disease.
Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom