Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
CT and MRI in acute neurology
Neurology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
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Time-critical presentation
Imaging must not delay airway stabilisation, glucose correction or seizure treatment, but suspected stroke, subarachnoid haemorrhage, intracranial bleeding, raised intracranial pressure, acute cord or cauda equina compression and deteriorating head injury require immediate senior-led imaging through the relevant pathway; a normal early scan never overrides clinical deterioration.
Purpose and principles
Choose and interpret emergency neuroimaging according to the clinical decision, understanding what non-contrast CT, angiography, perfusion and MRI sequences can and cannot establish.
Acute neuroimaging is a sequence of decisions. Non-contrast CT may first separate haemorrhage from ischaemia and detect mass effect; angiography then identifies treatable occlusion or aneurysm; perfusion may estimate salvageable tissue; MRI may resolve small infarction, inflammation, tumour or cord disease. The right order depends on onset, stability, local resources and what treatment changes now. More sequences are not automatically safer if they delay reperfusion or require unsafe transfer.
Interpreting a CT for acute stroke includes haemorrhage, early ischaemic change, established infarction, hyperdense artery, mass effect and alternative pathology. Clinical deficit may precede visible change. A small brainstem infarct can be obscured by posterior-fossa artefact, and a normal CT should not convert continuous focal symptoms into a benign diagnosis. CT angiography findings also need clinical correlation because chronic stenosis or occlusion may not explain the current event.
MRI signal reflects tissue properties and sequence design. Restricted diffusion with low ADC supports acute cytotoxic oedema, but seizure, abscess and other lesions can restrict. FLAIR timing may aid selected wake-up stroke decisions; susceptibility can show haemorrhage or thrombus; contrast enhancement can indicate disrupted blood–brain barrier but is non-specific. Radiological age estimates are probabilistic and must be reconciled with last known well.
Key points
Order an examination to answer a decision, not merely 'CT head'. State onset, deficit, trauma, cancer, infection, anticoagulation, pregnancy, renal function, implants and the suspected anatomical level.
Non-contrast CT is fast and excellent for acute blood, skull injury, hydrocephalus and major mass effect, but early ischaemia, posterior-fossa disease, encephalitis and diffuse axonal injury may be subtle or absent.
CT angiography maps arterial lumen and is central to detecting large-vessel occlusion, aneurysm and some dissections; its urgency is driven by reperfusion or haemorrhage decisions.
CT perfusion estimates tissue-level haemodynamics and can extend or refine thrombectomy selection in specified stroke pathways; motion and technical factors can create misleading maps.
MRI diffusion-weighted imaging is highly sensitive to acute infarction but is not infallible, particularly very early or in small posterior-fossa lesions. Match a negative study against the syndrome.
FLAIR highlights oedema, demyelination and many inflammatory lesions; susceptibility-sensitive sequences reveal blood products and microbleeds; T1, T2 and post-contrast images add anatomical and barrier information.
MRI is not simply 'better CT'. It takes longer, has access and monitoring constraints and requires device, metal, pregnancy, claustrophobia and physiological-safety screening.
Indications, selection and cautions
Acute intracranial haemorrhageRed flag
Fresh blood is usually hyperdense on non-contrast CT, with location suggesting intraparenchymal, subarachnoid, subdural or extradural compartments. Density evolves with time and severe anaemia can reduce conspicuity.
Early ischaemic strokeRed flag
Loss of grey–white differentiation, insular-ribbon change, sulcal effacement or a hyperdense artery may appear early, but absence of these signs does not exclude clinically significant acute ischaemia.
Large-vessel occlusionRed flag
CTA can show an abrupt arterial cutoff with distal reduced opacification and collateral patterns. Clinical severity and occlusion location inform immediate thrombectomy discussion; image acquisition must not become a reporting bottleneck.
Raised-pressure or mass-effect patternRed flag
Sulcal or ventricular effacement, midline shift, basal-cistern compression, obstructive hydrocephalus and herniation signs require urgent neurosurgical and airway assessment, whether caused by haemorrhage, tumour, infarct or infection.
Restricted diffusion
High signal on diffusion with corresponding low ADC often identifies acute infarction, but abscess, seizure-related change and cellular tumour can also restrict. Distribution and clinical timing determine meaning.
Inflammatory or demyelinating pattern
T2 and FLAIR lesions may affect white matter, cortex, limbic structures, brainstem or cord, with variable enhancement. Pattern guides differential but CSF, serology, tempo and specialist interpretation remain necessary.
Acute cord compression
Method and interpretation
01Non-contrast CT headFirst step
Why
Rapidly identify acute blood, mass effect, hydrocephalus, major infarction, fracture and some stroke mimics.
Interpretation and limitations
Sensitivity varies by pathology and time. Document symptom onset and anticoagulation, and use an early normal result only for the decision it can support—not as universal exclusion of neurological disease.
02CT angiography of head and neck
Why
Identify intracranial or cervical occlusion, stenosis, aneurysm and selected dissection within urgent vascular pathways.
Interpretation and limitations
Coverage and phases matter. Calcification, slow flow and motion can complicate interpretation; the stroke or neurosurgical team relates anatomy to deficit and intervention eligibility.
03CT perfusion
Why
Estimate infarct core and potentially salvageable hypoperfused tissue in defined reperfusion-selection scenarios.
Interpretation and limitations
Outputs depend on acquisition and algorithms and can be distorted by motion, poor cardiac output or chronic vascular delay. Apply local validated thresholds and specialist review.
04MRI brain with diffusion, ADC, FLAIR and susceptibility sequences
Why
Detect small or early infarction, haemorrhagic products and inflammatory, neoplastic or seizure-related abnormalities.
Interpretation and limitations
Sequence combinations distinguish true restriction from T2 shine-through and age lesions. A normal diffusion study reduces but does not eliminate acute ischaemia when timing and posterior-fossa syndrome remain compelling.
Clinical next steps
01Suspected acute strokeImage for reperfusion decisionsFirst stepSudden disabling focal deficit or posterior-circulation syndrome within a potential thrombolysis or thrombectomy pathway.+
1Activate the stroke team, establish last known well, glucose, deficit severity, anticoagulation and baseline function and proceed to urgent non-contrast CT without avoidable ward-based delay.
2Acquire CTA and perfusion or MRI according to local pathway, onset and thrombectomy criteria; communicate renal, contrast and device information without allowing routine tests to obstruct time-critical imaging.
3Stroke specialists integrate haemorrhage, infarct burden, vessel status, tissue imaging and clinical disability to decide reperfusion and monitoring, with repeat imaging if deterioration changes the question.
02Thunderclap headacheUse CT timing correctlyA headache reaching maximum intensity rapidly, especially with meningism, collapse, vomiting or focal dysfunction.+
Procedure and medicine safety
Opacifies vessels and enhancing tissues for angiography, perfusion and selected structural CT examinations that directly influence emergency treatment.
Iodinated CT contrast medium
Radiology selects the agent, concentration, injection rate and volume for the protocol, adjusting the plan to body habitus, vascular timing, kidney function and immediate diagnostic need.
Check previous hypersensitivity, renal context and interacting clinical risks under local policy; maintain emergency readiness for acute reactions and do not withhold life-saving imaging through an unbalanced theoretical-risk assessment.
Demonstrates blood–brain barrier disruption, vascularity and inflammatory, infectious, neoplastic or meningeal enhancement in selected brain and spine studies.
Gadolinium-based MRI contrast agent
Use only the radiologist-selected product and weight-based protocol when enhancement materially answers the question; record the exact agent and administered amount in the imaging record.
Review renal function, pregnancy, previous reaction and repeated exposure; follow MHRA restrictions and choose the lowest-risk effective agent because retention and nephrogenic systemic fibrosis risks differ between products.
Risks, monitoring and follow-up
Trend neurological findings during transfer and scanning; an image request does not replace airway, seizure, blood-pressure or glucose management.
After contrast, observe and treat acute hypersensitivity according to local radiology policy and document the agent and reaction accurately for future risk assessment.
Review creatinine when indicated by renal risk and the contrast protocol, while recognising that necessary emergency imaging may proceed after a benefit–risk decision.
Check the final radiology report and reconcile it with the provisional communication, documenting how incidental and discordant findings will be followed.
Repeat or change modality when symptoms progress and the initial study was early, technically limited or anatomically incomplete.
Communicate device conditions, contrast exposure and any sedation or anaesthesia at handover so post-scan monitoring and future imaging remain safe.
Special situations
Normal CT has boundaries
A normal non-contrast scan may answer 'no visible haemorrhage' while leaving acute ischaemia, seizure, encephalitis and metabolic disease unresolved. State which question was actually tested.
DWI is highly sensitive, not perfect
Very early, tiny or posterior-fossa infarcts can be diffusion-negative. Persistent, anatomically coherent deficits deserve specialist review and sometimes repeat imaging.
Ageing lesions is probabilistic
Diffusion, ADC, FLAIR, enhancement and blood-product evolution constrain timing but do not create an exact clock. Use last known well and clinical course in treatment decisions.
Implant details change access
Many pacemakers and other devices are conditionally safe at specified field strength and settings. Exact model, leads, abandoned components and local expertise matter more than a blanket label.
Posterior fossa challenges CT
Bone-related artefact and small anatomy reduce CT sensitivity around brainstem and cerebellum. Clinical posterior-circulation signs may justify CTA, MRI or repeat assessment despite a reassuring first scan.
Incidental is not explanatory
White-matter change, atrophy and old infarcts are common. They should not be used to explain abrupt symptoms unless distribution and chronology are compatible.
Common pitfalls
01
Writing 'rule out stroke' without onset, deficit or the treatment decision imaging must support.
02
Using a normal early CT to discharge a patient with persistent posterior-circulation signs.
03
Delaying CTA for routine renal blood results when the emergency stroke pathway supports immediate benefit–risk imaging.
04
Assuming MRI is always superior despite unsafe transport, monitoring limitations or reperfusion delay.
05
Treating any diffusion-bright lesion as infarction without checking ADC, distribution and seizure or abscess context.
06
Declaring all implanted devices MRI-incompatible without identifying the exact system and conditions.
07
Failing to review the final report after acting on a preliminary interpretation.
Practice
Two practice questions
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Question 1 of 2
0 correct
NeurologyOriginal SBA
Normal CT in acute stroke
A patient has persistent aphasia and right arm weakness beginning ninety minutes ago. Non-contrast CT shows no haemorrhage and no definite infarct. Which interpretation is most appropriate?
Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Safety is active rather than a tick box. Identify exact implant make and model, prior surgery, penetrating metal, infusion pumps, monitoring equipment, pregnancy, kidney function and ability to lie flat. Many contemporary devices are conditionally MRI-safe under specified settings, while unknown devices require formal investigation. In emergencies, radiology, physics, anaesthesia and the responsible specialty balance diagnostic benefit against delay and risk and select an alternative when appropriate.
Iodinated CT contrast and gadolinium-based MRI contrast have different risk profiles. The radiology team selects agent and dose after indication, allergy history, kidney function and urgency are considered.
In suspected subarachnoid haemorrhage, timing determines the value of CT and whether lumbar puncture is considered; follow NICE NG228 rather than applying one rule to every negative scan.
In head injury, use current NICE NG232 criteria for CT timing, anticoagulant and antiplatelet considerations and observation. Skull radiography is not a substitute for indicated CT.
Never delay thrombolysis or thrombectomy workflow for an elective-quality MRI when the local service uses CT-based selection, and never sedate an unstable patient for imaging without appropriate airway and monitoring expertise.
Review the actual report and images with clinical chronology, communicate unexpected deterioration to radiology and specialists and arrange repeat or alternative imaging when the first test does not answer the question.
Red flag
MRI can show extradural tumour, abscess, haematoma, disc disease and cord signal change. A clinical sensory level, bilateral signs or sphincter dysfunction determines urgency before imaging confirms anatomy.
05MRI spine with region-specific protocol
Why
Assess cord, cauda equina, epidural space, vertebrae, discs and soft tissues in compressive or inflammatory syndromes.
Interpretation and limitations
Image the level implied by examination, expanding coverage when signs are poorly localised or multifocal. Contrast decisions depend on suspected infection, tumour, inflammation and renal or pregnancy context.
06Formal MRI and contrast safety screening
Why
Identify device conditions, metallic foreign bodies, allergy history, renal risk, pregnancy and monitoring requirements before scanning.
Interpretation and limitations
A reported implant is not automatically a permanent contraindication. Obtain documentation and involve radiology or MRI physics; in unstable emergencies, choose the fastest safe modality that can answer the treatment question.
1
Stabilise and obtain urgent non-contrast CT, documenting exact onset and neurological findings; involve the appropriate emergency, stroke or neurosurgical team for positive imaging or deterioration.
2When a technically adequate CT is negative, follow NICE NG228: the need for lumbar puncture depends on whether scanning occurred within or after six hours and on ongoing clinical suspicion.
3If lumbar puncture is indicated, ensure at least twelve hours from symptom onset for bilirubin assessment and follow the receiving laboratory's sample and interpretation pathway.
03MRI-dependent emergencyCoordinate safety without avoidable delaySuspected acute cord compression, cauda equina syndrome, encephalitis, small posterior-fossa lesion or another question poorly answered by CT.+
1Define the anatomical and treatment question, stabilise physiology and notify radiology and the responsible surgical or neurological service before transport.
2Complete implant, metal, pregnancy, renal, contrast, monitoring and airway screening, obtaining exact device records and expert MRI-safety input rather than relying on patient recollection alone.
3AlternativeEscalationAgree scan coverage, contrast and sedation or anaesthetic support, use alternative imaging only when it can answer the urgent question, and escalate any wait that threatens neurological outcome.