Synopsis
Choose and interpret emergency neuroimaging according to the clinical decision, understanding what non-contrast CT, angiography, perfusion and MRI sequences can and cannot establish.
- 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.
- 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.
Key red flags
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.
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.
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.
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.
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.
Investigation priorities
Rapidly identify acute blood, mass effect, hydrocephalus, major infarction, fracture and some stroke mimics.
Management branches
Sudden disabling focal deficit or posterior-circulation syndrome within a potential thrombolysis or thrombectomy pathway.
- Activate 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.
- Acquire 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.
- Stroke 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.