Neuroimaging in Neurological Emergencies, CT Versus MRI
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Direct answer
Neuroimaging in neurological emergencies starts with non-contrast CT because it is fast and reliably shows acute haemorrhage, fracture and mass effect. MRI is preferred when the question is early ischaemia, encephalitis or spinal cord compression. In acute stroke, CT excludes bleeding before thrombolysis, and diffusion-weighted MRI detects early infarction more sensitively.
What you must remember
- First scan in trauma and bleeding: non-contrast CT is the fastest and most sensitive test for acute blood, fracture and hydrocephalus, so it is the first scan in head injury.
- Acute stroke: non-contrast CT excludes haemorrhage before thrombolysis, then CT angiography and perfusion imaging guide thrombectomy, with diffusion-weighted MRI more sensitive for early infarction.
- Hyperdense MCA sign: a bright middle cerebral artery on non-contrast CT suggests a thrombus and an acute ischaemic stroke.
- Subarachnoid haemorrhage: non-contrast CT within about six hours is highly sensitive, and a negative scan with a suspicious history needs lumbar puncture for xanthochromia, usually after about twelve hours.
- Encephalitis: MRI shows temporal and insular lesions more clearly than CT, and CT is usually done first to exclude a mass before lumbar puncture.
- Cord compression: urgent MRI of the whole spine with gadolinium is the investigation of choice in suspected metastatic cord compression, with CT myelography where MRI cannot be done.
- Extradural versus subdural: an extradural haematoma is biconvex and lens-shaped and does not cross sutures, while a subdural haematoma is crescentic and crosses sutures.
- Practical choice: CT is quicker, needs less monitoring and tolerates implants, whereas MRI suits stable patients when ischaemia, infection or cord disease is the question.
Choosing the first scan
Emergencies force a choice between speed and detail, and the first scan should answer the question that changes management in the next hour. The table sets out the usual first scan for common emergencies and the key sign to look for.
| Emergency | First scan | Key sign |
|---|---|---|
| Acute stroke | Non-contrast CT, then CT angiography | Hyperdense MCA sign, early loss of grey-white differentiation |
| Subarachnoid haemorrhage | Non-contrast CT | Blood in the basal cisterns and fissures |
| Encephalitis | CT, then contrast MRI | Temporal lobe and insular signal change |
| Metastatic cord compression | Urgent MRI of the whole spine | Epidural mass with cord compression |
| Head injury | Non-contrast CT | Lens-shaped extradural collection, fracture |
The table shows CT answering the bleeding and bony questions and MRI answering the tissue questions. Speed and safety decide the first scan, and MRI follows when CT is inconclusive.
Worked example: a 68-year-old man develops sudden right hemiplegia and aphasia two hours after onset. Non-contrast CT shows a hyperdense left middle cerebral artery and no bleed, so thrombolysis is considered, and CT angiography looks for a large vessel occlusion suitable for thrombectomy.
Where the examiner tests the choice
Questions describe an acute presentation and ask for the investigation of choice or the sign.
- Trap: Ordering MRI first in a suspected haemorrhagic stroke. Correct: Non-contrast CT comes first, because it excludes bleeding before thrombolysis and is quicker to perform.
- Trap: Treating a normal early CT as excluding ischaemic stroke. Correct: Early ischaemia often shows nothing on CT, so diffusion-weighted MRI is more sensitive in the first hours.
- Trap: Dismissing subarachnoid haemorrhage after a normal late CT. Correct: A normal CT within about six hours is reassuring, but a later or equivocal scan needs lumbar puncture for xanthochromia.
- Trap: Starting with CT of the spine in suspected metastatic cord compression. Correct: MRI of the whole spine with gadolinium is preferred, since it shows multiple levels and the extent of the lesion.
Frequently asked questions
Why is CT preferred in suspected acute stroke?
CT is preferred in suspected acute stroke because it is fast and detects intracerebral haemorrhage, which must be excluded before thrombolysis. Non-contrast CT also shows early signs such as a hyperdense middle cerebral artery. CT angiography and perfusion imaging then guide decisions about thrombectomy.
What is the hyperdense middle cerebral artery sign?
The hyperdense middle cerebral artery sign is a bright, dense artery on non-contrast CT that suggests thrombus in the middle cerebral artery. It is an early sign of acute ischaemic stroke, seen within hours of onset. The sign should prompt urgent vascular imaging and review for treatment.
When is MRI preferred over CT in neurological emergencies?
MRI is preferred over CT for encephalitis, early ischaemic stroke, posterior fossa lesions and spinal cord compression, because it shows tissue detail that CT can miss. Diffusion-weighted imaging detects early infarction, and gadolinium-enhanced MRI shows the extent of cord compression. CT remains the first scan when speed, monitoring or haemorrhage is the main concern.
How does CT distinguish extradural from subdural haematoma?
CT distinguishes extradural from subdural haematoma by the shape and boundaries of the collection. An extradural haematoma is biconvex and lens-shaped, and it does not cross the skull sutures. A subdural haematoma is crescentic and crosses sutures, so the two are told apart on a single scan.