Neuroimaging: choosing between CT, MRI and angiography

• When a neurological problem presents suddenly, computed tomography (CT) of the head is almost always the first imaging test, because it is fast, widely available and reliable for blood and skull fracture. A normal CT in the first hours does not exclude a cerebral infarct.
• Magnetic resonance imaging (MRI) gives finer detail of brain tissue, posterior fossa, white matter and spinal cord. It is the definitive test for many conditions (for example multiple sclerosis (MS) and small strokes), but it takes longer and needs careful safety screening.
• CT angiography (CTA), MR angiography (MRA) and digital subtraction angiography (DSA) answer the question of whether a vessel is blocked, narrowed, dilated or bleeding, and are the tests behind thrombectomy decisions.
• The choice of modality rests on four questions: how ill is the patient, what are we looking for, how quickly must we know, and what does the chosen test expose them to (radiation, contrast, magnet).
• Gadolinium and iodinated contrast are useful tools but are not given to everyone; pregnancy, kidney function, allergies and metal implants all change the decision.
• Follow one of the published appropriateness guides (ACR, NICE, RCR iRefer) rather than habit: they reduce unnecessary radiation and pick up the condition at the first scan.

By the end of this page you should be able to:

1. List the strengths, weaknesses and safety constraints of CT, MRI and angiography for neurological disease.
2. State when non-enhanced CT, CTA, MRI, MR venography and DSA are indicated, and the time limits that govern stroke imaging.
3. Interpret the typical appearances of ischaemic stroke, haemorrhage, brain tumour and multiple sclerosis on CT and MRI.
4. Apply the ACR Appropriateness Criteria, NICE and RCR iRefer concepts to headache, stroke and head trauma scenarios.
5. Decide which modality is appropriate in pregnancy, renal impairment, contrast allergy and suspected metal.
6. Explain why "normal imaging early in stroke" is expected, and why a scan is sometimes harmful rather than helpful.

A scan is a measurement, not a photograph. Understanding what it counts explains almost every rule that follows.

CT measures how much an X-ray beam is weakened by each part of the body. Dense material (bone, acute blood, contrast) weakens the beam strongly and appears bright (hyperdense). Water and oedema are darker (hypodense). The brain is scanned in slices; helical (spiral) CT acquires a whole volume in seconds.

MRI does not use X-rays. It detects the behaviour of hydrogen atoms in a strong magnetic field. Differences in water content and tissue architecture create T1-weighted, T2-weighted, FLAIR (fluid-attenuated inversion recovery), diffusion-weighted and other sequences. T2/FLAIR makes oedema and many lesions bright; T1 makes anatomy clear; diffusion-weighted imaging (DWI) makes acute ischaemia bright within minutes. No ionising radiation is used, but the magnet and radio waves impose their own safety rules.

Angiography answers a different question: where the blood vessels are, and whether they are open. CTA adds iodinated contrast to a CT scan timed to fill the arteries; MRA uses MRI-based flow or contrast; DSA is an invasive X-ray study in which dye is injected through a catheter and subtracts bone and tissue to show vessels.

Before ordering, work through four questions; each one eliminates options.

1. How sick is the patient? A dynamically unwell or uncooperative patient cannot stay still for a long MRI. Sedation for MRI in acute illness is slow and risky. CT wins on speed and monitoring.
2. What are we looking for? Acute blood and fracture are CT's strength. White-matter disease, small tumours and early stroke are MRI's. Vessel status needs CTA/MRA/DSA.
3. How much time do we have? Stroke and trauma have clock-driven rules (below).
4. What can we safely expose them to? Radiation, pregnant patients, children, kidney function, contrast allergy and metal implants all shift the choice.

Strengths. CT is the backbone of emergency neurology. It reliably detects acute blood, skull fracture, hydrocephalus and midline shift, and it reports in seconds. It is the gateway test before thrombolysis because the exclusion of haemorrhage is mandatory.

Weaknesses. Early ischaemic stroke can be invisible on CT for the first hours; findings may be as subtle as loss of insular grey-white distinction or a dense middle cerebral artery sign. Bone artefact hides the posterior fossa (brainstem, cerebellum) and the brain surface. CT is insensitive to small tumours, demyelination, cortical dysplasia and subtle infection.

What attenuation implies. Acute blood is hyperdense because deoxyhaemoglobin is protein-rich. Over days to weeks it becomes isodense and later hypodense as haemoglobin breaks down. Subarachnoid blood is bright in basal cisterns and sulci. A subacute or chronic subdural haematoma is often hypodense. Calcification (for example oligodendroglioma, craniopharyngioma, healed infection) is very bright and persists.

Strengths. MRI is the right test for parenchyma: tumour characterisation, MS and other demyelinating disease, abscess, herpes encephalitis, posterior fossa disease, epilepsy and cord lesions. DWI sees acute ischaemia within minutes and stays positive for days. FLAIR shows demyelinating plaques and oedema; T2 star (GRE/SWI) is sensitive to blood products (microhaemorrhages, cavernoma). Post-contrast T1 maps blood-brain barrier breakdown (active tumour, active inflammation).

Weaknesses. MRI is slower, requires stillness, and is limited by ferromagnetic implants, pacemakers (many are now conditional), cochlear implants and severe claustrophobia. Many inner-ear and skeletal implants are safe conditionally, but the whole patient and device list must be checked against the local safety database before scanning.

Sequences in one line each.

• T1: anatomy; fat bright; CSF dark.
• T2: water bright; lesions and oedema bright.
• FLAIR: water (CSF) suppressed, so periventricular lesions stand out.
• DWI/ADC: acute cytotoxic oedema is bright on DWI and dark on ADC.
• SWI/GRE: iron and blood products are dark.

CTA is CT with a timed intravenous iodinated bolus. It shows the circle of Willis, carotid and vertebral arteries, and the veins if venous timing is chosen. It is the standard test to confirm an arterial occlusion or aneurysm before mechanical thrombectomy, and can be run from the aortic arch to the vertex in seconds.

MRA shows larger arteries and veins without radiation, and is useful for younger patients, follow-up and non-acute screening, but it is slower, less available in the acute setting, and has lower spatial resolution than CTA.

DSA (catheter angiography) remains the gold standard for detailed vascular anatomy (small aneurysms, arteriovenous shunts, vasculitis) and allows treatment (coiling, stenting, thrombectomy) in the same session. It carries the risks of arterial puncture: stroke, groin haematoma, contrast reaction.

Radiation. CT uses ionising radiation. The principle is to keep dose as low as reasonably achievable while preserving diagnostic quality, and to avoid repeats. Children and pregnant patients are more radiosensitive. Whole-head low-dose protocols and single-phase imaging are preferred where adequate. MRI uses no ionising radiation and is preferred for repeated lifetime surveillance, in young people and in pregnancy when it answers the question.

Iodinated contrast (CTA/DSA). Exercise caution in renal impairment, in people with a previous contrast reaction, and in those with a bleeding tendency when arterial access is needed. Contrast-induced kidney injury is rare in normal renal function and more likely as renal function falls; many centres now apply an individual risk assessment rather than a fixed cut-off. Reactions range from mild skin flush to anaphylaxis. Metformin is usually stopped around the time of iodinated contrast in significant renal impairment and restarted after renal function is checked (follow the local protocol).

Gadolinium (MRI). Acute adverse reactions are uncommon. The main caution is in severe kidney disease: nephrogenic systemic fibrosis is a rare fibrosing disorder linked to some gadolinium-based agents in advanced renal failure. Current macrocyclic agents have a much better safety record. Gadolinium also deposits in brain tissue after repeated doses; this is not currently known to be harmful, but the indication for every gadolinium dose should be reviewed.

Pregnancy. Avoid ionising radiation in pregnancy unless the diagnosis cannot wait; if CT is necessary, shield the fetus and use the lowest achievable dose. Iodinated contrast crosses the placenta and should be used only when necessary; gadolinium crosses as well and is avoided unless essential. MRI without contrast is safe in all trimesters and is the preferred test in pregnancy for most neurological questions.

Contraindications to MRI. Unconditional or strong contraindications include pacemakers that are not MRI-conditional, some implantable cardioverter defibrillators, cochlear implants, certain ferromagnetic cerebral aneurysm clips, metallic foreign bodies in the orbit and severe unassessed claustrophobia. Always verify the make and model in the device database and the local protocol.

1. Unknown onset or wake-up stroke: NCCT rules out haemorrhage; CTA looks for a large vessel occlusion; CT perfusion (or MRI equivalent) estimates salvageable brain. This can extend thrombectomy use beyond 6 hours when the imaging supports tissue at risk. ^[1]
2. TIA or minor non-disabling stroke: no CT for the brain itself; MRI with diffusion-weighted and blood-sensitive sequences done the same day gives the territory of ischaemia and rules out haemorrhage. ^[1]
3. Candidates for carotid endarterectomy after TIA or non-disabling stroke need urgent carotid imaging. ^[1]
4. Confirmed arterial occlusion on CTA or MRA identifies people for thrombectomy (within 6 hours, or 6 to 24 hours with limited infarct core on imaging). ^[1]

• NCCT in the first hours: often normal. Subtle findings are a hyperdense middle cerebral artery sign, loss of the insular ribbon and early hypodensity. Exclude haemorrhage before thrombolysis. ^[1][2]
• CTA: confirms arterial occlusion (for example middle cerebral artery, internal carotid artery, basilar artery).
• MRI: DWI is bright within minutes of onset and stays positive for days; apparent diffusion coefficient (ADC) is dark. FLAIR becomes positive later, and its mismatch with DWI helps estimate time. Perfused but at-risk tissue appears on perfusion imaging. ^[1]
• Large vessel occlusion is what decides thrombectomy. CTA or MRA is mandatory when intervention is possible. ^[1]

• NCCT is the first and best test: acute blood is hyperdense, often with surrounding oedema. It is immediate, available 24 hours a day, and can be done in an unstable patient.
• Intracerebral haemorrhage: a hyperdense parenchymal clot, often with mass effect and intraventricular extension. NCCT can hint at a vascular cause (peripheral, irregular shape) but CTA may reveal an underlying spot sign or aneurysm. ^[1]
• Subarachnoid haemorrhage: blood in the basal cisterns, Sylvian fissure and interhemispheric fissure. If the NCCT is normal but the story is classic, lumbar puncture is required to look for xanthochromia.
• Chronic blood turns isodense then hypodense; a subdural collection may be mixed density.
• MRI is useful later to date the bleed and to find an underlying cavernoma or tumour (gradient echo / SWI is very sensitive to prior bleeds).

• MRI with contrast is the definitive study. Tumours often appear as a mass with surrounding T2/FLAIR oedema and patchy or ring-pattern enhancement after gadolinium, depending on type.
• T1: lesions are often mildly hypointense, becoming hyperintense if they contain fat, melanin, proteinaceous fluid or subacute blood.
• T2/FLAIR: oedema appears bright; many tumours are bright.
• DWI: high cellularity (for example lymphoma, medulloblastoma) shows restricted diffusion.
• MRS, perfusion and fMRI are research or tertiary tools rather than routine.
• CT is reserved for surgical planning (calcification, bone invasion) and for patients who cannot have MRI. A calcified intracranial mass on CT is classic for oligodendroglioma.

• MRI is required for diagnosis (McDonald 2024 criteria via NICE NG220): a combination of history, examination, MRI and (where needed) CSF findings. Lesions are typically ovoid, hyperintense on T2 and FLAIR, in periventricular (often a perpendicular "Dawson fingers" pattern), juxtacortical, infratentorial and spinal cord locations. ^[4]
• Active lesions enhance with gadolinium when the blood-brain barrier is open, which is how new and old lesions are separated.
• Brain atrophy and black holes (T1 hypointense lesions) accumulate over time and are a measure of tissue loss.
• Spinal cord MRI both supports the diagnosis and helps the differential diagnosis.
• A normal MRI in a patient with a typical first clinical event does not exclude future MS; follow-up and sometimes CSF oligoclonal bands are required. ^[4]

NICE CG150 recommends not offering neuroimaging solely for reassurance in diagnosed primary headache disorders. ^[3] It lists features that demand further investigation: sudden-onset "thunderclap" headache reaching maximum intensity within 5 minutes, new focal deficit, new cognitive dysfunction, change in personality, impaired consciousness, signs of giant cell arteritis or acute narrow-angle glaucoma, recent head trauma, orthostatic or cough/exercise-triggered headache, fever with worsening headache, substantial change in headache character, and (in new-onset headache) compromised immunity, malignancy history or vomiting. ^[3]

The ACR Appropriateness Criteria Headache: 2022 Update covers eight clinical scenarios for imaging choice (from acute life-threatening to chronic benign); headache disorders rank third globally among causes of disability. ^[5] In practice:

• Acute thunderclap headache: NCCT first, to look for subarachnoid blood; if negative but the story is compelling, lumbar puncture.
• Headache with new focal deficit, papilloedema, or altered cognition: MRI (or CT if MRI is unavailable), looking for mass, haematoma or venous sinus thrombosis.
• Chronic stable headache, normal examination: no imaging.
• First cluster headache bout: discussed with a clinician of appropriate headache experience before deciding on imaging. ^[3]
• Medication overuse: no scan; manage the drug problem.

Suspected TIA. Do not offer CT to a person with a suspected TIA unless there is clinical suspicion of an alternative diagnosis that CT could detect. After specialist assessment in the TIA clinic, consider MRI with diffusion-weighted and blood-sensitive sequences on the same day, to determine the territory of ischaemia and to exclude haemorrhage or other pathology. ^[1]

Acute stroke. Perform non-enhanced CT immediately for people with suspected acute stroke if any of the following apply: indications for thrombolysis or thrombectomy; being on anticoagulant treatment; a known bleeding tendency; depressed level of consciousness (Glasgow Coma Score below 13); unexplained progressive or fluctuating symptoms; papilloedema, neck stiffness or fever; severe headache at onset. If thrombectomy might be indicated, add CTA after the non-enhanced CT and CT perfusion (or MR equivalent) if beyond 6 hours of onset. All other patients are scanned as soon as possible and within 24 hours of symptom onset. ^[1]

Intervention timing. Thrombectomy is offered within 6 hours of symptom onset, and also between 6 and 24 hours when imaging shows limited infarct core volume; posterior-circulation occlusion is considered up to 24 hours. ^[1]

Vascular imaging. People with TIA who are candidates for carotid endarterectomy should have urgent carotid imaging; a stenosis of 50 to 99% on NASCET criteria (or 70 to 99% on ECST) is the threshold for surgery. ^[1]

How common is each stroke type. Of strokes, 87% are ischaemic, 10% intracerebral haemorrhage and 3% subarachnoid haemorrhage in US data. ^[6]

CT is the primary investigation of choice for acute clinically important traumatic brain injury; MRI is reserved for additional prognostic information and is not the primary investigation, for safety and logistics reasons. ^[2]

NICE NG232 (2023) lists CT head triggers for people 16 and over (within 1 hour of the risk factor being identified): GCS 12 or less initially; GCS less than 15 at 2 hours after injury; suspected open or depressed skull fracture; any basal skull fracture sign (haemotympanum, "panda" eyes, CSF leakage from the ear or nose, Battle's sign); post-traumatic seizure; focal neurological deficit; more than one episode of vomiting. A further group, if there has been loss of consciousness or amnesia, triggers CT within 8 hours if they are 65 or over, have a bleeding or clotting disorder, a dangerous mechanism of injury (for example a fall from more than 1 metre or 5 stairs), or more than 30 minutes of retrograde amnesia. Patients taking anticoagulants or antiplatelet agents (other than aspirin alone) may need CT within 8 hours, or within 1 hour if they present more than 8 hours after injury. ^[2]

The ACR Appropriateness Criteria Head Trauma (2016) state that noncontrast head CT is the most appropriate initial examination for minor, mild, moderate and severe acute closed head injury, and that MRI is the most appropriate initial examination in subacute to chronic traumatic brain injury, with CT having a complementary role. CTA or venography, or MRA or MR venography, is the most appropriate test in suspected vascular injury; high-resolution noncontrast skull base CT is the initial test for a suspected CSF leak, with cisternography reserved for problem solving. Paediatric criteria are a separate document. ^[7]

Radiation. NICE adds that every effort should be made to minimise radiation dose to the head and cervical spine while keeping image quality adequate, and that a provisional written radiology report should be available within 1 hour of a CT scan. ^[2]

Diagnosis uses a combination of history, examination, MRI and laboratory findings, and follows the 2024 revised McDonald criteria. ^[4] MRI (brain, and where indicated spinal cord) is therefore not an optional add-on but the central investigation: it is used for diagnosis, for treatment decisions, and for monitoring disease activity. Contrast is used to reveal active lesions.

• Metal and implants: see the MRI safety rules above; in an emergency the unstable patient goes to CT first.
• Children: radiation is reduced (NICE NG232 has separate paediatric CT criteria, and the ACR has paediatric criteria separately). ^[2]
• Pregnancy: prefer MRI without contrast; CT only if essential.
• Renal impairment: used to judge contrast, not to decide whether to scan; non-contrast studies are always available.
• Claustrophobia: pre-scan briefing, prone or supine alternatives, open MRI, or CT if it answers the question.

• Thunderclap headache reaching maximum intensity within 5 minutes: exclude subarachnoid haemorrhage.
• New focal deficit, altered consciousness, papilloedema: exclude mass, bleed, venous sinus thrombosis.
• Stroke within the thrombolysis or thrombectomy window: immediate NCCT plus CTA if intervention is possible.
• Head trauma with any NICE NG232 risk factor: CT within the stated time.
• New headache in an immunocompromised person, known cancer, or with vomiting: investigate.
• Seizure with fever or focal features: investigate for infection or mass.
• Pregnant patients with neurological symptoms: prefer MRI without contrast.

Neurological history, examination and localisation; stroke and transient ischaemic attack; TIA clinic pathway; head injury assessment and early management; headache diagnosis; multiple sclerosis management; brain tumour assessment; cerebral aneurysm and subarachnoid haemorrhage.