01Purpose and principlesWhat the assessment is for and the core concepts behind it.
CTA acquires arterial-phase contrast CT data rapidly and reconstructs the intracranial circulation. In confirmed SAH it can show the aneurysm sac, neck, parent vessel, nearby branches, calcification and thrombus while the preceding non-contrast scan shows the distribution of blood, intraventricular extension, hydrocephalus and mass effect. The paired interpretation is crucial: the artery study supplies a candidate lesion, while the blood pattern tests whether that candidate plausibly ruptured.
Catheter digital subtraction angiography passes a catheter into the arterial circulation and obtains selective, high-frame-rate images after contrast injection. Rotational acquisitions clarify complex neck and branch anatomy, small perforators, collateral flow and vasospasm, and the procedure may lead directly to coiling or another endovascular treatment. These advantages come with access-site, arterial-dissection, embolic, stroke, contrast and radiation risks, so NICE reserves DSA mainly for continuing suspicion after CTA has not shown the cause.
No angiographic test is an isolated rule-out. Tiny aneurysms, adjacent bone, motion, poor contrast timing, thrombosed sacs, vessel overlap and acute vasospasm can reduce sensitivity. Conversely, incidental aneurysms are common enough that a visible lesion must be reconciled with the haemorrhage distribution. A high-quality report therefore states diagnostic confidence, relevant limitations and whether urgent DSA, repeat imaging, treatment planning or investigation of a non-aneurysmal cause is needed.
Key points
- Non-contrast CT establishes the presence and distribution of acute blood; vascular imaging answers a different question: what vessel lesion caused it and how can it be treated?
- NICE NG228 says to offer head CTA without delay after confirmed subarachnoid haemorrhage to identify the cause and guide treatment.
- An aneurysm is the likely culprit only when it fits the blood pattern; if CTA is negative but suspicion remains, consider catheter DSA, or MRA when DSA is contraindicated.
- CTA is rapid and non-invasive; DSA adds selective high-resolution dynamic anatomy and possible same-session treatment but carries catheter, embolic, contrast and radiation risks.
- DSA offers high spatial and temporal resolution, selective projections and potential progression to endovascular treatment, but it is invasive and may still need repeat or alternative imaging when vasospasm, thrombosis or technique obscures a lesion.
- Report aneurysm location, size, neck, branch incorporation, multiplicity, vasospasm and access-relevant anatomy, and link the report to an explicit next action rather than treating imaging as a binary result.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Diffuse basal cisternal blood, asymmetric clot around a named arterial territory or associated intraventricular blood may point towards a rupture site and should be compared directly with the angiographic lesion.
A lesion whose side and arterial location do not fit the clot distribution may be non-culprit; an interventional neuroradiologist and neurosurgeon should review the mismatch without delay.
A negative CTA despite convincing aneurysmal SAH may reflect a very small, thrombosed or technically obscured aneurysm and is the key situation in which NICE advises considering DSA.
Perimesencephalic blood with negative vascular imaging, convexity haemorrhage, venous thrombosis, dissection, vascular malformation and vasculopathy have different differentials and follow-up strategies.
New focal deficit, severe headache, reduced consciousness, limb ischaemia or expanding access-site swelling after DSA requires immediate assessment for embolus, dissection, rebleeding or access haemorrhage.
03Method and interpretationA systematic approach to the test and its findings.
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.
- 01
CT angiography of the headFirst step - Why
- Rapidly identify an aneurysm or other arterial cause and define anatomy for immediate specialist planning.
- Interpretation and limitations
- A positive study becomes diagnostic of aneurysmal SAH only when the lesion is anatomically compatible with the blood pattern; assess technical adequacy and multiplicity.
- 02
Catheter digital subtraction angiographyPreferred - Why
- Provide the preferred specialist selective dynamic anatomy when aneurysm suspicion persists after negative or equivocal CTA.
- Interpretation and limitations
- Selective and rotational views can reveal a small or complex culprit and may lead to same-session treatment; a negative result is interpreted with haemorrhage pattern, technical quality and specialist follow-up plan.
- 03
Non-contrast CT head - Why
- Confirm haemorrhage and map the blood distribution before attributing a lesion seen on angiography.
- Interpretation and limitations
- Blood around a compatible arterial origin supports culprit assignment; hydrocephalus, haematoma and rebleeding also change urgency and procedural planning.
- 04
Magnetic resonance angiography - Why
- Provide a non-catheter vascular alternative when DSA is contraindicated or for selected follow-up questions.
- Interpretation and limitations
- MRA avoids arterial catheterisation and ionising radiation but is slower, motion-sensitive and less compelling than CTA in the acute NICE evidence review; it cannot automatically replace urgent DSA.
- 05
Renal function and contrast assessment - Why
- Identify modifiable iodinated-contrast and procedural risks without delaying emergency decisions unnecessarily.
- Interpretation and limitations
- Renal impairment or previous contrast reaction prompts risk mitigation and specialist modality selection; the danger of an unsecured aneurysm is considered alongside contrast risk.
- 06
Post-treatment vascular imaging - Why
- Assess residual or recurrent filling, parent-vessel patency and untreated aneurysms after coiling or clipping.
- Interpretation and limitations
- Modality and timing depend on the treatment, image artefact, occlusion result, other aneurysms and whether another intervention would be appropriate.
04Clinical next stepsHow the result changes management or prompts escalation.
01Acute imaging pathwayConfirmed subarachnoid haemorrhageFirst stepNon-contrast CT or CSF analysis has confirmed subarachnoid haemorrhage and a vascular cause must be defined.+
- 1Discuss urgently with the specialist neurosurgical centre and review the non-contrast blood distribution, neurological state, renal function, allergy history and any previous aneurysm treatment.
- 2Offer CTA without delay and compare every lesion with the blood pattern; if CTA is negative but aneurysm suspicion persists, consider catheter DSA, or MRA when DSA is contraindicated.
- 3If a compatible culprit is found, transfer the images immediately for joint endovascular and neurosurgical treatment planning; if the lesion is discordant, seek both specialist opinions without assuming causality.
- 4If CTA does not show the cause but aneurysm suspicion persists, consider DSA, or MRA when DSA is contraindicated, and document who will review a negative study and whether repeat imaging is required.
02Negative-study pathwayAngiogram-negative haemorrhageInitial CTA or DSA is negative despite confirmed subarachnoid blood.+
- 1Recheck the haemorrhage pattern and image quality for motion, contrast timing, adjacent bone, thrombosis, vasospasm or vessel overlap.
- 2AlternativeDistinguish a lower-risk perimesencephalic pattern from diffuse, focal or atypical haemorrhage that preserves stronger aneurysm or alternative vascular suspicion.
- 3AlternativeAgree specialist repeat or alternative vascular imaging and investigate relevant venous, malformation, inflammatory or traumatic causes rather than offering false reassurance.
03Safety pathwayNew deficit around catheter angiographyA patient develops neurological or access-site deterioration during or after DSA.+
- 1Stop and alert the interventional team immediately, establish the time and neurological deficit, and assess airway, circulation and the arterial access site.
- 2Use immediate angiographic or cross-sectional imaging as directed to identify thromboembolism, dissection, perforation, rebleeding or another intracranial complication.
- 3Treat the complication through the specialist neurovascular pathway and document contrast, radiation, devices, anticoagulants and the final vascular result.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Continue serial consciousness, pupil, cranial-nerve and limb assessments while vascular imaging is arranged; a diagnostic test must not interrupt surveillance for rebleeding or hydrocephalus.
- During DSA monitor arterial pressure, oxygenation, heart rhythm, sedation and the access limb, with immediate comparison of any neurological change to the pre-procedure baseline.
- After arterial access, inspect the puncture site and distal perfusion according to the device and local protocol, and investigate expanding swelling or haemodynamic change urgently.
- Record contrast exposure, renal-risk mitigation, procedural complications and the certainty with which a lesion explains the haemorrhage.
- Ensure a named specialist owns the result, treatment decision and any repeat or follow-up vascular imaging; an unexplained negative study is not a completed pathway.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Blood localises the culprit
The clot distribution is a second anatomical test. It can distinguish a plausible rupture site from a coincidental aneurysm on an otherwise impressive angiogram.
DSA is dynamic
Selective injections and rapid frames show filling, branch relationships, collateral routes and vasospasm in ways that static reconstructed images may not.
Negative means conditional
A negative angiogram lowers probability only to the extent permitted by its timing, technique, haemorrhage pattern and the lesions that the modality can resolve.
Imaging serves treatment
The useful report describes neck, dome, parent artery, branches, thrombus, calcification, multiplicity and access issues because these factors shape coiling and clipping feasibility.
Follow-up is individualised
Coils, clips, residual filling, non-culprit aneurysms, artefact and the likelihood of further treatment determine the modality and interval; there is no one universal schedule.
07Common pitfallsFrequent interpretation and management errors.
- 01
Ordering CTA before recognising that non-contrast CT is required to establish acute blood and its distribution.
- 02
Calling any aneurysm on CTA the culprit without anatomical concordance between the lesion and haemorrhage pattern.
- 03
Treating a negative CTA as definitive when diffuse aneurysmal-pattern SAH still carries a high-consequence occult lesion possibility.
- 04
Requesting invasive DSA without articulating the question, contraindications, access risks or how the result will change management.
- 05
Reporting technical findings without a documented specialist owner and next action for treatment, repeat imaging or alternative diagnosis.