Vascular Imaging in Stroke: CTA, Carotid Duplex, and Beyond
Vascular imaging is foundational to both acute stroke treatment and secondary prevention. In the hyperacute setting, CT angiography (CTA) identifies large vessel occlusions amenable to thrombectomy and detects tandem lesions requiring emergent intervention. For secondary prevention, imaging characterizes stroke etiology—from atherosclerotic stenosis to dissection, carotid web, and moyamoya—guiding targeted medical or surgical management. This review covers the role of CTA and carotid duplex ultrasound, with evidence-based guidance on when and how to image.
🔹 Bottom Line: Vascular Imaging in Stroke
- Acute CTA: Essential for LVO detection, collateral assessment, and tandem lesion identification in thrombectomy candidates.
- Etiology workup: CTA identifies dissection, carotid web, FMD, moyamoya, and aortic arch atheroma—often missed by other modalities.
- Carotid duplex: Non-invasive first-line for extracranial carotid stenosis; provides plaque characterization and hemodynamic data.
- Symptomatic stenosis ≥50%: CEA reduces stroke risk (NASCET NNT 6-8 for ≥70%); early intervention within 2 weeks is optimal.
- Asymptomatic stenosis: Modern medical therapy may rival revascularization in low-risk patients (ECST-2).
- Intracranial stenosis: Medical management superior to stenting (SAMMPRIS, CASSISS).
Part I: CT Angiography
CTA in Acute Stroke
CTA has become indispensable in the acute stroke workflow. Within minutes, it provides critical information for treatment decisions:
- LVO detection: Identifies occlusions of the ICA, M1/M2 MCA, basilar artery, and other targets for mechanical thrombectomy
- Collateral assessment: Multiphase CTA or delayed imaging grades pial collaterals, informing prognosis and treatment selection
- Tandem lesions: Detects concurrent extracranial ICA stenosis/occlusion requiring acute stenting or angioplasty
- Clot characteristics: Hyperdense clot, clot length, and location influence recanalization success
CTA for Stroke Etiology
Beyond acute intervention, CTA is a powerful diagnostic tool for determining stroke mechanism:
Atherosclerotic Disease
CTA quantifies stenosis degree in extracranial (carotid, vertebral) and intracranial vessels. Plaque morphology—calcified vs. soft plaque, ulceration, and surface irregularity—can be assessed, though with less detail than duplex or vessel wall MRI.
Cervical Artery Dissection
Dissection is a leading cause of stroke in young adults. CTA findings include:
- Intimal flap: Linear filling defect within the lumen
- String sign: Long, tapered stenosis
- Flame sign: Tapered occlusion at the skull base (ICA) or V3 segment (vertebral)
- Mural hematoma: Crescent-shaped wall thickening (better seen on fat-sat MRI)
- Pseudoaneurysm: Focal outpouching, often at the skull base
Treatment trials (STOP-CAD, TREAT-CAD) show no clear superiority of anticoagulation over antiplatelet therapy, though anticoagulation may benefit occlusive dissections.
Carotid Web
A carotid web is a shelf-like intraluminal projection at the posterior wall of the carotid bulb, thought to represent an intimal variant of fibromuscular dysplasia. CTA demonstrates a characteristic thin, linear filling defect best seen on sagittal reconstructions. Carotid webs are increasingly recognized as a cause of cryptogenic stroke, particularly in younger patients without traditional risk factors.
Fibromuscular Dysplasia (FMD)
FMD typically affects the mid-to-distal cervical ICA and vertebral arteries. The classic "string-of-beads" appearance results from alternating stenoses and dilations. Focal or tubular variants also occur. FMD is associated with dissection and intracranial aneurysms—screening of other vascular beds is recommended.
Moyamoya Disease
Moyamoya is characterized by progressive stenosis of the supraclinoid ICA and proximal ACA/MCA, with development of collateral "puff of smoke" vessels at the skull base. CTA shows:
- Bilateral ICA terminus stenosis/occlusion
- Prominent lenticulostriate and thalamoperforating collaterals
- Absent or diminished M1/A1 segments
DSA remains gold standard for staging. Surgical revascularization (direct or indirect bypass) is the definitive treatment.
Aortic Arch Atheroma
Complex aortic arch plaque (≥4mm thickness, ulcerated, or mobile) is a recognized embolic source. Extended CTA covering the aortic arch can identify high-risk features. The ARCH trial found no difference between antiplatelet and anticoagulation for secondary prevention, though the study was underpowered.
MR Angiography
MR angiography (MRA) complements CTA and offers distinct advantages in specific clinical scenarios. Time-of-flight (TOF) MRA provides non-contrast visualization of cervical and intracranial vessels, making it ideal for patients with contrast allergies or renal impairment. Contrast-enhanced MRA improves accuracy for extracranial stenosis and reduces flow-related artifacts that can cause overestimation of stenosis on TOF sequences.
Key applications for MRA include:
- Dissection: Fat-saturated T1-weighted sequences detect intramural hematoma with high sensitivity—often superior to CTA in the subacute phase when methemoglobin produces characteristic T1 hyperintensity
- Vessel wall imaging: High-resolution vessel wall MRI (VW-MRI) characterizes intracranial plaque burden, intraplaque hemorrhage, and enhancement (a marker of inflammation and instability)
- Moyamoya: MRA demonstrates the classic "ivy sign" on FLAIR (leptomeningeal collaterals) and progressive ICA terminus narrowing on follow-up
- Venous imaging: MR venography (MRV) for cerebral venous thrombosis evaluation
Limitations: MRA is more time-consuming, motion-sensitive, and less available in acute settings than CTA. TOF-MRA may overestimate stenosis due to turbulent flow, and metallic artifacts from stents or coils degrade image quality. For acute stroke with thrombectomy consideration, CTA remains the modality of choice.
Part II: Carotid Duplex Ultrasound
Indications and Advantages
Carotid duplex is the first-line imaging modality when the carotid artery is the vessel of interest. Key advantages include:
- Non-invasive: No radiation, no contrast
- Bedside capability: Can be performed in ICU or stroke unit
- Repeatable: Ideal for surveillance post-CEA/CAS
- Hemodynamic data: Provides velocity measurements and flow direction
- Plaque characterization: Assesses stability, ulceration, hemorrhage
ICA Stenosis Grading Criteria
The Society of Radiologists in Ultrasound (SRU) consensus criteria correlate duplex parameters with NASCET-style angiographic stenosis:
| Stenosis Grade | ICA PSV (cm/s) | ICA EDV (cm/s) | ICA/CCA PSV Ratio | Plaque Estimate |
|---|---|---|---|---|
| Normal | <125 | <40 | <2.0 | None |
| <50% | <125 | <40 | <2.0 | Visible plaque |
| 50–69% | 125–230 | 40–100 | 2.0–4.0 | Visible plaque |
| ≥70% | >230 | >100 | >4.0 | Significant plaque |
| Near-occlusion | Variable (may be low) | Variable | Variable | Severe narrowing, "string sign" |
| Occlusion | No flow detected | No flow | N/A | Complete obstruction |
PSV = peak systolic velocity; EDV = end-diastolic velocity; CCA = common carotid artery. Based on SRU Consensus Criteria (2003).
Beyond Stenosis: Plaque Characterization
Duplex provides information beyond stenosis degree that may influence risk stratification:
- Echolucent (hypoechoic) plaque: Lipid-rich, potentially unstable—associated with higher stroke risk
- Echogenic (hyperechoic) plaque: Calcified, generally more stable
- Ulcerated plaque: Surface irregularity or crater—marker of plaque rupture
- Intraplaque hemorrhage: Hypoechoic region within plaque—high-risk feature
- Plaque surface: Smooth vs. irregular—irregular surfaces carry higher embolic risk
Special Duplex Applications
Vertebral Artery Flow Direction
Reversal of vertebral artery flow indicates subclavian steal syndrome. Duplex can demonstrate retrograde flow in the ipsilateral vertebral artery with arm exercise or blood pressure differential >20 mmHg between arms.
Ophthalmic Artery Flow
In severe ICA stenosis or occlusion, reversed ophthalmic artery flow (from ECA to ICA territory) indicates collateral pathway recruitment—a marker of hemodynamic compromise and potentially higher stroke risk.
Post-Revascularization Surveillance
Duplex is standard for monitoring after CEA or CAS. Velocity criteria differ for stented arteries due to altered compliance. Restenosis >70% occurs in ~5-10% of CEA and up to 10% of CAS patients at 2 years.
Part III: Clinical Relevance
🔹 Extracranial Carotid Stenosis: What Imaging Means for Management
Symptomatic stenosis ≥50%:
- NASCET and ECST established CEA benefit—NNT 6-8 for ≥70% stenosis
- Greatest benefit when revascularization performed within 2 weeks of index event
- CREST: CAS and CEA equivalent overall; CAS has more stroke, CEA more MI; CEA preferred in patients ≥70 years
Asymptomatic stenosis ≥60-70%:
- ACAS/ACST: CEA reduced stroke risk, but NNT ~17 over 5 years
- ECST-2 (2025): No benefit of revascularization over optimized medical therapy in low-risk patients
- CREST-2 (2025): Intensive medical management alone was non-inferior to revascularization (CEA or CAS) for asymptomatic stenosis ≥70%
- Current trend: Modern medical therapy (high-intensity statin, BP control, antiplatelet) may be sufficient for many asymptomatic patients—risk stratification with imaging and clinical features is key
🔹 Intracranial Atherosclerotic Disease: Medical Therapy is Standard
- SAMMPRIS (2011): Intracranial stenting caused harm—30-day stroke/death 14.7% vs 5.8% with medical therapy
- CASSISS (2022): Confirmed no benefit of stenting over medical therapy at 3 years
- Standard of care: Aggressive medical management—DAPT for 90 days, high-intensity statin (LDL <70), BP control, risk factor modification
- Stenting reserved for refractory cases failing maximal medical therapy (limited evidence)
Practical Algorithm: When to Image What
| Clinical Scenario | Recommended Imaging | Rationale |
|---|---|---|
| Acute stroke, thrombectomy candidate | CTA head/neck (± multiphase) | LVO detection, collaterals, tandem lesions |
| Anterior circulation stroke, carotid source suspected | Carotid duplex → CTA if stenosis found | Non-invasive screening; CTA for surgical planning |
| Young stroke, suspected dissection | CTA + MRI with fat-sat T1 | CTA for stenosis/occlusion; MRI for mural hematoma |
| Cryptogenic stroke, embolic pattern | CTA arch to vertex | Carotid web, aortic atheroma, dissection |
| Intracranial stenosis suspected | CTA or MRA → vessel wall MRI if available | Stenosis quantification; plaque characterization |
| Post-CEA/CAS surveillance | Carotid duplex | Non-invasive, repeatable monitoring |
| Subclavian steal suspected | Carotid/vertebral duplex | Flow direction assessment with arm exercise |
References
- North American Symptomatic Carotid Endarterectomy Trial Collaborators. Beneficial effect of carotid endarterectomy in symptomatic patients with high-grade carotid stenosis. N Engl J Med. 1991;325:445-453.
- European Carotid Surgery Trialists' Collaborative Group. Randomised trial of endarterectomy for recently symptomatic carotid stenosis. Lancet. 1998;351:1379-1387.
- Executive Committee for ACAS. Endarterectomy for asymptomatic carotid artery stenosis. JAMA. 1995;273:1421-1428.
- Brott TG, et al. Stenting versus endarterectomy for treatment of carotid-artery stenosis (CREST). N Engl J Med. 2010;363:11-23.
- Chimowitz MI, et al. Stenting versus aggressive medical therapy for intracranial arterial stenosis (SAMMPRIS). N Engl J Med. 2011;365:993-1003.
- Gao P, et al. China Angioplasty and Stenting for Symptomatic Intracranial Severe Stenosis (CASSISS). JAMA. 2022.
- Grant EG, et al. Carotid artery stenosis: gray-scale and Doppler US diagnosis—Society of Radiologists in Ultrasound Consensus Conference. Radiology. 2003;229:340-346.
- ECST-2 Collaborative Group. Revascularisation versus medical treatment for asymptomatic carotid stenosis. Lancet. 2025.
- Brott TG, et al. Long-term results of stenting versus endarterectomy for carotid artery stenosis (CREST-2). N Engl J Med. 2025.
- Mandell DM, et al. Intracranial vessel wall MRI: principles and expert consensus recommendations. Stroke. 2017;48:e105-e123.