01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Duplex ultrasound is the first-line non-invasive test for extracranial carotid stenosis. It identifies plaque, visualises the lumen and estimates haemodynamic severity from spectral Doppler. The internal carotid artery peak systolic velocity is central, but end-diastolic velocity, the ICA-to-common-carotid velocity ratio, colour aliasing, waveform morphology and visible residual lumen make the classification more robust. A useful report communicates both the grade and how confidently it was obtained.
Percent stenosis must be tied to a measurement convention. NASCET compares the narrowest residual lumen with the normal distal ICA, whereas ECST estimates the original carotid bulb diameter. The same anatomical lesion therefore receives a higher ECST percentage; approximately 50% NASCET corresponds to about 75% ECST and 70% NASCET to about 85% ECST. NICE requires reports to state the method, and current ESC guidance recommends NASCET or its non-invasive equivalent.
Duplex measures flow rather than directly reproducing an angiographic diameter. Very severe disease can paradoxically lower velocity when the distal ICA collapses or flow approaches occlusion. Calcific shadowing can hide the lumen, while tortuosity can create angle error and focal acceleration. When a result will decide urgent CEA or CAS and the examination is limited or discordant, confirm anatomy promptly rather than averaging contradictory numbers.
Key points
- New weakness, aphasia or visual loss remains a stroke emergency; duplex does not replace immediate brain imaging and reperfusion assessment.
- Report urgent symptomatic stenosis promptly, including side, symptom concordance, measurement convention, technical limitations and whether confirmatory imaging is needed.
- Use a NASCET-equivalent assessment for intervention decisions: 50–69% is moderate and 70–99% severe, while complete occlusion is a separate category.
- Carotid duplex combines B-mode plaque imaging, colour flow and spectral Doppler velocities; no single number should override a technically poor or internally inconsistent study.
- Typical consensus velocity criteria use ICA PSV 125–230 cm/s for 50–69% and above 230 cm/s for at least 70%, but validated laboratory criteria and supporting parameters govern the final grade.
- Heavy calcification, tortuosity, tandem lesions, contralateral occlusion, low cardiac output and near-occlusion can distort velocities or obscure the residual lumen.
- Confirm treatment-determining uncertainty or discordance with CTA or MRA, chosen for renal function, contrast allergy, radiation, device and artefact constraints.
- Always interpret the scan beside symptom territory: an ipsilateral recent retinal or hemispheric event creates a different pathway from incidental asymptomatic plaque.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Visible plaque, luminal narrowing and raised ICA velocity with supporting ratio and waveform changes support a 50–69% NASCET-equivalent grade.
Marked focal acceleration, colour aliasing and supporting distal waveform changes suggest 70–99% disease when the lumen remains patent.
A threadlike distal ICA or unexpectedly low variable velocity despite extreme narrowing requires angiographic clarification rather than ordinary velocity grading.
No demonstrable lumen or flow across the ICA requires careful technical confirmation because mistaking trickle flow changes management.
Calcific shadowing, deep vessels, poor windows or inability to align the Doppler angle lowers confidence and must be declared.
Recent ipsilateral retinal or hemispheric symptoms make a lesion clinically urgent; dizziness alone does not establish carotid causality.
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
B-mode and colour duplexFirst step - Why
- Locate plaque, estimate lumen and identify disturbed flow
- Interpretation and limitations
- Plaque echogenicity, surface, colour filling and shadowing contextualise velocities; poor visualisation lowers certainty rather than proving severe disease.
- 02
Spectral Doppler velocities - Why
- Grade haemodynamic severity across the carotid bifurcation
- Interpretation and limitations
- Interpret ICA PSV with end-diastolic velocity, ICA/CCA ratio, angle correction and validated laboratory criteria; discordant components require review.
- 03
CT angiography - Why
- Confirm stenosis and map arch-to-intracranial anatomy
- Interpretation and limitations
- CTA shows lumen and calcification rapidly, but blooming can overstate disease and iodinated contrast or radiation may alter suitability.
- 04
MR angiography - Why
- Provide confirmatory arterial anatomy without ionising radiation
- Interpretation and limitations
- Contrast-enhanced or non-contrast methods can clarify disease, although motion, metallic devices and flow-related artefact may limit interpretation.
- 05
Brain CT or MRI - Why
- Define infarction, haemorrhage and competing cerebral pathology
- Interpretation and limitations
- Brain findings do not grade carotid stenosis but establish symptom relevance and whether urgent carotid intervention may be unsafe.
04Clinical next stepsHow the result changes management or prompts escalation.
01ManagementRead a carotid duplex systematicallyFirst stepA complete carotid duplex report is available for clinical interpretation+
- 1Confirm the indication, symptom side and timing before looking at the percentage so clinical urgency is framed correctly.
- 2Check that common, internal, external carotid and vertebral arteries were surveyed and note any unexamined or obscured segment.
- 3Review plaque and residual lumen, then integrate ICA PSV, end-diastolic velocity, ICA/CCA ratio, waveform and angle quality.
- 4Assign a NASCET-equivalent category only when parameters are coherent, explicitly separating near-occlusion and complete occlusion.
- 5State limitations and immediately communicate a treatment-relevant symptomatic result to the stroke or vascular team.
02EscalationDiscordant or limited duplexEscalationVelocity, visible lumen and supporting parameters disagree or anatomy is obscured+
- 1Ask whether angle error, arrhythmia, low output, tortuosity, tandem disease or contralateral occlusion could explain the discrepancy.
- 2Seek senior vascular-scientist review or targeted repeat imaging promptly if a correctable acquisition problem is likely.
- 3Use CTA or MRA when uncertainty remains and the result changes revascularisation selection or timing.
- 4Reconcile the final grade at multidisciplinary review rather than selecting whichever modality supplies the highest percentage.
03UrgencyRecent ipsilateral symptomsDuplex identifies plausible stenosis after retinal TIA, hemispheric TIA or non-disabling stroke+
- 1Ensure the patient is already in the urgent TIA or acute stroke pathway and has appropriate brain assessment.
- 2For stable 50–99% NASCET symptomatic stenosis, contact the carotid service without waiting for routine reporting cycles.
- 3AlternativeFor less than 50% NASCET disease, document that CEA is not indicated and continue secondary prevention and alternative-cause investigation.
- 4For large infarction, haemorrhage, major deficit or uncertain near-occlusion, request joint neurological and vascular interpretation before intervention.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- A vascular laboratory should audit its velocity criteria against reference imaging and report procedural outcome links where available.
- Compare serial studies using the same laboratory and grading convention; an unexplained category jump should trigger image review before labelling progression.
- After carotid revascularisation, ESC recommends baseline duplex within three months and annual assessment until two consecutive annual scans are stable, with later intervals individualised.
- Escalate new neurological symptoms immediately regardless of a recent reassuring duplex because plaque behaviour and non-carotid mechanisms can change.
- Document surveillance purpose, contralateral disease, life expectancy and whether detecting restenosis would realistically change management.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Velocity is a surrogate
Duplex infers narrowing from altered flow. It is powerful in validated hands but is not a direct angiographic calliper measurement and can fail at haemodynamic extremes.
Near-occlusion breaks simple rules
Distal ICA collapse can reduce flow and velocity despite extreme proximal narrowing, so a low PSV does not reliably exclude a critical lesion in this pattern.
Ratios add context
The ICA-to-CCA PSV ratio partly controls for global flow state, but a diseased CCA, contralateral occlusion or altered cardiac output can still distort interpretation.
Criteria evolve
The familiar SRU consensus uses 125–230 cm/s for moderate and above 230 cm/s for severe stenosis, while newer proposals use higher thresholds to reduce overestimation. Use the laboratory’s validated standard and disclose it.
Words drive decisions
A report should distinguish suspected, probable and confirmed occlusion, state technical limits, and name the measurement convention because these phrases alter referral and surgery.
07Common pitfallsFrequent interpretation and management errors.
- 01
Reporting a bare percentage without identifying NASCET or ECST convention, side, symptoms or technical confidence.
- 02
Calling severe stenosis from a single high PSV in a tortuous vessel without corroborating ratio, lumen and waveform findings.
- 03
Excluding severe disease because velocity is low when near-occlusion, distal collapse or low cardiac output is possible.
- 04
Mistaking calcific acoustic shadow for an occluded lumen or overlooking trickle flow because gain and scale were inappropriate.
- 05
Delaying treatment-determining confirmatory imaging for a recently symptomatic lesion until a routine surveillance slot.
- 06
Treating incidental asymptomatic plaque as the cause of non-focal dizziness and applying a symptomatic intervention threshold.