Carotid Duplex Ultrasound Velocity Criteria Reference
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Full bio →Ultrasound vs Direct Measurement: A Key Distinction
Duplex ultrasound does not directly measure NASCET percent stenosis. It measures blood flow velocities (peak systolic velocity, end-diastolic velocity, and velocity ratios). These velocity values are correlated with NASCET% stenosis categories through validated reference tables — but the relationship is probabilistic, not exact.
For direct NASCET% calculation, diameter measurements from CTA, MRA, or DSA are required. Use the NASCET Calculator with angiographic measurements. This reference explains how ultrasound velocity criteria map to NASCET categories when angiographic imaging is not yet available or as a screening tool.
Society of Radiologists in Ultrasound (SRU) Consensus Criteria
The SRU 2003 consensus criteria are the most widely used reference for carotid stenosis grading by duplex ultrasound. Primary criterion: ICA Peak Systolic Velocity (PSV). Secondary criteria confirm borderline findings.
| NASCET Category | ICA PSV | Additional criteria (any) |
|---|---|---|
| Normal (0%) | < 125 cm/s | — |
| < 50% stenosis | < 125 cm/s | No plaque or < 50% visible |
| 50–69% stenosis | 125–230 cm/s | ICA/CCA PSV ratio ≥ 2.0 |
| ≥ 70% to near-occlusion | > 230 cm/s | ICA/CCA ratio > 4.0, EDV > 100 cm/s |
| Near-occlusion | Variable (may be low) | Visible markedly narrowed lumen |
| Total occlusion | No detectable flow | — |
Bold the threshold: The ≥70% NASCET threshold is the most clinically critical — it corresponds to the CEA benefit threshold from the NASCET trial. A PSV of 230 cm/s has been the standard screening threshold for this category.
Additional Velocity Parameters
End-Diastolic Velocity (EDV)
EDV rises with increasing stenosis severity (stenotic jets maintain diastolic flow). EDV > 100 cm/s supports ≥70% stenosis when PSV is borderline.
| EDV | Stenosis implication |
|---|---|
| < 40 cm/s | Low-grade stenosis unlikely to be ≥50% |
| 40–100 cm/s | Moderate — correlates with 50–69% |
| > 100 cm/s | Severe — supports ≥70% |
ICA/CCA PSV Ratio
The ratio of ICA PSV to common carotid artery (CCA) PSV normalizes for cardiac output variation:
| ICA/CCA ratio | Stenosis implication |
|---|---|
| < 2.0 | < 50% stenosis |
| 2.0–4.0 | 50–69% stenosis |
| > 4.0 | ≥ 70% stenosis |
This ratio is particularly useful in patients with elevated cardiac output (high CCA velocities) where raw ICA PSV may overestimate stenosis, or in low cardiac output states where PSV underestimates stenosis.
Correlation Between PSV and NASCET%
A 2024 study (Georgianos et al., MDPI J Clin Med) found the optimal PSV cut-off for predicting ≥70% NASCET ICA stenosis was 200 cm/s (sensitivity 90.3%, specificity 93.8%). The standard SRU threshold of 230 cm/s has slightly lower sensitivity.
Important limitation: Concordance between direct angiographic NASCET measurement and ultrasound-derived stenosis grades is only moderate (κ = 0.25–0.32 for ≥70% and ≥80% thresholds). Ultrasound is a screening and surveillance tool — angiographic confirmation is standard before surgical intervention at most centers.
Near-Occlusion: Paradoxically Low Velocities
Near-occlusion (stenosis ≥95–99%) may produce paradoxically low or normal PSV because the hairline residual lumen cannot sustain the turbulent jet velocities seen in moderate-severe stenosis. In near-occlusion:
- Ipsilateral ICA PSV may be reduced
- Contralateral CCA and ICA may show increased velocities (compensatory)
- Color Doppler shows a markedly narrow color jet
Near-occlusion must be recognized on grayscale and color imaging — do not classify as “mild” based on low PSV. See also the near-occlusion discussion in the NASCET Measurement Technique reference.
Imaging Modality Comparison
| Modality | NASCET% direct? | Accuracy | Best use |
|---|---|---|---|
| Duplex ultrasound | No (velocity correlation) | Moderate (screening) | Initial evaluation, surveillance post-treatment |
| CTA (64-slice+) | Yes | High | Pre-surgical planning, confirming ultrasound findings |
| CE-MRA | Yes | High (slightly lower than CTA) | Contrast allergy alternatives, soft plaque characterization |
| DSA | Yes (gold standard) | Highest | Complex cases, discordant imaging, interventional planning |
For most patients, duplex ultrasound → CTA/MRA confirmation → surgery is the standard pathway. DSA is reserved for cases where non-invasive imaging is discordant or technically inadequate.
Post-Procedure (Stented Artery) Velocity Criteria
Stented carotid arteries have different velocity criteria than native vessels — stents alter wall compliance and flow patterns. Standard SRU criteria do not apply.
Published post-CAS criteria (Lal et al., J Vasc Surg):
- PSV > 450 cm/s: likely ≥50% in-stent restenosis
- PSV > 300 cm/s + ICA/CCA ratio > 4.0: consider further imaging
Institutions performing carotid stenting should maintain a local protocol for post-CAS surveillance with institution-validated velocity thresholds.
For clinical management based on stenosis degree identified by any modality, see Symptomatic Carotid Stenosis Management and CEA vs Carotid Artery Stenting.
Clinical note: This reference is for trained clinicians. Velocity criteria should be validated at each vascular laboratory using site-specific data. Treatment decisions require multidisciplinary assessment.