What is the guideline in carotid duplex scan interpretation for grading carotid artery stenosis?

Carotid Duplex Scan Interpretation Guidelines

Measurement Methodology

Use the NASCET (North American Symptomatic Carotid Endarterectomy Trial) method for measuring carotid stenosis severity, as the ECST (European Carotid Surgery Trial) and area methods overestimate stenosis and are not recommended. 1

The NASCET method remains the dominant standard against which all imaging modalities are compared and has been used in most modern clinical trials. 1

Stenosis Grading Criteria by Peak Systolic Velocity (PSV)

Carotid duplex ultrasound uses blood flow velocity as an indirect indicator of stenosis severity, not direct diameter measurement. 1 Two primary stenosis categories are defined:

Moderate Stenosis (50-69%)

• Primary criterion: PSV 125-230 cm/s in the internal carotid artery (ICA) 1
• However, recent evidence suggests raising the threshold to PSV ≥180 cm/s improves accuracy (sensitivity 93.3%, specificity 81.6%, accuracy 85.2%) compared to the traditional 125 cm/s threshold which significantly overestimates stenosis 1, 2
• Additional supporting criteria: ICA/CCA PSV ratio 2.0-4.0 1
• End-diastolic velocity (EDV) 40-100 cm/s in the ICA 1
• Sonographically visible plaque must be present 1

Severe Stenosis (≥70%)

• Primary criterion: PSV >230 cm/s in the ICA 1
• Additional supporting criteria: ICA/CCA PSV ratio ≥4.0 1
• EDV ≥100 cm/s in the ICA 1
• Plaque and luminal narrowing visualized by gray-scale and color Doppler 1

Critical Pitfalls and Limitations

Subtotal arterial occlusion may be mistaken for total occlusion on duplex ultrasound. 1 This is a critical distinction as it affects treatment decisions.

Distinguishing 70% stenosis from less severe stenosis can be difficult, which supports using corroborating imaging methods (CTA or MRA) in equivocal cases. 1

There is considerable overlap in velocity measurements between adjacent stenosis categories, as demonstrated by the standard deviation ranges in validation studies. 1

The traditional Society of Radiologists in Ultrasound Consensus Criteria (SRUCC) significantly overestimate stenosis severity when compared to angiography (moderate agreement only, κ = 0.42). 2

Recommended Approach for Improved Accuracy

For laboratories using SRUCC criteria, either raise the PSV threshold for ≥50% stenosis to ≥180 cm/s as a single parameter, OR require both PSV ≥125 cm/s AND ICA/CCA PSV ratio ≥2.0 to improve diagnostic accuracy. 1, 2

This modification improves test performance from 74.5% accuracy to 85.2-87.4% accuracy when validated against catheter angiography. 2

When Additional Imaging is Required

Obtain confirmatory imaging with CTA, MRA, or catheter angiography when:

• Duplex findings are equivocal or technically limited 1
• Noninvasive imaging yields discordant results 1
• Subtotal vs. complete occlusion needs clarification in symptomatic patients 1
• Planning endovascular intervention (requires imaging of aortic arch, proximal vessels, and distal anatomy) 1

MRA is reasonable for patients with renal insufficiency or extensive vascular calcification. 1

CTA is reasonable for patients unsuitable for MRA due to claustrophobia, pacemakers, or other incompatible devices. 1

Quality Assurance Requirements

All vascular laboratories must validate their duplex criteria against catheter angiography data to assess and improve accuracy. 1 No single imaging modality is uniformly superior across all institutions, as quality varies significantly. 1

Correlation of findings from multiple imaging modalities should be part of every laboratory's quality assurance program. 1