NASCET Carotid Stenosis Calculator

Measure the narrowest lumen and the normal distal ICA beyond the bulb, where the walls are parallel, on images perpendicular to the vessel. Do not use the bulb or a post-stenotic dilatation as the reference.

NASCET diameter stenosis = 100 × (1 − residual lumen / normal distal ICA lumen).

NASCET's analysis used the normal artery beyond the plaque and bulb. A reference just beyond the plaque was accepted for eligibility, not for the analysis calculation used here.

NASCET's 1991 methods figure prints −10% for a lumen wider than the distal reference and assigns 95% when distal narrowing from reduced flow invalidates the calculation. This calculator floors negative conventional results at 0% and reports near-occlusion without a percentage. Its display-rounding rules are software conventions; the methods paper does not specify fractional-percentage rounding.

The calculator uses three conventional-stenosis bands: less than 50%, 50% to <70% and 70% to <100%. Near-occlusion and total occlusion are graded separately.

ESO 2021: “mild: <50%, moderate: 50–69%, severe: 70–99%, near occlusion (defined as collapse of the distal lumen)”.

The 1991 NASCET methods paper used moderate for 30–69% and severe for 70–99%; the 1998 report separated patients below 50% from those at 50–69%.

Treatment decisions also depend on symptoms and other clinical factors; see the ESVS carotid guideline.

ESVS recommends corroborative CTA/MRA or a second-operator duplex when CEA is being considered, and CTA/MRA after duplex when CAS is being considered (ESVS 2023, §2.5).

Assess for near-occlusion before calculating a percentage. Distal ICA narrowing beyond a severe proximal stenosis makes the NASCET percentage inapplicable. Report near-occlusion without a percentage. This includes distal ICA narrowing with or without full collapse; a threadlike lumen is not required.

ESCR 2023: “it is an error to report a near-occlusion as 99% stenosis as in this specific situation the distal portion of the ICA may not be used as a denominator for the quantification of the degree of stenosis”. Johansson 2025: “When grading carotid stenosis, it is crucial to first assess if the case have CNO and only grade conventional stenoses with percent.”

Fox 2005 identified near-occlusion using at least two angiographic features: delayed ICA filling, collateral supply, reduced distal ICA relative to the opposite ICA, and reduced distal ICA relative to the ipsilateral ECA.

Johansson 2022: “Best new definition of full collapse was distal ICA diameter ≤ 2.0 mm and/or ICA ratio ≤ 0.42.” In Johansson's symptomatic CTA cohort, none of the 315 conventional ≥50% stenoses had a distal ICA diameter of 2.4 mm or less. The authors add that “some degree of model fitting can be suspected, validation is warranted.”

This calculator's caution at a distal ICA of 2.4 mm or less is an application choice based on that observation, not a validated diagnostic threshold. Diameter alone cannot diagnose or exclude near-occlusion, and the absence of the caution does not establish normal reference anatomy or rule out near-occlusion.

Bartlett 2006 reported CTA near-occlusion thresholds including a distal ICA of 3.5 mm or less (Table 1; its Results text gives ≤3.6), a residual lumen of 1.3 mm or less, a distal-to-contralateral ICA ratio of 0.87 or less and a distal ICA-to-ECA ratio of 1.27 or less, and concluded that identification requires “the judgment of the interpreter”. The calculator does not compute these criteria.

Anatomical variants can make an ICA small without collapse caused by a proximal stenosis. Johansson 2020 described ICA asymmetry associated with Circle of Willis variations that can mimic near-occlusion, and noted that “a measurement or measurements cannot be recommended to solely diagnose near-occlusion”.

In a published near-occlusion with full collapse (Gu 2020, Fig. 2), a 0.8 mm residual lumen and a 1.2 mm distal ICA would give 100 × (1 − 0.8/1.2) = 33.333…%. That calculation is invalid: it would read as less than 50% for a near-occlusion.

The near-occlusion box records the reader's imaging judgment; no diameter threshold checks it automatically. Leaving it unticked means near-occlusion was not identified, not that it was excluded. Two diameters cannot establish that the distal reference is unaffected or exclude near-occlusion.

NASCET uses an unaffected distal ICA reference; ECST uses the estimated original lumen at the stenosis. Their percentages are not interchangeable. Barnett and colleagues (1998) reported that NASCET values of 50% and 70% corresponded to ECST values of 75% and 85%, respectively. Treat these as approximate study-level correspondences, not a conversion for an individual patient's measurement.

Sources: Barnett 1998, Discussion; ESVS 2023, §2.4.

Duplex grading uses velocities together with plaque and other imaging features; it is not computed from these two diameters. IAC's 2023 recommendation raises the principal ≥50% PSV threshold to 180 cm/s and retains an alternative lower-PSV/raised-ratio pathway.

See the IAC carotid stenosis interpretation criteria.

Bartlett's CTA cohort associated a residual lumen of about 1.3 mm with 70% stenosis and about 2.2 mm with 50%; these regression estimates do not replace an individual distal reference.

Diameter and area percentages are different: the published 3.5/7.0 mm example gives 50% diameter stenosis, while 9.7/38.8 mm² gives 75% area reduction.

A 0% NASCET value does not exclude plaque. ESCR 2023 notes that the normal outward bulge of the lumen at the carotid bulb “may be lost due to the presence of disease but still not cause any measured stenosis given the location of the reference diameter”, and that “the presence of positive/negative remodeling should always be reported”.

The reference diameter must be unaffected; ESCR 2023 lists underfilling and near-occlusion as reasons it may not be. NASCET 1991: “Our results do not apply if the diameter of the carotid bulb or a segment with poststenotic dilatation is used as the denominator of this ratio in the measurement of stenosis (the severity of stenosis would be overestimated).”

ESCR 2023 also asks readers to identify tandem stenosis, “the simultaneous presence of severe carotid bifurcation stenosis and second stenosis of ≥50% in any downstream distal cerebral artery”. The two diameters entered here describe one stenosis only.

Measure in a plane perpendicular to the vessel's long axis; ESCR 2023: “Standard, axial, sagittal, or coronal planes are not applicable.” Image quality and calcium can limit lumen measurement, and modalities can disagree: Misiolek 2026 reported 72% on CTA and approximately 60% on DSA for the same right ICA.

For native atherosclerotic stenosis at the carotid bulb or proximal ICA. Not for dissection, fibromuscular dysplasia, carotid webs or in-stent restenosis.

How do you calculate NASCET carotid stenosis?

NASCET diameter stenosis is 100 × (1 − narrowest residual lumen / normal distal ICA lumen). Use measurements from the same artery and assess for near-occlusion first.

What is the difference between NASCET and ECST?

NASCET uses an unaffected distal ICA reference; ECST uses the estimated original lumen at the stenosis. Their percentages are not interchangeable.

Where do you measure the distal ICA for NASCET?

Use an unaffected ipsilateral ICA segment beyond the bulb and recognizable disease, where the walls are parallel. Do not use the bulb or a post-stenotic dilatation.

Why is NASCET not used for carotid near-occlusion?

Near-occlusion reduces the distal ICA diameter and makes the denominator unsuitable, so a NASCET percentage can underestimate disease. Report near-occlusion without a percentage.

What percentage of carotid stenosis is considered severe?

Severe conventional ICA stenosis is at least 70% and less than 100% by the NASCET method; near-occlusion and total occlusion are graded separately.

Can NASCET be measured on CT angiography?

Yes. On CTA, measure the narrowest lumen and an unaffected distal ICA segment in planes perpendicular to the vessel. Assess for near-occlusion before calculating a percentage.

Can ultrasound velocity alone determine the NASCET percentage?

No. Duplex estimates stenosis using velocities together with plaque and other imaging findings. This calculator uses lumen diameters, not Doppler velocities. See Duplex ultrasound.