NASCET Calculator

Q: What are the standard NASCET severity categories?

Stenosis is typically categorized as Mild (< 50%), Moderate (50% to 69%), Severe (70% to 99%), and completely occluded at 100%.

Q: What is the difference between NASCET and ECST?

NASCET uses the normal distal ICA as the denominator, while ECST uses the estimated original diameter of the carotid bulb. Therefore, ECST typically yields a mathematically higher stenosis percentage for the exact same lesion.

Q: Can the narrowest diameter be larger than the distal diameter?

No. The point of stenosis must be narrower than or equal to the normal distal artery. If it is larger, the measurement is likely incorrect, often due to measuring the naturally flared carotid bulb.

Carotid Measurements

Normal Distal ICA Diameter (mm)

Diameter where artery walls are parallel.

Narrowest ICA Diameter (mm)

Diameter at the point of maximum stenosis.

NASCET Stenosis Result

Degree of Stenosis

70.0%

Severity Category

Severe Stenosis

The NASCET Calculator is a specialized medical tool used by radiologists, neurologists, and vascular surgeons to determine the degree of internal carotid artery (ICA) stenosis. Based on the North American Symptomatic Carotid Endarterectomy Trial, this formula is considered the gold standard for grading carotid narrowing and deciding on surgical interventions.

How the NASCET Formula Works

The NASCET method calculates stenosis by comparing the diameter of the artery at its narrowest point to the diameter of the normal, healthy artery located further downstream (distal to the bulb). The formula is expressed as:

% Stenosis = (1 - (Narrowest ICA Diameter / Normal Distal ICA Diameter)) × 100

For example, if the normal distal artery measures 5.0 mm and the narrowest point of the stenosis measures 1.5 mm, dividing 1.5 by 5.0 gives 0.3. Subtracting 0.3 from 1 equals 0.7, which translates to a 70% stenosis. This method is preferred because it compares the stenosis against the true native size of the artery heading to the brain.

How to Use This Tool

Measure the normal distal Internal Carotid Artery (ICA) on your imaging study (Ultrasound, CTA, or MRA). This measurement should be taken well beyond the carotid bulb where the artery walls become parallel. Enter this value into the first input box.
Measure the lumen at the point of maximum stenosis. Enter this value into the second input box.
The calculator instantly processes the standard formula and outputs the precise percentage of stenosis.
The dashboard will automatically classify the result into standard clinical categories: Mild, Moderate, or Severe.

Frequently Asked Questions

What are the standard NASCET severity categories?

Based on the clinical trial guidelines, carotid stenosis is typically categorized into three main tiers. Mild stenosis is considered anything less than 50%. Moderate stenosis ranges from 50% to 69%. Severe stenosis is defined as 70% to 99%. A 100% result indicates complete occlusion of the artery.

What is the difference between NASCET and ECST?

Both are landmark trials for evaluating carotid stenosis, but they use entirely different measurement baselines. NASCET compares the narrowest point against the healthy distal ICA. The European Carotid Surgery Trial (ECST) compares the narrowest point against the estimated original outer diameter of the carotid bulb itself. Because the bulb is naturally wider, ECST calculations always result in a much higher percentage of stenosis for the exact same physical narrowing compared to NASCET.

Can the narrowest diameter be larger than the distal diameter?

No. Physically, the point of maximum stenosis cannot be wider than the normal, healthy distal artery. If your measurements show a wider stenosis point, you may be incorrectly measuring the widened carotid bulb instead of the distal ICA. The calculator will flag this as an error.