4.3 Carotid Velocity Criteria & Stenosis Staging
Key Takeaways
IAC recommends 180 cm/s as the primary lower PSV threshold for 50–69% native ICA stenosis, with plaque and secondary parameters considered.
PSV 125–180 cm/s with ICA/CCA ratio at least 2.0 and supportive plaque/flow findings can also support 50–69% stenosis.
Near-occlusion requires assessment of the residual lumen and reduced distal ICA caliber; velocities can be high or low.
Compensatory flow, cardiac disease, arrhythmia, tortuosity, and tandem lesions can create discordant measurements.
Use validated reconstruction-specific criteria for carotid stents and endarterectomy sites; near-occlusion does not automatically mandate surgery.
Native ICA grading requires a complete dataset
Internal carotid artery (ICA) stenosis is graded by combining visible narrowing with Doppler measurements. Record the highest representative peak systolic velocity (PSV) in the lesion, end-diastolic velocity (EDV), and the ICA/common carotid artery (CCA) PSV ratio using an appropriate distal CCA sample. Verify angle correction at or below 60 degrees and parallel to the actual flow stream. Describe plaque, residual lumen, distal caliber, and technical limitations.
The NASCET diameter method compares the narrowest residual lumen with the normal distal ICA, rather than the original bulb diameter. If the narrowest diameter is 1.5 mm and the normal distal diameter is 5.0 mm, diameter reduction is (1 − 1.5/5.0) × 100 = 70%. Duplex categories are supported by validated correlations with angiography; they are not an invitation to estimate an exact angiographic percentage from a single velocity.
Historical SRU versus the current IAC recommendation
The 2003 Society of Radiologists in Ultrasound (SRU) consensus used 125 cm/s as the lower PSV threshold for 50–69% stenosis. IAC's November 2023 modification raises that primary threshold to 180 cm/s, because using 125 alone overcalls moderate stenosis. The August 2026 IAC standards continue to strongly recommend the modification. Recognize the historical table if an examination question explicitly names it; use the stated framework rather than silently combining tables.
| Native ICA category | PSV, cm/s | Plaque/lumen | ICA/CCA PSV ratio | EDV, cm/s |
|---|---|---|---|---|
| Normal | <180 | No plaque | <2.0 | <40 |
| <50% | <180 | Plaque, <50% reduction | <2.0 | <40 |
| 50–69% | 180–230 | Substantial visible narrowing | 2.0–4.0 | 40–100 |
| Severe, below near-occlusion | >230 | Substantial narrowing | >4.0 | >100 |
| Near-occlusion | High, low, or undetectable | Marked narrowing with distal caliber reduction | Variable | Variable |
| Total occlusion | Undetectable | No patent lumen detected | Not applicable | Not applicable |
The IAC modification also permits PSV 125–180 cm/s with ratio at least 2.0 to support 50–69% stenosis when significant plaque and other stenotic features are present. This is a multiparameter interpretation. PSV of 160 cm/s without plaque or an elevated ratio does not establish moderate disease. Likewise, an isolated EDV should not override an incompatible image or a technically poor sample.
Worked interpretation
An ICA has PSV 185 cm/s, EDV 68 cm/s, ratio 2.8, and visible approximately 55% plaque-related narrowing. These findings are concordant with 50–69% native ICA stenosis. In contrast, PSV 245 cm/s, EDV 110 cm/s, ratio 4.5, and a clear severe lesion support the severe category. If the lumen appears nearly collapsed, assess near-occlusion separately instead of forcing the case into the velocity table.
Near-occlusion and complete occlusion
Near-occlusion is severe ICA stenosis with reduction of the distal ICA caliber; full threadlike collapse is one form, but not every case has a dramatic string sign. Velocity can be very high, low, or difficult to detect. Low velocity therefore does not exclude advanced disease. Routine NASCET percentage calculation becomes misleading when the reference distal artery has itself become reduced in caliber.
Search for residual flow with multiple windows, appropriately low color scale and wall filter, optimized gain, power Doppler, and spectral sampling. Avoid calling noise, flash artifact, or adjacent collateral flow a patent ICA. Conversely, color silence behind a calcified plaque is not proof of occlusion. Record what is visible, what is obscured, and whether additional imaging is needed.
For suspected total occlusion, demonstrate absent flow in an apparently occluded lumen after adequate optimization. A proximal high-resistance stump waveform, altered CCA flow, and collateral ECA flow are supportive rather than mandatory findings. An inadequate acoustic window makes the examination indeterminate, not diagnostic of occlusion.
Near-occlusion is not an automatic indication for urgent endarterectomy. Treatment depends on symptoms, distal collapse, recurrent events despite medical therapy, confirmatory imaging, and specialist assessment. Current ESVS guidance generally discourages routine CEA/CAS for symptomatic near-occlusion with distal collapse; selected recurrent-symptom cases may be considered after multidisciplinary review. New neurologic symptoms require urgent clinical assessment regardless of the ultrasound category.
Confounders and discordance
| Situation | Potential effect | Response |
|---|---|---|
| Contralateral ICA occlusion | Compensatory flow can increase ipsilateral velocity | Integrate plaque, ratio, EDV, and collateral findings; do not force a category |
| Low cardiac output or severe aortic stenosis | Bilateral low velocities or delayed upstrokes | Compare both sides and cardiac context |
| Aortic regurgitation | Altered contour, sometimes a bisferiens pattern or diastolic reversal | Describe the pattern and correlate clinically |
| Irregular rhythm | Beat-to-beat velocity variation | Sample representative cycles according to protocol and document rhythm |
| Tortuosity | Local acceleration and angle uncertainty | Sample in several planes and verify actual jet direction |
| Tandem lesions | Proximal/distal disease changes expected velocities | Examine the full accessible arterial course |
Ratios help with some systemic effects but do not mathematically cancel every confounder. For example, contralateral occlusion with PSV 245, ratio 2.9, EDV 82, and moderate plaque creates discordance. Repeat representative samples and report the inconsistency; a ratio below 4 does not conclusively prove 50–69% disease. Avoid basing a treatment recommendation on whichever parameter happens to support the desired category.
Native arteries and reconstructed arteries
A carotid stent changes wall compliance and may alter baseline velocities. Endarterectomy changes geometry, particularly with a patch. Use the laboratory's validated reconstruction-specific criteria and compare with the postoperative baseline. A native PSV threshold cannot simply be relabeled as an in-stent restenosis threshold. Different published studies use different thresholds and reference standards; they are not a universal rule requiring one exact PSV to diagnose every stent.
The next section teaches postoperative acquisition and reporting. Here the essential habit is to identify the vessel context before applying the table: native ICA, patched endarterectomy, stent, and near-occlusion are distinct interpretive problems.
Sources checked October 18, 2026
A native ICA has PSV 185 cm/s, EDV 68 cm/s, ICA/CCA PSV ratio 2.8, and plaque producing about 55% narrowing. Under the IAC-modified SRU framework, which category is best supported?
Normal
Less than 50% stenosis
50–69% stenosis
Severe stenosis below near-occlusion
With contralateral ICA occlusion, the patent ICA has PSV 245 cm/s, EDV 82 cm/s, ratio 2.9, and apparently moderate plaque. What is the best interpretation?
Near-occlusion is proven and immediate bypass is required
PSV alone proves severe stenosis
A narrow Doppler gate explains all findings
Compensatory flow may contribute; repeat representative samples and reconcile the discordant anatomy and Doppler parameters
Why should a PSV of 255 cm/s in a carotid stent not automatically be graded using the native ICA table?
Stents dissolve and invalidate Doppler
Ultrasound cannot penetrate stents
Restenosis occurs only with zero EDV
Stent mechanics and geometry change velocity relationships; use validated stent-specific criteria and the postoperative baseline
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