First and second year — the floor first, then every step
On Doppler, grade by the peak systolic velocity and the ICA/CCA ratio together with the grey-scale plaque; on CTA, measure by NASCET — the narrowest lumen against the normal distal ICA.
Orient first
NASCET compares the narrowest residual lumen with the normal ICA distal to the bulb; ECST compares it with the estimated original bulb diameter — they give different percentages for the same artery.
Doppler velocity criteria (e.g. the SRU consensus) grade in bands; the numbers depend on the criteria set your lab uses — verify.
Near-occlusion collapses the distal ICA and velocities can fall — a trap that makes a critical stenosis look milder.
Acquire the study
Linear probe; grey-scale, colour and spectral Doppler of the CCA, bulb, ICA and ECA; angle ≤ 60° parallel to flow.
The manoeuvre
Grey-scale in longitudinal and transverse planes: plaque location, surface (irregular, ulcerated) and echogenicity.
Spectral Doppler with angle correction ≤ 60°: peak systolic velocity at the tightest point and just beyond it; end-diastolic velocity.
ICA/CCA peak systolic velocity ratio — compare with the contralateral side.
Vertebral arteries on colour Doppler: direction of flow (subclavian steal).
What confirms it
A stenosis category consistent across velocity, ratio and plaque, or a NASCET percentage on CTA.
What licenses you to exclude it
A normal duplex with good windows excludes significant ICA stenosis at the bifurcation; intracranial disease needs other imaging.
The classic misread
Grading a contralateral compensatory high velocity as stenosis — compare both sides.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Carotid bulb · Expected flare of the bulb
the bulb is normally wider than the CCA — that flare is anatomy, not an aneurysm; plaque burden is reported with NASCET-style percent (vascular shelf)
Do not call a normal bulb an aneurysm. Stenosis measurement belongs on the vascular shelf (NASCET vs ECST), not as a millimetre of bulb width.
USG · CT
Diagnostic criteria
Internal carotid artery (percent stenosis) · NASCET versus ECST percent — how to report it
NASCET = (1 − residual lumen / distal ICA) × 100; ECST = (1 − residual lumen / estimated bulb) × 100. The same plaque is a HIGHER percent on ECST — state which method
State which method. On CTA/MRA measure residual lumen against the distal ICA (NASCET) or against a reconstructed bulb (ECST). Doppler criteria (next entry) are calibrated mainly to NASCET.
⚠️ A "70% stenosis" without the method is unreadable and can move a patient across an operating threshold. Doppler SRU bands below are NASCET-referenced.
USG · CT · MRI
Internal carotid artery · ICA PSV, EDV and ICA/CCA ratio (SRU consensus)
SRU 2003 consensus (NASCET-referenced): PSV <125 cm/s is <50%; 125–230 cm/s is 50–69%; >230 cm/s is ≥70%, with EDV and ICA/CCA ratio as concordant criteria
Angle-corrected Doppler ≤60° in the tightest ICA segment; CCA PSV in a disease-free mid-CCA. Concordance of PSV, EDV (>100 cm/s for ≥70%) and ratio (>4 for ≥70%) is the test, not one number.
Near-occlusion can DROP the PSV — a trickle with a collapsed distal ICA is not "mild". Contralateral occlusion raises ipsilateral velocities. Confirm the laboratory still uses SRU 2003 or a named successor. Versioned criterion — verify against the current edition before clinical use.
USG
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.