Carotid IMT, Plaque Characterization, and Stenosis Grading: A Sonographer's Reference

Carotid duplex is one of the most commonly performed vascular studies. New vascular sonographers spend significant time mastering carotid technique because the velocity criteria are specific, the anatomy is complex, and the clinical consequences of errors — for a patient with symptomatic carotid disease — are significant.

Anatomy Review

Carotid system (from proximal to distal):

• Common carotid artery (CCA) — bifurcates typically at C3–C4 level
• Carotid bifurcation — the "bulb" where the CCA divides
• Internal carotid artery (ICA) — supplies the brain; no branches in the neck
• External carotid artery (ECA) — supplies the face and scalp; gives multiple branches

Distinguishing ICA from ECA:

• ICA: larger diameter, no branches in the neck, located posterolaterally at the bifurcation, lower resistance waveform (forward diastolic flow)
• ECA: smaller, gives branches (superior thyroid artery first), located anteromedially, high resistance waveform (little or no diastolic flow)
• Temporal tap test: Tap the preauricular area (temporal artery territory) while sampling the ECA. Tapping creates oscillations visible in the spectral waveform — if the tapping artifact appears in your waveform, you're in the ECA. The ICA does not respond to the temporal tap.

Scanning Protocol

Patient position: Supine, neck slightly extended, head turned gently to the contralateral side (about 20–30° — don't force it).

Probe: 5–12 MHz linear transducer.

Systematic approach:

1. Begin at the clavicle (proximal CCA) in transverse
2. Follow the CCA proximally to distally with color Doppler
3. Identify the bifurcation, ICA origin, ECA origin
4. Sample: CCA mid, ICA proximal/mid/distal (if accessible), ECA proximal
5. Evaluate the vertebral artery (posterior to CCA)
6. Repeat on the contralateral side

Velocity Criteria for Carotid Stenosis (SRU 2003 Consensus)

The Society of Radiologists in Ultrasound (SRU) 2003 consensus criteria remain the standard for most institutions:

ICA Stenosis	ICA PSV	Additional Criteria	ICA/CCA Ratio
Normal	< 125 cm/s	No plaque	< 2.0
< 50%	< 125 cm/s	Plaque present	< 2.0
50–69%	125–230 cm/s	Plaque present	2.0–4.0
≥ 70% to near occlusion	> 230 cm/s	Plaque + turbulence	> 4.0
Near occlusion	Variable (may be low PSV)	Trickle flow, markedly narrowed lumen	Variable
Total occlusion	No flow	Echogenic lumen fill	N/A

Key point: ICA PSV > 230 cm/s reliably identifies stenosis ≥ 70% when combined with plaque on B-mode.

Near occlusion is the exception to velocity criteria: Severely narrowed vessels may have low velocities due to critical reduction in flow — the classic teaching is "don't let a slow velocity reassure you in a tight-looking vessel." Color Doppler may show a thin "trickle" of aliased color.

The ICA/CCA Ratio

The ICA/CCA ratio corrects for patients with globally elevated velocities (high cardiac output, anxiety, fever). Divide the peak ICA PSV by the peak CCA PSV (sample the distal CCA, 1–2 cm proximal to the bifurcation, not the bulb).

Ratio > 4.0 = ≥ 70% stenosis regardless of absolute velocity.

Vertebral Artery Evaluation

The vertebral artery runs through the cervical vertebral foramina (transverse processes) and is evaluated at its accessible portion (C3–C6 level).

Normal: Phasic, forward (antegrade) flow. Low-resistance waveform similar to ICA.

Vertebral stenosis: Elevated PSV (> 100 cm/s in the vertebral is concerning; > 140 cm/s is clearly elevated).

Vertebral steal (subclavian steal): The ipsilateral subclavian artery has a hemodynamically significant stenosis. Blood is "stolen" from the posterior circulation through the vertebral artery to feed the subclavian. The vertebral artery shows reversed, biphasic, or to-and-fro flow.

Plaque Characterization

Plaque morphology matters because it correlates with embolic risk — not all plaques causing the same degree of stenosis are equally dangerous.

Location: Most important is the ICA origin (just above the bifurcation). Document clock position (anterior, posterior, lateral, medial wall).

Surface character:

• Smooth: regular, flat surface
• Irregular: surface undulations
• Ulcerated: crater-like defect in the plaque surface (important — associated with emboli)

Echogenicity:

• Homogeneous hyperechoic (calcified): stable, fibrous, dense — acoustic shadow behind
• Heterogeneous: mixture of echogenic and hypoechoic components — increased risk
• Hypoechoic (soft plaque): lipid-rich, more prone to rupture — may be hard to see without good technique
• Anechoic intraplaque components: hemorrhage or lipid pools — highest risk

Documentation: "Heterogeneous plaque involving the posterior wall of the ICA origin, measuring [length × thickness], with irregular surface. No ulceration identified."

Carotid Intima-Media Thickness (cIMT)

cIMT is a surrogate marker for atherosclerosis risk and is measured in research and some preventive cardiology practices.

Measurement technique:

• Far wall of the CCA, 1 cm proximal to the bifurcation
• High-frequency probe, optimal B-mode resolution, image optimized for wall layers
• The intima-media complex appears as two echogenic lines (adventitia and intima) with a hypoechoic middle layer (media)
• Measure from the inner surface of the first echogenic line to the inner surface of the second
• Typically measure 3 sites, average, and compare to age-matched reference values

Normal cIMT: Varies by age, sex, and race. General guidelines:

• < 0.6 mm: low risk
• 0.6–0.9 mm: intermediate
• ≥ 1.0 mm: high risk (equivalent to subclinical plaque)
• Plaque (focal IMT ≥ 1.5 mm): established atherosclerosis

Note: cIMT measurement is highly technique-dependent and not universally performed in clinical practice. Know whether your institution includes it in the protocol.

Images to Capture for Carotid Study

B-mode:

• CCA sagittal (proximal, mid, distal)
• Bifurcation (transverse — color Doppler overlay)
• ICA sagittal (proximal, with any plaque)
• ECA sagittal
• Vertebral artery sagittal with color Doppler

Spectral Doppler:

• CCA mid (PSV, EDV, RI)
• ICA proximal PSV (+ angle, EDV)
• ICA/CCA ratio (calculated or image with both values)
• ECA PSV
• Vertebral artery (antegrade vs retrograde)

For any plaque:

• Two-plane images with measurements (length and thickness)
• Color Doppler through the stenosis to show the lumen narrowing
• Velocity at the point of maximum stenosis
• Post-stenotic turbulence if present

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SonoBuddy's Measurements section includes carotid stenosis velocity criteria table — use it for quick bedside reference when grading a stenosis.