Carotid artery duplex ultrasonography is the first-line noninvasive test for extracranial carotid stenosis. It combines gray-scale imaging, color Doppler, and spectral Doppler so you can map plaque, confirm flow direction, and grade narrowing with velocity and ratio criteria. This walkthrough covers a practical vessel survey, Doppler sampling, commonly cited velocity ranges, and the angle, sample-volume, and tandem-lesion errors that most often lead to misgrading.
When Carotid Duplex Is Indicated
Protocol a carotid duplex study when extracranial atherosclerotic disease would change management. Frequent clinical questions include anterior-circulation transient ischemic attack or ischemic stroke, amaurosis fugax, a cervical bruit, surveillance of known stenosis, and follow-up after carotid endarterectomy or stent placement. Some practices also assess the carotids before major cardiac or vascular surgery. The examination answers whether flow-limiting plaque or occlusion is present in the accessible extracranial carotid and vertebral arteries. It does not replace a complete neurologic evaluation.
Protocol for Carotid Artery Duplex Ultrasonography
A complete study documents both carotid systems and both vertebral arteries. Technique is as important as the numbers. Follow a written laboratory protocol aligned with AIUM and SRU practice parameters, and maintain quality assurance against correlative imaging. A structured review of extracranial technique is available in the Basics of Extracranial Carotid Artery Duplex Ultrasound course.
Patient position and transducer
Position the patient supine with the neck slightly extended and the head turned away from the side being scanned. A linear transducer in the 5 to 12 MHz range is typical. Use a lower-frequency linear or curved transducer for a thick neck or a deep vessel. Apply light pressure so you do not flatten the artery or obliterate the jugular vein.
Gray-scale and color survey
Start in transverse gray-scale from the clavicle to the mandible. Identify the common carotid artery (CCA), bifurcation, internal carotid artery (ICA), and external carotid artery (ECA). Document plaque location, length, and surface in transverse and longitudinal planes. Color Doppler then confirms vessel identity, flow direction, and aliasing at a stenosis.
Helpful landmarks include:
- The ICA is usually posterolateral, larger at the bulb, and has no extracranial branches
- The ECA is usually anteromedial and gives off branches; a temporal tap can confirm identity when waveforms overlap
- The vertebral artery lies between transverse processes; record antegrade versus retrograde flow
Survey the accessible CCA, bulb, proximal through distal cervical ICA, ECA origin, and proximal vertebral artery on each side. Standardizing Doppler sites across readers is easier when the lab uses a shared carotid duplex examination protocol.
Spectral Doppler sampling
Angle-corrected spectral Doppler drives stenosis grading. Keep the Doppler angle at 60 degrees or less and align the cursor with the flow stream, not only with the vessel wall. Place a small sample volume in the center stream. Record peak systolic velocity (PSV) and end-diastolic velocity (EDV) from proximal, mid, and distal CCA; proximal ECA; proximal, mid, and distal cervical ICA; any site of plaque or color aliasing; the true peak ICA velocity; and each proximal vertebral artery.
Calculate the ICA/CCA PSV ratio from a mid or distal CCA sample, not from a waveform taken immediately below a bulb lesion. If a stent is present, sample proximal to, within, and distal to the stent. Stent velocities run higher than native-vessel values, so apply stent-specific thresholds and compare with the baseline post-stent study. The Duplex Ultrasound of Carotid Stents course reviews those differences in detail.
Velocity Criteria and Carotid Duplex Scan Normal Values
Most North American laboratories grade native ICA stenosis with the 2003 Society of Radiologists in Ultrasound consensus criteria, which were correlated with NASCET-style angiographic narrowing. Other tables are in use, and some labs validate their own cutoffs. State which criteria you used, and do not mix tables in one report.
Commonly cited SRU ranges for the ICA are:
- Normal: ICA PSV less than 125 cm/s and no visible plaque
- Less than 50% stenosis: ICA PSV less than 125 cm/s with plaque
- 50% to 69% stenosis: ICA PSV 125 to 230 cm/s, ICA/CCA PSV ratio 2.0 to 4.0, and ICA EDV 40 to 100 cm/s
- 70% or greater stenosis: ICA PSV greater than 230 cm/s, ratio greater than 4.0, and ICA EDV greater than 100 cm/s
- Near occlusion: a string-like residual lumen with variable velocity
- Occlusion: no ICA flow, often with a high-resistance CCA waveform
Use PSV, EDV, and the ICA/CCA ratio together. An isolated velocity that disagrees with residual lumen or with the opposite side should trigger a second look at angle and sample site, not an automatic upgrade.
Normal CCA PSV often lies roughly between 50 and 100 cm/s but varies with age, cardiac output, and contralateral disease. The ICA is low resistance with ample diastolic flow. The ECA is higher resistance. Vertebral arteries normally show antegrade, low-resistance flow.
Key Imaging Findings and How to Report Them
Describe plaque as homogeneous or heterogeneous and note surface irregularity when gray-scale quality allows. Calcified plaque can shadow the lumen; change windows or use power Doppler before you call occlusion. Near occlusion shows a trickle of color or power Doppler through a hairline lumen and is managed differently from true occlusion, so document both the image and the Doppler tracing.
After endarterectomy, a patch or slightly ectatic bulb is expected; native criteria usually still apply unless your lab has patch-specific data. After stenting, do not apply native SRU cutoffs. Compare with the first post-stent duplex and with published stent ranges used by your laboratory.
Pitfalls That Lead to Misgrading
Most errors in duplex carotid ultrasound are technical rather than interpretive.
- Doppler angle: A small error near 60 degrees changes PSV a great deal. Stay at or below 60 degrees and align to the color jet, which may leave the wall in a bulb or a tortuous ICA.
- Missed jet: Peak velocity often sits at the distal end of plaque. Sample through and just beyond the stenosis. An oversized sample volume averages wall and lumen and underestimates PSV.
- Tandem or distal disease: Siphon stenosis can damp proximal velocities and hide a bifurcation lesion. A tardus-parvus ICA waveform with unexpectedly low PSV is a red flag.
- Contralateral occlusion and cardiac output: Compensatory flow can raise PSV without matching plaque. Low ejection fraction or severe aortic stenosis can lower velocities and mask a tight stenosis.
- Vessel identity: A large ECA collateral may mimic a patent ICA after occlusion. Seek branches, use a temporal tap, and confirm waveform morphology.
- Tortuosity and arrhythmia: Bends create angle error and focal velocity increase without plaque. Average several sinus beats rather than a single premature complex.
Duplex Versus Doppler: What the Terms Mean
Carotid duplex ultrasonography means anatomic imaging plus Doppler. Color and spectral Doppler provide physiology; B-mode provides plaque and residual lumen. Strictly, a Doppler-only tracing records velocity without a full gray-scale survey. Ordering language often uses carotid duplex, carotid Doppler, and duplex carotid ultrasound as if they were the same test. The study you perform should still be a complete duplex examination. Velocity without an image of the plaque, or an image without an angle-corrected waveform, is not sufficient to grade stenosis.
When protocol, angle discipline, and laboratory quality assurance are in place, carotid duplex is a reliable first-line test for extracranial ICA stenosis. Accuracy drops with angles above 60 degrees, ignored tandem or contralateral disease, and native cutoffs applied to stents. Discordant results and patients being considered for revascularization often need CTA, MRA, or catheter angiography. A careful survey and a consistent grading table keep reports usable at the bedside and in the reading room.
IAME offers accredited ultrasound CME courses on extracranial carotid duplex, including protocol-based and case-based modules for practicing clinicians.
Frequently Asked Questions
What does a carotid duplex ultrasound test for?
It evaluates extracranial carotid plaque, stenosis, near occlusion, and occlusion, and it documents vertebral flow direction. Spectral Doppler velocities and ratios grade ICA narrowing. Less common findings such as dissection can be seen, but everyday use is atherosclerotic disease of the accessible neck arteries.
What is the difference between a carotid duplex ultrasound and a carotid Doppler ultrasound?
Duplex combines gray-scale imaging with Doppler. Doppler alone is velocity recording without a full anatomic survey. Many orders say Doppler when a complete duplex study is intended. Stenosis grading requires both the plaque image and an angle-corrected spectral waveform.
What should I do to prepare for a carotid duplex scan?
No fasting or medication change is needed. Patients should wear an open-neck shirt and remove necklaces. Explain that gel will be applied and that they will lie still with the head turned while each side is scanned.
How long does a carotid duplex scan take?
A bilateral examination usually takes 20 to 45 minutes of scanning time. Tortuous vessels, heavy calcification, a short neck, or stent surveillance can take longer. Build in time to compare prior studies and to produce a complete report.
What are normal carotid ultrasound results?
A normal examination shows no plaque, an ICA PSV below the laboratory cutoff for stenosis (commonly less than 125 cm/s under SRU criteria), a low-resistance ICA waveform, a higher-resistance ECA waveform, and antegrade vertebral flow. Mild plaque with PSV still below 125 cm/s is typically reported as less than 50% stenosis rather than as a normal artery.
