A carotid ultrasound protocol is a structured, step-by-step examination of the neck’s carotid and vertebral arteries using sound waves to detect narrowing, plaque buildup, and abnormal blood flow. The exam combines gray-scale imaging with Doppler techniques to generate both pictures of the vessel walls and real-time measurements of how fast blood moves through them. Those velocity measurements, especially peak systolic velocity in the internal carotid artery, form the backbone of how stenosis is graded. The protocol sounds simple on paper, but getting reliable, reproducible results depends on everything from how the patient’s head is positioned to how the sonographer angles the transducer.
Who Should Get a Carotid Ultrasound
Not everyone benefits from this exam. Guidelines recommend screening people who have risk factors for carotid stenosis and who would be candidates for treatment, whether medical therapy or surgical intervention. Screening the general adult population with no significant risk factors is not recommended.1PubMed Central. Indications for carotid Doppler ultrasound in asymptomatic patients – are we ordering it correctly? The people most likely to benefit include those with a history of stroke or transient ischemic attack, those with a neck bruit heard on physical exam, and those with multiple cardiovascular risk factors like smoking, diabetes, high blood pressure, or high cholesterol. Post-surgical and post-stenting patients are another major group, since they need ongoing surveillance to catch restenosis early.
Patient Positioning and Equipment
Getting a good carotid ultrasound starts before the transducer touches the skin. The patient lies on their back on the scan bed with their head resting comfortably and the neck slightly extended. The head is rotated away from the side being examined, and a 45-degree wedge pillow can help standardize that rotation so images are more reproducible from visit to visit. Both the sonographer and the patient need to be positioned to allow a relaxed, unhurried scan.2Journal of the American Society of Echocardiography. Use of Carotid Ultrasound to Identify Subclinical Vascular Disease and Evaluate Cardiovascular Disease Risk
The transducer of choice is a high-frequency linear-array probe operating at a fundamental frequency of at least 7 MHz.3Journal of the American Society of Echocardiography. Use of Carotid Ultrasound to Identify Subclinical Vascular Disease and Evaluate Cardiovascular Disease Risk Higher frequencies provide better resolution of the vessel wall and plaque detail, which matters when you are trying to measure something as thin as the intima-media layer. The tradeoff is that higher frequencies penetrate less deeply, but for superficial arteries in the neck this is rarely a problem in most adults.
Gray-Scale B-Mode Imaging
The first part of the protocol uses standard gray-scale imaging to visualize the vessel anatomy. The sonographer sweeps through the common carotid artery, the carotid bulb (where the artery widens before splitting), and then follows both the internal and external carotid arteries as far distally as possible. Images are acquired in both transverse (cross-section) and longitudinal (lengthwise) planes.
One key measurement taken in B-mode is the intima-media thickness, or IMT, which tracks the combined thickness of the two innermost layers of the artery wall. A practical approach involves measuring IMT at multiple angles. In one widely referenced protocol, the common carotid artery was measured in transverse, posterolateral, and anterolateral views, with three measurements taken at each angle for both the near wall and the far wall.4PubMed Central. A practical protocol to measure common carotid artery intima-media thickness More extensive protocols extend these measurements to the bulb and internal carotid artery at five predefined angles.5PubMed. Extensive or restricted ultrasound protocols to measure carotid intima-media thickness: analysis of completeness rates and impact on observed rates of change over time Extensive protocols capture more data but take longer and have lower completion rates, so the choice depends on whether the exam is for clinical care or a research study tracking subtle changes over time.
B-mode also serves as the first pass for identifying plaque. The sonographer notes any focal thickening of the wall, its location, its approximate size, and its echogenicity, meaning whether it appears bright (calcified, fibrous) or dark (lipid-rich, potentially more vulnerable). This visual assessment sets the stage for Doppler evaluation.
Telling the Internal Carotid From the External Carotid
Correctly identifying which branch is the internal carotid artery and which is the external is surprisingly important and occasionally tricky, especially when anatomy varies. Several features help. The internal carotid artery sits posterior and lateral to the external carotid and is usually slightly larger. The external carotid has visible branch vessels coming off it, while the internal carotid has none in the neck. On Doppler, the internal carotid shows a low-resistance flow pattern with continuous forward flow throughout the cardiac cycle, because it feeds the brain, which always needs blood. The external carotid supplies muscles and skin, so its waveform has a higher-resistance, more pulsatile shape.6Ultrasonography. General principles of carotid Doppler ultrasonography
When anatomy is ambiguous, there is a useful trick called temporal tapping. While recording a Doppler waveform from the vessel in question, the sonographer or an assistant taps on the temporal artery in front of the ear on the same side. If the vessel being sampled is the external carotid, the tapping creates a distinctive serration pattern on the waveform. The internal carotid shows no such artifact.7Ultrasonography. General principles of carotid Doppler ultrasonography It is a simple, reliable maneuver that can save a sonographer from misidentifying a vessel.
Doppler Technique and Technical Settings
After gray-scale imaging, the protocol adds color Doppler and then spectral (pulsed-wave) Doppler to assess blood flow. Color Doppler overlays a map of blood flow direction and relative velocity onto the gray-scale image, making it easy to spot areas of turbulence or flow disturbance that suggest a narrowing. The sonographer adjusts the color velocity scale so that normal flow fills in smoothly while abnormal high-velocity jets or areas of aliasing stand out.
Spectral Doppler produces the waveform tracings that yield the actual velocity numbers used to grade stenosis. Several technical parameters directly affect accuracy: the Doppler angle, the size and placement of the sample volume, the color sampling window, the color velocity scale, and the color gain.8PubMed. Sonographic examination of the carotid arteries Of these, the Doppler angle is arguably the most critical. A standard protocol calls for a 60-degree angle between the ultrasound beam and the direction of blood flow. Deviating from this angle, especially above 60 degrees, introduces significant errors into velocity calculations. The sample volume should be kept small and placed mid-stream in the vessel lumen to capture the highest velocities without wall artifact.
Spectral waveforms are recorded at multiple sites along each carotid system: the proximal, mid, and distal common carotid artery; the carotid bulb; the proximal and mid internal carotid artery; and the external carotid artery. At each location, the sonographer records peak systolic velocity and end-diastolic velocity. Any focal area of increased velocity or visible stenosis gets additional sampling.
Grading Stenosis With Velocity Criteria
The most widely used grading system comes from the Society of Radiologists in Ultrasound (SRU) consensus conference. The primary measurement is peak systolic velocity (PSV) in the internal carotid artery, combined with visual assessment of plaque on gray-scale and color Doppler. Two secondary measurements, the ICA-to-common-carotid PSV ratio and end-diastolic velocity, serve as backup when technical or clinical factors make PSV alone unreliable.9PubMed. Carotid artery stenosis: gray-scale and Doppler US diagnosis–Society of Radiologists in Ultrasound Consensus Conference
The SRU categories break down as follows:
- Normal: PSV below 125 cm/s with no visible plaque or intimal thickening.
- Less than 50% stenosis: PSV below 125 cm/s but plaque or intimal thickening is visible.
- 50–69% stenosis: PSV between 125 and 230 cm/s with visible plaque.
- 70% or greater stenosis: PSV above 230 cm/s with visible plaque and lumen narrowing.
- Near-occlusion: a markedly narrowed lumen on color Doppler, often with paradoxically low or variable velocities.
- Total occlusion: no detectable lumen on gray-scale and no flow on any Doppler mode.
These criteria have held up well in validation studies. One institutional analysis found that using PSV above 230 cm/s to identify stenosis of 70% or greater had a sensitivity of about 95% and a specificity of about 84% when compared against angiography.10PubMed. Ultrasound imaging of carotid artery stenosis: application of the Society of Radiologists in Ultrasound Consensus Criteria to a Single Institution Clinical Practice In UK and Irish practice, about 90% of vascular labs use the PSV above 230 cm/s threshold for severe disease, and about 86% also incorporate the ICA/CCA velocity ratio.11Journal of Vascular Societies Great Britain & Ireland. Current practice in ultrasound grading of carotid artery stenosis in the UK and Ireland
Despite their widespread adoption, these criteria have drawn criticism for relying too heavily on PSV alone, which can be misleading in certain situations, such as contralateral occlusion, tandem lesions, or cardiac output abnormalities.12PubMed. Carotid artery stenosis: gray-scale and Doppler US diagnosis–Society of Radiologists in Ultrasound Consensus Conference That is exactly why the SRU consensus included the ratio and end-diastolic velocity as supplementary tools.
The Near-Occlusion Problem
One of the trickiest scenarios in carotid ultrasound is distinguishing near-occlusion from both total occlusion and conventional high-grade stenosis. A near-occluded internal carotid artery is so severely narrowed that the lumen collapses to a thin thread. Paradoxically, velocities can drop rather than rise, because so little blood is getting through. Standard ultrasound struggles with this: one study found that ultrasound was only about 22% sensitive for diagnosing near-occlusion, while being 99% specific.13PubMed Central. Near-occlusion is difficult to diagnose with common carotid ultrasound methods In that study, most near-occlusions missed by ultrasound were misclassified as conventional stenosis of 50% or greater.
A multicenter registry confirmed the difficulty, finding that ultrasound and angiography agreed on the diagnosis of near-occlusion in only about 44% of cases. When the artery had fully collapsed, nearly half of those cases were misread as complete occlusion on ultrasound. When the artery had not fully collapsed, about 40% were read as severe stenosis instead.14PubMed. Ultrasonographic and hemodynamic characteristics of patients with symptomatic carotid near-occlusion The clinical stakes here are high because near-occlusion and total occlusion call for very different management. A totally occluded carotid generally cannot be reopened surgically, while a near-occluded vessel sometimes can. When ultrasound suggests occlusion but the clinical picture does not quite fit, confirmatory imaging with CT angiography or MR angiography is warranted.
Color-flow duplex scanning performs well for total occlusion, with one study reporting 98% confirmation by angiography when ultrasound predicted complete occlusion.15PubMed. Detection of total occlusion, string sign, and preocclusive stenosis of the internal carotid artery by color-flow duplex scanning The problem is specific to near-occlusion, where the signal is subtle and the velocity criteria that work so well at other stenosis grades break down.
Plaque Characterization and Ulceration
Beyond measuring how tight a narrowing is, the protocol also evaluates plaque morphology, because not all plaques carry the same stroke risk. Smooth, calcified, echogenic plaques tend to be more stable, while dark (echolucent), irregular, or ulcerated plaques are considered more dangerous. Ulceration, in particular, signals a plaque surface that has broken down, exposing thrombogenic material to the bloodstream.
The most widely used ultrasound criteria for identifying ulceration require the defect to be at least 2 mm deep and long, with a well-defined base on B-mode and flow reversal within the crater on color Doppler. Newer criteria have relaxed the size requirement, defining ulceration as any cavity on the plaque surface where the surface echogenicity is lower than the adjacent plaque border.16PubMed Central. Imaging of the ulcerated carotid atherosclerotic plaque: a review of the literature Even with these criteria, ulceration is one of the harder features to detect reliably with standard ultrasound. Calcified plaque makes it worse, since heavy calcification throws an acoustic shadow that hides the vessel lumen and blocks Doppler signals entirely.17PubMed. Acoustic shadowing impairs accurate characterization of stenosis in carotid ultrasound examinations
Vertebral Artery Assessment
A complete carotid duplex protocol includes a look at the vertebral arteries, which run alongside the cervical spine and supply the back of the brain. The sonographer typically picks up the vertebral artery between the transverse processes of the cervical vertebrae and records a spectral Doppler waveform. The main things to note are the direction of flow and the waveform shape. Normal vertebral artery flow is antegrade, meaning it moves toward the head, with a low-resistance pattern.
Reversed flow in a vertebral artery raises concern for subclavian steal, a condition where a blockage in the subclavian artery upstream causes blood to flow backward down the vertebral artery to supply the arm. But completely reversed vertebral artery flow does not always mean subclavian steal. It can also result from occlusion of the proximal vertebral artery itself. One study found that while continuous reversed flow was always caused by subclavian steal, intermittent reversed flow could be caused by either subclavian steal or vertebral artery occlusion.18PubMed. Completely reversed flow in the vertebral artery does not always indicate subclavian steal phenomenon Provocative maneuvers, such as inflating a blood pressure cuff on the ipsilateral arm to induce reactive hyperemia, can help tease out the diagnosis by exaggerating the steal.19PubMed. Vertebral artery Doppler waveform changes indicating subclavian steal physiology
How Ultrasound Compares to Other Imaging
Carotid ultrasound is the first-line screening tool because it is noninvasive, involves no radiation, and is relatively inexpensive. But it does not match the accuracy of CT angiography or MR angiography for grading stenosis. In a comparative study of 170 patients using catheter angiography as the gold standard, the area under the curve for color Doppler ultrasound was 0.85, compared with values above 0.98 for both CT angiography and MR angiography, a statistically significant gap.20PubMed. Diagnostic accuracy of colour Doppler ultrasonography, CT angiography and blood-pool-enhanced MR angiography in assessing carotid stenosis Ultrasound is excellent as a screening and monitoring tool, but when surgical decisions hinge on the exact degree of stenosis, confirmatory cross-sectional imaging is standard practice. Agreement between different ultrasound studies and between ultrasound and MR angiography can vary, particularly around the 50–69% stenosis range.21PubMed. Carotid artery stenosis: accuracy of noninvasive tests–individual patient data meta-analysis
Protocol Adjustments After Stenting
Patients who have undergone carotid artery stenting need ongoing duplex surveillance, and the protocol changes in meaningful ways. The exam follows the same basic approach, using a 7 MHz linear-array transducer in sagittal and transverse planes, but velocity measurements are taken at more specific locations: the proximal and distal common carotid artery, then the proximal, middle, and distal portions of the stent itself, and finally the internal carotid artery beyond the stent. The key measurements include the maximum PSV within the stent, the velocity ratio along the stent’s length, and the ratio of stent PSV to proximal common carotid PSV.22Vascular Disease Management. Duplex Ultrasound Surveillance After Carotid Stent Angioplasty: When to Follow Up and What to Look for
The velocity thresholds for in-stent restenosis are higher than those used for native arteries. One study found that PSV above 300 cm/s, end-diastolic velocity above 90 cm/s, and an ICA-to-CCA ratio above 4 correlated well with greater than 70% in-stent restenosis.23PubMed. Ultrasound criteria for severe in-stent restenosis following carotid artery stenting Stents create a rigid tube within the artery that alters local flow dynamics, so simply applying native-artery criteria would overestimate the degree of narrowing. Labs performing post-stent surveillance need to be aware of these different thresholds.
Advanced Techniques for Plaque Vulnerability
Standard B-mode and Doppler tell you how tight a narrowing is and give a rough sense of plaque composition, but newer ultrasound techniques are being developed to go further. Two of the most studied are contrast-enhanced ultrasound (CEUS) and shear wave elastography (SWE). CEUS uses microbubble contrast agents injected intravenously to detect tiny blood vessels growing within a plaque, a feature called intraplaque neovascularization that is associated with plaque instability and rupture risk. SWE measures how stiff or soft a plaque is by tracking how shear waves propagate through it; softer plaques tend to be more vulnerable.
In a study comparing these techniques against surgical pathology, SWE had a sensitivity of about 87% and specificity of about 67% for detecting vulnerable plaques. CEUS performed similarly, with about 87% sensitivity and 58% specificity.24PubMed Central. Multiparametric ultrasound evaluation with CEUS and shear wave elastography for carotid plaque risk stratification Research combining both methods suggests they can quantitatively evaluate plaque vulnerability in a more comprehensive way than either alone.25PubMed. Evaluation of carotid plaque vulnerability with different echoes by shear wave elastography and CEUS There is also evidence that plaques become softer and more uneven as neovascularization increases, providing a direct link between the two measurements.26PubMed. Quantification of carotid plaque elasticity and intraplaque neovascularization using contrast-enhanced ultrasound and image registration-based elastography These modalities are not yet part of routine clinical protocols, but they represent where the field is heading.
Point-of-Care Carotid Ultrasound
Emergency departments are increasingly interested in using bedside, abbreviated carotid ultrasound for patients presenting with stroke symptoms. In one pilot study, emergency physicians performed carotid point-of-care ultrasound with a mean scan time of about six minutes. Sensitivity for detecting stenosis was 70% and specificity was about 87%, with moderate agreement between the bedside performers and expert reviewers.27PubMed Central. Can Emergency Physicians Perform Carotid Artery Point-of-Care Ultrasound to Detect Stenosis in Patients with TIA and Stroke? A Pilot Study Those numbers are not good enough to replace a formal vascular lab study, but they could help with early triage, flagging patients who need urgent full imaging while ruling out gross stenosis in others. This remains an evolving application, and no guidelines currently endorse it as a standalone diagnostic tool.
Pediatric Carotid Ultrasound
Carotid ultrasound protocols are not just for adults. In children with sickle cell anemia, stroke is a serious complication, and the extracranial internal carotid artery can develop stenosis or occlusion that contributes to stroke risk. A study of 236 children with sickle cell anemia found extracranial internal carotid stenosis or occlusion in about 5%, and the presence of stenosis was strongly associated with prior stroke, with an odds ratio above 35.28PubMed Central. Extracranial internal carotid arterial disease in children with sickle cell anemia Some centers now perform routine annual Doppler assessment of the extracranial carotid arteries in these children, alongside the more established transcranial Doppler screening.29PubMed. Evolution of Extracranial Internal Carotid Artery Disease in Children With Sickle Cell Anemia Adding a submandibular window to the standard transcranial Doppler protocol allowed one group to detect about 10% of patients at risk for extracranial carotid disease who would have been missed otherwise.30PubMed. Extracranial carotid arteriopathy in stroke-free children with sickle cell anemia: detection by submandibular Doppler sonography
Normal velocity ranges in children differ from adults, with the median extracranial internal carotid velocity in one sickle cell cohort sitting at 148 cm/s, and higher velocities correlating with younger age and higher white blood cell counts.31PubMed Central. Extracranial internal carotid arterial disease in children with sickle cell anemia These pediatric-specific baselines are important; applying adult velocity cutoffs to children would lead to widespread overdiagnosis of stenosis.
Sonographer Ergonomics
A topic that rarely makes it into clinical discussions of carotid ultrasound is the physical toll the exam takes on the person performing it. Sonographers hold a transducer in one hand, often with the scanning arm extended and elevated, while simultaneously manipulating controls with the other hand and maintaining pressure against the patient’s neck. Surveys of American and Canadian sonographers found that 84% reported musculoskeletal disorders in 1997, a figure that climbed to 90% by 2008.32PubMed. The importance of an ergonomic workstation to practicing sonographers Shoulder injuries, wrist problems, and neck pain are the most common complaints. Good protocol design includes adjustable bed height, proper chair support, and scan techniques that let the sonographer maintain neutral postures. None of this changes the diagnostic information, but it determines whether the person performing the scan can sustain a career doing it.