Carotid Artery Plaque on Ultrasound: A Patient's Guide to the 2020 ASE Guidelines for Measuring Atherosclerosis and Heart Risk

Table of Contents

• Key Points
• Background: Why the Carotid Artery Matters
• Rationale: Why Plaque Beats Simple Wall Thickness
• What These Guidelines Cover
• Defining Plaque: Two Distinct Types
• When Is Plaque "Clinically Significant"?
• How Plaque Is Measured: Two-Dimensional Techniques
• Method 1: The Plaque Score
• Method 2: Plaque Height (Thickness)
• Plaque Height and Patient Outcomes
• Method 3: Plaque Area
• Advanced and Emerging Techniques
• Putting Plaque Imaging into Practice
• Limitations and Open Questions
• Key Recommendations at a Glance
• Frequently Asked Questions
• Source Information

Key Points

• Carotid plaque is defined as protuberant (focal thickening into the lumen) or diffuse (CIMT 1.5 mm or greater in any carotid segment).
• Plaque thickness (height) is the recommended initial 2D measurement because it is simple, widely available, and highly reproducible.
• In the Three-city study of 5,895 older adults, plaque at one site was linked to a hazard ratio of 1.5 for cardiovascular events.
• Serial CIMT measurements are not recommended in asymptomatic patients unless the result meets diffuse plaque criteria (CIMT 1.5 mm or greater).
• Plaque information reclassifies risk beyond traditional factors, with some measures linked to 1.5 to 4.5 times higher risk of events or death.

Background: Why the Carotid Artery Matters

Atherosclerotic cardiovascular disease (CVD, the buildup of fatty plaque in arteries that can cause heart attack and stroke) remains the leading cause of illness and death worldwide. The carotid arteries in the neck offer a unique "window" into a patient's underlying cardiovascular risk, because they are easy to reach with ultrasound and they mirror what is happening in arteries elsewhere in the body.

The presence and degree of atherosclerosis — measured by whether plaque is visible in the carotid system — has been used to estimate, and sometimes reclassify, a person's cardiovascular risk. Carotid atherosclerosis also directly predicts other cardiovascular events. The most serious is stroke, which can result from narrowing of the vessel (luminal stenosis) or from plaque rupture, when a plaque breaks open and blocks blood flow.

Two different ultrasound approaches exist to assess risk this way:

• Carotid intima-media thickness (CIMT) — a measurement of artery wall thickness.
• Carotid arterial plaque assessment — looking for and measuring actual plaque deposits.

This document focuses on the second approach: how to quantify carotid plaque, when it is present, for the purpose of cardiovascular risk stratification (sorting patients into risk groups).

Rationale: Why Plaque Beats Simple Wall Thickness

The field has shifted from CIMT to plaque measurement for a clear reason: plaque assessment appears to offer greater risk-prediction benefit. This has led to a "paradigm shift" in which carotid ultrasound parameters are used for risk prediction.

The key insight is that CIMT and plaque may reflect two different biological processes. CIMT may mostly show the presence of cardiovascular risk factors such as high blood pressure. Carotid plaque, by contrast, is a sub-intimal process (starting beneath the innermost layer of the artery) and may better reflect atherosclerosis itself. It correlates with the total burden of atherosclerosis in the coronary arteries that feed the heart.

The largest portion of CIMT — roughly 99% in healthy individuals and about 80% in those with disease — consists of the medial layer, which is prone to non-atherosclerotic medial hypertrophy (thickening) commonly caused by aging and hypertension. Plaque, on the other hand, begins in the intimal layer and represents the atherosclerotic process itself. This is why plaque has been shown to predict cardiovascular events better than CIMT.

The high prevalence of carotid atherosclerosis in people who otherwise have a low Framingham risk score (a standard 10-year heart risk calculator) has potential implications for screening. Many people who look "low risk" on paper actually have hidden plaque.

CIMT still provides useful information even when no plaque is present. The ASE's earlier consensus statement on using carotid ultrasound to identify subclinical vascular disease continues to be endorsed. That document already covers the rationale for carotid ultrasound, scanning technique, reporting of results, and training of sonographers. This new document complements it by adding a standardized way to define and measure plaque.

Why is a new document needed? Since the 2008 consensus, carotid ultrasound technology has advanced tremendously. First came dedicated three-dimensional (3D) vascular ultrasound probes. More recently, a 3D matrix array probe for carotid ultrasound with companion analysis software has become available. This document is the first to provide systematic recommendations for standardizing plaque quantification.

What These Guidelines Cover

This consensus statement provides recommendations for two-dimensional (2D) and 3D quantification of carotid arterial plaque by ultrasound, for the basis of cardiovascular risk stratification.

It also discusses emerging techniques, including the role of ultrasound enhancing agents (UEA, injectable contrast agents that make blood and vessels show up more clearly) for assessing intraplaque neovascularization (new tiny blood vessels growing inside a plaque) and for analyzing plaque composition.

Defining Plaque: Two Distinct Types

Atherosclerotic plaques can be discrete, easy-to-spot lesions. But plaque can also be eccentric (off-center) and spread across the surface of the wall. In those cases, it is difficult to tell whether there is simply medial thickening or eccentric, diffuse plaque.

Because of this, different studies have used very different thresholds to define plaque. Definitions have ranged widely:

• One study defined plaque as focal thickening of the intima-media greater than 1 mm, protruding into the lumen, and at least twice as thick as the surrounding normal CIMT.
• Another study defined plaque as CIMT greater than 1.2 mm.
• The European Mannheim consensus defined plaque as focal thickening that encroaches into the lumen by 0.5 mm, or by 50% of the surrounding intimal-medial thickness, or where CIMT is greater than 1.5 mm.

Put together, definitions across studies have varied from 0.5 mm to more than 1.5 mm.

The writing panel chose a threshold slightly more conservative than the Mannheim consensus. They recommend ≥1.5 mm (rather than "greater than 1.5 mm") as the CIMT cutoff for diffuse plaque. This new Plaque Grading Consensus allows doctors to also identify and characterize small protruding plaque lesions that fall below the 1.5 mm threshold.

In plain terms: modern ultrasound can now resolve plaque lesions smaller than 1.5 mm in exquisite detail, allowing both measurement and even analysis of what the plaque is made of. The new grading system sets a framework for continued outcomes-based research across the full spectrum of plaque shapes, sizes, and types.

Recommendation #1. Carotid arterial plaque seen on ultrasound (with or without an ultrasound enhancing agent) should be defined in one of two ways:

1. Protuberant-type plaque — any focal thickening thought to be atherosclerotic in origin that encroaches into the lumen of any segment of the carotid artery.
2. Diffuse-type plaque — in diffuse vessel wall atherosclerosis, when carotid intima-media thickness measures ≥1.5 mm in any segment of the carotid artery.

Recommendation #2. Both protuberant and diffuse types of carotid plaque should be evaluated for cardiovascular risk stratification and for serial assessment of atherosclerosis (tracking it over time).

Recommendation #3. The carotid arterial wall should first be scanned visually for protuberant plaque. If none is found, then CIMT measurement should be performed to identify diffuse plaque, defined as CIMT ≥1.5 mm. If performed, CIMT should be measured as described in the earlier ASE consensus statement.

Importantly, the grading system does not reflect how blocked the vessel is. Instead, it aims to standardize the size of an individual plaque lesion so that results can be compared across studies. Grade I applies only to small protuberant lesions. This is because, if a plaque is non-protuberant (diffuse or eccentric) and less than 1.5 mm thick, it is currently impossible to tell whether the thickening is entirely medial or intimal. At a CIMT value of 1.5 mm or greater (Grades II and III), the framework attributes the thickness to diffuse atherosclerotic plaque and considers it a "plaque equivalent."

When Is Plaque "Clinically Significant"?

Some centers repeat CIMT evaluation even when no plaque is found, if CIMT is above the 75th percentile for a person's age, race, and gender. However, the evidence for how often to repeat testing is thin. Population studies have used a repeat interval of 2 to 5 years, though published evidence suggests that more frequent CIMT measurements could increase the precision of tracking CIMT progression.

Despite these methodological questions, the expert panel recommends against serial CIMT measurements for cardiovascular risk stratification — especially when the result does not meet the threshold for diffuse plaque (≥1.5 mm).

The panel acknowledged that, based on limited or anecdotal evidence, serial CIMT measurements may still have value in the hands of some experts for research, for monitoring progression or regression in specific cases, and as a potential tool to change patient behavior. It is also possible that over time a patient's CIMT could rise to ≥1.5 mm, meaning diffuse atherosclerotic plaque has developed by this definition. But the clinical usefulness of long-term CIMT monitoring is not yet established.

The panel suggests that a CIMT ≥1.5 mm be considered a clinically significant lesion for patients younger than 65 years of age. Plaque thickness (also called height in the long-axis view) was chosen as the initial measure to define plaque because it is widely available and can be measured in both protuberant and diffuse plaque. Other techniques, such as area and volume, apply mostly to protuberant-type lesions and are hard to define in plaques that are diffusely layered along the wall.

Recommendation #4. Serial CIMT measurements are not recommended in asymptomatic patients. Repeat measurements are not recommended unless the grade and CIMT meet the criteria for diffuse-type plaque (Grades II or III, with CIMT ≥1.5 mm) — in which case it is a plaque equivalent.

How Plaque Is Measured: Two-Dimensional Techniques

Interest in plaque quantification grew significantly when researchers discovered that the simple presence or absence of plaque added risk-stratification value beyond CIMT alone. This finding came from the ARIC Study, a large population study.

The logic is straightforward. If simply knowing whether plaque is present re-stratifies patients beyond traditional risk factors, then measuring how much plaque is there may personalize a patient's risk assessment even further. Several 2D techniques exist, and each has advantages and disadvantages.

Method 1: The Plaque Score

The plaque score is a semi-quantitative approach. The doctor visualizes and counts the total number of sites containing plaque along the common carotid artery (CCA), the carotid bulb, and the internal carotid artery (ICA), then adds them up.

This approach varies greatly among studies. Some investigators count plaque lesions in any visualized segment. Others count only lesions in easily identified segments, such as the distal first centimeter of the CCA, the bulb, and the proximal ICA.

Two important analyses from the Rotterdam Study — a prospective, population-based cohort of elderly people — used a unique process. The two studies included 4,217 and 6,389 participants. Both measured plaque at 6 locations in the carotid arteries: two sides each of the CCA, the bifurcation, and the ICA.

The total plaque score ranged from 0 to 6. It was calculated by adding the number of sites where plaque was detected, dividing by the total number of sites with a usable ultrasound image, and multiplying by 6 (the maximum number of sites). In the larger study of 6,389 people, scores of 0, 1, 2, and ≥3 points were considered to represent no, mild, moderate, or severe carotid atherosclerosis.

For risk prediction and standardization, the panel recommends that if a plaque score is being calculated, lesions limited to the distal 1 cm of the CCA, the bulb, and the proximal 1 cm of the ICA should be included in the counting.

The links to real patient outcomes are strong. In the Three-city study of 5,895 individuals aged 65 to 85 who were free of cardiovascular disease at the start, plaque at one site was linked to a hazard ratio (HR, a measure of how much more likely an event is over time) of 1.5 (95% confidence interval [CI] = 1.0–2.2). Plaque at two or more sites carried an HR of 2.2 (95% CI = 1.6–3.1; p < 0.001 for trend).

Adding plaque information to traditional risk factors also improved the area under the curve (AUC, a statistical measure of how well a prediction model sorts people correctly; 0.5 is no better than chance and 1.0 is perfect) for CVD prediction from 0.728 to 0.745 (p = 0.04). The net reclassification index (NRI, the share of patients moved into a more accurate risk category) was 13.7%. Another study of 367 men (mean age 78 ± 4 years) found that the hazard ratio for death over 4 years rose from 2.89 for a plaque score of 1–2 to 4.53 for a plaque score of 7–12.

Advantages. The plaque score is easy to perform and does not require advanced quantification software. Because individual lesions are simply seen and counted rather than measured, the angle or plane of imaging matters less. Even though it is a fairly rough reflection of plaque extent, it predicts risk better than simply reporting whether plaque is absent or present.

Disadvantages. The plaque score is semi-quantitative — it only counts the number of lesions. It does not consider the size of an individual plaque, which would better reflect the overall extent of atherosclerosis. Because lesions at distinct sites are counted, the score may overestimate when two "separate" plaques are actually contiguous (touching). Conversely, if a large contiguous area of plaque is counted as just one lesion, the score will underestimate the extent of disease.

Method 2: Plaque Height (Thickness)

Plaque thickness, or height, can be thought of as a variation on the maximal CIMT measurement. It differs in that it represents how far the plaque protrudes in a radial direction — outward from its origin along the vessel wall and into the lumen (the open channel where blood flows).

To measure it, some investigators suggest using electrocardiographic gating (timing the measurement to the heartbeat) so that measurements are taken at the same phase of the cardiac cycle each time. Typically, the doctor performs cross-sectional (transverse) sweeps to find plaque. Once a lesion is identified, electronic calipers (available with most software) are placed starting along the origin of the plaque at the vessel wall and extending into the lumen at right angles to the wall, along the most protruding part of that plaque.

The maximum plaque height or thickness among all identified lesions — seen in both the right and left carotid arteries — is then reported. Importantly, studies using this method do not sum the plaque heights. They report the single largest plaque height found anywhere in the patient.

Plaque height measured this way is highly reproducible. In the Northern Manhattan Study (NOMAS), the intra-class correlation coefficient (a statistical measure of agreement, where 1.0 is perfect) for inter-observer reliability was 0.77, and for intra-observer reliability it was 0.94.

Variability in this method comes from deciding where within the vessel wall the measurement should begin. To reduce variability across studies, the panel recommends beginning the measurement at the adventitial-medial layer, similar to how CIMT is measured.

Recommendation #5. Plaque thickness (height) should be measured as the initial 2D approach for quantifying carotid ultrasound plaque. Though plaque height is often measured from 2D images, it can be obtained from a 3D image when available, which overcomes the out-of-plane limitations of 2D imaging.

Recommendation #6. The maximal plaque height should be measured from the side where plaque is detected (unilateral) or from both the right and left carotid segments (bilateral), using a caliper placed at the adventitial plane and extending into the center of the lumen at right angles to the vessel wall. For standardization, this measurement should be taken from any segment of the long and short axis of the carotid artery (bulb, ICA, CCA), and the view and segment should be reported accordingly. The measurement begins at the same plane where CIMT measurement begins, to stay consistent with defining plaque beyond the CIMT threshold of 1.5 mm.

Plaque Height and Patient Outcomes

The document links maximum plaque height directly to clinical outcomes, reinforcing why this measurement matters. Higher plaque height has been associated with cardiovascular events in the studies reviewed by the panel, and the measurement's high reproducibility (intra-observer correlation of 0.94 in NOMAS) makes it practical for routine use. This is the core reason the panel selected thickness as the primary measurement tool.

Method 3: Plaque Area

Carotid plaque area is the most advanced of the 2D quantification methods. The process begins with a manual sweep of the carotid artery, typically scanning in cross section, to first identify plaque lesions. Once identified, the area is traced and measured, and the total value is reported.

Area measurements apply mainly to protruding lesions. They are harder to apply to plaques that are diffusely layered along the inner wall, which may be focal or diffuse wall calcification layered in concentric or eccentric patterns. Such lesions may or may not be atherosclerotic in origin.

Advanced and Emerging Techniques

Beyond simply measuring how much plaque is present, the guideline document explores the concept of plaque vulnerability — the idea that some plaques are more likely to rupture and cause events than others, regardless of size.

Several emerging methods are considered in the full document:

• Plaque neovascularization (new blood vessel growth inside plaque) and anatomy, assessed with contrast-enhanced ultrasound using ultrasound enhancing agents.
• Gene and biomarker approaches (noted in the authors' conflict-of-interest disclosures as an area of active research).
• Gray Scale Median (GSM) analysis — a computer-based method that measures how bright or dark plaque appears on ultrasound. Softer, echolucent (darker) plaque may signal higher risk.
• Pixel distribution analysis (PDA) — a related technique for characterizing plaque texture.
• Multi-modality assessment — combining ultrasound with other imaging such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), including fluorodeoxyglucose (FDG) PET, which detects metabolically active (inflamed) plaque.

These techniques aim to move beyond "how much plaque" toward "what kind of plaque" — a distinction that may matter greatly for predicting who will have a heart attack or stroke.

Putting Plaque Imaging into Practice

The document outlines where carotid plaque imaging fits in real patient care. Two scenarios receive particular attention:

1. Primary prevention in asymptomatic patients. People with no symptoms may still have hidden plaque that changes their risk category. The high prevalence of carotid atherosclerosis in people with otherwise low Framingham risk scores is exactly why screening for subclinical atherosclerosis has potential value.
2. Symptoms suggesting coronary artery disease, but normal non-invasive tests. When standard tests come back normal but a patient has symptoms, plaque imaging may help explain the picture and guide decisions.

The panel notes that a patient's CIMT may increase over time to ≥1.5 mm, signifying the development of diffuse atherosclerotic plaque by this definition. However, the clinical utility of long-term CIMT monitoring is not yet established.

Limitations and Open Questions

Several important gaps remain, and the panel is transparent about them.

• There is a lack of evidence on how often to repeat testing. The 2-to-5-year interval used in population studies is not firmly established.
• Debate continues over whether the transition from increased CIMT to plaque is one continuous process, or whether CIMT and plaque are truly separate conditions.
• It can be difficult to distinguish medial thickening from diffuse atherosclerotic plaque on an ultrasound image alone.
• Ultrasound cannot currently tell whether a non-protruding lesion under 1.5 mm thick is entirely medial or intimal.
• Further work is needed to standardize the plaque score across different clinics and studies.
• The value of serial CIMT measurements in expert hands rests on limited or anecdotal evidence.

The panel deliberately set a slightly more conservative threshold than the Mannheim consensus (≥1.5 mm rather than greater than 1.5 mm) and notes this creates a framework for continued research rather than a final answer.

Key Recommendations at a Glance

1. Define carotid plaque either as protuberant-type (focal thickening encroaching into the lumen) or diffuse-type (CIMT ≥1.5 mm in any carotid segment).
2. Evaluate both plaque types for risk stratification and for tracking atherosclerosis over time.
3. Scan first for protruding plaque. If none is found, measure CIMT to find diffuse plaque at the ≥1.5 mm threshold.
4. Do not perform serial CIMT measurements in asymptomatic patients unless the result meets the diffuse plaque criteria (Grades II or III, CIMT ≥1.5 mm).
5. Use plaque thickness (height) as the initial 2D measurement for quantifying carotid plaque.
6. Measure maximal plaque height with a caliper placed at the adventitial plane, extending into the center of the lumen at right angles to the wall, from any segment in the long and short axis views.

What does this mean for patients? If your doctor orders a carotid ultrasound, this guideline encourages a structured look for plaque — not just a quick check of artery thickness. If plaque is found, its height will likely be measured and reported. That number can help your care team decide whether you need more aggressive prevention, such as cholesterol-lowering medication or blood pressure control. If no plaque is found, a CIMT measurement may still be taken to look for diffuse disease.

Frequently Asked Questions

What is carotid plaque and why does it matter?

Carotid plaque is fatty, hardened buildup in the arteries of your neck. It matters because these arteries mirror what is happening elsewhere in your body. Plaque can narrow the vessel or rupture, blocking blood flow and causing a stroke. Measuring plaque helps estimate your risk of heart attack and stroke beyond traditional risk factors.

How is carotid plaque measured on ultrasound?

Doctors first scan visually for protruding plaque. If none is found, they measure carotid intima-media thickness (CIMT) to detect flat, diffuse plaque. The recommended initial measurement is plaque thickness, also called height, taken with a caliper from the vessel wall into the lumen. This method is simple, widely available, and highly reproducible.

What does a CIMT of 1.5 mm or more mean?

A CIMT of 1.5 mm or greater in any carotid segment is considered diffuse-type plaque, or a plaque equivalent. The panel suggests this be viewed as a clinically significant lesion for patients younger than 65. It does not reflect how blocked the vessel is, but it does indicate atherosclerosis and may lead to more aggressive prevention.

Will I need repeat ultrasound tests?

Serial CIMT measurements are not recommended for asymptomatic patients unless the result meets the criteria for diffuse plaque, meaning CIMT is 1.5 mm or greater. Even then, the evidence for how often to repeat testing is thin. Population studies have used intervals of 2 to 5 years, but this is not firmly established.

How much does plaque increase my risk of heart attack or stroke?

Studies cited show that some plaque measures are linked to 1.5 to 4.5 times higher risk of cardiovascular events or death. For example, in the Three-city study of 5,895 older adults, plaque at one site was linked to a hazard ratio of 1.5, and at two or more sites, 2.2. Adding plaque information improved risk prediction.

What happens if the ultrasound finds no plaque?

If no protruding plaque is seen, a CIMT measurement may still be taken to look for diffuse disease. If CIMT is below 1.5 mm, it does not meet the threshold for diffuse plaque. Your doctor may still consider other risk factors. The guideline encourages a structured look for plaque, not just a quick check of artery thickness.

My carotid ultrasound found plaque — when should I get a second opinion on my heart and stroke risk?

A second opinion can help when a carotid ultrasound reports plaque but the meaning of the measurement is unclear. Plaque thickness, or height, is the recommended initial measurement, and higher plaque height has been linked to cardiovascular events. Plaque information can reclassify risk beyond traditional factors, with some measures linked to 1.5 to 4.5 times higher risk of cardiovascular events or death. An independent review can confirm whether plaque was defined as protuberant or diffuse and whether the reported height was measured consistently. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: Assessment of Carotid Arterial Plaque by Ultrasound

Authors: Amer M. Johri, MD, FASE; Vijay Nambi, MD, FASE; Tasneem Z. Naqvi, MD, FASE; Steven B. Feinstein, MD; Esther S. H. Kim, MD, MPH, FASE; Margaret M. Park, ACS, RDCS, RVT, FASE; Harald Becher, MD, PhD; and Henrik Sillesen, MD, DMSc.

Author affiliations: Queen's University, Kingston, Ontario, Canada; Baylor College of Medicine, Houston, Texas; Mayo Clinic, Phoenix, Arizona; Rush Medical College, Chicago, Illinois; Vanderbilt University Medical Center, Nashville, Tennessee; Cleveland Clinic Heart and Vascular Institute, Cleveland, Ohio; University of Alberta Hospital, Mazankowski Alberta Heart Institute, Edmonton, Alberta, Canada; and Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.

Publication: Journal of the American Society of Echocardiography, August 2020, Volume 33, Number 8, pages 917–933. Copyright 2020 by the American Society of Echocardiography. DOI: 10.1016/j.echo.2020.04.021.

Endorsements: This document is endorsed by numerous international partners of the American Society of Echocardiography International Alliance, including societies from Argentina, ASEAN, the Asian-Pacific region, Australia, Canada, China, Brazil, Cuba, India, Iran, Italy, Japan, Korea, Mexico, the Philippines, Saudi Arabia, Thailand, Venezuela, and Vietnam.

Disclosures: Several authors reported relationships with commercial entities, including Bracco Imaging, Siemens, Philips, Ultromics, Lantheus Medical Imaging, Northwest Imaging Solutions, DIA Imaging, SonoGene, Amgen, Novo Nordisk, Cordex, and St. Jude Medical. Drs. Johri and Kim reported no actual or potential conflicts of interest related to this document.

This patient-friendly article is based on peer-reviewed research. It summarizes the publicly available excerpt of the guideline document, which covers plaque definitions, quantification methods, and clinical applications. The complete published guideline contains additional detail on 3D acquisition protocols, plaque composition analysis, gray scale median analysis, and multi-modality imaging.