Carotid Ultrasound: Why Measuring Plaque, Not Just Artery Wall Thickness, Matters for Your Heart and Brain Health

Table of Contents

• Key Points
• Why This Research Matters
• Two Measurements, Two Different Meanings
• How the Research Was Conducted
• Key Findings: What Large Studies Show
• The ELSA-Brasil Study: A Closer Look
• Genetics, Plaque, and New Therapies
• Measuring Treatment Response
• Clinical Implications: What This Means for Patients
• Limitations: What This Editorial Couldn't Prove
• Recommendations: What Patients Should Know
• Frequently Asked Questions
• Source Information

Key Points

• Carotid ultrasound measuring plaque burden, not just IMT, predicts heart attack and stroke risk far better.
• IMT is biologically distinct from plaque: it mostly reflects aging and demographics, while plaque reflects treatable risk factors.
• Plaque area or volume changes are larger and respond faster to treatment than IMT, making them better for monitoring.
• Statin therapy typically requires about 2 years of aggressive lipid lowering before plaque regression is seen.
• Lifestyle changes like weight loss and blood pressure reduction can measurably reduce artery vessel wall volume in 2 years.

Why This Research Matters

Your carotid arteries are like highways carrying oxygen-rich blood to your brain. When cholesterol and other substances build up inside them, they form plaque — a waxy deposit that can grow, rupture, and cause a heart attack or stroke. For years, doctors have used ultrasound to check these arteries, but there's been an important debate: which measurement actually predicts danger best?

Some doctors measure the thickness of the artery wall's inner two layers (the intima and media), a value called intima-media thickness (IMT). Others measure the actual plaque itself — either its area or volume. Still others measure stenosis, which is the percentage of narrowing of the artery opening.

This editorial, written by Dr. J. David Spence of Western University in London, Canada, was published in the journal Arteriosclerosis, Thrombosis, and Vascular Biology in 2015. It accompanies a large new study from Brazil (the ELSA-Brasil cohort, which included 9,792 participants) that found traditional heart risk factors explain surprisingly little of what IMT actually measures. The editorial pulls together evidence from multiple major studies to argue a critical point: these ultrasound measurements are not interchangeable — they capture biologically distinct processes in your body.

Two Measurements, Two Different Meanings

To understand the debate, you first need to know there are actually two different kinds of IMT measurements used in research — and they are not the same thing. As pointed out by Inaba and colleagues, it is crucial to distinguish between:

• Mannheim consensus IMT: Measured in the far wall of the distal common carotid artery, where there is no plaque. This measurement reflects thickening of the artery wall itself, not the buildup of plaque.
• ARIC-protocol IMT (used in the Atherosclerosis Risk in Communities study and related protocols): This method includes plaque thickness in the measurement at multiple locations, including the carotid bulb (the widened area where the artery branches).

This distinction matters enormously. IMT measured according to the Mannheim consensus does not represent atherosclerosis (the disease of plaque buildup). It is another phenotype entirely — a different biological trait.

There's also a key anatomical principle at work called compensatory enlargement (positive remodeling), first described by Glagov in 1987. As plaque builds up, the artery actually enlarges to accommodate it, so the opening (lumen) doesn't narrow — at least not at first. This means:

• Plaque burden reflects the effects of oxidative stress and a lifetime's exposure to heart risk factors.
• Stenosis (narrowing) reflects factors that cause plaque rupture and blood clotting (thrombosis).

An illustration of this principle: lipoprotein(a), or Lp(a) — a clotting factor — is associated with carotid stenosis and occlusion, but not with plaque burden. Different biological pathways, different measurements.

How the Research Was Conducted

This article is an editorial — a type of expert perspective article written by a leading researcher to comment on a new study and place it in the context of existing evidence. It is not a single clinical trial with patients. Instead, it reviews and synthesizes the findings of many large studies, meta-analyses, and clinical trials that together span decades of research.

The centerpiece is the ELSA-Brasil study by Santos and colleagues, a large Brazilian cohort study of 9,792 participants that used multiple regression analysis (a statistical method for determining how much of a measured value can be explained by various factors) to determine what predicts IMT. But the editorial also draws on evidence from:

• The Multi-Ethnic Study of Atherosclerosis (MESA) in the U.S.
• The Tromsø study in Norway (more than 6,000 participants)
• The Atherosclerosis Risk in Communities (ARIC) study
• The Northern Manhattan Study (NOMAS)
• The Cardiovascular Health Study (O'Leary et al.)
• Multiple meta-analyses combining data from thousands of patients worldwide

Key Findings: What Large Studies Show

The evidence is consistent and powerful: plaque measurement beats IMT measurement for predicting risk. Here are the specific findings from major studies.

Plaque Thickness Predicts Cardiovascular Risk

The Northern Manhattan Study, led by Rundek and colleagues, found that plaque thickness predicts cardiovascular risk. This may be why studies that include plaque thickness in their IMT measurement (like the ARIC protocol) tend to predict risk better — particularly in the elderly.

The ARIC Study: Added Value of Plaque

In the ARIC study, measuring IMT added only 0.08 to the "area under the curve" (a statistical measure of predictive accuracy, where 1.0 is perfect prediction and 0.5 is no better than chance) beyond traditional coronary risk factors. But when the presence of plaque was added, the improvement jumped to 0.17 — more than double. In plain terms, knowing whether plaque is present greatly improves risk prediction, while knowing the IMT adds relatively little.

Meta-Analyses: IMT Is a Weak Predictor

A systematic review and meta-analysis by Lorenz and colleagues (2007, published in Circulation) found that IMT measured without plaque is a weak predictor of cardiovascular risk. The PROG-IMT collaborative project (Lorenz, 2012, in The Lancet) went further: progression of IMT over time did not predict cardiovascular events. And a meta-analysis of 41 randomized trials by Costanzo and colleagues (2010, in the Journal of the American College of Cardiology) found regression of IMT did not predict reduction of cardiovascular events either. In other words, watching IMT go up or down tells you very little about whether you'll actually have a heart attack or stroke.

Plaque Area Is Stronger Than IMT

A meta-analysis by Inaba and colleagues (2012) concluded that plaque area was a stronger predictor of risk than IMT. Adams and Bojara, in a workplace health program study of more than 4,000 participants, found that plaque area and plaque type — but not IMT — predicted coronary stenosis and cardiovascular risk.

Coronary Calcium vs. IMT (MESA)

The Multi-Ethnic Study of Atherosclerosis (MESA) found that coronary calcium (a measure of calcified plaque in the heart's arteries) predicted cardiovascular risk in the overall population, but IMT did not.

Plaque Area for Excluding Heart Disease

Brook and colleagues (2006) reported that carotid total plaque area was more specific for excluding coronary artery stenosis (i.e., a negative test more reliably rules out significant heart artery narrowing) than either IMT or coronary calcium score.

A Study in Coronary Artery Disease Patients

Chan and colleagues (2003) found that among patients with known coronary artery disease, both total plaque area and impaired flow-mediated vasodilation (a measure of blood vessel health) predicted risk. In that study, IMT without plaque did not correlate with either measure and did not predict risk.

Plaque Echolucency, Not IMT, Predicts Residual Risk

A study by Uematsu and colleagues (2014) examined patients who had achieved their LDL ("bad") cholesterol goals on statin therapy but still had residual risk. The key predictor was carotid plaque echolucency (how dark the plaque appears on ultrasound, indicating softer, more dangerous plaque) — not IMT.

The Tromsø Study: A Landmark Comparison

The largest population-based study measuring both IMT and plaque burden side by side is the Tromsø study in Norway, with more than 6,000 participants. The results were striking:

• In the 7-year follow-up (6,226 persons), IMT in the common carotid did not predict coronary risk. IMT in the carotid bulb (which included plaque thickness) was a weak predictor. But total plaque area was a strong predictor of coronary risk.
• In the 10-year follow-up (6,584 men and women), IMT did not predict stroke, whereas total plaque area was a strong predictor of stroke.
• A 13-year follow-up (Herder and colleagues, 2012) found that total cholesterol, systolic blood pressure, and smoking were stronger predictors of progression of total plaque area than of IMT. By contrast, sex and age were stronger predictors of IMT — suggesting IMT mostly reflects aging and demographics, not treatable risk factors.

How Much Do Risk Factors Explain?

Previous studies reported that traditional coronary risk factors explained 52% to 57% of total plaque area, but only 13% of carotid stenosis. O'Leary and colleagues (Cardiovascular Health Study, using methods that included plaque thickness) found that coronary risk factors explained just 17% of IMT in the common carotid and 18% of IMT in the carotid bulb. The Northern Manhattan Study found only 11% of IMT was explained by traditional risk factors, with age and male sex accounting for most of the explained variance. Glucose and smoking (pack-years) also contributed, and LDL cholesterol was only marginally significant. Even an extended model — adding inflammatory biomarkers, adiponectin, homocysteine, and kidney function — explained only 16% of the variance, with adiponectin as the only additional significant contributor.

The ELSA-Brasil Study: A Closer Look

Into this landscape comes the ELSA-Brasil study (Santos et al., 2015), which the editorial accompanies. In this large Brazilian cohort of 9,792 participants, the researchers analyzed how much of IMT could be explained by various factors. Their findings added important nuance.

Santos and colleagues found that their comprehensive set of factors explained a higher proportion of IMT (a higher R² — the statistical "explained variance") than previous studies had reported. But the key insight was which factors did the explaining. Factors such as:

• Race
• Pulse pressure (the difference between systolic and diastolic blood pressure)
• Neck circumference

...made a greater contribution to predicting IMT (i.e., had higher beta values in the statistical models) than the traditional coronary risk factors like cholesterol, smoking, and blood pressure. Perhaps most tellingly, IMT was predicted less strongly among participants with a predicted coronary risk greater than 10% — the very people who most need accurate risk assessment.

Even with all these additional predictors, the total explained variance (the R²) rose to only 0.3 — meaning 70% of what IMT measures remains unexplained by known risk factors. This powerfully illustrates that IMT is a different biological phenomenon from plaque burden, which is far better explained (52-57%) by traditional risk factors.

Genetics, Plaque, and New Therapies

The biological distinctness of these ultrasound phenotypes is also visible in genetics. Dr. Spence notes that, as would be expected, genetic factors for IMT are different from those for plaque burden. Similarly, genetic factors affecting stenosis would be expected to differ, as they would preferentially affect plaque rupture and thrombosis. And genetic factors affecting coronary calcium scores would preferentially affect calcification. This means a person could inherit genes that thicken their artery walls without inheriting genes that cause dangerous plaque — and vice versa.

These distinctions have practical consequences for clinical trials. The recommended sample size for a study testing whether a therapy changes IMT is approximately 300 participants per group, followed for 2 years. Why so large and so long? Because the average progression of IMT is tiny — about 0.15 mm per year — while the spatial resolution (the smallest detail the ultrasound can reliably detect) of carotid ultrasound is about 0.3 mm. You're trying to measure a change that's half the size of your measuring tool's precision.

By contrast, two-dimensional (2D) and three-dimensional (3D) measurements of plaque burden change by much larger amounts:

• Total plaque area changes by about 10 mm² per year
• 3D total plaque volume changes by about 50 to 100 mm³ per year

These larger changes are much easier to measure reliably, making plaque measurements far more sensitive for detecting whether a treatment is working.

Measuring Treatment Response: Proof That Plaque Responds Faster

The editorial highlights several studies showing that plaque measurement can detect treatment effects quickly and with far fewer patients:

Statin Treatment: Rapid Plaque Regression

It was possible to show a significant change in carotid plaque volume with atorvastatin (a common statin medication) in just 3 months, using only about 20 patients per group — a dramatic contrast to the 300 patients and 2 years needed for IMT studies. In this randomized trial:

• Patients on placebo: total plaque volume increased by 16.81 ± 74.10 mm³
• Patients on atorvastatin: total plaque volume regressed by −90.25 ± 85.12 mm³
• The difference was statistically highly significant (P<0.001), meaning there's less than a 0.1% chance this result was due to random chance.

Furthermore, a study by Wannarong and colleagues (2013) found that progression of total plaque volume predicted cardiovascular events among patients attending vascular prevention clinics — but progression of IMT or plaque area did not.

Vessel Wall Volume: For Patients Without Plaque

For patients or study participants who don't yet have plaque, it's still possible to measure something called vessel wall volume (VWV) — the total volume of the artery wall itself. VWV also changes by amounts easy to measure in small groups over short time periods:

• Atorvastatin significantly reduced VWV in 3 months in a study with about 20 patients per group (Krasinski et al., 2009).
• Even weight loss and blood pressure reduction through diet alone showed significant VWV changes in just 2 years, with a mean reduction in VWV of −58.1 mm³ (95% confidence interval, −81.0 to −35.1 mm³; P<0.001).

How Long Should You Stay on Statins?

One of the most practical clinical takeaways from this editorial comes from a systematic review (Noyes and Thompson, 2014) of intravascular ultrasound and other methods. It found that atherosclerotic plaque regression with statin therapy — in studies that documented regression — occurred after an average time of 19.7 months. This suggests that patients should undergo about 2 years of aggressive lipid (cholesterol) reduction before considering any reduction in statin therapy.

Why Carotid Plaque Is Easier to Shrink Than Coronary Plaque

Carotid plaque is focal — it grows in patches. This means it can change in three dimensions: thickness, length, and circumferential extent. In fact, carotid plaques change in length (along the axis of blood flow) 2.4 times faster than they thicken. This three-dimensional flexibility makes them much more sensitive to treatment effects.

Coronary plaques (in the heart's arteries), by contrast, extend around the entire circumference of the artery and along the entire length of the ultrasound pullback — so they aren't focal. Their change over time reduces to a single dimension: average thickness. This is why carotid plaque is a more practical and sensitive target for measuring treatment response.

Clinical Implications: What This Means for Patients

For patients, these findings have several important implications:

1. Not all carotid ultrasound tests are equal. If your doctor orders a carotid ultrasound, ask what's being measured. A test that reports only IMT may be giving a very incomplete picture of your risk. A test that measures plaque burden (total plaque area or volume) is far more informative.
2. IMT is not a reliable marker of your risk factor control. Since traditional risk factors explain only a small fraction of IMT, and since IMT progression/regression doesn't reliably predict events, watching your IMT number go up or down may not tell you whether your treatment is working.
3. Plaque burden is a much better gauge of whether you're being protected. Changes in plaque area and volume respond quickly and measurably to effective treatment — and progression of plaque volume predicts future cardiovascular events.
4. Give statins time to work. Based on the landmark average of 19.7 months to regression, patients should expect to stay on aggressive lipid-lowering therapy for about 2 years before considering whether the dose can be safely reduced. Don't stop early.

Limitations: What This Editorial Couldn't Prove

It's important to understand the limits of this article:

• It's an editorial, not a clinical trial. It presents Dr. Spence's expert interpretation of existing research, not new patient data. Editorials are valuable for synthesis but are a form of expert opinion.
• The ELSA-Brasil study still leaves much unexplained. Even with added factors like neck circumference, pulse pressure, and race, the total explained variance (R²) was only 0.3 — meaning 70% of IMT variation remains unexplained. The editorial argues this proves IMT is biologically distinct, but it also highlights how little we understand about what drives IMT.
• Conflicting definitions across studies. Much of the confusion in this field stems from some studies measuring "IMT" in ways that include plaque and others not. This inconsistency has historically muddied the research literature.
• Many cited studies predate modern imaging. Some of the studies referenced were conducted with older ultrasound technology and methods.
• Potential conflicts of interest. Dr. Spence disclosed receiving grants from the Canadian Institutes of Health Research, the Heart & Stroke Foundation of Canada (Ontario), the National Institutes of Health, and industry grants (Pfizer and Merck). He also received lecture honoraria, travel support, and consulting fees from Bayer, Merck, and Boehringer-Ingelheim, and conducted contract research with multiple pharmaceutical companies. He is also a shareholder and officer of Vascularis Inc.
• Population diversity limits. While ELSA-Brasil adds valuable data from a Brazilian population, many of the other key studies (Tromsø, ARIC, NOMAS) were conducted in North American or European populations, which may not fully generalize worldwide.

Recommendations: What Patients Should Know

Based on this research, here are practical steps for patients and their families:

• Ask your doctor what your ultrasound actually measures. If you're getting a carotid ultrasound, ask whether it will measure plaque burden (plaque area or volume) and not just IMT. Plaque burden is the measurement that best predicts your risk of heart attack and stroke.
• Know your plaque status. If you have plaque, treatment should be aggressive and monitored. Plaque progression is a warning sign; plaque regression is a good sign.
• Don't be discouraged by a "stable" IMT — and don't be falsely reassured. IMT has been shown in meta-analyses to be a weak predictor, and changes in IMT don't reliably predict changes in your risk.
• Stick with statin therapy for at least 2 years. The evidence shows plaque regression typically takes an average of about 19.7 months of aggressive lipid lowering. If you're considering stopping or reducing your statin, discuss the 2-year timeline with your doctor first.
• Lifestyle changes work — and you can see them. The study showing weight loss and blood pressure reduction through diet reduced vessel wall volume by −58.1 mm³ in 2 years is proof that lifestyle changes produce measurable improvements in your arteries.
• Manage the factors that drive plaque. Unlike IMT (which is largely explained by age and sex), plaque burden is strongly driven by treatable factors: smoking, high blood pressure, and high cholesterol. These explained 52-57% of total plaque area in previous studies.

Frequently Asked Questions

What is the difference between IMT and plaque burden on a carotid ultrasound?

IMT measures the thickness of the artery wall's inner layers, but standard IMT protocols avoid plaque. Plaque burden measures the actual waxy buildup in the artery. They are biologically distinct: plaque reflects oxidative stress and lifetime risk factors, while IMT largely reflects aging and demographics. Measuring plaque predicts heart attack and stroke risk far better than wall thickness alone.

What should I ask my doctor before having a carotid ultrasound?

Ask whether the ultrasound will measure plaque burden — total plaque area or volume — rather than only IMT. A test reporting only IMT may give an incomplete picture of your risk. Also ask how the results will guide your treatment, since plaque progression or regression is a more reliable indicator of whether therapy is working.

Why is my IMT not changing even though I am on treatment?

IMT changes are tiny and slow, about 0.15 mm per year, and ultrasound resolution is only 0.3 mm. Large meta-analyses show IMT progression or regression does not reliably predict heart events. Plaque area or volume changes are much larger and respond quickly to effective treatment, so they are better for monitoring your response.

How long should I stay on statins to see plaque regression?

A systematic review found that atherosclerotic plaque regression with statin therapy, when it occurred, happened after an average of 19.7 months. Therefore, the editorial recommends about 2 years of aggressive cholesterol-lowering before considering reducing or stopping your statin. Always discuss this timeline with your doctor first.

Can lifestyle changes really make a measurable difference in my arteries?

Yes. A study showed that weight loss and blood pressure reduction through diet alone significantly reduced vessel wall volume by -58.1 mm³ in 2 years. This demonstrates that lifestyle changes produce measurable improvements in artery health. Plaque burden is also strongly driven by treatable factors like smoking, high blood pressure, and high cholesterol.

Does a normal IMT mean I have no risk of heart attack or stroke?

No. IMT alone is a weak predictor of cardiovascular risk. Even if your IMT is stable or normal, you could still have dangerous plaque. In studies, traditional risk factors explained only a small fraction of IMT, and IMT changes did not predict events. Measuring plaque burden is much more informative for assessing your true risk.

Source Information

Original article title: Spence Carotid Ultrasound Phenotypes Are Biologically Distinct

Author: J. David Spence, MD, Stroke Prevention and Atherosclerosis Research Centre, Robarts Research Institute, Western University, London, Canada.

Journal: Arteriosclerosis, Thrombosis, and Vascular Biology, 2015;35:1910-1913. DOI: 10.1161/ATVBAHA.115.306209

Accompanied article: Santos IS, Alencar AP, Rundek T, et al. "Low impact of traditional risk factors on carotid intima-media thickness: the ELSA-Brasil cohort." Arterioscler Thromb Vasc Biol. 2015;35:2054-2059.

© 2015 American Heart Association, Inc.

This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and is not a substitute for professional medical advice. Always consult your physician about your individual health situation.