What Are Normal Carotid Ultrasound Results?

A normal carotid ultrasound report describes arteries with smooth walls, no visible plaque, and blood flowing at velocities below 125 cm/sec in the internal carotid artery. Those are the benchmarks most vascular labs use, drawn from a widely adopted consensus published by the Society of Radiologists in Ultrasound. But “normal” on a carotid ultrasound is not a single number; the report covers several distinct measurements, and understanding what each one means gives you a much clearer picture of what your results actually say about your stroke risk.

What the Test Actually Measures

A carotid ultrasound (also called carotid duplex or Doppler) combines two types of imaging. The first is a grayscale picture of the artery walls, which lets the sonographer measure wall thickness and look for plaque deposits. The second uses Doppler technology to track the speed and direction of blood flow through the artery. Together, these two modes give your doctor information about the structure of the vessel and how freely blood is moving through it.¹

The exam typically covers the common carotid artery on each side of the neck, the point where it splits into two branches (the bifurcation), and both branches: the internal carotid artery, which feeds the brain, and the external carotid artery, which supplies the face and scalp. Some labs also check the vertebral arteries, which run alongside the spine toward the back of the brain. The whole scan usually takes 20 to 45 minutes and involves no radiation or needles.

Blood Flow Velocity and What Counts as Normal

The single most important number on a carotid ultrasound report is the peak systolic velocity, or PSV, of the internal carotid artery. This is the fastest speed blood reaches during each heartbeat. According to the consensus criteria from the Society of Radiologists in Ultrasound, a normal internal carotid artery has a PSV below 125 cm/sec with no plaque or wall thickening visible on the grayscale image.² When plaque narrows the artery, blood squeezes through the gap at higher speeds, the way water speeds up when you put your thumb over the end of a garden hose. Higher velocity signals greater narrowing.

Your report may also list the end-diastolic velocity (EDV), which is the slowest flow speed at the end of the heart’s resting phase, and the ratio of internal carotid velocity to common carotid velocity. In healthy arteries, that ratio varies with age and sex. One study of adults grouped by decade found mean ratios ranging from about 0.65 in younger men up to around 0.91 in older men, with women generally showing somewhat higher ratios at younger ages.³ These ratios matter because they help the sonographer confirm that an elevated velocity is truly abnormal and not just a quirk of individual anatomy.

The waveform shape also carries information. A healthy internal carotid artery produces a low-resistance waveform, meaning blood flows forward throughout the entire cardiac cycle, because the brain constantly demands blood. If the waveform looks abnormal but no plaque is present, the cause may be cardiac rather than arterial. Conditions like heart failure, arrhythmias, and aortic valve disease can all distort Doppler waveforms without any carotid artery problem being present.⁴

Intima-Media Thickness

Many carotid ultrasound reports include a measurement of the intima-media thickness, often abbreviated IMT or CIMT. This is the combined thickness of the two innermost layers of the artery wall. It serves as an early marker for atherosclerosis, the process of fatty buildup in artery walls, sometimes years before an actual plaque forms.

Normal IMT values depend heavily on your age and sex. In a study that established reference limits across age groups, values for women were about 0.47 mm in the 18-to-29 range, 0.59 mm in the 30s, 0.67 mm in the 40s, and 0.70 mm in the 50s. For men, the numbers were similar in the youngest group (0.47 mm) but climbed higher with age: 0.62 mm, 0.72 mm, and 0.80 mm for the 30s, 40s, and 50s respectively.⁵ Community-based research has consistently confirmed that IMT rises with age and is thicker in men than women, though that gender gap narrows after about age 75.⁶

Because the “normal” range shifts so much with age, researchers have argued that age-specific percentile tables are more useful than a single cutoff. Data from the VITA population study showed that IMT increases steadily across every age group, and the upper percentile limits for different decades do not overlap, reinforcing the need to compare your measurement against people your own age and sex rather than a universal threshold.⁷

Whether IMT alone should change your treatment is debated. A large analysis in the Framingham Offspring Study found that while both common and internal carotid IMT predicted cardiovascular events, only the internal carotid measurement modestly improved risk classification beyond traditional risk factors.⁸ Some guidelines still consider routine IMT measurement controversial, partly because the measurement technique varies between labs and because other markers like coronary artery calcium scoring and direct plaque detection may do a better job of sorting people into risk categories.⁹

What About Plaque

On a normal scan, the artery wall appears smooth, with clean layers and no bumps protruding into the channel where blood flows. Under normal conditions, the wall typically measures between 0.5 and 0.9 mm on a grayscale image. A clinical practice guideline from the American Society of Echocardiography defines plaque in two ways: either a focal thickening that bulges into the lumen of the artery, or a diffuse wall thickening where IMT reaches 1.5 mm or more in any segment.¹⁰

If your report says “no plaque identified,” that is good news. Population-based data from large cohort studies, including the Rotterdam Study and the Multi-Ethnic Study of Atherosclerosis, have shown that people without carotid plaque are at very low risk for atherosclerosis-related stroke or cardiovascular events.¹¹ A recently proposed classification system called Plaque-RADS puts this category at its lowest risk tier, consistent with the earliest stage of artery health.

When plaque is present, the report will typically describe its location, size, and surface characteristics. Soft or irregular plaques tend to be more concerning than hard, calcified ones because they are more prone to rupture and send debris toward the brain. That said, heavily calcified plaque creates its own diagnostic problem, which is covered below.

How Stenosis Is Graded

If narrowing is present, the consensus system classifies it into a handful of categories based primarily on velocity and the visual appearance of the artery. Understanding these tiers helps you interpret what your report means in practical terms:

Normal: PSV below 125 cm/sec, no plaque or wall thickening visible.
Less than 50% narrowing: PSV still below 125 cm/sec, but some plaque or wall thickening is seen.
50-69% narrowing: PSV between 125 and 230 cm/sec with visible plaque.
70% or greater narrowing: PSV above 230 cm/sec with visible plaque and a clearly narrowed channel.
Near-occlusion: the artery channel is dramatically narrowed, sometimes with paradoxically low or variable velocities.
Total occlusion: no blood flow detected at all on any Doppler mode.

These categories come from the Society of Radiologists in Ultrasound consensus conference and are used by most vascular labs in the United States.² The velocity thresholds were chosen because they correspond to the narrowing ranges that drive surgical and medical treatment decisions. The 70% mark is especially important because several landmark trials established that patients with symptoms and 70% or greater narrowing benefit from surgical or stenting procedures.

Near-occlusion is a tricky category. Because the artery is almost entirely blocked, flow may actually slow down rather than speed up, making it look paradoxically less severe on velocity readings alone. Research has found that no single ultrasound measurement reliably distinguishes a near-occlusion from a standard narrowing above 50%.¹² This is one reason your doctor may order a second imaging test if the ultrasound findings are ambiguous.

Vertebral Arteries and the Rest of the Picture

A thorough carotid ultrasound often includes a look at the vertebral arteries. These are smaller vessels that supply the back of the brain, and their flow direction matters. Normal vertebral artery flow moves upward, toward the head. If flow reverses direction, it can signal a steal phenomenon caused by narrowing of the subclavian artery, the large vessel in the upper chest that feeds the arm. Research on subclavian artery stenosis has shown that the vertebral artery waveform changes in a predictable pattern as the upstream narrowing worsens, progressing from a small notch in the waveform to partially reversed and then fully reversed flow.¹³ A normal report will note antegrade (forward) flow in both vertebral arteries.

When the Numbers Might Be Misleading

Carotid ultrasound is an excellent screening tool, but several factors can push readings away from reality. Being aware of these helps you understand why your doctor might want a follow-up scan or a different type of imaging.

Heavily calcified plaque is the most common culprit. Calcium deposits create an “acoustic shadow” on the image, essentially a blind spot where the ultrasound beam cannot penetrate. One study of arteries with significant calcification found that the usual velocity criteria became unreliable: when researchers used the standard velocity cutoff to predict severe narrowing, sensitivity dropped to roughly 23 to 33%, far too low for confident diagnosis.¹⁴ A separate study confirmed that a small shadow has only a minor effect on accuracy, but a large shadow severely degrades the correlation between ultrasound and CT angiography results.¹⁵

Beyond calcification, other situations can make ultrasound readings less straightforward. Contralateral occlusion (a complete blockage on the opposite side) can increase flow on the open side, making that artery’s velocities look artificially high. Tortuous or kinked vessels, tandem blockages at multiple sites, and coexisting heart conditions can all distort the numbers.¹⁶ The shadow itself is not just a technical annoyance, either. Population data from the Atherosclerosis Risk in Communities study found that the mere presence of acoustic shadowing from calcified plaques was independently associated with a roughly two- to fourfold higher risk of ischemic stroke, depending on sex, compared to having no visible plaque at all.¹⁷

Operator Skill and Lab Variability

Carotid ultrasound is considered operator-dependent, meaning the skill and technique of the person holding the probe can affect the measurements.¹⁸ A quality assessment across a regional vascular network found significant variance in velocity measurements between different sonographers, though no single operator was consistently producing outlier results across all measurements.¹⁹ Factors as mundane as image quality, probe angle, lighting conditions, and even operator fatigue can influence the final numbers.²⁰

This is why accredited vascular labs follow standardized protocols and why borderline results are often repeated or confirmed with a second imaging method. If your result sits right at a threshold, a small measurement error could tip it into a different category. A good vascular lab will note any technical limitations in the report itself.

When Your Doctor Orders a Second Test

Ultrasound is almost always the first-line test for suspected carotid disease because it is inexpensive, widely available, and involves no radiation or contrast dye. But it has blind spots. It cannot see the full length of the carotid arteries, particularly the portion deep inside the skull or the origin at the aortic arch in the chest.¹⁸ When a treatment decision hinges on the degree of narrowing, CT angiography or MR angiography is usually ordered to confirm the ultrasound findings and provide a more complete view of the vessel from chest to brain.²¹

Each confirmatory test has trade-offs. CT angiography uses radiation and iodine-based contrast dye, which can be a concern for people with kidney problems. MR angiography avoids radiation but is less accurate without contrast enhancement, and certain MRA techniques can overestimate the degree of narrowing or even mistake a severe stenosis for a complete blockage.²² Color-flow Doppler has also improved the ability to tell a near-total blockage from a complete one. Older grayscale-only scans correctly identified only about a quarter of near-occlusions, while color Doppler bumped that rate to about 94%.²³ That distinction matters a great deal because a near-total blockage may still benefit from surgery, while a complete occlusion generally does not.

Reading Your Own Report

Carotid ultrasound reports are not standardized across every hospital, but most share a common structure. You will typically see a section for each side of the neck, with velocity readings for the common carotid, internal carotid, and external carotid arteries. Plaque presence and characteristics will be noted, along with any wall thickness measurements. The impression or conclusion section at the bottom summarizes the findings in plain clinical terms, and this is where the stenosis grade appears.

A few things worth knowing when you read the report yourself. First, mild wall thickening in a middle-aged or older adult is extremely common and is not the same as having a plaque. As noted earlier, normal IMT rises with every decade, so a 55-year-old man with a measurement of 0.75 mm is well within the expected range for his age and sex. Second, asymmetric velocities between the two sides do not automatically mean one side is diseased. Anatomic variation, slight differences in probe angle, and even differences in blood pressure between the arms can create small side-to-side discrepancies. Third, a report that describes “less than 50% stenosis” is not necessarily alarming. This is a broad category that includes everything from barely detectable wall thickening to a moderate but hemodynamically insignificant plaque. The clinical significance depends on whether you have symptoms, what your other risk factors look like, and whether the finding is changing over time.

Why the Same Scan Can Look Different at Two Labs

If you have ever had a carotid ultrasound at one facility and then another, you may have noticed slightly different numbers or even different conclusions. Part of this comes down to the operator variability described above. But labs also differ in which velocity criteria they use. While most follow the Society of Radiologists in Ultrasound thresholds, some institutions validate their own internal criteria against surgical outcomes, which can shift the cutoff values by 10 to 20 cm/sec in either direction. Equipment differences, transducer frequency, and software algorithms for calculating velocities also play a role. None of this means the test is unreliable. It means that comparing results between labs requires a bit of caution, and trending over time is most accurate when done at the same facility with the same protocol.

Automated measurement tools are making their way into the field, with some labs using cloud-based software to measure artery diameter and wall thickness with less human variability. Early reproducibility data suggest these tools may reduce some of the operator-driven noise, though the technology is still maturing and relies on the quality of the images fed into it.²⁰

1
PubMed Central. General principles of carotid Doppler ultrasonography
2
PubMed. Carotid artery stenosis: gray-scale and Doppler US diagnosis–Society of Radiologists in Ultrasound Consensus Conference
3
PubMed. Normal reference values of ratios of blood flow velocities in internal carotid artery to those in common carotid artery using Doppler sonography
4
Journal for Vascular Ultrasound. Cardiac Effects on Peripheral Vascular Doppler Waveforms
5
PubMed. Normal values for intima-media thickness of the common carotid artery–an update following a novel risk factor profiling
6
PubMed. Age- and gender-associated determinants of carotid intima-media thickness: a community-based study
7
Journal of Thrombosis and Haemostasis. Age-adjusted reference limits for carotid intima-media thickness as better indicator of vascular risk: population-based estimates from the VITA project
8
PubMed Central. Carotid-wall intima-media thickness and cardiovascular events
9
PubMed Central. Role of Arterial Stiffness and Carotid Intima-Media Thickness on Subclinical Atherosclerosis and Cardiovascular Risk Assessment
10
Journal of the American Society of Echocardiography. Carotid Ultrasound Plaque Assessment Clinical Practice Guideline
11
JACC: Cardiovascular Imaging. Carotid Plaque-RADS: A Novel Stroke Risk Classification System
12
PubMed Central. Near-occlusion is difficult to diagnose with common carotid ultrasound methods
13
PubMed Central. Color Doppler Ultrasonography for the Evaluation of Subclavian Artery Stenosis
14
PubMed. Acoustic shadowing impairs accurate characterization of stenosis in carotid ultrasound examinations
15
PubMed Central. Assessment of carotid artery ultrasonography in the presence of an acoustic shadow artifact
16
PubMed. Pitfalls in Carotid Doppler Interpretation and How to Avoid Them
17
PubMed. Acoustic shadowing on B-mode ultrasound of the carotid artery predicts ischemic stroke: the Atherosclerosis Risk in Communities (ARIC) study
18
PubMed Central. Multimodality Imaging of Carotid Stenosis
19
PubMed Central. Levels of agreement in the measurements of carotid artery ultrasound across a regional vascular network
20
PubMed Central. Intra- and inter-operator reproducibility of automated cloud-based carotid lumen diameter ultrasound measurement
21
PubMed Central. Imaging modalities to diagnose carotid artery stenosis: progress and prospect
22
American Journal of Neuroradiology. Detection of Carotid Artery Stenosis: A Comparison between 2 Unenhanced MRAs and Dual-Source CTA
23
PubMed. Distinguishing carotid artery pseudo-occlusion with color-flow Doppler