What Is the ICA/CCA Ratio and Why Is It Important?

The ICA/CCA ratio is a comparison of blood-flow velocities between two sections of the carotid artery, used primarily to gauge how much a narrowing (stenosis) is choking off blood supply to the brain. It divides the peak systolic velocity measured in the internal carotid artery by the peak systolic velocity in the common carotid artery, and the resulting number helps clinicians decide whether a patient needs closer monitoring, medication adjustments, or surgery. The ratio matters because raw velocity readings can be misleading on their own, and this single calculation acts as a built-in correction for a range of patient-specific and technical variables that would otherwise cloud the picture.

How the Ratio Is Calculated

During a carotid duplex ultrasound, a sonographer measures how fast blood moves at several points along the neck arteries. The key measurement is peak systolic velocity, the fastest speed blood reaches during each heartbeat. The numerator of the ICA/CCA ratio is the highest PSV recorded in the proximal or mid internal carotid artery, and the denominator is the PSV in the distal common carotid artery.¹ The proximal ICA is chosen because that is where atherosclerotic plaque most commonly develops, right at or just beyond the point where the common carotid splits. And the distal CCA is chosen as the reference because it sits just upstream of that split, giving a baseline reading of what “normal” velocity looks like for that particular patient at that particular moment.

A healthy carotid bifurcation already has complex flow patterns including helical movement and brief zones of flow reversal, so some velocity variation is expected even without disease.² In a normal person, the ICA carries roughly 70% of the total common carotid flow, with the rest heading out through the external carotid artery to the face and scalp.³ When plaque narrows the ICA, blood squeezes through a tighter opening and accelerates, much like water spraying faster when you partially cover the end of a garden hose. The ratio captures that acceleration relative to the patient’s own baseline flow.

Why Not Just Use the Raw Velocity

The standalone ICA PSV is the primary workhorse of carotid ultrasound, and for straightforward cases it works well. Professional consensus guidelines from the Society of Radiologists in Ultrasound list ICA PSV as the first-line measurement, with the ratio and end-diastolic velocity serving as supportive parameters.⁴ But raw PSV can be thrown off by factors that have nothing to do with how tight the artery actually is. A patient with a very strong heart might naturally push blood faster, producing a higher PSV even through a mildly narrowed artery. A patient with heart failure might show deceptively low velocities even through a critically narrowed one.

The ratio helps neutralize these effects. Because both the numerator and the denominator are influenced by the same cardiac output, blood pressure, and heart rate, dividing one by the other cancels out much of the patient-specific noise. If a strong heart raises velocity everywhere, the ratio stays roughly the same. If a weak heart lowers velocity everywhere, the ratio again stays closer to the truth. One study examining heart failure patients found that while ICA and CCA velocities both dropped significantly as heart function declined, the ICA/CCA ratio remained relatively stable.⁵ That stability is exactly why the ratio exists.

The Standard Thresholds and What They Mean

Vascular labs around the world use a set of velocity thresholds to translate ultrasound numbers into stenosis categories that guide treatment. The two categories that matter most clinically are narrowing of at least 50% and narrowing of at least 70%, because large trials showed that surgery benefits patients crossing those thresholds under certain conditions. Joint UK recommendations, for example, use an ICA/CCA PSV ratio above 2 to indicate more than 50% narrowing and a ratio above 4 to indicate more than 70% narrowing.⁶

An early validation study comparing duplex ultrasound against angiography found that a ratio of 4.0 detected 70–99% stenosis with about 91% sensitivity and 87% specificity, giving an overall accuracy around 88%.⁷ Those numbers are solid for a non-invasive screening tool, though they also mean that a meaningful minority of patients will be misclassified in either direction. That is part of why the ratio is used alongside other parameters rather than in isolation.

The duplex parameters that labs report typically include ICA PSV, ICA end-diastolic velocity, the ICA/CCA PSV ratio, and sometimes an end-diastolic velocity ratio as well.⁸ This multi-parameter approach makes the overall grading more robust than relying on any single number.

When the Ratio Becomes Especially Important

There are specific clinical scenarios where raw PSV can mislead, and the ratio becomes the more trustworthy guide. The most well-documented is contralateral occlusion or severe stenosis. If one carotid artery is badly blocked, the opposite side often compensates by carrying more blood than usual. That compensation pushes up velocity on the “open” side, making a moderate narrowing look worse than it really is on PSV alone. Research has shown that a severe contralateral stenosis can artificially elevate ICA PSV by an average of about 84 cm/s.⁹ That is a large enough bump to push a patient into a higher stenosis category and potentially toward an operation they do not need.

Because the compensatory increase in flow affects both the ICA and the CCA on that side, the ratio absorbs much of the distortion. Consensus criteria specifically recommend using the ratio in patients with contralateral occlusion for this reason, though some researchers have questioned how reliable it remains when the contralateral narrowing is moderate rather than complete.¹⁰

Angle Errors and Technical Pitfalls

Ultrasound velocity measurements depend on the angle at which the sound beam meets the blood vessel. Standard practice calls for keeping this Doppler angle between 45 and 60 degrees. Even small deviations from the chosen angle can change the recorded velocity, and if the angle used in the ICA measurement differs from the one used in the CCA, the ratio itself gets distorted. One study found that velocity differences were statistically significant when the Doppler angle varied between measurements but were not significant when the angle was held constant.¹¹

Other sources of error include misalignment of the angle cursor with the actual direction of blood flow and a phenomenon called intrinsic spectral broadening, which is an artifact of the ultrasound beam’s width. An analysis of these two errors found that Doppler angle misalignment could produce velocity errors of up to about 33 cm/s, though this did not change whether a given stenosis was detected as significant or not.¹² The takeaway for patients is straightforward: the test is quite good, but its accuracy depends heavily on the skill and consistency of the person performing it. A well-trained sonographer using consistent technique is the single biggest factor in getting reliable numbers.

Why Thresholds Vary Between Labs

If you have carotid ultrasounds done at two different hospitals, you may notice that the threshold numbers they use to categorize your stenosis are not identical. This is not a mistake. Different labs have historically validated their duplex criteria against different imaging standards, whether older catheter-based angiography or newer CT angiography. One study comparing CT angiography–derived criteria to previously published ones found that while the criteria differed in some specifics, they aligned reasonably well with the widely used Society of Radiologists in Ultrasound thresholds.¹³

The development of these criteria traces back to pioneering work in the 1980s, when researchers compared spectral waveform measurements against independently read angiograms and established the first quantitative thresholds for classifying carotid disease. Those original criteria, based on PSV, end-diastolic velocity, and waveform characteristics like spectral broadening, have been widely adopted and modified by vascular laboratories worldwide.¹⁴ The ICA/CCA ratio was added later as an additional parameter to improve accuracy, and its thresholds have similarly evolved. A recent effort by the Intersocietal Accreditation Commission worked to optimize and standardize these criteria across accredited labs.¹

The Ratio After Carotid Stenting

Patients who have had a metal stent placed in their carotid artery present a unique challenge. Stents change the physical properties of the arterial wall, making it stiffer, and this alters the way blood flows and how velocities register on ultrasound. The standard thresholds validated for native (unstented) arteries tend to overestimate stenosis in stented ones. In-stent restenosis, where the artery narrows again inside the stent, requires its own set of criteria.

Research on stented carotids has shown that for the same degree of narrowing, velocities and ratios tend to run higher than in native arteries. For narrowing of 50–69%, one study found a mean ICA/CCA ratio of about 2.76 in stented arteries compared to 2.04 in unstented ones. For narrowing of 70% or more, the difference was even larger, with ratios averaging about 7.6 in stented arteries versus 4.5 in native ones.¹⁵ The optimal thresholds for detecting at least 50% in-stent restenosis were a PSV of 240 cm/s and a ratio of 2.45, while the thresholds for at least 70% in-stent restenosis were a PSV of 450 cm/s and a ratio of 4.3.

A separate study proposed slightly different numbers, suggesting a ratio above 2.7 for at least 50% in-stent restenosis and a ratio above 4.15 for at least 80% restenosis.¹⁶ The discrepancies between studies reflect differences in how the reference-standard imaging was done and in patient populations, but the general pattern is clear: if your lab applies the standard native-artery criteria to a stented carotid, it will flag more patients as having significant narrowing than actually do. Any follow-up ultrasound after carotid stenting should use stent-specific criteria.

Clues from the Other End of the Heartbeat

Most discussion of the ICA/CCA ratio focuses on peak systolic velocity, the fastest point in the cardiac cycle. But there are also end-diastolic velocity measurements taken during the slower phase between heartbeats, and these carry their own diagnostic value. End-diastolic velocity in the CCA itself has been investigated as a marker. One study found that for every increase of 10 cm/s in CCA end-diastolic velocity, the odds of a severe (70–99%) ICA stenosis being present dropped by nearly half. A CCA end-diastolic velocity at or below 19 cm/s was associated with a roughly 64% probability of severe ICA stenosis.¹⁷

This makes physiological sense. When a severe blockage sits downstream, the CCA “feels” the increased resistance, and diastolic flow slows as a result. These end-diastolic parameters can serve as additional diagnostic clues, particularly when direct visualization or measurement of the ICA itself is technically difficult due to calcified plaque, a high bifurcation, or patient anatomy that limits the ultrasound window.

Where the Ratio Hits Its Limits

Despite its usefulness, the ICA/CCA ratio is not a perfect proxy for stenosis severity, and there are situations where it can fail. One well-recognized scenario is near-occlusion, where the artery is so severely narrowed that blood flow actually slows to a trickle rather than accelerating. In that case, both PSV and the ratio may paradoxically drop, and the ultrasound can look deceptively normal or even suggest less disease than is actually present. Recognizing near-occlusion requires looking at the overall picture: a threadlike appearance of the artery on imaging, reduced flow volume, and sometimes a smaller-than-expected ICA diameter on the affected side.

Another limitation involves using the ICA PSV alone versus the ratio for clinical decisions. One study comparing the two approaches for selecting surgical candidates found that ICA PSV alone was adequate, but adding the ratio slightly improved agreement with the reference standard and was a reasonable addition to daily practice.¹⁸ The ratio is helpful, in other words, but rarely game-changing in isolation. Its real value comes from the way it anchors the PSV measurement to a patient-specific reference point.

The Stakes of Getting the Number Wrong

The practical consequence of carotid stenosis grading is whether a patient ends up in surgery, on medication, or in a surveillance program. For people with symptoms like a transient ischemic attack or minor stroke, a stenosis above 70% on the affected side is a strong indication for carotid endarterectomy. For asymptomatic patients, the decision is more fraught, and the thresholds matter enormously.

A study that tracked asymptomatic patients with ICA PSV between 230 and 429 cm/s, a range that consensus criteria would classify as at least 70% stenosis, found that many did well with medical therapy alone when monitored carefully. The researchers argued that a substantial number of patients would have undergone unnecessary surgery if the standard consensus velocity thresholds had been used to recommend the operation.¹⁹ This is a live debate in vascular surgery. Modern medical therapy, including aggressive cholesterol-lowering and blood-pressure management, has improved enough that some patients who would have clearly benefited from surgery decades ago may now do equally well without it. Accurate duplex grading, including the ratio, is part of how clinicians try to identify which patients genuinely need an operation and which can safely be watched.

The meta-analysis of Society of Radiologists in Ultrasound Doppler velocity criteria confirmed that the consensus cutpoints for 50% and 70% stenosis perform reasonably well against angiographic measurements, but accuracy is not perfect, and the specific performance varies across studies.²⁰ This variability underscores why most vascular specialists do not make surgical decisions based on a single ultrasound parameter. The ICA/CCA ratio is one piece in a puzzle that includes direct visualization of plaque, symptom history, and sometimes confirmatory imaging with CT or MR angiography. Its importance lies not in replacing those other tools but in providing a quick, reproducible, and patient-normalized signal that the artery is in trouble.

Non-Atherosclerotic Disease and the Ratio

The ratio was designed with garden-variety atherosclerotic plaque in mind, the kind that forms at the carotid origin. Other conditions can narrow the carotid artery in different locations. Fibromuscular dysplasia, for instance, tends to affect the mid-to-distal ICA rather than the origin. Because the standard ICA/CCA ratio takes its numerator from the proximal or mid ICA, it can miss disease that sits further downstream. The Intersocietal Accreditation Commission’s criteria specifically note that measuring the proximal-to-mid ICA helps distinguish atherosclerotic lesions from non-atherosclerotic conditions like fibromuscular dysplasia, which generally involves the more distal vessel.¹ If your doctor suspects a non-atherosclerotic cause of carotid narrowing, the standard ratio may not tell the whole story, and additional imaging is usually needed.