A blocked carotid artery appears on ultrasound as a visible mass of plaque narrowing the vessel, combined with disrupted blood flow patterns shown in color. The exact appearance depends on how severe the blockage is and what the plaque is made of, but sonographers look at three things simultaneously: the physical structure of the artery wall, the color-coded blood flow through the narrowed area, and the speed of that flow measured as a waveform.
How Plaque Looks on Gray-Scale Imaging
The first thing a sonographer sees is the B-mode image, which is the black-and-white, gray-scale picture of the artery itself. A healthy carotid artery appears as a dark, open tube with thin, smooth walls. When plaque builds up, it shows as material protruding into that open space, narrowing the channel where blood flows.
Plaque doesn’t all look the same. Its brightness on the screen reveals what it’s made of. Plaque that’s rich in fibrous tissue and calcium appears bright white (hyperechoic), sometimes casting a dark shadow behind it the way bone does. This type is generally considered more stable. Plaque that contains fat deposits or internal bleeding appears dark, almost blending into the blood-filled channel (hypoechoic). These darker plaques are the ones that raise concern because they’re more prone to rupturing and causing a stroke. Many plaques are a mix of both, appearing as a mottled combination of bright and dark areas.
The surface of the plaque also matters. On the ultrasound image, a smooth, regular surface suggests a stable plaque. An irregular or cratered surface, particularly one with a visible pocket or depression, may indicate an ulcerated plaque, where the surface has broken open. Standard B-mode ultrasound has moderate sensitivity for catching these ulcers, so contrast-enhanced ultrasound is sometimes used when the sonographer needs a closer look.
What Color Doppler Reveals About Flow
Color Doppler overlays the gray-scale image with red and blue coloring to show blood moving through the artery. Red typically indicates flow toward the ultrasound probe, blue indicates flow away from it. In a healthy carotid, you see a uniform fill of color flowing smoothly through the vessel.
When a blockage narrows the artery, the blood has to squeeze through a tighter space, and several visual changes appear. The most distinctive is called aliasing: a sudden shift from one color to another (red flipping to blue, or vice versa) right at the point of narrowing. This happens because the blood is moving so fast through the tight spot that the machine can’t accurately measure it, causing the signal to “wrap around” and display as reversed flow. Aliasing at a properly calibrated color scale is actually a quick visual flag that tells the sonographer exactly where the stenosis is.
Around and just beyond the narrowed segment, the color pattern becomes chaotic, with a mosaic of mixed colors representing turbulent flow. Instead of a clean column of one color, you see a swirl of red, blue, and green speckles. The more severe the narrowing, the more dramatic this turbulence becomes.
How Speed Measurements Grade the Blockage
The most precise part of the exam is spectral Doppler, which produces a waveform graph showing blood velocity over time. The sonographer places a small sampling cursor inside the artery at and around the narrowed area, and the machine traces a wave pattern with each heartbeat. The peak of each wave represents peak systolic velocity (PSV), the fastest blood flow during a heartbeat.
In a normal internal carotid artery, PSV is typically below 125 cm/s. As the artery narrows, blood accelerates through the tighter opening, and these velocity numbers climb predictably. The widely used Society of Radiologists in Ultrasound consensus criteria define the thresholds: a PSV above 125 cm/s, along with an elevated velocity ratio between the internal carotid and the common carotid artery of greater than 2.0, indicates at least 50% narrowing. A PSV above 230 cm/s with a ratio above 4.0 and end-diastolic velocity above 100 cm/s signals 70% or greater narrowing, which is the range where surgical intervention is most commonly considered.
These velocity ratios are important because they account for individual variation. Someone with naturally high or low cardiac output might have misleading absolute numbers, but comparing the speed inside the narrowing to the speed in the normal segment upstream gives a more reliable picture.
What a Complete Blockage Looks Like
A 100% occluded carotid artery has a strikingly different appearance. On the gray-scale image, the vessel lumen is filled with echogenic material rather than appearing as an open, dark channel. On color Doppler, there is no color fill at all within the blocked segment. The artery appears as a solid, silent tube.
Spectral Doppler confirms this: when the sonographer places the cursor inside the vessel, there is no detectable flow waveform. Upstream from the blockage, the common carotid artery often shows an abnormal, high-resistance waveform pattern because blood is essentially hitting a dead end.
One critical challenge is distinguishing a complete occlusion from a near-complete one, where a tiny trickle of blood still passes through. This distinction matters because a near-total blockage can still be treated surgically, while a fully occluded artery typically cannot. Standard ultrasound sometimes misses that thin thread of remaining flow, especially when heavy calcification in the plaque blocks the ultrasound signal. In these difficult cases, an ultrasound contrast agent (tiny microbubbles injected into a vein) can dramatically improve detection of minimal residual flow. In one study, contrast-enhanced ultrasound correctly reclassified 3 out of 10 arteries initially thought to be completely blocked, revealing a trickle of flow that was later confirmed by angiography.
Where Blockages Typically Appear
The carotid artery splits into two branches in the neck: the internal carotid, which supplies the brain, and the external carotid, which supplies the face and scalp. Plaque overwhelmingly favors the carotid bulb, the slight widening right at this branching point, and the first few centimeters of the internal carotid artery. This is where blood flow naturally slows and becomes turbulent, creating conditions that promote plaque buildup.
The sonographer identifies the correct vessel by checking the waveform shape. The internal carotid has a low-resistance pattern with continuous forward flow throughout the heartbeat, reflecting the brain’s constant demand for blood. The external carotid has a high-resistance pattern with sharp peaks and minimal flow between beats, similar to arteries supplying muscles. This distinction is important because misidentifying which vessel is narrowed would change the clinical significance entirely.
Early Wall Thickening Before Plaque Forms
Ultrasound can also detect the earliest stages of artery disease, before any visible plaque develops. The intima-media thickness (IMT) measurement captures the combined thickness of the artery’s two innermost layers. In healthy young adults, this measures around 0.4 to 0.5 mm. It thickens gradually with age: in men, the common carotid IMT averages about 0.48 mm in the early twenties and rises to around 1.13 mm by the late seventies. In women, the same measurement ranges from about 0.43 mm to 0.91 mm over the same span.
The carotid bulb is naturally thicker than the rest of the artery. In men in their seventies, bulb IMT averages 1.74 mm compared to 1.13 mm in the straight common carotid segment. A measurement of 1.5 mm or greater at any site is considered thickened and may encompass early plaque formation. These measurements appear on the ultrasound as a slightly wider bright line along the artery wall, visible even to a patient watching the screen during the exam.
What to Expect During the Exam
A carotid ultrasound requires no fasting or special preparation. You’ll lie on your back with your head turned slightly away from the side being examined. A technologist applies warm gel to your neck and presses a small handheld probe against the skin, sliding it along the course of the artery from your collarbone up toward your jaw. The exam covers both sides and typically takes 15 to 30 minutes. You may hear a pulsing “whoosh” sound when the sonographer turns on the audio for spectral Doppler, which is the amplified sound of your blood flow. Wearing a shirt with a low neckline and skipping necklaces or long earrings makes the process easier.