Vascular US (carotid, aorta, DVT)
- Vascular ultrasound is just ultrasound that cares about blood moving, so it leans hard on Doppler to turn flow into colors and squiggly waveforms.
- Three big jobs: grade narrowing in the carotids, measure and watch the abdominal aorta, and answer "is there a clot?" in the leg veins.
- The single most important DVT maneuver is also the dumbest-sounding one: press on the vein. A normal vein squishes flat; a clotted one won't.
- It's cheap, portable, radiation-free, and uses no contrast — but it's deeply operator-dependent and bowel gas or big body habitus can hide the very thing you're hunting for.
Most of radiology is about anatomy holding still long enough for you to photograph it. Vascular ultrasound is the rebel — it's obsessed with whether things are flowing, how fast, and which direction. That's because a blood vessel can look perfectly fine sitting there and still be quietly strangling the blood inside it. So we don't just take a picture of the pipe; we listen to the water.
The toolkit: gray-scale plus Doppler
Every vascular scan layers a few modes on top of each other. Gray-scale (plain B-mode) shows you the wall, the lumen, and any junk inside it. Then we add Doppler, which is the whole reason this exam exists.
If Doppler is new to you, it's worth a quick detour through Doppler in plain English — but the one-sentence version is this: when sound bounces off something moving toward you it comes back at a higher pitch, and away from you a lower pitch, exactly like an ambulance siren rising as it approaches and dropping as it passes. The machine measures that pitch shift and translates it into flow.
Two flavors do the heavy lifting:
- Color Doppler paints flow onto the image — conventionally red toward the probe, blue away (it's direction, not artery-versus-vein, a trap I'll come back to).
- Spectral Doppler plots speed over time as a waveform, so you can actually measure peak velocities.
Carotids: where speed equals stenosis
Here's the elegant trick of the carotid exam. Squeeze a garden hose and the water speeds up at the pinch — same volume, smaller opening. Arteries do the same thing. So when an artery narrows from plaque, blood screams through the tight spot, and that jump in velocity is something we can measure with spectral Doppler.
We scan the common, internal, and external carotid arteries, look at the plaque on gray-scale, and record peak systolic velocity (and often the ratio between segments). Faster flow generally means a tighter narrowing.
The exact velocity cutoffs used to sort stenosis into categories vary between labs and published consensus criteria, and they're calibrated against the angiographic grading that drives treatment decisions. The concept to hold onto: higher velocity, tighter stenosis. The specific numbers live in the dedicated grading page.
The actual category-by-category thresholds — and why the internal carotid is the one everyone obsesses over — belong to carotid disease.
Color is direction, not vessel identity. In the neck the artery and the adjacent jugular vein can both light up the same color depending on which way they happen to run relative to your probe. Don't read a color and announce "artery." Confirm with the waveform: arteries pulse, veins drift.
The aorta: a tape measure you can carry
For the abdominal aorta, ultrasound is mostly a ruler. You're measuring the diameter to screen for and follow an aneurysm — a ballooning weak spot in the wall. Crucially, you measure outer wall to outer wall, because an aneurysm can be lined with clot (mural thrombus) that makes the flowing channel look deceptively normal-sized while the true vessel is dangerously wide.
| Feature | Why it matters |
|---|---|
| Outer-to-outer diameter | The number that decides "normal," "watch it," or "fix it." |
| Mural thrombus | Hides true size if you only measure the flow channel. |
| Growth over time | Serial scans track whether an aneurysm is enlarging. |
Ultrasound is the workhorse for screening and surveillance here precisely because it's cheap, contrast-free, and repeatable. The catch is bowel gas — air scatters sound, so a gassy abdomen can blank out the very segment you need.
Ultrasound is fantastic for finding and following an aortic aneurysm, but a suspected rupture is a CT problem, not an ultrasound one. The numbers, thresholds, and surveillance intervals live in aortic aneurysm.
DVT: the press test
My favorite exam in all of vascular ultrasound, because the core maneuver requires no fancy physics at all. To hunt for a deep vein thrombosis (DVT) in the leg, you take the probe and push.
A normal vein is a floppy, low-pressure tube — press on it and the walls kiss together and it disappears, like flattening an empty drinking straw. A vein stuffed with clot can't collapse; it stays stubbornly round under the probe. That lack of compressibility is the headline finding. We walk down the leg pressing at intervals, and we add color and spectral Doppler as supporting evidence — flow should fill the vein and gently rise and fall with breathing.
A fresh clot can be nearly invisible on gray-scale — almost the same shade as flowing blood — which is exactly why we don't trust our eyes and trust the squish instead.
If a leg vein doesn't compress, treat it as clot until proven otherwise. Compressibility beats how the lumen "looks."
The honest limitations
Ultrasound is gloriously safe and portable, but it has a personality. It's intensely operator-dependent — the answer is only as good as the hands and the angle. Body habitus and bowel gas can defeat it, especially for the aorta. And it images a window, not the whole vascular tree, so when you need the big map — runoff vessels, the chest, surgical planning — you reach for CTA and MRA instead.
The throughline across all three exams: stop thinking of the vessel as a still object and start thinking of it as plumbing. Is the pipe wide enough, is the water moving, and how fast? Answer those three and you've done vascular ultrasound.