Vascular US (carotid, DVT, arterial)
- Vascular ultrasound is two tools in one box: grayscale to see the vessel wall and lumen, and Doppler to hear and color the blood moving through it.
- The single most important test for deep vein thrombosis is dead simple: press on the vein. A normal vein squashes flat. A clotted one won't.
- Carotid disease is graded mostly by speed — as a narrowing tightens, blood has to sprint through it, and Doppler velocities climb.
- Arteries and veins speak different Doppler languages: arteries are pulsatile and pushy, veins are quiet and breathe with you.
- Know the few "drop everything" findings: a non-compressible vein, a hugely elevated carotid velocity, or an absent arterial signal in a cold, pulseless leg.
Most ultrasound is about looking at things — a kidney, a gallbladder, a baby. Vascular ultrasound is different. Here you're studying plumbing, and plumbing has a job: move fluid from A to B. So you don't just ask "what does it look like?" — you ask "is it flowing, which way, and how fast?" That's the whole personality of vascular US, and once it clicks, the three big exams (veins, neck arteries, leg arteries) all fall out of the same playbook.
The two halves of the machine
Every vascular study runs on two modes working together. First, grayscale (the plain black-and-white picture) shows you the anatomy: the vessel wall, the open channel inside (the lumen), and anything clogging it. Second, Doppler measures motion — it's the part that detects blood cells whizzing past and turns them into color and sound.
Think of grayscale as the photo of the river and Doppler as the gauge that tells you which way the water's going and how fast. You need both. A vessel can look wide open on grayscale but be functionally dead if nothing's flowing, and it can have an ugly-looking wall but perfectly fine flow.
Doppler itself comes in two flavors you'll use constantly: color Doppler, which paints flow onto the picture (a quick "is there flow, and where?"), and spectral Doppler, which plots speed over time as a wiggly waveform (the actual numbers). If the color-versus-speed thing feels shaky, it's worth a quick detour through Doppler in plain English before going further.
DVT: the press test that runs the world
Here's the beautiful part. The headline test for deep vein thrombosis (DVT) barely needs the fancy modes at all. It's compression ultrasound: put the probe over a deep vein, push down, and watch.
A normal vein is a floppy, low-pressure tube — squash it with the probe and the walls kiss together completely, like pressing flat an empty water balloon. A vein stuffed with clot is firm and stubborn; press all you want and it stays round. That failure to compress is the finding. Everything else — the color, the waveforms, whether you can see the clot directly — is supporting evidence.
This is why the exam is sometimes called a "compression study." You walk the probe down the leg in short steps — squeeze, release, squeeze, release — checking that each segment of vein flattens. It's repetitive on purpose: a clot you skip over is a clot you miss.
Don't confuse a vein with its neighboring artery. Two quick tells: the artery pulsates and won't compress under normal pressure, while the vein is the squashable one. Push too hard before you've identified which is which and you'll collapse the vein, then wrongly reassure yourself it "compressed." Light touch first.
Carotids: when speed tells the story
For the neck, the question is usually "how narrowed is this artery?" — the stuff that throws clots to the brain. Grayscale shows you the plaque, but the grade of carotid stenosis leans heavily on velocity.
Why speed? Same reason your thumb over a garden hose makes the water shoot farther. Narrow the channel and the blood has to accelerate to get the same volume through. So as a stenosis tightens, the peak speed Doppler measures climbs — and radiologists use established velocity thresholds to sort mild from moderate from severe. (The exact cutoffs come from validated criteria, not vibes, so I won't quote numbers I can't anchor.)
Always check flow direction, not just speed. Reversed flow in the vertebral artery — blood running backward down it to feed an arm starved by a tight proximal subclavian narrowing — is the classic tip-off for subclavian steal. The waveform shape is doing real diagnostic work, not just decorating the screen.
Arterial vs venous: two different waveforms
The last skill is reading the shape of the spectral waveform, because arteries and veins flow in completely different rhythms.
| Feature | Normal artery | Normal vein |
|---|---|---|
| Driving force | The heartbeat | Low-pressure return to the chest |
| Waveform | Pulsatile, sharp systolic spikes | Gentle, undulating |
| Varies with | Each heartbeat | Breathing (phasicity) |
| Probe compresses it? | No (until you push hard) | Yes, flattens easily |
Veins are the mellow ones — flow rises and falls softly as you breathe, a feature called phasicity. Lose that gentle respiratory variation and the waveform goes flat and continuous, which hints at a blockage upstream (between the probe and the heart) that you can't see directly. Arteries are the dramatic ones: crisp, pulsatile spikes. In peripheral arterial disease, a tight upstream narrowing softens those sharp spikes into a lazy, rounded, delayed waveform — the tardus-parvus pattern, which is just Latin for "late and small."
The findings that make you move fast
Most of vascular US is methodical, but a handful of results mean stop scrolling and pick up the phone: a non-compressible deep vein (DVT, with its risk of traveling to the lungs), a markedly elevated carotid velocity (a high-grade stenosis perched to cause a stroke), and absent arterial flow in a cold, pulseless limb (acute limb ischemia, where the clock is muscle and nerve).
The unifying lesson is the same one we started with: vascular US isn't about pretty anatomy. It's about flow — present or absent, fast or slow, forward or backward. Keep asking that one question and the whole field gets a lot less intimidating.