Doppler ultrasound measures the direction and speed of blood as it moves through your vessels, using the change in pitch of sound waves bouncing off moving blood cells (the 'Doppler effect'). It displays this as colours (colour Doppler), graphs (spectral Doppler), or a distinctive whooshing sound. From this, a radiologist can tell whether flow is normal, reduced, blocked or turbulent — pointing to clots, narrowing or other problems.
Key takeaways
- Doppler measures the direction and speed of blood flow.
- It shows flow as colours, graphs, or a whooshing sound.
- It reveals whether flow is normal, reduced, blocked or turbulent.
- A radiologist interprets the pattern and reports to your doctor.
You’ve probably seen the flashes of red and blue on an ultrasound screen, or heard a rhythmic whooshing during a scan. That’s Doppler — the part of ultrasound that measures blood flow. Here’s what it’s actually showing.
What Doppler measures
Ordinary ultrasound shows structure — what a vessel looks like. Doppler adds movement: it measures the direction and speed of blood as it flows.[1] It works using the Doppler effect — the pitch of the sound waves changes as they bounce off moving blood cells — and a computer turns that into a picture of the flow.[1]
The three ways it’s shown
Doppler results appear in a few forms:[1]
- Colour Doppler — colours overlaid on the image show the speed and direction of flow (commonly one colour toward the probe, another away).
- Power Doppler — more sensitive, good at picking up slow or faint flow, though it doesn’t show direction.
- Spectral Doppler — a graph of flow speed over time, and often that distinctive whooshing sound, timed with your heartbeat.
What the patterns mean
From the flow, a radiologist can tell whether blood is moving normally, is reduced, is blocked, or is turbulent — which can point to:[1]
- A clot (blocked or absent flow in a vein)
- Narrowing of an artery (faster, turbulent flow at the narrow point)
- Reduced flow to an organ, or increased flow (which can signal infection or inflammation)
Getting the result
The sonographer captures the Doppler images and traces; a radiologist interprets the pattern and reports to your referring doctor, who explains what it means for you.[2] As with any scan, it’s read alongside your symptoms.
Frequently asked questions
What do the red and blue colours mean?
They show the direction of blood flow relative to the probe (and the shade often reflects speed) — not “arteries vs veins” or “good vs bad.” The radiologist interprets the full pattern.[1]
Why can I hear my blood during the scan?
That’s spectral Doppler converting your blood flow into sound — a normal part of how it measures the speed and rhythm of flow.[1]
Sources
- RadiologyInfo.org (RSNA & ACR) — General Ultrasound (Doppler) — www.radiologyinfo.org/en/info/genus
- RANZCR / InsideRadiology — Ultrasound (Coombs) — www.insideradiology.com.au/ultrasound-hp/
