Doppler Ultrasound
Simulate a medical Doppler ultrasound. Watch blood cells flow through a vessel while an ultrasound probe measures their velocity via the Doppler shift. The spectral waveform below shows velocity over time, just like a real clinical display.
Δf = 2 · f₀ · v · cos(θ) / c
About this lab
Medical Doppler ultrasound exploits the same frequency shift that makes an ambulance siren change pitch as it passes. When an ultrasound transducer emits sound waves at a known frequency f₀ toward moving blood cells, the reflected waves return at a different frequency. The shift depends on the blood velocity v, the angle θ between the ultrasound beam and the direction of flow, and the speed of sound in tissue c (approximately 1540 m/s): Δf = 2 · f₀ · v · cos(θ) / c.
The factor of 2 arises because the Doppler effect occurs twice: once when the moving blood cell receives the wave, and again when it re-emits it. The cosine term is critical in clinical practice — at 90° there is no measurable shift regardless of flow speed, which is why sonographers aim for beam angles below 60°. At 0° the full velocity is measured, but this angle is rarely achievable anatomically.
The spectral waveform display (velocity spectrum over time) is the primary diagnostic tool. In arteries, the pulsatile cardiac cycle creates a characteristic waveform: a sharp systolic peak followed by diastolic flow that varies by vascular bed. High-resistance beds like the femoral artery show reversed flow in early diastole, while low-resistance beds like the internal carotid maintain forward flow throughout. Venous flow is nearly steady, modulated gently by respiration.
Color Doppler assigns red to flow toward the transducer and blue to flow away — a convention borrowed from astronomical redshift and blueshift. This color mapping overlaid on the B-mode image lets clinicians rapidly identify vessel patency, stenoses, and abnormal flow patterns. The spectral broadening visible in the waveform indicates turbulence, a hallmark of stenotic disease.