Doppler Shift – The Angle Matters!

The Doppler shift equation describes the observed change in frequency or wavelength of a wave (such as sound, light, or radio waves) when the source of the wave and the observer are in relative motion. This phenomenon is known as the Doppler effect and is incredibly important to understand when utilizing ultrasound.

Doppler shift = [2*f*v*cos(θ)] / c where ‘f’ is the operating frequency, ‘v’ the velocity of blood, ‘θ’ the angle between the ultrasound beam and the direction of blood flow, and ‘c’ the velocity of sound in soft tissue (~1540 m/s).

Color-flow Doppler (CFD) is a form of pulsed wave Doppler where all the points within a “color box” are labeled with a color that describes the speed and direction (relative to the ultrasound transducer) of blood at that point. Blue and red coloration represents flow away and towards the probe, respectively.

This is a TEE clip of the descending thoracic aorta in long-axis. Note how the blood flow transitions from blue (flow away from the probe) to black (no flow) to red (flow towards the probe). Rest assured, it’s not like the blood flow erratically stopped halfway down the aorta. The reason why the color turns black is that at that point, blood flow is perpendicular to the doppler interrogation beam (θ = 90°). Because cos(90°) = 0, there is no doppler shift at that point.

When performing quantitative analyses utilizing Doppler techniques, keeping θ as low as possible is crucial!

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