The Doppler Equation

Doppler ultrasound interprets the frequency as well as the amplitude of returning sound waves. The Doppler frequency (fD) is defined as the difference between  the received and transmitted frequencies of the moving sound wave scatterers (RBCs).

The Doppler equation: f0, initial sound frequency; fr, return sound frequency; v, flow velocity; c, speed of sound.

The Doppler angle strongly influences the amount of return signal, with a maximum parallel to the flow (cos 0 = 1), and no signal perpendicular to the flow (cos 90 = 0).  The return signal is half strength at 60 degrees (cos 60 =0.5)

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1. "Introduction to Vascular Ultrasonography" Zweibel and Pellerito, ed. 5th ed. (2005).