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