Doppler effect results from the change in
High-Yield Explanation
An ultrasound beam insonating a blood vessel is paially reflected by red blood cells. If these are moving, there is a change in the frequency of the reflected pulse: an increase in frequency if the flow is towards the probe and a decrease if it is away from it. This is the Doppler effect and the changes in frequency referred to as the Doppler shift. (Ref: Grainger and Allison&;s diagnostic radiology 6th edition, page 63)