Fig. 1.

(a) The Doppler equation, used to calculate the velocity V of a moving target from the Doppler shift in the transmitted and received ultrasound signal, assumes the transducer is at a distance L from the target at an angle to the ultrasound beam θ and does not take into account the size of the aperture D. (b) In reality, the aperture has a finite width (D > 0), and therefore, there are a range of angles (θ − δ to θ + δ) that the beam subtends, causing a spread in the received Doppler shift from a target moving with constant velocity.