A method and system for determining a velocity of a moving object such as a body or a stream. The object is irradiated in a non-collinear configuration with multiple ultrasound waves overlapping in a region-of-interest (ROI) of the object. An response wave, resulting from the non-linear interaction among the incident waves and the object, is detected and the frequency variations of the response wave are determined. Data representing a Doppler-shift of this frequency is further determined and processed to calculate the velocity of the moving object.
