Ultrasonic frequency-domain system and method for sensing fluid flow

Pulses of ultrasound are focused in the patient's body to create an interrogation volume where a characteristic of blood flow is to be measured. The bandwidth of the back-scattered Doppler return signal is measured. In order to measure flow velocity independent of direction, the interrogation volume is generated substantially as a sphere in which the range dimension is set equal to the lateral dimensions (azimuth and elevation) of the interrogation signal. The Doppler bandwidth is then scaled to provide a direction-independent measurement of flow velocity. In order to determine the direction of flow, the interrogation volume is generated substantially as an ellipsoid. The long axis of the ellipsoidal interrogation volume is then rotated until the measured Doppler bandwidth is at a minimum, which is reached when the long axis is aligned with the flow direction. The interrogation volume is preferably rotated and translated using differential phasing of the ultrasonic signals from different transducer elements in a two-dimensional array.