A heterodyning demodulation technique for spatial quadrature

Conventional flow estimation techniques quantify the axial component of blood or tissue velocity by either directly or indirectly estimating any Doppler shift of the frequency of the reflected ultrasound pulse. We have previously described a means of extending flow estimation to two dimensions through a method known as spatial quadrature. This technique creates a lateral complex modulation in the ultrasound beam, and hence in the echoes of scatterers moving through the beam, that can be used to estimate the lateral component of the flow velocity. The introduction of this lateral modulation creates the need for a means to distinguish the conventional axial modulation of the ultrasound pulse from this new lateral modulation. We demonstrate a novel vector velocity estimator based on spatial quadrature that uses heterodyning to achieve axial-only and lateral-only demodulation, and demonstrate its function using experimental data captured in real time in a vessel phantom under laminar flow conditions. This estimator was used to construct velocity profiles with peak velocities between 8 and 55 cm/s at Doppler angles of 90° and 60°. Flow velocity profiles across the vessel were integrated to estimate flow rates that agreed with the known flow rates to within 3% and 6% relative error for flow at 90° and 60° Doppler angles