We have developed a vector Doppler system based on an array transducerusing a clinical ultrasound scanner with a research interface. In this system,vector Doppler is performed by splitting the array transducer into transmit andreceives sub-apertures, which are then steered to obtain velocity estimates atdifferent Doppler angles. The goal of this study was to characterize the size ofthe overlap region between the transmit and receive beams in both axial andlateral directions. We studied the effect of vector Doppler geometry, beamsteering angle, aperture size and the depth of focus on the size of beam overlapregion. Our results show that change in these parameters has minimal effect onthe axial widths when a single transmit beam and two steered receive beams areused. However, the lateral widths tend to decrease with depth under all thefactors under consideration. The average deviation of the beam widths was about7% and 17% along the axial and lateral directions respectively over all 48scenarios under consideration. Preliminary experiments on tissue did notsubstantially affect the beam overlap region. © 2010 IEEE.