The invention relates to an apparatus for flow estimation using synthetic aperture imaging. The method uses a Synthetic Transmit Aperture, but unlike previous approaches a new frame is created after every pulse emission. In receive mode parallel beam forming is implemented. The beam formed RF data is added to the previously created RF lines obtained by the same transmit sequence. The apparatus comprises a pulser (1) to generate a pulsed voltage signal, that is fed to the emit beam former (2). The emit beam former (2) is connected to the emitting transducer array (3). The ultrasound is reflected by the object (4) and received by the elements of the transducer array (5). All of these signals are then combined in the beam processor (6) to focus all of the beams in the image in both transmit and receive mode and the simultaneously focused signals are used for updating the image in the processor (7). The update signals are used in the velocity estimation processor (8) to correlate the individual measurements to obtain the displacement between high-resolution images and thereby determine the velocity.