Clutter reduction in plane wave synthetic aperture imaging

A synthetic aperture imaging method which uses a steered plane wave is advantageous in that the resolution can be improved by covering a wide area with a small number of transmissions, and that the calculation of focusing delay for transmit focus synthesis is relatively simple. Through simulation studies, we found, however, that the clutter level of this method is higher than that of SAI using virtual sources positioned in the imaging plane, and the main reason is considered to be that the area covered by plane wave (PW) is wider than that of the virtual source case so that the scattering occurs over a large area, and the received signal basically has higher clutter levels than the virtual source case. A straightforward solution to this problem would be to use narrower PWs. In this case, however, the number of transmissions needs to be increased to cover the same area, resulting in a lowered frame rate. In order to maintain the same frame rate while reducing the clutter level, we divide the aperture into several sub-apertures, and simultaneously transmit two PWs in spatially separated sub-apertures, and furthermore, we use an up-chirp signal for one PW and a down-chirp signal for the other PW to minimize the crosstalk between the two PWs. In addition, transmit apodization is employed to reduce the fringing effect of the transmit aperture, and a further reduction of clutter level is achieved.