Plane wave imaging using virtual sub-wavelength receiving elements

Abstract

Plane wave imaging is generally performed using linear array transducers which array pitch is larger than one wavelength, resulting in grating lobe artifacts. In addition, larger element width associated with linear array transducers reduces delay accuracy for delay-and-sum based plane wave beamformation and therefore causes higher sidelobes and lower imaging contrast. To suppress both of the artifacts, we propose a novel plane wave beamforming method based on a concept of virtual sub-wavelength receiving element and the derived correlation weighting. Each array element is divided into multiple virtual sub-elements with sub-wavelength pitch. The signals of the virtual sub-elements are synthesized using the signal received by their corresponding physical element. Delay-and-sum beamformation on receive is then done with the synthesized signals and corresponding delays of the virtual sub-elements. Such beamformation synthesizes a non-uniform sampled receive aperture with its mean effective array pitch smaller than one wavelength; thus the grating lobes can be reduced. Better receive delay estimation, because of smaller effective receive elements, can be achieved, which plays the role of sidelobe suppression. In addition, to further suppress grating lobes and sidelobes, virtual-receiving-element derived correlation weighting can be performed. With proper grouping of the virtual receiving elements, images formed with different groups of the virtual elements own similar mainlobe signals and different grating-lobe and sidelobe artifacts. Mainlobe signals can be distinguished from grating-lobe and sidelobe clutters using correlation among the beamformed RF data from different groups of the virtual elements. The correlation is then used as a pixel-by-pixel weighting factor to the image formed by all the virtual receiving elements, which retains the mainlobe signals and further suppresses the grating-lobe and sidelobe contributions.