Linear array beamformation using virtual sub-wavelength receiving elements

Generally, the grating-lobe artifacts can be avoided with array pitch smaller than one half of wavelength; however, due to cost or physical limitations, such a condition is hard to be satisfied in multi-dimension or high frequency array transducers. Besides grating lobes, side-lobe signals also affect imaging contrast and quality. To suppress the side-lobes, the window method such as Tukey window and Hamming window is widely used. To suppress grating lobes and side-lobes associated with linear arrays, we propose a novel linear array beamformation method based on a concept of virtual sub-wavelength receiving elements. Each array element is virtually divided into multiple sub-elements with sub-wavelength pitch. The signal detected by each virtual sub-element contributes to that of its physical element at different arrival time. Therefore, the signals of the virtual sub-elements from the same physical elements are approximated using the same signal received by their physical element. Delay-and-sum beamformation on receive is then done with the approximated signals and corresponding delays of the virtual sub-elements instead of using those of physical elements. Such a beamformation synthesizes a non-uniform sampled receive aperture with its mean effective array pitch smaller than one wavelength; thus the grating lobes can be suppressed. The sub-wavelength mean effective array pitch implies that better receiving delay estimation, because of smaller effective receive elements, can be achieved, which plays the role of side-lobe suppression. Wire phantom experiments using a research ultrasound engine were performed to verify our idea. Our method provided at least 5-dB suppression of the grating lobe and sidelobes, and rivaled the window method. Our method works for coherent plane wave compounding as well. Optimal sub-wavelength dividing of each physical element will also be presented.