A virtual point source pulse probing technique for suppressing grating lobes in large-pitch phased arrays

We have been investigating a recently developed technique called Phase Coherence Imaging (PCI) that suppresses grating lobes in large-pitch arrays by defining a weighting factor based on the instantaneous phase of received echoes. This could lessen many of the limitations associated with the fabricating of high-frequency micro-arrays. The technique is not very effective however, when used with conventional transmit beamforming because the received grating lobe echoes are narrowband. In previous work, we suggested and theoretically evaluated a new beamforming technique called "Pulse Probing" in order to generalize the application of PCI for suppressing grating lobes when using conventional transmit focusing. In this work, the experimental verification of the technique using a commercially available high-frequency linear array system (Vevo 2100, VisualSonics) is reported. We demonstrate that by pre-calculating PCI weighting factors using a defocused pulse from a virtual point source, and later applying them to conventional transmit beamforming, the grating lobes resulting from a 50 MHz, 64- element, 1.26 λ pitch phased array can be suppressed approximately 40 dB more than when using PCI alone. We have further experimentally shown that with this technique, grating lobes resulting from wire targets embedded in a tissue-mimicking phantom could be suppressed while the tissue speckle and the wire target were preserved.