Experimental verification of pulse-probing technique for improving phase coherence grating lobe suppression.

Fabrication of high-frequency phased-array ultrasound transducers is challenging because of the small element- to-element pitch required to avoid large grating lobes appearing in the field-of-view. Phase coherence imaging (PCI) was recently proposed as a highly effective technique to suppress grating lobes in large-pitch arrays for synthetic aperture beamforming. Our previous work proposed and theoretically validated a technique called pulse probing for improving grating lobe suppression when transmit beamforming is used with PCI. The present work reports the experimental verification of the proposed technique, in which the data was collected using a high-frequency ultrasound system and the processing was done offline. The data was collected with a 50-MHz, 256-element, 1.26 λ-pitch linear array, for which only the central 64-elements were used as the full aperture while the beam was steered to various angles. By sending a defocused pulse, the PCI weighting factors could be calculated, and were subsequently applied to the conventional transmit-receive beamforming. The experimental two-way radiation patterns showed that the grating lobe level was suppressed approximately 40 dB using the proposed technique, consistent with the theory. The suppression of overlapping grating lobes in reconstructed phased array images from multiple wire-phantoms in a water bath and tissue phantoms further validated the effectiveness of the proposed technique. The application of pulse probing along with PCI should simplify the fabrication of large-pitch phased arrays at high frequencies.