Adaptive beamforming for ultrasonic phased array focusing through layered structures

Successful realization of ultrasonic imaging through a multilayered composite barrier is hampered by scattering, attenuation, and multiple reflections of acoustic waves at and inside the barrier. These effects tend to distort the beam pattern produced by conventional phased arrays, defocusing the ultrasonic field transmitted through the barrier and causing image quality degradation and resolution loss. To compensate for the refraction and multiple reflection effects, we developed an adaptive beamforming algorithm for small-aperture linear phased arrays. After assessing the barrier´s local geometry, the method calculates a new timing distribution to refocus the distorted beam at its original location. The procedure is in fact a construction of a matched filter that automatically adapts the transmission pattern of the phased array to the local geometry of the barrier and cancels its distorting effect. In this work, the adaptive beamforming algorithms, in transmission mode, for the barriers in the form of a flat homogeneous layer, a layer with a smooth, randomly curved back surface and a two-layered combination of the above have been developed and experimentally verified on custom-engineered samples with prescribed acoustical properties. The algorithms were implemented on ULA-OP, an ultrasound advanced open-platform (University of Florence), controlling 64 active elements on a 128-elements phased array. Experimental measurements of original, distorted and corrected beam profiles confirm the ability of our algorithms to refocus the beam after passing through a scattering and refractive sample. Different excitation signals and windowing options introduced through ULA-OP were examined and compared.