A new adaptive beamforming method for ultrasonic imaging via small-aperture phased arrays through composite layered structures, such as human skull, is developed. If there is a scattering layer between the phased array and the imaged volume, acoustic phase aberration and wave refraction at undulating interfaces between the barrier and the rest of the propagation media can cause significant distortion of an ultrasonic image pattern produced by conventional beamforming techniques. This distortion takes the form of defocusing the ultrasonic field transmitted through the skull and causes loss of resolution, overall degradation of image quality and generation of non-informative final sonograms. To compensate for the phase aberration and refractional effects, an adaptive beamforming algorithm is developed and examined. After accurately assessing the skull's local geometry and sound speed, the method calculates a new timing scheme 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 and reception patterns of the phased array to the local geometry and acoustical properties of the skull and cancels its distorting effects. Results of numerical simulations, developed to accommodate and verify the proposed theory, are provided and discussed. The simulation results are verified experimentally by applying the method on realistic human skull phantoms in water immersion setups. The developed adaptive beamforming algorithms were implemented on an open-platform phased array controller and a lab prototype of the imaging system was delivered. Results of 2D and 3D adaptive imaging through skull phantoms via 2MHz linear and matrix phased arrays are presented.Ph.D.2016-11-30 00:00:0
