Diffraction-corrected synthetic aperture focusing for spherical ultrasonic radiators

Synthetic Aperture Focusing Techniques (SAFT) represent a special class of beam-steering algorithms. They are employed to improve the signal-to-noise ratio (SNR) and resolution of ultrasonic images. Such techniques have traditionally been implemented assuming a geometrical model that neglects the effects of diffraction. This dissertation accounts for diffraction effects by using an analytical model based on the Rayleigh-Sommerfeld formula. The model has been extended to characterize the acoustic fields in an immersed solid; a case frequently encountered in scanning acoustic microscopy applications. The results are then verified by comparing them with results obtained using a finite element model;The analytical model is used to evaluate the synchronization information necessary to perform SAFT. Studies conducted using the model reveal discrepancies resulting from employing the geometrical model. The diffraction model provides more accurate estimates of the changes in the amplitude of the field, which are ignored by the geometrical model. A modified SAFT algorithm utilizing these parameters is presented. The modified approach offers improvement in performance. The diffraction model is then used to characterize the resolution obtained using the modified SAFT;In addition to the diffraction model, the dissertation introduces a new scheme for performing SAFT using the envelope function. This method is faster and less expensive to implement compared to methods that employ the RF signal. Finally, a discussion of the limitations of the imaging system that prevent full exploitation of the algorithm is presented. Further improvements with regard to ultrasonic imaging in general and SAFT in particular are contingent on improving the performance of imaging and scanning equipment