Ballard, John Robert2012-06-202012-06-202012-04https://hdl.handle.net/11299/125955University of Minnesota Ph.D. dissertation. April 2012. Major: Electrical Engineering. Advisor: Emad S. Ebbini. 1 computer file (PDF); xi, 126 pages, appendix p. 125-126.Dual mode ultrasound arrays (DMUA) have recently been introduced in the field of image-guided noninvasive surgery using high intensity focused ultrasound (HIFU). The goal of this dissertation is to investigate the use of DMUAs for transthoracic targeting of abdominal tumors. DMUAs are capable of operating in both imaging and therapy modes using the same array transducers. The inherent registration between these coordinate systems allows for the use of image feedback in refocusing the DMUA at target point(s) while avoiding critical structures in the path of the HIFU beam. In the #12;rst part of this dissertation, we propose an image-based refocusing algorithm which minimizes the power deposition over critical structures while maintaining or improving the power deposition at the focal location(s) by taking advantage of the available acoustical window. This is of particular importance when targeting abdominal tumors located in the liver or kidneys that are partially obstructed by the rib cage. An optimal weighted minimum-norm least-squares solution is shown to achieve the goal of maintaining desired power level(s) at the target point(s) while minimizing exposure to the ribs. Measurements and simulation results show that the optimal solution channels the ultrasound energy to the target through the intercostals, thus avoiding the ribs. One possible limitation of this approach is the creation of a virtual array in the rib plane, which could result in unacceptably high grating lobes in the target plane. The second part of this dissertation explores the use of a multiple-frequency optimal synthesis approach to mitigate the grating-lobe problem. In addition to the use of simulation and basic measurements in verification, the thesis research includes an extensive investigation of the basic characteristics of HIFU-induced lesions using both single-frequency and multiple-frequency synthesis approaches. Lesion formation experiments using these approaches were performed both in vitro and in vivo using small-animal models. In addition, the synergistic effects of the use of multiple frequency excitation, including enhancement of the thermal rate due to cavitation and increased harmonic generation are discussed.en-USHIFUPhased ArrayTherapeutic UltrasoundThesis or Dissertation