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The feasibility of using intracavitary ultrasound phased arrays for thermal surgery of the prostate was investigated. A simulation study was performed which demonstrated the ability of phased arrays to generate necrosed tissue volumes over anatomically appropriate ranges (2-6 cm deep and >6 cm axially) and investigated the effects of varying frequency, sonication time, maximum temperature, and blood perfusion on the necrosed tissue volume. An advantage that phased arrays have over geometrically focused transducers is that they are able to electronically scan a single focus over a specified range very quickly. This study demonstrated that the necrosed tissue volume may be increased by more than a factor of 100 by using electronic scanning. Scan parameters that were investigated included foci spacing, scan width, perfusion, maximum temperature, and unequal weighting of the foci. An optimization was performed to select the foci weighting parameters such that a uniform thermal dose was achieved at the focal depth, providing a more uniformly heated target volume. Finally, the ability of linear ultrasound phased arrays to create necrosed tissue lesions was demonstrated experimentally in fresh beef liver using a single stationary focus and single focus scans generated by an aperiodic 0.83-MHz 57-element linear ultrasound phased array.