Noninvasive ultrasound surgery using spherically curved phased arrays.

Two distinct research investigations were performed in this dissertation. The first investigation concerned the development and experimental testing of two improved numerical models for calculating ultrasonic field distributions in layered medium. The new models take into account ultrasound wave reflection and refraction at tissue interfaces thereby improving the accuracy of ultrasound hyperthermia and surgery treatment planning. It was found that in most cases the effects of the soft tissue interfaces can be ignored; however, in some layered medium the acoustic focus may be shifted several millimeters off axis. This shift is important when sharply focused transducers are used for ultrasound surgery or for pulsed, high temperature hyperthermia. The second research investigation concerned ultrasound phased arrays. Theoretical and experimental studies of spherically curved square-element phased arrays for use in ultrasound surgery were performed. Simulation results were compared with experimental results for 16-element and 64-element arrays. It was shown that phased arrays could control the necrosed tissue volume by using closely spaced multiple foci. Noninvasive necrosis of a 3 x 3 x 3 cm³ tumor by multiple sonications with focused transducers and phased arrays were simulated. The effects of multiple sonications on near field heating was investigated by varying the delay time between consecutive pulses and the movement pattern of transducer within the focal plane. It was demonstrated that phased arrays can offer significantly shorter treatment times relative to single spherically curved transducers