| Summary: | The purpose of this study was to use the concept of calculated thermal dose to predict the necrosed tissue volume, and to evaluate the near-field heating due to application of multiple pulses to cover a large volume. In addition, overlaying tissue damage caused by sonicating a highly-perfused tissue seated only a few cms deep was evaluated. The thermal dose distribution during focused ultrasound exposure was calculated based on numerical models used for calculating ultrasound power distributions and the resulting temperature distributions in tissue. In vivo experiments in dogs and rabbits were conducted to obtain the reliability of the predictions. It was found that the lesion intensity threshold was almost independent of the frequency for transducers with an F-number of 1. It was found that the lesion size was practically perfusion independent for pulses 5 s or shorter. The lesion size increased with increasing pulse duration, acoustical power, and F-number, but decreased with increasing frequency at a constant focal intensity. The results shown in this dissertation can be used as a guide for selection of transducer parameters for ultrasonic surgery. The temperature elevation in the near-field was elevated. It was found that significant delays (20 s or longer) between the pulses must be introduced in order to avoid unwanted tissue damage in front of the focal zone. In addition, decreasing the pulse duration and F-number reduced the temperature elevation in front of the focus. It was shown that the damage to surrounding tissues when sonicating a highly-perfused tissue (such as kidney) seated a few cms deep, can be avoided by using transducer with an F-number of 0.8 and a frequency of about 1 MHz. The heating of the surrounding tissues can be reduced more by correct selection of the pulse duration and power. The tissue necrosis due to ultrasound was monitored using MRI imaging. A study of MR signal intensity charge with temperature of dog and rabbit tissues in vitro showed that MRI has the potential to monitor temperature non-invasively. The signal sensitivity with temperature was found to be about 1.2-1.7 %/°C.
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