Ultrasound surgery using multiple sonications--treatment time considerations.

The noninvasive surgery of a tumor by focused transducers or phased arrays with multiple sonications was simulated in this study. The effects of multiple sonications on near field heating were investigated by varying the delay time between consecutive pulses and the movement pattern of the transducer within the focal plane. To avoid normal tissue damage, sufficient delay time between consecutive pulses is necessary. It was found that the delay is dependent on acoustic power and pulse duration. The treatment time is often long when a strongly focused transducer is used to treat a large tumor, because the volume of necrosed tissue generated by the focused ultrasound beam is small requiring a large number of multiple sonications. To reduce treatment time, it is necessary to decrease the total number of pulses. This can be done by increasing the acoustic power and/or sonication time, Longer sonications induce a perfusion dependency on the temperature elevations. The acoustic power is limited by mammalian tissue temperatures which should not exceed 100 degrees C to avoid boiling and gas formation. An alternative way to increase the coagulated tissue volume is to modify the acoustic field such that a large tissue volume is exposed, while generating a more uniform temperature elevation to avoid high exposure limiting temperatures. This can be done effectively by using a phased array. This study demonstrated that phased arrays can offer significantly shorter treatment times than similar spherically curved transducers.