Focal spacing and near-field heating during pulsed high temperature ultrasound therapy

It has been proposed that high temperature short duration hyperthermia treatment would be perfusion insensitive and thus, significantly improved thermal exposure uniformity could be achieved. This study investigates the execution of such a treatment, which utilizes single spherically curved transducer and multiple sonications to cover the complete target volume. The spacing of neighboring pulses as a function of the transducer characteristics was studied utilizing computer simulations. In addition, the temperature elevation in front of the focal zone during multiple sonications was evaluated. It was found that significant delays (20 s or longer) between the sonications 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 build-up in front of the focus. The results were verified in vivo in dog's thigh muscle. This study is important not only for hyperthermia but also for ultrasound surgery, and indicates that each sonication system must be carefully evaluated for potential thermal damage outside of the target volume prior to implementation in therapy. © 1993