Effects of blood flow on temperature elevation in ultrasonic interstitial hyperthermia

Localized heating achieved at very low acoustic powers with an interstitial ultrasonic applicator is reported for perfused and non-perfused porcine brain. The applicator consisted of a piezoelectric transducer driven in cw mode at about 1.0 MHz with the acoustic output fed via a conical velocity transformer into a waveguide in the form of G18 spinal needle, which was clad in an acoustic insulator except for exposed tip that served as an antenna. Cylindrical symmetry of the applicator was used to find a solution of the bioheat transfer equation in diffusion approximation. Measurements on the porcine brain of the anaesthetized and then sacrificed animal showed little influence of blood perfusion on the hyperthermic temperatures in the vicinity of the applicator. The observed effects of thermal wash-out were in qualitative agreement with the presented solution.