Experimental evaluation of scanned focussed ultrasound hyperthermia models in canine muscle in vivo.

Abstract

A theoretical model for scanned focussed ultrasound hyperthermia was evaluated in canine muscle in vivo. This model is composed of two models: an ultrasonic power deposition model and a heat transfer model. One ultrasound model and two bio-heat transfer models were considered. (1) Ultrasound field distributions were measured using thermal techniques in both canine thighs in vivo and in water. The experimental results were compared with distributions obtained from a model based on the one dimensional integration of the Rayleigh-Sommerfeld diffraction integral. The comparisons showed that the model is a good approximation to the distributions measured in water. The main lobe profiles obtained in the muscle also agreed well with both model predictions and results measured in water. However, these in vivo distributions showed enlargement of the side lobes. It was also found that muscle interfaces produced considerable beam distortions and increased side lobes. These findings were verified by measurements of the peak intensity and the total acoustic power attenuation coefficients for passage of the beams through thighs that showed that the former was about 40% higher than the latter. Also, absolute intensities at the acoustic focus were measured in water with a hydrophone for 11 transducers ranging in frequency from 0.246 to 3.54 MHz. When these intensities were compared to model predictions, it was found that the model overestimated the peak intensity by a factor of less than 2. That is, the model can be used to obtain upper bounds for absolute intensity. (2) Steady state temperature profiles from a simple (uniform blood perfusion) three dimensional bio-heat transfer model (Pennes), and from a simple (isotropic thermal conductivity) three dimensional effective thermal conductivity model, were compared with temperatures measured during scanned focussed ultrasound hyperthermia experiments in canine thighs in vivo. The experimental data consisted of radial temperature profiles across single octagonal scans measured at different depths into the thighs. The results showed that the bio-heat transfer equation predicted the experimental trends qualitatively and that the effective thermal conductivity equation failed to do so. Both models failed to predict the influence of thermally significant vessels. A scanned focussed ultrasound model composed of the ultrasound model evaluated here and the bio-heat transfer equation, can be used to predict the major features of temperature fields for hyperthermia patient treatment planning.