INTENSITY DISTRIBUTION AND TEMPERATURE RISE FOR TRANSCRANIAL BRAIN ULTRASOUND HYPERTHERMIA

Abstract

The main purpose of this paper is to investigate the intensity distribution and the temperature responses when ultrasound beams are used for the intact brain tumor hyperthermia. A search method is developed to search for the appropriate transducer diameter and the input power level that induce a well focused high intensity region and an effective thermal dose region to enhance the goodness of the brain ultrasound hyperthermia treatments. This work employs a simplified model of a scanned ultrasound transducer power deposition (a cone with convergent/divergent shape). Multiple reflections and transmissions occurring at the interfaces are taken into consideration of calculating the intensity distribution as an ultrasound plane wave propagates through skull bone into brain tissue. The distributions of ultrasound intensity, specific absorption rate ratio, and temperature are used to determine the appropriateness of the transducer diameter and the input power level for a yielded set of tumor conditions. Simulation results demonstrate that (1) the maximum intensity ratio in the tumor region is larger than that in the bone region for all frequencies; (2) the treatments for larger tumors become more difficult to solve if the ratio of the diameter of transducer to the tumor diameter is less than 10 when one tries to use only high intensity ultrasound exposure to cause small lesions due to the cavitation effects. Results demonstrate the feasibility of inducing both a well focused high intensity region to produce local cavitation lesions and an effective thermal dose region to enhance the goodness of the intact brain ultrasound hyperthermia treatments. The results of this study can be a guideline for designing an optimal ultrasound heating system, arranging the transducers, and implementing further treatment planning for the transcranial brain tumor hyperthermia.