Relationship between acoustic aperture size and tumor conditions for external ultrasound hyperthermia.

External ultrasound hyperthermia is a very flexible modality for heating deep-seated tumors due to its deep penetration and focusing ability. However, under the constraints of the available acoustic aperture size for the ultrasonic beam, ultrasonic attenuation, as well as other anatomic properties, it may not be able to deliver sufficient ultrasonic energy to heat a large tumor located in a deep region without overheating the normal tissue between the tumor and the aperture. In this work, we employ a simulation program based on the steady-state bioheat transfer equation and an ideal ultrasound power deposition (a cone with convergent/divergent shape) to examine the relationship between the minimal diameter of the acoustic aperture and the tumor conditions. Tissue temperatures are used to determine the appropriate aperture diameter and the input power level for a given set of tumor conditions. Due to the assumed central axis symmetry of the power intensity deposition and anatomic properties, a two-dimensional (r-z) simulation program is utilized. Factors determining the acoustic aperture diameter and the input power level considered here are the tumor size, tumor depth, ultrasonic attenuation in tissue, blood perfusion, and temperature of the surface cooling water. Simulation results demonstrate that tumor size, tumor depth, and ultrasonic attenuation are major factors affecting the aperture diameter of the ultrasonic beam to obtain an appropriate temperature distribution, while blood perfusion and the temperature of the surface cooling water are the minor factors. Plots of the effects of these factors can be used as the guideline for designing an optimal ultrasound heating system, arranging the transducers, and planning further treatments.