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

External ultrasound hyperthermia is a very flexible modality for heating de ep-seated tumors due to its deep penetration and focusing ability. However, under the constraints of the available acoustic aperture size for the ultr asonic 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 betwee n 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 r elationship between the minimal diameter of the acoustic aperture and the t umor conditions. Tissue temperatures are used to determine the appropriate aperture diameter and the input power level for a given set of tumor condit ions. Due to the assumed central axis symmetry of the power intensity depos ition and anatomic properties, a two-dimensional (r-z) simulation program i s utilized, Factors determining the acoustic aperture diameter and the inpu t power level considered here are the tumor size, tumor depth, ultrasonic a ttenuation in tissue, blood perfusion, and temperature of the surface cooli ng 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, whi le 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 gu ideline for designing an optimal ultrasound heating system, arranging the t ransducers, and planning further treatments. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01505-9].