Determination of maximum response radius at therapeutic pressure levels.

Abstract

High-intensity, focused ultrasound therapy is a minimally invasive therapy technique that has shown potential in many therapeutic applications, especially when coupled with the cavitation of microbubbles. The purpose of this study was to determine the effect of pressure amplitude on cavitation resonance frequency/bubble size at therapeutic field levels. Earlier work has indicated that the resonance frequency depends on pressure amplitude; however, the investigation only considered pressure amplitudes up to 1 MPa [Ultrasonics 43, 113 (2004)]. Our study was conducted by simulating the response of bubbles using the Gilmore-Akulichev formulation to solve for the bubble response. The frequency of the sine wave varied from 1–5 MHz while the amplitude of the sine wave varied from 0.0001–13 MPa. The resonance size for a particular frequency of excitation and amplitude was determined by finding the initial bubble size that resulted in the maximum bubble expansion relative to the initial size for an air bubble in water prior to the first collapse. Preliminary simulations indicated that this metric gave the correct resonance size for small excitations. These simulations demonstrated a downshift in resonance size with increasing pressure amplitude.