Nonlinear behaviors of contrast agents relevant to diagnostic and therapeutic applications

The nonlinear properties of an encapsulated microbubble of a contrast agent were studied theoretically and experimentally. A modified nonlinear differential equation (Herring equation) was used to describe the radial oscillation of the microbubble and solved numerically. It was found that the nonlinear resonance frequency, at which the peak radial oscillation amplitude occurs, was a decreasing function of the acoustic amplitude of a driving ultrasonic pulse. Optical images of the contrast agent microbubbles under various ultrasonic exposure conditions: 1. sham exposure; 2. 2-MHz spatial peak acoustic pressure = 200 kPa, I(SATA) = 260 mW/cm(2), duty cycle = 7.5%, repetition period = 0.0266 ms; 3. 0.5-MHz spatial peak acoustic pressure = 200 kPa, I(SATA) = 130 mW/cm(2), duty cycle = 7.5%, repetition period = 0.1067 ms; have also shown that the lower-frequency ultrasound (US) excitation (0.5 MHz) is more effective in disruption of the microbubbles due to acoustic inertial cavitation than the higher frequency US (2 MHz).