Asymmetric Oscillation and Acoustic Response from an Encapsulated Microbubble Bound within a Small Vessel

Understanding the dynamics of ultrasonic excited microbubbles bound within microvessels is of significance for novel ultrasonic imaging, drug delivery and therapeutic biomedical applications. A finite element model (FEM) considering acoustic nonlinearity is developed to describe the asymmetric oscillation and acoustic response from an encapsulated microbubble bound within a small vessel. Numerical simulation is performed for a 2 μm encapsulated microbubble bound within 8-20 μm vessels using 2 MHz ultrasound excitation. The oscillation of the bound microbubble becomes more asymmetric under larger ultrasound pressure or within the smaller vessel. The normalized difference between the major and minor axes of epllipse is estimated to be 2.16% for the 8 μm vessel at an acoustic pressure of 0.5 MPa. In addition, the fundamental component of the acoustic scattering from the bound microbubble is enhanced by 6 dB while the second harmonic component is decreased by approximately 29 dB compared with the free microbubble.