Acoustic modeling of shell-encapsulated gas bubbles.

Existing theoretical models do not adequately describe the scatter and attenuation properties of the ultrasound contrast agents Quantison and Myomap. An adapted version of the Rayleigh-Plesset equation, in which the shell is described by a viscoelastic solid, is proposed and validated for these agents and Albunex. The acoustic transmission and scattering are measured in the frequency band from 1-10 MHz. The measured transmission is used to estimate two parameters, the effective bulk modulus, Keff, describing the elasticity, and the friction parameter, SF, describing the viscosity of the shell. For the scattering, the difference between measurements and calculations is < 3 dB. For Quantison, the effective bulk modulus is independent of the bubble diameter. For Albunex, it increases for decreasing bubble diameter. The nonlinear response of Quantison is minimal for acoustic pressures up to 200 kPa. For acoustic pressures above 200 kPa, the measured scattering abruptly increases. This increase reaches a level of 20 dB for an acoustic pressure of 1.8 MPa. This response cannot be predicted by the theoretical model developed in this article.