Acoustic properties of shell-encapsulated, gas-filled ultrasound contrast agents

Experiments have shown that the acoustic properties of shell-encapsulated ultrasound contrast agents deviate from gas-bubble theory. The shell elasticity reduces the particles´ compressibility, increasing their resonance frequency, and the shell viscosity increases the damping constant of the particle-liquid oscillating system. The aim of this study has been to find a model describing the acoustic scatter and attenuation from suspensions of contrast agent particles, at frequencies close to resonance for the particles. The shell is described as a visco-elastic material, with a complex Young´s modulus E_c given by the Kelvin-Voigt model: E_c=E+iωμ_E. Using a linear oscillator model for the particle-liquid system, the resonance frequency and damping constant were calculated, giving the scattering and extinction cross-sections as function of the particles´ bulk modulus K and volume viscosity μ_K. This model has been used to describe an experimental contrast agent from Nycomed, by comparing theoretical calculations with measured acoustic attenuation spectra. The experimental contrast agent was found to have bulk modulus K=2.2 MPa. Previously published results on Albunex were recalculated using the same model, and Albunex microspheres were found to have bulk modulus K between 1.5 and 5 MPa, increasing with decreasing microsphere diameters. By modifying the Rayleigh-Plesset equation, we found that this model also predicts acoustic scatter at the second harmonic frequency