Acoustical Characterization of Individual Phospholipid-based Ultrasound Contrast Agent Microbubbles

A quantitative acoustical study of individual ultrasound contrast agent microbubbles, excited at low acoustic pressure amplitudes, is presented. A total of 76 microbubbles were studied. Sound pressure amplitudes as low as 1 Pa, collected from the smallest bubbles (radius of 1.4 micrometer), were recorded with a signal-to-noise ratio of 17 dB. Higher signal-to-noise ratios of 31 dB were obtained for bubbles with radii larger than 2 micrometer. The results were compared to a model derived from the Rayleigh-Plesset equation. Two methods are presented to extract the viscoelastic shell parameters, i.e. bubble shell elasticity and bubble shell viscosity. Firstly, the individual time responses were fitted to the modified Rayleigh-Plesset equation, by adjusting the viscoelastic shell parameters through a best-fit optimization procedure. The second method uses a similar best-fit optimization procedure to fit a linearized analytical expression of the model to the scattered pressure amplitude in the fundamental frequency band as a function of initial bubble radius. The shell parameters found with both methods are in close agreement and a good correspondence with the shell parameters found in other studies on similar bubbles was found. The characterization of single bubbles provided more detailed information about the viscoelastic shell parameters as a function of bubble size, which confirms a size dependency of these parameters.