Nonlinear propagation of ultrasound through microbubble populations can generate artifacts and reduce contrast to tissue ratio in ultrasound imaging. The existing propagation model, which underestimates harmonic generation by an order of magnitude, was revised by incorporating a nonlinear constitutive equation for the coating into the description of the microbubble dynamics. Significantly better agreement with experiments was obtained, indicating that coating nonlinearity represents an important contribution to nonlinear propagation of ultrasound in microbubble populations. The results were found to be sensitive to the parameters characterizing the coating nonlinearity and thus accurate measurement of these parameters is required for accurate quantitative predictions.
The oscillation and destruction of microbubbles under ultrasound excitation form the basis of contrast enhanced ultrasound imaging and microbubble assisted drug and gene delivery. A typical microbubble has a size of a few micrometers and consists of a gas core encapsulated by a shell. These bubbles can be driven into surface mode oscillations, which not only contribute to the measured acoustic signal but can lead to bubble destruction. Existing models of surface model oscillations have not considered the effects of a bubble shell. In this study a model was developed to study the surface mode oscillations in shelled bubbles. The effects of shell viscosity and elasticity on the surface mode oscillations were modeled using a Boussinesq-Scriven approach. Simulation was conducted using the model with various bubble sizes and driving acoustic pressures. The occurrence of surface modes and the number of ultrasound cycles needed for the occurrence were calculated. The simulation results show a significant difference between shelled bubbles and shell free bubbles. The shelled bubbles have reduced surface mode amplitudes and a narrower bubble size range within which these modes develop compared to shell free bubbles. The clinical implications were also discussed.
An investigation into the effect of clinical ultrasound exposure on adherent microbubbles is described. A flow phantom was constructed in which targeted microbubbles were attached using biotin-streptavidin linkages. Microbubbles were insonated by broadband imaging pulses (centred at 2.25 MHz) over a range of pressures (peak negative pressure (PNP) = 60-375 kPa). Individual adherent bubbles were observed optically and classified as either being isolated or with a single neighbouring bubble. It is found that bubble detachment and deflation are two significant effects, even during low amplitude ultrasound exposure. Specifically, while at very low acoustic pressure (PNP < 75 kPa) 95% of the bubbles were not affected, at medium pressure (151 kPa < P < 225 kPa) 53% of the bubbles detached and at higher pressures (301 kPa < P < 375 kPa) 96% of the bubbles detached. In addition, more than 50% of the bubbles underwent deflation at pressures between 301 and 375 kPa. At pressures between 226 and 300 kPa, more adherent bubbles detached when there was a neighbouring bubble, suggesting the role of multiple scattering and secondary Bjerknes force on bubble detachment. The flow shear, primary and secondary Bjerknes forces exerted on each bubble were calculated and compared to the estimated forces acting on the bubble due to oscillations. The oscillation force is shown to be much higher than other forces. The mechanisms of bubble detachment are discussed.
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