Modeling subharmonic response from contrast microbubbles for imaging and noninvasive pressure estimation

Abstract

In order to characterize the behaviors of encapsulated contrast microbubbles, an approach will be described where progressively more sophisticated models have been developed guided by experimental observations. The development and characterization process includes independent estimation and validation components — attenuation is used to estimate the model material parameters, and then the estimated model is validated against independently measured subharmonic response. The subharmonic aided noninvasive pressure estimation that depends on experimentally observed decrease of subharmonic response of many commercial contrast agents with local hydrostatic pressure will also be critically examined. It will be shown that the basic bubble dynamics predicts either a decrease or an increase of subharmonic response with pressure increase depending on the excitation frequency and the bubble size. This finding indicates a lack of proper understanding of the underlying process. Finally, the minimum threshold excitation for subharmonic generation from an encapsulated microbubble will be revisited to show that in contrast to the classical perturbative result, it is not always obtained at twice the resonance frequency; instead it can occur over a range of frequency from resonance to twice the resonance frequency. The quantitative variation of the threshold with different models of encapsulation will be discussed. [support: NSF, NIH, DOD]