On the suitability of broadband attenuation measurement for characterizing contrast microbubbles

Sarkar

et al. 2010

Self Cite

117

136

Two nonlinear interfacial elasticity models-interfacial elasticity decreasing linearly and exponentially with area fraction-are developed for the encapsulation of contrast microbubbles. The strain softening ͑decreasing elasticity͒ results from the decreasing association between the constitutive molecules of the encapsulation. The models are used to find the characteristic properties ͑surface tension, interfacial elasticity, interfacial viscosity and nonlinear elasticity parameters͒ for a commercial contrast agent. Properties are found using the ultrasound attenuation measured through a suspension of contrast agent. Dynamics of the resulting models are simulated, compared with other existing models and discussed. Imposing non-negativity on the effective surface tension ͑the encapsulation experiences no net compressive stress͒ shows "compression-only" behavior. The exponential and the quadratic ͑linearly varying elasticity͒ models result in similar behaviors. The validity of the models is investigated by comparing their predictions of the scattered nonlinear response for the contrast agent at higher excitations against experimental measurement. All models predict well the scattered fundamental response. The nonlinear strain softening included in the proposed elastic models of the encapsulation improves their ability to predict subharmonic response. They predict the threshold excitation for the initiation of subharmonic response and its subsequent saturation.

Section: Determination Of Characterization Parameters Using Attenumentioning

confidence: 99%

Material characterization of the encapsulation of an ultrasound contrast microbubble and its subharmonic response: Strain-softening interfacial elasticity model

Paul

Sarkar

et al. 2010

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117

136

“…Experimentally measured attenuation data for that contrast agent is used for the procedure (Chatterjee et al, 2005b;Paul et al, 2010). The parameter values for Sonazoid bubbles are given in Table I.…”

Section: Mathematical Formulation and Numerical Solutionmentioning

confidence: 99%

Effects of encapsulation damping on the excitation threshold for subharmonic generation from contrast microbubbles

Sarkar

2012

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A recent study . J. Acoust. Soc. Am. 130,[3137][3138][3139][3140][3141][3142][3143][3144][3145][3146][3147] showed that in contrast to the analytical result for free bubbles, the minimum threshold for subharmonic generation for contrast microbubbles does not necessarily occur at twice the resonance frequency. Here increased damping-either due to the small radius or the encapsulation-is shown to shift the minimum threshold away from twice the resonance frequency. Free bubbles as well as four models of the contrast agent encapsulation are investigated varying the surface dilatational viscosity. Encapsulation properties are determined using measured attenuation data for a commercial contrast agent. For sufficiently small damping, models predict two minima for the threshold curve-one at twice the resonance frequency being lower than the other at resonance frequency-in accord with the classical analytical result. However, increased damping damps the bubble response more at twice the resonance than at resonance, leading to a flattening of the threshold curve and a gradual shift of the absolute minimum from twice the resonance frequency toward the resonance frequency. The deviation from the classical result stems from the fact that the perturbation analysis employed to obtain it assumes small damping, not always applicable for contrast microbubbles.

“…There, such linear models might be inappropriate, warranting more sophisticated nonlinear constitutive models for the encapsulation. Recently, considerable effort has been devoted toward developing rigorous models for the encapsulation (Chatterjee and Sarkar, 2003;Chatterjee et al, 2005b;Doinikov and Dayton, 2007;Hoff et al, 2000;Marmottant et al, 2005;Sarkar et al, 2005;Tsiglifis and Pelekasis, 2008). We have developed Newtonian (Chatterjee and Sarkar, 2003) and viscoelastic (Sarkar et al, 2005) (zero-thickness) interfacial rheological models of the encapsulation; the molecular nature of the encapsulation and thereby the directional anisotropy justified such a two-dimensional continuum model in contrast to finite thickness models, e.g., Church (1995); Hoff et al (2000); Doinikov and Dayton (2007).…”

Section: Introductionmentioning

confidence: 99%

Excitation threshold for subharmonic generation from contrast microbubbles

Sarkar²

2011

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Six models of contrast microbubbles are investigated to determine the excitation threshold for subharmonic generation. The models are applied to a commercial contrast agent; its characteristic parameters according to each model are determined using experimentally measured ultrasound attenuation. In contrast to the classical perturbative result, the minimum threshold for subharmonic generation is not always predicted at excitation with twice the resonance frequency; instead it occurs over a range of frequencies from resonance to twice the resonance frequency. The quantitative variation of the threshold with frequency depends on the model and the bubble radius. All models are transformed into a common interfacial rheological form, where the encapsulation is represented by two radius dependent surface properties-effective surface tension and surface dilatational viscosity. Variation of the effective surface tension with radius, specifically having an upper limit (resulting from strain softening or rupture of the encapsulation during expansion), plays a critical role. Without the upper limit, the predicted threshold is extremely large, especially near the resonance frequency. Having a lower limit on surface tension (e.g., zero surface tension in the buckled state) increases the threshold value at twice the resonance frequency, in some cases shifting the minimum threshold toward resonance.