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

Abstract

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.

Determination of the Interfacial Rheological Properties of a Poly(DL-lactic acid)-Encapsulated Contrast Agent Using In Vitro Attenuation and Scattering

“…The frequency for minimum subharmonic threshold for PLA agents is therefore expected to be between 5 and 6 MHz. Note however that we have recently shown that the minimum threshold shifts towards resonance away from twice its value for encapsulated microbubbles due to large damping (Katiyar and Sarkar 2012). We also showed that the threshold is rather flat in the region between resonance and twice its value.…”

“…The smaller dilatational viscosity for distribution 3 can be explained by noting that damping of a bubble increases as the radius decreases (Katiyar and Sarkar 2011; Katiyar and Sarkar 2012). Size distribution 3 has the largest fraction of smaller bubbles.…”

Encapsulated microbubbles and echogenic liposomes for contrast ultrasound imaging and targeted drug delivery

“…Hence, thermal damping is either neglected—assuming nearly isothermal or adiabatic oscillations—or included through an additional thermal viscosity term just like the viscous damping due the surrounding liquid. Usually thermal damping is negligible in comparison to the encapsulation damping for contrast agents [124, 125], but one must be aware that such assumptions might not always be valid. The mathematical descriptions of the interfacial rheological models proposed by our group are discussed in the next section.…”

“…Subharmonic imaging has also been investigated for high frequency imaging applications [159–162] and noninvasive blood pressure estimation [163–174]. We have been investigating scattered subharmonic response from contrast microbubbles both experimentally and through numerical simulations [110–112, 120, 125, 175]. Figure 4 below shows both the experimentally measured and simulated subharmonic responses from Sonazoid and PLA coated microbubbles.…”

“…Our previous studies [111, 120, 125] have shown that interfacial models with nonlinear elasticity term viz., EEM and MM, can predict subharmonic thresholds accurately. Nonlinear interfacial rheology was found to be required to describe the large amplitude oscillations of contrast microbubbles.…”