Characteristics of acoustic scattering from a double-layered micro shell for encapsulated drug delivery.
ABSTRACT This work examines the characteristic differences in acoustic scattering between air-filled double-layered encapsulating (DLE) shells and air-filled single-layered encapsulating (SLE) shells. The analysis shows that the presence of an outer layer softer than the inner layer results in a shift of the first monopole of the reflectivity-frequency response to a higher frequency and a reduction in the monopole peak; and it leads to a frequency-separation of the two dipoles that trace the monopole. The frequency shift and the peak reduction of the monopole and the frequency separation of the two dipoles all increase with the increasing thickness of the softer outer layer. The numerical results reveal that variations in the Lame constant of the model material for the protein albumin have little effect on the low-frequency scattering characteristics, while they affect the high-frequency scattering characteristics significantly. The authors have shown that this phenomenon is due to the fact that the model material for the protein albumin has a Lame constant substantially larger than its shear modulus. Their further numerical studies conclude that, for each DLE shell, one can construct an equivalent SLE shell, using a simple scheme originated from the mechanics of composite materials in the sense that the so-constructed SLE shell has essentially the same acoustic scattering characteristics as the DLE shell within a low frequency range.
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ABSTRACT: The theoretical understanding of encapsulated microbubble response to high-frequency ultrasound (HFUS) excitation is still limited although some novel experimental HFUS contrast imaging techniques have been well developed. In this paper, the higher-order modal (HOM) contributions to the scattered field are studied for such microbubbles driven by 1-100 MHz ultrasound. An exact solution of all small-amplitude vibrational modes of a single encapsulated microbubble in water is given by the wave scattering theory (WST) method and compared to results obtained from Church's Rayleigh-Plesset-like model for the small-amplitude radial oscillation of a microbubble in an incompressible fluid. From numerical results, we show that the HOM field contribution is significant for scattering properties from individual Nycomed microbubbles with normalized frequency > or = 0.2. It is also shown that the multiple scattering is strengthened for monodispersed Definity microbubbles of 3 microm radius at frequencies >40 MHz. However, comparisons between the authors' analyses and known experimental data for polydispersed Definity microbubbles indicate that the HOM contributions are insignificant in attenuation estimation at frequencies <50 MHz. In conclusion, the WST model analysis suggests that HOM scattering is an important consideration for single bubbles but may be less critical in the modeling of polydispersed Definity bubbles at high frequencies.The Journal of the Acoustical Society of America 10/2009; 126(4):1766-75. · 1.65 Impact Factor
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ABSTRACT: The evolution of a bubble confined inside a nonlinear micro vessel fully filled with a viscous liquid, subjected to a shock lithotripsy wave (SWL), is analyzed with a previously established asymmetrical model on bubble oscillation. Both the normal and shear stress components within the vessel wall are calculated. It is observed that although the shear stress induced by viscosity is far less than the normal stresses, hypertension patients are still at more risk than normal people in SWL because of the high blood pre-pressure and stiff vessel wall accompanying their high blood viscosity. Hence, safety of hypertensive patients with high blood viscosity must be taken into careful consideration in SWL. More detailed numerical results show that the increase of circumferential normal stress and strain in SWL is significantly larger than that of other stresses and strains. Large circumferential normal stress and strain are responsible for excessive dilation of vessel wall during asymmetrical oscillation of constrained bubbles, which implies that the vessel wall will rupture mainly in the form of a cleft along the vessel axial direction.Journal of Mechanics. 02/2008; 24(01):55 - 61.
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ABSTRACT: Microbubble contrast agents and the associated imaging systems have developed over the past 25 years, originating with manually-agitated fluids introduced for intra-coronary injection. Over this period, stabilizing shells and low diffusivity gas materials have been incorporated in microbubbles, extending stability in vitro and in vivo. Simultaneously, the interaction of these small gas bubbles with ultrasonic waves has been extensively studied, resulting in models for oscillation and increasingly sophisticated imaging strategies. Early studies recognized that echoes from microbubbles contained frequencies that are multiples of the microbubble resonance frequency. Although individual microbubble contrast agents cannot be resolved-given that their diameter is on the order of microns-nonlinear echoes from these agents are used to map regions of perfused tissue and to estimate the local microvascular flow rate. Such strategies overcome a fundamental limitation of previous ultrasound blood flow strategies; the previous Doppler-based strategies are insensitive to capillary flow. Further, the insonation of resonant bubbles results in interesting physical phenomena that have been widely studied for use in drug and gene delivery. Ultrasound pressure can enhance gas diffusion, rapidly fragment the agent into a set of smaller bubbles or displace the microbubble to a blood vessel wall. Insonation of a microbubble can also produce liquid jets and local shear stress that alter biological membranes and facilitate transport. In this review, we focus on the physical aspects of these agents, exploring microbubble imaging modes, models for microbubble oscillation and the interaction of the microbubble with the endothelium.Physics in Medicine and Biology 03/2009; 54(6):R27-57. · 2.70 Impact Factor