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.
"The encapsulated microbubbles for drug delivery usually have radii from 0.5 to 5μm with a shell-thickness between 10 and 250 nm and hence behave much like gas bubbles. There have been numerous theoretical investigations on the acoustic scattering and the nonlinear dynamics of UCAs in blood (Church 1995; Frinking et al. 1999; Allen et al. 2001; Allen et al. 2002; Hu et al. 2004; Stride and Saffari 2004; Qin et al. 2006). "
[Show abstract][Hide abstract] ABSTRACT: The existing models of the dynamics of ultrasound contrast agents (UCAs) have largely been focused on an UCA surrounded by an infinite liquid. Preliminary investigations of a microbubble's oscillation in a rigid tube have been performed using linear perturbation, under the assumption that the tube diameter is significantly larger than the UCA diameter. In the potential application of drug and gene delivery, it may be desirable to fragment the agent shell within small blood vessels and in some cases to rupture the vessel wall, releasing drugs and genes at the site. The effect of a compliant small blood vessel on the UCA's oscillation and the microvessel's acoustic response are unknown. The aim of this work is to propose a lumped-parameter model to study the interaction of a microbubble oscillation and compliable microvessels. Numerical results demonstrate that in the presence of UCAs, the transmural pressure through the blood vessel substantially increases and thus the vascular permeability is predicted to be enhanced. For a microbubble within an 8 to 40 microm vessel with a peak negative pressure of 0.1 MPa and a centre frequency of 1 MHz, small changes in the microbubble oscillation frequency and maximum diameter are observed. When the ultrasound pressure increases, strong nonlinear oscillation occurs, with an increased circumferential stress on the vessel. For a compliable vessel with a diameter equal to or greater than 8 microm, 0.2 MPa PNP at 1 MHz is predicted to be sufficient for microbubble fragmentation regardless of the vessel diameter; however, for a rigid vessel 0.5 MPa PNP at 1 MHz may not be sufficient to fragment the bubbles. For a centre frequency of 1 MHz, a peak negative pressure of 0.5 MPa is predicted to be sufficient to exceed the stress threshold for vascular rupture in a small (diameter less than 15 microm) compliant vessel. As the vessel or surrounding tissue becomes more rigid, the UCA oscillation and vessel dilation decrease; however the circumferential stress is predicted to increase. Decreasing the vessel size or the centre frequency increases the circumferential stress. For the two frequencies considered in this work, the circumferential stress does not scale as the inverse of the square root of the acoustic frequency va as in the mechanical index, but rather has a stronger frequency dependence, 1/va.
Physics in Medicine and Biology 11/2006; 51(20):5065-88. DOI:10.1088/0031-9155/51/20/001 · 2.76 Impact Factor
"Eventually, they might even allow ultrasound to replace much of contemporary and future radionuclide imaging for functional studies, thus avoiding the problems associated with radioactivity, and with relatively high spatial and temporal resolutions. For further information, refer to Bekeredian et al (2002), Chomas et al (2001), Hope Simpson et al (1999, 2001), Hu et al (2004), Hughes et al (2003), Kvikliene et al (2004) and Stride and Saffari (2004) "
[Show abstract][Hide abstract] ABSTRACT: Ultrasound imaging is now in very widespread clinical use. The most important underpinning technologies include transducers, beam forming, pulse compression, tissue harmonic imaging, contrast agents, techniques for measuring blood flow and tissue motion, and three-dimensional imaging. Specialized and emerging technologies include tissue characterization and image segmentation, microscanning and intravascular scanning, elasticity imaging, reflex transmission imaging, computed tomography, Doppler tomography, photoacoustics and thermoacoustics. Phantoms and quality assurance are necessary to maintain imaging performance. Contemporary ultrasonic imaging procedures seem to be safe but studies of bioeffects are continuing. It is concluded that advances in ultrasonic imaging have primarily been pushed by the application of physics and innovations in engineering, rather than being pulled by the identification of specific clinical objectives in need of scientific solutions. Moreover, the opportunities for innovation to continue into the future are both challenging and exciting.
Physics in Medicine and Biology 08/2006; 51(13):R83-98. DOI:10.1088/0031-9155/51/13/R06 · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A theoretical model for constrained oscillations of a cavitation bubble inside a vessel is developed, and an approximate solution, including the second-order effect when the vessel radius is significantly larger than the bubble, indicates that the effect of asymmetric bubble oscillation due to the vessel constraint is significant even with a very moderate initial wall velocity, and that this effect magnifies itself with time. Neglecting the effect of asymmetric oscillation leads to a substantial underestimate of the peak pressure exerted on the vessel due to the bubble oscillation, underscoring the importance of the asymmetric oscillation on the resulting vessel dilation, which has been viewed as a primary mechanism for the clinical injuries of capillary and small blood vessels in shock wave lithotripsy.
International Journal of Non-Linear Mechanics 03/2005; 40(2-40):341-350. DOI:10.1016/j.ijnonlinmec.2004.06.007 · 1.98 Impact Factor
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