Now showing 1 - 8 of 8
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    Characterization of ultrasound contrast microbubbles using in vitro experiments and viscous and viscoelastic interface models for encapsulation
    (01-07-2005)
    Sarkar, Kausik
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    Shi, William T.
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    Forsberg, Flemming
    Zero-thickness interface models are developed to describe the encapsulation of microbubble contrast agents. Two different Rheological models of the interface, Newtonian (viscous) and viscoelastic, with rheological parameters such as surface tension, surface dilatational viscosity, and surface dilatational elasticity are presented to characterize the encapsulation. The models are applied to characterize a widely used microbubble based ultrasound contrast agent. Attenuation of ultrasound passing through a solution of contrast agent is measured. The model parameters for the contrast agent are determined by matching the linearized model dynamics with measured attenuation data. The models are investigated for its ability to match with other experiments. Specifically, model predictions are compared with scattered fundamental and subharmonic responses. Experiments and model prediction results are discussed along with those obtained using an existing model [Church, J. Acoust. Soc. Am. 97, 1510 (1995) and Hoff et al., J. Acoust. Soc. Am. 107, 2272 (2000)] of contrast agents. © 2005 Acoustical Society of America.
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    Ultrasound-mediated destruction of contrast microbubbles used for medical imaging and drug delivery
    (01-01-2005) ;
    Jain, Pankaj
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    Sarkar, Kausik
    Micron-size bubbles encapsulated by a stabilizing layer of surface-active materials are used in medical ultrasound imaging and drug delivery. Their destruction stimulated by ultrasound plays a critical role in both applications. We investigate the destruction process of microbubbles in a commercially available contrast agent by measuring the attenuation of ultrasound through it. The measurement is performed with single-cycle bursts from an unfocused transducer (with a center frequency of 5 MHz) for varying pressure amplitudes at 50-, 100-, and 200-Hz pulse repetition frequencies (PRF) with duty cycles 0.001%, 0.002%, and 0.004%, respectively. At low excitation, the attenuation is found to increase with time. With increased excitation level, the attenuation level decreases with time, indicating destruction of microbubbles. There is a critical pressure amplitude (∼1.2 MPa) for all three PRFs, below which there is no significant bubble destruction. Above the critical pressure amplitudes the rate of destruction depends on excitation levels. But at high-pressure amplitudes the destruction becomes independent of excitation pressure amplitude. The results are interpreted to identify two different mechanisms of bubble destruction by its signature in attenuation, namely, slow dissolution by diffusion and catastrophic shell rupture. The different modes are discussed in detail with their implications in medical applications. © 2005 American Institute of Physics.
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    Numerical study of turbulent flow over an S-shaped hydrofoil
    (01-09-2008)
    Kumar, T. Micha Prem
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    In this paper, a numerical study of turbulent flow over the S-shaped hydrofoil at 0° angle of attack has been reported. Here, the flow takes place over concave and convex surfaces and is accompanied by the favourable and adverse pressure gradients and flow separation. Modelling such a flow poses a formidable challenge. In the present work four turbulence models, namely, k-ε- realizable, k-ω shear stress transport, v'2-f, and the Reynolds stress model (RSM) were examined. Simulations were performed on a structured grid using finite-volume method formulation. Commercial software was used for grid generation and numerical simulation. A comparison of the experimental data of Madhusudan et al. (Fluid Dyn. Res., 1994, 14(5), 241-258) and numerical predictions were made, and the suitability of turbulence models was ascertained for both the mean and turbulent quantities. It was seen that v'2-fworks better for predicting the mean quantities and the RSM for turbulent quantities. © IMechE 2008.
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    Analytical investigation of hydrodynamic cavitation control by ultrasonics
    (01-10-2006)
    Chatterjee, Anindya
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    In recent experimental work, Chatterjee and Arakeri have demonstrated that an imposed acoustic field of sufficiently high strength and frequency can suppress or control cavitation. In this paper, we analytically study the equation governing the dynamics of a single bubble in a time-varying pressure field, for parameter ranges representative of those experimental conditions. The governing equation is strongly nonlinear and intractable in general; however, for the parameter ranges of interest, we are able to nondimensionalize and scale the governing equation into a form that, though still strongly nonlinear, is amenable to analysis using the method of multiple scales (MMS) based on an arbitrarily chosen "small" parameter ∈. Removal of secular terms, a key step in the MMS, raises an interesting issue which we discuss. Second order MMS gives the slow average evolution of the bubble radius. Numerical solutions of the original equation match the MMS approximation well on time scales of scriptOsign (1/∈). The MMS approximation also provides insight into the roles played by relevant physical parameters in the system. Our results provide theoretical support for the abovementioned experimental results. © Springer 2006.
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    Characterization and ultrasound-pulse mediated destruction of ultrasound contrast microbubbles
    (01-12-2006)
    Sarkar, Kausik
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    Jain, Pankaj
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    Intravenously injected encapsulated microbubbles improve the contrast of an ultrasound image. Their destruction is used in measuring blood flow, stimulating arteriogenesis, and drug delivery. We measure attenuation and scattering of ultrasound through solution of commercial contrast agents such as Sonazoid and Definity. We have developed a number of different interfacial rheology models for the encapsulation of such microbubbles. By matching with experimentally measured attenuation, we obtain the characteristic rheological parameters. We compare model predictions with measured subharmonic responses. We also investigate microbubble destruction under acoustic excitation by measuring time-varying attenuation data. © 2006 American Institute of Physics.
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    On the suitability of broadband attenuation measurement for characterizing contrast microbubbles
    (01-06-2005) ;
    Sarkar, Kausik
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    Jain, Pankaj
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    Schreppler, Nathan E.
    Broadband attenuation measurement has been widely used for characterizing ultrasound contrast agents. Chen et al. (2002) recently suggested that broadband attenuation data depend on the center frequency of the broadband excitation pulse and, therefore, that they are not a reliable measure of the bubble behavior. We investigated the suitability of measurement of broadband attenuation as a characterizing tool using the contrast agent Definity® as a test case. Analyzing the attenuation data obtained with three broadband unfocused transducers with different center frequencies (2.25, 3.5 and 5 MHz), we found that attenuation is independent of the transducer used and matches in the overlap regions of any two transducers. Attenuation does not depend on excitation pressure amplitude as long as the excitation amplitude remains below a critical value (≈ 0.26 MPa), indicating that the measurement of broadband attenuation below critical excitation can, indeed, be used for characterization. Furthermore, the linear relationship of attenuation with concentrations of Definity® is also investigated. © 2005 World Federation for Ultrasound in Medicine & Biology.
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    Design and development of a modular valveless micropump on a printed circuit board for integrated electronic cooling
    (01-04-2009)
    Verma, P.
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    Nagarajan, T.
    With constraints on size, cost, reliability, and performance for liquid-based cooling systems, the design of modular micropumps suitable for an integrated thermal management system still remains a challenge. In this paper, the effectiveness of a low-cost valveless micropump-heat exchanger on a printed circuit board is investigated for electronic cooling. Signal conditioning and control electronics are integrated with the fluidic components on the substrate to form a compact modular unit. Piezoelectric actuation and conical diffusers are utilized to generate pulsating flow through a minichannel heat sink. With ethanol as the working fluid, the tested pump reached a maximum flowrate and a pressure head of 2.4 ml/min and 743 Pa at an input voltage of 6 VDC. Suitability of the system for active real-time temperature control has been demonstrated at two input heater power levels of 1.45 and 2.5W A maximum reduction of 57 per cent in the average heat sink surface temperature could be achieved at a maximum power consumption of 150 mW by the micropump. © IMechE 2009.
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    Modeling and characterization of encapsulated microbubbles for ultrasound imaging and drug delivery
    (11-09-2008)
    Sarkar, Kausik
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    Jain, Pankaj
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    Intravenously injected encapsulated microbubbles improve the contrast of an ultrasound image. Their destruction is used in measuring blood flow, stimulating arteriogenesis, and drug delivery. We measure attenuation and scattering of ultrasound through solution of commercial contrast agents such as Optison (GE Health Care, Princeton, NJ) and Definity (Bristol Meyer-Squibb Imaging, North Ballerina, MA). We have developed an interfacial rheology model for the encapsulation of such microbubbles. By matching with experimental data, we obtain the characteristic rheological parameters. We compare model predictions with other experiments. We also investigate microbubble destruction under acoustic excitation by measuring time-varying attenuation data. Three regions of acoustic pressure amplitudes are found: at low pressure, there is no destruction; at slightly higher pressure bubbles are destroyed, and the rate of destruction depends on a combination of PRF and amplitude. At a still higher pressure amplitude, the attenuation decreases catastrophically. The last two regimes correspond respectively to 1) slow destruction of bubbles due to increased gas diffusion and 2) complete bubble destruction leading to release of free bubbles. An analytical model for the bubble growth and dissolution will be presented. The effects of membrane permeability and elasticity on the stability of microbubbles are investigated. (Supported by DOD, NSF and NIH). © 2008 American Institute of Physics.