Kausik Sarkar | The George Washington University (original) (raw)

Papers by Kausik Sarkar

The Journal of the Acoustical Society of America, 2015

Broadband attenuation of ultrasound measured at different excitation pressures being different ra... more Broadband attenuation of ultrasound measured at different excitation pressures being different raises a serious theoretical concern, because the underlying assumption of linear and independent propagation of different frequency components nominally requires attenuation to be independent of excitation. Here, this issue is investigated by examining ultrasound attenuation through a monodisperse lipid-coated microbubble suspension measured at four different acoustic excitation amplitudes. The attenuation data are used to determine interfacial rheological properties (surface tension, surface dilatational elasticity, and surface dilatational viscosity) of the encapsulation according to three different models. Although different models result in similar rheological properties, attenuation measured at different excitation levels (4-110 kPa) leads to different values for them; the dilatation elasticity (0.56 to 0.18 N/m) and viscosity (2.4 × 10(-8) to 1.52 × 10(-8) Ns/m) both decrease with i...

Ultrasound in medicine & biology, Jan 6, 2016

In this Technical Note, we determine the interfacial rheological parameters of the encapsulation ... more In this Technical Note, we determine the interfacial rheological parameters of the encapsulation of the contrast agent Targestar P using ultrasound attenuation. The characteristic parameters are obtained according to two interfacial rheological models. The properties-surface dilatational elasticity (0.09 ± 0.01 N/m) and surface dilatational viscosity (8 ± 0.1E-9 N·s/m)-are found to be of similar magnitude for both models. Contrast microbubbles experience different ambient pressure in different organs. We also measure these parameters as functions of ambient pressure using attenuation measured at different overpressures (0, 100 and 200 mm Hg). For each value of ambient hydrostatic pressure, we determine the rheological properties, accounting for changes in the size distribution caused by the pressure change. We discuss different models of size distribution change under overpressure: pure adiabatic compression or gas exchange with surrounding medium. The dilatational surface elasticit...

ABSTRACT Because the components of a multiphase flow often exhibit different electrical propertie... more ABSTRACT Because the components of a multiphase flow often exhibit different electrical properties, a variety of probes have been developed to study such flows by measuring impedance in the region of interest.

This paper presents applications of boundary element methods to electrical impedance tomography. ... more This paper presents applications of boundary element methods to electrical impedance tomography. An algorithm for imaging the interior of a domain that consists of regions of constant conductivity is developed, that makes use of a simpler parametrization of the shapes of the regions to achieve efficiency. Numerical results from tests of this algorithm on synthetic data are presented, and show that the method is quite promising.

Ultrasonics, Mar 28, 2012

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2010

A viscoelastic drop falling through a Newtonian medium is simulated using a front tracking finite... more A viscoelastic drop falling through a Newtonian medium is simulated using a front tracking finite difference method. The drop viscoelasticity deforms the drop into an oblate shape. Further increase in viscoelasticity forms a dimple at the rear end of the drop. The dimple is a result of viscoelastic stresses which pulls the drop interface towards the center. The dimple becomes increasingly prominent as Deborah number or the capillary number is increased. An approximate analysis is executed to model the stress development along the axis of symmetry, specifically its increase near the rear end that governs dimple formation. The model also suggests a shift of the maximum of the viscoelastic stresses toward the centre of the drop with increasing Deborah number. For even higher values of Deborah number, the interface cannot balance the viscoelastic stresses and the dimple grows to make the drop unstable. Unstable cases accompany a decrease in velocity because of the formation of a globular shape at the end of the dimple. This results in a sudden increase in the cross-sectional area of the drop and simultaneous decrease in the settling velocity. Finally, we determine the critical Deborah number for transition from stable to unstable cases for varying capillary number.

ABSTRACT Micron-size gas bubbles encapsulated with lipids and proteins are used as contrast enhan... more ABSTRACT Micron-size gas bubbles encapsulated with lipids and proteins are used as contrast enhancing agents for ultrasound imaging. Biodegradable polymer poly-lactic acid (PLA) has recently been suggested as a possible means of encapsulation. Here, we report in vitro measurement of attenuation and scattering of ultrasound through an emulsion of PLA agent as well as theoretical modeling of the encapsulated bubble dynamics. The attenuation measured with three different transducers of central frequencies 2.25, 3.5 and 5 MHz, shows a peak around 2-3 MHz. These bubbles also show themselves to possess excellent scattering characteristics including strong non-linear response that can be used for harmonic and sub-harmonic contrast imaging. Our recently developed interfacial rheological models are applied to describe the dynamics of these bubbles; rheological model properties are estimated using measured attenuation data. The model is then applied to predict nonlinear scattered response, and the prediction is compared against experimental observation.

Bulletin of the American Physical Society, Nov 24, 2008

Interactions between viscous drops in a steady shear are numerically simulated using front tracki... more Interactions between viscous drops in a steady shear are numerically simulated using front tracking finite difference method. The results match well at low Reynolds number with the experimental observations of Guido and Simione (1998). At finite inertia, the drops behave differently ...

Physical review. E, Statistical, nonlinear, and soft matter physics, 2015

Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated... more Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using a front-tracking finite difference method. The membrane of the capsule is modeled using different hyperelastic constitutive relations. We also compare the pair interactions between drops to those between capsules. An increased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops after collision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separation than those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for both cases. We investigate pair-collisions between two heterogeneous capsules C_{1} and C_{2} with two different capillary numbers. The maximum deformation of C_{1} was seen to increase with increasing stiffness (decreasing capillary number) of C_{2}, even though the stiffness of C_{1} was kept fixed. The findings are similar for a drop-pa...

Broadband attenuation measurement has been widely used for characterizing ultrasound contrast age... more 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 ( approximately 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.

Physical Review D Particles and Fields, Jun 1, 2007

Expression for second post-Newtonian level gravitational deflection angle of massive particles is... more Expression for second post-Newtonian level gravitational deflection angle of massive particles is obtained in a model independent framework. Comparison of theoretical values with the observationally constructed values of post-Newtonian parameters for massive particles offers the future possibility of testing at that level competing gravitational theories as well as the equivalence principle. Advantage of studying gravitational deflection of massive particles over that of massless particles in testing gravity is discussed.

Physics of Fluids, Jun 1, 2009

Interactions between a pair of equal-size viscous drops in shear are numerically investigated at ... more Interactions between a pair of equal-size viscous drops in shear are numerically investigated at finite Reynolds number (Re=0.1-10). At low Reynolds number the simulation compares well with a previous experimental observation. Apart from the usual pairwise motion where drops driven by shear pass over each other (type I trajectory), finite inertia introduces a new type (type II) of reversed trajectory where drops approaching each other reverse their initial trajectories. The new trajectory is explained by a reversed streamline pattern observed around a single drop in an imposed shear, and is similar to what is also observed for rigid spheres at finite inertia. However, drop deformability introduces a nonuniform transition from one to the other type of trajectory—drops display type I trajectory for high and low capillary numbers and type II for intermediate capillary numbers. The phenomenon is explained by noting that increasing capillary number gives rise to competing effects—while it increases drop deformation and therefore increases resistance to sliding motion, it also increases drop flexibility, decreases inclination angle, and overall effect of the drop's presence is reduced, all helping them to slide by. The nonuniform behavior—type II trajectory for an intermediate range of capillary numbers—occurs only at Reynolds number above a critical value. Further increase in Reynolds number increases the range of capillary numbers for type II trajectory. For type I trajectory, terminal cross-stream separation increases linearly with increasing inertia indicating an enhanced shear induced diffusion. Increasing initial streamwise separation aids in reversed (type II) trajectory due to increased overlap with the reversed streamline zone. Increasing cross-stream distance expectedly facilitates (type I) sliding motion. For passing drops (type I trajectory), terminal cross-stream separation is not appreciably affected by capillary number and initial drop separation.

Journal of Non Newtonian Fluid Mechanics, Apr 30, 2010

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2007

We investigate a viscous/Oldroyd-B drop in an Oldroyd-B matrix. We compare simulated drop deforma... more We investigate a viscous/Oldroyd-B drop in an Oldroyd-B matrix. We compare simulated drop deformation and inclination with experimental observations by other groups. A non-monotonic change in the steady state drop deformation is observed with increasing Deborah number (De) and explained in terms of the competition between increased localized polymer stretching at the drop tips and the decreasing effects due to change in drop orientation angle. The transient drop orientation angle is found to evolve on the polymer relaxation time scale for high . The breakup of a viscous drop in a viscoelastic matrix is inhibited for small De, and promoted at higher De. The effect of polymeric to total viscosity ratio beta was seen to affect through the parameter beta De indicating a dominant role of the first normal stress difference. A viscoelastic drop in a viscoelastic matrix with matched relaxation time experiences less deformation compared to the case when one of the phases is viscous. But the inclination angle assumes an intermediate value between two extreme cases. Increased drop phase viscoelasticity compared to matrix phase leads to decreased deformation.

The retraction of a sheared drop when either the drop or the matrix phase is Oldroyd B is investi... more The retraction of a sheared drop when either the drop or the matrix phase is Oldroyd B is investigated. The retraction is initially faster and later slower with increasing drop viscoelasticity. The initial faster relaxation of viscoelastic drops is due to inward pulling viscoelastic stresses at the drop tip and the later slowing down is due to the slowly relaxing viscoelastic stresses at the equator. The behavior is captured well by three model ODEs for two principal viscoelastic stresses (along the tip and equatorial directions) and the deformation. Matrix viscoelasticity slows the relaxation of a Newtonian drop right from the beginning because of the slow relaxation of stresses near the drop tip with increasing Deborah number. For drops sheared in supercritical conditions, when initially stretched beyond a certain length, relaxation leads to neck formation with two bulbous ends resulting in drop break-up, while for less stretching, it relaxes back to its spherical state.

The Journal of the Acoustical Society of America, 2015

Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different or... more Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different organs, which can also change due to diseases. Pressure change can alter the material properties of the encapsulation of these microbubbles. Here the characteristic rheological parameters of contrast agent Definity are determined by varying the ambient pressure (in a physiologically relevant range 0-200 mm Hg). Four different interfacial rheological models are used to characterize the microbubbles. Effects of gas diffusion under excess ambient pressure are investigated in detail accounting for size decrease of contrast microbubbles. Definity contrast agent show a change in their interfacial dilatational viscosity (3.6 × 10(-8) Ns/m at 0 mm Hg to 4.45 × 10(-8) Ns/m at 200 mm Hg) and interfacial dilatational elasticity (0.86 N/m at 0 mm Hg to 1.06 N/m at 200 mm Hg) with ambient pressure increase. The increase results from material consolidation, similar to such enhancement in bulk properties...

Mechanical stimulation of bone increases bone mass and fracture healing, at least in part, throug... more Mechanical stimulation of bone increases bone mass and fracture healing, at least in part, through increases in proliferation of osteoblasts and osteoprogenitor cells. Researchers have previously performed in vitro studies of ultrasound-induced osteoblast proliferation but mostly used fixed ultrasound settings and have reported widely varying and inconclusive results. Here we critically investigated the effects of the excitation parameters of low-intensity pulsed ultrasound (LIPUS) stimulation on proliferation of MC3T3-E1 preosteoblastic cells in monolayer cultures. We used a custom-designed ultrasound exposure system to vary the key ultrasound parameters-intensity, frequency and excitation duration. MC3T3-E1 cells were seeded in 12-well cell culture plates. Unless otherwise specified, treated cells, in groups of three, were excited twice for 10 min with an interval of 24 h in between after cell seeding. Proliferation rates of these cells were determined using BrdU and MTS assays 24...

Volume 2, Parts A and B, 2004

ABSTRACT

Volume 2: Fora, Parts A and B, 2007

The Journal of the Acoustical Society of America, 2014

ABSTRACT Acoustic behaviors of five different polymer (polylactide) encapsulated microbubbles-PB1... more ABSTRACT Acoustic behaviors of five different polymer (polylactide) encapsulated microbubbles-PB127 (Point Biomedical), PH37, PH43, PH44, PH45 (Phillips Healthcare) with different shell-thickness-to-radius ratios (STRR) of 3.5, 30, 40, 65, and 100 nm/μm have been characterized. In vitro attenuation data were used to determine the interfacial rheological properties of their shells. Use of different models-Newtonian, viscoelastic, strain-softening, Marmottant, and Church-resulted in similar rheological properties. The shell elasticity and shell viscosity were found to increase with increasing shell thickness as expected. The nonlinear scattered response from these microbubbles were measured. Experimentally measured scattered subharmonic response were compared with the model predictions.

The Journal of the Acoustical Society of America, 2015

Broadband attenuation of ultrasound measured at different excitation pressures being different ra... more Broadband attenuation of ultrasound measured at different excitation pressures being different raises a serious theoretical concern, because the underlying assumption of linear and independent propagation of different frequency components nominally requires attenuation to be independent of excitation. Here, this issue is investigated by examining ultrasound attenuation through a monodisperse lipid-coated microbubble suspension measured at four different acoustic excitation amplitudes. The attenuation data are used to determine interfacial rheological properties (surface tension, surface dilatational elasticity, and surface dilatational viscosity) of the encapsulation according to three different models. Although different models result in similar rheological properties, attenuation measured at different excitation levels (4-110 kPa) leads to different values for them; the dilatation elasticity (0.56 to 0.18 N/m) and viscosity (2.4 × 10(-8) to 1.52 × 10(-8) Ns/m) both decrease with i...

Ultrasound in medicine & biology, Jan 6, 2016

In this Technical Note, we determine the interfacial rheological parameters of the encapsulation ... more In this Technical Note, we determine the interfacial rheological parameters of the encapsulation of the contrast agent Targestar P using ultrasound attenuation. The characteristic parameters are obtained according to two interfacial rheological models. The properties-surface dilatational elasticity (0.09 ± 0.01 N/m) and surface dilatational viscosity (8 ± 0.1E-9 N·s/m)-are found to be of similar magnitude for both models. Contrast microbubbles experience different ambient pressure in different organs. We also measure these parameters as functions of ambient pressure using attenuation measured at different overpressures (0, 100 and 200 mm Hg). For each value of ambient hydrostatic pressure, we determine the rheological properties, accounting for changes in the size distribution caused by the pressure change. We discuss different models of size distribution change under overpressure: pure adiabatic compression or gas exchange with surrounding medium. The dilatational surface elasticit...

ABSTRACT Because the components of a multiphase flow often exhibit different electrical propertie... more ABSTRACT Because the components of a multiphase flow often exhibit different electrical properties, a variety of probes have been developed to study such flows by measuring impedance in the region of interest.

This paper presents applications of boundary element methods to electrical impedance tomography. ... more This paper presents applications of boundary element methods to electrical impedance tomography. An algorithm for imaging the interior of a domain that consists of regions of constant conductivity is developed, that makes use of a simpler parametrization of the shapes of the regions to achieve efficiency. Numerical results from tests of this algorithm on synthetic data are presented, and show that the method is quite promising.

Ultrasonics, Mar 28, 2012

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2010

A viscoelastic drop falling through a Newtonian medium is simulated using a front tracking finite... more A viscoelastic drop falling through a Newtonian medium is simulated using a front tracking finite difference method. The drop viscoelasticity deforms the drop into an oblate shape. Further increase in viscoelasticity forms a dimple at the rear end of the drop. The dimple is a result of viscoelastic stresses which pulls the drop interface towards the center. The dimple becomes increasingly prominent as Deborah number or the capillary number is increased. An approximate analysis is executed to model the stress development along the axis of symmetry, specifically its increase near the rear end that governs dimple formation. The model also suggests a shift of the maximum of the viscoelastic stresses toward the centre of the drop with increasing Deborah number. For even higher values of Deborah number, the interface cannot balance the viscoelastic stresses and the dimple grows to make the drop unstable. Unstable cases accompany a decrease in velocity because of the formation of a globular shape at the end of the dimple. This results in a sudden increase in the cross-sectional area of the drop and simultaneous decrease in the settling velocity. Finally, we determine the critical Deborah number for transition from stable to unstable cases for varying capillary number.

ABSTRACT Micron-size gas bubbles encapsulated with lipids and proteins are used as contrast enhan... more ABSTRACT Micron-size gas bubbles encapsulated with lipids and proteins are used as contrast enhancing agents for ultrasound imaging. Biodegradable polymer poly-lactic acid (PLA) has recently been suggested as a possible means of encapsulation. Here, we report in vitro measurement of attenuation and scattering of ultrasound through an emulsion of PLA agent as well as theoretical modeling of the encapsulated bubble dynamics. The attenuation measured with three different transducers of central frequencies 2.25, 3.5 and 5 MHz, shows a peak around 2-3 MHz. These bubbles also show themselves to possess excellent scattering characteristics including strong non-linear response that can be used for harmonic and sub-harmonic contrast imaging. Our recently developed interfacial rheological models are applied to describe the dynamics of these bubbles; rheological model properties are estimated using measured attenuation data. The model is then applied to predict nonlinear scattered response, and the prediction is compared against experimental observation.

Bulletin of the American Physical Society, Nov 24, 2008

Interactions between viscous drops in a steady shear are numerically simulated using front tracki... more Interactions between viscous drops in a steady shear are numerically simulated using front tracking finite difference method. The results match well at low Reynolds number with the experimental observations of Guido and Simione (1998). At finite inertia, the drops behave differently ...

Physical review. E, Statistical, nonlinear, and soft matter physics, 2015

Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated... more Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using a front-tracking finite difference method. The membrane of the capsule is modeled using different hyperelastic constitutive relations. We also compare the pair interactions between drops to those between capsules. An increased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops after collision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separation than those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for both cases. We investigate pair-collisions between two heterogeneous capsules C_{1} and C_{2} with two different capillary numbers. The maximum deformation of C_{1} was seen to increase with increasing stiffness (decreasing capillary number) of C_{2}, even though the stiffness of C_{1} was kept fixed. The findings are similar for a drop-pa...

Broadband attenuation measurement has been widely used for characterizing ultrasound contrast age... more 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 ( approximately 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.

Physical Review D Particles and Fields, Jun 1, 2007

Expression for second post-Newtonian level gravitational deflection angle of massive particles is... more Expression for second post-Newtonian level gravitational deflection angle of massive particles is obtained in a model independent framework. Comparison of theoretical values with the observationally constructed values of post-Newtonian parameters for massive particles offers the future possibility of testing at that level competing gravitational theories as well as the equivalence principle. Advantage of studying gravitational deflection of massive particles over that of massless particles in testing gravity is discussed.

Physics of Fluids, Jun 1, 2009

Interactions between a pair of equal-size viscous drops in shear are numerically investigated at ... more Interactions between a pair of equal-size viscous drops in shear are numerically investigated at finite Reynolds number (Re=0.1-10). At low Reynolds number the simulation compares well with a previous experimental observation. Apart from the usual pairwise motion where drops driven by shear pass over each other (type I trajectory), finite inertia introduces a new type (type II) of reversed trajectory where drops approaching each other reverse their initial trajectories. The new trajectory is explained by a reversed streamline pattern observed around a single drop in an imposed shear, and is similar to what is also observed for rigid spheres at finite inertia. However, drop deformability introduces a nonuniform transition from one to the other type of trajectory—drops display type I trajectory for high and low capillary numbers and type II for intermediate capillary numbers. The phenomenon is explained by noting that increasing capillary number gives rise to competing effects—while it increases drop deformation and therefore increases resistance to sliding motion, it also increases drop flexibility, decreases inclination angle, and overall effect of the drop's presence is reduced, all helping them to slide by. The nonuniform behavior—type II trajectory for an intermediate range of capillary numbers—occurs only at Reynolds number above a critical value. Further increase in Reynolds number increases the range of capillary numbers for type II trajectory. For type I trajectory, terminal cross-stream separation increases linearly with increasing inertia indicating an enhanced shear induced diffusion. Increasing initial streamwise separation aids in reversed (type II) trajectory due to increased overlap with the reversed streamline zone. Increasing cross-stream distance expectedly facilitates (type I) sliding motion. For passing drops (type I trajectory), terminal cross-stream separation is not appreciably affected by capillary number and initial drop separation.

Journal of Non Newtonian Fluid Mechanics, Apr 30, 2010

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2007

We investigate a viscous/Oldroyd-B drop in an Oldroyd-B matrix. We compare simulated drop deforma... more We investigate a viscous/Oldroyd-B drop in an Oldroyd-B matrix. We compare simulated drop deformation and inclination with experimental observations by other groups. A non-monotonic change in the steady state drop deformation is observed with increasing Deborah number (De) and explained in terms of the competition between increased localized polymer stretching at the drop tips and the decreasing effects due to change in drop orientation angle. The transient drop orientation angle is found to evolve on the polymer relaxation time scale for high . The breakup of a viscous drop in a viscoelastic matrix is inhibited for small De, and promoted at higher De. The effect of polymeric to total viscosity ratio beta was seen to affect through the parameter beta De indicating a dominant role of the first normal stress difference. A viscoelastic drop in a viscoelastic matrix with matched relaxation time experiences less deformation compared to the case when one of the phases is viscous. But the inclination angle assumes an intermediate value between two extreme cases. Increased drop phase viscoelasticity compared to matrix phase leads to decreased deformation.

The retraction of a sheared drop when either the drop or the matrix phase is Oldroyd B is investi... more The retraction of a sheared drop when either the drop or the matrix phase is Oldroyd B is investigated. The retraction is initially faster and later slower with increasing drop viscoelasticity. The initial faster relaxation of viscoelastic drops is due to inward pulling viscoelastic stresses at the drop tip and the later slowing down is due to the slowly relaxing viscoelastic stresses at the equator. The behavior is captured well by three model ODEs for two principal viscoelastic stresses (along the tip and equatorial directions) and the deformation. Matrix viscoelasticity slows the relaxation of a Newtonian drop right from the beginning because of the slow relaxation of stresses near the drop tip with increasing Deborah number. For drops sheared in supercritical conditions, when initially stretched beyond a certain length, relaxation leads to neck formation with two bulbous ends resulting in drop break-up, while for less stretching, it relaxes back to its spherical state.

The Journal of the Acoustical Society of America, 2015

Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different or... more Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different organs, which can also change due to diseases. Pressure change can alter the material properties of the encapsulation of these microbubbles. Here the characteristic rheological parameters of contrast agent Definity are determined by varying the ambient pressure (in a physiologically relevant range 0-200 mm Hg). Four different interfacial rheological models are used to characterize the microbubbles. Effects of gas diffusion under excess ambient pressure are investigated in detail accounting for size decrease of contrast microbubbles. Definity contrast agent show a change in their interfacial dilatational viscosity (3.6 × 10(-8) Ns/m at 0 mm Hg to 4.45 × 10(-8) Ns/m at 200 mm Hg) and interfacial dilatational elasticity (0.86 N/m at 0 mm Hg to 1.06 N/m at 200 mm Hg) with ambient pressure increase. The increase results from material consolidation, similar to such enhancement in bulk properties...

Mechanical stimulation of bone increases bone mass and fracture healing, at least in part, throug... more Mechanical stimulation of bone increases bone mass and fracture healing, at least in part, through increases in proliferation of osteoblasts and osteoprogenitor cells. Researchers have previously performed in vitro studies of ultrasound-induced osteoblast proliferation but mostly used fixed ultrasound settings and have reported widely varying and inconclusive results. Here we critically investigated the effects of the excitation parameters of low-intensity pulsed ultrasound (LIPUS) stimulation on proliferation of MC3T3-E1 preosteoblastic cells in monolayer cultures. We used a custom-designed ultrasound exposure system to vary the key ultrasound parameters-intensity, frequency and excitation duration. MC3T3-E1 cells were seeded in 12-well cell culture plates. Unless otherwise specified, treated cells, in groups of three, were excited twice for 10 min with an interval of 24 h in between after cell seeding. Proliferation rates of these cells were determined using BrdU and MTS assays 24...

Volume 2, Parts A and B, 2004

ABSTRACT

Volume 2: Fora, Parts A and B, 2007

The Journal of the Acoustical Society of America, 2014

ABSTRACT Acoustic behaviors of five different polymer (polylactide) encapsulated microbubbles-PB1... more ABSTRACT Acoustic behaviors of five different polymer (polylactide) encapsulated microbubbles-PB127 (Point Biomedical), PH37, PH43, PH44, PH45 (Phillips Healthcare) with different shell-thickness-to-radius ratios (STRR) of 3.5, 30, 40, 65, and 100 nm/μm have been characterized. In vitro attenuation data were used to determine the interfacial rheological properties of their shells. Use of different models-Newtonian, viscoelastic, strain-softening, Marmottant, and Church-resulted in similar rheological properties. The shell elasticity and shell viscosity were found to increase with increasing shell thickness as expected. The nonlinear scattered response from these microbubbles were measured. Experimentally measured scattered subharmonic response were compared with the model predictions.