Biomedical subjects

Andrew J Szeri

Publications and source records attributed to Andrew J Szeri.

4 recordsLinked to original sources

PubMed2003

Optimization of acoustic scattering from dual-frequency driven microbubbles at the difference frequency.

The second harmonic radiation of acoustically driven bubbles is a useful discriminant for their presence in clinical ultrasound applications. It is useful because the scatter from a bubble at a frequency different from the driving can have a contrast-to-tissue ratio better than at the drive frequency. In this work a technique is developed to optimize the scattering from a microbubble at a frequency different from the driving. This is accomplished by adjusting the relative phase and amplitudes of the components of a dual-frequency incident ultrasound wave form. The investigation is focused primarily on the example of dual-mode driving at frequencies of 1 MHz and 3 MHz, with the scattering optimized at 2 MHz. Bubble radii of primary interest are 0.5 to 2 microm and driving amplitudes to 0.5 atm. Bubbles in this size range are sensitive to modulation of driving. It is shown that an optimal forcing scheme can increase the target response eightfold or more. This suggests new applications in imaging and in bubble detection.

Animals↗

PubMed2002

Argon rectification and the cause of light emission in single-bubble sonoluminescence.

In single-bubble sonoluminescence, repeated brief flashes of light are produced in a gas bubble strongly driven by a periodic acoustic field. A startling hypothesis has been made by Lohse and co-workers [Phys. Rev. Lett. 78, 1359 (1997)] that the non-noble gases in an air bubble undergo chemical reaction into soluble products, leaving only argon. In the present work, this dissociation hypothesis is supported by simulations, although the associated temperatures of about 7000 K seem too low for bremsstrahlung, which has been proposed as the dominant light emission mechanism. This suggests that emission from water vapor and its reaction products, heretofore not included, may play an important role.

Journal Article↗

PubMed2002

Optimal pulse-inversion imaging for microsphere contrast agents.

Microbubbles are used as contrast agents because they scatter incident ultrasound (US) efficiently. Pulse-inversion imaging makes use of the asymmetrical difference in response of a bubble to an US pulse and its symmetrical opposite to obtain improved contrast. In this theoretical work, the principle of pulse-inversion imaging is expanded upon by finding the optimal pulse shape to maximize the signal from a bubble. Given a limit on driving intensity, a suitable norm of the bubble response is maximized using optimal control theory to identify the proper pulse/antipulse driving protocol for a single bubble. The optimal pulse yields a several-fold increase in the normed response over the best sinusoidal pulse of the same driving intensity. It was found that the optimal driving for a single bubble also maximizes the nonlinear response of the cloud if its mean bubble size is the same as that of the single bubble.

Biomedical Enhancement↗

PubMed2002

Coupled dynamics of translation and collapse of acoustically driven microbubbles.

Pressure gradients drive the motion of microbubbles relative to liquids in which they are suspended. Examples include the hydrostatic pressure due to a gravitational field, and the pressure gradients in a sound field, useful for acoustic levitation. In this paper, the equations describing the coupled dynamics of radial oscillation and translation of a microbubble are given. The formulation is based on a recently derived expression for the hydrodynamic force on a bubble of changing size in an incompressible liquid [J. Magnaudet and D. Legendre, Phys. Fluids 10, 550-556 (1998)]. The complex interaction between radial and translation dynamics is best understood by examination of the added momentum associated with the liquid motion caused by the moving bubble. Translation is maximized when the bubble collapses violently. The new theory for coupled collapse and translation dynamics is compared to past experiments and to previous theories for decoupled translation dynamics. Special attention is paid to bubbles of relevance in biomedical applications.

Acoustics↗