Event № 1157

Type: Seminar

Name: Seminar Lecture for PhD

Title: On backscattering of a focused ultrasound beam from cells and on the dynamics of intra-membrane nanobubbles

Speaker: Michael Assa

Place: Amado 509, Technion

Ultrasound is extensively used in medicine for imaging. In the last decades therapeutic ultrasound is becoming increasingly popular whereas many ultrasound manipulations of cells and tissues are being studied. We have provided four years ago a comprehensive explanation for the generation of sub-cellular-level forces by an acoustic pressure wave. Using a physical model that incorporates molecular forces with bubble dynamics and gas diffusion, we predicted that ultrasound induces a pulsating bubble in the intra-membrane space between the two lipid leaflets. Experiments were conducted under the assumption that cells in culture when exposed to ultrasound will behave as a cluster of micro- and nano-bubbles. Cell monolayers in a 12-well plate were placed at the focal point of a focused ultrasound beam of 0.5MHz (CW) with pressure amplitude that varies in the range 0.1-2MPa (at the focus). Hydrophobic polystyrene particles with 10-20µm in diameter were tested as well. The backscattered beam on the ultrasound source surface was measured and analyzed. Backscattered pressure amplitudes that were detected by the ultrasound source acting as a receiver reached surprisingly high levels relative to the amplitudes at the focal point. This is typical for a cluster of gas bubbles, each of them acting like a point source that emits spherical waves; being synchronized sources, when they interact with each other they generate an interference peak at the receiver. Theoretical simulations of a spherical cell that is surrounded by an intra-membrane active space were developed analytically and solved numerically. We used Fick?s law for gas transport, and modified Rayleigh-Plesset integral momentum and energy equations for bubble dynamics. The simulated dynamics of pulsating gas pockets of the bilayer sonophores, together with Euler Momentum equation and mass conservation allowed to calculate the pressure amplitudes and the phase shift of the acoustic waves propagating away from the sonophores. Qualitative similarity was found between our predicted results and the measured backscattering from cells and particles that can be explained by pulsating nano- or micro- bubbles. Different cell types demonstrated different acoustic signatures of pressure amplitude and phase that most probably result from different membrane composition and structure. The qualitative agreement between experiments and theory provides a valuable support for the theory of ultrasound induced intra-membrane cavitation. Moreover, this new understanding might pave the way for the development of technology of ultrasound imaging at the cellular level.

SubmittedBy: Michael Assa , assami@tx

EventLink: Event № 1157