Observing ultrasound stimulated microbubble dynamics at MHz framing rates

Microbubble contrast agents were originally developed to enhance echogenicity in diagnostic sonography. However, their somewhat unique acoustic response and facility to transduct energy into spatially focussed regimes, together with the clinical demand for non-invasive adjuncts and options to conventional therapy, have seen interest in exploring the therapeutic potential of microbubbbles grow steadily within this past decade. For the purposes of the present study, we examined the ultrasonically stimulated response of many such microbubbles by controlling their spatial placement with bespoke optical traps. The objective was to improve our understanding of their statistical behaviour and to feed this information into realistic computational models so that processes might be more easily optimised. Hence, individual microbubbles of commercial [low-index] ultrasound contrast agent were isolated from a parent population by selection using a Laguerre-Gaussian optical trap, and subsequently manipulated to controllable micrometer level displacements from a solid surface. This configuration was then insonated using a 60ms burst of 1MHz ultrasound at a nominal peak pressure of 6.5MPa, and the subsequent bubble dynamics recorded using ultra high speed micro-photography at microsecond temporal resolutions. Any resultant damage induced at the target surface was also observed and characterised using atomic force microscopy.