Non-focal acoustic lens designs for cavitation bubble consolidation

Abstract

During shockwave lithotripsy, cavitation bubbles form on the surface of urinary stones aiding in the fragmentation process. However, shockwaves can also produce pre-focal bubbles, which may shield or block subsequent shockwaves and potentially induce collateral tissue damage. We have previously shown in-vitro that low amplitude acoustic waves can be applied to actively stimulate bubble coalescence and help alleviate this effect. A traditional elliptical transducer lens design produces the maximum focal gain possible for a given aperture. From experiments and simulation, we have found that this design is not optimal for bubble consolidation as the primary and secondary Bjerknes forces may act against each other and the effective field volume is too small. For this work, we designed and constructed non-focal transducer lenses with complex surface geometries using rapid prototyping stereolithography to produce more effective acoustic fields for bubble consolidation during lithotripsy or ultrasound therapy. We demonstrate a design methodology using an inverse problem technique to map the desired acoustic field back to the surface of the transducer lens to determine the correct phase shift at every point on the lens surface. This method could be applied to other acoustics problems where non-focused acoustic fields are desired.