Dynamics of Bubble Oscillation in Constrained Media and Mechanisms of Vessel Rupture in Shock Wave Lithotripsy

Abstract

Rupture of small blood vessels is a primary feature of the tissue injury associated with shock wave lithotripsy (SWL), and cavitation has been implicated as a potential mechanism. To improve our understanding of the damage mechanism, dynamics of SWL-induced bubbles in constrained media were investigated. Silicone tubing and cellulose hollow fibers (i.d.=0.2 ~ 1.5 mm) were used to fabricate vessel phantoms, in which test fluid containing cavitation nuclei were circulated. Intraluminal bubble dynamics were examined by high-speed imaging and passive cavitation detection (PCD). Compared to the large expansion and violent inertial collapse of SWL-induced bubbles in a free field, the expansion of bubbles inside the vessel phantom is greatly constrained, leading to asymmetric elongation along the vessel axis and much weakened collapse. Conversely, the rapid, large expansion of the bubbles significantly dilates the vessel wall, leading to consistent rupture of the hollow fibers after less than 20 shocks in a XL-1 lithotripter. The rupture is dose-dependent, and varies with the spatial location of the vessel phantom inside the lithotripter field. Further, when the large intraluminal bubble expansion was suppressed by inverted lithotripter shock waves (LSW), the rupture could be avoided even after 100 shocks. Theoretical calculation confirms that the propensity of vascular injury due to LSW-induced intraluminal bubble expansion increases with the tensile pressure of the shock wave, and with the reduction of the inner diameter of the vessel, as noted in previous animal studies.