Dynamics of bubble oscillation in constrained media and mechanisms of vessel rupture in SWL

Rupture of small blood vessels is a primary feature of the vascular injury associated with shock-wave lithotripsy (SWL) and cavitation has been implic ated as a potential mechanism. To understand more precisely the underlying mechanical cause of the injury, the dynamics of SWL-induced bubble dynamics in constrained media were investigated. Silicone tubing and regenerated ce llulose hollow fibers of various inner diameters (0.2 to 1.5 mm) were used to fabricate vessel phantoms, which were placed in a test chamber filled wi th castor oil so that cavitation outside the phantom could be suppressed. D egassed water seeded with 0.2% Albunex(R) contrast agent was circulated ins ide the vessel phantom, and intraluminal bubble dynamics during SWL were ex amined by high-speed shadowgraph imaging and passive cavitation detection v ia a 20-MHz focused transducer. It was observed that, in contrast to the ty pical large and prolonged expansion and violent inertial collapse of SWL-in duced bubbles in a free field, the expansion of the bubbles inside the vess el phantom was significantly constrained, leading to asymmetric elongation of the bubbles along the vessel axis and, presumably, much weakened collaps e. The severity of the constraint is vessel-size dependent, and increases d ramatically when the inner diameter of the vessel becomes smaller than 300 mum. Conversely, the rapid, large intraluminal expansion of the bubbles cau ses a significant dilation of the vessel wall, leading to consistent ruptur e of the hollow fibers (i.d. 200 mum) after less than 20 pulses of shock wa ve exposure in a XL-1 lithotripter. The rupture is dose-dependent, and vari es with the spatial location of the vessel phantom in the lithotripter fiel d. Further, when the large intraluminal bubble expansion was suppressed by inversion of the lithotripter pressure waveform, rupture of the hollow fibe r could be avoided even after 100 shocks. Theoretical calculation of SWL-in duced bubble dynamics in blood confirms that the propensity of vascular inj ury due to intraluminal bubble expansion increases with the tensile pressur e of the lithotripter shock wave, and with the reduction of the inner diame ter of the vessel. It is suggested that selective truncation of the tensile pressure of the shock wave may reduce tissue injury without compromising t he fragmentation capability of the lithotripter pulse. (E-mail: pzhong@acpu b.duke.edu) (C) 2001 World Federation for Ultrasound in Medicine & Biology.