The Characteristics of Cavitation Bubbles Induced by the Secondary Shock Wave in an HM-3 Lithotripter and Its Effect on Stone Comminution Yufeng Zhou, Jun Qin, and Pei Zhong Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, 27708-0300, USA Abstract. The characteristics of cavitation bubbles induced by the secondary shock wave (SW) in an electrohydraulic Dornier HM-3 lithotripter were investigated using simultaneous high-speed imaging and passive cavitation detection techniques. The results are compared with those produced by the primary SW and several differences have been observed. First, similar collapse time of the secondary bubble cluster was observed both in free field and near a solid surface. Second, bubbles in the secondary cluster are smaller in size but with higher density. These small bubbles do not expand significantly and/or aggregate strongly near a solid surface as their counterpart produced by the primary SW. As a result, their collapse is weaker with few microjets formation. By applying a strong suction flow aimed at the tip of the lithotripter electrode, the production of the secondary SW could be disturbed but without significant influencing the primary SW and associated bubble activities based on pressure waveform measurement and cavitation detection by light transmission. With the suction flow, stone comminution after 250 shocks was reduced from the original configuration, suggesting a potential effect of the secondary bubble cluster on stone comminution in SWL. INTRODUCTION The first shock wave lithotripter was invented in early 1980s and soon obtained the approval from FDA for clinical use in the United States. Since then shock wave lithotripsy (SWL) has rapidly emerged as the primary treatment choice for most urinary tract calculi worldwide [1]. Based on the means of shock wave generation, all commercial lithotripters can be categorized in three types: electrohydraulic (EH), electromagnetic (EM) and piezoelectric (PE) lithotripters. In comparison to EM and PE lithotripters, a uniqueness of the EH lithotripters is the production of two shock waves, one from the rapid plasma expansion and the other from the inertial collapse of the plasma bubble between electrode tips after each spark discharge. It is unknown whether this unique feature of the EH lithotripters could lead to performance differences among different types of lithotripters [2,3]. In this study, the characteristics of the bubble dynamics induced by the secondary SW in an HM-3 lithotripter were investigated and compared with those produced by the primary SW both in free field and near a solid surface. By using a strong suction flow directed at the tip of the lithotripter electrode we were able to disturb the plasma bubble
oscillation at F 1, and therefore to evaluate the effect of the secondary SW on stone comminution in an HM-3 lithotripter in vitro. METHODS FIGURE 1. A schematic diagram of the experimental set up with high-speed shadowgraph, passive cavitation detection, light transmission, pressure measurement, and suction flow near the electrode tips. The experiments were carried out in an unmodified Dornier HM-3 lithotripter (Figure 1). To disturb the generation of the secondary SW, a suction flow (~ 2.7 liters/s) was applied to the electrode tips (Fig. 1). The pressure waveforms at the lithotripter focus (F 2 ) were measured using a fiber optic probe hydrophone (FOPH) attached to a 3-D translation stage and aligned parallel to the lithotripter axis. The acoustic emission signals associated with cavitation bubbles in the free field and near a solid surface were measured by using a 2.25-MHz focused hydrophone and a PCB pressure transducer, respectively. The dynamics of LSW-induced cavitation bubbles were captured using high-speed shadowgraph imaging and measured by light transmission technique [4,5]. RESULTS AND DISCUSSION The collapse time (t c ) of the plasma bubble induced by the spark discharge at F 1 varies from 2.3 to 4.4 ms when the output voltage of the lithotripter increases from 16 to 24 kv (Figure 2a). Based on Raleigh s formula for bubble collapse, these values correspond approximately to a plasma bubble with a maximum diameter of 2.3 to 4.4 cm. In comparison, within the corresponding output voltage range t c for bubble clusters induced by the primary SW at F 2 increases from 229 to 334 μs in free field. The spark discharge occurs in a small region at F 1 and the focusing SW spreads the
acoustic energy into a much larger area around F 2 (-6 db beam width of HM-3 lithotripter at 20 kv is about 16 mm). Therefore, the size of SW-induced bubble is much smaller than that of the plasma bubble. In addition, although t c of the secondary bubble cluster increases with the output voltage both the value and the slope are significantly smaller than those produced by the primary SW (Fig. 2b). For example, at 20 kv t c of the first bubble cluster near a solid surface (444±36 μs) is about 1.7 times as that in free field (267±21 μs). However, the corresponding values produced by the secondary SW in both conditions are similar (202±11 μs in free field and 248±20 μs near a solid surface). The boundary condition seems to have little influence on the secondary bubble cluster. Time Delay between Two Shock Waves (ms) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 16 18 20 22 24 Output Voltage (kv) Bubble Collapse Time (μs) 700 600 500 400 300 200 100 2nd bubble cluster near PCB 1st bubble cluster near PCB 2nd bubble cluster in free field 1st bubble cluster in free field 16 18 20 22 24 Output Voltage (kv) FIGURE 2. Time delay between the primary and secondary shock waves, and collapse time of bubble clusters in free field and near a solid surface induced by the primary and secondary shock waves. FIGURE 3. Representative bubble dynamics induced in an HM-3 lithotripter at 20 kv by the primary shock wave in free field, near the surface of a PCB transducer, and those induced by the secondary shock wave (c) in free field, (d) near the surface of a PCB transducer. The number above each image is the time delay for the shock wave propagation from F 1. The frame size is about 12 9 mm.
The peak positive/peak negative pressure of the primary and secondary SW at F 2 measured by using a FOPH are 47/ 8 MPa, and 6.2/ 2.1 MPa, respectively. Because of the instability of the collapse strength of the plasma bubble, there are large variations in the profile of the secondary SW. Furthermore, representative sequences of the bubble dynamics produced by the primary and secondary SWs at F 2 both in free field and near a solid surface (i.e., PCB transducer) were obtained using the high-speed shadowgraph system (in Figure 3). In comparison to the bubble dynamics produced by the primary SW there are several differences in the secondary bubble cluster. First, although the shock front of the secondary SW is visible, it is not well focused as the primary SW. The convex edge wave of the secondary SW is also not clear (t = 180 μs in Fig. 3c). This observation may be due to the fact that the collapse of the plasma bubble at F 1 may not be symmetric, and thus different portions of the reflected shock wave may be out of phase when arriving at F 2. Second, higher bubble density was observed in the secondary bubble cluster, presumably from the bubble nuclei generated by the collapse of the primary SW induced bubbles. However, because of the lower peak pressure in the secondary SW, the bubbles in the secondary cluster do not expand and/or aggregate strongly as their counterpart induced by the primary SW either in free field or near a solid surface. Third, when the bubbles in the secondary bubble cluster collapse, the collapse strength is weaker with few microjets formation. Based on the measurement from the PCB transducer, the collapse strength of the secondary bubble cluster is about 1/6 as that of the primary bubble cluster. Finally, after the primary inertial collapse of the secondary bubble cluster almost no rebound bubbles were observed. 5 4 spark discharge AC Amplitude (V) 3 2 1 0 1st bubble cluster rebound of 1st bubble cluster Pump on + offset 3V Pump off -1-2 reflected wave 1st shock wave 2nd bubble cluster 0 1 2 3 4 5 Time (ms) FIGURE 4. Comparison of the peak positive and negative pressure of the primary shock wave (waveform shown in the inset) and light transmission signals associated with primary and secondary bubble clusters produced with and without suction flow. Because the suction tubing blocks a portion of the reflected shock wave, the output voltage of the lithotripter was increased from 20 kv to 22 kv for compensation. The measured pressure waveforms and light transmission signals with suction pump on or off were similar with no statistically significant differences in the measured peak positive and negative pressures (p > 0.05 in Fig. 4a). Therefore, the contribution of the primary SW to stone comminution should remain the same independent of the pump
condition. However, when the suction flow was applied, the light transmission signal from the secondary bubble cluster almost disappeared (Fig. 4b) and few bubbles could be observed in the high-speed shadowgraph images. It was found that the suction flow could remove consistently the secondary bubble cluster during the SWL treatment. After 250 shocks, the stone comminution efficiency produced by HM-3 lithotripter at 22 kv with suction flow (32.19±3.53%) is smaller than that without the suction flow (41.23±7.07%). Major difference between those two groups is that the secondary bubble cluster may help to break large-size fragments (> 4 mm) and to produce more small pieces (< 2 mm). This result indicates that the secondary SW, although much weaker compared to the primary SW, does contribute to stone comminution in SWL, which may be one of the reasons why EH lithotripters such as HM-3 has better performance than EM and PE machines. Stone Fragment Percentage (%) 50 40 30 20 10 p=0.001 p=0.29 Pump off Pump on p=0.23 p=0.03 (c) 0 <2 2-2.8 2.8-4 >4 Size of Fragment (mm) FIGURE 5. The percentage of stone fragments treated after 250 shocks at 22 kv, and the representative photos of fragments treated with and (c) without suction flow. REFERENCES 1. C. Chaussy, Extracorporeal shock wave lithotripsy: New aspects in the treatment of kidney stone disease, S Karger, Basel, 1982. 2. J. E. Lingeman, Urolithiasis 24, 185-211, (1997). 3. P. Zhong, L. Cioanta, F. H. Cocks, and G. M. Preminger, J. Acoust. Soc. Am. 101, 2940-2950 (1997). 4. S. L. Zhu, F. H. Cocks, G. M. Preminger, and P. Zhong, Ultrasound in Med. & Biol. 28, 661-671, (2002). 5. Y. F. Zhou and P. Zhong, J. Acoust. Soc. Am. 113, 586-597 (2003).