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. Author manuscript; available in PMC: 2013 Apr 1.
Published in final edited form as: Ultrasound Med Biol. 2012 Apr;38(4):601–610. doi: 10.1016/j.ultrasmedbio.2011.12.022

CHARACTERISTICS OF THE SECONDARY BUBBLE CLUSTER PRODUCED BY AN ELECTROHYDRAULIC SHOCK WAVE LITHOTRIPTER

Yufeng Zhou *, Jun Qin †, Pei Zhong ‡
PMCID: PMC3572244  NIHMSID: NIHMS360893  PMID: 22390990

Abstract

This study investigated the characteristics of the secondary bubble cluster produced by an electrohydraulic lithotripter using high-speed imaging and passive cavitation detection techniques. The results showed that (i) the discrepancy of the collapse time between near a flat rigid boundary and in a free field of the secondary bubble cluster was not as significant as that by the primary one; (ii) the secondary bubble clusters were small but in a high bubble density and nonuniform in distribution, and they did not expand and aggregate significantly near a rigid boundary; and (iii) the corresponding bubble collapse was weaker with few microjet formation and bubble rebound. By applying a strong suction flow near the electrode tip, the production of the secondary shock wave (SW) and induced bubble cluster could be disturbed significantly, but without influence on the primary ones. Consequently, stone fragmentation efficiency was reduced from 41.2 ± 7.1% to 32.2 ± 3.5% after 250 shocks (p <0.05). Altogether, these observations suggest that the secondary bubble cluster produced by an electrohydraulic lithotripter may contribute to its ability for effective stone fragmentation.

Keywords: Electrohydraulic lithotripter, Bubble cluster cavitation, Secondary shock wave, Stone fragmentation

INTRODUCTION

The first clinical extracorporeal shock wave lithotripter (i.e., Dorner HM3, Dornier Med Tech, Singapore) was introduced by Dornier in the early 1980s and utilizes an underwater spark discharge for shock wave (SW) generation based on the electrohydraulic (EH) principle and a truncated ellipsoidal reflector for SW focusing. Since then, shock wave lithotripsy (SWL) has rapidly emerged worldwide as the treatment of choice for most urinary tract calculi (Chaussy and Fuchs 1989). Subsequently, different types of (2nd and 3rd gneration) lithotripters were introduced with new means for SW generation and focusing (i.e., electromagnetic [EM] generator with an acoustic lens and self-focusing piezoelectric [PE] generator), patient coupling (i.e., water cushion) and stone localization (i.e., ultrasound or fluoroscopic imaging alone or in combination). Despite these technological advances in lithotripter design, the fundamental principle remains the same (Coleman and Saunders 1989). Most interestingly, clinical studies in the past decade have often demonstrated that the original Dornier HM3 is more effective in stone comminution with lower stone recurrent rate among all commercial models, and is considered the gold standard by practicing urologists (Lingeman et al. 2003; Gerber et al. 2005). Therefore, understanding the critical factors leading to the performance difference is of importance for technical improvement in the design of modern SWLs.

In contrast to EM and PE lithotripters, a unique feature of the EH lithotripters is the production of two SWs of different strengths in sequence after each spark discharge. The underwater spark discharge vaporizes the surrounding fluid, generating the primary SW that emanates from the electrode tips (i.e., F1 of the ellipsoidal reflector) and concomitantly a large plasma bubble (~20 mm in diameter), which collapses in several milliseconds, generating a secondary SW, and consequently leads to the occurrence of another cavitation event (Zhong et al. 1997). Because the secondary bubble cluster has not been studied in detail, its effect on stone fragmentation in SWL is unknown.

In this study, the bubble cluster dynamics induced by the secondary SW in a Dornier HM3 lithotripter was characterized by pressure measurement, cavitation detection via passive cavitation detection, light transmission and high-speed shadowgraph, and the results were compared with those associated with the primary bubble cluster, both in a free field and near a flat rigid boundary. Moreover, using a strong suction flow aimed at the tip of the lithotripter electrode, the production of the secondary SW and associated bubble cluster at the lithotripter focus could be effectively diminished, without significantly altering the generation of the primary SW and associated bubble activities. In vitro stone fragmentation experiments showed that with the removal of the secondary bubble cluster the resultant stone comminution produced after 250 shocks decreased significantly from 41.2 ± 7.1% to 32.2 ± 3.5% (p < 0.05). Altogether, it is suggested that the secondary bubble cluster may lead to the better stone fragmentation ability of an EH lithotripter.

MATERIALS AND METHODS

Lithotripter

The experiments were carried out in a Dornier HM3 electrohydraulic lithotripter. To disturb the inertial collapse of the plasma bubble between the electrode tips, a strong suction flow (~2.7 L/s with a mean velocity of 2.7 m/s in the pipe) was applied via a water pump (FP5172-1/2HP, Flotec, Delavan, WI, USA) with poly-vinyl chloride tubing (D = 35.75 mm) aiming at F1 (Fig. 1). To minimize the effect of suction flow on the cavitation activities in the focal region of the lithotripter, a 115-μm-thick acoustic, transparent, polyester membrane was placed at the bottom of the testing tank. The lithotripter output voltage was increased from 20 to 22 kV to compensate for energy loss (~30% reduction in the peak positive pressure according to our waveform measurement) cause by the shadowing of the suction tubing inside the lithotripter reflector (Zhou et al. 2005).

Fig. 1.

Fig. 1

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Schematic diagram of the experimental setup in the Dornier HM3 lithotripter for pressure measurement, passive cavitation detection, high-speed shadowgraph and light transmission in a vertical view. A strong suction flow was applied close to the electrode tip to disturb the production of the secondary SW.

Pressure measurement

The pressure waveforms of both the primary and secondary SWs were measured using a fiberoptic probe hydrophone (FOPH 500, RP Acoustic, Leutenbach, Germany) and recorded by a digital oscilloscope (LeCroy 9314M, Chestnut ridge, NY, USA). The hydrophone was accurately aligned with F2 using a mechanical point designed by Dornier that coincides with the geometrical focus of the HM3 lithotripter and allows the operator to determine the lithotripter focus under bioplanar fluoroscopic imaging (Zhou and Zhong 2003).

Bubble cavitation detection

The acoustic emission (AE) signals associated with bubble cluster induced by both SWs in a free field were measured by a 2.25-MHz focused hydrophone (V304-SU, D = 25 mm, F = 101.6 mm, Olympus-IMS, Waltham, MA, USA), which was aligned confocally with F2, and then amplified by a broadband amplifier (5052 Pulse/Receiver, Olympus-IMS, Waltham, MA, USA) with a high-pass filtering frequency of 1 MHz to eliminate signals caused by the vibration of the passive cavitation detection (PCD) sensing element in the lithotripter field (Zhong et al. 1997). In addition, a flat pressure sensor (119B02, D =6.4 mm, PCB Piezotronics Inc., Depew, NY, USA), whose ceramic element is encapsulated in a steel housing for ruggedness in ballistic pressure measurement, was used to simulate a solid boundary at the lithotripter’s geometrical focus as cylindrical kidney calculi (Pishchalnikov et al. 2003) and to detect bubble activities as well (Zhou et al. 2005).

High-speed shadowgraphy

A laser pulse (MiniLaseI, New Wave Research, Sunnyvale, CA, λ = 512 nm, tp = 6 ns) formed a parallel beam through the focal region of the HM-3 lithotripter by a concave lens and a Schlieren mirror (Zhou et al. 2005). By adjusting the trigger delay of the laser, a high-speed CCD camera (GP-MF552, Panasonic, Secaucus, NJ, USA) and a frame grabber (DT3155, Data Translation, Marlboro, MA, USA) using a delay generator (DG535, Stanford Research System, Sunnyvale, CA, USA) with respect to the spark discharge picked up by a fast photo-detector (PDA50, Thorlabs, Newton, NJ, USA), a series of images of shock wave propagation and bubble dynamics could be captured (Zhou and Zhong 2003). When studying the secondary bubble cluster, a transducer (f0 = 5 MHz, D = 3 mm, V1091, Olympus-IMS) was attached on the inner surface of HM3 reflector with a distance of ~40 mm to the electrode tip to detect the plasma collapse at F1 as the reference time.

Light transmission

The dynamics of SW-induced bubble clusters were monitored continuously by light transmission technique. An expanded illumination light produced by a Helium-Neon laser (λ = 632.8 nm, 0.95 mW, Uniphase, Manteca, CA, USA) was transmitted through the lithotripter focal region, and subsequently focused onto the fast photo-detector (PDA50, Thorlabs, Newton, NJ, USA). An iris diaphragm (ID36, Thorlabs) was mounted before the focusing lens to limit the laser beam size to 15 mm (Zhou et al. 2005).

Stone comminution

Spherical stone phantom (D = 10 mm) made of plaster-of-Paris with a powder to water mixing ratio of 2:1 by weight was used with established protocol (Zhou and Zhong 2003). Before placing it into a holder with a disposable rubber finger cot (QRP, Tucson, AZ, USA) at the bottom, each stone phantom was weighed in the dry state and then immersed in the degassed water until fluid saturated the stone. The holder was connected to the hydraulic gantry of the HM3 lithotripter so that the stone phantom could be aligned to F2 under the guidance of biplanar fluoroscopic imaging. A total of 250 SWs at the output voltage of 22 kV were delivered to the stone phantom at a pulse repetition rate of 1 Hz. Afterwards, all fragments of stone phantoms were carefully removed from the finger cot, spread out into a layer on paper and let dry at room temperature overnight. The dry fragments were separated through a series of ASTM standard sieves (W.S. Tyler, Mentor, OH, USA) with 4-, 2.8- and 2-mm grids, respectively. The stone comminution efficiency was determined by the percentage of fragments <2 mm, which can be discharged spontaneously after clinical SWL treatment (Chaussy et al. 1982).

RESULTS

Pressure waveform

The representative pressure waveforms of the primary and secondary SWs produced at the focal point of the HM3 lithotripter at 20 kV are shown in Figure 2 with their characteristic parameters summarized in Table 1. The primary SW had a leading compressive wave with high amplitude (46.83 ± 2.5 MPa) and short duration (2.17 ± 0.28 μs) followed by a much weaker (−7.47 ± 1.79 MPa) but longer (4.65 ± 0.35 μs) tensile trail. Because of the instability of the inertial collapse of the plasma bubble at F1, the secondary SW at F2 had a great variation both in pressure waveform profile and its characteristics. The peak positive and negative pressures of the secondary SW were 6.1 ± 1.82 MPa and −2.1 ± 0.93 MPa, with duration of 0.68 ± 0.22 μs and 0.54 ± 0.18 μs, respectively. Despite of the low signal-to-noise ratio (SNR) of the secondary SW, it could be discerned because of its sharp shock front with a rise time of 94.8 ±78.3 ns, which was on the same order as that of the primary one (51.2 ± 8.4 ns). Furthermore, it was found that no matter the suction pump status (on or off), there was no significant difference on the peak positive and negative pressures of the primary SW, and the pressure waveforms were similar (Zhou et al. 2005). Therefore, the effect of the primary SW and its bubble cluster on stone comminution may remain the same.

Fig. 2.

Fig. 2

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The waveforms of (a) the primary and (b) the secondary SWs at the geometric focal point of the HM3 lithotripter at the output voltage of 20 kV measured by fiber-optical probe hydrophone in the degassed water (O2 concentration <4 mg/L).

Table 1.

Peak pressure and temporal parameters of the primary and secondary shock wave produced by the HM3 lithotripter at 20 kV

SW p+ (MPa) p− (MPa) tr (ns) t+ (μs) t− (μs)
Primary 46.83 ± 2.50 −7.47 ± 1.79 51.2 ± 8.4 2.17 ± 0.28 4.65 ± 0.35
Secondary 6.1 ± 1.82 −2.1 ± 0.93 94.8 ± 78.3 0.68 ± 0.22 0.54 ± 0.18
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p+ = peak positive pressure; p− = peak negative pressure; tr = rise time of the shock front; t+ = positive pulse duration; t− = negative pulse duration (n = 6).

Measurement was under the guidance of International Electrotechnical Commission (IEC) standard.

Cavitation detection

In the traces measured by PCB transducer, the primary and secondary SWs, their induced collapse and sometimes rebounds of bubble clusters around a rigid boundary were observed (Fig. 3a). The inertial collapse time, tc, is usually defined as the duration between inception and/or growth of cavitation bubbles and their inertial collapse, either in the free field or near a flat rigid boundary (Zhong et al. 1997). Similarly, tc of the plasma bubble between the electrode tips could also be detected and was found to rise from 2.3–4.4 ms with the increase of the output voltage from 16–24 kV (Fig. 3b). According to Rayleigh’s model, the plasma bubble is estimated to be several centimeters in size (Crum 1984). In comparison, tc of bubble cluster induced by the primary SW at F2 varied from 229–334 μs in the free field. Therefore, cavitation bubbles induced by the primary SW had already been in the phase of dissolution when the secondary SW arrived at F2. In addition, it was found that although tc of the secondary bubble cluster increased with the output voltage, its energy-dependence (i.e., increase from 186 to 269 μs near the PCB transducer within the output voltage range of 16 to 24 kV) was significantly less than that of the primary one (i.e., increase from 285 to 611 μs in Fig. 3c). Therefore, the bubble cavitation induced by the primary SW may have a dominant role in both stone fragmentation and renal injury during SWL treatment. Furthermore, tc of the primary bubble cluster near the PCB transducer (444 ± 36 μs) was about 1.7 times as that in the free field (267 ± 21 μs) at 20 kV. However, the corresponding values of the secondary bubble cluster were similar (202 ± 11 μs in the free field and 248 ±20 μs near the PCB transducer). The boundary condition seems to have little influence on the secondary bubble cluster. The strength of bubble collapse could also be estimated qualitatively by the PCB transducer, whose response is not fast enough to resolve the peak impact pressure generated by a collapsing bubble because its resonant frequency is only ~400 kHz (Vogel and Lauterborn 1988). It was found that the collapse strength of the secondary bubble cluster (i.e., from 113.2 ± 35.5 to 158.3 ± 42.6 mV) was lower than those of the inertial cavitation (i.e., from 500.9 ± 111.5 to 1433.9 ± 346.5 mV) and the first rebound bubbles (i.e., from 221.7 ± 105.7 to 308.1 ± 71 mV) induced by the primary SW with the output voltage increasing from 16–24 kV (Fig. 3d).

Fig. 3.

Fig. 3

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(a) A representative signal measured by the PCB transducer at the focal point of HM3 at the output voltage of 20 kV; (b) the dose dependency of the delay time between two SWs; (c) the comparison of dose dependency of the bubble collapse time both in the free field and near a solid boundary produced by the primary and secondary SWs; (d) the collapse strengths produced by the primary and secondary bubble clusters; and (e) the relationship of peak positive and negative pressures with output voltage. Twenty samples were recorded in each condition for statistical analysis.

High-speed shadowgraph

Representative sequences of bubble dynamics produced by the primary and secondary SWs both in the free field and near the PCB transducer in the HM3 lithotripter at 20 kV are shown in Figure 4. After the passage of the primary SW (t = 180 μs), the existing bubble nuclei in water grew up rapidly and then collapsed violently (Fig. 4a), which is characterized as the production of AE signals (i.e., circular rings expanding outward from the collapse sites) and microjet (i.e., along the propagation direction of AE signal at t = 450 μs). During the growing period, individual bubbles were almost spherical in geometry (t = 220 μs in Fig. 4a). The bubble coagulation led to substantially increased maximum bubble size, Rmax, and tc (t = 300 μs in Fig. 4a). After the inertial bubble collapse, a few bubble nuclei may rebound with irregular geometry and ensuing weak collapse (t = 500 and 600 μs in Fig. 4a). When a stone phantom was located at the focus, the primary SW-induced bubbles would be attracted around its surface as a result of the surface tension or the radiation force of SWs (Vakil et al. 1992; Choi et al. 1993). Moreover, SW reflection at the interface of water/stone wound provides additional acoustic energy for inertial bubble growth. The bubble cluster at the proximal face typically formed an envelope or blister covering most of the PCB transducer and would then constrict close to the contacting point and take a “mushroom cloud” shape (Pishchalnikov et al. 2003) when generating AE signals (t = 580 μs) and a microjet towards stone surface (t = 600 μs in Fig. 4b). Therefore, its collapse time is much longer than that in the free field, which was verified by cavitation detection in Figure 3. After the inertial collapse, the rebound bubble was found to be moving away from the stone surface (t = 800 μs in Fig. 4b).

Fig. 4.

Fig. 4

Fig. 4

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Representative sequences of high-speed shadowgraphs of bubble dynamics (a) in a free field and (b) near a solid boundary produced by the primary SW, and (c) in a free field and (d) near a solid surface produced by the secondary SW in the focal region of the HM3 lithotripter at the output voltage of 20 kV. The number above each image frame indicates the time delay in μs after the trigger event. SW propagates from left to right and the frame size is about 12 × 9 mm.

Because of the trigger method, there was about a 25-μs time delay (the SW propagation time from the electrode tip to the immersed ultrasound probe) after the secondary SW production. Compared with the bubble dynamics in the free field produced by the primary SW, there are several differences of the secondary bubble cluster (Fig. 4c):

  1. Although the shock front of the secondary SW was visible in the shadowgraphs, it looked more like a plane wave, not as well focused as the primary SW. In addition, the convex edge waves, the diffraction wave from the aperture of the ellipsoidal reflector, were not seen following closely to the shock front (t =155 μs). These discrepancies may be caused by the asymmetric collapse of the plasma bubble at F1 and subsequently phase mismatch of the focusing secondary SWs at F2.

  2. The secondary SW-induced bubbles were nonuniformly but densely distributed, presumably where the primary bubble cluster had collapsed to provide nuclei (t = 165 μs). They grew up very quickly in only a few microseconds.

  3. Because of the lower peak pressure of the secondary SW, the secondary bubble cluster did not expand as strongly as their counterpart induced by the primary SW. Although initially these bubbles were close to each other and some bubbles may merge, coalescence into a single one in symmetrical shape was not found. When the secondary bubble cluster came to collapse, they generated individual AE signals without microjet formation (t = 385 μs). After the inertial bubble cavitation, little rebounds of the secondary bubble cluster were observed.

  4. The characteristics of the secondary bubble cluster dynamics near the PCB transducer were similar to those in the free field (Fig. 4d), which means no difference between tc in both conditions.

  5. Collapse of the secondary bubble cluster happened more randomly, not always at the center of the proximal stone surface (t = 405 μs).

Light transmission

Representative light transmission signals through the focal region of the HM3 lithotripter at 22 kV are shown in Figure 5. Besides the SW-induced cavitation bubble cluster, light transmission method was sufficiently sensitive to pick up the shock front of the incoming SW and the reflected waves. The strong compressive wave changes the optical index of the fluid and forms a dark spot in the shadowgraphs (Fig. 5) or a negative spike in the light transmission signal. After the collapse of the primary bubble cluster, there were several rebounds of bubble nuclei and usually only the first rebound could be measured clearly by bubble cavitation detection techniques. Although the secondary bubble cluster has a shorter lifetime than the primary one, it induced more changes in the light transmission signal, suggesting a larger bubble area in the focal region. Overall, the light transmission is a highly sensitive method in cavitation bubble detection. When the water pump was turned on, the signal from the secondary bubble cluster almost disappeared with no significant changes on the primary one (Fig. 5) and such a disappearance was consistent throughout SWL treatment when the flat rigid boundary or stone phantom was aligned to the lithotripter’s geometrical focus (data not shown).

Fig. 5.

Fig. 5

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Representative light transmission signal near a solid boundary in the HM3 at 22 kV with suction flow on and off.

Stone fragmentation

The effect of the secondary SW and bubble cluster on stone comminution was evaluated by using an established in vitro phantom system that mimics kidney stone fragmentation in the renal pelvis. After 250 shocks, the stone comminution efficiency with the pump on (32.2 ± 3.5%) was smaller than that with the pump off (41.2 ± 7.1%). A major difference between those two groups was when the secondary SW presented fewer large-size fragments (>4 mm, 16.4 ± 5.6% vs. 23.4 ± 8.4%) and more small pieces (<2 mm) were produced (Fig. 6). This result indicates that the secondary SW and its induced bubble cavitation, although much weaker compared with the primary ones, does contribute to stone comminution in SWL, which may be one of the reasons EH lithotripters, such as the Dornier HM3, have better performance than EM and PE machines.

Fig. 6.

Fig. 6

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(a) Comparison of the size distribution of fragments after 250 shocks at 22 kVat different suction pump status, and the representative photos of the treated stone fragments treated with (b) pump off and (c) pump on.

DISCUSSION

The characteristics of the secondary bubble cluster were investigated in this study and found to be significantly different from those of the primary one. The secondary SW-induced bubbles have much higher density but are distributed in several separated regions. Although those cavitation bubbles are close to each other, significant aggregation from several small bubbles into a large one seldom occurs either in the free field or near the rigid boundary, such as kidney calculi. The differences may be caused by two factors: the distribution and properties of the cavitation bubbles and the profile of the secondary SW. The primary SW can induce strong cavitation activities in the focal region of an EH lithotripter. Bubble nuclei in water grow up from several micrometers to a few millimeters, and then collapse violently. Because the bubble dissolution time (on the order of seconds) is much longer than the delay time between these two SWs (a few hundred milliseconds), the fragments of large bubbles by the inertial collapse of the primary cavitation, although not visible, would provide the nuclei for the secondary SW arriving at F2 (Crum 1984; Sapozhnikov et al. 2002). Therefore, the secondary cavitation bubbles are nonuniformly distributed and presumably concentrated in the site of the last cavitation process. In addition, the tensile energy of the secondary SW was too weak to coagulate them into a single bubble blister as the primary bubble cluster near the rigid surface. When these bubbles collapsed, they behaved individually (Fig. 4c), which results in the weak collapse strength and no significant change of the bubble collapse time compared with that in the free field.

After the maximum expansion, the bubble collapses violently and generates a localized pressure wave, “secondary shock waves” (Lauterborn and Bolle 1975; Delacrétaz et al. 1991; Coleman et al. 1992; Kodama and Takayama 1998), whereas the primary wave shock is referred as one caused by the water breakdown with response to the spark discharge. Therefore, there are two types of secondary shock waves in SWL: one from the collapse of plasma bubble between the electrode tips at F1 and the other from the collapse of bubbles induced by the primary shock wave at F2. There are several differences between these two types. First, the secondary SW emitted from F1 will hit the ellipsoidal reflector of EH lithotripter and be focused at F2 as the primary one to generate other bubble cavitation activities, although with different characteristics as illustrated in this study. The secondary SW at F2 may affect the collapse of the rebound primary bubble cluster and lead to the formation of microjet along the propagation of the SW. However, this type of secondary SW is a diverging source with the amplitude decaying exponentially with the propagation distance and will not generate more cavitation bubbles. Second, there is only one large plasma bubble in the size of centimeter at F1. In comparison, multiple bubbles in the size of millimeter exist at F2 in the free field although bubble coalescence may occur. With the presence of stone or rigid boundary individual bubbles combine to form clusters at the proximal and distal ends and at the sides of stone (Pishchalnikov et al. 2003). Therefore, multiple secondary SWs will be produced at F2, but only one at F1.

The stone fragmentation in SWL is the consequence of dynamic material fracture in response to both the stress waves and cavitation, which work synergistically rather than independently (Zhu et al. 2002). The stress wave would extend preexisting flaws or randomly distributed microcracks and eventually lead to the stone breakage under repeated SW bombardments if the accumulated stress-intensity factor at the crack tip exceeds the materials’ fracture toughness (Lokhandwalla and Sturtevant 2000). Cavitation damage is characterized by surface erosion with numerous minute pittings produced by the violent collapse of bubbles near a stone surface (Coleman et al. 1987; Crum 1988; Philipp et al. 1993; Rink et al. 1994). Cavitation can weaken the structure of the stone surface, making it much more susceptible to the impact of ensuing SWs and associated bombardments of cavitation bubbles, but at a much slower rate of stone comminution. Although the secondary SW is too weak to generate strong stress waves inside the calculus, its tensile pressure exceeds the threshold of 0.5–1 MPa for the inertial cavitation in water (Fowlbes and Crum 1988). It is expected that the collapse of the secondary bubble cluster is not strong, only 1/6 qualitatively as that of the primary bubble cluster (Fig. 3d), which would also weaken the solid structure and make the following SWs more effective. In addition, the cracks or minute pittings on the stone surface will attract and stabilize bubbles, which are exempt from surface tension, dissolution or overpressure in the fluid and have a much longer lifetime (Sapozhnikov et al. 2002). Therefore, the collapse of the bubbles attached at the crack site will become stronger for the ensuing SWs because of the larger size of bubble formed. In our study, the removal of the secondary bubble cluster led to the reduced comminution efficiency and more large-size fragments. It is suggested that the unique feature of the secondary bubble cluster in EH lithotripter may be one of the reasons that differentiates its performance from EM and PE machines. Tandem pulse techniques are available for lithotripters with adjustable delay time (i.e., a few hundred microseconds) between two pulses (Xi and Zhong 2000; Loske et al. 2002; Dreyer et al. 2003). Although the amplitude of the second pulse is often smaller than the first, the stone fragmentation efficiency can be enhanced significantly. Altogether, generation of two SWs could be a method of increasing stone comminution efficiency of SWL.

Although the secondary bubble cluster covers a larger area in the focal region than the primary one, the bubble nuclei collapse individually rather than as a whole. If the secondary bubble cluster can be merged, the size of the single bubble will become comparable with or even larger than that of the primary one. It has been observed that low-amplitude ultrasound burst can help two bubbles attract to and contact with each other, exchange the gas contents, and finally coagulate into a single bubble (Postema et al. 2004). If a low-amplitude but long pulse is first delivered to coalesce the secondary bubble cluster and then followed by another strong but short pulse to accelerate the bubble collapse, such as from an auxiliary acoustic source, two strong inertial cavitation events may happen in one spark discharge and the consequent stone comminution efficiency could be increased. This hypothesis will be verified in the following studies.

CONCLUSIONS

Significant differences were found between the primary and secondary bubble cluster in an electrohydraulic lithotripter: (i) The secondary bubbles were small in size, concentrated in several locations and in a higher bubble density; (ii) their collapse was asymmetric with the production of weaker acoustic emissions and few microjets; (iii) their cavitation activities have little dependence on the boundary conditions. Removal of the secondary bubble cluster led to the reduced stone fragmentation efficiencies. Altogether, although the secondary SW and its bubble cavitation are weak in comparison to the counterpart primary ones, they play a role in stone comminution and cause the performance difference between electrohydraulic and electromagnetic, piezoelectric lithotripters.

Acknowledgments

This work was supported in part by NIH grant RO1-DK52985. The authors thank the valuable discussion with Drs. W. Neal Simons and Georgii N. Sankin in this study.

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