JOURNAL OF ENDOUROLOGYVolume 12, Number 4, August 1998Mary Ann Liebert, Inc.

Transient Cavitation and Acoustic Emission Produced byDifferent Laser Lithotripters

PEI ZHONG, Ph.D.,1,2 HON-LEUNG TONG, M.S.,1 FRANKLIN HADLEY COCKS, Sc.D.,1MARGARET S. PEARLE, M.D., Ph.D.,3 and GLENN M. PREMINGER, M.D.2

ABSTRACT

Transient cavitation and Shockwave generation produced by pulsed-dye and holmium:V AG laser lithotripterswere studied using high-speed photography and acoustic emission measurements. In addition, stone phantomswere used to compare the fragmentation efficiency of various laser and electrohydraulic lithotripters. Thepulsed-dye laser, with a wavelength (504 nm) strongly absorbed by most stone materials but not by water,and a short pulse duration of I ¿¿sec, induces plasma formation on the surface of the target calculi. Subse-quently, the rapid expansion of the plasma forms a cavitation bubble, which expands spherically to a maxi-mum size and then collapses violently, leading to strong Shockwave generation and microjet impingement,which comprises the primary mechanism for stone fragmentation with short-pulse lasers. In contrast, theholmium laser, with a wavelength (2100 nm) most strongly absorbed by water as well as by all stone materi-als and a long pulse duration of 250 to 350 /isec, produces an elongated, pear-shaped cavitation bubble at thetip of the optical fiber that forms a vapor channel to conduct the ensuing laser energy to the target stone(Moss effect). The expansion and subsequent collapse of the elongated bubble is asymmetric, resulting in weakShockwave generation and microjet impingement. Thus, stone fragmentation in holmium laser lithotripsy iscaused primarily by thermal ablation (drilling effect).

INTRODUCTION

SEVERAL TYPES OF LASERS, including pulsed-dye,alexandrite, and holmium:YAG, are currently used for in-

tracorporeal lithotripsy.1 These lasers differ significantly inwavelength, pulse duration, pulse energy, and therefore themechanism of stone fragmentation and treatment efficiency.1,2Although the physical processes in laser treatment are gener-ally believed to include an initial optical breakdown in wateror on the stone surface, with simultaneously generated plasmaexpansion and cavitation bubble formation, followed by subse-quent Shockwave emission and microjet formation produced bybubble collapse, the exact mechanism of action has not alwaysbeen clearly elucidated. A fundamental understanding of thelaser-stone interaction is critical for the effective and safe use

of laser lithotripters, as well as for technical improvement inlasertripsy technology.

In this study, we utilized high-speed photography combinedwith acoustic pressure measurements to characterize the tran-sient cavitation bubble oscillation and associated acoustic emis-sion generated by holmium and pulsed-dye laser lithotripters.Stone fragmentation produced by different laser lithotripters, as

well as by an electrohydraulic lithotripter (EHL) using a stan-dard stone phantom, was compared. The implications of thestudy results relating to the improvement of lasertripsy tech-nology are discussed.

MATERIALS AND METHODS

Figure 1 shows schematically the experimental set-up usedfor high-speed photography of the transient cavitation bubbleoscillation and for the measurement of concomitant Shockwavesgenerated during lasertripsy.

Departments of Mechanical Engineering and Materials Science1 and Urologie Surgery,2 Duke University Medical Center, Durham, NorthCarolina.

3Department of Urology, The University of Texas Southwestern Medical Center, Dallas, Texas.

371

372 ZHONG ET AL.

A A' ViewDigitalOscilloscope

Glass Tank

StonePhantom

P OpticalFiber

-l[^CHI CH2 TRIG

O3=M^

DigitalDelay Generator

IN AuB CUDO Oi

High-SpeedCamera

Halogen Lampvil Ground Glass A'< Optical

N StoneFiber Phantom

Laser LithotripsyDevice

FIG. 1. Experimental arrangement for high-speed photography and acoustic emission measurements during laser lithotripsy.

Laser LithotriptersIn this study, two different holmium (1210-VHP, Trimedyne;

and VP-Select, Coherent), one pulsed-dye (MDL-2000, Can-dela), and one alexandrite (ML-300, Laser Photonics) laserlithotripters were used. The physical features of theselithotripters are summarized in Table 1, which reveals an out-

put energy ranging from 0.1 to 3.5 J/pulse, a repetition rate from3 to 60 pulses/sec, and an optical fiber from 300 to 365 pm indiameter. The optical fiber, attached to an x-y-z translationalstage, was aligned perpendicular to the surface of a calcium ox-

alate monohydrate stone sample, which was immersed in a

transparent glass tank (10 X 10 X 10 cm) filled with saline(20°C). Using the translational stage, the distance between thefiber tip and the stone surface can be adjusted continuously with

a precision of 0.01 mm. In addition, the Trimedyne holmiumlaser lithotripter can be operated in a double-pulse mode, whichreleases two pulses in sequence with an approximately 2.2-msecdelay between them.

High-Speed PhotographyA high-speed rotating drum camera (Model 350, Cordin

Inc.), equipped with a TV zoom lens (f = 17.5-105 mm, Fuji-non) and a 2X teleconverter (Cosmicar, ASAHI Precision), was

used to capture the oscillation of the transient cavitation bub-bles produced by the holmium and pulsed-dye lasers. This high-speed camera was operated at a framing rate of 10,000 to 20,000frames per second (FPS), providing a continuous recording ofthe whole cavitational event generated during lasertripsy. Trig-

Table 1. Physical Properties of Various Laser Lithotripters

Manufacturer(Model)

LaserType

PulseMode

PulseEnergy

(mJ)

PulseDuration

(p¿ec)

PulseRate(Hz)

Wavelength(nm)

FiberDiameter

(pm)Trimedyne

(1210-VHP)Coherent

(VP Select)Candela

(MDL 2000)Laser

Photonics(ML 300)

Holmium

Holmium

Pulsed dye

Alexandrite

Single/doubleSingle

Single

Single

500-1,000

500-1,000

140

50-120

350

250

1.2

1

3-60

5-10

10

8-10

2100

2100

504

755

365

365

300

300

CAVITATION AND ACOUSTIC EMISSION FROM LASER LITHOTRIPTERS 373

Quantz Fiber

LaserLithotripsy

Device

Plastic Tube with4 (2 mm Dia.) Holesat Bottom

Stone FragmentsLess than 2 mm

3-DimensionalPositioner

Water Tank

FIG. 2. Experimental set-up for stone fragmentation test using different laser lithotripters.

gering between the laser and the high-speed camera was syn-chronized using a digital delay generator (DS534, Stanford Re-search Inc.). A 1000 W halogen lamp and a 4-mm thick groundglass were used to provide homogeneous illumination for thehigh-speed camera. The size of the cavitation bubble in eachhigh-speed sequence was measured in reference to the diame-ter of the optical fiber.

Acoustic Emission Measurements

Shockwave pulses generated during the oscillation of a cavi-tation bubble were measured using a polyvinylidene fluoride nee-dle transducer (Precision Acoustics LTD) with a bandwidth of20 MHz and a sensitivity of 14.5 mV/MPa. The needle trans-ducer was placed 5 mm from the tip of the fiber and aligned per-pendicular to its axis (see Fig. 1). The acoustic signal, capturedby the needle transducer, was registered on a digital oscilloscope(LeCroy 9314) with a bandwidth of 100 MHz at single capturemode. Simultaneous high-speed imaging of bubble oscillationand acoustic emissions measurements were carried out to corre-

late bubble dynamics with concomitantly generated Shockwaves.

Stone FragmentationFigure 2 shows schematically the experimental set-up used

to compare stone fragmentation produced by different modesof lasertripsy. Stone phantoms (0.5 X 0.5 X 0.5 mm) made ofplaster of Paris were placed in a plastic tube with an array of2-mm holes drilled in the bottom. Each stone sample under-went 3 minutes of lithotripsy treatment at the selected power

FIG. 3. Representative high-speed sequences of transient cav-itation bubble oscillations produced by Trimedyne holmiumlaser lithotripter at 0.5 J and operated in single-pulse mode (A)in free water, taken at 10,000 FPS, and (B) near stone surfacein water, taken at 20,000 FPS.

374 ZHONG ET AL.

setting, during which debris <2 mm was filtered out. After-ward, residual fragments >2 mm were collected from the tubeand allowed to air dry for 48 hours, then weighed to determinethe mass loss of the stone sample as a result of lithotripsy.

RESULTS

Cavitation Bubble DynamicsHolmium Laser: Single-Pulse Mode. Figure 3A shows a

representative high-speed sequence of transient cavitation bub-ble oscillation generated in water by the Trimedyne holmiumlaser lithotripter at an output pulse energy of 0.5 J. Followingthe release (t = ~0 /¿.sec) of the laser pulse, a cavitation bub-ble was produced at the tip of the optical fiber. The bubble was

observed to expand to a maximum size in about 200 itsec (frame4) and then collapsed quickly (frame 5), followed by a rapidrebound at about t = 400 jixsec (frame 6) before its disintegra-tion. The expansion of the bubble was nonspherical, with thebubble elongated along the fiber axis (maximum lateral diam-eter 0.7 mm). This elongated, pear-shaped bubble is character-istic of the holmium laser, with its long pulse duration, becauseof the continued laser-induced vaporization of water at the apexof the expanding bubble.3 This phenomenon, known as theMoss effect, helps to create a vapor channel to conduct the laserenergy directly onto the stone surface.3 As the bubble collapses,it tends to move away from the tip of the fiber.

When the fiber tip was placed < 1 mm from a stone surface,as is often the case during clinical lasertripsy, a transient cav-

itation bubble was also produced following the release of thelaser pulse (Fig. 3B). The bubble was seen to expand both lat-

FIG. 4. Representative high-speed sequence of transient os-cillation of cavitation bubbles produced in free water by Tri-medyne holmium laser lithotripter at 0.5 J, operated in double-pulse mode.

FIG. 5. Representative high-speed sequence of transient os-cillation of cavitation bubbles produced near stone surface byTrimedyne holmium laser lithotripter at 0.5 J, operated in dou-ble-pulse mode.

erally and backward along the fiber, presumably because thestone surface retards forward expansion. Compared with bub-ble formation in free water, the lateral expansion of the bubbleproduced near a stone surface was found to be increased by

FIG. 6. Typical high-speed sequence of transient cavitationbubble oscillation generated near calcium oxalate monohydratestone surface by Candela pulsed-dye laser lithotripter at pulseenergy of 100 mJ.

CAVITATION AND ACOUSTIC EMISSION FROM LASER LITHOTRIPTERS 375

about 14%. In addition, the expanded bubble surface appearedto be irregular; therefore, the subsequent collapse of the bub-ble was less coherent and less violent.

Holmium Laser: Double-Pulse Model. When the Trime-dyne holmium laser was operated in double-pulse mode, two

oscillating cavitation bubbles were produced in sequence, sep-arated by a time delay of approximately 2400 /xsec corre-

sponding to the delay between the release of the two laser pulses(Fig. 4). The dynamics of each bubble oscillation is similar andcan be characterized by an initial expansion and elongation ofthe bubble along the fiber axis to a maximum size in about 200/xsec, followed by the collapse of the bubble in another 200/xsec and a subsequent rapid rebound before its final disinte-gration. The total oscillation period of each individual bubbleis <600 /xsec, which is significantly shorter than the time in-terval between the releases of two adjacent laser pulses. There-fore, no interactions between the first and the second cavitationbubbles were visible.

When the double-pulse laser was fired near a stone surface,two distinctly separated cavitation bubble oscillations were pro-duced (Fig. 5). The dynamics of individual bubble oscillationswere found to be similar, with no apparent interaction. In bothFigure 3B and Figure 5, flaking off of stone debris can be ob-served before the collapse of the expanded cavitation bubble,but no significant stone fragmentation was produced after thebubble collapse. This observation suggests that stone fragmen-

tation during holmium lasertripsy is produced by ablation of thesurface material, not by cavitation-induced Shockwave and mi-crojet impingement.

Pulsed-Dye Laser. Figure 6 shows a representative high-speed sequence of a transient cavitation bubble oscillation pro-duced by the Candela pulsed-dye laser lithotripter at an outputpulse energy of 100 mJ. A calcium oxalate monohydrate stone(label "S" in Fig. 6) was affixed to a Plexiglas plate and placednear the fiber tip. At the wavelength of 504 nm, the laser en-

ergy is strongly absorbed by most stone materials but not bythe interposed water. Therefore, in contrast to the strong ab-sorption of holmium laser energy by water and the subsequentformation of a cavitation bubble around the fiber tip, the pulsed-dye laser produces a transient cavitation bubble on the stonesurface consequent to vaporization of stone material and sub-sequent plasma formation and expansion. As shown in Figure6, this transient cavitation bubble expanded spherically to a

maximum diameter of approximately 3.9 mm in about 200 /xsecand then collapsed violently. Although some fine powders ofstone debris were observed after the initial laser irradiation(frames 3 and 4), most stone fragmentation was produced bythe violent collapse of the cavitation bubble (frames 6-10). Be-cause of its short pulse duration (1.2 /xsec), the pulsed-dye laser-induced cavitation bubble expansion is spherical, and the dy-namics of the bubble oscillation are similar to those generatedby EHL using underwater spark discharge.4

ß:M1—.5 its5.9 V

ë M2—.5 RS5.8nV

Las

++^

er Trigger SignalH-H

Shockwavle Puls

t+++ T+++ ++++

W**^* y^liW*ttA»

-t-H-

kij^j..

f+++

w*-->

++++

K«M

B

ft: HI—.5 pis5.0 V

g:\a-.5 ms5.8nV

Laser Trigger SignalH-H-

MMM

++++

U»*tShdckwawe Pulse

444

***** 4»

++++

+***»

++++ ++++ ++++ ++++

FIG. 7. Acoustic emission signals measured simultaneously during transient cavitation bubble oscillations shown in Figure 3.A. In free water. B. Near stone surface.

376 ZHONG ET AL.

fl:H1—.5 FIS5.0 V

M-5 FIS

5.0IW

Laser++++

Tfigg'-i—^

•*»*

er

»VlWr

Shockwave Pulses

Sighal

list

** »»•FW rappppi

¡2nd+4++

—

-

2nd

4m«

B L¿ser JTrigder SJign¿f+++

• «Ml

+4+1stT+++

( Wrfp»

Shockwave Pulses

H-H-

1st

444

_»

..

^^]^^|^|^^_—J

2nd

i n| I

f44+

2nd

H44 +444

FIG. 8. Acoustic emission signals measured simultaneously during transient cavitation bubble oscillations shown in Figures 5and 6. A. In free water. B. Near stone surface.

Acoustic Emission

Figure 7 shows the acoustic emission signals, recorded si-multaneously during the transient bubble oscillations producedin free water (see Fig. 3A) and near a stone surface (Fig. 3B),respectively, by the Trimedyne laser operated in the single-pulse mode. Only one Shockwave pulse above the noise levelwas detected in about 800 to 900 /¿sec after the trigger signalof the lithotripter. Considering that there is a 250- to 300-psectime delay between the leading edge of the trigger signal andthe release of the laser and an additional 100 /xsec for the for-mation of the bubble,3 this Shockwave pulse was found to begenerated by the rebound of the bubble after its primary col-lapse. The average peak pressure values of the Shockwavepulses, measured 5 mm from the tip of the fiber in water andnear a stone surface, were 330 and 172 kPa, respectively.

For the transient cavitation bubble oscillations produced bythe Trimedyne laser operated in the double-pulse mode (seeFigs. 4 and 5), the simultaneously measured acoustic emissionsignals revealed two distinctively separated Shockwave pulses(Fig. 8). Each acoustic pulse was produced in about 800 to 900/xsec after the leading edge of each trigger signal, indicatingagain that these acoustic emission signals were generated bythe rebounds of individual bubbles after their primary collapse.

Table 2 summarizes the peak pressure values of the acousticemission signals associated with the primary bubble collapse/re-

bound generated by different intracorporeal Shockwavelithotripters. Clearly, acoustic emission produced duringholmium lasertripsy is much weaker than the corresponding ones

generated during pulsed-dye lasertripsy and EHL. These differ-ences in acoustic emission are consistent with the photographicevidence that holmium laser-induced cavitation bubbles have ir-regular geometry and asymmetric expansion and collapse, lead-ing to weak acoustic emission generation on bubble collapse.4In contrast, cavitation bubbles generated by the pulsed-dye laserand EHL have spherical geometry and symmetric expansion andcollapse, leading to strong acoustic emission generation at bub-ble collapse.4 The reduction in acoustic emission signals fromthe free water medium to that near a stone boundary duringholmium lasertripsy is likely caused by the partial absorption ofthe laser energy by the stone surface, resulting in a reduction inthe amount of energy partitioned into bubble generation.

Stone FragmentationThe results of stone fragmentation produced by the Trime-

dyne and Coherent holmium laser lithotripters are shown in Fig.9. A linear relation between the pulse energy and stone frag-mentation, measured by weight loss during 3 minutes of laser-tripsy, was identified. In addition, for the Trimedyne lithotripter,it was found that the same stone fragmentation could beachieved by operating with either the single-pulse mode at 1.0

CAVITATION AND ACOUSTIC EMISSION FROM LASER LITHOTRIPTERS 377

Table 2. Shockwaves Generated by Laserand Electrohydraulic Lithotripters*

LithotripterIn Water

(MPa)

*Measured 5 mm from the probe tip.tFrom reference 9.

Near a Stone(MPa)

Trimedyne holmium laser (500 mJ)Single-pulse mode 0.269 ± 0.073 0.176 ± 0.084Double-pulse mode

First pulse 0.352 ± 0.242 0.241 ± 0.169Second pulse 0.364 ± 0.189 0.190 ± 0.066

Candela pulsed-dye laser (100 mJ) NA 58.5-130+Nortech electrohydraulic lithotripter (40%) 30.3 ± 4.6 7.9 ± 0.9

J/pulse or the double-pulse mode at 0.5 J/pulse. This finding issupported by the high-speed photographic evidence that thereis no interaction between the two subsequent cavitation bub-bles produced in the double-pulse mode; thus, stone fragmen-tation is determined by the total laser energy delivered to thestone during the treatment. Because the lateral expansion of thecavitation bubble, a primary mechanism of ureteral wall injury,is proportional to the pulse energy, the double-pulse modewould have the advantage of producing the same stone frag-mentation efficiency, while reducing the potential risk forureteral wall injury, compared with single-pulse mode opera-tion. In addition, at the same pulse energy level, the extent ofstone fragmentation produced by the Coherent laser lithotripterwas between the values produced by the Trimedyne laser us-

ing the single- and the double-pulse modes.Using the standard stone phantom, the efficiency of stone

fragmentation produced by different intracorporeal laserlithotripters and EHL was compared (Table 3). The results showthat for this particular stone phantom, EHL and alexandrite

220

200 |-180

160

¡g 120O

O Trimedyne (S) Trimedyne (D)A Coherent (S) I

lasertripsy appear to be most effective, followed by pulsed-dyeand holmium lasers. However, this conclusion should not begeneralized, because the absorption of alexandrite and pulsed-dye laser energy and their resultant stone fragmentation effi-ciency will depend on the composition of the stone, whereasthe holmium laser can fragment stones of all compositions, in-cluding cystine and pale calcium oxalate stones.1

DISCUSSION

Using high-speed photography and acoustic emission mea-

surements, we have characterized the transient cavitation bub-ble oscillation and resultant Shockwave generation by pulsed-dye and holmium laser lithotripters. Although both types oflasers induce optical breakdown and subsequent Shockwavegeneration, the physical details of the process differ signifi-cantly. For the pulsed-dye laser at 504 nm, with its strong ab-sorption by most stone materials but weak absorption by wa-

ter, the incident laser pulse is absorbed directly by the targetstone material, leading to the formation of a cavitation bubbleon the stone surface. Because of the short pulse duration andhigh energy flux density, the deposition of the laser energy israpid and highly localized, producing a spherical bubble thatexpands symmetrically to a large size and then collapses vio-lently. Consequently, a strong Shockwave emission, a high-

Table 3. Stone Fragmentation Efficiency

0.4 0.6 0.8 1.0

Pulse Energy (J)1.2 1.4

Lithotripter Type

OutputSetting

(J)

FragmentationEfficiency(mgl3 min)

FIG. 9. Stone weight loss v pulse energy produced by twodifferent holmium laser lithotripters.

Coherent holmium

Trimedyne holmium(single-pulse mode)

Trimedyne holmium(double-pulse mode)

Candela pulsed-dyeLaser Photonics

alexandriteNortech EHL

0.50.81.00.50.81.00.50.81.0

100 mJ100 mJ

80%

71 ± 18117 ± 30140 ± 1447 ± 1075 ± 3893 ± 2688 ± 10

127 ± 37175 ± 26197 ± 20705 ± 125

227 ± 54

378 ZHONG ET AL.

speed microjet, or both are generated on bubble collapse, whichhave been found to be the primary mechanisms of stone frag-mentation.5 Similar physical processes occur for the alexandritelaser because of its short pulse duration and a wavelength (755nm) that is absorbed by most stone materials.6,7

In contrast, the holmium laser (at a wavelength of 2100 nm),although absorbed by all stone materials, is most strongly ab-sorbed by water. Thus, the incident laser pulse will vaporizewater immediately after its release, leading to the formation ofa cavitation bubble at the tip of the optical fiber. In addition,with its long pulse duration, the holmium laser causes contin-ued vaporization of the water near the bubble wall distal to thefiber, producing an elongated, pear-shaped bubble. This phe-nomenon, known as the Moss effect, creates a vapor channelthat conducts the ensuing laser energy to a further distance.3,8Therefore, when the holmium laser pulse is released near a

stone, a cavitation bubble is formed between the fiber tip andthe stone surface, which effectively conducts the laser energyto the target stone. On the other hand, because of the asym-metric bubble expansion and collapse, the resultant Shockwaveemission and microjet impingement are weak and do not con-

tribute significantly to stone fragmentation. Previous studieshave shown that the primary mechanism of stone fragmenta-tion in holmium lasertripsy is thermal ablation, although thefluid movement produced by bubble collapse helps to dispersethe fragments.5

In this study, a linear correlation between the pulse energyand resultant stone fragmentation has been demonstrated dur-ing holmium lasertripsy (Fig. 9). This correlation is indepen-dent of the operation mode (single or double) of the lithotripter,indicating that stone fragmentation is determined by the totalamount of laser energy delivered to the target stone during thetreatment. For the Trimedyne 1210-VHP lithotripter, the same

amount of laser energy can be delivered at a high pulse energylevel using the single-pulse mode or at a half of the pulse en-

ergy level using the double-pulse mode. Stone fragmentationtests using a standard phantom have shown that an equal amountof fragmentation can be produced. However, at lower pulse en-

ergy levels, the lateral expansion of the cavitation bubble in-duced during lasertripsy is significantly reduced. Therefore,these findings suggest that the double-pulse mode has an ad-vantage over the single-pulse mode in producing the same stone

fragmentation efficiency with a reduced potential for ureteralwall injury.

Our study results also indicate that the short-pulse alexan-drite and pulsed-dye lasers work quite effectively when the en-

ergy is well absorbed by the target stone material. This effi-cient fragmentation is attributed to the strong cavitation bubbleexpansion and collapse, with resultant strong Shockwave andmicrojet impingement effects. The main disadvantage of thealexandrite and pulsed-dye lasers, however, is that the treatment

efficiency varies depending on stone composition. The alexan-drite laser is less effective for pale urinary stones than for darkstones,6 whereas the pulsed-dye laser is ineffective for cystineand calcium oxalate monohydrate stones.1 In contrast, the treat-ment efficiency of the holmium laser does not vary significantlywith stone composition, and this laser can be used to fragmentall types of stones, including hard cystine and calcium oxalate

monohydrate stones. The primary disadvantage of the holmiumlaser is its low fragmentation power, manifest by its pure"drilling" effect on stone material, and the lack of a Shockwaveeffect.

Technically, an ideal laser lithotripter should combine thedrilling power of a holmium laser and the fragmentation powerof a short-pulse laser, which utilizes the "explosive" power ofan expanding cavitation bubble. The holmium laser mode can

be used to produce holes (weak spots) in the target stone,whereas the short-pulse laser mode can be used to break up thestone from those weak spots into small fragments. Future in-vestigations should aim at incorporating the best features ofboth laser systems.

ACKNOWLEDGMENTS

This work was supported in part by NIDDK Grant POl-DK20543, by the Whitaker Foundation, and by an educationalgrant from Trimedyne, Inc. We would also like to thank JoshMalenbaum and Pat Clark for their excellent technical assis-tance.

REFERENCES

1. GrocelaJ, DretlerSP: Intracorporeal lithotripsy: instrumentation anddevelopment. Urol Clin North Am 1997;24:13

2. Watson GM: A survey of the action of lasers on stones. In: SteinerR (ed): Laser Lithotripsy: Clinical Use and Technical Aspects.Berlin: Springer-Verlag, 1988, pp 15-24

3. Asshauer T, Rink K, Delacretaz G: Acoustic transient generationby holmium-laser-induced cavitation bubbles. J Appl Phys1994;76:5007

4. Zhong P, Tong, HL, Cocks FH, Preminger GM: Transient oscilla-tion of cavitation bubbles near stone surface during electrohydrauliclithotripsy. J Endourol 1997; 11:55

5. Schafer SA, Durville FM, Jassemnejad B, Bartels KE, Powell RC:Mechanisms of biliary stone fragmentation using the Ho:YAG laser.IEEE Trans Biomed Eng 1994;41:276

6. Strunge C, Brinkmann R, Flemming G, Engelhardt R: Interspersionof fragmented fiber's splinters into tissue during pulsed alexandritelaser lithotripsy. Lasers Surg Med 1991 ; 11:183

7. Steiger E, Geisel G: Dual-wavelength-alexandrite-laserlithotripsy:in-vitro results of urinary calculi fragmentation. SPIE (Lasers inUrology) 1994;2129:151

8. Jansen ED, Asshauer T, Frenz M, et al: Effect of pulse duration on

bubble formation and laser-induced pressure waves during holmiumlaser ablation. Lasers Surg Med 1996; 18:278

9. Rink K, Delacretaz G, Salathe RP: Fragmentation process inducedby microsecond laser pulses during lithotripsy. Appl Phys Lett1992;61:258

Address reprint requests to:Pei Zhong, Ph.D.

Dept. of Mechanical Engineering and Materials ScienceP.O. Box 90300Duke University

Durham, NC 27708

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