Investigation on cavitation bubble dynamics induced by clinically available Ho:YAG lasers
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Karl Stock
Abstract
Background and objective: Endoscopic laser lithotripsy is the preferred technique for minimally invasive destruction of ureteral and kidney stones, and is mostly performed by pulsed holmium:yttrium-aluminum-garnet (Ho:YAG) laser irradiation. The absorbed laser energy heats the water creating a vapor bubble which collapses after the laser pulse, thus producing a shock wave. Part of the laser energy strikes the stone through the vapor bubble and induces thermomechanical material removal. Aim of the present study was to visualize the behavior and the dynamics of the cavitation bubble using a specially developed ultra-short-time illumination system and then to determine important characteristics related to clinically used laser and application parameters for a more detailed investigation in the future.
Materials and methods: In accordance with Toepler’s Schlieren technique, in the ultra-short-time-illumination set-up the cavitation bubble which had been induced by Ho:YAG laser irradiation at the fiber end, was illuminated by two Q-switched lasers and the process was imaged in high contrast on a video camera. Cavitation bubbles were induced using different pulse energies (500 mJ/pulse and 2000 mJ/pulse) and fiber core diameters (230 μm and 600 μm) and the bubble dynamics were recorded at different times relative to the Ho:YAG laser pulse. The time-dependent development of the bubble formation was determined from the recordings by measuring the bubble diameter in horizontal and vertical directions, together with the volume and localization of the center of the bubble collapse.
Results: The results show that the bubble dynamics can be visualized and studied with both high contrast and high temporal resolution. The bubble volume increases with pulse energy and with fiber diameter. The bubble shape is almost round when a larger fiber core diameter is used, and elliptical when using a fiber of smaller core diameter. Moreover, the center of the resulting bubble is slightly further away from the fiber end and the center of the bubble collapse for a smaller fiber core diameter.
Conclusion: The experimental set-up developed gives a better understanding of the bubble dynamics. The experiments indicate that the distance between fiber tip and target surface, as well as the laser parameters used have considerable impact on the cavitation bubble dynamics. Both the bubble dynamics and their influence on the stone fragmentation process require further investigation.
Zusammenfassung
Hintergrund und Zielsetzung: Die endoskopische Laser-Lithotripsie ist eine bevorzugte Technik zur minimal-invasiven Entfernung von Harnleiter- und Nierensteinen. Dabei wird meist mit einem Ho:YAG-Laser über Lichtleiter auf den Stein eingestrahlt. Die absorbierte Laserenergie erhitzt das Wasser und eine Dampfblase entsteht, die nach dem Laserpuls kollabiert und eine Stoßwelle erzeugt. Ein Teil der Laserenergie tritt durch die Dampfblase hindurch auf den Stein und induziert thermo-mechanischen Abtrag. Ziel der vorliegenden Studie war es, das Verhalten und die Dynamik der Kavitationsblase durch einen speziell entwickelten Ultra-Kurzzeit-Beleuchtungsaufbau zu visualisieren, charakteristische Kenngrößen zu ermitteln und aus der Kavitationsblasendynamik in Abhängigkeit von klinischen Applikationsparametern Fragestellungen für weiterführende Untersuchungen zu identifizieren.
Material und Methodik: Bei dem entwickelten Ultra-Kurzzeit-Beleuchtungsaufbau wird basierend auf der Toeplerschen Schlierenmethode die Dampfblase, die am Ende eines Lichtwellenleiters durch Einstrahlung von Ho:YAG-Laserlicht entsteht, mit zwei gütegeschalteten Lasern belichtet und der Prozess mit hohem Kontrast auf eine Videokamera abgebildet. Dazu wurde unter Verwendung verschiedener Pulsenergien (500 mJ/Puls und 2000 mJ/Puls) und Lichtwellenleiter mit verschiedenen Kerndurchmessern (230 μm und 600 μm) Ho:YAG-Laserlicht eingestrahlt und die Dynamik der Kavitationsblase zu unterschiedlichen Zeiten relativ zum Ho:YAG-Laserpuls aufgezeichnet. Aus den Aufnahmen wurde der zeitliche Verlauf der Blasendurchmesser in horizontaler und vertikaler Richtung, das Volumen sowie das Zentrum des Blasenkollapses bestimmt.
Ergebnisse: Die Ergebnisse zeigen, dass mit dem entwickelten experimentellen Aufbau die Dynamik der Kavitationsblase mit hohem Kontrast und hoher zeitlicher Auflösung untersucht und visualisiert werden kann. So nimmt das Volumen der Dampfblase mit der Pulsenergie und mit dem Faserkerndurchmesser zu. Dabei ist bei größerem Faserkerndurchmesser die Kavitationsblase nahezu rund, bei kleinerem Durchmesser elliptisch. Zudem ist das Zentrum der entstehenden Dampfblase bei kleinerem Faserkerndurchmesser etwas weiter vom Faserende entfernt und dementsprechend auch das Zentrum des Blasenkollapses.
Fazit: Der entwickelte Beleuchtungsaufbau ist für Untersuchungen zum Verständnis der Dynamik von Kavitationsblasen nützlich. Die ersten Versuche lieferten Hinweise, dass sowohl der Abstand zwischen Faserende und Zielstruktur als auch die gewählten Laserparameter unterschiedliche Kavitationsblasendynamiken zur Folge haben, deren Einfluss auf die Steinzertrümmerung weiterer Untersuchungen bedarf.
Acknowledgments
The authors would like to thank StarMedTec GmbH, Starnberg, Germany for support with the laser device, optical fibers and laser surgical equipment.
Conflict of interest statement
Conflict of interest statement: All authors state no conflict of interest. All authors have read the journal’s Publication Ethics and Publication Malpractice Statement available at the Journal’s website and hereby confirm that they comply with all parts applicable to the present scientific work.
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©2016 by De Gruyter
Articles in the same Issue
- Frontmatter
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Articles in the same Issue
- Frontmatter
- Editorial
- From optical bioimaging to clinical biophotonics
- Magazine section
- Snapshots
- Review
- Photodynamic therapy for cancer of the pancreas – The story so far
- Original contributions
- Fluorescence imaging for photodynamic therapy of non-melanoma skin malignancies – A retrospective clinical study
- Hydrogen peroxide detection in viable and apoptotic tumor cells under action of cisplatin and bleomycin
- Light enhancement of in vitro antitumor activity of galactosylated phthalocyanines
- Investigation on cavitation bubble dynamics induced by clinically available Ho:YAG lasers
- Preliminary research reports
- Enhancement of OCT imaging by blood optical clearing in vessels – A feasibility study
- Effect of temperature regime and compression in OCT imaging of skin in vivo
- Congress announcements
- Congresses 2016
