Effect of pulse duration on photomechanical response of soft tissue during Ho:YAG laser ablation

E. Duco Jansen; Massoud Motamedi; T. Joshua Pfefer; Thomas Asshauer; Martin Frenz; Guy P. Delacretaz; George S. Abela M.D.; Ashley J. Welch

doi:10.1117/12.209071

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12 May 1995 Effect of pulse duration on photomechanical response of soft tissue during Ho:YAG laser ablation

E. Duco Jansen, Massoud Motamedi, T. Joshua Pfefer, Thomas Asshauer, Martin Frenz, Guy P. Delacretaz, George S. Abela M.D., Ashley J. Welch

Author Affiliations +

Proceedings Volume 2395, Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems V; (1995) https://doi.org/10.1117/12.209071
Event: Photonics West '95, 1995, San Jose, CA, United States

Abstract

Mechanical injury during pulsed holmium laser ablation of tissue is caused by rapid bubble expansion and collapse or by laser-induced pressure waves. In this study the effect of pulse duration on the photomechanical response of soft tissue during holmium:YAG laser ablation has been investigated. The dynamics of laser-induced bubble formation was documented in water and in transparent polyacrylamide tissue phantoms with a water concentration of 84%. Holmium:YAG laser radiation ((lambda) equals 2.12 micrometers ) was delivered in water or tissue phantoms via an optical fiber (200 or 400 micrometers ). The laser was operated in either the Q- switched mode ((tau) _p equals 500 ns, Q_p equals 14 +/- 1 mJ, 200 micrometers fiber, H_o equals 446 mJ/mm²) or the free-running mode ((tau) _p equals 100 - 1100 microsecond(s) , Q_p equals 200 +/- 5 mJ, 400 micrometers fiber, H_o equals 1592 mJ/mm²). Bubble formation was documented using a fast flash photography setup while simultaneously a PVDP needle hydrophone (40 ns risetime), recorded pressures. The effect of the pulse duration on the photomechanical response of soft biological tissue was evaluated by delivering 5 pulses of 800 mJ to the intimal side of porcine aorta in vitro, followed by histologic evaluation. It was observed that, as the pulse duration was increased the bubble shape changed from almost spherical for Q-switched pulses to a more elongated, cylindrical shape for the longer pulse durations. The bubble expansion velocity was larger for shorter pulse durations. A thermo- elastic expansion wave was measured only during Q-switched pulse delivery. All pulses that induced bubble formation generated pressure waves upon collapse of the bubble in water as well as in the gel. The amplitude of the pressure wave depended strongly on the size and geometry of the laser-induced bubble. The important findings of this study were (1) the magnitude of collapse pressure wave decreased as laser pulse duration increased, and (2) mechanical tissue damage is reduced significantly by using longer pulse durations (> 460 microsecond(s) , for the pulse energy used).

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E. Duco Jansen, Massoud Motamedi, T. Joshua Pfefer, Thomas Asshauer, Martin Frenz, Guy P. Delacretaz, George S. Abela M.D., and Ashley J. Welch "Effect of pulse duration on photomechanical response of soft tissue during Ho:YAG laser ablation", Proc. SPIE 2395, Lasers in Surgery: Advanced Characterization, Therapeutics, and Systems V, (12 May 1995); https://doi.org/10.1117/12.209071

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