Effect of pulse duration on bubble formation and laser-induced pressure waves during holmium laser ablation

Background and Objective: One concern during laser ablation of tissue is the mechanical injury that may he induced in tissue in the vicinity of the ablation site. This injury is primarily due to rapid bubble expansion and collapse or due to laser-induced pressure waves. In this study, the effect of laser pulse duration on the thermodynamics of bubble formation and accompanying acoustic pressure wave generation has been investigated. Study Design/Materials and Methods: Q-switched holmium:YAG laser pulses (pulse duration 500 ns, pulse energy 14 mJ) and free-running holmium:YAG laser pulses (pulse duration 100-1,100 μs, pulse energy 200 mJ) were delivered in water and tissue phantoms via a 200- and 400-μm fiber, respectively. The tissue phantoms consisted of polyacrylamide gels with varying mechanical strengths. Bubble formation was recorded with a fast flash photography setup, while acoustic transients were measured with a needle hydrophone. Results: It was observed that, as the pulse length was increased the bubble shape changed from almost spherical for Q-switched pulses to a more elongated cylinder shape for longer pulse durations. The bubble expansion velocity was larger for shorter pulse durations. Only the Q-switched pulse induced a measurable thermo-elastic expansion wave. All pulses that induced bubble formation generated pressure waves upon collapse of the bubble in gels as well as in water. However, the magnitude of the pressure wave depended strongly on the size and geometry of the induced bubble. Conclusion: The magnitude of the collapse pressure wave decreased as laser pulse duration increased. Hence it may be possible to reduce collateral mechanical tissue damage by stretching the holmium laser pulse.