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 be induced in tissue in the vicinity of the ablation site. This injury is primarily due to rapid bubble ex pansion and collapse or due to laser-induced pressure waves. In this s tudy, the effect of laser pulse duration on the thermodynamics of bubb le formation and accompanying acoustic pressure wave generation has be en investigated. Study Design/Materials and Methods: Q-switched holmiu m:YAG laser pulses (pulse duration 500 ns, pulse energy 14 mJ) and fre e-running holmium:YAG laser pulses (pulse duration 100-1,100 mu s, pul se energy 200 mJ) were delivered in water and tissue phantoms via a 20 0- and 400-mu m fiber, respectively. The tissue phantoms consisted of polyacrylamide gels with varying mechanical strengths. Bubble formatio n was recorded with a fast flash photography setup, while acoustic tra nsients were measured with a needle hydrophone. Results: It was observ ed that, as the pulse length was increased the bubble shape changed fr om almost spherical for Q-switched pulses to a more elongated cylinder shape for longer pulse durations. The bubble expansion velocity was l arger for shorter pulse durations. Only the Q-switched pulse induced a measurable thermo-elastic expansion wave. All pulses that induced bub ble 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. Conc lusion: The magnitude of the collapse pressure wave decreased as laser pulse duration increased. Hence it may be possible to reduce collater al mechanical tissue damage by stretching the holmium laser pulse. (C) 1996 Wiley-Liss, Inc.