Pulsed laser ablation of biological tissue: Influence of laser parameters and tissue properties on thermal and mechanical damage

Many medical disciplines make use of lasers to ablate or remove tissue. Pulsed lasers are used to achieve precise tissue ablation with relatively little thermal damage to adjacent tissue. Given its nature of deposition of relatively large amounts of energy in tissue in a short time, pulsed laser ablation is quite explosive and violent. Consequently, there are many concerns about possible side effects in the form of unwanted tissue damage. The goal of this study was to determine and describe phenomena that take place during pulsed infrared (holmium and thulium) laser ablation of biological soft tissue.;Thermographic and histologic analysis of laser irradiated human aorta samples was used to correlate thermal events and tissue damage to laser irradiation conditions. Significant temperature rises and zones of tissue damage were observed owing to thermal superposition during multiple pulse delivery of free-running holmium, Q-switched holmium and excimer laser radiation.;Both holmium and excimer laser ablation produced rapidly expanding and collapsing vapor bubbles which were documented by fast flash photography. Since infrared laser radiation is strongly absorbed by (tissue) water, the content of holmium laser induced bubbles is believed to be water vapor. It was inferred that excimer laser induced bubbles contain primarily water vapor as well. The threshold radiant exposure for water vaporization was much lower than theoretically predicted.;A novel ablation model was developed based upon the concept of partial vaporization. The model, which assumes vaporization of only a fraction of the disk of water under a delivery fiber, rather than vaporization of the entire disk, was used to calculate bubble volumes. A good agreement was found between the model and experimental data.;The temperature dependence of the water absorption peak around ;Finally, fast flash photography was combined with hydrophone measurements, to examine the effect of laser pulse duration on bubble formation and pressure wave generation in water and tissue phantoms. It was shown that pressure wave amplitudes may be minimized by stretching the laser pulse.