Mechanical and thermal effects in striated muscle irradiated by ns-laserpulses

The therapeutic application of laser light is required to minimize defects in the non-irradiated tissue. The primary mechanism of interaction is determined by the duration of laser action. In the case of continuous wave laser light a tissue layer surrounding the irradiated volume is thermally affected. With the pulses of a Q-switched laser (duration some ns) tissue cutting will be obtained by the laser-induced breakdown. To be able to distinguish between thermal and mechanical effects by histological examination, experiments were performed with laser pulse durations of 8ns and 100μs under the same conditions with a Nd-YAG laser at 1064nm. The beam was focused through air below the tissue surface. The beam geometry in the focal region was identical for both cases. The defective region after irradiation could be divided into four zones surrounding a crater. In the μs-experiments the zones corresponded to the temperature distribution in the tissue, so the changes were all classified as thermal. In the ns-experiments, in general larger craters were found. Increasing the number of pulses to 200 the picture is similar to that produced with μs-pulses. These results show that a few ns-pulses suffice to form a crater. Additional ns-pulses lead to heat accumulation and produce thermal lesions like those of the μs-case.