레이저의 피부 흡수에 의한 열 및 기계적 효과 발생에 관한 연구

Abstract

This thesis describes a study of the generation of the thermal and mechanical effects induced by laser absorption in tissues. This was a basic study executed prior to the development of highly precise, multi-functional, and minimal- or non-contact haptic interface technologies. The ultimate goal of the study is to understand and analyze the thermal and mechanical effects generated by laser absorption in tissues. First, the optical constants and anatomical structures of the tissues were investigated. Based on the results of these investigations, simulations of the propagation, scattering, and absorption of a laser beam in the tissue were performed using a numerical technique based on the Monte Carlo method. Subsequently, a thermal diffusion equation was solved to determine the temperature distribution caused by the absorbed laser energy. Finally, a study of the generation of the mechanical effects induced by the absorption of a pulsed-laser beam and the resulting abrupt increase in the temperature was conducted by using a thermoelastic equation. The absorbed power density exhibited larger values at wavelengths around 400 nm and 1450 nm and has been found to be determined mainly by the absorption coefficient of the tissue. The penetration depth was maximum at wavelengths around 1300 nm and has been found to be determined mainly by the transport mean free path. The spatial and temporal distributions of the temperature within the tissue were simulated for laser beams of 0.68 mm in diameter and with wavelengths of 532 nm, 809 nm, 905 nm, and 1064 nm at pulse widths of 5 ns and 100 ms. The simulated results provide a quantitative relationship between the maximum temperature increase and the recommended maximum permissible exposure values given in the international laser safety standards. An analytical solution to the thermoelastic wave equation was introduced to estimate the transient stress distributions generated by the temperature increase in soft tissue through the absorption of the pulsed-laser beam, as well as to estimate the maximum stress values. The results of this study provide important information on the laser parameters for the efficient generation of optomechanical effects in tissues. Investigations using 532-nm, 809-nm, 905-nm and 1064-nm lasers and numerical simulations of the thermoelastic wave equation are currently in progress.