Analyzing Thermal and Mechanical Effects of Pulsed Laser Irradiation on Tissues
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Pulsed lasers are known for their spatial and temporal specificity in delivering heat energy to the tissues. This is useful in laser ablation treatment mechanism where damage to the healthy tissues is highly undesired. However, the efficacy of the process is limited by the damage caused by the pulsed laser. A pulsed laser has both photothermal and photomechanical interaction with tissues. Photothermal interaction is caused by the rise in temperature due to the laser irradiation. This includes the denaturation of proteins, increased mitochondrial membrane permeability and ultimately vaporization. Photomechanical interaction causes the generation of pressure waves produced as a result of the pulse-laser interaction. This arises due to the thermoelastic expansion of the tissues due to heating. Photothermal and photomechanical interactions combined lead to damage in the tissues and are a potential threat to the surrounding tissues. In this thesis, the effects of both the mechanisms are studied using finite element models of the skin. A three-layered model of the skin is considered which is irradiated upon by a focused Nd:YAG infrared laser beam. The finite-element solver COMSOL Multiphysics is used to simulate the thermal and mechanical interaction due to the laser irradiation. Thermal effects of the irradiation are evaluated using the Arrhenius damage integral and the equivalent thermal dose administered to the tissue. The results obtained are validated using the histology results when mouse tissues are irradiated with a focused beam Q-switched Nd:YAG laser which leads to temperature rise and tissue removal. The mechanical interaction is evaluated in terms of the stress generated in the tissue during the laser ablation damage. Results obtained here are useful in characterizing the parameters of laser ablation like the repetition rate, laser power and pulse width. This helps us in optimizing the laser ablation process for a more effective treatment with minimum damage to surrounding tissues.