Computational Study on Thermal Responses of Laser Irradiated Blood Perfused and Vascularized Tissues

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Lasers find extensive applications in medicine and biology. The laser-tissue interaction has remained an active area of research. Its usage has been explored in the treatment of anomalies such as port-wine stain and diseases like cancers. Treatment is performed by the thermal effects of laser irradiation, and the process is known as laser induced hyperthermia or laser induced thermal therapy. It is important therefore to understand heat transfer and radiative transfer inside biological tissue. During any laser induced heating of biological tissues, two of the major concerns are (i) skin surface overheating and (ii) undesired thermal damage of healthy tissue. In present study, a quantitative evaluation of the effect of heating by laser irradiation in biological tissue is performed. The objective of laser induced hyperthermia and thermal therapy procedures is to treat the affected tissue while minimizing the thermal damage to the surface of the skin and the surrounding healthy tissues. Using the contact surface cooling method, skin surface temperature rise can be prevented during laser irradiation of tissue. During any such procedures, skin surface cooling is desirable to prevent its damage. Optical window surface contact cooling and cryogenic spray cooling are widely used methods for the skin surface cooling. Optical windows are materials which are transparent for the wavelength ranging from ultra violet to near infra-red, and they are used for providing a transparent and highly conducting contact cooling material. In the present study, four optical windows are examined for their suitability during contact cooling applications. The four optical window used are sapphire, yttrium aluminium garnet, lithium tantalate, and magnesium oxide doped lithium niobate. In addition to the optical windows, two cryogens (R1234yf and liquid CO2) are also studied for spray type cooling and in combination with optical window cooling. The modelling of laser propagation and absorption in biological tissue is done by solving the bioheat and radiative transfer equation. In order to study heat transfer phenomena inside biological tissue when the tissue surface is irradiated by the laser, numerical simulations are conducted. The radiative transfer equation was solved by the discrete ordinate method...
Supervisor: Subhash C. Mishra