MATHEMATICAL-MODELING OF DYNAMIC COOLING AND PREHEATING, USED TO INCREASE THE DEPTH OF SELECTIVE DAMAGE TO BLOOD-VESSELS IN LASER TREATMENTOF PORT-WINE STAINS

Based on the assumption that the maximum irradiance allowed during las er treatment of port wine stains is limited by the temperature rise at the dermoepidermal junction, we theoretically investigated how much t he irradiance could be increased by dynamically cooling the skin surfa ce. The heat condution equation was solved numerically in cylindrical coordinates using a skin model composed of four layers. The laser-ligh t absorption was calculated using Monte Carlo simulations. The transie nt thermal behaviour of the skin was modelled when cooling with water at a temperature of 0 degrees C and with liquid nitrogen at a temperat ure of -196 degrees C. With cooling, an increase in the maximum irradi ance by a factor of 2.3-3.6 was theoretically permitted depending on t he irradiation time, wavelength and mode of cooling. The corresponding increase in vessel selective damage depth was predicted to be 0.4-0.5 mm. A new concept for increasing the depth of vessel selective damage is introduced where the initial temperature profile of the skin is re shaped by using not only surface cooling but also laser irradiation. B y pre-irradiating the skin with near-infrared light without selective absorption by the tissue chromophores in conjunction with surface cool ing, a maximum temperature at a depth of 1 mm from the dermoepidermal junction was theoretically achieved. A subsequent 0.1 s pulse from a f requency doubled Nd:YAG laser is theoretically shown to selectively de stroy vessels up to a depth of 0.8 mm from the dermoepidermal junction . By pre-heating at 1064 nm and treating at 532 nm in conjunction with surface cooling, the theoretical results indicate that the Nd:YAG las er can compete in effectiveness with the flashlamp-pumped dye laser in the treatment of port wine stains.