Mathematical modelling of dynamic cooling and pre-heating, used to increase the depth of selective damage to blood vessels in laser treatment of port wine stains.

Based on the assumption that the maximum irradiance allowed during laser treatment of port wine stains is limited by the temperature rise at the dermoepidermal junction, we theoretically investigated how much the irradiance could be increased by dynamically cooling the skin surface. The heat condution equation was solved numerically in cylindrical coordinates using a skin model composed of four layers. The laser-light absorption was calculated using Monte Carlo simulations. The transient thermal behaviour of the skin was modelled when cooling with water at a temperature of 0 degrees C and with liquid nitrogen at a temperature of -196 degrees C. With cooling, an increase in the maximum irradiance by a factor of 2.3-3.6 was theoretically permitted depending on the 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 reshaped by using not only surface cooling but also laser irradiation. By pre-irradiating the skin with near-infrared light without selective absorption by the tissue chromophores in conjunction with surface cooling, a maximum temperature at a depth of 1 mm from the dermoepidermal junction was theoretically achieved. A subsequent 0.1 s pulse from a frequency doubled Nd:YAG laser is theoretically shown to selectively destroy 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 laser can compete in effectiveness with the flashlamp-pumped dye laser in the treatment of port wine stains.