Er:YAG laser skin resurfacing using repetitive long‐pulse exposure and cryogen spray cooling: II. Theoretical analysis

Abstract

Background and Objective: To analyze the effects of laser pulse duration and cryogen spray cooling (CSC) on epidermal damage and depth of collagen coagulation in skin resurfacing with repetitive Er:YAG laser irradiation. Study Design/Materials and Methods: Evolution of temperature ®eld in skin is calculated using a simple onedimensional model of sub-ablative pulsed laser exposure and CSC. The model is solved numerically for laser pulse durations of 150 and 600 msec, and 6 msec cryogen spurts delivered just prior to (``pre-cooling''), or during and after (``post-cooling'') the 600 msec laser pulse. Results: The model indicates a minimal in¯uence of pulse duration on the extent of thermal effect in dermis, but less epidermal damage with 600 msec pulses as compared to 150 msec at the same pulse¯uence. Application of pre-or post-cooling reduces the peak surface temperature after laser exposure and accelerates its relaxation toward the base temperature to a different degree. However, the temperature pro®le in skin after 50 msec is in either example very similar to that after a lower-energy laser pulse without CSC. Conclusion: When applied in combination with repetitive Er:YAG laser exposure, CSC strongly affects the amount of heat available for dermal coagulation. As a result, CSC may not provide spatially selective epidermal protection in Er:YAG laser skin resurfacing.

Er:YAG laser skin resurfacing using repetitive long‐pulse exposure and cryogen spray cooling: I. Histological study*

“…According to this model, it is primarily the increase in ablation threshold that enables a larger heat deposition at longer pulse durations. We address brie¯y this effect also in the theoretical consideration in Part II of this article [40].…”

“…As the CSC spurts cool the super®cial skin layer, they counteract the heating effect of the Er:YAG laser, thus diminishing the coagulative effects in deeper skin layers. A more detailed analysis of this effect, using a simple theoretical model of laser heating and cryogen cooling of skin, is presented in Part II of this article [40].…”

“…For a few examples, the laser pulses were attenuated externally with one or two microscope slides. (Note that the¯uence values reported in our preliminary presentations were underestimated [1,2], as they were estimated from the manufacturer-speci®ed laser footprint of 2.8 mm. Subsequent measurements of the laser beam pro®le yielded spot diameters around 2 mm at half-maximum, depending on the laser operation regimen.…”

Thermal Modelling for Laser Treatment of Port Wine Stains

“…As the cryogen reaches the tissue surface, tissue experiences a quick cooling process to reach low temperatures (Kelly et al, 2005). To model laser PWS with CSC, a convective thermal boundary condition is usually used (Aguilar et al, 2002;Jia et al, 2007;Li et al, 2007a,b;Majaron et al, 2001;Pfefer et al, 2000). Due to lack of experimental data, the early models of laser PWS have employed a constant heat transfer coefficient to quantify the short-pulsed cooling process (Aguilar et al, 2002;Majaron et al, 2001;Pfefer et al, 2000).…”

“…To model laser PWS with CSC, a convective thermal boundary condition is usually used (Aguilar et al, 2002;Jia et al, 2007;Li et al, 2007a,b;Majaron et al, 2001;Pfefer et al, 2000). Due to lack of experimental data, the early models of laser PWS have employed a constant heat transfer coefficient to quantify the short-pulsed cooling process (Aguilar et al, 2002;Majaron et al, 2001;Pfefer et al, 2000). Recently, extensive experimental and numerical investigations have been conducted to characterize the cryogen spray (Pikkula et al, 2001;Zhou et al, 2008a,b) and to quantify the convection heat transfer during spray (Aguilar et al, 2003a,b;Franco et al, 2004Franco et al, , 2005Jia et al, 2004Jia et al, , 2007.…”