Optimal cryogen spray cooling parameters for pulsed laser treatment of port wine stains

Background and Objective: In dermatologic laser therapy, cryogen spray cool ing (CSC) is a means to protect the epidermis while leaving dermal structur es susceptible to thermal damage. The purpose of this study was to determin e optimal spurt duration, tau(s), and optimal delay, tau(d), between the cr yogen spurt and laser pulse when using CSC in treatment of port wine stain birthmarks. Study Design/Materials and Methods: A finite difference method is used to c ompute temperature distributions in human skin in response to CSC. Optimal tau(s) and tau(d) are determined by maximizing the temperature difference b etween a modeled basal layer and an imaginary target chromophore. Results: The model predicts an optimal tau(s) of 170-300 msec and approxima tely 400 msec for shallow (150 mu m) and deeper (400 mu m) targets, respect ively. Spraying for longer than the optimal tau(s) does not critically impa ir cooling selectivity. For a spurt duration of 100 msec, optimal delays ar e 5-10 msec and 25-70 msec for a shallow and deep basal layer, respectively . Conclusion: In the absence of knowledge about the lesion anatomy, using a t au(s) of 100-200 msec and no delay is a good compromise. A delay is justifi ed only when basal Payer and target chromophore are relatively deep and the optimal spurt duration cannot be applied, e.g., to avoid frostbite. Lasers Surg. Med, 27:165-170, 2000 (C) 2000 Wiley-Liss, Inc.