Optimal Cryogen Spray Cooling Parameters for Pulsed Laser Treatment of Port Wine Stains Wim Verkruysse, PhD, 1 * Boris Majaron, PhD, 1,2 B. Samuel Tanenbaum, PhD, 3 and J. Stuart Nelson, MD, PhD 1 1 Beckman Laser Institute and Medical Clinic, University of California, Irvine, California 2 Joz ˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia 3 Department of Engineering, Harvey Mudd College, Claremont, California Background and Objective: In dermatologic laser therapy, cryogen spray cooling (CSC) is a means to protect the epidermis while leaving dermal structures susceptible to thermal damage. The purpose of this study was to determine optimal spurt duration,  s , and optimal delay,  d , between the cryogen spurt and laser pulse when using CSC in treat- ment of port wine stain birthmarks. Study Design/Materials and Methods: A finite difference method is used to compute temperature distributions in human skin in response to CSC. Optimal  s and  d are determined by maximizing the temperature difference between a modeled basal layer and an imaginary target chromophore. Results: The model predicts an optimal  s of 170–300 msec and approxi- mately 400 msec for shallow (150 m) and deeper (400 m) targets, respectively. Spraying for longer than the optimal  s does not critically impair cooling selectivity. For a spurt duration of 100 msec, optimal delays are 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  s of 100–200 msec and no delay is a good compromise. A delay is justified only when basal layer and target chromophore are rela- tively deep and the optimal spurt duration cannot be applied, e.g., to avoid frostbite. Lasers Surg. Med. 27:165–170, 2000 © 2000 Wiley-Liss, Inc. Key words: spatial cooling selectivity; basal layer; target chromophore; human skin; spurt duration; delay of laser pulse; vascular lesion; heat trans- fer coefficient INTRODUCTION In laser treatment of subepidermal skin le- sions, target chromophores such as blood vessels in port wine stain (PWS) birthmarks are coagu- lated through selective light absorption [1]. Ow- ing to the broad absorption spectrum of melanin, the epidermal layer is a competing absorber of the laser light and, therefore, susceptible to thermal damage. Cryogen spray cooling (CSC) was intro- duced to cool and protect the epidermis from ther- mal damage, while maintaining sufficiently high target temperatures for selective thermal damage [2,3]. The benefits of CSC have been demon- strated clinically in laser treatment of PWS [3,4], hemangiomas [5,6], and rhytides [7]. Contract grant sponsor: Institute of Arthritis and Musculo- skeletal and Skin Diseases at the National Institutes of Health; Contract grant number: AR43419; Contract grant sponsor: Department of Energy; Contract grant sponsor: Of- fice of Naval Research; Contract grant sponsor: National In- stitutes of Health; Contract grant sponsor: Beckman Laser Institute and Medical Clinic. *Correspondence to: Wim Verkruysse, PhD, Beckman Laser Institute and Medical Clinic, 1002 Health Sciences Road East, Irvine CA 92612. E-mail: wverkru@bli.uci.edu Accepted 20 March 2000 Lasers in Surgery and Medicine 27:165–170 (2000) © 2000 Wiley-Liss, Inc.