Franco Walfre, Liu Jie, Romero-Méndez Ricardo, Jia Wangcun, Nelson J Stuart, Aguilar Guillermo
Department of Mechanical Engineering, University of California, Riverside, California 92521, USA.
Lasers Surg Med. 2007 Jun;39(5):414-21. doi: 10.1002/lsm.20511.
Cryogen spray cooling (CSC) has become an integral part of dermatologic laser surgery because of its ability to remove selectively large amounts of heat from human skin in short periods of time, thereby protecting the epidermis from unintended thermal injury. The objective of the present study is to investigate the extent of lateral epidermal protection afforded by a cryogen spray during laser irradiation.
CSC experiments on skin phantoms are conducted using a commercial nozzle (GentleLase, Candela) to characterize epidermal cooling in time and space; namely, surface temperatures and heat fluxes during a 60 milliseconds spurt and 30 milliseconds delay. Numerical methods are used to model the light distribution (755 nm), heat diffusion and thermal injury within the epidermis and dermis. A 755 nm laser (GentleLase, Candela) was used to assess in vivo the extent of lateral epidermal protection against irradiation in human skin.
The commercial nozzle produced an uneven deposition and spread of liquid cryogen, thereby creating zones of high and low heat extraction on the surface. Numerical and in vivo studies show that 18 mm diameter laser beams may induce skin injury at the periphery of the irradiated areas. However, a 10 mm diameter beam provides the safest therapy because only the zone of highest heat extraction is exposed to laser irradiation. Beyond 10 mm, heat extraction is no more than a third of the maximum heat extraction within this diameter.
Accumulation of heat within the epidermis is always greater at the laser beam periphery, away from the CSC nozzle tip, where heat extraction is lowest. Therefore, there is risk of thermal injury at the beam periphery when there is a mismatch between the skin protected by CSC and that exposed to laser irradiation. For the cooling and irradiation sequences considered herein, heat extraction provided by a 60 milliseconds spurt/30 milliseconds delay correctly matches the heating profile of a 10 mm diameter beam.
冷冻喷雾冷却（CSC）已成为皮肤激光手术不可或缺的一部分，因为它能够在短时间内从人体皮肤中选择性地去除大量热量，从而保护表皮免受意外热损伤。本研究的目的是探讨冷冻喷雾在激光照射期间对表皮横向保护的程度。
使用商用喷嘴（GentleLase，Candela）对皮肤模型进行CSC实验，以表征表皮在时间和空间上的冷却情况；即60毫秒喷射和30毫秒延迟期间的表面温度和热通量。采用数值方法对表皮和真皮内的光分布（755nm）、热扩散和热损伤进行建模。使用755nm激光（GentleLase，Candela）在体内评估对人体皮肤照射时表皮横向保护的程度。
商用喷嘴产生的液态冷冻剂沉积和扩散不均匀，从而在表面形成高吸热区和低吸热区。数值研究和体内研究表明，直径18mm的激光束可能在照射区域周边引起皮肤损伤。然而，直径10mm的光束提供了最安全的治疗，因为只有最高吸热区暴露于激光照射。超过10mm，吸热量不超过该直径内最大吸热量的三分之一。
在远离CSC喷嘴尖端的激光束周边，表皮内的热量积累总是更大，此处吸热量最低。因此，当CSC保护的皮肤与暴露于激光照射的皮肤不匹配时，光束周边存在热损伤风险。对于本文所考虑的冷却和照射顺序，60毫秒喷射/30毫秒延迟提供的吸热量与直径10mm光束的加热曲线正确匹配。
