Intermittent cryogen spray cooling for optimal heat extraction during dermatologic laser treatment
Boris Majaron, Lars O Svaasand, Guillermo Aguilar, J Stuart Nelson
PHYSICS IN MEDICINE AND BIOLOGY
Abstract
Fast heat extraction is critically important to obtain the maximal benet of cryogen spray cooling (CSC) during laser therapy of shallow skin lesions, such as port wine stain birthmarks. However, a lm of liquid cryogen can build up on the skin surface, impairing heat transfer due to the relatively low thermal conductivity and higher temperature of the lm as compared to the impinging spray droplets. In an attempt to optimize the cryogen mass ux, while minimally affecting other spray characteristic...Mores, we apply a series of 10 ms spurts with variable duty cycles. Heat extraction dynamics during such intermittent cryogen sprays were measured using a custom-made metal-disc detector. The highest cooling rates were observed at moderate duty cycle levels. This conrms the presence, and offers a practical way to eliminate the adverse effect of liquid cryogen build-up on the sprayed surface. On the other hand, lower duty cycles allow a substantial reduction in the average rate of heat extraction, enabling less aggressive and more efcient CSC for treatment of deeper targets, such as hair follicles.
keywords:maximal; laser treatment; mass flux; duty cycles; Intermittent; skin surface; sprays; spray cooling; optimal heat extraction; dermatologic; liquid cryogen; spray characteristics; spurts
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