Extended theory of selective photothermolysis.

BACKGROUND AND OBJECTIVE: We present a new theory of selective thermal damage of non-uniformly pigmented structures in biological tissues. Spatial separation of the heavily pigmented areas and the target requires limitation of the pigment temperature and heat diffusion from the pigmented to the targeted areas. STUDY DESIGNS/MATERIALS AND METHODS: A concept of selective target damage by heat diffusion is presented for three target geometries: planar, cylindrical, and spherical. An in vitro experiment is described in which the dependence of thermal damage on pulsewidth at constant fluence was evaluated. RESULTS: The in vitro experiment showed that the size of the damage zone for similar hair follicles was pulsewidth-independent over a very broad range of pulsewidths (30-400 ms). We formulated a new theory (extended theory of photothermolysis) to interpret the experimental results. CONCLUSIONS: Based on this new theory, the treatment pulsewidth for non-uniformly pigmented targets is significantly longer than the target thermal relaxation time (TRT). The theory provides new recommendations for photoepilation and photosclerotherapy parameters.

Evaluation of cooling methods for laser dermatology.

BACKGROUND AND OBJECTIVE: Skin cooling is used to protect the epidermis in a variety of laser dermatology procedures, including leg vein treatment, hair removal, and port wine stain removal. Spray and contact cooling are the two most popular methods, but similarities and differences of these techniques are not well understood. STUDY DESIGN/MATERIALS AND METHODS: A theoretical model of skin cooling is presented for two different regimens: "soft" cooling in which freezing of the skin is not permitted and "hard" cooling in which the skin can be frozen to a given depth. Spray and contact cooling were also compared experimentally using an in vitro model. RESULTS: For a fixed skin surface temperature, spray and contact cooling theoretically produce the same cooling profile in the skin. Anatomic depth of cooling depends on the time for which either the spray or contact is applied. In vitro experiments caused temperature at the simulated basal layer to be between -5 and +5 degrees C for both spray (tetrafluoroethane, boiling point -26 degrees C) and contact (-27 degrees C sapphire plate) cooling. The theoretical precooling analysis shows hard mode to be faster and more selective than soft mode; however, cooling time for hard mode must be carefully controlled to prevent irreversible epidermal damage caused by freezing. CONCLUSIONS: Both spray and contact cooling provide efficient skin cooling. The choice of cooling method depends on other factors such as the target depth, cost, safety, and ergonomic factors.

Contact cooling of the skin.

Skin precooling can be used to reduce epidermal thermal damage in laser procedures (such as hair removal) where the target structures are located up to several millimetres below the skin surface. We have developed and experimentally verified a computational model that describes contact precooling of a multilayered skin structure prior to laser irradiation. The skin surface is assumed to be brought into thermal contact with a cold plate made of material with a high thermal conductivity. The approximate analytical solution for the skin temperature is obtained by considering the plate as a local heat sink. The time evolution of temperature (in both the skin and the plate) is simulated numerically to yield the optimal cooling parameters. To experimentally verify the numerical results of the model, we performed direct measurements of skin temperature for contact cooling with a sapphire plate held at several different temperatures in the range +10 to -30 degrees C.