Radial and temporal variations in surface heat transfer during cryogen spray cooling

A new mathematical model for accurate quantification of cryogen spray cooling in cutaneous laser surgery using realistic boundary conditions

Laser dermatologic surgery is widely used for various skin diseases. Prior to the laser treatment, transient cryogen spray cooling (CSC) is employed to prevent unwanted thermal damage of skin tissue due to melanin absorption in the epidermis. Accurate quantification on the cooling effect of CSC is essential in the subsequent laser energy dosage prediction. Based on the existing experimental data, we present a new dynamic correlation in terms of non-dimensional heat convection coefficient as a function of the dimensionless time and dimensionless spatial location. The effects of transient spray cooling and the thermal properties of experimental substrate are included in the correlation. The heat transfer within the skin tissue is calculated by a heat conduction equation. Meanwhile, the dynamic correlation of heat convection coefficient is incorporated into the heat conduction equation as a convection boundary condition. Simulation results demonstrate that the correlation correctly reflects several important characteristics of CSC and the model predictions are in consistent with the experimental observations. Numerical results indicate that the CSC time duration should be short enough to maintain selective cooling in epidermis while long enough to avoid the non-uniform spatial cooling on skin surface. A shorter spray distance (for example z = 30 mm) is found to have better cooling effect than a longer spray distance (z = 60 mm). The dynamic correlation in terms of non-dimensional heat convection coefficient proposed in this study can be used in accurate quantification of the effect of CSC. Optimal cooling duration of 75-100 ms and short spray distance of 30 mm are recommended for CSC clinical application.

Fluid and thermal dynamics of cryogen sprays impinging on a human tissue phantom

Cryogen spray cooling (CSC) protects the epidermis from unintended heating during cutaneous laser surgery. The present work investigated the time-dependent flow characteristics of cryogen sprays and correspondent thermal dynamics at the surface of a human tissue phantom. First, a numerical analysis was carried out to evaluate an epoxy block substrate as a human tissue phantom. Next, the velocity and diameter of cryogen droplets were measured simultaneously and correlated with surface temperature of the human tissue phantom during CSC. Finally, velocity and diameter measurements were used to compute the spray number, mass, and kinetic energy fluxes, and temperature measurements were used to compute the surface heat flux. Numerical modeling showed that the thermal response of our phantom was qualitatively similar to that of human stratum corneum and epidermis; quantitatively, thermal responses differed. A simple transformation to map the temperature response of the phantom to that of tissue was derived. Despite the relatively short spurt durations (10 ms, 30 ms, and 50 ms), cryogen delivery is mostly a steady state process with initial and final fluid transients mainly due to the valve dynamics. Thermal transients (16 ms) are longer than fluid transients (4 ms) due to the low thermal diffusivity of human tissues; steady states are comparable in duration ( approximately 10 ms, 30 ms, and 50 ms) although there is an inherent thermal delay ( approximately 12 ms). Steady state temperatures are the lowest surface temperatures experienced by the substrate, independent of spurt duration; hence, longer spurt durations result in larger exposures of the tissue surface to the same lower, steady state temperature as in shorter spurts. Temperatures in human tissue during CSC for the spray system and parameters used herein are estimated to be approximately -19 degrees C at the stratum corneum surface and >0 degrees C across the epidermis.

A model-based approach to the evaluation of flame-protective garments

High-quality protective garments are essential in many occupational assignments, e.g., in the petrochemical industry and for firefighting. The problem addressed in this paper is how to objectively asses the performance of protective garments designed to resist fire. We present a system based on a flame mannequin equipped with an array of temperature sensors that provide information about the temperature on the mannequin's surface during exposure to fire. Particular attention is devoted to reconstructing the heat flux reaching the mannequin's surface on the basis of the available temperature records (the inverse heat-conduction problem). The estimated heat flux is used in a skin-simulation model to predict the level of injury that would occur in human exposed to the same heat. The paper includes a novel computational procedure based on a detailed model of the temperature sensor and a numerical solution of the accompanying heat equation. The effectiveness of the solution is demonstrated with experimental data.

Extent of lateral epidermal protection afforded by a cryogen spray against laser irradiation

BACKGROUND AND OBJECTIVES

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.

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Thermal responses of ex vivo human skin during multiple cryogen spurts and 1,450 nm laser pulses

BACKGROUND AND OBJECTIVE

Although cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery, concern has been expressed that CSC may induce cryo-injury. The objective of this study is to measure temperature variations at the epidermal-dermal junction in ex vivo human skin during three clinically relevant multiple cryogen spurt-laser pulse sequences (MCS-LPS).

STUDY DESIGN/MATERIALS AND METHODS

The epidermis of ex vivo human skin was separated from the dermis and a thin-foil thermocouple (13 microm thickness) was inserted between the two layers. Thermocouple depth and epidermal thickness were measured using optical coherence tomography (OCT). Skin specimens were preheated to 30 degrees C before the MCS-LPS were initiated. Three MCS-LPS patterns, with total cryogen spray times of 38, 30, and 25 milliseconds respectively, were applied to the specimens in combination with laser fluences of 10 and 14 J/cm(2), while the thermocouple recorded the temperature changes at the epidermal-dermal junction.

RESULTS

The thermocouple effectively recorded fast temperature changes during three MCS-LPS patterns. The lowest temperatures measured corresponded to the sequences with longer pre-cooling cryogen spurts. No sub-zero temperatures were measured for any of the MCS-LPS patterns under study.

CONCLUSIONS

The three clinically relevant MCS-LPS patterns evaluated in this study do not cause sub-zero temperatures in ex vivo human skin at the epidermal-dermal junction and, therefore, are unlikely to cause significant cryogen induced epidermal injury.

Improvement of port wine stain laser therapy by skin preheating prior to cryogen spray cooling: A numerical simulation

BACKGROUND AND OBJECTIVES

Although cryogen spray cooling (CSC) in conjunction with laser therapy has become the clinical standard for treatment of port wine stain (PWS) birthmarks, the current approach does not produce complete lesion blanching in the vast majority of patients. The objectives of this study are to: (1) experimentally determine the dynamic CSC heat flux when a skin phantom is preheated, and (2) numerically study the feasibility of using skin preheating prior to CSC to improve PWS laser therapeutic outcome.

STUDY DESIGN/MATERIALS AND METHODS

A fast-response thin-foil thermocouple was used to measure the surface temperature and thus heat flux of an epoxy skin phantom during CSC. Using the heat flux as a boundary condition, PWS laser therapy was simulated with finite element heat diffusion and Monte Carlo light distribution models. Epidermal and PWS blood vessel thermal damage were calculated with an Arrhenius-type kinetic model.

RESULTS

Experimental results show that the skin phantom surface can be cooled to a similar minimum temperature regardless of the initial temperature. Numerical simulation indicates that upon laser irradiation, the epidermal temperature increase is virtually unaffected by preheating, while higher PWS blood vessel temperatures can be achieved. Based on the damage criterion we assumed, the depth and maximum diameter of PWS vessels that can be destroyed irreversibly with skin preheating are greater than those without.

CONCLUSIONS

Skin preheating prior to CSC can maintain epidermal cooling while increasing PWS blood vessel temperature before laser irradiation. Numerical models have been developed to show that patients may benefit from the skin preheating approach, depending on PWS vessel diameter and depth.