Cryogen spray cooling is used to prevent epidermal thermal damage during port-wine stain laser therapy, despite the limited understanding of the fluid dynamics, thermodynamics, and heat transfer characteristics of cryogen sprays. In recent studies, it has been suggested that the heat flux through human skin could be increased by changing physical parameters such as nozzle-to-skin distance, nozzle diameter, and/or by depositing cryogen in sequential spurts. These changes affect spray parameters such as droplet diameter, velocity, and spray temperature. Therefore, in order to optimize new nozzle designs, it is necessary to explore the influence that these fundamental spray parameters have on heat extraction. In this paper, various valve/nozzle configurations were characterized. A Phase Doppler Particle Analyzer was used to determine the average diameter, velocity, and droplet concentration of various cryogen sprays. The mass flux delivered by each valve/nozzle configuration was also measured, along with the average spray temperature. A custom-made device consisting of an insulated metallic disk was used to measure the heat extracted by different sprays. The results showed that there are significant differences in the heat extracted by the different valve/nozzle configurations. These variations are proportionally influenced by mass fluxes. Strong correlations were also observed between average droplet velocities and heat extraction. These findings indicate that mass flux has a dominant effect on heat extraction from human skin during cryogen spray cooling. It is also apparent that kinetic and thermal energies are other parameters to be considered when optimizing heat extraction.

The purpose of this study was to investigate the dynamics of laser light attenuation during cryogen spray cooling (CSC). Two detection schemes were used to approximate collimated and diffuse light transmittance measurements of continuous-wave (λ = 594 nm) and pulsed (λ = 585 nm) laser light during application of short (20-100 ms duration) cryogen spurts on a glass substrate. High-speed video images were also obtained during CSC. Collimated light transmittance varied considerably during CSC. Comparison of collimated and total transmitted light detection indicated that the diffuse component was substantial. Light attenuation occurred despite transparency of the liquid cryogen layer. Light scattering by cryogen results in a diverging laser beam incident on the skin surface. Since specular reflectance at the cryogen-skin interface may differ for diffuse light, further study of light scattering during CSC is warranted. Due to the differences in optical properties of glass and skin, experiments on skin need to be performed to extrapolate our results to the clinical scenario. For dermatologic procedures such as laser port wine stain and vascular lesion removal, hair removal, and nonablative skin rejuvenation, recommended τd are 10-80 ms. This range of τd appears to be appropriate, although more studies are required to arrive at a definite conclusion. © 2002 SPIE.