Selective cooling of biological tissues: application for thermally mediated therapeutic procedures

Abstract

The ability to control the degree and spatial distribution of cooling in biological tissues during a thermally mediated therapeutic procedure would be useful for several biomedical applications of lasers. We present a theory based on the solution of the heat conduction equation that demonstrates the feasibility of selectively cooling biological tissues. Model predictions are compared with infrared thermal measurements of in vivo human skin in response to cooling by a cryogen spurt. The presence of a boundary layer, undergoing a liquid-vapour phase transition, is associated with a relatively large thermal convection coefficient (approximately 40 kW m-2 K-1), which gives rise to the observed surface temperature reductions (30-40 degrees C). The degree and the spatial-temporal distribution of cooling are shown to be directly related to the cryogen spurt duration.

Non-invasive determination of port wine stain anatomy and physiology for optimal laser treatment strategies

“…When the diameter increases from the minimum with constant pulse duration, the blood volume to be heated increases, so a higher fluence is required to achieve an average vessel temperature of 70 • C. Additionally, the effect of heat conduction becomes less important as τ t L . Similar curves have been calculated analytically by Svaasand et al (1995a), who assumed that instantaneous heating of the red blood cells to 100 • C occurs. The results (figure 5) suggest that laser irradiated (λ = 577 nm) 30 µm diameter PWS blood vessels (or 40 µm at 585 nm, see de Boer et al 1996), require the lowest fluence (0.45 ms pulse) for irreversible injury; therefore, vessels of this diameter are the deepest vascular dermal structures that can be irreversibly injured with such pulse durations.…”

“…Furthermore, in the green-yellow spectral region (500-600 nm) the penetration depth in whole blood (41% haematocrit) is 30-50 µm, which corresponds to the diameter of moderate-sized PWS vessels. Therefore, light propagation in PWS skin depends on the fractional blood volume and vessel diameter (Svaasand et al 1995a, Verkruysse et al 1997.…”

“…Equation (6) has been formulated (van Gemert et al 1995a, p 811), assuming that all blood is homogeneously distributed over the dermis. However, recent work (Svaasand et al 1995a, Verkruysse et al 1997 has shown that the homogeneous distribution assumption overestimates absorption by blood because red blood cells at the centre of the vessel absorb less light than those at the wall. Consequently, the vessel diameter, at constant dermal fractional blood volume, has a significant effect on φ .…”

“…Epidermal melanin concentration is important, although cooling the skin surface using short (ms) cryogen spurts can protect the basal layer during laser irradiation, allowing use of a higher radiant exposure and deeper vascular injury than would otherwise be possible (Anvari et al 1995a.…”

Description and Analysis of Treatments for Port-wine Stain Birthmarks

“…Commonly used forms of contact cooling at present include plates of conductive material (eg, glass or sapphire) continuously cooled by a recirculating refrigerant. 16 Nelson et al 17,18 developed cryogen spray cooling (CSC) as an efficient and effective means of achieving selective epidermal protection. Cryogen (tetrafluoroethane [C 2 H 2 F 4 ]; boiling point=−26.2°C) spurts, 10 to 50 milliseconds in duration, are delivered to the skin surface immediately before laser exposure.…”