Abstract
Selective photothermolysis with pulsed lasers is presumably the most successful therapy for port-wine stain birthmarks (flammeus nevi). Selectivity is obtained by using an optical wavelength corresponding to high absorption in blood, together with small absorption in tissues. Further on, the pulse length is selected to be long enough to allow heat to diffuse into the vessel wall, but simultaneously short enough to prevent thermal damage to perivascular tissues. The optical wavelength and pulse length are therefore dependent on vessel diameter, vessel wall thickness and depth in dermis. The present work demonstrates that in the case of a 0.45 ms long pulse at 585 nm wavelength, vessels of 40–60Μm require minimum optical fluence. Smaller vessels require higher fluence because the amount of heat needed to heat the wall becomes a substantial fraction of the absorbed optical energy. Larger vessels also require a higher dose because the attenuation of light in blood prevents the blood in the centre of the lumen from participating in the heating process. It is shown that the commonly used optical dose in the range of 6–7 J cm−2 is expected to inflict vessel rupture rather than thermolysis in superficially located vessels. The present analysis might serve to draw guidelines for a protocol where the optical energy, wavelength and pulse length are optimized with respect to vessel diameter and depth in dermis.
Highlights
The principle of laser-induced photothermolysis is to inflict damage on microvessels of the skin in such a manner that the temperature of the surrounding normal dermis is maintained below the threshold for damage [1]
These results indicate that in the case of a 0.45 ms pulse, only the inner 5-7/~m of the vessel wall is heated above the temperature required for thermal denaturation, ie 60-70 ~ even when the blood if heated to the boiling point at the beginning of the pulse
When the irradiant dose is limited to the epidermal damagethreshold limit of 6 J cm - 2, the blood in these vessels will be heated to boiling point down to about 400 #m and 500 ~m for, respectively, 577 nm and 585 nm wavelength
Summary
The principle of laser-induced photothermolysis is to inflict damage on microvessels of the skin in such a manner that the temperature of the surrounding normal dermis is maintained below the threshold for damage [1]. The light distribution in vessels with comparable or larger diameters than the optical penetration depth will decay with distance from the irradiated perimeter.
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