UC Irvine

Individual blood vessels in the chick chorioallantoic membrane (CAM) were selectively coagulated through photothermolysis, using pulsed laser irradiation at 585 nm. Pulse durations were chosen to be 0.45 ms and 10 ms, which correspond to the thermal relaxation times in blood vessels of 30 μm and 150 μm diameter, respectively. The short pulses, at an energy density (or light dose) D=3 Jcm-2, caused permanent occlusion of vessels of 40 μm diameter or less, while larger caliber vessels (60-120 p.m) required D=4-5 Jcm-2. The long-duration pulses, at D=7 Jcm-2, caused coagulation of the larger diameter vessels; the small caliber vessels and capillaries showed resistance to photothermolysis and required multiple exposures to achieve coagulation. The dose vs diameter (D vs d) relationship for coagulation was calculated for the two pulse shapes. The energy deposited in a cylindrical absorber of diameter d by an optical field, incident perpendicular to the vessel, was expressed analytically and compared with the energy required to coagulate a blood vessel of the same lumen diameter. When thermal diffusion is incorporated into the model, our findings can be accounted for quantitatively. This information will be of use for improving the laser treatment of port wine stains and other vasculopathies. A surprising observation was that arterioles were damaged at lower incident energy densities than venules having the same lumen diameter, despite the fact that absorbance in oxygen-rich and oxygen-poor blood is the same for 585 nm radiation.