LASER-PULSE DURATION MUST MATCH THE ESTIMATED THERMAL RELAXATION-TIMEFOR SUCCESSFUL PHOTOTHERMOLYSIS OF BLOOD-VESSELS

The relationship between photothermal damage to blood vessels of diame ter, d, and laser pulse duration, t(p), was verified in a series of st udies using the chick chorioallantoic membrane (CAM). A total of 879 i ndividual CAM blood vessels (d=50-130 mu m) was irradiated, using a la ser pulse duration of 0.45 or 10 ms. Laser-induced vascular damage was observed in real time, recorded on videotape, and evaluated in a doub le-blind fashion. Permanent damage was confirmed by inspection 24 h af ter laser exposure. Under the conditions of this experiment, only when laser pulse durations are approximately equal to the estimated therma l relaxation times (tau) of the CAM microvessels can the critical core intravascular temperature, necessary to destroy vessels irreversibly, be achieved and sustained for sufficient time. Shorter pulse duration s are more effective for damaging smaller blood vessels; conversely, l onger pulse durations are more effective for damaging larger diameter vessels.