Pulsed delivery of laser energy in experimental thermal retinal photocoagulation

Retinal lesions produced with a pulsed laser beam of 1-20 kHz frequency and 10-100% duty cycle were compared with lesions produced with a continuous wave (cw) laser of the same peak power and total energy. Photocoagulation was applied to the retina of three black pigmented rabbits using krypton red laser (647.1 nm) equipped with an acousto-optical modulator to convert cw laser emission to a pulsating beam. An optical fiber fed the laser beam into an optical system delivering a collimated beam of predetermined divergence; the animal's eye focused this beam to a 50-pm spot on the retina. Peak power was kept constant at 0.2 W, and energy was kept constant at 20 mJ. After 7 months the animals were sacrificed and retinal tissue examined by light microscopy. The central section of each lesion was identified and photographed. For lesions with the same energy per pulse and the same pulse duration, the most influential factor, in the frequency range of 1-20 kHz, appeared to be the duty cycle: the smaller the duty cycle, the smaller the lesion, and vice versa. In other words, the shorter the time interval between consecutive pulses, the larger were the pulsed laser lesions.

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