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C. Sramek, J. Brown, L.-S. Leung, T. Leng, D. V. Palanker; Enhanced Safety of Retinal Photocoagulation by Spatial and Temporal Modulation of Laser Beam. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4268.
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© ARVO (1962-2015); The Authors (2016-present)
Decreasing pulse duration in photocoagulation helps to confine the treatment zone and reduce pain by limiting heat diffusion. However, narrowing of the safe therapeutic window (TW) limits the short end of safe pulse duration. We enhance safety of photocoagulation by improving uniformity of the temperature distribution using modulated pulses and a ring-shaped beam.
A numerical model of retinal heat conduction was used to calculate pulse shapes and beam profiles that optimize the temperature course during treatment. A waveform generator was used to produce corresponding shaped pulses of 532 nm radiation with a modified PASCAL laser. A ring-shaped beam with adjustable depth of modulation was created. In-vivo acute thresholds for mild coagulation and rupture for both treatment systems were measured in 30 rabbit eyes and compared with those for conventional pulses of constant power and top-hat beam.
The width of the TW was significantly improved (28%, p<0.003) for the shaped pulses of 5 and 10 ms, compared with conventional square pulse: 2.4 vs. 1.9 at 5 ms, and 2.8 vs. 2.2 at 10 ms. A similar significant (p<0.05) increase in TW was found with a ring-shaped beam at 10 - 50ms pulse durations (3.0 vs. 2.3 at 10 ms, 3.3 vs. 2.9 at 20 ms, 4.8 vs. 3.5 at 50 ms), compared to a conventional top-hat beam.
Safe therapeutic range of retinal photocoagulation can be increased by temporal modulation of the laser pulse and by spatial modulation of the beam shape. With the experimental pulse and beam shapes, a TW close to 3.0 was achieved with 10 ms pulses, similar to 20 ms exposures with conventional laser parameters. This improvement allows for a two-fold increase in the speed of patterned scanning laser photocoagulation.
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