Micropulse modulation in retinal laser therapy was intended to confine tissue heating around the light-absorbing layers, such as RPE and choroid, while the transparent retina is heated less as a result of slow heat diffusion. Current implementations use micropulses of 100-300μs at 500Hz, with overall pulse envelope of 100-300ms. The effect of such modulation compared to continuous-wave (CW) is not well characterized and misleading comparisons are made in the literature between exposures of different average power or overall duration. In this study, we modeled and measured the retinal tissue response to pulse trains with duty cycles from 4% (80μs pulse at 500Hz) to CW at overall envelope of 200ms and 20ms. Three thresholds of tissue response were measured in Dutch-belted rabbits: immediate (<3s after laser delivery) and delayed (1-5min) ophthalmoscopic visibility of lesions corresponding to photoreceptor damage, as well as fluorescein angiography visibility indicating RPE damage. Both the model and experimental results show that tissue response to micropulse modulation with long pulse envelope (200ms) is not significantly different from CW exposures at the same average power and duration. Heat confinement is improved with lower duty cycle (2%) and shorter pulse envelope (20ms), however further decrease in exposure duration raises the temperature dangerously close to vaporization. Pulse modulation cannot improve the therapeutic range of non-damaging thermal therapy since it is defined by the Arrhenius integral, regardless of the time course of hyperthermia. However, it does allow greater thermal stress to the RPE and underlying choroid while avoiding damage to neural retina.
Jenny Wang, Yi Quan, Roopa Dalal, and Daniel V. Palanker, "Tissue response to micropulse modulation in retinal laser therapy (Conference Presentation)," Proc. SPIE 10045, Ophthalmic Technologies XXVII, 1004508 (Presented at SPIE BiOS: January 28, 2017; Published: 16 May 2017); https://doi.org/10.1117/12.2251042.5370275073001.
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