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R. Brinkmann, K. Schlott, L. Ptaszynski, S. Koinzer, M. Bever, A. Baade, R. Birngruber, R. Johann; Gentle Retinal Photocoagulation by Automatic Temperature Control. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4274.
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The damage extent of the retinal photocoagulations depends on the temperature increase and the time of irradiation. So far, the temperatures are unknown due to intraocular variations in light transmission and RPE/choroidal pigmentation. Thus in practice, often too large burns are produced, which can lead to extended scotoma and bleeding in the worst case. The aim of this project focuses on a dosimetry system, which automatically generates a desired coagulation strength for every single coagulation spot. This widely improves the therapy and relieves the ophthalmologists from any dosimetry control.
Experiments were performed on porcine eyes in vitro and dutch belted rabbits in vivo. Coagulations were performed with a standard clinical Nd:YAG-laser (Visulas, 532 nm), equipped with an interface to cease emission upon trigger. Simultaneously, low energy Q-switched Nd:YLF-laser pulses were applied to excite thermoelastic pressure waves from the RPE/choroid which are detected with a transducer embedded in the contact lens. The pressure amplitudes are used to calculate the temperature increase in realtime and the laser shut down.
The ED50 threshold temperature to for minimal visible lesions were determined for different laser irradiation times by the Probit algorithm. An exponential dependence similar to the Arrhenius curve was found, however, threshold temperatures are much lower than predicted by Arrhenius: 65°@50ms and 55°@300ms. Feeding the treatment laser to automatically cease irradiation at threshold consequently led to barely visible to invisible lesions in vivo. Spots with almost constant diameters could be achieved when shifting the threshold curve to stronger damage. Sizes smaller than the target spot diameter were reproducibly generated which is otherwise almost impossible to obtain.
These preliminary results show very promising towards realization of an automatic dosimetry system to achieve homogenous well defined retinal coagulations, almost independent of the individual retina and eye properties.
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