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Ralf Brinkmann, Kerstin Schlott, Stefan Koinzer, Marco Bever, Alex Baade, Lars Ptaszynski, Yoko Miura, Johann Roider, Reginald Birngruber; Temperature Controlled Retinal Photocoagulation In Vivo. Invest. Ophthalmol. Vis. Sci. 2011;52(14):547.
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The strength of retinal photocoagulations depends on the temperature increase and the time of irradiation. So far, the temperatures during irradiation are unknown due to intraocular variations in light transmission and RPE/choroidal pigmentation. Thus in practice, irregular and often too large coagulations are produced, which can lead to extended scotoma and bleeding in the worst case. This project focuses on a dosimetry system, which automatically generates a desired coagulation strength for every single coagulation spot.
Coagulations were performed on chincilla bastard rabbits with a clinical Nd:YAG-laser (Zeiss Visulas 532s) at a laser slitlamp (Zeiss, SL 130) giving a retinal laser spot diameter of 132 µm. The laser was modified with an interface to cease emission upon trigger. Simultaneously, low energy Nd:YLF-laser pulses (523 nm) were repetitively 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 and to trigger the laser shut down. In the dosimetry mode, the trigger to cease laser emission was adjusted on a retinal spot diameter of the same size as the laser spot diameter.
The ED50 threshold temperatures at the center of the spot for minimal visible lesions were determined for different irradiation times by the Probit algorithm. An exponential dependence similar to that predicted by the Arrhenius formalism of thermal damage was found. Threshold temperatures of 65°@50ms and 55°@300ms were determined. Above threshold the retinal spot diameter linearly increased from threshold around 20 mW to 450 µm at 200 mW. Operating the laser in the dosimetry mode in the range between 20 to 200 mW, lesion diameter of 153 +/- 26 µm were achieved, thus spots within a small diameter range almost constant diameters. Finally very first temperature measurements on patients will be shown.
This study shows encouraging results for an automatic dosimetry system giving nearly equally sized coagulations, independent of the retinal pigmentation. Such a system would widely improve the therapy by relieving the ophthalmologists from any dosage control and most likely will reduce discomfort and pain for the patients.
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