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Ralf Brinkmann, Stefan Otto Johannes Koinzer, Alex Baade, Wadim Schwarzer, Kerstin Schlott, Yoko Miura, Johann Roider; Automatically Power Controlled Retinal Photocoagulation. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5666.
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© ARVO (1962-2015); The Authors (2016-present)
The strength of retinal photocoagulation lesions mainly depends on the temperature increase at the irradiation site. Owing to intraocular variations in light transmission and RPE/choroidal pigmentation, the absorbed laser power at the irradiation site and thus the temperature rise is unknown. We recently presented an optoacoustic temperature feedback technique which switches the laser off according to a time dependent Arrhenius algorithm in order to achieve uniform coagulations. In this work, we present a power control modus with a fixed exposure time, which is advantageous when combining it with a patterned irradiation system.
Irradiation was performed on rabbits with a new Nd:YVO4-laser system (Zeiss Visulas Series Prototype, 532 nm). A Nd:YLF-laser (Crystalaser, 523 nm, 75 ns, 1 kHz) simultaneously served to excite thermoelastic pressure waves. The pressure is detected by an ultrasonic transducer embedded in the contact lens and is used to calculate the temperature rise. Irradiations are performed with predefined exposure times. The laser power is automatically varied during irradiation in order to achieve a preset peak temperature at the end of the laser exposure. The lesions sizes were examined on color fundus images acquired with a fundus camera one hour after irradiation.
In the first series lesions were created with a calculated retinal spot diameter of 133 µm with an overall irradiation time of 150 ms/spot and a preset peak temperature of 65 °C. With the implemented algorithm, a mean peak temperature of 67 °C with a standard deviation of 4.2 °C was achieved. The variability of automatically chosen laser power of 47 +/- 11 mW shows that the method takes the individual variations of light transmission and absorption into account. The mean lesion diameter was determined to be 149 +/- 17 μm. The results with an overall irradiation time of 50 ms were even more accurat within +/- 2 °C from different set temperatures and will be presented in detail on the conference as well.
The achieved data are promising for a feedback controlled treatment, which allows to automatically generate preselected sublethal temperature rise, or minimal ophthalmoscopically invisible thermal damage, or defined and uniformly visible coagulations. This technique seems to be the ideal method to unburden the ophthalmologist from any manual dosing and allowing a fast and reproducible treatment also in a patterned modus.
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