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R. Brinkmann, J. Stalljohann, B. Weber, K. Schlott, J. Kandulla, R. Birngruber; Retinal Temperature Determination During Photocoagulation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4165.
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Retinal laser photocoagulation is an established therapy for a variety of retinal diseases. The extent of the coagulations depends on the temperature increase and the time of elevated temperatures during photocoagulation. However, this temperature rise is unknown since it does not only depend on the laser settings but also on the unknown retinal/choroidal pigmentation as well as the transmission characteristics of the radiation through the whole eye. So far, clinical dosimetry is performed post radiation by estimating appropriate power-time settings for the next spot in order to achieve whitish lesions. Due to intraocular changes in pigmentation and transmission, often too large burns are produced, which can lead to extended scotoma and bleedings in the worst case. This work investigates a non-invasive technique for an online temperature monitoring during photocoagulation. Far aim of the research is an automatic on-line dosimetry system by means of regulating the treatment laser onto minimal invasive and well defined coagulations.
Optoacoustic techniques are used to determine the temperature increase during photocoagulation. Therefore, Q-switched Nd:YLF-laser pulses (527 nm, 200 ns, 200 Hz) are applied to excite the emission of pressure waves from the retina, which are detected with an ultrasonic transducer embedded in the contact lens. The pressure amplitude can be used to calculate the temperature. The ns-pulses are transmitted to the eye via the same slitlamp and fiber as the treatment laser radiation of a cw Nd:YAG-laser (532 nm). Experiments are performed on enucleated pig globes.
Irradiation with constant power of 175 mW onto 400 µm spots of medium pigmented eyes leads to a temperature rise of 28 K after 500 ms with a logarithmic temperature rise over time as expected from the theory. Applying pulses of 200 ms with different power, we found a maximal temperature rise of 0.13 K/mW. At the onset of retinal denaturation, a change in the pressure curves is observed. From the measured temperature/time history at threshold, the Arrhenius constants for retinal tissue denaturation can directly be calculated.
These are the first non-invasive measurements demonstrating an online monitoring of the temperature/time history during retinal laser photocoagulation. The data achieved show very promising towards realization of an automatic dosimetry system to achieve homogenous well defined retinal coagulations almost independent of retina and eye properties.
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