Abstract

P 263

Automatic dosimetry control for gentle retinal photocoagulation

Ralf Brinkmann1, Jens Langejürgen1, Kerstin Schlott1, Marco Bever2, Katharina Herrmann2, Stefan Koinzer3, Johann Roider3, Reginald Birngruber1
1Institut für Biomedizinische Optik, Universität zu Lübeck, Lübeck, 2Medizinisches Laserzentrum Lübeck, Lübeck, 3Augenklinik, Campus Kiel, Universitätsklinikum Schleswig-Holstein, Kiel

Objective
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. 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. The aim of this project focuses on an automatic dosimetry system for gentle photocoagulation, which automatically gives the appropriate coagulation strength for every single coagulation spot, relieving the ophthalmologists from any dosimetry control.
Methods
Optoacoustic techniques are used to determine the temperature increase during photocoagulation in realtime. Therefore, Q-switched Nd:YLF-laser pulses (527 nm, 200 ns, 1 kHz) are applied to excite the emission of thermoelastic 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 probe pulses are transmitted to the eye via the same slitlamp and fiber as the treatment laser radiation of a cw Nd:YAG-laser (Zeiss Visulas, 532 nm). Experiments are performed on enucleated porcine globes and dutch belted rabbits in vivo.
Results
Irradiation with a constant power of 175 mW onto 400 µm spots of medium pigmented eyes lead to a temperature rise of 35 K after 500 ms with an approximately logarithmic temperature rise over time as expected from the heat diffusion theory. Applying pulses of 200 ms with different powers, we found a temperature rise of 0.18 K/mW at the end of the irradiation period. Data from rabbit eyes in vivo will be reported as well as first results towards the laser dosimetry control.
Conclusions
The experiments confirm the possibility to non-invasively determine the temperature/time course at the retina during 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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