Noninvasive temperature measurements during laser irradiation of the retina with optoacoustic techniques

In all laser treatments at the fundus of the eye the temperature increase is unknown. In order to optimize the treatment modalities, a noninvasive online temperature determination is preferable. Method: Applying laser pulses to the fundus, thermoelastic stress waves are emitted based on the thermal expansion of the heated tissue, mainly the retinal pigment epithelium (RPE). The amplitude of the thermoelastic wave is proportional to the thermal expansion coefficient, which linearly depends on temperature between 30-80°C for water. The method was evaluated for selective RPE-treatment in vitro and clinically using the μs-laser pulses for treatment and temperature determination simultaneously. Conventional laser photocoagulation was investigated in vitro using an Ar Ion laser for coagulation and low-energy N₂-pumped dye laser pulses to probe the temperature. Results: In all cases, sufficient pressure amplitudes were detected either by a needle hydrophon in vitro or by a contact lens with embedded transducer during treatment. Depending on the treatment parameter, temperature increase of 60°C were evaluated from the pressure transients. All temperatures detected are in close agreement to heat diffusion calculations. Conclusion: We demonstrated a noninvasive online method to detect retinal temperatures during laser treatments. This technique can be adapted to photocoagulation, PDT and TTT.