Optoacoustic temperature determination at the fundus of the eye during transpupillary thermotherapy

Transpupillary Thermotherapy (TTT) is a retinal laser treatment targeting on occult neovascular membranes. However, the induced temperature increase during irradiation is not exactly known. Model calculations show that differences in pigmentation and vascular perfusion should lead to a different temperature rise under same treatment parameters. In order to evaluate the temperature profiles during TTT, we developed a non-invasive optoacoustic method. The experiments were carried out on enucleated porcine eyes (ex-vivo) and rabbits (in-vivo). Simultaneously to the irradiation with a TTT-laser (λ=810nm, P≤3W) through a slit lamp, pulses from a pulsed dye laser (λ=500nm, τ≈3ns, E≈5μJ) were applied onto the irradiated area. This probe irradiation induces an ultrasonic pressure wave, its amplitude depends on the temperature of the absorbing media. We measured this pressure wave by an ultrasonic transducer integrated into a contact lens. The induced pressure maximum for porcine RPE samples was found to raise by 14.6% between 37°C and 50°C. Using this relation we determined optoacoustically the temperature increase during TTT. A comparison of the acoustic data with simultaneous temperature measurements by a thermo couple positioned in the choroid showed accordance within ±1.2°C. Differences in pigmentation led to a 1.8 fold induced temperature increase between weak and heavy pigmented eyes. First results in the in-vivo animal study showed an average temperature increase of (2.45 ±±0.16)°C for an irradiation of 1W/cm ² . In conclusion the developed optoacoustic method seems adequate for real-time temperature determination during retinal laser treatments and might serve as a dosimetry control for TTT.