Noninvasive optoacoustic online retinal temperature determination during continuous-wave laser irradiation

Abstract

The therapeutic effect of most retinal laser treatments is initiated by a transient temperature increase. Although crucial to the effectiveness of the treatment, the temperature course is not exactly known due to individually different tissue properties. We develop an optoacoustic method to determine the retinal temperature increase in real time during continuous-wave (cw) laser irradiation, and perform temperature calculations to interpret the results exemplary for transpupillary thermotherapy (TTT). Porcine globes ex vivo and rabbit eyes in vivo are irradiated with a diode laser (λ=810nm, P⩽3W, ϕ=2mm) for 60s. Simultaneously, pulses from a N2-laser pumped dye laser (λ=500nm, τ=3.5ns, E≈5μJ) are applied on the retina. Following its absorption, an ultrasonic pressure wave is emitted, which is detected by a transducer embedded in a contact lens. Using the previously measured temperature-dependent Grüneisen coefficient of chorioretinal tissue, a temperature raise in porcine eyes of 5.8°C∕(W∕cm2) after 60s is observed and confirmed by simultaneous measurements with an inserted thermocouple. In a rabbit, we find 1.4°C∕(W∕cm2) with, and 2.2°C∕(W∕cm2) without perfusion at the same location. Coagulation of the rabbit’s retina occurs at ΔT=21°C after 40s. In conclusion, this optoacoustic method seems feasible for an in vivo real-time determination of temperature, opening the possibility for feedback control retinal laser treatments.