Noninvasive optoacoustic temperature determination at the fundus of the eye during laser irradiation

In all fundus laser treatments of the eye, the temperature increase is not exactly known. In order to optimize treatments, an online temperature determination is preferable. We investigated a noninvasive optoacoustic method to monitor the fundus temperature during pulsed laser irradiation. When laser pulses are applied to the fundus, thermoelastic pressure waves are emitted, due to thermal expansion of the heated tissue. Using a constant pulse energy, the amplitude of the pressure wave increases linearly with an increase in the base temperature of between 30 and 80°C. This method was evaluated in vitro on porcine retinal pigment epithelium (RPE) cell samples and clinically during selective RPE treatment with repetitive microsecond laser pulses. During the irradiation of porcine RPE with a neodymium-doped yttrium (Nd:YLF) laser (527 nm, 1.7 μs, 500 Hz repetition rate, 160 mJ/cm ²) an increase in the base temperature of 30±4°C after 100 pulses was found. During patient treatments, a temperature increase of 60±11°C after 100 pulses with a 500-Hz repetition rate and 7±1°C after 30 pulses with 100 Hz at 520 mJ/cm ² was found. All measured data were in good agreement with heat diffusion calculations. Optoacoustic methods can be used to noninvasively determine retinal temperatures during pulsed laser treatment of the eye. This technique can also be adapted to continuous-wave photocoagulation, photodynamic therapy and transpupillary thermotherapy, or other fields of laser-heated tissue.