Automatic Temperature Control for Retinal Laser Exposures

Abstract

Purpose : Retinal laser treatments lack an objective dosing control, particularly for sub visible irradiations becoming more and more popular. However, laser effects vary largely due to inter and intraocular variations of light scattering and pigmentation at the fundus, leading to strong variations of the induced temperatures. We developed an optoacoustic method to determine the temperature rise during laser irradiation and demonstrated the technique in a clinical trial. The method was now been extended with an automatic closed-loop feedback control system in order to automatically obtain a desired target temperature preselected by the ophthalmologist.

Methods :
A commercial treatment laser (532nm) is used. The laser fiber is coupled to a control unit containing a pulsed probe laser (523 nm, 75 ns, 3 kHz) to excite thermoelastic pressure waves at the retina, and a fast modulator to modify the power of the therapy laser with a frequency of 3 kHz. Both beams are transmitted by the same fiber to a laser slitlamp. The acoustic amplitudes contain the current retinal temperatures and are recorded by an ultrasonic transducer embedded in a standard contact lens. A control software calculates the required laser power in real-time in such a way that the desired temperature rise is reached. Experiments were performed on freshly enucleated pig eye globes and RPE-choroid-sclera explants. Target temperatures between 40°C and 70°C were selected for irradiation times between 50 and 200 ms and a spot diameter of 200 µm.

Results : Depending on the control settings, the aim temperature was reached within 10-100 ms. Thereafter it is kept constant until the end of the irradiation period. For an irradiation time of 100 ms for example, target temperatures of 40, 50 and 60 °C were achieved with accuracies of 1.6 +/- 1.4 % after 78 +/- 9 ms, 2.1 +/- 1.1 % after 74 +/- 12 ms and 5.0 +/- 2.6 % after 41+/- 20 ms, respectively. When adjusting the controller towards a fast temperature rise, target temperatures were reliably reached within 30 ms with fluctuations thereafter of about +/- 1 °C around the target temperature.

Conclusions : A device automatically controlling the temperature rise during retinal laser therapies has been developed and demonstrated. It can be adapted to every commercially available continuous wave retinal laser. The method allows a reliable thermal impact for visible or subvisible coagulations and especially for sublethal hyperthermia.

This is a 2021 ARVO Annual Meeting abstract.