Abstract
Purpose :
Photocoagulation of the retina is currently performed with different continuous wave (cw) and pulsed laser modalities. It is well known that the extent of laser induced tissue damage differs by inter and intraocular variations of scattering and fundus pigmentation. However, the influence of different laser modalities on the tissue denaturation has not been investigated in detail so far. Therefore, this work investigates the induced damage range on retinal pigment epithelium (RPE) explants between cw and pulsed irradiation in correlation to the temperature rise measured by optoacoustics (OA).
Methods :
RPE/choroid/sclera explants from porcine eyes were irradiated via laser slitlamp on a 200 µm spot diameter with a cw laser at 532 nm wavelength, and with a Q-switched frequency doubled Nd:YLF laser at 523 nm with a pulse duration of 160 ns at a repetition rate of 10 kHz. The pulsed heating excites thermoelastic pressure waves, which contain the present RPE temperature. The pressure waves were detected with an ultrasonic transducer embedded in a contact lens and processed for real-time temperature evaluation. In an open-loop setup, the power of the cw laser and the pulsed laser were kept constant until the end of the irradiation period, where the final temperature was noticed. For a closed-loop feedback control, the pulse energy was actively modulated with respect to quickly achieve a desired aim temperature and by then keep this temperature constant for the rest of the irradiation time. The laser-induced RPE-cell damage was evaluated with Calcein-AM assay.
Results :
Temperatures up to 70°C could be achieved in both modalities as well as with open- and closed-loop temperature-controlled irradiations. An average power of 60 mW in an open-loop setup for example revealed the same temperature of 44 °C in the cw mode as well as in the pulsed mode at the end of an 800 ms irradiation period at the same site of a low absorbing sample. The slopes of the temperature rise were almost identical.
Conclusions :
It is shown that cw as well as pulsed irradiation with high repetition rate can be used to heat the tissue in a similar way. When using the same average laser power, the same temperature rise is achieved. The pulsed heating provides the unique feature that it additionally excites optoacoustic pressure waves, which can be used for real-time temperature monitoring and control.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.