Abstract
Purpose :
Most laser therapies at the retina are performed with continuous wave lasers (CWL) and micropulsed lasers (MPL) using 500 Hz repetitive pulsing. However, proper dosing is a severe problem when aiming for less damaging, invisible applications. Therefore test lesions near the vascular arcades are applied to achieve visible spots. Based on this a reduced power for the macular target area is calculated. Due to the strong intraocular variations of light scattering and fundus pigmentation, this strategy is highly error-prone. Since objective real-time optoacoustic (OA) temperature measurement and control systems have already been demonstrated, this work investigates the adaptation of such techniques for different laser systems.
Methods :
A low pulse energy Q-switched laser (QSL, Crystalaser, 160 ns pulse duration) can be used to induce temperature-depended ultrasound waves at the target area, which are measured with a transducer embedded in a Mainster contact lens. The QSL was optically coupled to a CWL (Zeiss Visulas 532s) and a MPL (Iridex IQ 577), respectively, and was also operated alone at 10 kHz serving for temperature elevation and measurement simultaneously. Porcine RPE/choroid/sclera explants served as probes and were irradiated via slitlamp on a 200 µm spot diameter. RPE-cell damage was evaluated with a Calcein-AM vitality assay.
Results :
Temperature measurements could be demonstrated with the QSL. The mean temperature rise was highly comparable between CWL and MPL, e.g. a temperature rise of 45 °C for an average power of 30 mW and an irradiation time of 200 ms was achieved with both systems, resulting in the same area of thermal damage. Comparison of the CWL with the QSL alone e.g. showed the same temperature rise over a range up to 800 ms with the same average power of 60 mW.
Conclusions :
Real-time temperature monitoring can be obtained for cw and micropulsed laser systems. With the same average power the same average temperature course and damage area are obtained, which likely will result in the same therapeutic effect. Thus temperature based laser power dosing seems very appropriate. For future applications and especially for low or non-damaging exposure, we propose the use of a stand alone high frequency QSL-laser, for which a close loop control onto a predefined temperature rise has already been demonstrated. A combined CWL/QSL system with closed loop automatic temperature control is already in clinical study.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.