Laser photocoagulation has been a treatment method for retinal diseases for decades. Recently, studies have demonstrated therapeutic benefits for subvisible effects. A treatment mode based on an automatic feedback algorithm to reliably generate subvisible and visible irradiations within a constant irradiation time is introduced. The method uses a site-individual adaptation of the laser power by monitoring the retinal temperature rise during the treatment using optoacoustics. This provides feedback to adjust the therapy laser power during the irradiation. The technique was demonstrated on rabbits in vivo using a 532-nm continuous wave Nd:YAG laser. The temperature measurement was performed with 523-nm Q-switched Nd:YLF laser pulses with 75-ns pulse duration at 1-kHz repetition rate. The beam diameter on the fundus was 200 μm for both lasers, respectively. The aim temperatures ranged from 50°C to 75°C in 11 eyes of 7 rabbits. The results showed ophthalmoscopically invisible effects below 55°C with therapy laser powers over a wide range. The standard deviation for the measured temperatures ranged from 2.1°C for an aim temperature of 50°C to 4.7°C for 75°C. The ED50 temperature value for ophthalmoscopically visible lesions in rabbits was determined as 65.3°C. The introduced method can be used for retinal irradiations with adjustable temperature elevations.
The induced thermal damage in retinal photocoagulation depends on the temperature increase and the time of irradiation. The temperature rise is unknown due to intraocular variations in light transmission, scattering and grade of absorption in the retinal pigment epithelium (RPE) and the choroid. Thus, in clinical practice, often stronger and deeper coagulations are applied than therapeutically needed, which can lead to extended neuroretinal damage and strong pain perception. This work focuses on an optoacoustic (OA) method to determine the temperature rise in real-time during photocoagulation by repetitively exciting thermoelastic pressure transients with nanosecond probe laser pulses, which are simultaneously applied to the treatment radiation. The temperature-dependent pressure amplitudes are non-invasively detected at the cornea with an ultrasonic transducer embedded in the contact lens. During clinical treatment, temperature courses as predicted by heat diffusion theory are observed in most cases. For laser spot diameters of 100 and 300 μm, and irradiation times of 100 and 200 ms, respectively, peak temperatures range between 70°C and 85°C for mild coagulations. The obtained data look very promising for the realization of a feedback-controlled treatment, which automatically generates preselected and reproducible coagulation strengths, unburdens the ophthalmologist from manual laser dosage, and minimizes adverse effects and pain for the patient.
Retinal photocoagulation is a long time established treatment for a variety of retinal diseases, most commonly applied for
diabetic macular edema and diabetic retinopathy. The damage extent of the induced thermal coagulations depend on the
temperature increase and the time of irradiation. So far, the induced temperature rise is unknown due to intraocular
variations in light transmission and scattering and RPE/choroidal pigmentation, which can vary inter- and intraindividually
by more than a factor of four. Thus in clinical practice, often stronger and deeper coagulations are applied than
therapeutically needed, which lead to extended retinal damage and strong pain perception. The final goal of this project
focuses on a dosimetry control, which automatically generates a desired temperature profile and thus coagulation
strength for every individual coagulation spot, ideally unburden the ophthalmologist from any laser settings. In this paper
we present the first realtime temperature measurements achieved on patients during retinal photocoagulation by means of
an optoacoustic method, making use of the temperature dependence of the thermal expansion coefficient of retinal tissue.
Therefore, nanosecond probe laser pulses are repetitively and simultaneously applied with the treatment radiation in
order to excite acoustic waves, which are detected at the cornea with an ultrasonic transducer embedded in the contact
lens and then are processed by PC.