Visualizing retinal photocoagulation by real-time OCT measurements may considerably improve the understanding of thermally induced tissue changes and might enable a better reproducibility of the ocular laser treatment. High speed Doppler OCT with 860 frames per second imaged tissue changes in the fundus of enucleated porcine eyes during laser irradiation. Tissue motion, measured by Doppler OCT with nanometer resolution, was correlated with the temperature increase, which was measured non-invasively by optoacoustics. In enucleated eyes, the increase of the OCT signal near the retinal pigment epithelium (RPE) corresponded well to the macroscopically visible whitening of the tissue. At low irradiance, Doppler OCT revealed additionally a reversible thermal expansion of the retina. At higher irradiance additional movement due to irreversible tissue changes was observed. Measurements of the tissue expansion were also possible in vivo in a rabbit with submicrometer resolution when global tissue motion was compensated. Doppler OCT may be used for spatially resolved measurements of retinal temperature increases and thermally induced tissue changes. It can play an important role in understanding the mechanisms of photocoagulation and, eventually, lead to new strategies for retinal laser treatments.

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.

SRT induces a cytokine profile, which may improve the flux across Bruch's membrane, slows down progression of early AMD by RPE regeneration, and inhibits the formation of choroidal neovascularization. The cytokine release depends on the size and density of applied laser spots.

Laser coagulation is a treatment method for many retinal diseases. Due to variations in fundus pigmentation and light scattering inside the eye globe, different lesion strengths are often achieved. The aim of this work is to realize an automatic feedback algorithm to generate desired lesion strengths by controlling the retinal temperature increase with the irradiation time. Optoacoustics afford non-invasive retinal temperature monitoring during laser treatment. A 75 ns/523 nm Q-switched Nd:YLF laser was used to excite the temperature-dependent pressure amplitudes, which were detected at the cornea by an ultrasonic transducer embedded in a contact lens. A 532 nm continuous wave Nd:YAG laser served for photocoagulation. The ED50 temperatures, for which the probability of ophthalmoscopically visible lesions after one hour in vivo in rabbits was 50%, varied from 63°C for 20 ms to 49°C for 400 ms. Arrhenius parameters were extracted as ΔE=273 J mol(-1) and A=3 x 10(44) s(-1). Control algorithms for mild and strong lesions were developed, which led to average lesion diameters of 162 ± 34 μm and 189 ± 34 μm, respectively. It could be demonstrated that the sizes of the automatically controlled lesions were widely independent of the treatment laser power and the retinal pigmentation.

ABSTRACT.Purpose: To examine spectral domain optical coherence tomographic (OCT) and histological images from comparable retinal photocoagulation lesions in rabbits, and to correlate these images with comparable OCT images from patients. Methods: 508 rabbit lesions were examined by HE-stained paraffin histology. 1019 rabbit lesions versus 236 patient lesions were examined by OCT, all at the time-points 1 hr, 1 week and 4 weeks after photocoagulation. We analysed 100 lm lesions (in humans) and 133 lm lesions (in rabbits) of 200 ms exposures at powers titrated from the histological threshold up to intense damage. Lesions were matched according to morphological criteria. Results: Dome-shaped layer alterations, retinal infiltration by round, pigmented cells, outer nuclear layer interruption, and eventually full thickness retinal coagulation are detectable in histology and OCT. Horizontal damage extensions are found 1½ times larger in OCT. More intense irradiation was necessary to induce comparable layer affection in rabbit OCT as in histology. Restoration of the inner retinal layers is only shown in the OCT images. Comparable primary lesions caused more pronounced OCT changes in patients than in rabbits during healing. Conclusions: Optical coherence tomographic images indicate different tissue changes than histologic images. After photocoagulation, they show wider horizontal damage diameters, but underestimate axial damage particularly during healing. Conclusions on retinal restoration should not be drawn from OCT findings alone. Retinal recovery after comparable initial lesions appears to be more complete in rabbit than in patient OCTs.