Photocoagulation is a laser treatment widely used for the therapy of several retinal diseases. Intra- and inter-individual
variations of the ocular transmission, light scattering and the retinal absorption makes it impossible
to achieve a uniform effective exposure and hence a uniform damage throughout the therapy. A real-time
monitoring and control of the induced damage is highly requested. Here, an approach to realize a real time
optical feedback using dynamic speckle analysis is presented. A 532 nm continuous wave Nd:YAG laser is
used for coagulation. During coagulation, speckle dynamics are monitored by a coherent object illumination
using a 633nm HeNe laser and analyzed by a CMOS camera with a frame rate up to 1 kHz. It is obvious that
a control system needs to determine whether the desired damage is achieved to shut down the system in a
fraction of the exposure time. Here we use a fast and simple adaption of the generalized difference algorithm
to analyze the speckle movements. This algorithm runs on a FPGA and is able to calculate a feedback value
which is correlated to the thermal and coagulation induced tissue motion and thus the achieved damage. For
different spot sizes (50-200 μm) and different exposure times (50-500 ms) the algorithm shows the ability to
discriminate between different categories of retinal pigment epithelial damage ex-vivo in enucleated porcine
eyes. Furthermore in-vivo experiments in rabbits show the ability of the system to determine tissue changes in
living tissue during coagulation.
Katharina Bliedtner, Eric Seifert, Leoni Stockmann, Lisa Effe, and Ralf Brinkmann, "Towards real time speckle controlled retinal photocoagulation," Proc. SPIE 9693, Ophthalmic Technologies XXVI, 96931A (Presented at SPIE BiOS: February 14, 2016; Published: 5 April 2016); https://doi.org/10.1117/12.2212703.
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