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R Brinkmann, N Koop, M Oezdemir, C Alt, G Schuele, CP Lin, R Birngruber; Selective RPE Damage By Means of a Rapidly Scanned cw Laser Beam . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2535.
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Purpose: Selective photodamage of the retinal pigment epithelium (RPE) is a new technique to treat a variety of retinal diseases which are thought to be associated with a declined function of the RPE. By repetitively irradiating the fundus with a train of green µs-laser pulses, only the strong absorbing RPE cells are damaged while surrounding tissue, especially the photoreceptors are spared. The aim of this study was to investigate selective RPE damage thresholds by an alternative approach: We used a cw laser system and repetitively scanned the beam across the RPE with the corresponding speed to obtain µs- illumination times. Methods: The radiation of an Ar+ laser (514 nm) was transmitted through a 25 µm core diameter fiber to a self developed scanner unit. The beam with a spatially top-hat spot diameter of 18 µm was repetitively scanned across porcine RPE samples in vitro providing an irradiation time of 1.6 µs per point on the central scan axis. The threshold radiant exposures for RPE-cell damage for different number of exposures were investigated by means of a fluorescence viability assay. Results: The damage threshold radiant exposure was found to be 306 mJ/cm2 when applying 10 exposures with a repetition rate of 500 Hz. The threshold decreases with the number of exposures, a saturation was found at 135 mJ/cm2 with more than 500 scans applied. The depth of focus in the axial direction, defined by an increase of the threshold radiant exposure by 20%, was 350 µm. Conclusion: It could be demonstrated that selective RPE cell damage can be achieved with a laser scanning system. The damage threshold radiant exposures are higher than for the µs pulsed application. However, a clinical application at a slit-lamp or with a retina laser scanning device seems to be an attractive alternative to the pulsed irradiation due to the higher flexibility of a scanning device with respect to the irradiation geometry and pattern.
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