Experimental And Theoretical Study On Biological Effect Of 532nm Laser On Rabbit Retina

Objective 532nm laser is the optimal light source in HMME-PDT on benign vascular diseases. In order to apply HMME-PDT to CNV, the detailed experimental and theoretical data on biological effect induced by 532nm laser on retina is necessary. So, the aim of this study is to evaluate the relation between irradiation dose of 532nm laser and effect on ratina in rabbits, and analyze mechanisms of laser irradiation by establishing retinal thermal model, which is utilized for predicting temperature distribution.Method & Results (1) Experimental study on biological effect of 532nm laser on rabbit retina: twenty-five New Zealand albino rabbits and four Chichilan grey rabbits underwent irradiation with a frequency-doubled Nd:YAG laser (532 nm) or copper vapour laser( 578.2nm) coupled to a slit lamp biomicroscope using a spot size of 2mm. The power density ranged from 500 to 2000 mW/cm2 with exposure time of 100 to 300 seconds. Fundus photography, fluorescein angiography, light and electron microscopy were performed on rabbit eyes at lh, 24h, 3d, 7d, 14d and 28d after irradiation. Severe lesions companied with subretinal hemorrhages were observed 24h after laser irradiation of 200-300 seconds with power density more than 1600 mW/cm~2. Strong dye leakage and blocked fluorescence were also noted in the lesion. All layers of retina were destroyed. The choriocapillaris and large choroidal vessels were occluded by thrombosis. Moderate retinal discoloration and hemorrhages could be observed 24h after laser irradiation with power density between 1200 mW/cm~2 and 1600 mW/cm~2 for 100-200 seconds, or with 1000 mW/cm~2 for 300 seconds. Fluorescein angiography showed dye leakage and mild blocked fluorescence. The outer and inner segments were destroyed and vacuolated; outer and inner nuclear layers were pyknotic and exudation was noted in subretinal space. The power density needed to produced mild lesions or dye leakage were 1000-1200mW/cm~2 for 100-200 seconds. The inner and outer photoreceptor segments appeared short and diaorganized,while outer nuclear cells were strikingly absent. The power density was a key factor to determine the level retinal damage under the same energy density. For example, the retinal damage was more severe (P<0.05) with power density of 2000mW/cm~2 (t=100s) than power density of 1000mW/cm~2 (t=200s). In albino rabbits, the threshold dost was 119.58J/cm~2 with the exposure duration of 100s, while the threshold dose was 76.47J/cm~2 in pigmented rabbits. There was 1.56-fold threshold dose difference between pigmented and albino rabbits. When light microscopic examination was used, the minimal visible lesions of albino rabbits were produced with power density of 800mW/cm~2 and energy density of 80J/cm~2. Electron micrographs of the lamellar structure of outer segments showed disrupted but recovered 28 days after irradiation with power density of 600mW/cm~2and energy density of 60J/cm~2. There were more serious lesions, which appeared earlier, produced by 578.2nm laser than 532nm laser, especially in the choroid. The damage was most severe 24h after laser irradiation, then retinal repair could be noted and hemorrhage or exudation was absorbed. Outer and inner nuclear layer and photoreceptor recovered gradually and fiber regeneration also underwent.( 2 ) Mathematical model for laser induced thermal effect on retina: Homogeneous layer retinal models of albino and pigmented rabbits were presented to analyze the light distribution. Based on retinal optical and thermal properties, laser heat source term expression was modified and a convective heat loss due to blood flow in the choroid was included in heat transfer equation. The finite element method realized by Matlab software was used to solve the heat transfer nonlinear partial differential equation. The retinal temperature was calculated with different laser parameters such as different wavelength (532nm, 578nm and 690nm), power density (200-2000 mW/cm~2), spot size (lmm, 2mm and 3mm) and different pigmented fundus. Thermal damage integral (Q) was also calculated by Arrhenius equation. The temperature increased and reached heat balance in a few seconds following laser irradiation, then decreased to normal body temperature in a few seconds after the exposure was ended. The more power density was, the higher retinal temperature was induced. The balanced temperature reduced exponentially when the distance in the...