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Retinal Microvasculature Imaging Research Based-on the Liquid Crystal Adaptive Optics

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Tutor: XuanLi
School: Graduate Schoo,Chinese Academy of Sciences
Course: Optics
Keywords: Adaptive optics,Liquid crystal wave-front corrector,Retinalmicrovascular imaging
CLC: O439
Type: PhD thesis
Year:  2013
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The retinal capillaries are the only microvasculature which could be imaged invivo and non-invasively. Any blood pathological destruction, or rather any organs andendocrine diseases will change the blood components and lead to vasculopathy. So thehigh resolution retinal microvacular imaging is significant for pathology research anddiagnose of the critical disease. However, due to aberration of human eyes, theresolution of the common inspection equipment was only about15-20¦Ìm, which canminot meet the demand for early diagnosis. The application of adaptive optics couldresolve the limit of human eye aberration. With AO, the imaging resolution could beclose to the optical diffraction limit. Although deformable mirror was widely used inadaptive optics systems, but it is less practicality in the field of fundus imagingbecause of its low spatial resolution, small stroke,large size and costly. In this paper,the liquid crystal spatial light modulator has been used as a wavefront corrector andthe high resolution retinal microvasculature imaging system based on liquid crystaladaptive optics has been studied.In order to improve the universality of the system, the factors,which affect theaccuracy of wavefront sensor has been discussed comprehensively.The deep low orderaberrations of the human eye has been corrected by trial lens. And then, the focalplane of the illumination light source has been adjusted to the fundus to increase the signal to noise ratio of the wavefront sensor. The position of the focal plane of theillumination light source can be monitored by introducing a parameter, which isrelated to the morphological characteristics of the light spot in the wavefront sensor.On this basis, the modified centroid algorithm has been used and the depth of focus ofthe illumaination light has been shortened to further eliminate the influence of theunmodulated beam and the reflection of the multilayers in retina.The mechanism of the retinal microvasculature and the stability of the humaneyes has been analysed through experiments. The system has been designed based on0diopter. After compansation the chromatic aberrations of the eye, which scale isabout1Diopter, the image plane of the capillaries has been searched by theharmlessness near infrared light source at the same time of imaging. Living humaneye fundus image experiments were carried out and the clear images of the retinalcapillaries, which diameters are about7¦Ìm have been obtained.On the basis of the above analysis and testing, the polarization properties of thehuman eye has been discussed. By using the polarized light to illuminate the fundus,the energy utilization of the light reflected from fundus has been improved by20%.Further more, the actual resolution of the system has been discussed and the resultshows that the actual resolution of the system is about3.5micrometers. It indicatedthat the system has the ability to distinguish the smallest retinal capillaries. Because ofthe center obscuration ratio of the annular stop has been optimized, the contrast of theimage has been increased from0.13to0.23. In experiments, the fundus had beenexposured by short pulse, in order to avoid the image blur caused by eye movements.The signal to noise ratio has been improved by the superposition of fundus imagesand the contrast of the image after superposition has beed improved to0.3.
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