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

Diameter of the smallest retinal vessel is of about 4~6μm. Many human diseases(such as diabetes, hypertension, etc.) will cause the blood constituent changes, resulting in changes of retinal micro-vascular morphology. Therefore, the retinal micro-vascular imaging can provide valuable information for early diagnosis of disease and health screening. Aberrations can be corrected real-time using adaptive optics technology. The resolution can be up to 3μm under 6mm pupil diameter. The wavefront corrector is commonly used in adaptive optical system is the deformable mirror. In recent years, the liquid crystal wavefront corrector is applied in the retinal imaging with the advantages of its high resolution, small volume. But the imaging quality of retinal micro-vascular, especially for about 6mm diameter is not good, which can not satisfy the demands of clinical diagnosis. In retinal micro-vascular imaging there are some problems, which are the difficulties of the lighting focal plane, the imaging focal plane and the low contrast. In order to get the high resolution imaging of retinal micro blood vessels, this paper combine the optical characteristics of the eye, and improve micro-vascular focal plane and micro-vascular imaging contrast were studied.According to the physiological characteristics of human eyes, the visual accommodation errors can be eliminated using the visual target that locate in front ofthe eye of 1 meter; when the human see clearly the target and the Shack Hartmann wavefront sensor is used as an auxiliary means, the defocus aberration PV value controlled at less than 0.1μm, the illumination light can accurately focus the focal plane; Providing a method to calculate the axial magnification of the liquid crystal adaptive optics imaging system. With the different axial length of the individual, the magnification of the system can be calculated quickly and effectively. According to the distribution characteristics of micro-vascular and the photoreceptor as the reference, the camera can be moved to the vicinity of micro-vascular imaging focal plane. In addition, using the designing of telecentric structure of 1D light target, we can achieve precisely positioning of the horizontal position of the fundus.Analyzing the relationship between the isoplanatic patch and wavelength, selecting 561 nm light as the micro-vascular imaging light source, the isoplanatic patch is of 0.8 degree; the axicon lens is designed to form the annular illumination. Avoiding the loss of energy and realizing the size of the annular illumination light continuous adjustable, which make it can be applied to the different size of the pupil; Simulating the effect of the lateral shift of the pupil in wavefront detection. When the lateral shift is less than 0.1mm, the shift of the light spot on the Shack- Hartmann is less than 1/6 of the sub aperture size, the wavefront reconstruction residual error is less than λ/14, so the effect can be ignored; Based on the vascular focal plane positioning method, designing with the focal plane positioning function of retinal micro-vascular liquid crystal adaptive optics imaging system. Using the 808 nm light for accurately positioning, and 561 nm light for high contrast imaging, finally the micro-vascular imaging experiments are carried out, retinal micro-vascular images are obtained.Based on the accurate positioning of the micro-vascular, to improve the imaging quality of retinal micro-vascular, we analyze the influencing factors of retinal vascular imaging contrast. Quantitative analysis of the effect of intraocular stray light on the retinal image quality, the use of polarized light illumination, 1D light target, and the confocal pinhole, we can weaken the effect of the intraocular straylight; Using depolarization imaging, eliminating the influence of the reflected light of the vessel wall, increasing the intensity difference between the vessel and the background, the contrast of the image is improved from 0.11 to 0.25, increased 1.3 times.Through the above research, it is easy to position the retinal micro-vascular focal plane. We design the adaptive optics retinal imaging system, which has the function of focal plane positioning, based on the liquid crystal wavefront corrector. Using this imaging system, we get the high resolution micro-vascular images.