Jianxin Shen

High-resolution retinal imaging with micro adaptive optics system

Based on the dynamic characteristics of human eye aberration, a microadaptive optics retina imaging system set is established for real-time wavefront measurement and correction. This paper analyzes the working principles of a 127-unit Hartmann-Shack wavefront sensor and a 37-channel micromachine membrane deformable mirror adopted in the system. The proposed system achieves wavefront reconstruction through the adaptive centroid detection method and the mode reconstruction algorithm of Zernike polynomials, so that human eye aberration can be measured accurately. Meanwhile, according to the adaptive optics aberration correction control model, a closed-loop iterative aberration correction algorithm based on Smith control is presented to realize efficient and real-time correction of human eye aberration with different characteristics, and characteristics of the time domain of the system are also optimized. According to the experiment results tested on a USAF 1951 standard resolution target and a living human retina (subject ZHY), the resolution of the system can reach 3.6 LP/mm, and the human eye wavefront aberration of 0.728λ (λ=785 nm) can be corrected to 0.081λ in root mean square (RMS) so as to achieve the diffraction limit (Strehl ratio is 0.866), then high-resolution retina images are obtained.

Automatic fundus images mosaic based on SIFT feature

An automatic fundus image mosaic algorithm based on SIFT feature is presented, to overcome the characteristics of low contrast and the geometric distortion between different field of view of the fundus images. The SIFT feature of fundus images are extracted and described using vectors to determine the matching feature point pairs between two images, outlier point pairs are rejected using MLESAC algorithm employed RST model, the transformation matrix is then computed according to purified matching point pairs between images, at last image registration and image mosaic is implemented. The mosaic results of multiple images obtained by fundus camera show that the algorithm has good robustness and stability, and high-precision automatic fundus image mosaic can be achieved.

Research on human retinal cell image restoration

In order to improve the observation effect and further study on the human retinal cell image, image processing technology is used for restoration from the distortion and degradation of images, which are acquired through retinal cell imaging system based on adaptive optics. In this paper, two commonly used image restoration algorithms were introduced: Wiener filter and Lucy-Richardson. On this basis, a semi-blind deconvolution image restoration algorithm based on Sobel operator is proposed. The edges detected by Sobel operator were processed differently from other region, which could both effectively remove the noise, and maintain the image detail well. Finally, frequency domain function and point sharpness function are introduced to evaluate the images which are restored with the above algorithms, the results show that the Sobel based semiblind deconvolution algorithm can achieve quite better results in restoration of cell image.

Human Eye Aberration Measurement and Correction Based on Micro Adaptive Optics System

In view of the characteristics of human eye aberrations, a set of adaptive optics system for real-time wave- front measurement and correction is designed. The system realizes wave-front reconstruction based on the model reconstruction algorithm of Zernike polynomials by obtaining the spot pattern of human eye wave-front aberrations with the aid of Hartmann-Shack wave-front sensor and accessing to high precision centroid data by adaptive centroid detection method. Meanwhile, the need for real-time correction of different characteristics of human eye aberrations is met through the flexible adjustment of parameters of α, β and W with the aid of closed-loop iterative control algorithm based on singular value decomposition, combined with the features influencing function matrix and human eye wave-front aberrations, with micro- machined membrane deformable mirror serving as wave-front corrector. The measurement and correction of wave-front aberrations of human eyes is experimented on the platform of micro adaptive optics system, the result of which indicates that the system can meet accurate measurement and real-time correction of human eye wave-front aberrations with a wide- range application prospect in ophthalmic adaptive optics technology. Keywords-human eye wave-front aberration; adaptive optics; micro-machined membrane deformable mirror; Hartmann-Shack wave-front sensor; centroid detection; wave-front reconstruction; wave-front correction

Fast automatic fundus images registration and mosaic based on Compute Unified Device Architecture

In order to overcome the characteristics of low contrast, non-uniform illumination, limited field of view (FOV), and the geometric distortion between different FOV of the fundus images, a fast automatic fundus image registration and mosaic algorithm based on Compute Unified Device Architecture (CUDA) is presented. Firstly fundus images are enhanced by homomorphism filtering, then the Scale Invariant Feature Transform (SIFT) features in effective FOV are extracted and matched between images with CUDA speeded up. With CUDA application, point pairs are purified using random sample consensus (RANSAC) algorithm employed perspective model, transformation matrixes are computed according to the matching point pairs of surrounding FOV images to the central, image registration and image fusion is implemented to get fundus panoramic image finally. The automatic registration and mosaic results of multiple FOV images obtained by fundus camera show that the algorithm is robust and stability with registration accuracy up to pixel level, the algorithm speed upgrade 10 to 30 times, high-precision automatic fundus image mosaic can be achieved.

High Resolution Retinal Imaging Based on Ocular Aberration Measurement and Compensation

Ocular retinal imaging is a major diagnostic modality for retinal disease, and can play a critical role for diagnosing systemic diseases such as diabetes and eye-specific diseases such as macular degeneration and diabetic retinopathy, the leading causes of blindness. In order to get high-resolution retinal imaging and develop the low-cost and compact retinal imaging system, we employ micro adaptive optics, which is consisted of wavefront sensor, wavefront corrector and control system. In this paper, the theory, design and testing of the ocular retinal microscopy is detailed, with an emphasis on the eye wavefront aberration describing, aberration detecting method with Hartmann-Shack wavefront sensing and close-loop aberration compensating by micromachined membrane deformable mirrors(MMDM).The ocular retinal microscopy experimental setup is built, the retinal cell imaging had been snapped. It is showed in this work that the ocular retinal microscopy based on adaptive optics system can enable diffraction-limited imaging of micro-scale features of the retina, through real-time compensation of aberrations introduced by the eye.

Wavefront Aberration Correction with Conjugated Combination Models of Zernike Modes

In view of the coupling relationship between the Zernike modes over the concentric circle domain within the unit circle and the wavefront counteraction of those coupling modes within the reconstruction pupil region, the linear combination models of Zernike modes are presented. The modes in each model share with negative correlation, same coefficient sign and prescribed amount. Finally, the experiments are implemented on AO system, the results show that it’s feasible and advantageous to use the combination models in the process of aberration correction.

An Adaptive Estimating Centroid Method for Measurement of Human Eye Aberration

A new adaptive method is given in this paper to estimate the centroid of images caught by Hartmann-Shack wavefront sensor after analyzing the reason of causing error. This method is based on the morphological filter, dynamic tracing of spot window, and iterative approximation algorithm. It is more accurate and adaptive than general methods. The experimental results show that the method is with strong anti- noise ability, and obviously improves the spot recognition rate and centroid detection accuracy, which effectively extends the applicability of Hartmann-Shack wavefront sensor.