Hao

Does Retinal Vascular Geometry Vary with Cardiac Cycle

PURPOSE Changes in retinal vascular parameters have been shown to be associated with systemic vascular diseases. In this study, we assessed the physiologic variations in retinal vascular measurements during the cardiac cycle. METHODS Fundus images were taken using electrocardiogram-synchronized retinal camera at nine distinct cardiac points from 15 healthy volunteers (135 images). Analyses of retinal vessel geometric measures, including retinal vessel caliber (individual and summary), tortuosity, branching angle, length-diameter ratio (LDR), and optimality deviation, were performed using semiautomated computer software. Repeated-measures ANOVAs were used to obtain the means and to estimate the variation of each cardiac point compared with cardiac point 1. RESULTS There was a significant variation of the caliber of the individual arteriolar and venular vessels. However, there was no significant variation found for vessel caliber summary, represented by the central retinal arteriolar equivalent (CRAE) and the central retinal venular equivalent (CRVE). There was also no significant variation found for tortuosity and branching angle, and LDR showed none or very little variations at different cardiac points: variations in caliber ranges between 0 and 4.1%, tortuosity 0 and 1.5%, branching angle 0 and 3.5%, and LDR 0 and 2%; all values for variations, P > 0.1; linear trend, P > 0.5; and nonlinear trend, P > 0.8. CONCLUSIONS This study showed that there were minimal variations in the CRAE, CRVE, tortuosity, and branching angle that are clinically used for two-dimensional measures of retinal vascular geometry during cardiac cycles. However, there was significant variation in the caliber of the individual vessels over the cardiac cycle.

Zone Specific Fractal Dimension of Retinal Images as Predictor of Stroke Incidence

Fractal dimensions (FDs) are frequently used for summarizing the complexity of retinal vascular. However, previous techniques on this topic were not zone specific. A new methodology to measure FD of a specific zone in retinal images has been developed and tested as a marker for stroke prediction. Higuchis fractal dimension was measured in circumferential direction (FDC) with respect to optic disk (OD), in three concentric regions between OD boundary and 1.5 OD diameter from its margin. The significance of its association with future episode of stroke event was tested using the Blue Mountain Eye Study (BMES) database and compared against spectrum fractal dimension (SFD) and box-counting (BC) dimension. Kruskal-Wallis analysis revealed FDC as a better predictor of stroke (H = 5.80, P = 0.016, α = 0.05) compared with SFD (H = 0.51, P = 0.475, α = 0.05) and BC (H = 0.41, P = 0.520, α = 0.05) with overall lower median value for the cases compared to the control group. This work has shown that there is a significant association between zone specific FDC of eye fundus images with future episode of stroke while this difference is not significant when other FD methods are employed.

A method for visualization of fine retinal vascular pulsation using nonmydriatic fundus camera synchronized with electrocardiogram.

Pulsatile changes in retinal vascular geometry over the cardiac cycle have clinical implication for diagnosis of ocular and systemic vascular diseases. In this study, we report a Vesselness Mapping of Retinal Image Sequence (VMRS) methodology to visualize the vessel pulsation and quantify the pulsatile motions in the cardiac cycle. Retinal images were recorded in an image sequence corresponding to 8 segments of the cardiac cycle using a nonmydriatic fundus camera (Canon CR45, Canon Inc., Japan) modified with ECG-synchronization. Individual cross-sectional vessel diameters were measured separately and the significance of the variations was tested statistically by repeated measures analysis of variance (ANOVA). The graders observed an improved quality of vessel pulsation on a wide region around the optic disk using the VMRS. Individual cross- sectional vessel diameter measurement after visualization of pulsatile motions resulted in the detection of more significant diameter change for both arterioles (3.3 μm, P = 0.001) and venules (6.6 μm, P < 0.001) compared to individual measurement without visualization of the pulsatile motions (all P values > 0.05), showing an increase of 2.1 μm and 4.7 μm for arterioles and venules, respectively.

Retinal Vessel Diameter Measurement Using Unsupervised Linear Discriminant Analysis

An automatic vessel diameter measurement technique based on linear discriminant analysis (LDA) has been proposed. After estimating the vessel wall, the vessel cross-section profile is divided into three regions: two corresponding to the background and one to the vessel. The algorithm was tested on more than 5000 cross-sections of retinal vessels from the REVIEW dataset through comparative study with the state-of-the-art techniques. Cross-correlation analyses were performed to determine the degree to which the proposed technique was close to the ground truth. The results indicate that proposed algorithm consistently performed better than most of other techniques and was highly correlated with the manual measurement as the reference diameter. The proposed method does not require any supervision and is suitable for automatic analysis.

Retinal vessel diameter measurement using multi-step regression method

Study of retinal blood vessel features, especially the caliber, has been widely used for risk level assessment of certain disease conditions. In this work a vessel diameter measurement technique based on Gaussian modeling has been proposed. This method adaptively combines a series of second and higher order Gaussians to model the vessel profile and uses the sigma parameter of the best fitted Generalized Gaussians to the boundaries, to measure the vessel caliber. One advantage of this technique is that, unlike other methods, it does not assume the vessel profile as a symmetrical Gaussian-like shape with similar blurriness levels at the edges. The technique has been tested on 580 cross-sections from normal and pathological public datasets with mixed quality fundus images. Comparison was made against two gold standard techniques and a ground truth obtained manually by three experts to sub-pixel accuracy.

Automatic retinal vessel profiling using multi-step regression method

Caliber of the retinal blood vessel is widely used for risk assessment of cardiovascular diseases. Accurate and automatic caliber measurement requires a precise model to be made for the vessel profile. In this paper, we present a new approach for retinal vessel profiling in which the background noise, uneven illuminations and specular reflections have all been considered. In this method, regression analysis is performed with a series of second-order Gaussians to filter and up-sample the original vessel profile. This is then segmented to identify and represent the vessel edges by two Generalized Gaussian functions. The technique has been applied to retinal images and the results have been verified and compared with the state of the art automatic techniques.

Improved retinal photography method and visualization of multiple retinal images

The current instantaneous retinal photography method has large variations in the retinal vessel diameter measurement due to retinal vessel pulsation which has been reported associated to the cardiac cycle. The previous limited studies show limitation on their methodologies. To better understand and analyze the variation in the cardiac cycle, we developed an Electrocardiogram (ECG) synchronized retinal photography method which overcomes the previous limitations. Our method is able to detect ECG signal and calculate R-R interval in real time. The retinal camera can be triggered electronically and automatically at the designated time delays based on the real-time calculated R-R interval. Our results demonstrate this method is able to provide the visualization of the retinal vessel pulsations. It will be helpful for studying the relation of ophthalmology and cardiology. It also provides a possible solution to eliminate the cardiac cycle related photography variation in the retinal images analysis.

Using color histogram as the trait of retina biometric

Retina biometric has applications on high security access control systems. The interference of lighting and deformation of viewpoint in the retina image acquisition have impact on the process of retina feature extraction and matching. In this paper, we propose a method using color histogram as the trait of retina biometric. The color histogram has shown the feature of deformation invariant. The enhancement using discrete wavelet transform decomposition minimizes the difference within the individuals and increases the correlation between the retinal images of the same person. This method provides a solution against the variation within the individuals and lower false rejection rate in the retina biometric.

Loss of calibre information during vessel segmentation

To obtain precise calibre measurements, there are two general approaches: segment the vessels and measure the diameter, and to measure the diameter from the greyscale images. While there is a claim that vessel segmentation does not solve diameter measurement problems it has not been proven experimentally. The aim of this work is two-fold: first, to prove that the resulting edges from good vessel segmentation may not be suitable for precise diameter measurement, and, second, to propose a method for compensating for the imprecision in the calibre measurement from the segmentation.

Mathematical verification of summary formula in retinal vessel diameter measurement

Our previous study found the retinal vessel diameter measurement using summary formula has no significant difference for the multiple retinal images of the same person, while the individual vessel measures show the significant difference. It was speculated that the revised formula summarizing the individual measures and therefore against variation. This paper uses the mathematical models to verify this speculation. The impacts of the sample size, branching coefficients, different graders and grader/systemic errors are analyzed and discussed. Our results confirmed the revised formula is robust against the variation which exists on the individual measures. However, the results also demonstrate 1) the revised formula is not able to remove the systemic errors and the variations caused by the different graders. 2) the measures summarized by the revised formula may not be suitable for monitoring the changes on the individual retinal vessels.