Assessment of a Novel Semi-Automated Algorithm for the Quantification of the Parafoveal Capillary Network

Introduction

We present a novel semi-automated computerized method for the detection and quantification of parafoveal capillary network (PCN) in fluorescein angiography (FA) images.

Material and Methods

An algorithm detecting the superficial parafoveal capillary bed in high-resolution grayscale FA images and creating a one-pixel-wide PCN skeleton was developed using MatLab software. In addition to PCN detection, capillary density and branch point density in two circular areas centered on the center of the foveal avascular zone of 500μm and 750μm radius was calculated by the algorithm. Three consecutive FA images with distinguishable PCN from 56 eyes from 56 subjects were used for analysis. Both manual and semi-automated detection of the PCN and branch points was performed and compared. Three different intensity thresholds were used for the PCN detection to optimize the method defined as mean(I)+0.05*SD(I), mean(I) and mean(I)-0.05*SD(I), where I is the grayscale intensity of each image and SD the standard deviation. Limits of agreement (LoA), intraclass correlation coefficient (ICC) and Pearson's correlation coefficient (r) were calculated.

Results

Using mean(I)-0.05*SD(I) as threshold the average difference in PCN density between semi-automated and manual method was 0.197 (0.316) deg^-1 at 500μm radius and 0.409 (0.562) deg^-1 at 750μm radius. The LoA were -0.421 to 0.817 and -0.693 to 1.510 deg^-1, respectively. The average difference of branch point density between semi-automated and manual method was zero for both areas; LoA were -0.001 to 0.002 and -0.001 to 0.001 branch points/degrees², respectively. The other two intensity thresholds provided wider LoA for both metrics. The semi-automated algorithm showed great repeatability (ICC>0.91 in the 500μm radius and ICC>0.84 in the 750μm radius) for both metrics.

Conclusion

This semi-automated algorithm seems to provide readings in agreement with those of manual capillary tracing in FA. Larger prospective studies are needed to confirm the utility of the algorithm in clinical practice.

Ultrasound Elastography in Ocular and Periocular Tissues: A Review

Ultrasound elastography has become available in everyday practice, allowing direct measurement of tissue elasticity with important and expanding clinical applications. Several studies that have evaluated pathological and non-pathological tissues have demonstrated that ultrasound elastography can actually improve the diagnostic accuracy of the underlying disease process by detecting differences in their elasticity. Ocular and periocular tissues can also be characterized by their elastic properties. In this context, a comprehensive review of literature on ultrasound elastography as well as its current applications in Ophthalmology is presented.

Multimodal Imaging for the Assessment of Geographic Atrophy in Patients with "Foveal" and "No-Foveal" Sparing

INTRODUCTION

The aim of the study was to evaluate the applicability of optical coherence tomography (OCT) angiography (OCTA) for measuring geographic atrophy (GA) areas in age-related macular degeneration (AMD) patients with "foveal" and "no-foveal" sparing disease and compare it to other imaging modalities.

METHODS

A multimodal imaging protocol was applied, using infrared (IR) imaging, fundus autofluorescence (FAF), OCTA, and en-face OCT in 35 eyes of 23 AMD patients with GA. Patients were classified into 2 groups, with and without foveal sparing disease. GA area measurements for all imaging modalities were compared for each group separately.

RESULTS

The measured GA area was estimated to be 6.68 ± 3.18 mm2 using IR; 6.99 ± 3.09 mm2 using FAF; 6.56 ± 3.11 mm2 using OCTA, and 6.65 ± 3.14 mm2 using en-face OCT. There was no statistically significant difference in the GA area between different modalities (p = 0.977). When separate analysis was conducted for patients with "foveal" and "no-foveal" sparing disease, although GA measurements in FAF imaging displayed higher numerical values than the other modalities, especially in patients with foveal sparing, no statistically significant difference in the GA area was found between the different imaging modalities in either group (p = 0.816 for foveal sparing; p = 0.992 for no-foveal sparing group).

CONCLUSIONS

OCTA can be reliably used in the assessment of GA in AMD patients with and without foveal sparing disease. For both groups, measurements are comparable to IR, en-face OCT, and FAF, despite the fact that the latter recorded larger area of GA, mainly in the foveal sparing cases.

Quantification of effective orbital volume and its association with axial length of the eye. A 3D-MRI study

Objective: To measure the effective orbital volume (EOV) from magnetic resonance images, and investigate its relationship with axial length (AL) in those parameters. Methods: Cross-sectional, 3D-MRI study. 54 eyes of 54 patients (25 males) were included in this work. Patient weight, height and head circumference were also measured. Orbital and eyeball volumes were calculated after image segmentation. The difference between those values volume was assessed, estimating the EOV for each eye. Results: Mean eyeball volume was 7.83 ± 2.27 mm3, mean orbital volume 26.81 ± 0.59 mm3 and EOV 21.64 ± 0.19 mm3. The orbital volume was significantly higher in the male group (Wilcoxon signed-rank tests Z=-1.51, p<0.001; Z=-3.57, p<0.001 respectively). EOV was significantly correlated with AL in both males (r=-0.71, p<0.001) and females (r=-0.73, p<0.001), whereas it was also significantly associated with patient height (r=0.261, p=0.03). Associations between EOV and other age, axial and anthropometric characteristics were not statistically significant. Conclusions: Findings of this study could be of valuable importance in various clinical situations in which quantification of orbital volume is needed, such as orbital decompression in Graves' orbitopathy, volume restoration in orbital fractures or other orbital reconstructive surgery. In surgical interventions, clinical relationships should be carefully taken under consideration to avoid iatrogenic injury. Abbreviations: EOV = Effective orbital volume, AL = Axial length, ROI = Region of interest.

Comparison of digital color fundus imaging and fluorescein angiographic findings for the early detection of diabetic retinopathy in young type 1 diabetic patients

PURPOSE

To compare the findings from digital 7-field color fundus (CF) photography and fundus fluorescein angiography (FFA) in young patients with diabetes mellitus (DM) type 1 without known diabetic retinopathy.

METHODS

In this prospective, observational cohort study, 54 type 1 diabetic patients were recruited. Participants had been diagnosed with diabetes mellitus (DM) for at least 6 years, had Best Corrected Visual Acuity of 20/25 or better and did not have any known retinal pathology. One hundred and seven eyes were analyzed. All patients underwent a complete ophthalmic examination in the Retina Service of a University Eye Clinic including digital CF imaging and FFA.

RESULTS

The mean age of the patients was 18.6 years. Mean duration of DM was 11.3 years, and mean haemoglobin A1c (HbA1c) level was 8.6%. Of the 107 eyes, 8 eyes (7.5%) showed microvascular abnormalities on CF images, while FFA images revealed changes in 26 eyes (24.3%). Hence, 18 of the 26 eyes showing abnormalities on FFA did not show any abnormalities on CF images. Mean DM duration in the patient group with detectable microvascular changes was found to be significantly higher compared to patients without changes, while no difference in HbA1c levels, serum lipid levels or blood pressure was observed.

CONCLUSIONS

Comparison of digital CF and FFA findings for the detection of diabetic microvascular changes in type 1 diabetic patients showed that FFA reveals more information about retinal vascular pathology for early detection of diabetic retinopathy.

Quantification of parafoveal capillary network using a semi-automated algorithm

OBJECTIVE

The quantification of the morphology of the parafoveal capillary network (PCN) in fluorescein angiography (FA) images using a novel semi-automated computerized method.

MATERIAL AND METHODS

Using the MatLab R2011 a software we developed an algorithm that detects automatically the parafoveal capillary bed and its branch points as depicted in FA images creating simultaneously an one-pixel-wide skeleton of it. The detection process starts after delineating manually the foveal avascular zone in a cropped 1500μm*1500μm subimage resulting from the original FA image. Thereafter the algorithm calculates the capillary density and the branch points in a circle area with 1000μm radius. The method was also applied on FA images from subjects without diabetes mellitus, diabetics without diabetic retinopathy (DR) signs, patients with non-proliferative DR and patients with proliferative DR in order to assess the PCN morphology metrics for the studied groups.

RESULTS

The PCN density and the parafoveal capillary branch points were estimated for the mentioned subject groups and any significant differences among them were assessed as well.

CONCLUSIONS

The described method could serve as a potential tool for the diagnosis and monitoring of PCN diseases and subclinical abnormalities. The assessed metrics reflect the capillary abnormalities in the central 1000μm area across different DR stages.