Quantifying Retinal Area in Ultra-Widefield Imaging Using a 3-Dimensional Printed Eye Model.

PURPOSE: To study the effects of different axial lengths on ultra-widefield imaging to determine the presence of distortion in images despite software correction and calculate an enlargement factor based on angular location. DESIGN: Experimental image analysis study. STUDY OBJECTS: Three 3-dimensional printed model eyes simulating eyes with axial lengths of 22, 24, and 26 mm. Each model has a grid of rings 9° apart centered at the posterior pole. METHODS: Single-center study performed at the National Institute for Health Research Moorfields Biomedical Research Centre (London, UK). Each model was imaged using Optos 200TX (Optos, Dunfermline, UK). Two images for each model eye that were corrected using V2 Vantage Pro software (Optos) were used for analysis and the average values obtained. Each image inter-ring area was measured using ImageJ to obtain a measured image area in pixel and square millimeters. This was compared with the true calculated object inter-ring area and an enlargement factor was determined. MAIN OUTCOME MEASURES: Measured image inter-ring area in pixels and square millimeters. True calculated object inter-ring area in square millimeters. RESULTS: The enlargement factor of the rings gradually increases toward the periphery with factors of 1.4 at 45° and 1.9 at the equator. The axial lengths did not affect the enlargement factor of the rings imaged in 3 different model eyes (P = 0.9512). The anterior equator exhibits a significant distortion despite the software correction. CONCLUSION: The enlargement factor depends on angular location and not axial length. The enlargement factors can be used in clinical practice to more accurately measure area in ultra-widefield imaging.

Long-term Repeatability and Reproducibility of Phosphene Characteristics in Chronically Implanted Argus II Retinal Prosthesis Subjects.

PURPOSE: Previously published literatures of acute studies on few subjects have shown contradictory evidence on the reproducibility and characteristics of the elicited phosphenes, despite using the same stimulating parameters with epiretinal electrode arrays. In this study, we set out to investigate the long-term repeatilibity and reproducibility of phosphenes in subjects chronically implanted with the Argus II retinal prosthesis (Second Sight Medical Products, Inc., Sylmar, CA, USA). DESIGN: Retrospective interventional case series and reliability study. METHODS: Six Argus II subjects of >5 years implantation from a single site participated. The 4-electrode cluster ("quad") closest to fovea was stimulated in each subject with a fixed biphasic current. Perceived phosphenes were depicted relative to subjective visual field center. The stimulus was applied at reducing time intervals from 20 minutes to 1 second. Two sets of stimulations were performed on the same day and 2 further sets repeated on a separate visit >1 week apart. RESULTS: Each subject depicted phosphenes of consistent shapes and sizes, and reported seeing the same colors with the fixed stimulating parameters, irrespective of the interstimuli intervals. However, there is a wide intersubject variation in the phosphene characteristics. Four subjects drew phosphenes in the same visual field quadrant, as predicted by the quad-fovea location. Two subjects depicted phosphenes in the same hemifield as the expected locations. CONCLUSION: Phosphenes for each subject were consistently reproducible in all our chronically implanted subjects. This has important implications in the development of long-term pixelated prosthetic vision for future devices.

Posterior Segment Distortion in Ultra-Widefield Imaging Compared to Conventional Modalities.

BACKGROUND AND OBJECTIVES: To describe posterior segment distortions in Optos ultra-widefield images (Optos 200TX; Optos, Dunfermline, United Kingdom) compared to Topcon retinal camera images (Topcon, Tokyo, Japan) using optic disc dimensions and exploring a proposed method for correcting these distortions. PATIENTS AND METHODS: Comparative image analysis study on 20 eyes with color fundus images from Optos and Topcon. A model eye with vertical and horizontal grids imaged with Optos in the conventional position and rotated 90° was analyzed. RESULTS: Mean vertical-to-horizontal disc diameter ratios were 0.956 [95% CI, 0.919-0.993] for Optos and 1.083 [95% CI, 1.045-1.121] for Topcon (P < .001). This was 0.910 in the conventional position and 1.072 with the object rotated 90° for the model eye with Optos and 1.008 and 0.999, respectively, using Topcon. The average of the measurements taken using both images from Optos yielded a ratio of 0.987. CONCLUSION: Optos incorporates a consistent horizontal stretch to images. Combining images taken at right angles reduces the distortion. [Ophthalmic Surg Lasers Imaging Retina. 2016;47:644-651.].

In Vivo 3-Dimensional Strain Mapping of the Optic Nerve Head Following Intraocular Pressure Lowering by Trabeculectomy.

PURPOSE: To map the 3-dimensional (3D) strain of the optic nerve head (ONH) in vivo after intraocular pressure (IOP) lowering by trabeculectomy (TE) and to establish associations between ONH strain and retinal sensitivity. DESIGN: Observational case series. PARTICIPANTS: Nine patients with primary open-angle glaucoma (POAG) and 3 normal controls. METHODS: The ONHs of 9 subjects with POAG (pre-TE IOP: 25.3±13.9 mmHg; post-TE IOP: 11.8±8.6 mmHg) were imaged (1 eye per subject) using optical coherence tomography (OCT) (Heidelberg Spectralis, Heidelberg Engineering GmbH, Heidelberg, Germany) before (<21 days) and after (<50 days) TE. The imaging protocol was repeated for 3 controls in whom IOP was not altered. In each post-TE OCT volume, 4 tissues were manually segmented (prelamina, choroid, sclera, and lamina cribrosa [LC]). For each ONH, a 3D tracking algorithm was applied to both post- and pre-TE OCT volumes to extract IOP-induced 3D displacements at segmented nodes. Displacements were filtered, smoothed, and processed to extract 3D strain relief (the amount of tissue deformation relieved after TE). Strain relief was compared with measures of retinal sensitivity from visual field testing. MAIN OUTCOME MEASURES: Three-dimensional ONH displacements and strain relief. RESULTS: On average, strain relief (averaged or effective component) in the glaucoma ONHs (8.6%) due to TE was higher than that measured in the normal controls (1.07%). We found no associations between the magnitude of IOP decrease and the LC strain relief (P > 0.05), suggesting biomechanical variability across subjects. The LC displaced posteriorly, anteriorly, or not at all. Furthermore, we found linear associations between retinal sensitivity and LC effective strain relief (P < 0.001; high strain relief associated with low retinal sensitivity). CONCLUSIONS: We demonstrate that ONH displacements and strains can be measured in vivo and that TE can relieve ONH strains. Our data suggest a wide variability in ONH biomechanics in the subjects examined in this study. We further demonstrate associations between LC effective strain relief and retinal sensitivity.