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A False Color Fusion Strategy for Drusen and Geographic Atrophy Visualization in Optical Coherence Tomography Images

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A False Color Fusion Strategy for Drusen and Geographic Atrophy Visualization in Optical Coherence Tomography Images A FALSE COLOR FUSION STRATEGY FOR DRUSEN AND GEOGRAPHIC ATROPHY VISUALIZATION IN OPTICAL COHERENCE TOMOGRAPHY IMAGES QIANG CHEN, PHD,* THEODORE LENG, MD, MS,† SIJIE NIU, MS,* JIAJIA SHI, BS,* LUIS DE SISTERNES, PHD,‡ DANIEL L. RUBIN, MD, MS‡ Purpose: To display drusen and geographic atrophy (GA) in a single projection image from three-dimensional spectral domain optical coherence tomography images based on a novel false color fusion strategy. Methods: We present a false color fusion strategy to combine drusen and GA projection images. The drusen projection image is generated with a restricted summed-voxel projection (axial sum of the reflectivity values in a spectral domain optical coherence tomography cube, limited to the region where drusen is present). The GA projection image is generated by incorporating two GA characteristics: bright choroid and thin retina pigment epithelium. The false color fusion method was evaluated in 82 three-dimensional optical coherence tomography data sets obtained from 7 patients, for which 2 readers independently identified drusen and GA as the gold standard. The mean drusen and GA overlap ratio was used as the metric to determine accuracy of visualization of the proposed method when compared with the conventional summed-voxel projection, (axial sum of the reflectivity values in the complete spectral domain optical coherence tomography cube) technique and color fundus photographs. Results: Comparative results demonstrate that the false color image is more effective in displaying drusen and GA than summed-voxel projection and CFP. The mean drusen/GA overlap ratios based on the conventional summed-voxel projection method, color fundus photographs, and the false color fusion method were 6.4%/100%, 64.1%/66.7%, and 85.6%/100%, respectively. Conclusion: The false color fusion method was more effective for simultaneous visualization of drusen and GA than the conventional summed-voxel projection method and color fundus photographs, and it seems promising as an alternative method for visualizing drusen and GA in the retinal fundus, which commonly occur together and can be confusing to differentiate without methods such as this proposed one. RETINA 0:1–13, 2014 ge-related macular degeneration (AMD) affects an Evaluation of color fundus photographs (CFPs) is Aestimated 30 to 50 million individuals worldwide1 currently the gold standard for measuring drusen in and is the most common cause of legal blindness nonneovascular AMD and for visualizing and assess- among elderly individuals in developed countries.2,3 ing GA.9 Total drusen area and maximum drusen size One of its clinical characteristics and, in most scans, are estimated by visual inspection of drusen in CFPs, the first clinical finding is the presence of drusen (“dry with comparison with a set of standardized circles.10 AMD”),4 which are focal deposits of extracellular However, it can be challenging to reliably localize material located between the basal lamina of the retina drusen against the varying background of the pigments pigment epithelium (RPE) and the inner collagenous of the macula, RPE, and choroid.11 In addition, it is layer of Bruch’s membrane.5 The advanced form of difficult to make reproducible quantitative measure- AMD associated with severe vision loss is character- ments of drusen and GA in CFPs. ized by the development of macular neovasculariza- Optical coherence tomography (OCT) enables the tion (“wet AMD”) and geographic atrophy (GA).6–8 differentiation of retinal structures such as drusen and 1 Copyright ª by Ophthalmic Communications Society, Inc. Unauthorized reproduction of this article is prohibited. 2 RETINA, THE JOURNAL OF RETINAL AND VITREOUS DISEASES 2014 VOLUME 0 NUMBER 0 GA in the depth axis. The latest spectral domain OCT costly. In addition, the SVP images produced by the (SD-OCT) systems are able to acquire high-speed, methods by Stopa et al only give information about high-resolution, high-density three-dimensional (3-D) drusen extent, but no information about drusen thick- images covering the central macula,12 with the advan- ness, which could be useful for characterizing drusen. tage over other imaging modalities for dry AMD that Gorczynska et al18 proposed a method of generating the same scan pattern can be used to observe both a series of projection OCT fundus images from each drusen and GA while obtaining reproducible quantita- single OCT cube by selectively summing different tive data on both abnormalities.13 Khanifar et al14 cat- retinal depth levels, which enhanced contrast and egorized drusen ultrastructure in AMD using SD-OCT visualized outer retinal pathology not visible with and correlated the tomographic and photographic dru- standard fundus imaging or OCT fundus imaging sen appearance. A high-density scan pattern also techniques. With this method, however, drusen are allows the visualization of drusen and GA on an separated into several projected fundus images OCT fundus image, which represents an en face summed at different retinal depth levels, and cannot summed-voxel projection (SVP) of all the B-scans be directly visualized in a single image. from the SD-OCT data set.14–16 The OFIs can be used Another form of drusen visualization in an en face to register the SD-OCT data sets to fundus photo- image was originally proposed with the name of graphs or other en face retinal imaging modalities. restricted summed-voxel projection (RSVP).19,20 The This facilitates calibration of the color fundus images RSVP includes the automatic segmentation and pro- so that exact correlations can be achieved between the jection of the voxels around the RPE, together with retinal cross-sectional geometry seen on the OCT B- image processing techniques such as “brightening” of scans and the retinal landmark seen on en face imaging voxels underneath drusen. This process greatly and the color images. improve drusen visualization in an en face image The SVP fundus image is not ideal for drusen and provides a rapid way of assessment of drusen in visualization because most drusen are not visible with the whole macula volume other than inspecting B-scan this technique15; these small abnormalities are often by B-scan. obscured when the image volume is collapsed when The primary retinal layer affected by the evolution making these projections. Stopa et al17 overcame of GA is still not very clear; the RPE, choriocapil- some of these limitations by locating pathologic ret- laris, or photoreceptors (PR) layers can present inal features with color marking in each OCT image deformities in patients affected by GA.13,21 However, before the image volume was collapsed along the most histopathologic studies suggest that the initial depth axis to produce the SVP. By this way, they event in GA occurrence is RPE cell loss, followed preserved the delineation of pathologic features in with ensuing PR cell death and choriocapillaris atro- SVP visualizations. Another technique recently intro- phy.22–25 Bearelly et al21 studied the PR-RPE inter- duced into OCT imaging devices is the “slab SVP,” face in GA using SD-OCT to test in vivo whether a semiautomated method to restrict the SVP to a sub- SD-OCT provides adequate resolution for reproduc- volume of the retina near the RPE layer (Carl Zeiss ible measurement of the PR layer at the margins of Meditec, Inc, unpublished data); however, the user GA, and whether the relationship between PR layer needs to annotate the image to localize the RPE. and RPE at those margins could be delineated suc- Manually annotating pathologic features in a stack cessfully. This study emphasized the direct associa- of SD-OCT images in the time-pressured and high- tion between GA and cell loss or “thinning” of PR volume setting of clinical care is time-consuming and and RPE as observed in SD-OCT images. The GA can also be visualized in en face SVP fundus images From the *School of Computer Science and Engineering, Nanjing as a bright and more homogeneously delimited University of Science and Technology, Nanjing, China; †Byers Eye Institute at Stanford, Stanford University School of Medicine, Palo region, because of the mentioned cell loss and con- Alto, California; and ‡Department of Radiology and Medicine (Bio- sequent increased penetration of light into the choroid medical Informatics Research), Stanford University School of Med- coat, together with the constant high reflection of icine, Stanford, California. 26 Supported by a grant from the Fundamental Research Funds for light from the choroid coat. However, some partic- the Central Universities (Grant 30920140111004), the Bio-X Inter- ular cases with highly reflective retinal layers above disciplinary Initiatives Program of Stanford University, a grant the RPE complex complicate and obscure GA visu- from the National Cancer Institute, National Institutes of Health (Grant U01-CA-142555), and Qing Lan Project. alization, making the use of SVP fundus imaging for None of the authors have any conflicting interests to disclose. GA inspection suboptimal. Reprint requests: Qiang Chen, PhD, School of Computer In this article, we present a new combined drusen Science and Engineering, Nanjing University of Science and Technology, Xiao Lingwei 200#, Nanjing 210094, China; e-mail: and GA visualization method that enhances the [email protected] conspicuity of GA lesion by incorporating RPE loss Copyright ª by Ophthalmic Communications Society, Inc. Unauthorized reproduction of this article is prohibited. FALSE COLOR FUSION OF DRUSEN AND GA CHEN ET AL 3 and increase of
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