Imaging Figure 3: Right Eye of a 72-year-old Man with Drusen that Increased in Volume during 16 Months of Follow-up A B CD
Volume: 0.085 mm³ E F G H
Volume: 0.108 mm³
A–D: Colour fundus images, retinal pigment epithelium (RPE) map, RPE elevation map and horizontal B-scan from the baseline visit; E–H: Colour fundus images after 16 months of follow-up. The volume within a 3 mm circle increased from 0.085 to 0.108 mm3 (C and G). Some areas of drusen disappeared during the follow-up period (B and F, white arrow) while the total drusen volume increased over time.
This new algorithm creates a drusen map from a scan pattern of 40,000 uniformly spaced A-scans organised as 200 A-scans in each B-scan and 200 horizontal B-scans, covering an area of 6 x 6 mm centered in the fovea. The algorithm uses the actual RPE geometry and compares this RPE segmentation map with a virtual map of the RPE free of deformations (RPE floor). Using these two maps, the algorithm creates an elevation map, which permits reproducible measurements of drusen area and volume. Using this algorithm, Yehoshua et al. reported the natural history of drusen in AMD. Based on their findings, drusen were shown to undergo three different growth patterns. In most eyes, drusen were found to increase in volume and area. Drusen could also remain stable or they could dramatically decrease over time. When these drusen decreased, they could develop into GA or neovascular AMD, or they could decrease resulting in no residual defect in the macula.19
algorithms, results in the ability to generate maps of the internal limiting membrane and the RPE, which provides information on the RPE geometry and therefore a unique perspective of drusen. A novel algorithm developed to identify RPE elevations, such as drusen, has been shown to be highly reproducible in the quantification of drusen area and volume.11
significant regression of drusen as demonstrated using the algorithm. The colour fundus image, a representative B-scan over the drusen, the RPE segmentation map and the RPE elevation map with a 3 mm circle at baseline (see Figures 2A–2D), at six months (see Figures 2E–2H) and at one year (see Figures 2I–2L) of follow-up, respectively, are shown. The drusen area and volume decreased over time as shown most convincingly in the RPE segmentation and elevation maps. Drusen volume decreased from 0.125mm3 at baseline to 0.067mm3 at six months and to 0.039 mm3 at one year of follow-up.
Figure 3 shows an example of an eye with increasing drusen volume and area over 16 months. The volume within the 3 mm circle increases from 0.085 to 0.108 mm3. It is interesting to note that some areas of drusen disappear during the follow-up interval (Figures 3B and 3F, white arrow) while the total drusen volume increases over time. This example demonstrates that drusen are dynamic.
Geographic Atrophy
The formation of GA impairs visual function, impacts the quality of life and may result in blindness.20
GA is seen clinically as one or more
An example of the segmentation algorithm used to measure drusen is shown in Figure 1. In this figure, the 6 x 6 mm scan area (white box) was superimposed on a colour fundus image of a macula containing drusen (see Figure 1A). A representative B-scan is shown with the boundaries of the actual RPE segmentation and the interpolated RPE floor identified in red and yellow, respectively (see Figures 1B and 1C). In Figure 1D, the OFI is shown, which represents the summation of the reflected light from each A-scan when viewed en face. This topic will be discussed later in this article. A 3D RPE segmentation map is shown along with the RPE elevation map, which represents the difference between the actual RPE and the RPE floor for each B-scan in the data set and this map is used to generate the area and volume of the drusen (see Figures 1E and 1F).
Figures 2 and 3 represent the clinical use of SD-OCT when observing patients with dry AMD. Figure 2 shows an example of an eye with
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well-demarcated areas of hypopigmentation or depigmentation due to the absence or severe attenuation of the underlying RPE. The larger, deeper choroidal vessels are more readily visualised through the atrophic patches, which also lack photoreceptors and choriocapillaris. The natural history of GA has been described as a progressive condition that evolves through stages with loss of vision occurring over years.8,21,22 The initial size and configuration of the atrophy appear to influence its progression rate. Average linear rates of growth of 140–200 μm/year from the GA margin have been reported and the area of GA may enlarge by up to 3 mm2 or more annually.20,21,23–25
Multiple imaging
Other imaging modalities, such as fluorescein angiography (FA), fundus autofluorescence (FAF) and SD-OCT are now used to evaluate and quantify GA. Although these imaging modalities
EUROPEAN OPHTHALMIC REVIEW
modalities have been used to document and quantify the area of GA. Historically, colour fundus photography was used to image GA; however, the use of colour photos can be challenging due to the reported difficulty in detecting and accurately delineating GA.21,26,27
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