Circularity index as a risk factor for progression of geographic atrophy.

OBJECTIVE: To develop a parameter that can assess the relative rate of progression of geographic atrophy (GA) based on the hypothesis that noncircular configuration of the atrophic lesion may be a risk factor for enlargement. DESIGN: Cohort study. PARTICIPANTS: Digitized color photographs of 593 eyes with GA from the Age-Related Eye Disease Study (AREDS). METHODS: A novel parameter called the "Geographic Atrophy Circularity Index" (GACI) was developed on the basis of area and perimeter measurements to categorize the irregularity of the shape of GA. The GACI ranges from 0.0 to 1.0 and is categorized into 3 groups: 0.25 (very irregular), 0.25 to <0.75 (partly irregular), and ≥ 0.75 (circular). MAIN OUTCOME MEASURES: Growth rate of GA. RESULTS: The mean growth rate in the 3 categories was 0.40 (± 0.18), 0.36 (± 0.30), and 0.21 (± 0.22) mm/year, respectively (P < 0.001). By adjusting for known confounders, baseline area, duration of GA, and configuration, GACI categories were significantly associated with increased growth rate of GA (P < 0.001). CONCLUSIONS: The GACI was associated with the progression rate of GA and may be a useful measure for clinical trial eligibility. The association also suggests that enlargement of GA may be related to the extent of the junctional zone of damaged retinal pigment epithelium, which increases with noncircularity for a given GA area.

Effect of optical coherence tomography scan decentration on macular center subfield thickness measurements.

PURPOSE: To investigate the effect of optical coherence tomography macular grid displacement on retinal thickness measurements. METHODS: SD-OCT macular scans of 66 eyes with various retinal thicknesses were selected. Decentration of the 1-, 3-, 6-mm-diameter macular grid was simulated by manually adjusting the distance between center of the fovea (cFovea) and center of the grid (cGrid). Center subfield thickness (CSF) between the internal limiting membrane and the top of the retinal pigment epithelium was measured along the displacement distance where the grid was displaced in eight cardinal directions from the cFovea in steps of 100 µm within the central 1-mm subfield and then by 200 µm within the inner subfields. One-way/mixed-effects repeated-measures ANOVA models were used to determine changes of CSF (ΔCSF) as a function of displacement distance (for α = 0.05, power = 0.80 and effect size = 0.1). The interactions between the displacement distance and direction, center point thickness (CPT), and foveal contour were also analyzed. RESULTS: The CSF measurement showed statistically significant error when the displacement distance between cFovea and cGrid exceeded 200 µm. The direction of displacement did not affect the ΔCSF-distance relationship, while the CPT and foveal contour significantly affected the relationship, in that some subgroups showed slightly larger tolerance in the displacement distance up to 300 µm before reaching significant ΔCSF. CONCLUSIONS: Small displacement distances of the macular grid from the cFovea affect CSF measurements throughout a broad range of thicknesses and retinal contour alterations from disease. Accurate registration of OCT scans or post hoc repositioning of the grid is essential to optimize CSF accuracy.

Development of a semi-automatic segmentation method for retinal OCT images tested in patients with diabetic macular edema.

PURPOSE: To develop EdgeSelect, a semi-automatic method for the segmentation of retinal layers in spectral domain optical coherence tomography images, and to compare the segmentation results with a manual method. METHODS: SD-OCT (Heidelberg Spectralis) scans of 28 eyes (24 patients with diabetic macular edema and 4 normal subjects) were imported into a customized MATLAB application, and were manually segmented by three graders at the layers corresponding to the inner limiting membrane (ILM), the inner segment/ellipsoid interface (ISe), the retinal/retinal pigment epithelium interface (RPE), and the Bruch's membrane (BM). The scans were then segmented independently by the same graders using EdgeSelect, a semi-automated method allowing the graders to guide/correct the layer segmentation interactively. The inter-grader reproducibility and agreement in locating the layer positions between the manual and EdgeSelect methods were assessed and compared using the Wilcoxon signed rank test. RESULTS: The inter-grader reproducibility using the EdgeSelect method for retinal layers varied from 0.15 to 1.21 µm, smaller than those using the manual method (3.36-6.43 µm). The Wilcoxon test indicated the EdgeSelect method had significantly better reproducibility than the manual method. The agreement between the manual and EdgeSelect methods in locating retinal layers ranged from 0.08 to 1.32 µm. There were small differences between the two methods in locating the ILM (p = 0.012) and BM layers (p<0.001), but these were statistically indistinguishable in locating the ISe (p = 0.896) and RPE layers (p = 0.771). CONCLUSIONS: The EdgeSelect method resulted in better reproducibility and good agreement with a manual method in a set of eyes of normal subjects and with retinal disease, suggesting that this approach is feasible for OCT image analysis in clinical trials.

Effect of image compression and resolution on retinal vascular caliber.

PURPOSE: Changes in retinal vascular caliber measured from digital color fundus photographs have been independently associated with systemic outcomes in epidemiologic studies, but the effect of image resolution and compression on vascular measurements has not been previously evaluated. METHODS: To explore image compression, 40 natively digital fundus images were selected with good photo quality, high spatial resolution, and no previous image compression. Using Adobe Photoshop, these images were compressed at progressively higher levels up to 147:1, and then retinal vascular caliber was measured at each level using semiautomated software. To examine resolution, 40 fundus photographs acquired on high-resolution film were scanned with settings corresponding to 10, 7, 5, 3, and 1 megapixel fundus cameras. After adjusting for scale factor, vascular caliber was measured at each level of resolution. Data were analyzed by comparing the calculated central retinal arteriole equivalent (CRAE) and the central retinal venular equivalent (CRVE) of the original and altered images, using repeated measures ANOVA. RESULTS: CRAE became significantly wider with increasing levels of compression at the 25:1 threshold (~1 µm wider, P < 0.001) and was ~5 µm wider with 147:1 compression. CRVE also increased, but less than CRAE. Using 7 (megapixel)-MP resolution as the standard, CRVE was significantly narrower at the 5-MP simulation (~2 µm, P < 0.001) and was ~12 µm narrower at the 1-MP simulation. CRAE also decreased, but less than CRVE. CONCLUSIONS: Increasing digital image file compression and decreasing fundus image spatial resolution led to skewed measurements of the retinal vascular caliber.

Signal quality assessment of retinal optical coherence tomography images.

PURPOSE: The purpose of this article was to assess signal quality of retinal optical coherence tomography (OCT) images from multiple devices using subjective and quantitative measurements. METHODS: A total of 120 multiframe OCT images from 4 spectral domain OCT devices (Cirrus, RTVue, Spectralis, and 3D OCT-1000) were evaluated subjectively by trained graders, and measured quantitatively using a derived parameter, maximum tissue contrast index (mTCI). An intensity histogram decomposition model was proposed to separate the foreground and background information of OCT images and to calculate the mTCI. The mTCI results were compared with the manufacturer signal index (MSI) provided by the respective devices, and to the subjective grading scores (SGS). RESULTS: Statistically significant correlations were observed between the paired methods (i.e., SGS and MSI, SGS and mTCI, and mTCI and MSI). Fisher's Z transformation indicated the Pearson correlation coefficient ρ ≥ 0.8 for all devices. Using the Deming regression, correlation parameters between the paired methods were established. This allowed conversion from the proprietary MSI values to SGS and mTCI that are universally applied to each device. CONCLUSIONS: The study suggests signal quality of retinal OCT images can be evaluated subjectively and objectively, independent of the devices. Together with the proposed histogram decomposition model, mTCI may be used as a standardization metric for OCT signal quality that would affect measurements.