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S. E. Harris, L. D. Hubbard, J. K. White, J. A. Elledge, R. P. Danis, D. W. Thayer; Linear Calibration of Digital Retinal Images Based Pragmatically on Standard Disc-Macula Distance. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2243. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To introduce and test pragmatic linear calibration of digital retinal images, based on standardized disc to macula (D-M) distance, as an alternative to the current fixed calibration method.Background: Clinical trials use retinal images for linear and area measurements of retinal abnormalities. Historically, reading centers have calibrated spatial resolution by using magnification factors from manufacturers (sometimes unreliable, always requiring verification) and tracking each camera’s optics and software (labor-intensive and uncertain). Previously we have shown that D-M distance averages 4.5 mm (JA Elledge, ARVO abstract 2005), and is less variable proportionately than optic disc size, a feature previously used for calibration.
For pragmatic calibration, we used an image centered between disc and macula, clearly showing the centers of both. We located their exact centers by concentrically placing, around each, a circle with a central dot. Then we drew a line between the disc and macula central dots. Assuming a standard D-M distance of 4.5 mm, we calculated a calibration factor in pixels/mm. This factor is applied to all images of that eye at that visit, but each visit is calibrated independently. To compare fixed and pragmatic calibration, we selected 66 subjects (study eyes) from an age-related macular degeneration study, having both baseline (BL) and 12 month (12m) images, and total lesion areas quantified. After pragmatically recalibrating both BL and 12m visit images, we recalculated lesion areas at BL and 12m, and change in area between visits. To assess repeatability of the pragmatic approach, we selected a separate set of 36 eyes, which 4 graders each independently calibrated.
BL lesion area was highly correlated between fixed and pragmatic approaches (R = 0.97), as was BL-to-12m change in lesion area (R = 0.93). In cases with different pragmatic factors between visits, superimposing BL and 12m images confirmed actual differences in scale. We explored cataract extraction and photographic technique as sources of variable magnification. In the repeatability study, pair-wise concordance correlations between graders were all ≥ 0.99.
Pragmatic calibration produces similar area measurements to fixed calibration. Difference between the two methods is mostly explained by real magnification change that the fixed method cannot capture. The pragmatic approach is more accurate, highly repeatable, simple, and would not require verification or tracking of multiple camera systems.
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