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J Sugita, VE Evans, JM Tiffany, AJ Bron, N Yokoi; Digital Image Analysis With Video Meniscometry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):95.
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Purpose: Reflective meniscometry is a non-invasive method for measuring tear meniscus curvature, used in the diagnosis of dry eye. The newly developed video meniscometer (Yokoi, Cornea, 2000), allows the measurement of changes in lower meniscus curvature over time. However, analysis of temporal data by manual measurement of multiple, printed images is labour intensive. To overcome this, we have devised an alternative, digital method, using image analysis software. The manual and digital techniques were compared in this study. Methods: The meniscometer records the reflected image of a horizontally striped target onto videotape. The system was calibrated by capturing images reflected from the internal, air-glass surfaces of a series of glass capillary tubes of known internal radius (range: 0.204 to 1.100 mm). Images were generated, either by printing the freeze-frame image, or by grabbing digital images using a computer equipped with a Pinnacle Studio capture board (Pinnacle System Inc., Germany). The total width of 3 bands, 2 black and one white, was measured in each case. Printed images were measured manually using a magnifying loupe (x10), whereas digital images were measured using Scion Image Analysis Software (Scion Corporation, U.S.A). Results: Image line spacing was linearly related to the standard capillary tube radius of curvature for both digital and manual measurement (R2= 0.9986 and 0.9993, respectively). The digital and manual methods showed a high correlation (R2= 0.999). The difference versus the mean of each measurement method was plotted to determine the measurement bias and agreement between the two techniques. The bias (0.0002 mm) was not significantly different from zero (paired T-test) indicating a high level of agreement between the two methods. Digital image analysis shortened the measurement period by 33% compared with manual analysis. Conclusion: Digital image measurement allows analysis of the change in meniscus curvature over time more easily than manual measurement. This technique is being used in temporal studies of tear meniscus curvature in normal and dry eye subjects.
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