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Jyotsna Maram, Trefford Simpson, Luigina Sorbara, Kostadinka K. Bizheva, Song S. Eun; An Apparent Imaging Artifact Occurring At The Edge Of A Contact Lens When Imaged With Oct; The Underlying Tissue Appears Erroneously Discontinuous. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6540.
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Discontinuity of the underlying conjunctiva at the edges of contact lenses observed in OCT images may be real or an artifact.This is because a continuous surface appears displaced when the refractive index of the leading medium changes e.g.at the edge of a contact lens.Therefore we examined this effect when the edges of the contact lenses were imaged on a continuous surface i.e. glass slide using UHR-OCT.
2D images of the edges of selected marketed silicone and conventional hydrogel lenses(refractive index from 1.41 to 1.51) were taken and placed on a glass slide concave side down.The room temperature was controlled to minimize dehydration and 5 images for each lens were taken using an UHR-OCT system operating at 800nm with 3.2µm (axial) and 10µm (lateral) resolution at the rate of 47,000 A-scans/s.The displacement of the glass slide beneath the lens edge was measured using ImageJTM and the thickness of the edges of the contact lenses were measured in a similar way.
There was a noted difference in the position of the glass plate under the edges of the contact lenses(figure1). The range of displacement was from 42.8 to 60.8 microns.Using a path length matching method we were able to relate that the displacement of the glass slide is a function of the thickness of the edges and refractive indices of the lenses.
When contact lenses are imaged in-situ using UHR-OCT the conjunctival tissue appears displaced beneath the edge of the contact lens.This experiment demonstrates that this displacement is an artifact of OCT imaging;a continuous flat surface(glass slide)appears discontinuous(and curved).This is a function of the refractive index and also the thickness at the contact lens edge.Therefore, caution should be exercised when concluding that the edge of a contact lens indents the underlying conjunctiva;it might not be affecting the tissue morphology at all.
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