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Florian Beer, Andreas Wartak, Bernhard Baumann, Michael Pircher, Christoph K Hitzenberger; Three-dimensional corneal layer thickness measurement with PS-OCT using a conical scan pattern. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1975.
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© ARVO (1962-2015); The Authors (2016-present)
To demonstrate the capabilities of PS-OCT with conical scanning to segment corneal layers (epithelium (EP), Bowman’s layer (BL) and stroma) across the entire cornea (limbus to limbus) and to analyze the reproducibility of layer thickness measurements.
A swept-source PS-OCT system operating at 1045 nm central wavelength was used for imaging. The instrument provides an A-scan rate of 100 kHz and an axial resolution of 5.4 µm in air. A refractive index of 1.385 was used to calculate the thickness in tissue and was verified by comparing total corneal thickness values with Pentacam® data. Conical scanning allows for almost perpendicular beam incidence on the corneal surface and provides good signal quality over the entire field of view. The anterior and posterior EP boundaries and the posterior corneal surface are segmented by standard methods in the co-polarized channel. The light backscattered by the stroma is partly depolarized which can be used to reliably segment the posterior surface of BL in the cross polarized channel. Therefore 3D thickness maps of corneal layers can be computed. These maps were analyzed in 13 sectors. For that, all values in one sector were averaged and the standard deviation (SD) of repeated measurements was calculated for each sector. Both eyes of 20 healthy volunteers were imaged at least three times to test this method and to verify reproducibility.
EP, BL and stroma were segmented (cf. Fig 1 a) and mapped (cf. Fig 1 b-d). Parts of the cornea were obstructed by the eyelid and eyelashes, thus, sectors in the top region had to be excluded (cf. Fig 1 b-g). The SD of repeated measurements was < 1 µm for EP and BL in the central sectors but increased in the periphery. The mean thicknesses of EP and BL over the entire cornea are 54.4 µm and 12.3 µm respectively. The measured thicknesses are in good agreement with literature.
Thicknesses of EP, BL and stroma of 40 healthy eyes were measured with PS-OCT using a conical scan pattern. The results are in agreement with literature and show good reproducibility. The detection of thickness changes of corneal layers might be a valuable tool for early detection and diagnosis of corneal diseases.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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