Fan and optical distortions can significantly affect image reconstruction in OCT cross-sectional images, and the same can strongly obstruct corneal topographic reconstruction. The fan distortion is associated with the OCT scanning system, which includes a pair of separate scanner mirrors and optical elements. Optical distortions are related to imaging optics and can cause, for example, pincushion or barrel effects in projection OCT images. When flat surfaces are imaged with OCT, they become curved. However, this effect can be minimized by careful adjustment of the optical setup. Following Ortiz et al.
], we have adjusted the position of the objective lens with respect to scanners to decrease the magnitude of fan distortion. During this procedure a flat surface was imaged on a screen in the preview mode in two orthogonal directions. Careful adjustment of the objective lens position enabled us to flatten reconstructed images, which are presented in
and for two perpendicular directions. To investigate the influence of fan and optical distortion of our OCT system in three dimensions, we performed a simple experiment using 2D gridlines printed on paper, which were then measured by the SS OCT system. shows an en face
projection image generated from 3D OCT data. The view is limited to the 9 mm area (the same area size used for elevation map calculation). A slight defocusing is visible at the edge of the scanned area. However, the shape of the grid is not significantly distorted. We can assume that for demonstration of the applicability of SSOCT for corneal topography the optical distortions are negligible.
SS OCT images of a flat surface measured in two perpendicular directions, (a) horizontally and (b) vertically, after optical distortion correction. (c) En face projection image from 3-D OCT data for graph paper in a 9 mm × 9 mm area.
To verify the performance of the SS OCT system for anterior segment imaging, a reference sphere was measured (EyeSys Technologies) (
). The reference sphere was measured at nine different randomly set tilt-shift positions (-), which was consistent with the scanning protocol used for patient examination. Since the reference sphere consists only of one surface, the postprocessing was performed without the refraction correction step. The reference shape was segmented automatically by using custom designed software written in LabView. In the method proposed by Gora et al.
] the pupillary plane is used for mathematical correction of tilt or misalignment of the measured eye with respect to the optical axis of the instrument. As the reference sphere has no pupil, we used the basal surface of the target.
Fig. 3 (a–b) central OCT images of the reference sphere; (c) photograph of the reference sphere with 7.94 mm radius of curvature. (d) Radius of curvature for nine different OCT measurements; diamonds, mean value; red line, radius given by manufacturer. (more ...)
For each of nine measurements the best fit sphere was found, and its radius of curvature was calculated (). A mean value of 7.96 mm (standard deviation of 0.07 mm) was consistent with the value given by the manufacturer of the reference sphere (7.94 mm) and the value measured by the Placido-based topographer (7.94 mm). The 0.07 mm error for a 7.96 mm radius will produce a 0.37 D error.
Quantitative analysis of three corneas from three different patients (keratoconus, a cornea with superficial postinfectious scar, and a cornea 5 months after penetrating keratoplasty) was performed with three different instruments.
shows the topographic analysis of a keratoconic cornea measured with three different instruments. The confirmation of the clinically significant keratoconous is based on Placido topography in which typical pattern is detected on anterior sagittal curvature map. We revealed good agreement between anterior elevation topographies provided by Pentacam and SS OCT in this case (). The central keratometry value (K1) and axis in a flat meridian is almost the same for Placido-based topography, Pentacam, and SS OCT, but readings for a steep meridian (K2) show differences above 1 D and above 20° between the Placido-based topographer and elevation-based systems (
). Agreement of the results from elevation topographers is also high for posterior corneal topography and pachymetry.
Qualitative evaluation of a keratoconic cornea with three different instruments. K1, K2, central keratometry readings. The red lines on Scheimpflug images correspond to the lateral size of cross-sectional images for SS OCT.
Central keratometry readings and central thickness for selected corneal pathologies; all patients measured with three different instruments
shows results of the examination of the cornea with a scar in the anterior stroma after superficial infectious ulcer. Placido topography shows irregular astigmatism, and keratometric values are quite different from elevation-based systems. Pentacam shows an area below the reference shape in the center and paracentrally, whereas SS OCT topography shows a more regular anterior surface. Posterior topography patterns are similar for both instruments; thus a distinct decrease of corneal thickness in the center is visible on Pentacam pachymetry map compared with that calculated by the SS OCT system ().
Qualitative evaluation of a cornea with superficial postinfectious scar with three different instruments. K1, K2, central keratometry readings. The red lines on Scheimpflug images correspond to the lateral size of cross-sectional images for SS OCT.
show magnified tomograms of the cornea with a scar measured with the Pentacam HR and the SS OCT prototype. Blue light from the 475 nm LED of the Pentacam imaging system is strongly reflected back from the opaque scarring tissue. The signal is oversaturated, which results in difficulties in segmentation of the anterior surface. Moreover, the posterior surface delineation is poor. In contrast to SS OCT, Pentacam segmentation causes a falsely decreased thickness of the central cornea (). On SS OCT images, both anterior and posterior surfaces are clearly visible and easy to delineate. In addition, because of its better resolution and sensitivity, SS OCT offers a much better view of the corneal structure. In this pathological case, SS OCT images reveal that the lack of superficial stromal tissue due to the scarring process is compensated by increased thickness of the epithelium ((b)), and the resultant corneal thickness is larger than that measured by the Pentacam instrument ().
Cross-sectional images of the cornea with superficial postinfectious scar: (a) Scheimpflug image measured by Pentacam HR; (b) SS OCT cross-sectional image. Red lines delineate the segmented corneal boundaries.
Results obtained from the eye 5 months after penetrating keratoplasty are shown in
. Placido reflections are much distorted. Only a couple of rings in the center could be partially distinguished and analyzed by the computer. Despite the fact that part of the cornea is covered with the upper lid, the Placido system provides a curvature map of the whole cornea.
Fig. 7 Qualitative evaluation of a cornea 5 months after penetrating keratoplasty with three different instruments. K1, K2, central keratometry readings. The red lines on Scheimpflug images correspond to the lateral size of cross-sectional images from SS OCT. (more ...)
Also, the differences in the central keratometry reading are very large in comparison with Pentacam and SS OCT. Anterior topographies from Pentacam and SS OCT show comparable patterns, but the elevated central island is located slightly higher on the SS OCT map, resulting in a significant difference in central keratometry readings.
The area where the donor and host tissues are interconnected is more elevated on maps generated with SS OCT. As a result, SS OCT shows much thicker pachymetry in the area of graft-host junction.
A closer look on a single cross-sectional image has to be done to fully understand origin of differences in elevation maps generated with both Pentacam and SS OCT.
shows tomograms of the corresponding parts of the cornea after penetrating keratoplasty acquired with two different instruments. The Scheimpflug image is moderately clear, and segmentation of the tissue by the software is not correct ((a)). This inaccuracy is caused by the simplified segmentation algorithm’s assuming a certain level of corneal smoothness, which is obviously not true in the case of very complex corneal topography like that after penetrating keratoplasty. SS OCT provides tomograms with a more homogenous distribution of the backscattered intensity. Therefore it is easier to apply a segmentation algorithm that does not assume the smoothness of the corneal surface ((b)).
Cross-sectional images of the cornea after penetrating keratoplasty: (a) Scheimpflug image measured by Pentacam HR, (b) SS OCT cross-sectional image. Red lines delineate the segmented corneal boundaries.