Whereas surface corneal curvature (power) is best expressed by Placido imaging, overall corneal shape, including spatial thickness profiles, is best expressed by computed tomography. Various imaging systems are available that take multiple slit images and reconstruct them into a corneal-shape profile, including anterior and posterior corneal elevation data (Fig 1-12). These include scanning-slit technology, Scheimpflug-based imaging systems, and anterior segment optical coherence tomography (OCT). To represent shape directly, color maps may be used to display a z-height from an arbitrary plane such as the iris plane; however, in order to be clinically useful, corneal surface maps are plotted to show differences from best-fit spheres or other objects that closely mimic the normal corneal shape (Fig 1-13). In general, each device calculates the best-fit sphere for each map individually. For this reason, comparing elevation maps is not exact because they frequently have different referenced best-fit sphere characteristics.
Figure 1-12 Different options for corneal imaging. All images are of the same patient taken at the same visit. A, Placido disk–based corneal curvature map showing axial and tangential curvature maps as well as the elevation map and the Placido rings image. Recall that this mapping technology analyzes only the surface characteristics of the cornea. B, Optical coherence tomography (OCT) image of the same cornea shown in A. Note that the corneal thickness profile (of the stroma as well as the epithelium) is well demonstrated, but the overall surface curvature is not. Had this patient previously undergone either LASIK or Descemet membrane–stripping keratoplasty (DSEK), which he has not, the demarcation line would have been well imaged with this technology. C, Corneal tomography image using dual Scheimpflug/Placido–based technology of the same patient and eye shown in A and B. The surface curvature, pachymetry, and anterior and posterior elevation mappings are demonstrated. Numerical values are shown along the right side. D, Wavescan image from a device like that illustrated in Fig 1-1A, taken of the fellow eye to that represented in A, B, and C. Note that this map does not show any corneal surface contours or features but rather provides information about the optics of the entire ocular system. As such, it can provide information on the refractive error and aberrations of the entire eye.
(Courtesy of M. Bowes Hamill, MD.)
Elevation-based tomography is especially helpful in refractive surgery for depicting the anterior and posterior surface shapes of the cornea and lens. With such information, alterations to the shape of the ocular structures can be determined with greater accuracy, especially postoperative changes.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.