Corneal Tomography
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.
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.