The cornea, the clear, dome-shaped front surface of the eye, focuses light onto the retina. Its precise curvature and transparency are essential for clear vision. Corneal tomography is an advanced imaging technique that provides a detailed, three-dimensional (3D) assessment of the cornea. This technology is valuable for understanding the cornea’s structure and detecting subtle changes that might impact vision.
Corneal Tomography Explained
Corneal tomography generates a 3D model of the cornea by capturing multiple cross-sectional images. It measures both the anterior (front) and posterior (back) surfaces, as well as its thickness. This provides a complete picture of the corneal shape and internal structure.
The technology used includes rotating Scheimpflug cameras or optical coherence tomography (OCT). A Scheimpflug device, like the Pentacam, captures 25 to 50 images of the eye’s anterior segment in about 2 seconds. These images reconstruct a detailed 3D corneal image. This 3D capability reveals information not obtainable from simpler 2D methods.
Distinguishing Tomography from Topography
Corneal tomography and topography are distinct imaging techniques, though often confused. Corneal topography maps only the curvature of the front surface, similar to a geographical map. It uses reflected concentric rings to create a 2D representation of the anterior corneal shape, useful for assessing astigmatism and surface irregularities.
Corneal tomography provides a 3D picture by imaging both the front and back surfaces, along with its thickness profile. Consider a mountain: topography shows its visible surface, while tomography provides a full 3D model, revealing hidden slopes and internal structure. Visualizing the back surface and entire thickness is important for diagnosing conditions not apparent on the front surface alone.
Interpreting Tomography Maps
Corneal tomography systems generate color-coded maps providing specific corneal information. These maps help eye care professionals interpret the collected data. Common maps include elevation, pachymetry, and curvature maps.
Elevation Maps
Elevation maps display the corneal surface height relative to a reference sphere. They highlight subtle irregularities or localized bulges. Both anterior (front) and posterior (back) elevation maps are generated, with posterior elevation sensitive for detecting early corneal changes. Warmer colors (red, orange) indicate points above the reference, while cooler colors (blue) represent areas below it.
Pachymetry Maps
Pachymetry maps illustrate corneal thickness across its diameter. They highlight the thinnest point and its distribution. Normal thickness maps show a concentric pattern, with the thinnest point often slightly inferior-temporal to the center. Central corneal thickness and the thinnest point’s location are analyzed.
Curvature Maps
Curvature maps display the steepness and shape of the corneal surfaces. Warm colors indicate steeper areas, while cool colors represent flatter regions. These maps provide information on corneal power and astigmatism.
Clinical Significance of Corneal Tomography
Corneal tomography is important for diagnosing and managing various eye conditions due to its detailed 3D imaging. It reveals subtle structural changes.
Keratoconus Detection
A primary application is diagnosing and monitoring keratoconus, a progressive thinning and bulging of the cornea. Tomography detects early keratoconus, even in subclinical stages, where changes may not be evident with traditional topography. It reveals characteristic changes in posterior elevation and localized corneal thinning. Monitoring these parameters assesses disease progression and guides treatment, such as collagen cross-linking.
Refractive Surgery Screening
Corneal tomography is standard for screening patients considering refractive surgery like LASIK and PRK. It identifies individuals at risk for postoperative corneal ectasia, a complication where the cornea weakens and bulges after surgery. By assessing corneal surfaces and thickness, tomography helps ensure the cornea is healthy enough for surgery, minimizing adverse outcomes. Identifying subtle risk factors, like abnormal posterior elevation or an unusually thin cornea, is important for patient safety.
Other Applications
Beyond keratoconus and refractive surgery screening, tomography diagnoses and manages other corneal ectasias. It also evaluates the cornea after various eye surgeries, providing assessment of corneal shape and thickness post-procedure. The data aids in understanding the corneal response to surgical interventions.