Fundamental Concepts in Corneal Imaging
Mehdi Roozbahani, MD; Marcony R. Santhiago, MD, PhD; William J. Dupps, MD, PhD; and J. Bradley Randleman, MD
The cornea is a unique structure of equal importance optically and anatomically to the function of the eye. Given its anterior-most location, it is uniquely available to measure and to alter surgically to induce specific refractive outcomes.
Corneal imaging takes on many different functions. We routinely assess corneal curvature, corneal refractive power, sublayer corneal anatomy, and ocular aberrations in both normal corneas and those with pathology. Assessing and analyzing corneal imaging requires an examiner with experience to understand the implications of subtle findings and to evaluate signs of weakening or pathology accurately. Possible inferences about the structural integrity, derived from the interpretation of the image, inform about the possibility of surgical application with the maintenance of the optical and physiological function. Failure to recognize critical findings in their subtlest presentations can lead to incorrect diagnoses, inaccurate or improper treatment plans, and iatrogenic corneal disease.
Corneal imaging has become an integral part of eye care in cornea clinics. One of the main applications of corneal imaging has been in the identification of ectatic corneal disorders in patients with visual complaints and in those presenting for refractive surgical evaluation. Much of this text will be directed toward those evaluations; however, there are multiple other uses for advanced corneal imaging, and this text will cover those as well.
The purpose of this book is to guide the reader through the process of analyzing corneal images using a variety of different techniques, technologies, and individual devices to create a complete picture of the cornea’s basic methods and pathological processes that present to clinicians on a daily basis.
Table 1-1
ELEMENTS OF CORNEAL IMAGING
| PROPERTY | EXAMPLES |
| Surface geometry (anterior or posterior) | Curvature |
| Elevation | |
| Calculated refractive properties | Power |
| Astigmatism: Regular vs irregular | |
| Aberrations: Total and corneal | |
| Thickness | Corneal thickness |
Total: Central and regional | |
Epithelial | |
Bowman | |
Stromal | |
| Cellular and extracellular matrix anatomy | Confocal microscopy |
| Biomechanical | Static or dynamic assessment of biophysical properties |
In the 1880s, Antonio Placido performed the first photokeratoscopy. He photographed the reflected image of an altering concentric light and dark rings (also known as mires). These sets of dark and light rings were later called Placido disc, and the imaging called photokeratoscopy. If there is pathology on the corneal surface, it compromises the anterior corneal surface, and rings will not look round and circular. Rings look closer together in the steeper areas and more widely spaced in the flatter areas of the cornea. This was the first effort to record information from the corneal curvature. The photokeratoscope was not precise and suffered from being qualitative. It was also not able to provide information about the amount of the curvature. In 1896, Gullstrand used a numerical algorithm to perform the first quantitative assessment of corneal curvature on Placido disc photokeratoscopes.
Figure 1-1. Photokeratoscope image showing ring reflections from a normal cornea (left) and a cornea with keratoconus (right). Note the symmetric distribution of rings in the normal eye contrasted with the uneven ring distribution in the left image, with the inferior paracentral rings spaced closely together (black arrow).
Almost 100 years later, in the 1980s, the development of new refractive surgery procedures, such as epikeratophakia and later the excimer laser, gained attention. The only way to evaluate the effectiveness of these procedures, however, was by measuring visual acuity. Stephen Klyce, PhD, endeavored to achieve a better and more accurate method for the imaging process, which culminated in modern corneal topography, when computers were utilized for analysis of the images recorded by videokeratoscopy. Computer programs were able to appraise several points on the corneal surface through color-coded maps. This advance helped the examiners to access the cornea more quickly, practically, and repeatedly, and also to interpret and compare the corneal curvature data more conveniently through the detailed information displayed. Soon, computerized videokeratoscopes (commercially named topography devices) became the preferred method for corneal evaluation.
Figure 1-2. Videokeratoscope image showing raw ring images on the left and calculated focal curvature measures on the right.
Figure 1-3. (A) Photokeratoscope photographic image of the cornea from a patient with keratoconus. Note the significant distortion in the mires, especially centrally, in the area of maximal corneal warpage and steepening. (B) Raw data as collected from the photokeratoscope image shown in A and then digitized using computer software. (C) Smooth data after transformation from the raw data shown in B using computer generated programming from a research computer at Louisiana State University by Stephen Klyce, PhD, used to develop algorithms to convert the shapes into corneal curvature data. (Reproduced with permission from Stephen Klyce, PhD. From: Klyce SD. Computer-assisted corneal topography. High-resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci. 1984;25[12]:1426-1435.) Today, most corneal imaging devices are based on corneal topography or corneal tomography and provide highly processed, color-coded images to assist with clinical evaluation. Topographers provide information from the anterior corneal surface obtained directly by reflection, whereas tomographers reconstruct full-thickness sectional images of the cornea and then reconstruct these images to show corneal thickness and shape. BASIC DEFINITIONS AND TERMINOLOGY The shape of the cornea has toricity (difference between curvature of the principle meridians) and asphericity (varying curvature and refractive power across the cornea from center to periphery). The corneal apex is the point of the highest curvature and vertex is the highest point on the corneal surface. The definition of the apex is related to the curvature while the definition of the vertex is related to the shape of the cornea. Mixing up the definitions can lead to confusion in interpretation of the maps. Multiple terms used in corneal imaging can be confusing, even to advanced users. A select list of important terms is discussed here: Figure 1-4. Placido ring images from the right and left eyes of a patient. These are the reflection data captured and used to generate color-coded maps. Figure 1-5. (A) Raw Scheimpflug images from the Scheimpflug (Pentacam) and (B) dual Scheimpflug/Placido (Galilei) devices. While the images are slightly different, in both, one can see the anterior cornea, posterior cornea, anterior iris, and lens features are clearly visible and in focus. These are the data used to generate color-coded maps. There are important concepts that are often confused, interchanged, or poorly understood that limit basic understanding of corneal imaging processes. A brief clarification follows; for a more detailed description the authors recommend the following resource: Roberts CJ. Introduction. In Kilic A, Roberts CJ, eds. Corneal Topography: From Theory to Practice. Kugler Publications; 2013:x-xix. Toricity: Meridian Versus Axis When discussing corneal astigmatism, the terms meridian and axis unfortunately often get used interchangeably when they are not and, typically (if used correctly), should represent physical locations 90 degrees apart from one another. In the cornea, a corneal meridian is a physical location that represents a virtual half circle running in a specific orientation. An axis describes an imaginary line about which something rotates. Regarding the cornea, the most commonly described corneal meridian is the principle power meridian, which is the meridian with maximal corneal curvature (and, therefore, power centrally). The axis is the line around which a cylinder rotates that would neutralize the principal meridian. As a cylinder has no refractive power along its axis, the cylinder axis and the principle corneal meridian are always 90 degrees apart. As this book addresses corneal imaging instead of refraction, we will endeavor to use the term meridian throughout since we will not, in general, be talking about cylindrical lenses applied to correct astigmatism. Figure 1-6. Composite image demonstrating the orientation of the steep corneal meridian and the axis of cylinder perpendicular to that meridian. Curvature: Axial Versus Tangential Corneal surface curvature is typically shown in either axial or tangential curvature maps. These methods deduce curvature for any point on the cornea differently and thus the maps appear different. In simple terms, tangential curvature can be thought of as a focal or local curvature, a measure that provides specific information of curvature at a focal point, independent of any adjacent points, while axial curvature can be thought of as an averaged value, taking into account the points between the specific point being measured and the optical axis. In this way, axial curvature represents an averaged, or smoothed, measure of curvature, while tangential curvature represents a focal, unaltered measure of curvature. Both map styles play a role in corneal evaluations. Most clinicians prefer axial maps for general evaluations as these present an overall interpretation of corneal shape with less focal noise/irregularities to overlook. For specific foci, however, tangential maps are superior in their presentation. Tangential maps can be particularly useful in identifying postoperative patterns, the extent of focal corneal pathology, and when assessing local curvature changes in ectatic corneas. Figure 1-7. (A) Placido image showing tangential (left) and axial (right) curvature maps of a patient who underwent myopic LASIK. The pattern of central flattening is more obvious in the tangential map. Figure 1-7. (B) Scheimpflug image showing axial (upper) and tangential (lower) curvature maps of a patient who underwent myopic LASIK. The pattern of central flattening is more obvious in the tangential map. (C) Scheimpflug image showing axial (upper) and tangential (lower) curvature maps of a patient with inferior steepening. The focal nature of the steepening is more distinct in the tangential map. Sagittal curvature is measured perpendicular to the plane of measure for tangential curvature and, as such, has no clinical meaning. The term, however, has inadvertently persisted in some devices and many reference sources, when in reality, the correct term to use would be axial. Therefore, the term sagittal will not be used in this text, even when this term is displayed in the maps arising directly from various devices. Curvature Versus Power Many individuals use corneal curvature and power interchangeably; however, they are different terms with different meanings. The 2 measures are directly related in the central cornea, and both are measured in diopters, which exacerbates the confusion. Curvature is an independent measure, however, while power requires information on the angle of incidence of light rays and the refractive media through which the light passes. Because of the small angle of incidence in the central cornea, when the curvature increases, refractive power increases as well. In the corneal periphery, however, this relationship is no longer relevant. When curvature decreases peripherally, refractive power increases due to the larger angle of incidence. This phenomenon gives rise to the clinically relevant concept of spherical aberration. Figure 1-8. (A and B) Scheimpflug images of 2 separate patients showing corneal curvature (left) and total corneal power (right) highlighting the relationship between these variables. Note how the local numbers seen on each map more closely coincide centrally but deviate from one another peripherally, with total corneal power being greater than axial curvature in the periphery. Curvature Versus Elevation Both anterior and posterior corneal surfaces can be represented in terms of curvature or elevation. Curvature can be thought of as an independent measure, while elevation, requires a reference shape against which to compare the surface being evaluated. A variety of reference shapes can be employed, and unique reference shapes produce different map appearances. The best reference surface shape to use is hotly debated. Different shapes will create individual elevation maps, and all elevation map data must be interpreted within the scope of the reference shape employed. Curvature maps are more sensitive to alterations in the tear film and epithelial irregularities than elevation maps. Figure 1-9. (A) Placido image of a patient with a history of myopic LASIK showing the relationship in this case between corneal curvature (left) and anterior corneal elevation (right) maps.
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