Disclosure
Imaging techniques of the cornea are developing rapidly, mainly because of recent advances in refractive surgery. To understand the significance of new imaging techniques, the relevant principles of corneal optics are reviewed. The discussion of the most common clinical method of Placido-based corneal topography emphasizes important concepts of its clinical interpretation. The principles and clinical significance of several emerging technologies are reviewed.
Different concepts are used to characterize optical properties of the cornea.
The keratometric diopter is a concept inherited from keratometry and is calculated simply from radii of curvature, as follows: K=refractive index of 337.5/radius of curvature. This concept is a simplification ignoring the fact that the refracting surface is air-tear interface, and it does not account for the oblique incidence of incoming light in the corneal periphery. As a result, it miscalculates a true corneal refractive index of 1.376 to 1.3375 to correct for some of these factors. That is why these diopters more correctly are termed keratometric diopters to distinguish them from the diopters expressing more precisely the true refractive power at certain corneal point. Corneal shape The average anterior and posterior corneal power is 48.6 diopter (D) and –6.8 D, respectively. To simplify it in clinical practice or in keratometry, a substitution with one refractive surface with the resulting corneal power of 42-44 keratometric D often is used. The average cornea changes little with age. It flattens about 0.5 D by age 30 years and steepens about 1 D by age 70 years. During adulthood, an average cornea is steeper in the vertical meridian by about 0.5 D compared to the horizontal meridian, which contributes to higher incidence of with the rule astigmatism in young adults. This difference between vertical and horizontal curvature diminishes with age, finally disappearing at age 70 years. Lenticular changes contribute significantly to the higher incidence of against the rule astigmatism with age. Normal cornea is a prolate surface, ie, steeper in the center and flatter in the periphery. Oblate surface (eg, surface after myopic laser photorefractive keratectomy) is flatter in the center and steeper in the periphery. The visually significant area of the corneal surface is approximately the area with the same diameter as the pupil size. The pupil diameter decreases with age. A large variation exists between people in any age group. In one large study, the average pupil size in individuals aged 20 and 80 years was 4.5 mm and 3.5 mm, respectively, in bright illumination. In dim illumination, the average pupil size in individuals aged 20 and 80 years was 8 mm and 5 mm, respectively. This finding is clinically important because most refractive lasers treat an area with a 6.5-mm diameter with a surrounding blending zone. The average central corneal thickness is approximately 550 mm. In one study, the average thickness in temporal, nasal, inferior, and superior quadrants of the cornea was 590, 610, 630, and 640 mm, respectively. The thinnest site on the entire cornea is located on an average 0.9 mm from the visual axis most commonly in the inferotemporal quadrant. Mechanical properties The mechanical properties of human corneal tissue are less well understood. The intact central corneal thickness of 250 mm is considered enough to ensure long-term mechanical stability of the cornea. The peripheral thickness is not well studied, but it is certainly clinically important in some refractive procedures as intracorneal rings or radial and astigmatic keratotomy. With the advances in corneal imaging and widespread refractive surgeries, corneal behavior likely will be understood better.
Corneal topography instruments used in clinical practice most often are based on Placido reflective image analysis. This method of imaging of the anterior corneal surface uses the analysis of reflected images of multiple concentric rings projected on the cornea. Corneal topography – Principles Multiple light concentric rings are projected on the cornea. The reflected image is captured on charge-coupled device (CCD) camera. Computer software analyzes the data and displays the results in a variety of formats Interpretation of topographic maps Every map has a color scale that assigns particular color to certain keratometric dioptric range. Never base an interpretation on color alone. The value in keratometric D is crucial in the clinical interpretation of the map and has to be looked at with the interpretation of every map. Absolute maps have a preset color scale with the same dioptric steps, dioptric minimum and maximum assigned to the same colors for particular instrument. These maps allow direct comparison of 2 different maps. Although, because the steps are in large increments (generally 0.5 D), their disadvantage is that they do not show subtle changes of curvature and can miss subtle local changes (eg, early keratoconus). Normalized maps have different color scales assigned to each map based on the instrument software that identifies the actual minimal and maximal keratometric dioptric value of a particular cornea. The dioptric range assigned to each color generally is smaller compared to the absolute map, and, consequently, maps show more detailed description of the surface. The disadvantage is that the colors of 2 different maps cannot be compared directly and have to be interpreted based on the keratometric values from their different color scales. Corneal topography measurements Corneal topography most commonly is thought of as Placido-based reflective image analysis. It does not directly measure the x, y, and z coordinates of the points on the corneal surface that usually are used for characterization of objects in 3-D space. Instead, it typically measures the deviation of reflected rings and primarily calculates the curvature of the corneal surface points in axial direction. Curvature/power map Surface curvature measures how fast the surface bends at a certain point in a certain direction. Axial curvature (formerly termed sagittal curvature) measures the curvature at a certain point on the corneal surface in axial direction relative to the center. It requires the calculation of the center of the image, which cannot be measured directly. Meridional curvature (formerly termed tangential and instantaneous curvature) measures the curvature at a certain point on the corneal surface in meridional direction relative to the other points on the particular ring. Meridional curvature maps always are more sensitive measures of local curvature change. Axial curvature maps can be derived from meridional maps. Axial value at a certain point equals the average meridional curvature along the radius from the map center to the point of interest, thereby approximating the average refractive power. Axial and meridional maps should be displayed theoretically in the units of radii of curvature (ie, mm) at each corneal surface point. For ophthalmologists who better understand more clinically used units of D in ophthalmic practice, the instruments display curvature in units of keratometric D and constitute so-called axial or meridional power maps. Elevation map Elevation is not measured directly by Placido-based topographers, but certain assumptions allow the construction of elevation maps. Elevation of a point on the corneal surface displays the height of the point on the corneal surface relative to a spherical reference surface. The reference surface in most instruments was chosen to be a sphere. Best mathematical approximation of the actual corneal surface called best-fit sphere is calculated by instrument software for every elevation map separately. The same surface may appear different when mapped against different reference surfaces. Consequently, it is difficult to compare directly 2 elevation maps that likely have slightly different best-fit spheres as reference values, and comparison only can be intuitive. To understand intuitively the theoretical difference between the elevation and curvature, it is easiest to imagine a corneal surface on Image 6. It is clear that the spherical curvature A-B and C-D are the same, although the elevation is quite different. As in laser refractive surgery, the refractive power is changed by removing tissue from the corneal surface, and elevation data appear more relevant for calculation of ablation depth and optical zones. Notice that in practice the colors in the same region of elevation and axial curvature maps often are reversed. The vertical 90° axis is steeper in those incidences of with the rule corneal astigmatism. Therefore, the superior area typically is depressed (blue on elevation map and steeper, red on axial map) as seen on Image 5. The bow tie pattern on the axial map is just a different mathematical representation of an oval spherocylindrical surface. It is not seen on the elevation map that is created from the x, y, and z coordinates of the usual representation of data in a 3-D world. Interpretation of topographic indexes Simulated K (SimK): Simulated keratometry measurements characterize corneal curvatures in the central 3-mm area. The steep simulated K-reading is the steepest meridian of the cornea, using only the points along the central pupil area with 3-mm diameter. The flat simulated K-reading is the flattest meridian of the cornea and is by definition 90° apart. These readings give an idea about the central corneal curvature that is frequently visually most significant. The 3-mm diameter was chosen primarily from historical reasons for the purpose of comparison with standard keratometry that is used for analysis of 4 central points, 3.2 mm apart. The index of asphericity: The index of asphericity indicates how much the curvature changes upon movement from the center to the periphery of the cornea. A normal cornea is prolate (ie, becomes flatter toward the periphery) and has the asphericity Q of -0.26. A prolate surface has negative Q values and positive oblate surface values. Most myopic laser vision corrections change the anterior corneal surface from prolate to oblate. Indexes characterizing the uniformity and optical quality of the anterior corneal surface: Currently, best spherocylindrical correction that is used in glasses, soft contact lenses, and laser vision correction and are reflected in keratometric indexes do not correct all the optical aberrations of the corneal surface. The best visual acuity also requires uniform smooth anterior corneal surface. A variety of these indexes try to relate the variability of the actual values of the anterior corneal surface obtained by corneal topography to the optical quality and potential best visual acuity that would be permitted by the anterior corneal surface. These indexes can be thought of in clinical practice as different mathematical estimates of the visual disturbance that can be expected to be caused by the amount of irregularities of the anterior corneal surface. Many indexes specific for each instrument exist. Examples include the following: surface regularity index (SRI), corneal uniformity index (CUI), predicted corneal acuity (PCA), and point spread function (PSF). It also is important to realize that patients with normal corneal indexes can have poor vision caused by disturbances in any other part of the optical system of the eye. Limitations of corneal topography: The error of corneal topography is under optimal conditions in the range of ±0.25 D or 2-3 mm, but, in those abnormal corneas seen in clinical practice, it often is ±0.50-1.00 D due to several limitations. The imaging requires an intact epithelial surface and tear film. Some of the errors of the Placido-based systems are as follows: focusing errors, alignment and fixation errors with induced astigmatism, difficulty to calculate the position of the center from the small central rings, increased inaccuracy toward the periphery because the accuracy of each point depends on the accuracy of all preceding points, and other errors. Different technologies use different measurement methods and algorithms; thus, the output data are not directly comparable. Also, the technologies undergo constant modifications, and the results of studies comparing the instruments are outdated quickly and difficult to interpret for practical clinical purposes. In addition, normal spherocylindrical surface imaging techniques should be able to characterize a variety of more complex optical surfaces, as follows: very steep or flat corneas, keratoconus with local steepening, sharp transition zones after uncomplicated refractive surgeries, diffusely irregular surfaces after penetrating keratoplasty, complex surfaces after complicated refractive surgeries with decentered ablations, and central islands. These optical surfaces are more difficult to measure. For example, 7 commercial topographers have been tested on nonspherical models, and errors greater than 4 D may occur. Corneal topography in normal corneas The normal cornea flattens progressively from the center to the periphery by 2-4 D, with the nasal area flattening more than the temporal area. The topographic patterns of the 2 corneas of an individual often show mirror-image symmetry, and small variations in patterns are unique for the individual. The approximate distribution of keratographic patterns described in normal eyes includes the following: round (23%), oval (21%), symmetric bow tie typical for regular astigmatism (18%) (see Image 1 and Image 5), asymmetric bow tie (32%) (see Image 2), and irregular (7%). Most important clinical applications of corneal topography Detection of corneal pathologic conditions most importantly keratoconus and pellucid marginal degeneration: Corneal topography often is used clinically for detecting and evaluating of the severity of keratoconus (see Image 3). In clinical practice, the topographic diagnosis of keratoconus often is suggested by high central corneal power, large difference between the power of the corneal apex and of the periphery, and disparity between the 2 corneas of a given patient. Corneal topography has detected changes suggestive of early keratoconus in many patients without classic clinical manifestations of this disease that often are classified as subclinical keratoconus. Corneal topography is not sufficient for the definitive diagnosis of keratoconus because other disorders, such as corneal warpage from contact lens wear, can simulate early keratoconus. No history of contact lens wear, corneal stromal thinning, and other clinical signs would confirm the diagnosis of keratoconus. Several quantitative indices to diagnose keratoconus topographically have been proposed. For example, central steepening greater than 47.2 D, inferior-superior asymmetry index over 1.2 D, and high index of irregular astigmatism have been suggested for topographic diagnosis of keratoconus. Automatic diagnosis of corneal disorders based on corneal topography by the instruments has been improved recently with the use of mathematical algorithms called neural networks. These algorithms are programmed to improve by themselves with experience, and, after a period of training on several hundred corneal topographies, they are reported to be highly specific. Screening before refractive surgeries: The detection of keratoconus is of particular importance in patients who plan to undergo refractive surgery. Evaluation of irregular astigmatism especially after penetrating keratoplasty: Corneal topography is mostly valuable for detection of postoperative astigmatism, planning of removal of sutures, and postoperative fitting of contact lenses. Evaluation of the effects of corneal refractive surgeries: Corneal topography is useful for evaluation of effects and stability of all refractive procedures (see Image 4). The value for guiding refractive treatments currently is evaluated. Contact lens fitting: Corneal topography is especially valuable in fitting complex corneal surfaces (eg, after penetrating keratoplasty). Raster photography topography method The posterior apical radius (PAR) imaging device projects a regular pattern of lines or grid of known geometry onto the anterior corneal surface that is viewed from an offset angle. The topical fluorescein is used to stain the tear film, and cobalt blue light is used to illuminate the pattern. The accuracy and reproducibility is in the range of Placido-based systems, but the advantage is that the PAR system does not require an intact epithelial surface. |
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The Orbscan corneal topography system uses a scanning optical slit design that is fundamentally different from the corneal topography that analyzes the reflected images from the anterior corneal surface. The high-resolution video camera captures 40 light slits at the 45° angle projected through the cornea similarly as seen during slit lamp examination. The instrument's software analyzes 240 data points per slit and calculates the corneal thickness and posterior surface of the entire cornea. The anterior surface of the cornea initially was calculated in this manner; however, since the calculation from the reflected images used by corneal topography is more precise, the current version of Orbscan is using the latter method and is a combination of reflective corneal topography and optical slit design. Orbscan currently is the most common instrument in clinical practice that allows the analysis of the corneal thickness and posterior surface of the entire cornea (see Image 5) and has the ability to detect abnormalities in those areas. Currently, limited knowledge of the 3-D structure and the mechanical behavior of the cornea, especially after different types of refractive procedures, are available. More detailed knowledge of this behavior could improve the outcomes of refractive procedures. With the interpretation of the abnormalities of the posterior surface of the cornea, it is important to realize that the posterior corneal surface contributes 7 times less compared to the anterior surface to the refractive power of the cornea. The refractive clinical significance of the abnormalities of the posterior surface currently is being investigated. The accuracy and repeatability of the instrument is reported to be below 10 mm and, under optimal conditions, in the range of 4 mm in the central cornea and 7 mm in the peripheral cornea. In clinical practice, it is even more dependent than corneal topography on many factors, such as the limited movement of the patient's eye, ability of patients to keep the eye wide open, optically clear cornea, and the presence of corneal abnormalities. The other limitations of current optical slit technology are the inability to detect interfaces (eg, after LASIK flap) and the longer time of image acquisition and processing compared to standard Placido-based topography.
The conventional microscope collects all the light reflected back from the object, and out of focal plane information above and below areas of interest creates noisy images with limited resolution; therefore, a need to section the specimens thinly exists. This method does not allow in vivo imaging. In confocal microscopy, the pinhole source of light and its conjugate pinhole detector limit the collected out of focus signal. The field of view of such design is very small and full field of view must be built up by scanning. It can be achieved by rotating discs with thousands of optically conjugate source-detector pinholes. Some of the confocal systems use laser beams as the point source and are called laser confocal scanning microscopes. The advantages of confocal microscopy are as follows: (1) high resolution in the range of 1 mm and the ability to image cells of corneal epithelium, epithelial nerve plexus, different parts of stroma, and endothelium; and (2) in vivo imaging without the need to mechanically section the cornea. The limitation is the necessary to obtain multiple sections to evaluate larger areas of cornea and the time of image acquisition and processing. Time to acquire a single image is typically less than 1 second, and the observation time in clinical settings is around 10 minutes. The maximal depth of imaging is around 2.7 mm. In clinical practice, confocal microscopy has been used to identify Acanthamoeba cysts and trophozoites, fungi, and other pathogens in the cornea. In clinical research, it can be used for evaluation of responses and complications after refractive procedures, corneal healing processes, and a variety of corneal diseases. In experimental studies, it can be used in any application requiring the imaging of the cornea at microscopic level.
Optical coherence tomography (OCT) of the cornea is an optical method of cross-sectional scanning based on reflection and scattering of light from the structures within the cornea. Measuring different reflectivity from structures within the cornea by a method of optical interferometry produces the cross-section image of the cornea. In optical interferometry, the light source is split into the reference and measurement beam. The measurement beam is reflected from ocular structures and interacts with the reference light reflected from the reference mirror, a phenomenon called interference. The coherent or positive interference characterized by an increased resulting signal is measured by the interferometer, and, subsequently, the position of the reflecting structure of the eye can be determined. The resolution of 6-25 mm allows both imaging and calculating of the thickness of corneal epithelium, LASIK flap, and 3-D structure of the cornea under normal or pathologic conditions at this precision level.
High-frequency high-resolution ultrasound biomicroscopy (UBM) is similar in principal to traditional immersion B-scan ultrasonography. In general, the resolution and depth of penetration are affected by transducer frequency. The traditional ultrasonography of the whole eye uses a 10 MHz transducer, with approximately 150 mm resolution. Higher frequency of a 50 MHz transducer increases the tissue resolution to 50 mm but only at the expense of decreasing tissue penetration depth of 4-5 mm sufficient to image the anterior segment. Scanning of the cornea is possible with a 100 MHz transducer that increases the tissue resolution to less than 20 mm. The tissue resolution of 5-20 mm distinguishes the layers of the cornea and the corneal flap after LASIK and characterizes the 3-D structure of the cornea at this precision level under normal or pathologic conditions. Another advantage is that the imaging of the anterior segment is possible even through edematous or scarred corneal tissue.
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