Applications and Set Up
Mehdi Roozbahani, MD; Marcony R. Santhiago, MD, PhD; William J. Dupps, MD, PhD; and J. Bradley Randleman, MD
As mentioned in Chapter 1, there are a variety of imaging modalities available to clinicians today and a variety of aspects of the cornea that can be imaged. Although there are numerous elements in the essential equipment set up that are common to most technologies, some differences must be recognized, as the information from different devices may not be interchangeable in many circumstances.
In this chapter, we will explain some fundamental concepts such as color-coded mapping (including color steps and scales), discuss individual technology categories, and show a variety of examples of imaging artifacts that, if not recognized, will negatively impact image interpretation.
Before any pattern analysis or clinical decision making is undertaken, each user must become familiar with their respective imaging devices and learn how to set up device output for maximal information. The specific user’s manual contains valuable information on how to set up each output map. Going through each device available on the market in detail is beyond the scope of this text. We recommend the reader review the user manuals and become acquainted with your particular devices, making sure the appearance of the map output is set up to optimize your clinical decision making.
Figure 2-1. Example of a set-up screen from the Scheimpflug Pentacam device (Oculus) showing some of the various parameters that can be adjusted.
Map Colors
Placido-based topography machines enabled the incorporation of raw data from thousands of points into a 2-dimensional color-coded map that would later help surgeons and clinicians recognize the familiar pattern of corneal diseases instantly. Color codes are used for curvature, elevation, and pachymetry maps. When using an absolute scale, mild/neutral temperature colors (green and yellow spectrum) represent points very close to the normal values (points close to reference surface on elevation maps; normal curvatures on curvature maps); warmer colors (red and orange spectrum) represent points higher than normal values (points higher than reference surface on elevation maps; steeper parts of the cornea on curvature maps); and cooler colors (blue and purple spectrum) represent spots lower than normal values (points lower than the reference surface on elevation maps; flatter parts of the cornea on curvature maps). In terms of pachymetry, cooler colors represent thicker values, mild colors are used for normal pachymetry values, and warm colors for thinner than average values.
Color Steps
A color step is the amount of change needed to require a different color output. Color steps can be altered for curvature, thickness, and elevation data. Among these, curvature is one of the most sensitive to changes in terms of the information relayed. For curvature, most devices range from 0.25 diopter (D) color steps to 2.0 D color steps. The images generated from different color steps can look remarkably different despite representing the same corneal shape. While there are valid arguments for each step, we recommend using 0.5 D color steps to maximize detection of focal curvature differences (better than 1.0 D or 1.5 D steps), while reducing the noise obtained with 0.25 D scales.
Figure 2-2. (A) Placido image showing the same cornea in 0.25 D (upper left), 0.5 D (upper right), 1.0 D (lower left), and 1.5 D (lower right) color steps. The patterns appear remarkably different despite being generated from the same raw data. (B) Placido image showing a patient with inferior steepening that is masked in 1.5 D color steps (left) but evident in the 0.5 D color step setting (right).
Figure 2-2. (C and D) Scheimpflug image of a patient who presented for LASIK evaluation. Note the different appearance of the anterior curvature map (upper left) between the maps with (C) 1.5 D output and (D) 0.5 D output. Figure 2-2. (E and F) Dual Scheimpflug/Placido image of a patient with central keratoconus. Note the different appearance of the anterior curvature map (upper left) between the maps with (E) 1.5 D output and (F) 0.5 D output.
Color Scales
Color-coded maps provide qualitative information about the cornea. On the maps, dioptric range, step size (intervals), and number of colors can be adjusted. Different scales can be chosen for displaying the color-coded maps:
- Absolute scale: Provides a spectrum of colors for the whole range of diopters that the machine is able to measure. A specific color is assigned for each diopter by the device manufacturer. The relation between numbers and colors remains constant in all images generated in the absolute scale. The absolute scale ranges typically from 10 D to 90 D; thus, many of the colors are never utilized as they lie outside the physiologic range of the cornea.
- Fixed scale: Similar to the absolute scale, with each color being affixed to a specific value that does not vary, but with a narrower range of diopters assigned colors to cover most physiologic conditions. Beyond the assigned range any values will retain the warmest or coolest color assigned depending on the direction that is exceeded.
- Relative (normalized) scale: The relative scale adjusts the color range output based on the specific cornea being measured. Thus, any particular color is not a fixed value but is assigned based on the relative curvature and steepness of the cornea being measured. Advantages of this scale include the ability to show an expanded range of values in eyes at extreme ends of the normal spectrum. Disadvantages of the relative scale include lack of color standardization, so that any shade of red, green, yellow, etc does not hold specific meaning, and the potential to minimize focal curvature changes over normal physiologic ranges.
Figure 2-3. (A) Placido images of the right and left eyes from the same patient taken 10 years apart. On initial review, it appears that there has been progression in steepening in both eyes, given the shift from green colors (upper) to red colors (lower). On closer inspection, however, the top maps are shown in a relative scale (normalized), while the bottom maps are shown in a fixed scale. There are no changes in steep or flat K, or in steepest K (number not shown), and there is no change in pattern in either eye.
While it is beyond the scope of this book to provide a comprehensive review of every imaging device on the market, the following sections will highlight critical features for the major categories of imaging devices, including Placido-based topography, scanning slit tomography, Scheimpflug-based tomography and its variations (combined Scheimpflug/Placido and dual Scheimpflug/Placido devices), and anterior segment optical coherence tomography (AS-OCT).
There are a variety of commercially available Placido-based devices. Although manufacturers might offer various maps to make their product unique, they utilize similar imaging concepts. We introduce some of the popular maps in Placido-based devices here.
Curvature Maps
Curvature maps are mostly labeled as “power” maps in topography machines. Curvature is the reciprocal of the radius of curvature in meters. The normal unit for curvature is m-1, but it usually converts to keratometric diopter, which is more familiar to clinicians. This should not be confused with the refractive power map, which also displays refractive power in diopters (D). Based on the axis that is used for obtaining the radius of curvature, there are at least 2 different methods to calculate the curvature of every point on the cornea:
- Axial: To calculate the curvature map, the radius of curvature (R) must be known. In the axial map, R is the distance between each point on the anterior corneal surface and the optical center of the eye, which is on the optical axis. In this method, the curvature of every point on the corneal surface calculates from the same reference axis and considers cornea as a spherical surface. Due to using a common reference axis for all points on the surface, small changes on this display may not be visible. Smoothening of the data makes qualitative analysis in the screening of the refractive surgery candidates easier.
- Tangential (instantaneous): Instead of using the same axis for R, in this method, R is calculated for each point on the corneal surface locally, without any overall reference. The tangential map points out small differences between 2 adjacent points and represents local changes more obviously. Although this map could be noisier than the axial map, it may be more sensitive for detecting peripheral diseases at earlier stages. Every point on this map is calculated independently, so it is potentially less affected by misalignment.
Note: There are some devices that list “sagittal curvature” and/or use that term interchangeably with axial curvature; this nomenclature is incorrect. Sagittal curvature is not a relevant curvature for the evaluation of the cornea; however, since it is in use in some devices, we mention it here. For the remainder of the book, only the term “axial” will be used.
Elevation Maps (3D Map)
Placido-based devices reconstruct the corneal shape by creating elevation data from curvature. As opposed to the meticulous and precise concept of triangulation, which is used in elevation-based tomographers, the concept of “angle of refraction” is used by Placido-based topographers to calculate the elevation map. The latter is not as accurate as the triangulation and reliability of the numbers are less robust on Placido disc elevation maps.
Refractive Power Map
These power maps are calculated by ray tracing method and applying Snell’s law. The refractive power map shows increasing power from center to periphery, which happens due to the change in the angle of incidence of the light rays. It demonstrates the local power change in every point of the cornea needed to correct for a given refractive error.
Mean Curvature Map
These maps show the average curvature of the 2 principal curvatures for any given point on the cornea. Whereas axial and tangential views are defined along individual meridians (or slices of the cornea), the mean curvature map contains curvature information along both the radial and azimuthal directions of every single data point. As the mean curvature map provides the average curvature in all directions around the data point (axial and tangential curvature is only along the semi-meridian), it suppresses corneal astigmatism and is insensitive to patient fixation. The theoretical most important advantage of the mean curvature map is that it may better detect subtle shape abnormalities because it is a more faithful description of 3-dimensional shape and has the ability to suppress some nonpathological features (eg, corneal astigmatism) while preserving pathological features (eg, keratoconus).
The Atlas topographer (Zeiss), one of the most commonly utilized Placido-based corneal topography devices in use today, provides various maps, displays and indices.
- Simulated keratometry (SimK): Measurement that simulates the readings of a standard keratometer. These readings are computed in a similar way to the calculations performed by keratometers commonly used to measure the central 3 mm, assuming the principal axes have a 90-degree shift.
- Eccentricity: A measure determined by the shape of the cornea. The eccentricity indicates the departure of the peripheral curvature from the apical radius and so defines the degree of asphericity.
- Q value: A measure of the shape of the cornea. It is defined as the inverse (or negative) of the shape factor. Q is an index of asphericity and 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). A prolate surface has negative Q values and an oblate surface has positive values. Most myopic laser vision corrections change the anterior corneal surface from prolate to oblate, while hyperopic ablations create a hyperprolate surface.
- Shape factor: A measure of the asphericity of the cornea. Shape factor gives a measure of the flattening or steepening of the cornea along its flattest meridian. This value should be used in conjunction with the corneal irregularity measurement (CIM) value to gain the most information about the cornea. According to the manufacturer, this factor can be used to determine whether the cornea is more oval or elliptical. Shape factors are unique and different from eccentricity in that it is possible to calculate a negative, or oblate, shape as well as a positive, or prolate, shape. Negative Shape factors imply an oblate cornea (flatter in the center, steeper in the periphery). Positive shape factors imply a prolate cornea (steeper in the center, flatter in the periphery). When the shape factor reaches 0.7 and above, the cornea begins to exhibit a more conical shape, and usually indicates some sort of pathology or abnormal shape. Shape factor is useful in contact lens fitting and in identifying unusual or pathological corneal shapes. Shape factor is based on a best-fit general ellipsoid to the corneal surface. This takes into account the effects of astigmatism, and the tip and tilt of the cornea symmetry axis. The value of both the CIM and shape factor measurements can be seen with serial topography over time. Judging whether these statistical indices are increasing or decreasing over time may indicate a worsening pathology or a healing trend.
- CIM: A statistical measurement that uses topographic data from the central area of the cornea and compares it to the best-fit surface found. It determines the regularity or irregularity of the corneal surface used for vision. The CIM measurement determines how much the actual corneal surface varies from a smooth fitted ellipsoidal toric surface as root-mean-square (RMS) error in microns. The fitted surface includes any regular astigmatism that may be present. The higher the irregularity index, the more uncorrectable or uneven the surface is optically, thereby highlighting irregular astigmatism that often results in visual distortions. CIM uses the thousands of data points within the first 14 rings of the corneal topography data to determine the difference in height or elevation between the patients cornea and the best matching, perfect model ellipsoidal toric cornea. The difference between the perfect model and the actual cornea is measured in microns and the standard deviation (SD) is taken. This is defined as CIM. Higher CIM values, then, would tend to indicate a worsening pathology, such as keratoconus. High CIM indices can also be observed in post–refractive surgery corneas due to the irregularity that the surgical procedure and subsequent healing process cause. CIM also measures the type of irregular astigmatism that cannot be corrected with spectacle lenses. CIM values range from 0.35 to 0.80 in normal shaped corneas, and 1.0 and higher in abnormally shaped corneas. The higher the value, the more irregular the corneal surface is. When CIM values are measured at 2.0 µm (RMS) and above, severe corneal distortion is usually present.
- Toric keratometric mean, also known as Ro: The average corneal curvature at the apex. The apical position is the intersection of the symmetry axis of the best-fit ellipsoidal toric with the cornea. Two principal curvatures are calculated at the apex for the flattest and steepest meridian and their mean is determined. This is described as the mean value of apical curvature.
- Horizontal visible iris diameter (or white-to-white) measurement: Measures corneal diameter.
- Toric keratometric mean, also known as Ro: The average corneal curvature at the apex. The apical position is the intersection of the symmetry axis of the best-fit ellipsoidal toric with the cornea. Two principal curvatures are calculated at the apex for the flattest and steepest meridian and their mean is determined. This is described as the mean value of apical curvature.
Figure 2-4. (A) Labeled overview image from the Atlas topographer with various specific points labeled. Multiple indices are listed in the legend to the right. (B) Placido overview image showing basic display maps, including tangential curvature (upper left), axial curvature (upper right), elevation (lower left) and corneal wavefront (lower right).
Figure 2-4. (C) Placido overview image showing basic display maps, including axial curvature (upper left), corneal wavefront (upper right), keratometry (lower left), and raw rings image (lower right). (D) Placido image showing corneal wavefront data. (E) Placido screening indices. In this patient with ectatic corneal disease the indices identify the pattern as post-hyperopic ablation, and individual metrics are normal or mildly suspicious. (F) Placido difference maps. The change in curvature between first (left upper) and last (left lower) visits is shown in the right map. Figure 2-4. (G) Composite Placido image showing the maps of the same individual taken on different Placido devices within the same 2 weeks. Note that while individual values are slightly different, the patterns are analogous between devices.
Scanning Slit Imaging (Orbscan II)
The slit scanning principles were explained in the first chapter. During every scan, 20 vertical slits of light (12.5 × 0.3 mm) are projected from the right and 20 from the left on the cornea at the fixed angle of 45 degrees to the device axis. Each slit extracts data from 240 points. Acquisition time is about 1.5 seconds. The overlapping pattern of slits are recorded by a high-resolution video camera. The device can spatially localize the anterior and posterior of the cornea and provides a 3-dimensional map. Measuring the curvature is performed directly by measuring the reflection of the Placido disc mires from the corneal surface.
The typical scanning slit output is seen in the Orbscan II (Bausch + Lomb) quad map, which shows anterior elevation (anterior float), posterior elevation (posterior float), regional pachymetry, and keratometric power (axial curvature) on 2-dimensional color-coded maps. There is a middle box that contains the information about central 3 and 5 mm of the cornea including the irregularity in those regions. SimK, thinnest point, pupil diameter, anterior chamber depth, and Kappa angle are also displayed (see Figure 1-12).
Scheimpflug Imaging (Pentacam)
The Pentacam was introduced in 2002 by Oculus (Oculus Optikgeräte GmbH). The Scheimpflug camera rotates around the optical axis of the eye, while a slit of light is focused on the cornea. The slit of light shines from one side of the limbus to the opposite side on a specific angle at every moment. The angle between the rotatory camera and the slit of light is fixed and the slit of light rotates with the camera. There is another fixed camera located in the center to detect fixation loss by looking at the pupil counter. Over the course of 2 seconds, the device captures up to 50 images and provides information of up to 25,000 elevation points. The device’s processor merges the slit images together to reconstruct a 360-degree map of the cornea and anterior segment. Pentacam HR (high resolution) is a newer version that measures up to 138,000 elevation points and captures up to 100 images per scan. Taking advantage of meticulous triangulation techniques, curvature and power of the cornea is calculated from the elevation data.
Curvature Maps: Placido Versus Scheimpflug
As discussed in Chapter 1, curvature can be determined directly through reflection-based technology or indirectly by surface reconstruction of tomographic images. In the past, there was a concern about surface reconstruction due to the limitations of the algorithms in use. Today, however, there is less difference between devices and, for most corneas, Scheimpflugbased curvature effectively conveys curvature details with adequate accuracy.
Figure 2-5. Composite image of a patient with keratoconus showing anterior curvature in similar fixed scales from Placido (left) and Scheimpflug (right) imaging. Note that the curvature pattern is analogous between devices.
The Pentacam, one of the most commonly utilized corneal tomography devices in use today, provides various maps, displays, and indices.
Figure 2-6. (A) Scheimpflug overview map showing a single raw Scheimpflug image (upper), basic data (right box), the 3-dimensional reconstruction of the cornea and anterior chamber (lower color image), and a single selectable overview image. In the upper left corner, there is an image showing the relative camera position to the eye where camera centration can be confirmed. (B) Scheimpflug single large color map. In this case, anterior axial curvature is shown; however, any of the available maps can be viewed in this format. (C) Scheimpflug “Show 2 Exams” display showing, in this case, anterior axial curvature (upper) and back elevation (lower), in addition to various data points (side boxes). The show 2 map is a convenient way to view both eyes to look for symmetry or other interesting features.
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