Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

The Pentacam Ocular Scanner is a specialized camera which utilizes Scheimpflug imaging to accomplish with a variety of ophthalmic applications. Scheimpflug imaging was patented by Theodor Scheimpflug in 1904 after he discovered that when the planes within a camera intersect rather then be placed in parallel, the depth of focus is extended.

In a typical camera, three imaginary surfaces exist: the film plane, lens plane and sharp image plane. These are parallel to each other such that the image of the object placed in the plane of sharp focus will pass through the lens plane perpendicular to the lens axis, and fall on to the film plane. The depth of focus is limited in such a camera.

In a Scheimpflug camera, the three planes are not parallel but intersect in a line, called the “Scheimpflug line”. When the lens is tilted such that it intersects the film plane, the plane of sharp focus also passes through the Scheimpflug line, extending the depth of focus. Note that this results in mild image distortion, which is then corrected by the Pentacam system.

A two-dimensional cross-sectional image results, as shown in Figure 2.1. When performing a scan, two cameras are used to capture the image. One centrally located camera detects pupil size and orientation, and controls fixation. The second rotates 180 degrees to capture 25 or 50 images of the anterior segment to the level of the iris, and through the pupil to evaluate the lens. 500 true elevation data points

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

are generated per image to yield up to 25,000 points for each surface. Data points are captured for the center of the cornea, an area that placido disc topographers and slitscanning devices are unable to evaluate.

Elevation data measured using this technique has several advantages. Because it is independent of axis, orientation and position, it yields a more accurate representation of true corneal shape. Thus, the Pentacam’s curvature map, because it is not sensitive to position, is theoretically more accurate. The elevation maps are created using one of three reference bodies: a best fit sphere, an ellipse of revolution, and toric. The best fit sphere calculation approximates the sphere as accurately as possible to the true nature of the cornea. This facilitates comparison between other topographers but is not the best fit for the aspheric cornea.

The ellipsoid of revolution is calculated from the keratometry eccentricity and the mean central radius. This reference shape correlates well with the true shape of the normal cornea.

The toric is based on the central radii and keratometry eccentricity as well. The flat and steep radii are automatically used. The toric is a good estimation for astigmatic corneas. The toric ellipsoid float display best facilitates pattern recognition of abnormalities on the front and back surfaces, such as found in keratoconus. Figures 2.2 to 2.4 show the same astigmatic eye mapped using the

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 2.3: Elevation map using the ellipse reference body

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

three different reference bodies. The cone is easily detected using the toric ellipse reference body.

The default view presented following data capture is the “overview display”, which includes the Scheimpflug image, three-dimensional model, an additional map, keratometry, pupil, anterior chamber measurements, and pachymetry. An example of an overview display in a patient with a history of conductive keratoplasty is shown in Figure 2.5. From here, other displays are typically chosen relative to the reason for scanning: surgical vision correction, keratoconus evaluation, cataract or anterior chamber evaluation, etc.

The refractive and topographic displays for the patient in Figure 2.5 are shown in Figures 2.6 and 2.7. Single maps can also be viewed by selecting the preferred single display.

The two major meridians, determined using the 3 mm ring and at 90 degrees from each other, are listed as K1 (flat) and K2 (steep), with corresponding radii R1 and R2. The mean radius and mean keratometry are the arithmetic average of the corresponding measures. “Astig” describes the central corneal astigmatism. The mean radii of the 7mm and 9 mm rings are described as the “Rper”.

The Q-value describes the corneal shape factor, or the eccentricity of the cornea. A value of –0.26 is ideal. Highly prolate corneas with significantly higher negative values may suggest keratoconus or hyperopic treatment. Positive

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CHAPTER 2: PENTACAM

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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