Refractive surgery has advanced to an extraordinary level. By surgically modifying the shape of the cornea and by implanting artificial lenses into the eye, the visual acuity of most individuals can now be improved to a level better than “20/20”.

An important prerequisite for being able to perform vision correction at this level of accuracy is to know the shape and dimensions of the cornea in minute detail and with exacting precision. Until recently, in order for opticians to prescribe eyeglasses that would give their customers adequate far or near vision it was sufficient to know the average curvature of the central cornea. Today, ophthalmologists require data on the “instantaneous curvature” of a cornea and its thickness at each point across the entire surface of the cornea in order to know its optical power. For surgical procedures that involve implanting an artificial lens into the anterior chamber of the eye (i.e. the space between natural lens and cornea), the spatial dimensions of the anterior chamber, in particular the distance between cornea and lens, and the location of the sulcus and the ciliary angle need to be known.

Methodologies

Several methods for measuring the dimensions and shape of ocular structures have been known for years, and they all have their specific advantages and limitations. Ultrasound imaging is widely used in medicine as it can penetrate opaque tissue, but its limited resolution and the need for direct contact with the tissue to be studied, render it less than ideal for ocular precision measurement. Optical imaging techniques are the method of choice for studying the optically transparent ocular tissues (cornea, lens, aqueous and vitreous humour).

Placido topography. Corneal topography1, or videokeratography, has become a widely used technique for measuring the topography (shape) of the corneal surface. Its principle is based on a reflecting disk with illuminated concentric rings which are reflected off the eye (Figure 1). Analysis of the reflected image (Figure 2) allows one to calculate the topography of the anterior surface of the cornea. As it is based on the light that is reflected from the corneal surface, this method is not capable of studying the posterior surface and the thickness of the cornea. Also, due to geometrical constraints, only the central part of the cornea is covered by the Placido image, as seen in Figure 2.

Scheimpflug photography. Observation of the cornea and anterior chamber under slit illumination and in a Scheimpflug camera arrangement is another classical observation method that has been in use for a long time. In a Scheimpflug camera, the object plane, lens plane, and image plane all intersect in a straight line, as illustrated in Figure 3. The principle advantage of this camera geometry, which was invented by Theodor Scheimpflug in 1904, is that the entire image plane is observed in focus. With the illumination focused in a plane going through the apex and with the lens of the camera at 45°, an optical section of the entire anterior segment of the eye, from the anterior surface of the cornea to the posterior surface of the lens can be obtained (Figure 4), allowing measurement of anterior and posterior corneal topography, anterior chamber depth, as well as anterior and posterior topography of the lens. By rotating the camera and taking images at different angles, a complete three-dimensional view can be constructed. Practical limitations of Scheimpflug imaging are limited accuracy for measuring corneal curvature and sensitivity to centration and alignment of the optical axes of camera and eye.

Dealing with systematic errors

Conceivably, the inherent advantages of the two methods might be combined and their limitations overcome simply by taking measurements with both devices and combining the results. However, this approach poses the problem of superimposing data taken at different times and with different optical alignment. The GALILEI Dual Scheimpflug Analyzer by Ziemer Ophthalmic Systems, Port, Switzerland, was designed with the goal of achieving “the best of both worlds.” That is, to combine the advantages of Placido and Scheimpflug imaging in a single device, thus eliminating most, if not all, sources of systematic errors.

Dual Scheimpflug imaging. A major limitation of Scheimpflug photography is that a correct representation of corneal thickness (pachymetry) is only obtained when the illuminating slit beam is centered precisely on the corneal apex. With any deviation from perfect centration, the camera “sees” a pachymetry that is either lower or higher than the true values, depending on the sign of the decentration. By using two optically identical cameras for viewing the slit from opposite directions (Figure 5), and by averaging the measurements of the two cameras, a correct pachymetry, independent of any centration errors, can easily be obtained (Figure 6). It is important to realize that when taking measurements on human eyes there is always some decentration due to rapid saccadic eye movements. Compensation of errors due to movement by Dual Scheimpflug imaging therefore only works when the corresponding opposite images are taken at exactly the same time, as it is the case in the GALILEI. Simply put, the Dual Scheimpflug technology overcomes the systematic error of any misalignment or eye motion that is unavoidable in a single Scheimpflug camera system. An additional advantage of the dual camera arrangement is that the cameras have to rotate through only 180° to cover the entire 360° view; hence measurement time is cut in half.

Surface reconstruction from Placido data. As shown in Figure 7, the surface reconstruction from Placido lines depends on the exact knowledge of the distance between the corneal apex and the Placido disk. Even a small error of ±0.5 mm produces curvature errors of half a diopter. The GALILEI determines this distance exactly from the simultaneously recorded Scheimpflug data, resulting in correct curvature measurements, independent of apex–to-disk distance variations.

Correction of motion and cyclotorsion. For a complete eye exam, the GALILEI Analyzer collects 16 or more Scheimpflug images, a “top view” reference image, and two Placido images at 0° and 90° while the camera rotates through 180°. The complete data-taking process takes less than a second to complete. During this time, the patient’s eye may move vertically, horizontally and forward and backwards (translation), and it also rotates (cyclotorsion). GALILEI’s patented motion detection and compensation module determines correction values for each scan with a precision of ±3µm, based on iris patterns.

Data processing

The first step in the analysis of the image data collected in a GALILEI scan consists in detecting the bright and dark edges that represent the ring reflections in the Placido images and the key features in the Scheimpflug images (anterior/posterior surfaces of cornea and lens).

An advanced image contrast-based edge-tracking algorithm is used to detect the edges of each ocular tissue (cornea, iris, and lens) in the images and convert these data into three-dimensional coordinates. The algorithm includes self-checks and a computed image quality index.

Sub-pixel edge detection. The light from the slit projector, with its specific beam profile, is scattered in the epithelium and to a lesser degree in the stroma. Additionally, camera sensor noise degrades the image. The resulting image of an edge is therefore a gradient with a characteristic shape extending over several pixels. A parabolic fitting process, depicted in Figure 8, determines the position of the edge within 1/10 of the width of a pixel.

Merging of Placido and Scheimpflug images; 3D reconstruction. The aligned, motion-corrected image data coordinates are merged to generate a 3D model of the anterior segment of the eye using a proprietary advanced computational algorithm. ANISA (“Adaptive Numeric Interpolation for Surface Analysis”) delivers a stable 3D model, which covers a large corneal diameter and minimizes interpolation artifacts.

Ray-tracing. Total Corneal Refractive Power is calculated by ray-tracing (Figure 9) through the anterior surface and posterior surface using Snell’s Law and the thin lens equations. Total Corneal Power calculated via ray tracing is the most accurate representation of the refractive behavior of the cornea and is therefore the method of choice for determining the required power for intraocular lens implantation.

Data presentation. A Placido image superimposed with detected edges (Figure 10), and a series of Scheimpflug images with detected edges (Figure 11) are usually only displayed for verifying image and edge tracking quality.

To aid interpretation of the massive amount of information generated from over 120,000 data points, GALILEI generates several different kinds of topographical maps and a large number of numerical indices that characterize specific properties of the eye that are of interest to the ophthalmologist. Several of these maps, together with an appropriate choice of indices, are combined into specific reports. Some illustrative examples are discussed in the next section.

Applications

Refractive report. The standard “refractive report” provides a good summary overview. It includes four standard maps: (A) curvature of the anterior corneal surface, labeled in diopters; (B) pachymetry, in micrometers; (C) elevation of the anterior surface relative to a best-fit sphere (BFS), in micrometers; and (D) elevation of the posterior surface relative to BFS. Each map is color-coded to allow easy visual identification of prominent features. Various color scales are available according to the user’s preference. Most color scales are arranged so that normal ranges of values appear in green shades. On the right, a series of key values (usually referred to as indices) summarize the characteristics of the eye being evaluated.

The report in Figure 12 shows a regular, healthy cornea of a young, myopic subject with a slight astigmatism and a slightly thicker than average cornea. The thinnest spot of the cornea is 560µm and is marked with an “О” symbol on each map, while the pupil center is marked “+”. The pupil diameter (3.26mm) is also marked on all maps.

IOL Power report. The standard IOL Power report features an axial curvature map (A, same as in the refractive report); ray-traced total corneal power TCP (B); total corneal wavefront map (C); and the Placido image (D). It helps to detect the irregularities in the cornea that need to be considered when determining the optimal intraocular lens to be fitted. The example report in Figure 13 illustrates an eye with irregularly formed, biomechanically unstable cornea due to Pellucid Marginal Degeneration (PMD). The axial curvature map (A) exhibits the typical “crab-claw” shaped curvature in the center of the cornea which is associated with PMD. The distorted, noncircular Placido rings (D) and the characteristic wavefront aberrations (C) are also typical for PMD.

Wavefront report. From the Total Corneal Power (TCP) information, GALILEI can compute the corneal wavefront. The information is displayed in a combination of numerical and graphical information (Figure 14). The spherical aberration contribution from the cornea can be used by the cataract surgeon to support the selection of the appropriate IOL for lens replacement surgery.

Densitometry. GALILEI’s eye metrics software includes a densitometry tool (Figure 15) which permits opthalmologists to visualize and quantify opacifications in the lens and the cornea. By moving a cursor line across the Scheimpflug image, localized densitometry curves may be observed. The tool allows the ophthalmologist to assess cataract density and corneal opacities (e.g. scars).

Z-LASIK surgery planning report. (Figure 16) “Z-LASIK” is a premium procedure for the surgical correction of visual errors, based on the Ziemer FEMTO LDV Crystal Line™ femtosecond laser and a state-of-the-art excimer laser. For planning and documenting a safe and effective surgery, the GALILEI Z-LASIK Surgery Planning report combines all relevant diagnostic information and the key surgical decisions in a convenient report that can be shared with the patient.

Summary

With its combination of optical imaging technologies and utilization of advanced image processing techniques, the GALILEI Dual Scheimpflug Analyzer has become an enabling technology for several fields of high-precision ocular microsurgery, including refractive corneal and lens-replacement surgery. These capabilities have given surgeons a deeper understanding of corneal pathologies such as Keratoconus and Pellucid Marginal Degeneration.