PAR Corneal Topography System Research Articles

Comparison of Topographic Technologies in Anterior Surface Mapping of Keratoconus using Two Display Algorithms and Six Corneal Topography Devices

ABSTRACT Purpose To evaluate anterior surface topographic technologies and display algorithms in mapping keratoconus. Materials and methods A total of 27 eyes of 17 subjects clinically diagnosed with keratoconus were imaged on six topographers: EyeSys, Alcon EyeMap, Keratron, TMS-1, Orbscan and PAR corneal topography system. Axial distance (AD) and instantaneous radius of curvature (IROC) algorithms were generated, and the cone apex was determined manually using a cursor. Intermachine comparisons for cone magnitude (steepest curvature), as well as cone location in radius and meridian were performed for each display algorithm using both AD and IROC. Significance (p < 0.05) was determined using repeated measures analysis of variance (ANOVA) on successive mean values. Maps were also evaluated for processability, defined by the ability to reconstruct a reasonable map for each subject, not map quality. Results There were no significant differences between successive means for cone location in either radial or meridional directions. For AD, Orbscan was greater than both small mire Placido devices (Keratron and TMS-1), which were not different from each other. The small mire devices had significantly greater curvature magnitude than the large mire placido devices (EyeSys, Alcon EyeMap) which were not different from each other. Finally, PAR was significantly lower than the large mire Placido devices. For IROC, the pattern was the same with the exception that the Orbscan was not different than the small mire Placido devices in curvature magnitude. For processing success, the PAR had 100% processability, and all other devices were between 73 and 77%. Conclusion In monitoring keratoconus, evaluation of change over time is fundamental to treatment decisions, making understanding of topographic technology differences in mapping keratoconic corneas extremely important. How to cite this article Markakis GA, Roberts CJ, Harris JW, Lembach RG. Comparison of Topographic Technologies in Anterior Surface Mapping of Keratoconus using Two Display Algorithms and Six Corneal Topography Devices. Int J Kerat Ect Cor Dis 2012;1(3):153-157.

Radius of curvature and asphericity of the anterior and posterior surface of human cadaver crystalline lenses

The purpose of this study was to measure the shape of the anterior and posterior surface of human cadaver lenses in situ using a corneal topography system. Measurements were performed on 13 pairs of eyes using the PAR Corneal Topography System (PAR-CTS). The age of the donors ranged from 46 to 93 years, with an average age of 76·4 years. Anterior lens topography was measured after excision of the cornea and iris. Posterior lens topography was measured after sectioning the posterior segment and adherent vitreous. The PAR-CTS files providing raw surface height were exported for analysis. In each surface, 18 meridians separated by 10° were fitted using conic sections to obtain values of the apical radius of curvature ( R) and shape factor ( p). The average apical radius of curvature and asphericity were R=10·15±1·39 mm and p=4·27±1·39 for the anterior surface and R=−6·25±0·79 mm and p=−0·64±1·85 for the posterior surface. A significant variation of the radius of curvature and shape factor as a function of the meridian angle (lens astigmatism) was found in some lenses. Contrary to previous findings, the anterior lens surface was found to steepen toward the periphery.

Topographic pattern and apex location of keratoconus on elevation topography maps.

To compare topography pattern and apex location in elevation and axial curvature topographic maps of keratoconic corneas. We prospectively evaluated 72 corneas of 42 patients who had one or more corneal findings of keratoconus with the elevation and axial curvature displays of the PAR Corneal Topography System (PAR-CTS) and 66 of these corneas with the axial curvature display of the Tomey Topographic Modeling System (TMS-1). Topography maps were evaluated for topography pattern and location of the cone apex. Axial curvature displays of the PAR-CTS and the TMS-1 showed good concordance in terms of topographic patterns (96% for right, 86% for left corneas) and apex locations of cones (92% for right, 80% for left corneas). On the other hand, low concordances were noted when comparing topographic patterns (35.3% for right, 36.8% for left corneas) and apex locations (47% for right, 38% for left corneas) on curvature and elevation mode displays of PAR-CTS. Apices were found in the inferotemporal quadrant in 65% of corneas evaluated with the PAR-CTS. This distribution is significantly different from the apex location in axial curvature maps (p < 0.04). Twenty-nine percent of corneas that showed an apex on the axial curvature mode of the PAR-CTS had a normal pattern, without a detectable cone apex, on the elevation mode display. The results of this study clearly show the difference between elevation and curvature-based corneal topographic evaluation of keratoconus. Unlike axial curvature maps, the majority of apices on elevation maps are clustered in the inferotemporal quadrant. This new information about apex location in keratoconic corneas provided by elevation topography may have better diagnostic specificity than regional differences of curvature on axial curvature maps. Because elevation mapping shows the physical location of the cone, it may improve results of contact lens fitting and surgical management.

Comparative study of the elevation topography of complex shapes

Purpose: To compare the accuracy and precision of the elevation topography from two commercially available instruments using videokeratoscopy or rasterstereography.Setting: University of Ottawa Eye Institute, Ottawa General Hospital, Ontario, Canada.Methods: Repeated measurements of elevation topography of six calibrated surfaces were done with the PAR Corneal Topography System (CTS) and the Tomey Topographic Modeling System (TMS-1). The shapes simulated normal (A: aspheric, B: spherocylindric) and postsurgical corneas (C: hyperopic, D: myopic, E: central island, F: phototherapeutic keratectomy). Surface shapes were described by parametric equations. Equation parameters associated with each elevation measurement were determined by best-fit analysis. Measurement precision was assessed by the standard deviation of the difference between the fitted and the measured data. Fitted parameters were compared with nominal values obtained from an independent calibration of each surface. The root mean square error (RMSE) of the deviation of the fitted from the nominal surfaces was used to evaluate the accuracy of each instrument.Results: The accuracy of the CTS exceeded that of the TMS-1 for all surfaces measured. The RMSE values (µm) were (A: 0.1, 6.5), (B: 0.3, 3.8), (C: 1.1, 11.8), (D: 5.0, 43.0), (E: 1.2, 3.2) and (F: 2.2, 17.5) for the CTS and TMS-1, respectively. The differences in the measured data from the fit surface were generally smaller with the TMS-1.Conclusion: Quantitative analysis of elevation measurements showed that the CTS represented surface topography more accurately than the TMS-1.

PAR Corneal Topography System (PAR CTS): the clinical application of close-range photogrammetry.

The PAR Corneal Topography System (CTS) is a computer-driven corneal imaging system which uses close-range photogrammetry (rasterphotogrammetry) to measure and produce a topographic map of the corneal surface. The PAR CTS makes direct point-by-point measurements of surface elevation using a stereo-triangulation technique. The CTS uses a grid pattern composed of horizontal and vertical lines spaced about 0.2 mm (200 microns) apart. Each grid intersection comprises a surface feature which can be located in multiple images and used to generate an (x,y,z) coordinate. Unlike placido disc-based videokeratoscopes, the PAR CTS requires neither a smooth reflective surface nor precise spatial alignment for accurate imaging. In addition to surface elevation, the PAR CTS computes axial and tangential curvatures and refractive power. Difference maps are available in all curvatures, refractive power, and in absolute elevation.

Accuracy of the PAR corneal topography system with spatial misalignment.

The PAR Corneal Topography System is a computerized corneal imaging system which uses close-range raster photogrammetry to measure and produce a topographic map of the corneal surface. Raster photogrammetry is a standard method of extracting object information by projecting a known pattern onto an object and recording the distortion when viewed from an oblique angle. Unlike placido disc based videokeratoscopes, the PAR system requires neither a smooth reflective surface nor precise spatial alignment for accurate imaging. We studied both the accuracy of the system with purposeful misalignment (defocusing) of the test object and determined the ability to image freshly deepithelialized, keratectomized, and photoablated corneas. The PAR system was both accurate and reproducible in imaging calibrated spheres within a defined zone in space. Whole cadaver eyes were imaged both before and immediately after removal of the epithelium, lamellar keratectomy, and laser photoablation. The system demonstrated the ability to image irregular, deepithelialized, and keratectomized corneas. The ability to maintain accuracy without precise alignment and the facility to image freshly deepithelialized and keratectomized corneas may make the system suitable for intraoperative refractive monitoring.

Intraoperative raster photogrammetry—the PAR Corneal Topography System

The PAR Corneal Topography System (CTS) is a computer-driven corneal imaging system that uses close-range raster photogrammetry to measure and produce a topographic map of the corneal surface. The CTS determines distortion in a projected two-dimensional grid. Unlike Placido-disc-based videokeratoscopes, the PAR CTS produces a true topographic map (elevation map) and requires neither a smooth reflective surface nor precise spatial alignment for accurate imaging. Because the system uses two noncoaxial optical paths, it can be integrated into other optical devices. A modified CTS was integrated into an experimental erbium: YAG photoablatioe laser. The CTS successfully imaged corneas before, after, and during laser photoablation. Its ability to image nonreflective surfaces and to be integrated into other optical systems may make it suitable for intraoperative refractive monitoring.