Comparison of Ophthalmic Model Eye Radius of Curvature using Swept Source Optical Coherence Tomography and Corneal Topography

Abstract

 

Purpose

 

The purpose of this experiment is to compare methods for obtaining radius of curvature (ROC) of a model eye. The first method is to use a Corneal Topographer which projects a two-dimensional array of spots onto the model eye surface and then analyzes the spot pattern to determine the ROC. The second method uses an Axsun swept-source laser Optical Coherence Tomography (OCT) system with a scanning optical probe to provide a direct measurement of the model eye surface. The fitted ROCs and the residual will be compared using these methods.

 

Methods

 

Utilizing a swept-source based OCT setup operating at 1060nm, a model eye lens was measured. This setup consists of a reference arm and a sample arm which uses two galvanometer mirrors to scan across the sample surface. For this test, a single B-scan across the surface which included the vertex was analyzed, and the resultant data was fitted to a sphere. Additionally, an analysis and comparison of the surface fit residuals from both methods was performed. The depth range for the OCT is 3.7mm. The commercially-based iDesign corneal topographer uses a large cone to reflect a series of light spots off of the model eye surface and into a detector. Analysis of the relative position of the spots yields a measurement of the surface curvature. A series of repeat measurements were taken with both methods to compare repeatability.

 

Results

 

As expected, the surface results from the OCT were more noisy and yielded larger surface residuals than from the more conventional spot-based topography; however, repeat measurements showed that the OCT best fit sphere ROC was comparable (6.16μm 1σ) to the topographer best fit sphere repeatability (5.33μm 1σ).

 

Conclusions

 

A direct comparison of the measurement of the spherical ROC was performed using a swept-source OCT and a Corneal Topographer. The ROC of curvature measured with the OCT is 7.780mm +/- 6.16μm and 7.797mm +/- 5.33μm for the topographer with the discrepancy in the averages due to calibration differences. Therefore the OCT is a viable method for measuring the radius of curvature.

 

 

Figure 1: Figure 1: OCT B-scan result with surface residual after best fit sphere (7.78mm) removal

 

 

Figure 2: Corneal topography Elevation map after removal of best fit sphere of 7.79mm

 

Keywords: 681 refractive surgery: corneal topography • 682 refractive surgery: other technologies