Corneal pachymetric topography

Abstract

Purpose: The authors developed a system for producing topographic pachymetric maps of the corneal epithelium and anterior scar tissue.

Method: The system uses high-frequency ultrasound scanning enhanced by digital signal processing. Ultrasonic echo data from consecutive parallel B-scans of the cornea spaced at 250-microns intervals are digitized and stored. Using the I-scan (obtained by computing the analytic signal magnitude of the deconvolved ultrasound signal), layer thickness measurements are made with a precision of 2 microns (standard deviation) at 120-microns intervals along each scan plane. The data are stored as an array, z(x,y), mapping thickness, z, onto horizontal and vertical (x,y) spatial coordinates. Pachymetric maps are then constructed by plotting local thickness, represented by a color scale, against measurement point position.

Results: Examples of a normal cornea, a contact lens-wearing cornea, Reis-Bückler dystrophy, and postphotorefractive keratectomy are presented. Areas with significant subepithelial scarring and general epithelial thickening in a subject with Reis-Bückler dystrophy are mapped. Unevenness in the epithelial thickness profile of the cornea in a subject after photorefractive keratectomy is shown, relative to the fellow (untreated) cornea.

Conclusion: This technique provides the corneal surgeon with a new tool for the topographic evaluation of the thickness of anterior corneal layers in normal and pathologic corneas with high precision. In addition, the technique is not limited to optically transparent tissue.