A Video System For Measuring Light Scattered From The Corneal Stroma

Abstract

Abstract: : Purpose: Stromal scatter is an indicator of the optical quality of the cornea. In this study we developed an instrument based on a photoslitlamp to measure scattered light through the depth of the stroma. Methods: The instrument was constructed from a Zeiss photoslitlamp with a flash illuminator. A low–light video camera was mounted on the photo arm and synchronized with the flash to integrate and record one frame through a video digitizer, after being triggered by the user. The microscope was aligned with the cornea along the visual axis of the eye with the aid of a fixation target and Purkinje images from two light–emitting diodes, one fixed to each side of the objective of a biomicroscope. A custom program identified the anterior and posterior surfaces of the cornea and calculated corneal thickness (CT). It then divided the corneal image into 4 equally thick layers (Q1 – Q4) and calculated mean intensity for each. Image intensity was adjusted for variations in the flash intensity and sensitivity of the microscope and camera by using measurements from a fluorescent glass standard. Fifty corneas from 25 normal subjects were examined, as well as 27 corneas from 21 patients with penetrating keratoplasty (PK). Seven of these corneas (6 patients) had late endothelial failure (LEF). Results: All differences between the normal group and both PK groups were significant (p < 0.05). Only Q2 and Q3 were significantly different between LEF and non–LEF PK corneas. All means were compared by using the Rank–Sum test adjusted for 3 comparisons by the Bonferroni method. 

Conclusions: This non–contact video system can be used to measure corneal scatter objectively. With this design we can resolve scatter through the depth of the cornea and acquire an image in a few microseconds, the duration of the flash, to minimize movement artifacts.

Keywords: imaging/image analysis: clinical • optical properties • cornea: clinical science