The potential of OCT for corneal imaging was first shown by Izatt et al.
23 with the prototype OCT system that was later developed into the commercially available system. More recently, Drexler et al.
24 have shown images of the cornea with even higher resolution obtained with a prototype ultrahigh-resolution system. With a few very minor modifications, a commercially available system (OCT; Humphrey Zeiss) originally designed for retinal imaging can also be used to image the cornea. Several studies give examples of various situations in which OCT was useful in the evaluation of corneal conditions, especially after refractive surgery.
16 25 26 27 28 In addition, OCT may be used to make measurements of corneal thickness and has an advantage over more established methods of measuring corneal thickness, in that it is a noncontact and noninvasive technique. Corneal thickness measured with OCT has been found to correlate very well with measurements from ultrasound pachymetry, as shown by Bechmann et al.
10 who found that in normal subjects, measurements obtained with OCT were consistently lower than those obtained by ultrasound by a constant value of approximately 50 μm, but that the actual correlation coefficient between the two methods was very high. Correlation was also high in a group of patients with corneal edema, indicating the clinical potential of this relatively new imaging technique. This implies that OCT can be used immediately after corneal surgery and may be a useful tool for evaluating the success of corneal surgery.
Although the system used in this study (OCT; Humphrey-Zeiss) is known to yield repeatable and reproducible measurements of retinal and nerve fiber layer thickness,
11 12 13 14 15 we thought it also necessary to rigorously assess its performance in measuring corneal thickness, especially because this commercial system is not actually designed to perform imaging of the anterior segment. In our study, we evaluated repeatability, intersession reproducibility, and interoperator reproducibility as suggested by Bland and Altman,
22 who based their definitions on the standards set by the British Standards Institution.
17 18
The coefficients of repeatability for vertical and horizontal scanning were less than 3% in all subjects in this part of the study. This indicates that during the same scanning session, corneal thickness measurements are repeatable, and there is therefore no need to acquire a large number of readings for a reliable estimate of CCT.
Coefficients of interoperator and intersession reproducibility were 0.18% and 1.11%. The observation that the intersession reproducibility is slightly higher than the interoperator reproducibility may be an indication of diurnal variation in corneal thickness between different sessions. Although we attempted to acquire the second set of scans at approximately the same time of day, but on a different day from the first, it appears that there was nevertheless a small amount of variation. Marsich and Bullimore
1 investigated the performance of three devices for determining corneal thickness and found that the Orbscan slit lamp pachymetery system (Orbscan Topography System I; Bausch & Lomb, Inc., Salt Lake City, UT) showed the best repeatability (what we have termed intersession reproducibility) with 95% LoA of −10 to +17 μm. We obtained 95% LoA of −16 to +7 μm from the intersession data. This indicates that the performance of OCT is comparable to that of established methods of evaluating corneal thickness.
Our average CCT was 526 ± 28 μm, almost identical with the 530 ± 32 μm obtained by Bechmann et al.
10 29 in normal subjects. This group found that the interpatient variability with OCT was 4.90 μm, whereas interpatient variability was 32 μm, similar to our findings. A study by Radhakrishnan et al.
30 describes a higher-resolution OCT system designed specifically for corneal imaging. This study quotes a corneal thickness of 488 μm measured in a single volunteer. This thickness is lower than that obtained in our study. Whether this is because the measurement came from a single subject or because the commercial system overestimates corneal thickness in comparison with this new high-resolution system remains to be seen. Our study showed no gender-related differences in average corneal thickness; however, our sample sizes (seven men and seven women) were rather small.
In our study we limited ourselves to assessing only CCT. Peripheral measurements would have been inaccurate for two reasons: First, at peripheral points the OCT beam would not have been perpendicular to the corneal surface, and measurements therefore would have had to be corrected for refraction at the air–cornea interface; second, the radius of curvature seen on the corneal scans is overestimated, because the system is designed to image a concave rather than convex structure and therefore exaggerates the curvature of the cornea. CCT measurements, however, have been shown to be both repeatable and reproducible, and this indicates that the commercially available OCT system in its present state and with practically no modifications is a reliable and useful tool in assessing and monitoring corneal conditions.