Corneal pachymetry mapping with high-speed optical coherence tomography

Abstract

Objective: To map corneal thickness before and after LASIK with optical coherence tomography (OCT).

Design: Cross-sectional observational study.

Participants: Forty-two eyes of 21 normal subjects undergoing LASIK.

Methods: A high-speed (2000 axial scans/second) 1.3-microm-wavelength corneal and anterior segment OCT prototype was used for corneal scanning. The scan pattern consisted of 10-mm radial lines on 8 meridians centered on the vertex reflection. The entire scan pattern of 1024 a-scans was acquired in 0.5 seconds. We developed automated computer processing for 3-dimensional corneal reconstruction and measurement. Corneal thickness was measured normal to the anterior surface and presented as color pachymetry maps and zonal statistics. The maps were divided into a central zone (<2 mm) and 3 annular areas (pericentral, 2-5 mm; transitional, 5-7 mm; peripheral, 7-10 mm), which were further divided into quadrantal zones. The average, minimum, and maximum corneal thicknesses were computed for zones within the 7-mm diameter. Optical coherence tomography and ultrasound pachymetry were measured 3 times at the preoperative and 3-month postoperative visits. Reproducibility was assessed by the pooled standard deviations (SDs) of the repeated measurements.

Main outcome measures: Optical coherence tomography pachymetric map and zonal statistic, and ultrasound pachymetry.

Results: Before LASIK, central corneal thicknesses (CCTs) were 546.9+/-29.4 microm (mean +/- SD) for OCT and 553.3+/-33.0 microm for ultrasound. After LASIK, CCTs were 513.7+/-44.5 microm for OCT and 498+/-46.6 microm for ultrasound. Optical coherence tomography and ultrasound CCT were highly correlated (Pearson correlation r = 0.97 before LASIK and 0.98 afterwards). Optical coherence tomography CCT was slightly less than ultrasound CCT before surgery (mean difference, -6.4 microm; 95% limits of agreement, -23.2 to 10.4 microm) but slightly greater after LASIK (15.7 microm; -1.6 to 33 microm). These differences were statistically significant, but no more than the CCT measurement differences between ultrasound pachymeters. The reproducibility of the OCT zonal pachymetry averages was roughly 2 microm.

Conclusions: High-speed OCT provided noncontact, rapid, reproducible pachymetric mapping over a wide area of the cornea. It is equivalent to ultrasound for CCT measurement before and after LASIK. This technology could be valuable for planning keratorefractive procedures and diagnosis of corneal diseases.

Figure 1

A, Optical coherence tomography scan pattern for pachymetry mapping shown on a frame of charge-coupled device video image. The pattern consisted of 8 radial lines of 10-mm length. B, Three-dimensional reconstruction of the cornea from 8 dewarped cross sections.

Figure 2

A, Cross-sectional optical coherence tomography (OCT) image from 1 of the 8 radial line scans. B, An OCT axial scan from pericentral cornea plotted on a logarithmic scale. Arrows mark the signal peaks at the anterior (left) and posterior (right) corneal boundaries located by the automated algorithm. C, Dewarped OCT cross section overlaid with detected corneal boundaries.

Figure 3

A corneal pachymetry map. d = diameter; I = inferior; N = nasal; S = superior; T = temporal.

Figure 4

Pre-LASIK optical coherence tomography (OCT) central corneal thickness compared with the ultrasound (US) pachymetry measurement. OCT_Mean = average OCT corneal thickness over the central zone. A, Regression analysis. y = x is the line of equality. The 95% confidence interval (CI) of the slope was 0.789 to 0.936. The 95% CI of the y intercept was 29 to 110.1 μm. The R² of the fit was 0.939. B, Bland–Altman plot. The distribution of the mean (x axis) and the difference (y axis) between US pachymetry and OCT reflects the level of agreement. The mean difference was −6.4 μm, with 95% limits of agreement from −23.2 to 10.4 μm. SD = standard deviation.

Figure 5

Post-LASIK optical coherence tomography (OCT) central corneal thickness compared with the ultrasound (US) pachymetry measurement. OCT_Mean = average OCT corneal thickness over the central zone. A, Regression analysis plot. y = x is the line of equality. The 95% confidence interval (CI) of the slope was 0.862 to 1.018. The 95% CI of the y intercept was 6.7 to 84.3 μm. The R² of the fit was 0.964. B, Bland–Altman plot. The distribution of the mean (x axis) and the difference (y axis) between US pachymetry and OCT reflects the level of agreement. The mean difference was 15.7 μm, with 95% limits of agreement from −1.6 to 33 μm. SD = standard deviation.