Corneal pachymetry mapping with high-speed optical coherence tomography

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-mu m-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 mu m (mean +/- SD) for OCT and 553.3 +/- 33.0 mu m for ultrasound. After LASIK, CCTs were 513.7 +/- 44.5 mu m for OCT and 498 +/- 46.6 mu m 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 mu m; 95% limits of agreement, -23.2 to 10.4 mu m) but slightly greater after LASIK (15.7 mu m; -1.6 to 33 mu m). 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 Am. 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 keratorefractiveprocedures and diagnosis of corneal diseases.