title: The repeatability of corneal and corneal epithelial thickness measurements using optical coherence tomography.
abstract: OBJECTIVE
  The purpose of this study is to examine within and between session repeatability of clinical optical coherence tomography (OCT) imaging for anterior segment morphometry.
  
  
  METHODS
  Images of the corneal apex of each eye in 18 subjects were obtained using a Humphrey Zeiss OCT imager. Subjects viewed a target positioned to ensure that scans were orthogonal to the ocular surface and each image, consisting of 100 adjacent sagittal scans, analyzed using custom software. Repeatability data were analyzed using intraclass correlation coefficients (ICCs), correlation coefficient of concordance (CCC, perfect test-retest agreement ICC or CCC = 1.0), and coefficients of repeatability (COR, 95% confidence interval of test-retest differences). To account for each eye, the multivariate repeatability statistic Iota was estimated.
  
  
  RESULTS
  Mean central corneal and epithelial thickness of 32 eyes (OD and OS combined) is 536 +/- 26 microm (standard deviation [SD]) and 52 +/- 3 microm (SD) with 5th and 95th percentile thicknesses of 507 and 591 microm for central cornea and 48 and 57 microm for central epithelial. Worst case within session repeatability was defined as repeatability between images with greatest differences in mean thickness within a session. Corneal thickness worst case ICC was 0.95 and COR was +/- 9.98 microm. Epithelium worst case ICC was 0.36, CCC was 0.12, and COR was +/- 11.11 microm. First image between session corneal thickness had an ICC = 0.98 and a COR = 10.83 microm, whereas epithelium ICC = 0.38, CCC = 0.37, and COR was +/- 12.84 microm. When we compared the average of the first three tests with the first three retest images, corneal ICC was 0.98 and COR was +/- 10.64 microm and epithelium ICC = 0.73, CCC = 0.72, and COR was +/- 6.53 microm. Iota (multivariate repeatability, using eye as a factor) for the cornea was at least 0.96 (worst case) and increased to at least 0.98 when within-session image data were averaged. Iota for epithelium measures ranged from 0.29 when first images were compared with 0.57 when within-session image data were averaged.
  
  
  CONCLUSIONS
  There is very good repeatability of corneal thickness measurement using OCT; even the worst case measurements are similar between sessions. On the other hand, this is not the case for epithelium measurements, and if multiple images within a session are acquired, the worst case results demonstrate how important it is to optimize each OCT scan and also average multiple scans to maximize intersession repeatability.
authors.affiliation: Centre for Contact Lens Research, School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.
authors.forename: Sokpheaktra
authors.initials: S
authors.lastname: Sin
authors.forename: Trefford L
authors.initials: TL
authors.lastname: Simpson
mesh_terms: D000293:Adolescent; D000328:Adult; D003315:Cornea / Q000648:ultrastructure; D019573:Epithelium, Corneal / Q000648:ultrastructure*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D041623:Tomography, Optical Coherence / Q000379:methods*