Author reply

Abstract

We thank Pong et al for their interest in our work. A number of studies,1Zhao P.S. Wong T.Y. Wong W.L. et al.Comparison of central corneal thickness measurements by Visante anterior segment optical coherence tomography with ultrasound pachymetry.Am J Ophthalmol. 2007; 143: 1047-1049Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 2Li Y. Shekhar R. Huang D. Corneal pachymetry mapping with high-speed optical coherence tomography.Ophthalmology. 2006; 113: 792-799.e2Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar including ours,3Li E.Y. Mohamed S. Leung C.K. et al.Agreement among 3 methods to measure corneal thickness: ultrasound pachymetry, Orbscan II, and Visante anterior segment optical coherence tomography.Ophthalmology. 2007; 114: 1842-1847Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar have demonstrated that central corneal thickness (CCT) obtained from Visante anterior segment optical coherence tomography (OCT) is thinner compared with that measured by ultrasound pachymetry. However, our recent study comparing slit-lamp OCT (SLOCT) and Visante OCT did not reveal significant difference in CCT between SLOCT and ultrasound pachymetry,4Li H. Leung C.K. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar which is different from the finding by Pong et al. The discrepancy in CCT measurement between the 2 anterior segment OCT instruments could be attributed to the differences in the choice of refractive indexes in the calculation of corneal thickness and different algorithms for image analysis. The corneal refractive index adopted by the SLOCT is 1.376, which is derived from the model of Gullstrand of the human eye for visible light, whereas a group index of 1.389 was used in Visante OCT.5Lin R.C. Shure M.A. Rollins A.M. et al.Group index of the human cornea at 1.3-micron wavelength obtained in vitro by optical coherence domain reflectometry.Opt Lett. 2004; 29: 83-85Crossref PubMed Scopus (78) Google Scholar Because of the geometry of the scan probe and the refraction at smooth surfaces of the eye, algorithms for dewarping are incorporated in the analysis software of Visante OCT and SLOCT to correct for image misalignment. It is uncertain whether these algorithms adjust equivalently. Different versions of the built-in analysis software could also result in different measurement values. In our study, the Heidelberg Eye Explorer viewer v1.4.1.0 (SLOCT viewer v1.0.1.7) was used in the analysis.4Li H. Leung C.K. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar In a recent study by Kim et al6Kim H.Y. Budenz D.L. Lee P.S. et al.Comparison of central corneal thickness using anterior segment optical coherence tomography vs ultrasound pachymetry.Am J Ophthalmol. 2008; 145: 228-232Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar using software version v1.5.9.0, they found that the SLOCT CCT measurement was significantly lower than that measured by ultrasound pachymetry. We analyzed our dataset4Li H. Leung C.K. Wong L. et al.Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and Visante optical coherence tomography.Ophthalmology. 2008; 115: 796-801Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar again using the latest software version v1.5.10.0 (SLOCT viewer v.1.0.2.0). To our surprise, the CCT analyzed with software v1.5.10.0 (SLOCT viewer v1.0.2.0) was significantly lower than that analyzed by v1.4.1.0 (SLOCT viewer v1.0.1.7). A significant difference was also found between SLOCT and ultrasound pachymetry (mean difference, −4.4±9.1 μm; P = 0.001). Figure 1 (available online at http://aaojournal.org) illustrates an anterior segment OCT image analyzed by software v1.4.1.0 (SLOCT viewer v1.0.1.7) and v1.5.10.0 (SLOCT viewer v1.0.2.0). It is notable that a difference of 17 μm (CCT) was found despite the fact that the same image was analyzed. It is obvious that a different set of parameters has been implemented for CCT measurement in the SLOCT software upgrade. We should pay careful attention in the interpretation of CCT obtained from different imaging systems, different models, and even different versions of analysis software. Better standardization is needed. Corneal ThicknessOphthalmologyVol. 115Issue 12PreviewWe read with interest the articles by Li et al1 and Li et al2 on the measurements of central corneal thickness (CCT) between ultrasound pachymetry (USP) and optical coherence tomography (OCT). Li et al1 reported that Visante anterior segment OCT underestimated CCT by 14.74 μm compared with that measured with USP. Another Li et al2 compared the corneal thickness measurement by the 2 commercially available anterior segment OCT systems—Visante anterior segment OCT and slit-lamp OCT (SLOCT)—with the gold standard USP. Full-Text PDF