September 2014
Volume 55, Issue 9
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Letters to the Editor  |   September 2014
Author Response: Caveats to Obtaining Retinal Topography With Optical Coherence Tomography
Investigative Ophthalmology & Visual Science September 2014, Vol.55, 5732. doi:10.1167/iovs.14-15386
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      Jaeryung Oh; Author Response: Caveats to Obtaining Retinal Topography With Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2014;55(9):5732. doi: 10.1167/iovs.14-15386.

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      © ARVO (1962-2015); The Authors (2016-present)

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We appreciate the comments of Kuo et al. 1 concerning our article, 2 in which we assessed retinal topography in myopic eyes using spectral-domain optical coherence tomography (SD-OCT). Kuo et al. 1 highlighted caveats to obtaining absolute retinal topography with OCT. We agree with their comment that the absolute topographic measurement is not allowed in the current OCT system. However, it does not mean that we measured the absolute retinal topography in this study. As described in the methods section, topographic characteristics were determined as the difference in geometry depicted with a pseudocolor scale, not with absolute values. Topographic measurement, including variation of shape and morphology, is valid to characterize the topographic patterns in myopic eyes, and is not limited to the measurement of absolute value in geometric variation. 
Kuo et al. 1 presented how the position of an eye relative to the OCT device affects the eye's appearance in the OCT image. They featured three representative images in which two line scan images were obtained during the eccentric fixation. In the two line scan images, the macular center was not located in the scan center, which is the typical feature of an image with eccentric fixation. In the previous study, 3 we showed how eccentric fixation in diseased eyes influenced the scan center in OCT images, and we presented how to recognize fixation error and determine the amount of unusual fixation in the OCT image. It may be useful to determine the images with error from loss of fixation. Stable fixation during acquisition of an OCT image is mandatory to obtain an OCT image without errors from fixation loss or eccentric fixation. In this study, we presented retinal topography using a volume scan. The volume scan requires greater acquisition time than a line scan; therefore, it requires higher stable fixation to be without errors. 
We also considered that an alignment between an eye and OCT at the time of image acquisition may influence the result regarding the slope and central zone topography. Therefore, as described in the discussion section, we compared the slope between images from a three-dimensional–volume scanning protocol and a line-scanning protocol, which were obtained at different time points of the same day. We did not find differences in slope between images, which may mean that the images were obtained from patients with stable fixation. Our results may also indicate that the current scanning protocol, which uses an internal fixator, is not really influenced by fixation error when the scan is normally and purposefully obtained from patients with stable fixation without eccentric fixation. 
Even with recent advancements in the SD-OCT system, SD-OCT images are still not free from various errors. 4 We agree with the comment by Kuo et al. 1 that we need to examine the error when obtaining retinal topography, just as in other uses of OCT images. According to general principles, the current method using volume scanning of SD-OCT images to determine the retinal topography of myopic eyes is valid. 
References
Kuo AN Carrasco-Zevallos O Toth CA Izatt JA. Caveats to obtaining retinal topography with optical coherence tomography. Invest Ophthalmol Vis Sci . 2014; 55: 5730–5731. [CrossRef] [PubMed]
Oh IK Oh J Yang KS Lee KH Kim SW Huh K. Retinal topography of myopic eyes: a spectral-domain optical coherence tomography study. Invest Ophthalmol Vis Sci . 2014; 55: 4313–4319. [CrossRef] [PubMed]
Oh IK Oh J Kim SW Huh K. Fixation and photoreceptor integrity in optical coherence tomography. Optom Vis Sci . 2012; 89: E1000–E1008. [CrossRef] [PubMed]
Park JH Oh J Yang KS Kim SW Huh K. Errors in thickness comparison maps from 3D optical coherence tomography. Ophthalmic Surg Lasers Imaging . 2012; 43: 275–283. [CrossRef] [PubMed]
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