Comparison of Fourier-Domain and Time-Domain Optical Coherence Tomography for Assessment of Corneal Thickness and Intersession Repeatability GAURAV PRAKASH, AMAR AGARWAL, SOOSAN JACOB, DHIVYA ASHOK KUMAR, ATHIYA AGARWAL, AND RUMKI BANERJEE ● PURPOSE: To compare Fourier-domain optical coher- ence tomography (FD-OCT) with time-domain OCT (TD-OCT) for agreement in corneal pachymetry in the central (2-mm) zone and paracentral (2- to 5-mm) zone, minimum corneal thickness, and instrument intersession reliability in candidates with normal cornea. ● DESIGN: Prospective comparative trial. ● METHODS: The study was performed at a tertiary care ophthalmic hospital. One hundred normal candi- dates with no ocular pathology except mild refractive error were included. FD-OCT and TD-OCT were used for pachymetric analysis within 15 minutes of each other. Repeat testing on both machines was done on the next day. One eye from each patient was randomly selected for analysis of both comparison and reliability. ● RESULTS: The mean thickness was significantly more for FD-OCT (difference being 5.8, 11.1, and 19.2 m for central, paracentral, and minimal thickness). Highest correlation, best linear fit, and smallest range of 95% limits of agreement (LoA) was seen for central corneal thickness, followed by paracentral and minimum corneal thickness (R 2 values for the 3 best fit lines were 0.93, 0.86, and 0.58, respectively). In terms of intersession reliability, FD-OCT was better (coefficients of reliability <1.5% vs 4.5% of TD-OCT) with smaller 95% LoA range, suggesting its better performance. ● CONCLUSIONS: The corneal thickness measurements between TD-OCT and FD-OCT were highly correlated but not similar. Conversion equations may be used for central and paracentral, but not for minimum, corneal thickness. Even though both the OCT instruments had good reliability, FD-OCT was better. (Am J Ophthal- mol 2009;148:282–290. © 2009 by Elsevier Inc. All rights reserved.) A CCURATE ASSESSMENT OF CENTRAL AND PARA- central corneal thickness is essential in varied clinical settings. Preoperative planning of refrac- tive surgery requires accurate calculations of thickness of the cornea, the flap, and the residual bed. 1 Correct assessment of central thickness is required in glaucoma evaluation and as an indirect evidence of functionality of endothelial cells. 2,3 Anterior segment optical coherence tomography (AS-OCT) is a noncontact device that pro- vides central and paracentral pachymetric data, along with biometric information of the anterior segment and other details. The use of OCT for anterior segment analysis was first published around the year 2000. 4,5 Since then, AS-OCT has demonstrated usefulness in diverse clinical situations including refractive surgery, uveitis, and phacoemulsifica- tion wound assessment. 6 –10 The comparison of AS-OCT with other devices and its acceptable repeatability is established. 11–16 There are 2 principles of data acquisition and processing in AS-OCT: time-domain (TD) and the Fourier-domain (FD). In the TD-OCT, there is a mechanical moving part, which performs the A-scan, and the information along the longitudinal direction is accumulated over the course of the longitudinal scan time. 17–19 FD-OCT has recently been introduced to ophthalmic practice. This device has a higher resolution and acquisition speed than the TD- OCT. In FD-OCT, the light from the reference arm interferes with the reflected light, generating spectral interference fringes that eventually undergo Fourier trans- formation. 17–19 Therefore, information in an entire A-scan area can be acquired by a charge-coupled device (CCD) camera simultaneously. This increases the acquisition rate manifold without any physical movement. The A-scan acquisition rate is limited by the CCD camera frame transfer rate and the computer calculation time to perform the Fourier transform of the CCD-acquired raw data into A-scan information. The 2 acquisition methods have been compared for evaluation of retinal layers in normal and diseased condi- tions. 20 –23 However, to the best of our knowledge, there is no published study comparing the assessment of corneal pachymetry by the 2 methods. We used 2 commercially available machines based on TD (Carl Zeiss Meditec Inc, Dublin, California, USA) and FD principles (RTVue; Optovue, Fremont, California, USA). The aims of the current study were to compare the corneal thickness in the central (2 mm) zone and the Accepted for publication Mar 6, 2009. From Dr Agarwal’s Eye Hospital and Eye Research Centre, Chennai, India. Inquiries to Amar Agarwal, Dr Agarwal’s Eye Hospital and Eye Research Centre, 19, Cathedral Road, Chennai 600 086, India; e-mail: dragarwal@vsnl.com © 2009 BY ELSEVIER INC.ALL RIGHTS RESERVED. 282 0002-9394/09/$36.00 doi:10.1016/j.ajo.2009.03.012