This study aimed to evaluate a new method to measure SFCT, using enhanced depth imaging (EDI)-OCT, and compare the interpersonal variability among different methods.
Senior medical students (n = 82) at the Keimyung University Medical School volunteered for this non-interventional study during their ophthalmology rotation between January and August 2019. During their attendance, the students were introduced to ophthalmic examination devices, including the OCT system. They reviewed the OCT images of normal subjects and those with various ophthalmic pathologies with the attending professors (KTK and JHJ). Following the introduction and OCT, the students were asked to measure the SFCT on OCT images of five pre-selected standard subjects, using the three methods described below. We have obtained written informed consent from all students before starting the measurements.
Standard Subjects
For the standard images, five eyes of five pre-selected healthy participants with no proven ophthalmic or systemic ailments were examined with a swept-source OCT (SS-OCT; DRI TRITON, Topcon Corp., Tokyo, Japan). All participants underwent a complete ocular examination, including refraction, slit-lamp biomicroscopy, intraocular pressure, and fundoscopy, to exclude ophthalmic abnormalities. The participants included a 60-year-old man with a spherical equivalent (SE) of +1 diopter as image 1 (Figure 1a), a 67-year-old man with a SE of +2 diopter as image 2 (Figure 1b), a 46-year-old man with a SE of -3.25 diopter as image 3 (Figure 1c), a 43-year-old woman with a SE of -0.375 diopter as image 4 (Figure 1d), and a 58-year-old man with a SE of -11 diopter as image 5 (Figure 1e). These images were chosen as standards because the OCT quality was better than 86/100 as defined by the OCT software. We obtained informed consent from all participants. The institutional review board of Keimyung University approved the study procedures (IRB: 2019-12-056), which adhered to the tenets of the Declaration of Helsinki.
SS-OCT and Scan Protocols
After pupillary dilation, 64 to 128 averaged horizontal scan images in a 12-mm transverse scan range were obtained with the SS-OCT system for each standard image. The single images were averaged by the system software to improve the image quality. The wavelength-swept laser light source was centered at 1050 nm. The transverse resolution of the OCT was 20 μm, the in-depth, in-tissue resolution was 8 μm, and the maximum scan velocity was 100,000 A-scans per second.
Choroidal Thickness Measurement
SFCT was defined as the distance between the outer border of the hyperreflective line representing the RPE and the choroidoscleral interface. The automated built-in calibration software of the IMAGEnet® 6 viewer (Topcon., Tokyo, Japan) was used to determine the distance between these lines. The students were asked to measure the SFCT with the caliper tool embedded in the viewer program. Measurements were performed in three different ways. In method 1, to see an intuitive way of measurement, students that were not yet exposed to the other known method were asked to measure the SFCT, using their own methods, without receiving any specific instructions from the authors (Figure 2a). For method 2, the conventional method, commonly used in previous studies, was followed. The students were asked to measure the SFCT perpendicular to the RPE (Figure 2b). Finally, the new method, or method 3, measured SFCT along a virtual line connecting the deepest point of the fovea (the umbo) and the most elevated point of the ellipsoid zone (the elevation) (Figure 2c). The students were provided a separate room and enough time to perform the SFCT measurements. For inter-image and inter-method comparisons, the angle between the SFCT measurement line and the vertical line on the OCT image was calculated using ImageJ ver. 1.50i (National Institutes of Health, Bethesda, MD, USA) (Figure 3).
Interpersonal Variability and Statistical Analyses
We obtained SFCTs, measured by the 82 independent observers, using three methods in five standard OCT images. The interpersonal variability among the three SFCT measurement methods was assessed using the intraclass correlation coefficient (ICC). Statistical analysis was performed using the Statistical Package for the Social Sciences version 19.0.1 (IBM Corp., Armonk, NY, USA.). Statistical significance was defined as P < 0.05.