Variability of Retinal Thickness Measurements in Tilted or Stretched Optical Coherence Tomography Images

Abstract

PURPOSE: To investigate the level of inaccuracy of retinal thickness measurements in tilted and axially stretched optical coherence tomography (OCT) images. METHODS: A consecutive series of 50 eyes of 50 patients with age-related macular degeneration were included in this study, and Cirrus HD-OCT images through the foveal center were used for the analysis. The foveal thickness was measured in three ways: (1) parallel to the orientation of the A-scan (Tx), (2) perpendicular to the retinal pigment epithelium (RPE) surface in the instrument-displayed aspect ratio image (Ty), and (3) thickness measured perpendicular to the RPE surface in a native aspect ratio image (Tz). Mathematical modeling was performed to estimate the measurement error. RESULTS: The measurement error was larger in tilted images with a greater angle of tilt. In the simulation, with axial stretching by a factor of 2, Ty/Tz ratio was > 1.05 at a tilt angle between 13 degrees to 18 degrees and 72 degrees to 77 degrees , > 1.10 at a tilt angle between 19 degrees to 31 degrees and 59 degrees to 71 degrees , and > 1.20 at an angle ranging from 32 degrees to 58 degrees . Of note with even more axial stretching, the Ty/Tz ratio is even larger. Tx/Tz ratio was smaller than the Ty/Tz ratio at angles ranging from 0 degrees to 54 degrees . The actual patient data showed good agreement with the simulation. The Ty/Tz ratio was greater than 1.05 (5% error) at angles ranging from 13 degrees to 18 degrees and 72 degrees to 77 degrees , greater than 1.10 (10% error) angles ranging from 19 degrees to 31 degrees and 59 degrees to 71 degrees , and greater than 1.20 (20% error) angles ranging from 32 degrees to 58 degrees in the images axially stretched by a factor of 2 (b/a = 2), which is typical of most OCT instrument displays. CONCLUSIONS: Retinal thickness measurements obtained perpendicular to the RPE surface were overestimated when using tilted and axially stretched OCT images. TRANSLATIONAL RELEVANCE: If accurate measurements are to be obtained, images with a native aspect ratio similar to microscopy must be used.