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

Abstract

Purpose : Despite having a higher axial than tranverse resolution, optical coherence tomography (OCT) images are typically displayed in an axially-stretched manner. As a result, thickness measurements perpendicular to the plane of the retina will be over-estimated unless this stretching is taken into account. The purpose of this study is to investigate the magnitude of inaccuracy in retinal thickness measurements from tilted and axially-stretched OCT images.

Methods : A consecutive series of 50 eyes of 50 patients with age-related macular degeneration were included in this retrospective IRB-approved study. OCT B-scans (Cirrus HD-OCT) through the foveal center were used for this analysis, and the rotation angle of the B-scan was computed. The foveal thickness was measured in three ways: (a) parallel to the orientation of the A-scan (Tx), (b) perpendicular to the retinal pigment epithelium (RPE) surface in the instrument-displayed aspect ratio image (Ty), and thickness measured perpendicular to the RPE surface in a native (“true”) aspect ratio image (Tz). Mathematical modeling was also performed to estimate the measurement error at various angles of tilting and axial stretching.

Results : The average rotation angle was 12.8 ± 6.3 degrees. FTx (224 ± 52µm) and FTy (235 ± 54µm) were significantly larger than FTz (222 ± 51µm) (both P < 0.0001), and FTy was significantly larger than FTx (P < 0.0001). Moreover, the FTy/FTz ratio was 1.06 ± 0.05, and was larger than the FTx/FTz ratio of 1.01 ± 0.01 (P < 0.0001). Both FTx/FTz and FTy/FTz ratios increased as the angle increased, however, the increase in FTx/FTz was more gradual than for FTy/FTz. FTy/FTz ranged from 1.00 to 1.21. In the simulation, with axial stretching by a factor of 2, Ty/Tz ratio was > 1.05 at a tilt angle between 13 to 77 degrees, > 1.10 at a tilt angle between 19 to 71 degrees, and >1.20 at an angle ranging from 32 to 58 degrees. Of note, with more axial stretching, the Ty/Tz ratio rises further. Tx/Tz ratio was smaller than the Ty/Tz ratio at angles ranging from 0 to 54 degrees. The actual patient data showed good agreement with the simulation.

Conclusions : Retinal thickness measurements obtained perpendicular to the RPE surface were overestimated when using tilted and axially-stretched OCT images. If accurate measurements are to be obtained, images with a native aspect ratio similar to microscopy should be used.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.