title: Variability of Retinal Thickness Measurements in Tilted or Stretched Optical Coherence Tomography Images.
abstract: OBJECTIVE
  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° to 18° and 72° to 77°, > 1.10 at a tilt angle between 19° to 31° and 59° to 71°, and > 1.20 at an angle ranging from 32° to 58°. 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° to 54°. 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° to 18° and 72° to 77°, greater than 1.10 (10% error) angles ranging from 19° to 31° and 59° to 71°, and greater than 1.20 (20% error) angles ranging from 32° to 58° 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.
  
  
  CONCLUSIONS
  If accurate measurements are to be obtained, images with a native aspect ratio similar to microscopy must be used.
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA ; Department of Ophthalmology, David Geffen School of Medicine at the University of California-Los Angeles, Los Angeles, California, USA.
authors.forename: Akihito
authors.initials: A
authors.lastname: Uji
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA ; Department of Ophthalmology, David Geffen School of Medicine at the University of California-Los Angeles, Los Angeles, California, USA.
authors.forename: Nizar Saleh
authors.initials: NS
authors.lastname: Abdelfattah
authors.affiliation: Retina Vitreous Associates Medical Group, Beverly Hills, California, USA.
authors.forename: David S
authors.initials: DS
authors.lastname: Boyer
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA ; Department of Ophthalmology, David Geffen School of Medicine at the University of California-Los Angeles, Los Angeles, California, USA.
authors.forename: Siva
authors.initials: S
authors.lastname: Balasubramanian
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA ; Department of Ophthalmology, David Geffen School of Medicine at the University of California-Los Angeles, Los Angeles, California, USA.
authors.forename: Jianqin
authors.initials: J
authors.lastname: Lei
authors.affiliation: Doheny Image Reading Center, Doheny Eye Institute, Los Angeles, California, USA ; Department of Ophthalmology, David Geffen School of Medicine at the University of California-Los Angeles, Los Angeles, California, USA ; Retina Vitreous Associates Medical Group, Beverly Hills, California, USA.
authors.forename: SriniVas R
authors.initials: SR
authors.lastname: Sadda
keywords: optical coherence tomography; retina; retinal thickness