title: The cellular origins of the outer retinal bands in optical coherence tomography images.
abstract: OBJECTIVE
  To test the recently proposed hypothesis that the second outer retinal band, observed in clinical OCT images, originates from the inner segment ellipsoid, by measuring: (1) the thickness of this band within single cone photoreceptors, and (2) its respective distance from the putative external limiting membrane (band 1) and cone outer segment tips (band 3).
  
  
  METHODS
  Adaptive optics-optical coherence tomography images were acquired from four subjects without known retinal disease. Images were obtained at foveal (2°) and perifoveal (5°) locations. Cone photoreceptors (n = 9593) were identified and segmented in three dimensions using custom software. Features corresponding to bands 1, 2, and 3 were automatically identified. The thickness of band 2 was assessed in each cell by fitting the longitudinal reflectance profile of the band with a Gaussian function. Distances between bands 1 and 2, and between 2 and 3, respectively, were also measured in each cell. Two independent calibration techniques were employed to determine the depth scale (physical length per pixel) of the imaging system.
  
  
  RESULTS
  When resolved within single cells, the thickness of band 2 is a factor of three to four times narrower than in corresponding clinical OCT images. The distribution of band 2 thickness across subjects and eccentricities had a modal value of 4.7 μm, with 48% of the cones falling between 4.1 and 5.2 μm. No significant differences were found between cells in the fovea and perifovea. The distance separating bands 1 and 2 was found to be larger than the distance between bands 2 and 3, across subjects and eccentricities, with a significantly larger difference at 5° than 2°.
  
  
  CONCLUSIONS
  On the basis of these findings, we suggest that ascription of the outer retinal band 2 to the inner segment ellipsoid is unjustified, because the ellipsoid is both too thick and proximally located to produce the band.
authors.affiliation: Vision Science and Advanced Retinal Imaging Laboratory, University of California, Davis Eye Center, Sacramento, California, United States.
authors.forename: Ravi S
authors.initials: RS
authors.lastname: Jonnal
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana, United States.
authors.forename: Omer P
authors.initials: OP
authors.lastname: Kocaoglu
authors.affiliation: Vision Science and Advanced Retinal Imaging Laboratory, University of California, Davis Eye Center, Sacramento, California, United States.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Zawadzki
authors.affiliation: Vision Science and Advanced Retinal Imaging Laboratory, University of California, Davis Eye Center, Sacramento, California, United States.
authors.forename: Sang-Hyuck
authors.initials: SH
authors.lastname: Lee
authors.affiliation: Vision Science and Advanced Retinal Imaging Laboratory, University of California, Davis Eye Center, Sacramento, California, United States.
authors.forename: John S
authors.initials: JS
authors.lastname: Werner
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana, United States.
authors.forename: Donald T
authors.initials: DT
authors.lastname: Miller
mesh_terms: D001485:Basement Membrane / Q000033:anatomy & histology; D003941:Diagnostic Techniques, Ophthalmological; D006801:Humans; D008953:Models, Anatomic; D012160:Retina / Q000033:anatomy & histology*; D017949:Retinal Cone Photoreceptor Cells / Q000166:cytology; D055211:Retinal Photoreceptor Cell Inner Segment; D055214:Retinal Photoreceptor Cell Outer Segment; D041623:Tomography, Optical Coherence / Q000379:methods
keywords: adaptive optics; morphometry; nomenclature; optical coherence tomography; photoreceptor morphology