title: Scale Adjustments to Facilitate Two-Dimensional Measurements in OCT Images.
abstract: OBJECTIVE
  To address the problem of unequal scales for the measurement of two-dimensional structures in OCT images, and demonstrate the use of intra¬ocular objects of known dimensions in the murine eye for the equal calibration of axes.
  
  
  METHODS
  The first part of this work describes the mathematical foundation of major distortion effects introduced by X-Y scaling differences. Illustrations were generated with CorelGraph X3 software. The second part bases on image data obtained with a HRA2 Spectralis (Heidelberg Engineering) in SV129 wild-type mice. Subretinally and intravitreally implanted microbeads, alginate capsules with a diameter of 154±5 μm containing GFP-marked mesenchymal stem cells (CellBeads), were used as intraocular objects for calibration.
  
  
  RESULTS
  The problems encountered with two-dimensional measurements in cases of unequal scales are demonstrated and an estimation of the resulting errors is provided. Commonly, the Y axis is reliably calibrated using outside standards like histology or manufacturer data. We show here that intraocular objects like dimensionally stable spherical alginate capsules allow for a two-dimensional calibration of the acquired OCT raw images by establishing a relation between X and Y axis data. For our setup, a correction factor of about 3.3 was determined using both epiretinally and subretinally positioned beads (3.350 ± 0.104 and 3.324 ± 0.083, respectively).
  
  
  CONCLUSIONS
  In this work, we highlight the distortion-related problems in OCT image analysis induced by unequal X and Y scales. As an exemplary case, we provide data for a two-dimensional in vivo OCT image calibration in mice using intraocular alginate capsules. Our results demonstrate the need for a proper two-dimensional calibration of OCT data, and we believe that equal scaling will certainly improve the efficiency of OCT image analysis.
authors.affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
authors.forename: Marina
authors.initials: M
authors.lastname: Garcia Garrido
authors.affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
authors.forename: Regine L
authors.initials: RL
authors.lastname: Mühlfriedel
authors.affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
authors.forename: Susanne C
authors.initials: SC
authors.lastname: Beck
authors.affiliation: BTG International Germany GmbH, Alzenau, Germany.
authors.forename: Christine
authors.initials: C
authors.lastname: Wallrapp
authors.affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
authors.forename: Mathias W
authors.initials: MW
authors.lastname: Seeliger
mesh_terms: D000818:Animals; D005123:Eye / Q000033:anatomy & histology; D049452:Green Fluorescent Proteins; D007091:Image Processing, Computer-Assisted / Q000706:statistics & numerical data; D051379:Mice; D057507:Mice, 129 Strain; D009887:Ophthalmoscopy / Q000706:statistics & numerical data; D012160:Retina / Q000033:anatomy & histology; D012171:Retinal Vessels / Q000033:anatomy & histology; D041623:Tomography, Optical Coherence / Q000706:statistics & numerical data*
chemical_list: D049452:Green Fluorescent Proteins