title: The influence of contour line size and location on the reproducibility of topographic measurement with the Heidelberg Retina Tomograph.
abstract: BACKGROUND
  The recommended contour line (CL) location with the Heidelberg Retina Tomograph (HRT) is on the inner edge of Elschnig's scleral ring. This study investigated HRT parameter reproducibility when: (i) the CL size is altered relative to Elschnig's ring; (ii) the CL is either redrawn or imported between images.
  
  
  METHODS
  Using the HRT, seven 10 degrees images were acquired for 10 normal volunteers and 10 primary open angle glaucoma (POAG) subjects. A CL was drawn on one image for each subject using Elschnig's scleral ring for reference and imported into subsequent images. The CL diameter was then (a) increased by 50 microns; (b) increased by 100 microns; and (c) decreased by 50 microns. To investigate the effect of the method of contour line transfer between images a CL was: (1) defined for one image and imported to 6 subsequent images; (2) drawn separately for each image.
  
  
  RESULTS
  Parameter variability improved as the size of the CL increased for the normal group relative to Elschnig's ring but was unchanged in the POAG group. The export/import function (method 1) resulted in better parameter reproducibility than the redrawing method for both groups.
  
  
  CONCLUSIONS
  The exporting and importing function resulted in better parameter variability for both subject groups and should be used for transferring CLs across images for the same subject. Increasing the overall CL size relative to Elschnig's scleral ring improved the reproducibility of the measured parameters in the normal group. No significant difference in parameter variability was observed for the POAG group. This suggests that the reproducibility of HRT images are affected more by the variation in topography between images than change in CL definition.
authors.affiliation: Neurosciences Research Institute, School of Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK. e.j.roff@aston.ac.uk
authors.forename: E J
authors.initials: EJ
authors.lastname: Roff
authors.forename: S L
authors.initials: SL
authors.lastname: Hosking
authors.forename: D A
authors.initials: DA
authors.lastname: Barnes
mesh_terms: D016022:Case-Control Studies; D005260:Female; D005902:Glaucoma, Open-Angle / Q000473:pathology*; D006801:Humans; D008297:Male; D018613:Microscopy, Confocal / Q000379:methods; D008875:Middle Aged; D019721:Ophthalmoscopes / Q000592:standards; D009898:Optic Disk / Q000473:pathology*; D015203:Reproducibility of Results; D014054:Tomography / Q000379:methods* / Q000592:standards