title: Optical quality assessment in normal and forme fruste keratoconus eyes with a double-pass system: a comparison and variability study.
abstract: OBJECTIVE
  To comparatively evaluate the image quality in normal and forme fruste keratoconus patients using a double-pass system.
  
  
  METHODS
  Fourteen normal eyes and 15 eyes with forme fruste keratoconus were included. Corneal tomographic parameters were obtained with optical coherence tomography. Image quality characteristics were determined by an optical quality analysis system (OQAS). Two readings were obtained from each eye. The following parameters were analysed: modulation transfer function cutoff frequency (MTF), Strehl ratio and OQAS values (OV) at contrasts of 100% (OV-100), 20% (OV-20) and 9% (OV-9). Repeatability coefficient (RC), coefficient of variation (CVw) and intraclass correlation coefficient (ICC) were evaluated.
  
  
  RESULTS
  There were no significant differences in central corneal thickness, anterior best-fit sphere and posterior best-fit sphere in both groups. There was a significant difference between the mean MTF (p=0.001), Strehl ratio (p<0.001), OV-100 (p=0.001), OV-20 (p<0.001) and OV-9 (p<0.001) values between both groups. The coefficients of variation were 4.10 (MTF) and 13.77 (Strehl ratio) in normal eyes and 14.53 (MTF) and 14.97 (Strehl ratio) in eyes with forme fruste keratoconus. The ICC values were 0.99 and 0.95 for MTF and, 0.80 and 0.91 for Strehl ratio, in normal and forme fruste keratoconus eyes, respectively. The corresponding repeatability values were 3.53 and 6.15 for MTF and, 0.06 and 0.04 for Strehl ratio, respectively.
  
  
  CONCLUSIONS
  OQAS has the potential to be a useful diagnostic tool in cases with early keratoconus. The repeatability of a double-pass (DP) system was good in normal as well as forme fruste keratoconus eyes.
authors.affiliation: Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, Hong Kong.
authors.forename: Cong
authors.initials: C
authors.lastname: Ye
authors.affiliation: Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, Hong Kong.
authors.forename: Paul Ka-Fai
authors.initials: PK
authors.lastname: Ng
authors.affiliation: Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, Hong Kong Centre for Eye Research Australia, University of Melbourne, Melbourne, Victoria, Australia.
authors.forename: Vishal
authors.initials: V
authors.lastname: Jhanji
mesh_terms: D000328:Adult; D003315:Cornea / Q000503:physiopathology*; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D007640:Keratoconus / Q000503:physiopathology*; D008297:Male; D015203:Reproducibility of Results; D041623:Tomography, Optical Coherence; D014785:Vision, Ocular / Q000502:physiology*; D014792:Visual Acuity / Q000502:physiology*; D055815:Young Adult
keywords: Cornea; Imaging