title: Distortion correction of OCT images of the crystalline lens: gradient index approach.
abstract: OBJECTIVE
  To propose a method to correct optical coherence tomography (OCT) images of posterior surface of the crystalline lens incorporating its gradient index (GRIN) distribution and explore its possibilities for posterior surface shape reconstruction in comparison to existing methods of correction.
  
  
  METHODS
  Two-dimensional images of nine human lenses were obtained with a time-domain OCT system. The shape of the posterior lens surface was corrected using the proposed iterative correction method. The parameters defining the GRIN distribution used for the correction were taken from a previous publication. The results of correction were evaluated relative to the nominal surface shape (accessible in vitro) and compared with the performance of two other existing methods (simple division, refraction correction: assuming a homogeneous index). Comparisons were made in terms of posterior surface radius, conic constant, root mean square, peak to valley, and lens thickness shifts from the nominal data.
  
  
  RESULTS
  Differences in the retrieved radius and conic constant were not statistically significant across methods. However, GRIN distortion correction with optimal shape GRIN parameters provided more accurate estimates of the posterior lens surface in terms of root mean square and peak values, with errors <6 and 13 μm, respectively, on average. Thickness was also more accurately estimated with the new method, with a mean discrepancy of 8 μm.
  
  
  CONCLUSIONS
  The posterior surface of the crystalline lens and lens thickness can be accurately reconstructed from OCT images, with the accuracy improving with an accurate model of the GRIN distribution. The algorithm can be used to improve quantitative knowledge of the crystalline lens from OCT imaging in vivo. Although the improvements over other methods are modest in two dimension, it is expected that three-dimensional imaging will fully exploit the potential of the technique. The method will also benefit from increasing experimental data of GRIN distribution in the lens of larger populations.
authors.affiliation: Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland. damian.siedlecki@pwr.wroc.pl
authors.forename: Damian
authors.initials: D
authors.lastname: Siedlecki
authors.forename: Alberto
authors.initials: A
authors.lastname: de Castro
authors.forename: Enrique
authors.initials: E
authors.lastname: Gambra
authors.forename: Sergio
authors.initials: S
authors.lastname: Ortiz
authors.forename: David
authors.initials: D
authors.lastname: Borja
authors.forename: Stephen
authors.initials: S
authors.lastname: Uhlhorn
authors.forename: Fabrice
authors.initials: F
authors.lastname: Manns
authors.forename: Susana
authors.initials: S
authors.lastname: Marcos
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Parel
mesh_terms: D000065:Accommodation, Ocular; D000293:Adolescent; D000328:Adult; D000368:Aged; D000465:Algorithms; D002102:Cadaver; D002648:Child; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D007908:Lens, Crystalline / Q000033:anatomy & histology* / Q000502:physiology; D008297:Male; D008875:Middle Aged; D012029:Refraction, Ocular; D015203:Reproducibility of Results; D041623:Tomography, Optical Coherence / Q000379:methods*; D055815:Young Adult