title: Precise thickness measurements of Bowman's layer, epithelium, and tear film.
abstract: OBJECTIVE
  To visualize corneal microstructure such as tear film, epithelium, and Bowman's layer in three dimensions with spectral domain optical coherence tomography (SDOCT) exhibiting 1.3 μm axial resolution at 100,000 A-scans/s. This enables measurement of epithelial and Bowman layer thickness across an area of 8.4 mm × 8.4 mm and measuring the tear film thickness at the central cornea.
  
  
  METHODS
  We designed a high-performance SDOCT system, which uses a broad bandwidth TiSapph Laser and a high-speed complementary metal-oxide-semiconductor detector technology, providing a resolution in tissue of 1.3 μm and an acquisition speed of 100,000 A-scans/s. Such speed and resolution is a prerequisite if precise anatomy is to be determined. The high resolution gives access to corneal microstructure such as the epithelium layer as well as the boundaries of Bowman's layer and stroma. Even more interestingly, the tear film can be distinguished on the surface of the cornea. The Bowman's layer and epithelial thickness for both eyes of nine subjects have been measured out of which two subjects underwent photorefractive keratectomy treatment.
  
  
  RESULTS
  Three-dimensional volumes of the human cornea have been recorded in vivo at an A-scan rate of 100,000 scans/s. Epithelial thickness was measured to be 55.8 ± 3.3 μm and Bowman's layer thickness 18.7 ± 2.5 μm in normal eyes. Epithelial thickness in the eyes after refractive surgery was measured to be 68.2 ± 5.0 μm. The Bowman layer was degenerated in these eyes. The average tear film thickness of four eyes was 5.1 ± 0.5 μm.
  
  
  CONCLUSIONS
  Using a high-performance SDOCT system with high-imaging speed and ultrahigh resolution, we produced precise thickness maps of the epithelium and for the first time of the Bowman's layer. Such a system will give insight into high-fidelity three-dimensional corneal microstructure helping to precisely plan refractive surgery. It may furthermore yield new perspectives on studying and understanding tear film dynamics.
authors.affiliation: Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria.
authors.forename: Tilman
authors.initials: T
authors.lastname: Schmoll
authors.forename: Angelika
authors.initials: A
authors.lastname: Unterhuber
authors.forename: Christoph
authors.initials: C
authors.lastname: Kolbitsch
authors.forename: Tuan
authors.initials: T
authors.lastname: Le
authors.forename: Andreas
authors.initials: A
authors.lastname: Stingl
authors.forename: Rainer
authors.initials: R
authors.lastname: Leitgeb
mesh_terms: D000328:Adult; D050541:Bowman Membrane / Q000033:anatomy & histology* / Q000378:metabolism; D019781:Corneal Topography; D019573:Epithelium, Corneal / Q000033:anatomy & histology* / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D012016:Reference Values; D012030:Refractive Errors / Q000175:diagnosis; D015203:Reproducibility of Results; D013666:Tears / Q000737:chemistry*; D041623:Tomography, Optical Coherence / Q000379:methods*