title: Noninvasive visualization and analysis of parafoveal capillaries in humans.
abstract: OBJECTIVE
  To demonstrate a noninvasive method to visualize and analyze the parafoveal capillary network in humans.
  
  
  METHODS
  An adaptive optics scanning laser ophthalmoscope was used to acquire high resolution retinal videos on human subjects. Video processing tools that enhance motion contrast were developed and applied to the videos to generate montages of parafoveal retinal capillaries. The capillary network and foveal avascular zone (FAZ) were extracted using video and image analysis algorithms. The capillary densities in the zone immediately outside the FAZ were calculated and the variation in density as a function of direction was investigated. Extracted FAZ geometries were used to calculate area and effective diameters. The authors also compared their method against fluorescein angiography (FA) for one subject.
  
  
  RESULTS
  The parafoveal capillaries were clearly visible when the motion contrast in noninvasive videos was enhanced. There was a marked improvement in the contrast of the parafoveal capillaries when compared to the unprocessed videos. The average FAZ area was 0.323 mm(2), with an average effective diameter of 633 microm. There was no variation in capillary density near the FAZ in different directions.
  
  
  CONCLUSIONS
  Using motion cues to enhance vessel contrast is a powerful tool for visualizing the capillary network, in the absence of contrast agents. The authors demonstrate a tool to study the microcirculation of healthy subjects noninvasively.
authors.affiliation: University of California, Berkeley and University of California, San Francisco, Joint Graduate Group in Bioengineering, Berkeley, California 94720-2020, USA. johnny@berkeley.edu
authors.forename: Johnny
authors.initials: J
authors.lastname: Tam
authors.forename: Joy A
authors.initials: JA
authors.lastname: Martin
authors.forename: Austin
authors.initials: A
authors.lastname: Roorda
mesh_terms: D000328:Adult; D000465:Algorithms; D002196:Capillaries / Q000502:physiology; D005451:Fluorescein Angiography; D005584:Fovea Centralis / Q000098:blood supply*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007834:Lasers; D008833:Microcirculation / Q000502:physiology; D019721:Ophthalmoscopes; D012039:Regional Blood Flow / Q000502:physiology; D012171:Retinal Vessels / Q000502:physiology*; D014741:Video Recording