title: Variation associated with measurement of retinal vessel diameters at different points in the pulse cycle.
abstract: OBJECTIVE
  To assess the variability in retinal vessel measurements at different points in the pulse cycle.
  
  
  METHODS
  A healthy white male aged 19 years had 30 digitised images taken at three distinct points in the pulse cycle over a one hour period. A pulse synchronised ear clip trigger device was used to capture images at the desired point in the pulse cycle. Two trained graders measured the retinal vessel diameter of one large arteriole, one large venule, one small arteriole, and one small venule 10 times in each of these 30 images.
  
  
  RESULTS
  Within an image, variability was similar between graders, pulse point, and vessel type. Across images taken at the same point in the pulse period, the change from the minimum to maximum measurement was between 6% and 17% for arterioles and between 2% and 11% for venules. In addition, measurements of small vessels had greater changes than large vessels and no point in the pulse period was more variable than another. Ignoring pulse cycle increased variability across images in the large venule, but not in the other vessel types. Mixed effect models were fit for each of the vessel types to determine the greatest source of variability. Controlling for pulse point and grader, the largest source of variability for all four vessels measured was across images, accounting for more than 50% of the total variability.
  
  
  CONCLUSIONS
  Measurements of large retinal venules is generally less variable than measurements of other retinal vessels. After controlling for pulse point and grader, the largest source of variation is across images. Understanding the components of variability in measuring retinal vessels is important as these techniques are applied in epidemiological studies.
authors.affiliation: UW-Madison Department of Ophthalmology and Visual Sciences, University of Wisconsin, 610 N. Walnut Street, 4th floor WARF, Madison, WI 53726, USA. knudtson@epi.ophth.wisc.edu
authors.forename: M D
authors.initials: MD
authors.lastname: Knudtson
authors.forename: B E K
authors.initials: BE
authors.lastname: Klein
authors.forename: R
authors.initials: R
authors.lastname: Klein
authors.forename: T Y
authors.initials: TY
authors.lastname: Wong
authors.forename: L D
authors.initials: LD
authors.lastname: Hubbard
authors.forename: K E
authors.initials: KE
authors.lastname: Lee
authors.forename: S M
authors.initials: SM
authors.lastname: Meuer
authors.forename: C P
authors.initials: CP
authors.lastname: Bulla
mesh_terms: D000328:Adult; D001160:Arterioles / Q000033:anatomy & histology / Q000502:physiology; D003941:Diagnostic Techniques, Ophthalmological; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008297:Male; D010781:Photography; D011674:Pulse; D015203:Reproducibility of Results; D012171:Retinal Vessels / Q000033:anatomy & histology* / Q000502:physiology*; D014699:Venules / Q000033:anatomy & histology / Q000502:physiology