title: Circle of Willis: morphologic variation on three-dimensional time-of-flight MR angiograms.
abstract: OBJECTIVE
  To establish normal reference values for the presence of the anatomic variants of the circle of Willis and average diameters for its component vessels by using three-dimensional time-of-flight magnetic resonance (MR) angiography and to determine whether age- or sex-related differences exist in the circle's anatomy.
  
  
  METHODS
  One hundred fifty volunteers were grouped according to age: those aged 20-25 years (n = 50) and those aged 60-88 years (n = 100). All subjects underwent three-dimensional time-of-flight MR angiography of the arterial circle at 1.5 T. The anatomic variants of the anterior and posterior parts of the circle were determined separately, the completeness of the entire circle was assessed, and the diameters of all component vessels were measured.
  
  
  RESULTS
  On MR angiograms, 111 (74%) subjects demonstrated a complete anterior part of the circle, 78 (52%) demonstrated a complete posterior part of the circle, and 63 (42%) demonstrated an entirely complete circle of Willis (complete anterior and posterior parts of the circle combined). The presence of an entirely complete circle of Willis was slightly higher in younger persons and in women. Most vessel diameters were smaller in women, except for the diameter of the posterior communicating artery. Statistically significant differences were found in vessel diameters between the younger and the older age groups.
  
  
  CONCLUSIONS
  The authors determined normal reference values for morphologic variants and diameter measurements of the circle of Willis specific to three dimensional time-of-flight MR angiography.
authors.affiliation: Department of Radiology, University Hospital Utrecht, The Netherlands.
authors.forename: M J
authors.initials: MJ
authors.lastname: Krabbe-Hartkamp
authors.forename: J
authors.initials: J
authors.lastname: van der Grond
authors.forename: F E
authors.initials: FE
authors.lastname: de Leeuw
authors.forename: J C
authors.initials: JC
authors.lastname: de Groot
authors.forename: A
authors.initials: A
authors.lastname: Algra
authors.forename: B
authors.initials: B
authors.lastname: Hillen
authors.forename: M M
authors.initials: MM
authors.lastname: Breteler
authors.forename: W P
authors.initials: WP
authors.lastname: Mali
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D002941:Circle of Willis / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D018810:Magnetic Resonance Angiography; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012727:Sex Characteristics