title: Sonography of the fetal posterior fossa: false appearance of mega-cisterna magna and Dandy-Walker variant.
abstract: OBJECTIVE
  To establish that incorrect scanning of the fetal posterior fossa may falsely create the appearance of a mega-cisterna magna (MCM) or Dandy-Walker variant (DWV).
  
  
  METHODS
  After routine sonography, 100 consecutive fetuses (gestational age range, 25-40 weeks) without abnormality underwent additional posterior fossa scanning in an attempt to create the appearance of an MCM (anteroposterior diameter larger than 10 mm) or DWV. Sonograms were obtained in a plane inferior to or angled more coronally than the routine axial plane.
  
  
  RESULTS
  The posterior fossa was depicted in 93 fetuses. Pseudo-MCM occurred in 35 (38%) of 93 fetuses: 12 (30%) of 40 fetuses aged 25.0-29.9 weeks, 11 (50%) of 22 fetuses aged 30.0-34.9 weeks, and 12 (39%) of 31 fetuses aged 35.0-40.0 weeks. Pseudo-DWV occurred in 40 (43%) of 93 fetuses: 20 (50%) of 40 fetuses aged 25.0-29.9 weeks, 12 (55%) of 22 fetuses aged 30.0-34.9 weeks, and eight (26%) of 31 fetuses aged 35.0-40.0 weeks.
  
  
  CONCLUSIONS
  Sonography of the posterior fossa in an angled semi-coronal plane should be avoided because it may create an appearance that mimics an abnormality.
authors.affiliation: Department of Radiology, Brigham and Women's Hospital, Boston, MA 02115.
authors.forename: F C
authors.initials: FC
authors.lastname: Laing
authors.forename: M C
authors.initials: MC
authors.lastname: Frates
authors.forename: D L
authors.initials: DL
authors.lastname: Brown
authors.forename: C B
authors.initials: CB
authors.lastname: Benson
authors.forename: D N
authors.initials: DN
authors.lastname: Di Salvo
authors.forename: P M
authors.initials: PM
authors.lastname: Doubilet
mesh_terms: D016477:Artifacts; D002946:Cisterna Magna / Q000002:abnormalities / Q000000981:diagnostic imaging*; D003388:Cranial Fossa, Posterior / Q000000981:diagnostic imaging*; D003616:Dandy-Walker Syndrome / Q000000981:diagnostic imaging*; D003951:Diagnostic Errors; D005260:Female; D005315:Fetal Diseases / Q000000981:diagnostic imaging; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D016216:Ultrasonography, Prenatal