title: MRI tight posterior fossa sign for prenatal diagnosis of Chiari type II malformation.
abstract: BACKGROUND
  Chiari type II malformation (CMII) is one of three hindbrain malformations that display hydrocephalus. We have observed that cerebrospinal fluid (CSF) signal in the posterior fossa, which is always apparent on normal fetal MR images, is not visible in a fetus with CMII. We use the term 'tight posterior fossa' for this MR imaging finding, and evaluate the diagnostic value of this finding on fetal MR images.
  
  
  METHODS
  Included in the study were 21 fetuses which underwent brain MR imaging at 1.5 T using two-dimensional balanced turbo-field-echo (2-D balanced-TFE) in the axial and sagittal planes. Postnatal diagnoses were CMII (n=5), CNS abnormalities other than CMII (n=8), and no abnormality (n=8). A tight posterior fossa was defined as an absent or slit-like water signal space around the hindbrain in the posterior fossa on both sagittal and axial MR images.
  
  
  RESULTS
  All CMII fetuses displayed a tight posterior fossa on MR images. Hydrocephalus was visualized in all CMII fetuses and myelomeningocele in four fetuses, but hindbrain herniation was visualized only in two of five fetuses. The CSF signal surrounding the hindbrain was clearly visible in all the other 16 fetuses, including five with hydrocephalus not associated with CMII, although it was slightly narrower in a fetus with a cloverleaf skull than in the normal fetuses.
  
  
  CONCLUSIONS
  Tight posterior fossa in the presence of hydrocephalus is a useful and characteristic finding of CMII on fetal MRI.
authors.affiliation: Department of Radiology, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan. annekumi@cameo.plala.or.jp
authors.forename: Kumiko
authors.initials: K
authors.lastname: Ando
authors.forename: Reiichi
authors.initials: R
authors.lastname: Ishikura
authors.forename: Masayo
authors.initials: M
authors.lastname: Ogawa
authors.forename: Miyuki
authors.initials: M
authors.lastname: Shakudo
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Tanaka
authors.forename: Kyoko
authors.initials: K
authors.lastname: Minagawa
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Takada
authors.forename: Satoshi
authors.initials: S
authors.lastname: Yamamoto
authors.forename: Masayuki
authors.initials: M
authors.lastname: Fujiwara
authors.forename: Shozo
authors.initials: S
authors.lastname: Hirota
mesh_terms: D001139:Arnold-Chiari Malformation / Q000175:diagnosis*; D003388:Cranial Fossa, Posterior / Q000473:pathology*; D003937:Diagnosis, Differential; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011247:Pregnancy; D011296:Prenatal Diagnosis; D012189:Retrospective Studies