MRI tight posterior fossa sign for prenatal diagnosis of Chiari type II malformation.
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.
Ando
Kumiko
K
Department of Radiology, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan. annekumi@cameo.plala.or.jp
Ishikura
Reiichi
R
Ogawa
Masayo
M
Shakudo
Miyuki
M
Tanaka
Hiroyuki
H
Minagawa
Kyoko
K
Takada
Yoshihiro
Y
Yamamoto
Satoshi
S
Fujiwara
Masayuki
M
Hirota
Shozo
S
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