Document Detail

MRI of the fetal posterior fossa.
MedLine Citation:
PMID:  15565345     Owner:  NLM     Status:  MEDLINE    
MRI is a useful tool to complement US for imaging of the fetal posterior fossa (PF). In France, the discovery of a PF malformation in the fetus frequently leads to termination of pregnancy (80% in a personal series). However, despite improved accuracy in the diagnosis of PF abnormalities, prognosis remains uncertain. The first objective of this review is to document the normal MRI landmarks of the developing fetal PF. Because of their thinness, the visibility of the cerebellar fissures is dramatically delayed on MRI compared to macroscopic data. An important landmark is identification of the primary fissure of the vermis, normally seen at around 25-26 weeks' gestation (WG) on the sagittal slice, separating the larger posterior lobe from the anterior lobe (volume ratio around 2:1). The prepyramidal and secondary fissures are usually only identifiable after 32 WG and the hemispheric fissures are difficult to see until the end of pregnancy. Considering the signal changes, high signal on T2-weighted (T2-W) sequences is seen from 25 WG in the posterior part of the brain stem (tegmentum and ascending sensory tracts) related to myelination. The low signal intensities seen within the cerebellum on T2-W images correspond to high cellularity of grey matter (deep nuclei), as there is no myelination within the white matter before 38 WG. The second objective is to highlight the signs highly predictive of a poor neurological prognosis. Lack of pontine curvature or vermian agenesis without a PF cyst (small volume of PF) is greatly associated with poor neurological status. The third objective is to propose a diagnostic strategy in difficult cases where prognosis is important, e.g. the Dandy Walker continuum. Analysis of the cerebellum is often impossible if a PF cyst is present (whatever its nature) as the mass effect usually blurs the foliation and even impairs evaluation of the normal ratio between the posterior and anterior lobes of the vermis. Isolated cerebellar hypoplasias raise the question of prognosis and genetic counselling. Such uncertainties require an amniocentesis and a careful search for other anomalies (cerebral and extracerebral). Unilateral abnormalities of a cerebellar hemisphere can be associated with good neurological status if they are isolated. The final objective is to discuss other rare PF fetal abnormalities, such as vascular malformations and tumours.
Catherine Adamsbaum; Marie Laure Moutard; Christine André; Valérie Merzoug; Solène Ferey; Marie Pierre Quéré; Fanny Lewin; Catherine Fallet-Bianco
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Publication Detail:
Type:  Journal Article; Review     Date:  2004-11-23
Journal Detail:
Title:  Pediatric radiology     Volume:  35     ISSN:  0301-0449     ISO Abbreviation:  Pediatr Radiol     Publication Date:  2005 Feb 
Date Detail:
Created Date:  2005-01-17     Completed Date:  2005-07-14     Revised Date:  2005-11-16    
Medline Journal Info:
Nlm Unique ID:  0365332     Medline TA:  Pediatr Radiol     Country:  Germany    
Other Details:
Languages:  eng     Pagination:  124-40     Citation Subset:  IM    
Department of Radiology, St Vincent de Paul Hospital, 82 avenue Denfert Rochereau, 75674 Paris Cedex 14, France.
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MeSH Terms
Cranial Fossa, Posterior / abnormalities*,  embryology,  pathology*
Fetal Diseases / diagnosis
Gestational Age
Magnetic Resonance Imaging / methods*

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