subject: MRI of the Fetal Central Nervous System [print this page] MRI of the Fetal Central Nervous System MRI of the Fetal Central Nervous System
MRI had been used to characterize the developing brain as early as 1983 ( Gilles et al. 1983 ). The use of the HASTEtechnique has provided excellent anatomic information about the fetal central nervous system (CNS) without the need forfetal paralysis.Ultrasound examination remains theprimary method of prenatal imaging but, especially in the brain, MRI is becoming an invaluable adjunct, providingsuperior characterization of fetal brain maturation, and morphology. In most instances, MRI is used for fetuses in which aCNS abnormality has already been demonstrated or is suspected by ultrasonography.
The most commonCNS abnormalities diagnosed by ultrasound examination and referred for evaluation by MRI include cysts,DandyWalker malformation or variant, agenesis of the corpus callosum, hydrocephalus, vein of Galen aneurysm,encephaloceles, and otherwise malformed brains.Ventriculomegaly is a common indication for fetal MRI. T 2-weighted sequences provide excellent imaging of the size andconfiguration of the ventricles.
Criteria for ventriculomegaly in fetal MRI is based on ultrasound data demonstratingventricles of a width of at least 10 mm. However, the size of the ventricles tends to be slightly larger when obtained byMRI than the measurements obtained in the same fetus by ultrasound examination. Once ventriculomegaly is identified, adetailed examination of the entire fetus should be performed to detect associated anomalies. When ventricularenlargement is due to a myelomeningocele, MRI is particularly good for demonstrating the Chiari II malformation, theextent of the neural-tube defect, and the level of the sac.
MRI provides excellent tissue discrimination in defining congenital CNS abnormalities such as calvarial defects,differentiating hemangioma or lymphangioma from encephalocele or meningocele, and demonstrating partial or completeagenesis of the corpus callosum. The finding of an abnormality of the corpus callosum is rarely isolated and oftenindicates the presence of other cerebral abnormalities.
Vein of Galen aneurysm is a vascular malformation that may be misdiagnosed as a cyst or a mass if not examined usingcolor Doppler on ultrasound. MRI can demonstrate not only the vascular anatomy, but also the condition of the brain,which is of concern because these lesions can be associated with encephalomalacia and macrocrania. The presence ofencephalomalacia in cases of vein of Galen aneurysm indicates a poor prognosis.MRI can be helpful in further delineating the anatomy.
