Imaging the fetal central nervous system

Abstract

The low prevalence of fetal central nervous system anomalies results in a restricted level of exposure and limited experience-- for most of the obstetricians involved in prenatal ultrasound. Sonographic guidelines for screening the fetal brain in a systematic way will probably increase the detection rate and enhance a correct referral to a tertiary care center, offering the patient a multidisciplinary approach of the condition. This paper aims to elaborate on prenatal sonographic and magnetic resonance imaging (MRI) diagnosis and outcome of various central nervous system malformations. Detailed neurosonographic investigation has become available through high resolution vaginal ultrasound probes and the development of a variety of 3D ultrasound modalities e.g. ultrasound tomographic imaging. In addition, fetal MRI is particularly helpful in the detection of gyration and neurulation-- anomalies and disorders of the gray and white matter.

Keywords: brain malformation; central nervous system; fetal imaging; prenatal MRI.

Fig. 1. The transthalamic and transcerebellar reference planes are used for structured analysis of the fetal brain in the second trimester.

Fig. 2. Measurement of the lateral ventricle

Fig. 3. MRI axial view of ventriculomegaly associated with bilateral polymicrogyria (encircled in white). Gestational age 27 weeks.

Fig. 4. Large, but isolated choroid plexus cyst (arrow head) mimicking a ventriculomegaly.

Fig. 5. Spinal neural tube defects are mainly detected by the presence of the “lemon” (yellow arrows) and “banana” sign (dashed arrows), or scalloping of the frontal bones and obliteration of the cisterna magna with hypoplasia of the cerebellum (a). Severe lesions are characterized by spinal dysraphism and are more easily detectable at the level of the spine it self (b). MRI axial view of a fetal meningomyelocoele in a mother with a body mass index of 42. Gestational age 20 weeks (c).

Fig. 6a. Alobar holoprosencephaly is characterized by a large single ventricle (*), fused thalami (arrowhead) and absence of other midline structures. In addition varying degree of facial abnormalities can be present.

Fig. 6b. MRI axial view of lobar holoprosencephaly in a fetus of 31 weeks. Note the absent septum pellucidum, hypoplastic anterior interhemispheric fissure, non-separation of the frontal lobes, rudimentary developed anterior horns and partial fusion of the basal ganglia.

Fig. 7. Absent cavum septum pellucidi, teardrop shaped ventricles and an abnormal underdevelopped pericallosal artery pattern illustrated by color Doppler flow are characteristic for agenesis of the corpus callosum.

Fig. 8. MRI midsagittal view of a Dandy-Walker malformation, showing an enlarged retrocerebellar space (*), cerebellar hypoplasia and upward displacement of the tentorium.

Fig. 9. MRI coronal view of polymicrogyria (encircled in black) in a proven congenital CMV infection. Also note the hypoplastic corpus callosum.

Fig. 10a. Vascular anomalies are uncommon, but the most frequently seen on prenatal ultrasound is the arterio-venous malformations of the vene of Galen. 3D color Doppler imaging allows to create a 3D rotational cast of the A-V malformation.

Fig. 10b. MRI midsagittal view at gestational age 31 weeks (left) and 36 weeks (right) of an aneurism of the Galen vein (arrow head).

Fig. 11a. A suprasellar arachnoid cyst (arrow head) exerts pressure of the circle of Willis and the optic chiasma. Power Doppler imaging shows the extension of the circle of Willis at the base of the cyst.

Fig. 11b. MRI axial (left) and coronal (right) view of a voluminous mixed solid-cystic lesion in the left cerebrum. Note the shift of the midline to the contralateral side. Postoperative biopsy revealed a glioblastoma.

Fig. 12a. Ventriculomegaly, irregular lining of the ventricular walls, and a large intraventricular blood clot (arrow head) are signs of an intraventricular hemorrhage.

Fig. 12b. MRI axial view of supratentorial ventriculomegaly (gestational age 24 weeks) with small blood remnant in the left posterior horn, best depicted on a dedicated sequence (right) and barely visible on the conventional T2 image (left).

Fig. 13. MRI axial view of subcortical tubers in both frontal lobes (encircled in white) in a fetus (gestational age 32 weeks) with multiple cardiac rhabdomyomas. Tentative diagnosis of tuberous sclerosis.