5 Perinatal Imaging


5 Perinatal Imaging

5.1 Introduction

Perinatal imaging includes both fetal and neonatal imaging for both congenital defects and acquired abnormalities. Perinatal imaging utilizes ultrasonography to a greater extent than do other applications of neuroradiology, taking advantage of the thin skull and sonographic windows (in particular the anterior fontanelle) of fetuses and neonates, the absence of ionizing radiation in ultrasonography, and the ability to perform the study without sedation and at the patient’s bedside if needed. Because clinical examinations in this age group can be challenging, the appropriate interpretation of imaging studies can significantly aid in the care of these young patients.

5.2 Fetal Imaging

Fetal evaluation of abnormalities of the central nervous system (CNS) is most commonly done to evaluate areas of uncertainty on screening examinations done with ultrasonography at various stages of pregnancy. After an abnormality is identified or suspected on a screening examination, a more detailed fetal sonographic evaluation can be performed to characterize the abnormality. When there is need for further clarification of a suspected finding and/or associated abnormalities, fetal magnetic resonance imaging (MRI) can be performed. Fetal MRI is typically performed at 1.5 teslas or a lower field strength. Recent investigations have begun to use a field strength of 3 teslas for fetal MRI, but the safety and benefits of this have not yet been established.

One of the most common indications for more detailed fetal ultrasonography is for ventriculomegaly, which can be associated with a variety of congenital and acquired abnormalities. Fetal ventriculomegaly is typically defined as a transverse dimension of the atrium of the lateral ventricle that exceeds 10 mm. A finding of ventriculomegaly should prompt a detailed survey of the entire CNS 1 ; however, it is occasionally an isolated finding. When ventriculomegaly is present, a follow-up examination should be done to evaluate for its progression, which may result in the need for postnatal shunting (see Chapter 11).

Hydrocephalus can be seen in the setting of germinal matrix hemorrhage (GMH) in utero, which is typically the result of severe maternal stressors, such as an automobile accident or exposure to cocaine (Fig. 5.1). Ventriculomegaly can be related to malformations of the posterior fossa, including a Chiari type II malformation (Fig. 5.2) or a malformation within the Dandy–Walker spectrum (Fig. 5.3), or to supratentorial abnormalities, including agenesis of the corpus callosum (Fig. 5.4). Congenital malformations of the CNS are further discussed in Chapters 3 and 4.

Fig. 5.1 Fetal germinal matrix hemorrhage (GMH). (a) A fetal abdominal ultrasonographic examination performed at approximately 36 weeks, gestational age shows ventriculomegaly of both lateral ventricles and the third ventricle, with echogenic material in the atria of the lateral ventricles. (b,c) Axial single-shot T2 W images and (d) coronal T2 W image show hypointense material in the body of the left lateral ventricle (b,d), extending into the atrium of the left lateral ventricle (c).
Fig. 5.2 Fetal Chiari type II malformation with myelomeningocele. (a) Ultrasonographic examination of the fetal head in the axial plane obtained at approximately 19 weeks of gestation shows inward bowing of the frontal calvaria (green arrowheads), resulting in an appearance described as resembling that of a lemon. There is also an arclike configuration of the cerebellum, described as resembling a banana (green arrows). (b) Fetal ultrasonographic examination of the lower lumbar region in the axial plane demonstrates splaying of the posterior elements, with a cystic posterior protrusion representing a myelomeningocele. Neural elements are seen extending to a posterior echogenic structure, the neural placode. (c) Fetal sagittal MRI scan with fast imaging employing steady-state acquisition (FIESTA) shows crowding of the posterior fossa with absence of a cisterna magna (green arrow), as well as a cystic-appearing myelomeningocele (green arrowheads).
Fig. 5.3 Fetal Dandy–Walker malformation. (a) Axial–oblique fetal ultrasonographic examination shows splaying of the cerebellar hemispheres with communication between the fourth ventricle and cystically dilated posterior fossa without intervening vermis. (b) Axial single-shot T2 W image from a fetal MRI scan confirms the Dandy–Walker malformation and ventriculomegaly.
Fig. 5.4 Fetal agenesis of the corpus callosum. (a) Axial fetal ultrasonographic image of the head shows enlargement of the atria of both lateral ventricles. (b) Sagittal Doppler ultrasonographic image shows a low position of the distal branches of the anterior cerebral artery. (c) Axial single-shot T2 W magnetic resonance image of the brain shows parallel lateral ventricles and preferential enlargement of the atria (colpocephaly). (d) Coronal single-shot T2 W magnetic resonance image shows a vertically oriented third ventricle in communication with the interhemispheric fissure without intervening corpus callosum, and a “longhorn” appearance of the third ventricle and lateral ventricles.

A Chiari type II malformation is nearly always associated with a lumbosacral myelomeningocele (Fig. 5.2), although this can be difficult to see if the back of the fetus is abutting the placenta or the amniotic sac/uterus. A Chiari type II malformation is associated with effacement of the cisterna magna on an axial view of the posterior cranial fossa, resulting in a finding referred to as a “banana sign.” Owing to the low intracranial pressures in fetuses with a Chiari type II malformation, there is slight inward bowing of the frontolateral aspect of the calvarium, resulting in a finding referred to as a “lemon sign.” Cerebellar ectopia through an enlarged foramen magnum cannot always be seen with ultrasonography; it may be better revealed by magnetic resonance imaging (MRI).

Although a Chiari type malformation is marked by a small posterior fossa, the Dandy–Walker spectrum of malformations results in cystic enlargement of the posterior cranial fossa. The extent of vermian hypoplasia in such a malformation can be difficult to determine with ultrasonography, and MRI performs better for this purpose.

Ventriculomegaly can also be seen with congenital supratentorial malformations, and particularly with agenesis of the corpus callosum (ACC), in which there is preferential enlargement of the atria and occipital horns of the lateral ventricles secondary to a loss of parieto-occipital white matter volume (Fig. 5.4), a condition known as colpocephaly. In ACC, a midsagittal view can show absence of the corpus callosum and a radiating gyral pattern, and a coronal view can show a typical high-riding third ventricle. Agenesis of the corpus callosum is commonly associated with an interhemispheric cyst (“cystic meningeal dysplasia”). 2 When accompanied by a cyst in a female fetus, ACC raises the possibility of Aicardi syndrome, in which there are also abnormalities of the eye.s. Literatur

An additional indication for more detailed fetal CNS imaging arises from failure to visualize the septi pellucidi in a screening examination. During development there is typically a cavum septum pellucidum. Absence of the septi pellucidi can be seen in the constellation of findings known as septo-optic dysplasia (SOD), in which the optic nerves are hypoplastic. Both endocrine abnormalities and pituitary malformation, in particular ectopic neurohypophysis, and possibly a schizencephalic cleft, may occur in SOD. Hypoplasia of the optic nerve and an ectopic neurohypophysis can be difficult to confirm on prenatal imaging, and postnatal ophthalmic examination and endocrine testing may be indicated for their detection, as may also be postnatal MRI. At times, absence of the septum pellucidum can be an isolated finding without presumable pathologic consequences, but this must be considered a diagnosis of exclusion.

Septo-optic dysplasia is considered to possibly represent the mildest form of a disorder within the holoprosencephaly spectrum. Although the absence of a septum pellucidum can occasionally be an isolated finding, it should prompt an investigation for other features of disorders within this spectrum of holoprosencephalic disorders

The identification of a more profound parenchymal abnormality makes it important to fully characterize the other (extracranial) findings in the affected fetus, which have prognostic implications for the fetus and possibly genetic implications for any future children. The more severe disorders in the holoprosencephaly spectrum can have an appearance that is confusing to one who is not familiar with the abnormality (refer to Fig. 3.13, Fig. 3.14, Fig. 3.15, and Fig. 3.16). 4 The more severe disorders within the spectrum (e.g. , alobar holoprosencephaly) tend to have poor postnatal prognoses.

A defect in closure of the rostral neural tube can result in an open cranial vault and exposure of the developing tissue of the patient’s central nervous system (CNS) to amniotic fluid, resulting in injury to the tissue and failure of formation of the brain (Fig. 5.5). This condition is known as anencephaly (literally “no brain”).

Fig. 5.5 Fetal anencephaly. Sagittal ultrasonographic image of the fetal head shows the eyes (double green arrowhead) and chin/face (green arrowheads). There is absence of a cranial vault above the level of the eyes (red arrow).

In another condition, known as hydranencephaly (literally “water in place of the brain”) there is in utero occlusion of both internal carotid arteries, resulting in necrosis of nearly the entirety of the supratentorial parenchyma, other than the thalami and possibly the inferior occipital lobes, which may be supplied by the posterior circulation. The brainstem and cerebellum are typically normal. Because the cerebral hemispheres will have originally formed and cleaved, there will be a normal falx cerebri (Fig. 5.6); however, the post natal prognosis is poor. The presence of the falx cerebri can differentiate hydranencephaly from alobar holoprosencephaly.

Fig. 5.6 Hydranencephaly. (a) Axial computed tomographic image of the head of a 4-month-old male shows nearly complete absence of the supratentorial parenchyma, except for the thalami (arrowhead) and occipital lobes (arrows), both of which areas are supplied by the posterior circulation. The normally separated thalami, as well as the presence of the falx cerebri (short arrow), indicate that the patient’s disorder does not belong to the holoprosencephaly spectrum. (b) Axial computed tomographic image of the posterior fossa shows a relatively normal appearance of the brainstem and cerebellum, which are supplied by the posterior circulation.

A defect in the calvarium can result in protrusion of the meninges and CSF (meningocele) or brain parenchyma (encephalocele) (Fig. 5.7). An encephalocele can be associated with a phenotype resembling a Chiari type II malformation, known as a Chiari type III malformation. An occipital encephalocele in the setting of renal anomalies and polydactyly can be seen in Meckel–Gruber syndrome.

Fig. 5.7 Encephalocele. Sagittal T2 W magnetic resonance image of the head of a 7-month-old boy shows brain parenchyma protruding through a defect in the occipital bone, with surrounding meninges and CSF.

A large central CSF space without a finding of significant parenchyma can be seen in the setting of severe hydrocephalus, which is important to differentiate from other causes because it has the potential for nearly complete normalization after shunting. The thin rim of peripheralized parenchyma may be difficult to see on ultrasound, which is why fetal MRI can help differentiate devastating conditions like alobar holoprosencephaly and hydranencephaly from a potentially treatable severe hydrocephaly.

It is important to note that the normal sulcation pattern of the brain occurs predominantly in the second half of gestation, and it can be very easy to incorrectly diagnose lissencephaly prior to term gestation. This is important to remember for prenatal imaging, as well as for postnatal imaging in very premature infants.

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May 28, 2020 | Posted by in NEUROLOGICAL IMAGING | Comments Off on 5 Perinatal Imaging

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