Role of Magnetic Resonance Imaging in Obstetric Imaging

Absent Cavum Septi Pellucidi

The CSP is a rectangular, midline, fluid-filled space between the frontal horns of the lateral ventricles. The more posterior extension (CSP et vergae) should not be mistaken for an intracranial cyst. Demonstration of the CSP is required in routine US evaluation because it is a marker for normal midline development and should always be seen between 18 and 37 weeks’ gestation. The differential diagnosis for an absent CSP is extensive ( Box 19.1 ) and warrants a detailed workup.

Dysgenesis (abnormal development) and agenesis (complete absence) of the corpus callosum are important causes of an absent CSP. In dysgenesis, coronal views show a thin, abnormal corpus callosum connecting the cerebral hemispheres. In agenesis, coronal images show an obvious absence of the corpus callosum between the cerebral hemispheres, the frontal horns have a “Texas longhorn” or “trident” shape, and the third ventricle is elevated and contiguous dorsally with the interhemispheric fissure ( Fig 19.2 ). In the sagittal plane, the cingulate gyrus is absent and the remaining gyri are radially oriented in a “sun ray” appearance. A midline lipoma or cyst may be present. Cysts may be large and associated with the AVID complex ( a symmetric v entriculomegaly, i nterhemispheric cyst, and d ysgenesis of the corpus callosum) ( Fig. 19.3 ).

Holoprosencephaly, described as alobar, semilobar, and lobar in decreasing order of severity, is a disorder of midline development with absent CSP and varying degrees of communication of the ventricles across the midline. In syntelencephaly (the middle interhemispheric variant of holoprosencephaly), the CSP is absent and the posterior frontoparietal areas are fused across the midline, but the frontal and occipital horns are separate.

Septo-optic dysplasia (SOD) is a difficult prenatal diagnosis; apparently isolated absence of the CSP may be the only sonographic finding. On MRI, the corpus callosum is present but may be thinned. On the coronal view, the frontal horns have a flat-topped or squared shape and point inferiorly as they drape over the fornices. Fusion of the fornices may also occur (see Fig. 19.2D ). Optic nerve hypoplasia and pituitary insufficiency are common; thus it is important to evaluate the optic nerves, which should be approximately the same thickness as an extraocular muscle. Any fetus with the suspected diagnosis of SOD requires a thorough postnatal ophthalmologic examination and endocrine workup.

Ventriculomegaly

Ventriculomegaly is defined as mild, moderate, or severe based on the diameter of the lateral ventricle (≥10 mm but ≤12 mm, mild; 12–15 mm, moderate; >15 mm, severe). When severe, it may be difficult to differentiate from hydranencephaly, as shadowing and reverberation artifact from the cranium may make it difficult to determine whether there is residual cerebral mantle. MRI is unaffected by the bony skull vault and provides excellent images of both hemispheres. It readily differentiates the lethal hydranencephaly (absence of supratentorial cerebral tissue) from severe hydrocephalus with thinned cerebrum (e.g., aqueductal stenosis), which can be shunted after delivery ( Fig. 19.4 ). Schizencephaly is a disorder of neuronal migration that results in a wedge-shaped, gray matter–lined cleft that extends from the inner table of the skull to the lateral ventricle. The cleft may be very large (giant open-lip defect), which may be confused with ventriculomegaly, or quite subtle (smaller open-lip or closed-lip defects). MRI easily shows the low-signal-intensity gray matter lining the cleft, which confirms schizencephaly. In an area of porencephaly (tissue destruction) the defect is not lined by gray matter.

Mild to moderate ventriculomegaly has a broad differential diagnosis ( Box 19.2 ). The Society for Maternal-Fetal Medicine recommends consideration of MRI in these cases “when this modality and expert radiologic interpretation are available” while acknowledging that “it may be of less value if the patient has had a detailed ultrasound performed by an individual with specific experience and expertise in sonographic imaging of the fetal brain.” A 2019 meta-analysis confirmed a lower rate of CNS anomaly detection exclusively on MRI in fetuses that had undergone dedicated neurosonography but also concluded that early MRI had an excellent diagnostic performance in identifying additional CNS anomalies and that third-trimester MRI was better for cortical, white matter, and hemorrhagic abnormalities. A 2020 international multicenter study supported the practice of MRI assessment in every fetus with a prenatal diagnosis of ventriculomegaly noting that, if ventriculomegaly was isolated on neurosonography, parents could be reassured of the low risk of an associated anomaly. If dedicated neurosonography is not available, referral for fetal MRI at designated facilities may provide critical additional information for appropriate counseling.

While the CSP is absent in some of these entities, ventriculomegaly is often the more obvious finding; agenesis of the corpus callosum is a classic example. An elevated third ventricle or cerebrospinal fluid between the cerebral hemispheres can be mistaken for the CSP, but the teardrop-shaped, dilated atria of the lateral ventricles (colpocephaly) on axial views and the steer horn configuration on coronal views are characteristic findings that point to the correct diagnosis. That said, even fetuses with apparently isolated complete or partial agenesis of the corpus callosum by dedicated neurosonography still benefit from MRI. A systematic review and meta-analysis from 2021 noted that “cortical and posterior fossa anomalies are among the most common anomalies detected exclusively at MRI, thus confirming the crucial role of fetal MRI in determining the prognosis of these fetuses.”

Abnormal cortical development can present with mild ventriculomegaly. Forms of cortical dysplasia include lissencephaly (lack of gyral formation with an abnormally smooth brain), pachygyria (abnormally broad, flattened gyri with a thick cortex), polymicrogyria (multiple small gyri), cobblestone lissencephaly (fine nodular cortical development), and gray matter heterotopia (arrested neural migration anywhere between the germinal matrix and cortex). Gray matter heterotopia can form bands (a layer of gray matter between the ventricles and cortex creating a double cortex appearance) or nodules, which may be subependymal or subcortical. Subependymal tubers (hamartomas) in tuberous sclerosis do not follow gray matter signal intensity, whereas subependymal heterotopic nodules do; therefore MRI can differentiate these conditions. Migrational anomalies are often associated with syndromes (e.g., Miller-Dieker syndrome, Walker-Warburg syndrome) and autosomal recessive inheritance patterns, so making the most definitive diagnosis possible is crucial for patient counseling.

Hypoxic-ischemic injury often presents with mild ventriculomegaly, which becomes progressive with advancing gestation. Associated areas of hemorrhage are easily detected as increased signal intensity on T1WI with corresponding low signal intensity on T2WI ( Fig 19.5 ). More sophisticated techniques such as DWI can demonstrate diffuse ischemic injury earlier than US; this information can be very important in countries where termination is legal at any gestational age. In countries where late pregnancy termination is not allowed, knowledge of severe brain injury or malformation is still useful for delivery planning and proof that the insult did not occur during labor or delivery.

Idiopathic mild ventriculomegaly (10–12 mm) is more common in male fetuses, especially at greater than 20 weeks’ gestation, and is thought to be secondary to a larger calvarial volume. If truly isolated, the outcome is good, but this is a diagnosis of exclusion; subtle abnormalities that may only be seen on MRI must be excluded.

Posterior Fossa Fluid Collection

Detailed, multiplanar US evaluation of the posterior fossa and brainstem is difficult due to shadowing from the skull base. With MRI, the cerebellar hemispheres, vermis, fourth ventricle, midbrain, medulla, and pons are easily evaluated.

A posterior fossa fluid collection presents a diagnostic challenge; the differential is broad ( Box 19.3 ), and arriving at a final diagnosis can be difficult even with postnatal MRI. The first step is to determine whether the posterior fossa is too big (e.g., expanded by a cyst) or the cerebellum is too small (e.g., cerebellar hypoplasia).