MRI Assessment of Fetal Brain Anomalies

Introduction:

Prenatal imaging has come a long way in providing crucial insights into the development and health of a growing fetus. Among these imaging techniques, magnetic resonance Imaging (MRI) has established itself as a potent tool for evaluating fetal brain anomalies. While ultrasound remains the primary imaging modality for routine prenatal care, MRI offers a more detailed and comprehensive evaluation of the fetal brain, particularly in cases where there is suspicion of complex or rare abnormalities. In the realm of fetal surgery, fetal MRI serves as a pivotal tool for identifying ideal candidates for surgical interventions and furnishing vital anatomical information essential for meticulous surgical planning.

Why Consider Fetal Brain MRI?

Ultrasound is the preferred method for routine fetal screening and assessing brain anatomy. Even when MRI is available, it is important to carefully evaluate ultrasound findings, especially when focusing on the fontanelle. In specific situations, particularly when using T2-weighted images exclusively, and when a highly skilled operator performs ultrasound via the transvaginal route, MRI may not offer superior results to sonography.

However, even with skilled operators, ultrasound can sometimes miss abnormalities due to local conditions (maternal obesity, fetal position, reduced amniotic fluid) or technical challenges (near-field reverberation artifact, which hinders proximal ventricle assessment) or because the anomaly is subtle. In such cases, MRI proves beneficial, and prior studies have shown its advantages over ultrasound in select cases. MRI, in particular, excels in assessing certain aspects of brain maturation and myelination, making it a valuable tool for diagnosing various fetal brain abnormalities. However, it is important not to think that MRI can replace ultrasound completely. MRI should only be done after a skilled ultrasound exam to add extra information.

What Are the Indications for Fetal Brain MRI?

The primary indications for fetal brain MRI include:

1. A history of severe brain abnormalities in a previous pregnancy, despite a normal ultrasound examination. MRI is performed to detect subtle signs of recurrence.

2. An isolated abnormality was identified on ultrasound, typically ventriculomegaly or corpus callosum agenesis, prompting MRI to search for potential overlooked abnormalities.

3. An abnormality was diagnosed via ultrasound that could not be fully assessed due to technical issues such as maternal obesity, and fetal positioning. MRI is used to complement the ultrasound findings.

4. High risk of developing a brain abnormality, particularly in cases of fetal infection (commonly cytomegalovirus, varicella, and toxoplasmosis) or ischemic damage (in-utero death of a monochorionic twin, twin-to-twin transfusion syndrome). Some centers routinely offer MRIs in such cases.

What Are the Common Fetal Brain Anomalies Detected by MRI?

The common fetal brain anomalies that can be detected through MRI include:

• Ventriculomegaly: Enlargement of the fluid-filled spaces within the fetal brain. Ventriculomegaly, which is the enlargement of the lateral ventricles, typically defined as an atrial width of 10 mm or more, is frequently noticed during fetal ultrasound scans and often leads to a referral for fetal MRI. While ultrasound is good at detecting ventriculomegaly and other brain anomalies, it can sometimes produce false positives. Fetal MRI is valuable in improving the accuracy of these diagnoses, making it a more specific tool in such cases.

• Agenesis of the Corpus Callosum: Partial or complete absence of the structure connecting the two brain hemispheres.

• Holoprosencephaly: A condition where the fetal brain fails to divide properly into two hemispheres. MRI is recommended to assess the extent of fusion and any related abnormalities.

• Neural Tube Defects: Abnormalities resulting from incomplete closure of the neural tube, leading to conditions like spina bifida.

• Intracranial Hemorrhage: Bleeding within the fetal brain, often due to trauma or vascular issues.

• Dandy-Walker Malformation: A cystic enlargement of the fourth ventricle, typically associated with cerebellar abnormalities.

• Cephaloceles: This condition happens when there is a gap in the skull bone, allowing intracranial structures to extend outward, covered by skin. Among these, meningoencephalocele, which involves the extracranial extension of brain tissue, meninges, and cerebrospinal fluid (CSF), are quite common. MRI has been found to be a valuable addition to ultrasound in assessing the contents of the sac, including any protruding dural venous sinuses, and in detecting associated anomalies, which are found in approximately half of all cases.

• Hydranencephaly: This condition occurs when most or all of the cerebral cortex is missing due to problems with blood flow. However, some parts of the brain, like the posterior fossa structures, thalami, and interhemispheric falx, are still there. This condition can lead to a larger head because the brain keeps making cerebrospinal fluid (CSF). MRI helps doctors tell the difference between hydranencephaly and other brain issues like hydrocephalus, schizencephaly, and holoprosencephaly.

• Schizencephaly: This condition means there's a gap in the brain tissue that goes from the inner lining to the outer covering of the brain (ependymal surface to pia mater). There are two types: open-lipped and closed-lipped. An open-lipped schizencephaly has a gap with fluid in it, while a closed-lipped one has the gap walls touching each other. The gap walls are made of gray matter. MRI helps tell schizencephaly apart from encephaloclastic porencephaly, where the gap walls are made of white matter. MRI is useful for making this distinction.

Beyond the Common Abnormalities -

While ultrasound is effective at detecting common fetal brain anomalies, MRI is particularly valuable when investigating complex or rare conditions. Some examples are listed below:

1. Polymicrogyria: A condition characterized by excessive small, abnormal folds in the brain's surface. MRI can provide detailed information about the extent and location of these abnormalities.

2. Cortical Dysplasia: Abnormal development of the cerebral cortex. MRI can reveal the specific areas affected and their impact on brain function.

3. Chiari Malformations: Structural defects in the cerebellum and brainstem. MRI can help identify the type and severity of Chiari malformation.

4. Heterotopias: Abnormal clusters of neurons in the wrong location within the brain. MRI can visualize these clusters and their impact on brain structure.

5. Tumors: MRI is essential for characterizing fetal brain tumors, including their size, location, and tissue characteristics.

What Are the Advantages of Fetal MRI Over Ultrasound?

MRI offers several advantages over ultrasound for evaluating fetal brain anomalies:

• High Resolution: MRI provides exceptionally detailed images, allowing for a closer examination of brain structures, including gray and white matter, gyri, sulci, and various anatomical regions.

• Multi-Planar Imaging: MRI enables imaging in multiple planes, offering a comprehensive view of the fetal brain from different angles. This is particularly useful for visualizing complex structures and abnormalities.

• Soft Tissue Contrast: MRI excels in differentiating between various types of soft tissues. This is crucial for identifying subtle anomalies, especially in cases where ultrasound may be inconclusive.

• Functional Assessment: Functional MRI (fMRI) can provide insights into brain functionality and connectivity, which can be valuable for assessing the severity and potential outcomes of certain anomalies.

Conclusion:

Fetal MRI is a valuable and widely accessible tool that plays a crucial role in narrowing down potential causes and, in many instances, confirming specific diagnoses in cases of suspected fetal brain abnormalities. This advancement has significantly enhanced the accuracy of counseling sessions, empowering expecting parents to make informed decisions during the prenatal period and prepare for what lies ahead after birth. Moreover, fetal MRI contributes to planning the delivery process by providing critical insights that emphasize the necessity for neonatal interventions and specialized care. This ensures that the most effective treatment options can be promptly available at the time of delivery. The role of fetal MRI is an ever-evolving one in the fields of maternal-fetal medicine, neurology, neurosurgery, and neonatology. It holds the potential to contribute to better outcomes for patients in these areas in the future.