Among the anomalies and conditions best evaluated by fetal MRI are the following:
▸ Ventriculomegaly. Dilatation of the cerebral ventricles is a relatively common finding by prenatal diagnosticians. Although it is usually well visualized with ultrasound, ventriculomegaly may be accompanied by other associated abnormalities that may remain undetected with sonographic evaluation.
When ventriculomegaly is isolated with no other accompanying anatomical defects, the long-term prognosis is excellent. If there are associated abnormalities, however, the prognosis is significantly compromised, with much worse neurodevelopmental outcomes.
Fetal MRI can help identify those additional abnormalities. Studies from Europe and in the United States have documented significant percentages of cases in which apparently isolated ventriculomegaly was identified on the ultrasound, but was then found to be associated with additional anomalies on the follow-up MRI.
Even in cases with borderline ventricular dilatation, subtle but significant developmental abnormalities are frequently overlooked by ultrasound. MRI diagnosis can facilitate better counseling and prognostication regarding outcome, and can aid in the timely development of management strategies.
▸ Other brain anomalies. MRI can be advantageous for precisely visualizing deep structures of the brain, especially as gestational age advances and the skull becomes calcified. Sometimes, MRI enables visualization of deeper structures—such as the optic chiasma, pituitary stalk, and the pituitary—that are not visible on ultrasound.
Fetal MRI is also advantageous for visualizing subtle lesions of the brain, such as parenchymal infarcts and hemorrhage, and other abnormalities of cortical development. Such subtle anomalies can nevertheless be very consequential to long-term neurologic performance.
In our institution, we order an MRI whenever we see an anomaly of the brain. A persistently and significantly small fetal head with normal-appearing sonographic anatomy may, for example, reveal a lissencephaly syndrome on MRI exam. In patients with a significant family history of brain abnormalities, a confirmatory MRI of the fetal brain, despite a normal sonographic appearance, may be justifiable.
▸ Masses in the neck. MRI is thought to be particularly useful in assessing masses of the fetal neck and the potential for airway obstruction. Limitations of tissue differentiation on ultrasound may preclude a determination of the extent of infiltration of a neck mass. The panoramic view and tissue differentiation of the MRI may overcome this limitation.
These qualities are used to good advantage in determining whether a neck mass is infiltrating or obstructing the fetal airway, and whether it has the potential to prevent spontaneous breathing at delivery. Should such a situation be confirmed prenatally, an EXIT (ex utero intrapartum treatment) procedure can be planned. In this procedure, the fetus's head and shoulders are delivered and the placenta is left attached (maintaining umbilical circulation and fetal oxygenation) while a surgical intubation or tracheoscopy procedure is performed.
▸ Diaphragmatic hernia. Congenital diaphragmatic hernia is among the most common congenital thoracic lesions. Herniation of the abdominal viscus and organs into the chest can lead to compression of the lungs and lung hypoplasia at birth, precluding normal respiration. When the liver is also herniated into the chest, the chances of survival are sharply reduced.
Although possible, it can be difficult to determine herniation of the liver into the chest with ultrasound. MRI easily identifies thoracic displacement of the liver and therefore has prognostic value in congenital diaphragmatic hernia.
Limitations, Future Promise
Prenatal MRI does, however, have limitations. Because the technique is based on contrast between water and fat/lipids, it generally does not provide good quality images before about 24 weeks of gestation—a time period in which neurons, for instance, have not yet undergone significant myelination. Ultrasound, in contrast, tends to be quite effective earlier in pregnancy, which is a distinct advantage.
Availability of MRI technology and specific interest and expertise in fetal MRI also are significantly restricted, compared with ultrasound. Furthermore, MRI technology is significantly more costly than ultrasound at this time.
None of these limitations is immutable. All will likely be addressed or at least attenuated with the passage of time.
Just as important will be the development of a team approach to the use of MRI for fetal anomaly detection. Such an approach would involve embracing the expertise of the obstetrician in fetal anatomy and fetal anomalies in general. The interpretation of fetal MRI images should involve not only radiologists and pediatric subspecialists, such as pediatric neurologists, but also fetal medicine specialists working together.
The greatest promise of fetal MRI lies with further advances in so-called functional MRI. This has the potential to provide information not only about structural features of the anatomy, but about the function of various tissues as well. MRI studies could capitalize, for instance, on the fact that tissue that is injured or developmentally abnormal will have differences in metabolism, compared with normal tissue.