Womb with a view

State-of-the-art MRI imaging facilitates prognosis and pre-delivery planning for improved surgical outcomes

Prenatal ultrasound has been the cornerstone of fetal screening for more than three decades, providing the first indications of fetal anomalies. More recently, advances in MRI imaging during pregnancy provide greater anatomical detail, facilitating the accuracy of prenatal diagnosis and predelivery planning to avoid potentially devastating events during or after birth.¹ Fetal imaging enables the clinical team to proactively prepare for interventions prior to or immediately after birth.

"When we're planning to operate, the MRI provides a roadmap of the placenta and the position of the umbilical cords. This is critical for surgical planning," says pediatric and fetal surgeon Amy J. Wagner, MD, from the Fetal Concerns Center at Children’s Wisconsin. "We also have a better expectation of the health of the baby when the baby is born, so we can assemble the appropriate team to ensure the baby does well."

Not all imaging centers are the same, says pediatric radiologist Kevin P. Boyd, DO. "The need for subspecialty direction for fetal MRI is an absolute must on all levels."

Advantages of Fetal MRI

MRI can effectively image the moving fetus, allowing non-sedated studies in the second and third trimesters. It offers a noninvasive diagnostic examination that does not involve ionizing radiation and has no known associated negative side effects or reported delayed sequels.²

Advantages compared to ultrasound include excellent tissue contrast, a large field of view, no limitations due to overlying bone or obesity (unless the mother cannot fit into the MRI), and multiplanar imaging regardless of fetal lie.

"The imaging experience is not as simple of just putting the mother into a MRI scanner," explains Dr. Boyd. "We have a team of experienced technologists and nurses that cater to the needs of the patient in order to make the process as easy as possible."

For example, before the exam, the team discusses the exam with the patient, letting her know what to expect. She is allowed to eat and drink before the test, and she can bring a support person, as well as music and movies to help her relax during the study.

The scan requires that the mother lie still on her side. It takes 30 to 45 minutes, depending on fetal movement, and sequences are tailored based on the clinical indication. During the exam, the team uses position strategies and focused protocols to answer questions as quickly as possible so as to limit unnecessary table and scan time.

The images are all reviewed and interpreted by subspecialty, board-certified pediatric radiologists. "Our team of radiologists are in direct contact with the referring physicians, providing imaging interpretation and post-processing in a timely fashion to allow the family and team to make decisions as soon as possible," Dr. Boyd says.

The most common indications for fetal MRI at Children’s Wisconsin include brain, spinal and airway anomalies; cervical soft tissue masses, congenital diaphragmatic hernia (CDH), congenital pulmonary airway malformation (CPAM), and twin-twin transfusion syndrome. A full list of MRI indications is shown in Table 1.

Congenital Diaphragmatic Hernia and MRI

Infants with CDH are born with abnormal lung tissue and vasculature in both lungs, putting them at risk for pulmonary hypoplasia and pulmonary hypertension. Thus, an accurate prediction of survival in the prenatal period is important so families can be prepared for the challenges that may arise after pregnancy, including whether the baby should receive extracorporeal membrane oxygenation (ECMO) support. The two most important factors in prognosis are the size of the lung and the position of the liver, both of which can be determined with fetal MRI.

Lung hypoplasia is determined by the lung-to-head ratio, which is the measurement of the cross-sectional area of the contralateral lung in a four-chamber cardiac view divided by the head circumference. The head circumference normalizes the lung measurement for gestational age.³

Fetal lung volume (FLV) is determined based on MRI results, with the total lung area of each slice multiplied by the slice thickness to obtain a volume. The volumes are totaled with observed/expected ratios to allow for changes in gestational age.⁴ There is a clear correlation between neonatal survival and ECMO requirement and prognostic sonographic and MRI parameters.⁵ There is also a clear correlation between MRI-calculated FLV and survival.⁶

Liver position is another MRI-directed measurement that provides a surrogate for the degree of lung hypoplasia. Historically, the liver position is reported as down or up, with the latter carrying a poor prognosis. More recently, the percentage of intrathoracic liver is measured to predict severity.⁵

Fetal MRI Indications

Source: Fundamental and Advanced Fetal Imaging: Ultrasound and MRI, First Edition. Kline-Fath B, Bahado-Singh R, Bulas D, eds. LWW Publishers:Philadelphia, PA. 2015.

Fetal MRI for Surgical Interventions

Dr. Wagner highlights the importance of fetal MRI for surgical planning with a case in which the Children’s Wisconsin fetal surgery team received a referral of a fetus with CPAM. The mass was so large it had completely replaced one lung and was pushing the heart to the side, thus compressing the other lung and affecting the trachea. This, of course, would have impacted the baby’s ability to breathe after birth. Because of the detail provided by the MRI, Dr. Wagner says, "we had the operating room on standby and a team in the delivery room in case we had to open the baby’s chest and remove the mass right there."

An MRI is also critical in ex utero intrapartum treatment (EXIT) procedures, in which babies must be delivered in a controlled fashion on placental support until an airway can be established. "The MRI provides very detailed information about the airway," Dr. Wagner says.

The fetal imaging program is just one part of the multidisciplinary approaches that the Children’s Wisconsin Fetal Concerns Center offers. Dr. Wagner points out that not only do all clinicians have special pediatric and fetal training, but "the collaborative nature and dedication of everyone here is remarkable."

¹ Saleem SN. Fetal MRI: An approach to practice: A review. J Adv Res. 2014 Sep;5(5):507-23.
² Stecco A., Saponaro A., Carriero A. Patient safety issues in magnetic resonance imaging: state of the art. Radiol Med. 2007;112:491–508.
³ Peralta CFA, et al., Assessment of Lung Area in Normal Fetuses at 12-32 weeks. Ultrasound Obstet Gyenecol 2005; 26:718-724
⁴ Rypens F, et al. Fetal Lung Volume: Estimation at MR Imaging – Initial Results.
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⁵ Kilian AK, et al. Congenital Diaphragmatic Hernia: Predictive Value of MRI Relative Lung-to-Head Ratio Compared with MRI Fetal Lung Volume and Sonographic Lung-to-Head Ratio. AJR 2009; 192:153-158
⁶ Victoria T, et al. Use of Magnetic Resonance Imaging in Prenatal Prognosis of the Fetus with Isolated Left Congenital Diaphragmatic Hernia. Prenatal Diagnosis 2012; 32:715-723