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MRI of Fetal Chest Useful As Adjunct to Ultrasound


 

LAS VEGAS — Magnetic resonance images of the fetal chest can be a clinically useful addition to ultrasound to examine lung masses and identify underdeveloped lungs, Dr. Erika Rubesova said at a symposium on emergency medicine sponsored by Stanford (Calif.) University.

With MRI, “You will have a better characterization of the chest masses and you can perform measurements of the lung,” said Dr. Rubesova, a radiologist at the university.

A fetal MRI provides a greater tissue contrast than ultrasound, and features such as lung volume and signal intensity are easier to see, she noted.

As for the safety of a fetal MRI, the safety committee of the Society for Magnetic Resonance Imaging recommends that the risks and benefits of fetal MRI be assessed on a case-by-case basis and that MR procedures are indicated in pregnant women if other nonionizing imaging techniques are inadequate or if the MRI can provide information that could only be otherwise acquired using radiation technology. However, the Food and Drug Administration states that the safety of MR during pregnancy has not been proved definitively, Dr. Rubesova said.

“The FDA does not require a contraindication to the use of MRI for fetal imaging in device labeling,” Dr. Julia Carey-Corrado, an ob.gyn. at the FDA's Center for Devices and Radiological Health, said in an interview. “But the FDA does recommend that device labeling contain the following statement: 'The safety of magnetic resonance examination has not been completely established for embryos and fetuses,'” she said.

“We view ultrasound as the standard of care for fetal imaging, but MR can be viewed as a reasonable second-line imaging modality if you aren't getting enough information from ultrasound and you are concerned about a complex abnormality,” Dr. Carey-Corrado added.

To perform an MRI of the fetal lung, place the patient in the most comfortable position possible and focus on the fetal lung as best you can to minimize the blurriness associated with fetal movement, Dr. Rubesova said.

Dr. Rubesova usually uses 1.5-T and T2-weighted images. “You should be able to see both of the lungs and the airway,” she said. “And the diaphragm sometimes appears as a dark line above the liver.”

Congenital lung lesions fall into three broad categories: congenital cystic adenomatoid malformations, sequestrations, and bronchogenic cysts.

A congenital cystic adenomatoid malformation (CCAM) usually occurs early in fetal development, and the lesions are categorized based on size. In general, lesions larger than 2 mm are associated with a better prognosis for the infant than smaller lesions, so the ability to measure the lesions based on MRI data is useful for clinicians.

Sequestrations (also known as bronchopulmonary sequestrations) occur when a piece of the developing lung branches off from the main airway (but remains connected to it) and the lung fails to develop normally. Bronchogenic cysts form when a branch of the developing airway separates completely from the main bronchotracheal tree.

Data collected by researchers at Brown University, Providence, R.I., suggest that 1 in 3,000 infants has a congenital lung lesion. These masses compress the developing lung, and they may displace other organs in the chest. Large lung masses may cause fetal heart failure in severe cases because the pressure of the masses causes an abnormal accumulation of fluid around the heart, lungs, or abdomen.

The “horseshoe lung” is a characteristic image that is associated with CCAM, sequestrations, and bronchogenic fistulae. A fetal MRI can show the horseshoe shape of an underdeveloped lung, and the lung masses appear as areas of high signal intensity on a T2-weighted image, Dr. Rubesova noted.

There is no rush to perform fetal lung MRI procedures in cases of large lesions where the prognosis is good and termination of the pregnancy is unlikely, Dr. Rubesova said. The best time to get an accurate fetal MRI of these lesions is late in the third trimester because the fetus has less room to move, so the image is sharper. In these cases, the MRI helps parents and physicians plan for neonatal care that will allow the lungs to develop as completely as possible.

The outcome for most newborns with congenital lung masses is good, although congenital lung hypoplasia accounts for 10%–15% of all neonatal deaths, Dr. Rubesova noted. Sometimes the masses will shrink substantially by the time of birth, and in other cases the lesions can be surgically removed after birth to reduce the risk of recurrent infections such as pneumonia.

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