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For other articles and previous issues click here. January 24, 2005 Fetal
MRI: Seeing What Ultrasound Doesn’t Ultrasound is and will remain the primary fetal monitoring modality. But MRI is beginning to fill a niche in situations where ultrasound doesn’t provide enough information. Fetal motion once limited MRI’s utility for imaging in the womb. But today’s ultrafast MRI techniques have all but eliminated that problem. New MRI technology enables images to be acquired in less than one second and provide multiplanar views that help physicians more accurately make diagnoses. Such technology has led to increased usage of fetal MRI, which can lead to earlier diagnosis of conditions affecting the baby and has proven useful in planning fetal surgery and designing postnatal treatments. Adjunct to Ultrasound “Ultrasound is still the first screening method in pregnancy and it will continue to be,” says Deborah Levine, MD, associate professor of radiology at Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, where she uses MRI in her research into the fetal central nervous system (CNS) and the soft palate. “But there are selected indications for MRI.” Given its long record of safety, utility, and cost-effectiveness, ultrasound will remain the modality of first choice when screening the fetus. But Levine and others are investigating situations where MRI can provide information sonography cannot. “But they always have to have an ultrasound first,” emphasizes Levine. Researchers also use MRI to add to the available knowledge of fetal abnormalities and develop more effective strategies for dealing with these conditions. Typical scenarios for fetal MRI include after a suspected anomaly was spotted with ultrasound and if the mother is at risk for a condition that can’t be visualized well with sonography. “We use MRI as a problem-solving tool, when ultrasound is saying something to us but we don’t quite know what it is or what it means,” says Damien Grattan-Smith, MD, medical director of radiology for Children’s Health Care in Atlanta, where fetal MRI has been used for the past 18 months. Ultrasound remains far too useful, effective, and widely available, and it also images in real time. “Ultrasound is such a powerful test for obstetrics because it is really like a physical exam since it is in real time and operators can immediately see what is there,” says Donald G. Mitchell, MD, a neuroradiologist and director of the MRI division at Thomas Jefferson University Hospital in Philadelphia. “Also, there’s an infinite adjustability of the angle. The person who is scanning can move the transducer wherever they want, at whatever angle.” At Jefferson, clinicians usually perform a second ultrasound before turning to MRI. “We get a lot of referrals for high-risk pregnancies, and even though the woman already had an ultrasound somewhere, the first thing we do is make sure they had a really good ultrasound,” Mitchell says, “and then we’ll even repeat the ultrasound.” However, Mitchell adds that there are certain things that can’t be done effectively with ultrasound, such as obtaining a picture of the whole fetus or clearly delineating CNS defects. “That [imaging CNS defects] is probably the single biggest thing,” he adds. Blurry ultrasound images or obscured structures can arise due to factors such as fetal position, a mother’s obesity, or the amount of amniotic fluid, Levine notes. One of the main problems with ultrasound is shadows from bone, which can be especially problematic when trying to image the fetal brain and chest. “When imaging the fetal brain, it is hard to get sound waves through the skull because it is ossified. It’s a bone,” she points out. “You can’t visualize the brain parenchyma. Similarly, with the fetal chest, the ribs can cause shadowing or the spine can be in the way, so it can be difficult to see the entire lung. With MRI, you can just see it without a problem. You get direct visualization of the structure in question.” Diagnosing Cleft Lip and Palate The research demonstrated that MRI is an effective adjunct to ultrasound for specific circumstances. More importantly, better detection of cleft lip and palate prenatally will lead to screening for the associated syndromes and chromosome abnormalities. In addition, it helps determine surgery and can prepare parents for their child’s appearance. In the study report, which appeared in the July 2004 issue of the American Journal of Roentgenology, the researchers stated that reported detection rates of ultrasound for cleft lip and palate fell within a 16% to 93% range. Obviously, such a range is far too broad to be useful. In addition, they pointed out that MRI was less dependent on the factors that affect an ultrasound exam (eg, fetal position, amniotic fluid). Advantages and Safety MR’s imaging capabilities can help physicians plan for delivery and manage airways during delivery. The clear images of head or neck masses provide information about how these relate to nearby structures and indicates whether airways are compressed. The additional information about abnormalities can be helpful in planning for in-utero interventions and improving outcomes. In addition, MRI can be a method used to monitor response to therapy. Since ionizing radiation is involved, MRI is a valuable tool for diagnosing pregnant women. Studies have yet to report any adverse effects to the mother or fetus. Further, risk for any delayed sequelae is extremely small or nonexistent. “With MRI, there are some theoretical risks, but I think the MRI is something that is worth any theoretical risk,” says Smith. “The risk-to-benefit ratio certainly outweighs any risk.” In fact, MRI can be safely performed throughout the pregnancy. However, most clinicians avoid using it during the first trimester, as a developing embryo is more susceptible to damage by outside forces. In addition, use of gadolinium as a contrast agent is not recommended because it can cross the placenta. Procedure and Technology The type of imaging, she says, depends on what clinicians are looking for, but it essentially involves scanning the fetus on three planes: sagittal, axial, and coronal. “We make sure we’ve lined up the imaging to how the fetus is lying and not to the maternal anatomy,” Levine says. “After we get the images, we inform the patients about what we see.” No maternal preparation or fetal sedation is required. Sometimes mothers may fast for several hours before a procedure to reduce fetal movement. However, with the newer, ultrafast MRI technology, movement rarely affects a scan. Today, clinicians using fetal MRI work with single-shot
fast-spin echo techniques, which have nearly eliminated problems
of fetal movement, even though fetal motion generally occurs throughout
the examination. Standard MRI sequences used by clinicians at various
centers include the following: “With the imaging techniques that we use, we get a single slice at a time,” says Levine about the HASTE technique she and her colleagues employ. “Each slice is achieved in less than a second, so you don’t need to give any maternal or fetal sedation. The slices provide really good contrast of the fetal anatomy, so you can see the structures of the fetus particularly well.” In addition to the soft palate, Levine and colleagues use MRI to visualize fetal lungs and CNS, structures particularly difficult to image with ultrasound. CNS Research The purpose of this, she explains, is to determine whether the additional information provided by MRI—the shape of the ventricles, the cortical development, the volume of the brain parenchyma—can be used to correlate with postnatal outcome. It’s a large-scale study. The first five years of the NIH grant were focused only on the additional information provided by MRI. The researchers have now moved into the second five years, which will involve the postnatal follow-up. “From the first five years, we’ve collected a lot of information about the initial data provided by MRI for the CNS abnormalities,” she relates. “We can see a lot more abnormalities, and we see that the type of information that MRI provides can change patient management and neonatal care.” Other recent research Levine has worked on involves chest abnormalities. “We’ve shown that you can see lungs better with MRI and that you can see thoracic abnormalities differently on MRI than on US [ultrasound],” she reports. However, she points out that MRI does not change management as much as it does for fetal brain abnormalities. Future Directions Ultrafast MRI techniques have all but eliminated the need for such strategies. However, the fetal heart remains a challenge. “The heart can beat twice per second and even more, so it’s hard to capture a crisp image of this fetal organ, even with existing fast MRI techniques,” says Mitchell. “One of the problems with ultrasound is that, by its nature, it is point-of-contact. Therefore, you can’t see the whole fetus. But, since the fetal heart is small, the problem of getting a clear window isn’t that much of a problem. So, for the foreseeable future, ultrasound is still the way to look at the fetal heart. You still get much better detail with ultrasound.” However, Mitchell believes MRI techniques will only get faster and eventually eliminate this remaining motion problem. As far as other new directions, Levine expects functional MRI to be used more frequently in research. Appropriately, she and her colleagues have already started moving in that direction. “We just started a project on placental oxygenation to try to detect growth restriction in utero to try and better diagnose preclampsyia. Information will be coming out soon.” Beyond the technology itself, Levine sees the need for cost analysis studies involving fetal MRI. “Right now, we can see who it has helped and who it hasn’t,” she says. “But now we need to look at how much it costs to get this information.” — Dan Harvey is a freelance writer based in Wilmington, Del. He is a frequent contributor to Radiology Today. |
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