WASHINGTON -- While ultrasound is still the primary tool for diagnosing fetal central nervous system abnormalities, MRI is an attractive complement when the ultrasound image is unclear or cerebral and spinal abnormalities appear, experts said at the annual meeting of the American Society of Neuroradiology.
"Fetal MRI is an extremely useful adjunct to ultrasound for those of us who are counseling these patients," said Dr Mary E Norton, who is director of the prenatal diagnosis center at the University of California, San Francisco.
Though MRI has many applications during pregnancy, its most important role is to provide more accurate information to parents.
"Our goal is to help the parents make a well-informed decision," said Dr Erin M Simon, a pediatric neuroradiologist at the Children's Hospital of Philadelphia.
Many patients use findings about birth defects to make decisions about whether to terminate a pregnancy, noted Dr Nor ton. "Clearly, it's critically important that the information that we give to them is accurate," she said.
MRI is used primarily to evaluate fetal abnormalities. At the Children's Hospital of Philadelphia and the University of California, San Francisco, Medical Center fetal MRI is performed to confirm CNS abnormalities and to assess abnormalities not detected by ultrasound.
It's also used as an adjunct to invasive fetal therapy and as an alternative to fetal autopsy. MRI works particularly well in identifying ventriculomegaly, lesions within the posterior fossa, callosal anomalies and abnormalities in cerebral myelination, migration, and sulcation, Dr. Norton said.
Fetal spinal MRI is used to characterize primary congenital anomalies, neoplastic processes, and CNS abnormalities that secondarily affect the spine. Dr. Simon said her team has used the technique to identify several types of primary spinal anomalies, including diastematomyelia, segmental spinal dysgenesis, and the myelomeningocele spectrum.
As an adjunct to invasive fetal therapy, MRI is used before surgery to rule out other abnormalities for which no intervention would improve outcomes. After surgery it's used to monitor response to the therapy, Dr. Norton said.
MRI requires no maternal preparation, fetal sedation, or contrast media. Fetal movement can be decreased by having the mother not eat or drink for several hours before the procedure. The imaging can take 45-60 minutes; the amount of time depends on the number of fetuses and their activity level, according to Dr. Norton.
Dr. Simon noted that MRIs usually aren't performed until well into the second trimester. The technique should be used only when there is a suspected abnormality and when any potential risk from the MRI is outweighed by the risks associated with the abnormality and the potential benefit from the information MRI can provide.
Both Dr. Norton and Dr. Simon have used MRI to provide additional information for in utero surgical interventions. MRI is used to make decisions about twin-twin transfusion syndrome.
Intervention options include removing amniotic fluid and laser ablation of the placental anastomoses.
In twin-twin transfusion cases, Dr. Norton said her team has used MRI to rule out ischemic injury when there has been a spontaneous demise of one twin.
Before therapy it is also used when one twin is hydropic to rule out an injury that would predict a poor outcome regardless of intervention.
After intervention, MRI is used to rule out complications from the therapy.
At the Children's Hospital of Philadelphia and the University of San Francisco, San Diego, MRI is used to provide information before fetal myelomeningocele repair surgery. Terminal myelocystoceles are usually associated with other caudal cell mass anomalies and must be distinguished from myelomeningoceles. It is therefore important to look on the fetal MRI for additional lesions, such as an omphalocele, a cloacal bladder exstrophy, an imperforate anus, and nonspecific pelvic and renal abnormalities, according to Dr. Simon.
Colleagues of Dr. Simon at the Children's Hospital of Philadelphia found that the perinatal survival rate for in utero myelomeningocele repair was 93%, on the basis of the initial analysis of short-term clinical outcome data for 43 fetuses. All had thoracic or lumbar lesions that were repaired at less than 26 weeks of gestation.
Reversal of hindbrain herniation was seen in all fetuses, and 63% had better lower-extremity function than one would expect to see on the basis of their spinal deficits. The study will be published in the American Journal of Obstetrics and Gynecology, she said.
A prospective, multicenter trial on the efficacy of in utero myelomeningocele repair is now underway, Dr. Simon said at the meeting.
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