MRI can be an extremely effective modality for making complex central nervous system diagnoses in the fetus after 30 weeks of gestation, according to researchers at the University of Texas Southwestern Medical Center.
"MRI seems most beneficial later in pregnancy for CNS lesions. So far, it is very accurate for weight estimate at later gestations, and it seems to do a good job of evaluating the normal fetal anatomy near term," said Dr. Diane M. Twickler, a professor of radiology and ob/gyn at UT Southwestern.
Twickler and colleagues took blinded retrospective measurements of biparietal diameter, head circumference, and cerebellar width using MRI. They assigned these measurements a gestational age based on median sonographic measurements.
"We have been looking at fetal biometry, weight, and the anatomic fetal survey to try to determine which of the components of the recommended fetal ultrasound examination the MRI can perform," she said.
The researchers retrospectively compared the MRI measurements with sonographic and clinical assessment of gestational age in 22 cases of fetuses with normal central nervous systems. They also compared MRI measurements in 25 fetuses with CNS abnormalities with ultrasound measurements obtained within one week.
MR measurements of gestational age were closely correlated with ultrasound measurements in 22 fetuses with no suspected CNS abnormalities. Out of the 25 fetuses with CNS abnormalities, MRI and ultrasound exhibited a significant correlation between biparietal diameter and head circumference measurements. The results of the study were published in the April issue of the American Journal of Roentgenology.
MRI holds several benefits over ultrasound, according to Twickler, including a resolution not limited by attenuation from interfaces such as bone, an ability to scan in multiple fetal planes, and a global field-of-view.
MRI cost more than an ultrasound exam, however, and is not at the point of real-time imaging, she said.
"I see fetal MRI acting as the second opinion in the setting of some ultrasound findings in the second trimester and maybe even replacing ultrasound in the third trimester," Twickler said.
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