Diagnostic Imaging
August 2003

OVERREAD

Superior resolution boosts MR's role in fetal imaging

Ultrasound's dominance is challenged in a number of areas, including head and CNS imaging

With growing evidence of superior resolution, MRI is making slow but steady headway in the ultrasound-dominated field of fetal imaging.

In the experience of pediatric radiologists at Children's Hospital of Philadelphia, MRI definitely improves the accuracy of diagnosis in fetal abnormalities. The Center for Fetal Diagnosis and Treatment at the hospital now augments standard ultrasound for diagnosis and screening of fetal abnormalities with ultrafast MR imaging. Sequences of choice include fast gradient-echo, half-Fourier single-shot turbo spin-echo, and echo-planar imaging.

Experience with both imaging modalities enables physicians to see when MR has advantages over well-performed ultrasound scans, said Dr. Anne M. Hubbard, a pediatric radiologist at Children's. Hubbard spoke during a postgraduate course at the Society for Pediatric Radiology meeting in San Francisco in May.

"The fetal brain is seen with MRI so much better than with ultrasound," she said.

MRI was also superior for posterior fossa imaging, in which ultrasound can detect abnormalities only if the head is precisely positioned.

Both MRI and ultrasound performed well in spina bifida, one of the most common birth defects, although ultrasound was better able to describe the level of dysraphia present, according to Hubbard. MRI was helpful in surgical planning for sacrococcygeal teratomas.

One of the areas in which ultrasound outperformed MRI was imaging of pulmonary sequestration. Advanced ultrasound techniques such as color Doppler more accurately imaged the aberrant vessels feeding the sequestration, she said.

The hospital's experience corresponds to the results of a study conducted in Leuven, Belgium, that assessed the value of adding fast MRI to ultrasound in prenatal diagnosis. The researchers used T2-weighted HASTE MR to image 40 nonconsecutive fetuses that had initially been diagnosed by ultrasound as having an abnormality.

In the study, published online by European Radiology on April 15, they reported that MRI provided additional information in 21 fetuses and improved both prenatal counseling and pregnancy management in 15 pregnancies.

"With MRI, we are able to produce good-quality images, and 99% of the time we are able to obtain quite beautiful images," Hubbard said.

Researchers at the University of Texas Southwestern Medical Center have been evaluating the effectiveness of MRI in fetal imaging for several years. In a study published in the February 2003 issue of the American Journal of Obstetrics and Gynecology, they reported that MRI's ability to add information to ultrasound findings in the setting of suspected fetal central nervous system lesions is significantly increased with increased gestational age.

"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.

In another study, 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.

Her group 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.

MRI 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.

An MRI exam costs more than an ultrasound exam, however, and cannot offer real-time visualization.

"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.