MR may be sounder than sonography for catching fetal abnormalities

Nov 28, 2004

CHICAGO - When ultrasound fails to turn up fetal abnormalities, MRI may hold the answers, according to two presentations Monday at the RSNA meeting. Radiologists from Italy assessed the accuracy of MRI, in comparison to sonography, for detecting fetal malformations. Another group from Brazil did a prospective study comparing the two modalities, as well as the accuracy of MR fetal biometry.

"Ultrasound is the screening modality of choice for fetal imaging … however, there are cases when the sonographic diagnosis is limited," wrote Dr. Riccardo Manfredi's group in their poster. Manfredi and co-authors are from the Policlinico Agostino Gemelli at the Università Cattolica del Sacro in Rome.

The study included 47 pregnant women with 49 fetuses. The mean gestational age was 29 weeks. Inclusion criteria included diagnosed or suspected fetal malformation on ultrasound. The indications for MR included ventriculomegaly in the majority of fetuses (16 of 49), as well as cranial bone abnormalities and Dandy-Walker variant.

MRI was performed on a 1.5-tesla scanner using a phased-array coil. The protocol included a half Fourier RARE sequence along the three orthogonal planes according to the mother's long axis and three orthogonal planes according to the fetal body segment. Postnatal diagnostic imaging, postnatal follow-up, surgery, or autopsy was used to confirm all diagnoses.

According to the results, MRI confirmed ventriculomegaly in eight of 16 cases. It also detected additional problems, such as partial or complete callosal agenesis (4/16), intracranial hemorrhages (2/16), and cerebellar abnormality (1/16). In craniopagus fetuses, the fusion of the temporal lobe and sagittal sinus was well-defined, the authors said. Outside of the brain, MRI confirmed two cases of adenomatoid cystic malformations in the lungs, six cases of renal disease, and one hepatic cyst. MRI failed to turn up one case of Dandy-Walker variant.

The group described one case of cystic lymphangioma of the upper chest in which the ultrasound scan showed a hypoechoic, complex cystic mass lateral to the chest. MR imaging was better able to depict the structure of the lesion and its relationship with the fetal chest because of its larger field-of-view. This information was vital for surgical planning, the researchers said.

During a discussion of the poster, Manfredi said that the MR exams took between 15-20 minutes, depending on how active the fetus was. Manfredi said he'd successfully calmed down a moving fetus by encouraging the mother to relax.

Fetal MR results can go a long way to aid in treatment planning and reassure parents. However, the exam is covered by Italy's national healthcare system, unlike in the U.S., where patients experience pushback from insurance carriers for such expensive tests.

In a second poster presentation, Dr. João Kazan-Tannús, Ph.D., and colleagues shared the results of their prospective study comparing fetal MRI to ultrasound for assessing the face, trunk, and fetal limbs. The group is from the University of São Paolo.

For this research, 66 fetuses were selected based on morphologic ultrasound. MRI was performed on a 1.5-tesla scanner (Signa Horizon LX, GE Healthcare, Waukesha, WI) using FSE, a TR > 30,000 msec, TE = 60-70 msec, and a 5-mm slice thickness. The whole fetus was imagined in three planes. Measurements of the head circumference, biparietal diameter of the skull, and the abdominal circumference were also done. Ultrasound was performed with a 3.5 MHz transducer (HDI 5000, Philips Medical Systems, Bothell, WA).

"For comparison purposes, MRI and ultrasound were performed on all fetuses in sagittal, coronal, and axial planes of the whole body," the group wrote.

The images were evaluated and scored based on the feasibility of axial, coronal, and sagittal images of the skull, thorax, abdomen, and pelvis (1 = feasible). A quantitative score for 25 anatomic details and organs (0 = not seen and 3 = detailed exam) was also assigned to the MR images.

The authors reported superior results with fetal MR, particularly for evaluating the posterior fossa, the eyes, the lateral ventricles, the gyri, and the kidneys. The modality also proved better for looking at white and gray matter differentiation, in the liver, the nose, mouth, hands, and ears. In one case, ultrasound showed polyhydramnios, while MR revealed the same as well as a liver cyst. In another instance, MR showed a giant exophytic lymphangioma in addition to the thoracic mass extending throughout the neck seen on ultrasound.

Ultrasound turned in better results in the heart, choroid plexus, and intestines, as well as in the analysis of the corpus callosum, the authors reported. In one instance, the MR exam yielded a diagnosis of a multicystic right kidney, oligohydramnios, and a clubfoot. Ultrasound added left kidney rotation and multicystic kidney dysplasia on the right.

MR and ultrasound offered similar information in cases of diaphragmatic hernia and polycystic kidneys. "The concomitance of pathologies reduced the diagnostic accuracy, especially in ultrasound," they wrote.

They concluded that MR's capability to produce multiplanar studies gave it the edge over ultrasound. In addition, MRI correlated well with biometry and ultrasound, even adding new data for specific organs. The group said that they have developed a single optimized MR protocol that can be used between 17-30 weeks gestational age.

During the discussion of the poster, Kazan-Tannús said that fetal brain MR exams took about 15 minutes, while whole-body exams took 20-30 minutes. He cautioned that the exam might not progress that quickly if real-time imaging is required to compensate for excessive fetal movement.

Both of these studies build on previous research, making a solid case for fetal MR from head to toe. In a paper at the 2004 American Academy of Pediatrics meeting in October in San Francisco, Dr. Orit Glenn praised MR for allowing "direct visualization of the developing brain parenchyma." She also pointed out that prenatal MR is not susceptible to the same limitations as ultrasound, and has a higher contrast resolution, resulting in better differentiation between normal and abnormal tissue. Glenn is from the University of California, San Francisco.

More recently, Dr. Susan Connolly and colleagues from Massachusetts General Hospital in Boston used MR to assess the imaging features of the femur in fetal pigs. They found that during fetal development, "the cartilaginous epiphysis of the distal femur transformed from an oval to a bicondylar structure."

Other musculoskeletal features that were readily apparent on MR as the fetus grew included perichondrial structures, marrow cavitation, and bone bark. These results may be transferred to human imaging in the near future, making it possible for physicians to make a prenatal diagnose of skeletal abnormalities, such as skeletal dysplasia, clubfeet, or deficiencies, they concluded (Radiology, November 2004, Vol. 233:2, pp. 505-514).

"Improvements in MR imaging -- the most important being fast and very fast sequences with gradient-echo techniques -- have allowed the increased use of this modality for fetal imaging, and have obviated the use of sedation to decrease fetal motion," wrote Dr. Kimberly Applegate in an accompanying editorial. "Future use of fetal MR imaging will likely include refinement of current techniques to allow better investigation of normal and abnormal fetal musculoskeletal MR imaging characteristics" (Radiology, November 2004, Vol. 233:2, pp. 305-306).

By Shalmali Pal
AuntMinnie.com staff writer
November 29, 2004

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