MR provides safe and effective fetal imaging with appropriate use

May 11, 2005

Although ultrasound remains the method of choice for fetal screening and follow-up, MRI is emerging as a valuable tool in certain cases of potential congenital abnormality. Used safely and in appropriate circumstances by trained radiology staff, fetal MRI can add weight to critical diagnostic decisions, according to speakers at the European Congress of Radiology in March.

Although ultrasound remains the method of choice for fetal screening and follow-up, MRI is emerging as a valuable tool in certain cases of potential congenital abnormality. Used safely and in appropriate circumstances by trained radiology staff, fetal MRI can add weight to critical diagnostic decisions, according to speakers at the European Congress of Radiology in March.

Radiologists in most European countries are generally not involved in fetal diagnostics, which is done by obstetricians, said Dr. Isabella Bjorkman-Burtscher of Lund University Hospital, Sweden. But radiologists, because of their familiarity with conventional MRI, should control the specialized field of fetal MRI.

"It is our duty to know what it is possible to do with MRI in pregnancy. It is clear that pressure from clinicians to carry out these scans is going to increase dramatically in the near future. It is an extremely useful technique, and if you have to tell clinicians that this is not a service your department can provide, they are going to be very disappointed," Bjorkman-Burtscher said.

As MR technology shifts toward higher field imaging, little evidence exists that 3T fields affect fetal development, said Dr. Penny Gowland. Only one study to date has indicated a plausible mechanism for interaction between a magnetic field and biological tissue. That investigation involved frog embryos placed within a 16T static field.

"But we're not likely to be scanning at 16T in the near future, so this isn't likely to be a problem. And there isn't any evidence that for fetuses, rather than embryos, there is any risk," said Gowland, a reader in experimental physics at the Sir Peter Mansfield MR Centre, University of Nottingham, U.K.

Nonetheless, radiologists should resist switching from 1T or 1.5T to 3T for fetal studies, she said. The key issue is the associated effects of scanning at 3T, not the static field strength itself. Fetal MRI often involves rapid imaging sequences that induce a significant radiofrequency heating effect. The higher the field strength, the higher the specific absorption rate (SAR) of heat energy. Practical measures such as good ventilation within the scanner bore can prevent the mother from overheating, but if the fetal temperature rises, the heat will take far longer to dissipate. More research is needed to evaluate temperature changes and heat loss mechanisms within the womb.

"Although the SAR limit is 4 W/kg, a 1.5W/kg sequence applied for 15 minutes will cause a 0.5 degrees C rise in temperature in an adult. So you need to distribute high SAR sequences in between low SAR sequences to try to reduce the heating load to the subject," Gowland said.

Acoustic noise, another concern often associated with high-field MRI, is not a major issue in fetal imaging. Absorption in the mother's abdomen and fluid within the fetal ear reduce transmission of sound to the fetus. Use of acoustic foam on the scanner table also helps diminish transmission of sound.

The most significant side effect of fetal MRI can actually occur at any field strength. Supine scanning of a pregnant woman in the third trimester may result in aortocaval compression of the major arteries by the fetus, Gowland said. This compression can cause dizziness in the mother and may reduce uterine-placental blood flow.

Used carefully, however, MRI is a safe and effective method of fetal imaging. The question for radiologists is when such high-end imaging is needed.

"Certainly, at the moment, with the rapid advances in both 3D ultrasound and fetal MRI, the jury is out on which modality is best for different applications," Gowland said.

MR first proved valuable in fetal diagnosis for imaging fetal central nervous system defects. It is particularly helpful in evaluating possible brain malformations, as scan results are independent of fetal head position, maternal wall thickness, and the gestation period, said Dr. Catherine Garel, a pediatric radiologist at Hopital Robert Debre in Paris. Ultrasound evaluations of the fetal brain can be problematic if the mother-to-be is obese or the examination is performed toward the end of pregnancy. Visualizing certain midline structures, such as the corpus callosum, can be challenging in these cases.

Some midline structures, including the chiasma and optic nerves, olfactory bulbs, and pituitary gland, are always difficult or impossible to see on ultrasound. An MR examination can show these structures or reveal that they are missing. Both modalities clearly identify other midline anatomic features, such as septum pellucidum cyst. Ultrasound alone should be sufficient to diagnose septal agenesis, Garel said.

Ultrasound diagnosis of hemorrhage in the cerebral parenchyma may be problematic if the bleeding site is in a blind spot due to fetal head position, she said. MRI should provide a comprehensive assessment of the entire area. T1- and T2-weighted sequences may reveal signs of an older hemorrhage, which should not be confused with other pathologies.

MRI adds additional information to evaluation of the brain's surface. It can confirm diagnosis of schizencephaly, which is far more difficult to identify on ultrasound. Fetal MRI should also provide sufficient information from the posterior fossa to diagnose brainstem hyperplasia.

"But one must be careful to avoid pitfalls," Garel said. "The line between malformations and lesions acquired

in utero is sometimes blurred. These acquired lesions may sometimes mimic cerebral malformations."

With the availability of ultrafast sequences that overcome problems of movement artifacts, MRI has proved to be extremely versatile in the radiology department at the Medical University of Vienna. While CNS indications still account for about 50% of fetal MR imaging at the university clinic, the technology has been used in investigations of the face, neck, thoracic cavity, and abdominal organs, as well as the maternal tissues of the placenta, umbilical cord, amniotic fluid, and uterine wall, said Prof. Daniela Prayer, a radiologist at the university. The wide spectrum of tissues with specific characteristics on various sequences requires sensitive adjustment of parameters and the use of additional sequences for optimal imaging quality.

Fetal abdominal imaging is no longer based solely on T2-weighted scans, Prayer said. Virtual colonoscopy exploits the T1-weighted hyperintensity of meconium. Postprocessing of T1-weighted sequences results in clear images of the fetal bowels, enabling the diagnosis of malformations of the meconium-filled intestines. Dynamic gradient-echo sequences allow real-time imaging of intestinal peristalsis, which helps define the level of intestinal stenosis or occlusion. In this technique, the ingested amniotic fluid serves as a contrast agent. Similarly, MR can demonstrate impaired swallowing in cases of esophageal stenosis.

The use of diffusion-weighted sequences has provided new insights into the functional status of fetal and extrafetal organs. Normal renal tissue, for example, is characterized by anisotropy and diffusion-weighted hyperintensity on the source images. These signal properties help determine whether kidneys are functional and/or ectopic.