Heron Werner, MD & Pedro Daltro, MD
Clínica de Diagnóstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) – FIOCRUZRio de Janeiro – Brazil
Dorothy I. Bulas M.D.
Professor of Radiology and Pediatrics
Children"s National Medical Center
George Washington University Medical Center
111 Michigan Ave, NW, Washington D.C. 20010
The first study using MRI in gestation was done by Smith in 1983. Later, its use grew progressively in obstetrics and fetal medicine, especially in the evaluation of the central nervous system (CNS) of the fetus. This is due to fact that the fetal CNS is not easily evaluated by ultrasound because of the ossification of the calvaria in the third trimester, besides frequent inappropriate fetal position (Smith et al., 1983; Benson et al., 1991).
Although ultrasound continues to be the most used modality of prenatal exam routine because of its low cost, more equipment availability, safety, high sensibility and good real time capacity of analysis, MRI is potentially good at the morphological evaluation of those fetuses that are not easily studied by ultrasound (Williamson et al. 1989). Vadeyar et al. 2000 registered fetal heart beats before and during MRI exam in 10 fetuses at gestational ages between 37 to 41 weeks, demonstrating no significant alterations.
Safety committees have devised guidelines which say that pregnant employees of MRI centers can go into the exam room without any risk. Nevertheless, they must avoid the exam room during the use of a radiofrequency fields. Some centers still do not recommend these pregnant employees to stay in the exam room during their first trimester of pregnancy. In the USA, they recommend their pregnant employees to be well informed about MRI. Then, they will be able to take their own decision about staying or not in the unit (Brunelle et al., 1998).
Despite the fact that ultrasound has been reaching great progress with the improvement of probes and more recently, with 3D images, such a technology can have its image quality degraded, especially during the third trimester of gestation. The main inherent problem of ultrasound is, according to Poutamo et al. 1999:
Acoustic skull cap shadow, which prejudices mainly the study of the posterior fossa.
Presence of the cephalic pole in the maternal pelvis, especially in the transverse insinuations, prejudicing the sagittal evaluation of the cephalic pole. In some of these cases, the problem can be solved by the transvaginal ultrasound probe.
Distance enlargement between the ultrasound probe and the cerebral structures in the cases of an important hydrocephaly.
Intracranial hyperechoic images, such as tumors and hemorrhage which can prejudice the rest of the cerebral anatomy evaluation.
Acoustic shadow presence from the jaw and the base of the cranium which hinders a good evaluation of the cervical region.
Slight difference of echogenicity between tissues such as the esophagus and the trachea.
Strong polyhydramnios or oligohydramnios.
Poutamo et al. 1999 studied 27 fetuses at gestational ages over 20 weeks by ultrasound and MRI. MRI added additional information in 9 (47%) out of 19 cases with CNS anomalies and in 6 (75%) out of 8 cases of cervical anomalies. In 8 cases, the posterior fossa anatomy could not be well evaluated by ultrasound. In the 8 suspected cases of cervical anomaly, the polyhydramnios and the extreme deflexion of the fetal cephalic pole were the main limitation to a good echographic study. In all 8 cases, detailed images at the pharynx and esophagus were not satisfactory by ultrasound. So, MRI provided a better visualization of the posterior fossa, eliminating possible artifacts caused by acoustic shadows, facilitating the diagnosis of the cerebellar hypoplasia and hemorrhagic lesions alongside a good contrast between the trachea, esophagus and soft tissues of the cervical region.
Most recommendations for fetal MRI are related to CNS pathologies. The evaluation of the various stages of fetal cerebral formation proved to be poor by ultrasound. At present, the use of fetal MRI enables us to study the fetal CNS with good sensibility at the end of the second and third trimesters.
The embryogenesis of the brain is already substantially completed at about 20 weeks of gestation. In the first weeks of fetal development, the cerebral hemispheres surface is smooth, because the gyration depends on the conclusion of the neuronal migration, which will be completed near the term of pregnancy. The MRI study of sulcation requires knowledge of the patterns of maturation of sulci and gyri according to fetal age. The MRI results can be compared to the anatomy atlases (Figure 1).