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 fetal liver is the most readily visible abdominal organ by MRI, which in part relates to its size and to its position below the high signal intensity of the fetal lungs. It possesses a homogeneous intermediate intensity signal on T2-weighted. The hepatocyte chemical composition varies with gestational age. This is due to glycogen augmentation which occurs at the end of pregnancy. So, the signals emitted by the liver will significantly change within the 24th and 40th week of gestation. The portal and hepatic vein can also be visible at the end of the pregnancy. The gallbladder is detected as a bright and cystic structure on T2-weighted. The structures of the upper digestive apparatus can be viewed by MRI because of the ingested amniotic fluid (Shinmoto 2000). The signal intensity of proximal small bowel is different from that of distal small bowel and colon. The bowel loops are identified as serpiginous structures in high signal on T2 and low signal in T1. Garden et al. 1991 showed the sigmoid and transverse column with variable signals on T1 and T2. This signal variation is probably due to the presence or not of meconium. The spleen is frequently depicted with a signal intensity similar to that of the liver (Figure 55 - 70).

The kidneys are easily evaluated by MRI, but their visualization is more difficult before 20th week of gestation due to their small size and low contrast with retroperitoneal fat. The bladder is easily visualized by MRI due to its degree of repletion. The scrotum and penis are often recognized in the male fetus, but the female genital organs are hardly detected (Shinomoto et al. 2000) (Figure 71-84).

The musculoskeletal system can be well detailed in the third trimester. The upper and lower extremities can be easier identified. Obviously, the quality of image will be depended by the degree of movement occurring during the scan (Figure 85-91).

MRI has been used also in the evaluation of the placenta, especially in the cases of placenta previa and placental masses such as chorioangioma (Figure 94). The maternal sagittal view offered by MRI shows with a good definition the cervix and the placental margin. The placenta has a high signal on T2, which we can see easily the myometrial-placental interface (Figure 95). The MRI can also help the ultrasound in the evaluation of the placenta accreta, increta and percreta, which is very important to define the intrapartum morbidity and mortality (Figure 96, 97, 99, 100). The umbilical vessels and the insertion into the placenta are well demonstrated in MRI. The amniotic fluid has a low signal on T1-weighted and a high signal on T2-weighted images (Figure 92, 93, 98).
Accurate fetal imaging is an essential part of maternal medicine. MRI of the fetus has been developed over the last 20 years and fulfills the criteria for imaging the fetus such as security, noninvasive and capacity to provide a good tissue definition. The decision to perform MRI is taken by a multidisciplinary team. The mother must be aware of the advantages and limitations of this imaging modality.

Table 1: Chronology of the cerebral development

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