Fetal MR in the evaluation of pulmonary and digestive system pathology

Incomplete division of the foregut into the ventral respiratory portion and the dorsal digestive portion by the tracheoesophageal septum results in esophageal atresia. The most common condition is atresia with distal tracheoesophageal fistula (90% of cases) [12].

MRI findings are polyhydramnios and absent or very small stomach. However, a small stomach may be a normal finding or may have other causes (deglution disorders, facial defects, cervicofacial tumors, CNS lesions, oligohydramnios, etc.). Occasionally the proximal segment of the esophagus can be seen as a hyperintense pouch in T2-weighted sequences, especially in sagittal images along the midline of the fetal thorax [13, 14] (Fig. 8). Tracheoesophageal fistulas are not normally seen on MRI.

Associated anomalies are present in 50% to 70% of cases, and esophageal atresia is sometimes associated with chromosomopathies, especially Down’s syndrome [12].

Duodenal obstruction can be caused by atresia or stenosis, which might be caused by vascular impairment during gut development, intraluminar diaphragm, annular pancreas, intestinal malrotation with Ladd’s band, or volvulus of the middle portion of the intestine. Duodenal atresia and annular pancreas are often associated [15].

In cases of duodenal atresia or marked duodenal obstruction, MRI shows dilatation of the stomach and the portion of the duodenum proximal to the obstruction as hyperintense structures in T2-weighted sequences (Fig. 9). When obstruction is due to malrotation with Ladd’s band or volvulus, the loops of the small intestine can be seen lying mostly on the right side of the abdomen while the colon is on the left (Fig. 9); the colon is best visualized on T1-weighted sequences. The stomach and duodenum may be normal sized or even small when the fetus also has esophageal atresia. Polyhydramnios is almost always present.

Approximately 50% of cases of duodenal obstruction have associated gastrointestinal, genitourinary, cardiac, skeletal, or chromosomal anomalies; trisomy 21 is often associated [15‐17].

Atresia is probably caused by vascular impairment during gut development [18]. Atresia is most common in the distal ileum, followed by the proximal jejunum; multiple atresia is not uncommon.

MRI detects dilatation of the small intestine proximal to the atresia; the stomach and duodenum may also be dilated. The signal intensity of the contents of the intestine varies in relation to the site of obstruction; the more distal the obstruction is, the lower the signal intensity on T2-weighted images and the higher the signal intensity on T1-weighted images [3] (Fig. 10). The colon may be small in caliber or impossible to identify. The more proximal the lesion is, the more likely the fetus will have polyhydramnios.

Extraintestinal anomalies are less frequently associated than in duodenal atresia, and the most common associations are with other intestinal anomalies that might be related to the cause of the atresia and to atresias in other locations [15, 19]. A higher incidence of cystic fibrosis has been reported in children with intestinal atresia [20].

Small bowel atresia can be very difficult to differentiate from meconium ileus, volvulus, or Hirschsprung’s disease.

This condition is caused by functional obstruction of the distal ileum by very dense meconium. It is the earliest manifestation of cystic fibrosis. Nearly all newborns with meconium ileus have cystic fibrosis, and 10% to 15% of all patients with cystic fibrosis have meconium ileus [21].

The MRI findings include dilatation of small bowel loops secondary to meconium impaction, microcolon, and polyhydramnios. Ascites may be present when the condition is complicated by intestinal perforation.

Meconium ileus is often associated to gastrointestinal anomalies like volvulus, jejunoileal atresia, intestinal perforation, and meconium peritonitis.

The differential diagnosis should include jejunoileal atresia, volvulus, and Hirschsprung’s disease.

This condition results from intestinal perforation during fetal life. The release of meconium and digestive enzymes into the peritoneal cavity causes chemical peritonitis resulting in an inflammatory reaction and the formation of fibrous tissue that may calcify. Sometimes the inflammatory response seals the perforation spontaneously.

In most cases, meconium peritonitis is associated to meconium ileus, intestinal atresia, or intestinal volvulus. Intestinal ischemia due to occlusion or mesenteric thrombosis, which may be idiopathic or secondary to intrauterine infection, can also cause intestinal perforation and consequent meconium peritonitis. As meconium ileus is a frequent cause of meconium peritonitis, cystic fibrosis is common in these fetuses.

Ascites and dilatation of the small intestine can be detected with MRI. This is best done using T2-weighted sequences (Fig. 11). Peritoneal calcifications are difficult to appreciate on MRI, as they tend to be small and linear. When seen, they are hypointense in both T1- and T2-weighted sequences. A meconium pseudocyst may result from a contained perforation [22]. Polyhydramnios may also be present.

Fetal MRI allows assessment of the rectum and its contents, and can also provide additional information that can be useful for the diagnosis of anal atresia and other anomalies in the spectrum of cloacal malformations.

Atresia of the colon and anorectal atresia are uncommon and, as in other atresias, vascular impairment seems the most likely etiology. Intestinal loops above the atresia may be dilated, and meconium accumulated above the atresia may cause high signal intensity on T1-weighted sequences (Fig. 12) [22]. The differential diagnosis should include other causes of intestinal dilatation, atresia of the colon, and Hirschsprung’s disease.

Cloacal malformations represent a spectrum of developmental defects that usually affect female fetuses. In these malformations, the urinary tract, the vagina, and the rectum converge above the level of the perineum, creating a common channel with a single external opening. It should be suspected in cases of dilation and high position of the distal bowel and abnormalities in the genitourinary system [23].

The heterotaxy syndromes are a group of rare anomalies characterized by an abnormal arrangement of thoracic and abdominal organs. Abdominal anomalies are very common in this syndrome (multiple spleens, absent spleen, central or left-sided liver, small stomach, central or right-sided stomach, esophageal atresia, duodenal atresia, biliary atresia, intestinal malrotation); heart defects and abnormal bronchial morphology are also common [24].

MRI findings depend on the type of anomaly and on associated anomalies. The easiest to detect is the size and position of the stomach; the position of the liver and heart are also easy to determine, although possible associated heart defects cannot usually be evaluated by MRI. Depending on gestational age, thoracic and abdominal systemic vessel anomalies can also be detected [13]. Asplenia and polysplenia can be difficult to evaluate with MRI (Figs. 13 and 14).

Duplications of the digestive tube are uncommon congenital abnormalities found anywhere along the alimentary tract from the tongue to the anus. Intestinal duplication takes place on the mesenteric side of the intestine and does not usually communicate with the intestinal lumen. The most common site of duplication is the distal portion of the ileum, followed by the distal portion of the esophagus and stomach.

Duplication cysts rarely cause intrauterine intestinal occlusion; however, they may cause intestinal occlusion or abdominal pain after birth, due to volvulus, invagination, or bleeding of the cyst.

On MRI, they are seen as rounded intraabdominal cystic masses adjacent to the bowel. They are hyperintense on T2-weighted sequences and hypointense on T1-weighted sequences [22, 25] (Fig. 15). The rest of the intestinal loops tend to be normal in shape, and polyhydramnios is not usually associated.

T1-weighted sequences can be useful for differentiating these anomalies from dilated intestinal loops. They can be difficult to differentiate prenatally from other abdominal cysts like mesenteric cyst, choledochal cyst, and specially ovarian cysts. Ovarian cysts are common during the prenatal period; they are usually detected in the third trimester of pregnancy [26]. Most are unilateral although they can be bilateral. Overstimulation of the ovaries by placental and maternal hormones seems to play an important role in their formation. In the fetal period, they may be simple cysts or they can become complicated with torsion and bleeding. On MRI, simple cysts are seen as an intraabdominal cystic structure that is hyperintense on T2-weighted sequences and hypointense on T1-weighted sequences. When complicated by torsion and/or bleeding, the signal intensity on T2-weighted sequences is lower, and fluid-fluid levels can be detected within the cyst due to hemorrhage or detritus (Fig. 16). Hypointense images in the wall of the cyst are occasionally seen in T2-weighted sequences due to dystrophic calcifications associated with infarction. Approximately 50% of ovarian cysts disappear during gestation or after birth [26].

Prenatal masses in the liver and spleen are rare; they are usually cysts. In the liver, they may appear within the parenchyma or “hang” from the lower edge. They may be single or multiple; multiple cysts usually occur in autosomal recessive polycystic kidney disease. Single cysts usually correspond to a ciliated foregut cyst or epidermoid cyst. Splenic cysts are almost always epidermoid cysts.

MRI shows rounded structures within the liver or spleen that are hyperintense on T2-weighted sequences, unless they contain blood, in which case fluid-fluid levels can be detected in their interior (Fig. 17).

Neuroblastoma and teratoma are the most common solid neoplasms in infants. More than 90% of prenatally diagnosed neuroblastomas are adrenal in origin, although they may also be located in the thorax or cervical region. About half of these tumors are cystic or heterogeneous [27]. With MRI, solid (Fig. 18), cystic, or more complex lesions can be seen [28] below the diaphragm. Unlike extralobar bronchopulmonary sequestration they are normally in the right side and are almost always detected in the third trimester [10]. These tumors can disappear before birth though they can also metastasize, especially to the fetal liver [27].

Another neonatal tumor is sacrococcygeal teratoma, which appears as cystic, solid, or mixed masses that arise from the sacrococcygeal region. Sacrococcygeal tumors can be completely external, have external and intrapelvic components, or reside predominantly within the pelvis. MRI can help in detecting these lesions and in differentiating them from other solid or cystic masses in the pelvis.

Gastroschisis consists of the herniation of fetal abdominal viscera into the amniotic cavity secondary to a small defect in the abdominal wall involving all the layers of the abdominal wall. It nearly always affects the right side, and the umbilical cord is inserted in its normal position.

MRI easily detects the evisceration of abdominal structures [29, 30]. T2-weighted sequences are the most useful because the abdominal structures located outside the abdominal cavity are clearly visualized against the hyperintense amniotic fluid (Fig. 19). The axial plane is best to show the defect in the abdominal wall and the position of the umbilical cord. T1-weighted sequences are useful for determining the position of the large intestine. Volumetric images can be helpful in determining whether the eviscerated organs have a peritoneal lining and the site of umbilical cord insertion. These two findings are important in differentiating gastroschisis from omphalocele (Fig. 20).

Associated anomalies are rare, although intestinal anomalies or complications are common, especially intestinal stenosis or atresia, possibly due to the direct effect of amniotic fluid on the intestinal loops or ischemic problems secondary to compromise of the mesenteric vessels due to the relatively small size of the abdominal wall defect [31].

In omphalocele, embryologic development is detained at a point in which the developing intestines are located outside of the abdominal cavity. The viscera protrude through a central abdominal wall defect. The herniated viscera are lined with a membrane consisting of two layers: the peritoneum and the amnion. The umbilical cord inserts into the apex of the omphalocele rather than at its normal position. Omphalocele varies considerably in size.

MRI usually enables accurate diagnosis of omphalocele, and its contents are easily determined [30]. MRI also usually shows the linings of the omphalocele and the point of umbilical cord insertion clearly. T2-weighted sequences are the most useful for studying omphalocele, although T1-weighted sequences help in detecting the large intestine, and volumetric sequences help in determining the point of insertion of the umbilical cord (Fig. 20b).

Intestinal complications are less common than in gastroschisis, possibly because the abdominal structures are not in contact with the amniotic fluid and because the defect in the abdominal wall is usually large, making compromised mesenteric blood flow less likely. Omphalocele is often associated with other anomalies and sometimes with chromosomopathies, especially trisomies 18 and 13 [32].