Magnetic Resonance Imaging
Findings
Prenatal MRI enhances the fetal anatomic evaluation; it can be a valuable adjunct to ultrasonography before surgical intervention for selected life-threatening fetal defects. MRI helps in corroborating and refining the ultrasonographic diagnosis of complex fetal defects.
The prognosis and mortality rate in omphalocele is determined more by the presence of associated anomalies, such as cardiovascular and chromosomal defects, than by the omphalocele itself. Prenatal MRI can be used to screen for anomalies such as complex cardiac defects and nervous system anomalies.
Degree of Confidence
The diagnostic accuracy of MRI has been reported to be superior to that of sonography in selected cases of cerebral malformations because of the high resolution of the soft tissues and because of its more global depiction of complex fetal disorders.
Unlike ultrasonography, MRI is not operator dependent. Imaging, however, is dependent on magnet strength and the pulse sequences chosen. The field of view obtained with MRI is larger than that obtained with sonography; the large field of view allows good anatomic orientation. MRI is not limited by maternal obesity or oligohydramnios. Fetal movement can make MRI difficult.
MRI is considered safe for the developing fetus. At present, no clinical or experimental evidence suggests that MRI causes teratogenic or other adverse affects during pregnancy, although a few studies in laboratory animals have shown that prolonged, high-level exposure to electromagnetic radiation might result in teratogenicity. A recommendation from the National Institute of Health Consensus Development Conference states, "MRI should be used during the first trimester pregnancy only when there are clear medical indications and when it offers clear advantage over other modalities."
False Positives/Negatives
The fine detail of internal structures of the abdomen and pelvis is not well demonstrated with fast T2-weighted sequences.
Ultrasonography
Findings
An omphalocele is diagnosed when a fetal anterior midline abdominal mass is demonstrated. The mass consists of abdominal contents that have herniated through a midline central defect at the base of umbilical cord insertion. The mean size of the defect is 2.5-5 cm. The image below depicts a mass with a smooth surface and contains abdominal viscera, usually the liver, bowel, and stomach.
Sagittal antenatal scan in a 13-week-old fetus shows an omphalocele-containing liver.
Sagittal antenatal scan on a 13- or 14-week-old fetus shows an omphalocele-containing liver.
The covering of the mass, which comprises the peritoneum and amnion, may rarely rupture. The membrane is not always visible. Wharton jelly may be detectable as a hypoechoic lining between the layers of the covering of the membrane.
The umbilical cord attaches to the apex of the herniated mass, where the umbilical vein can be seen within the mass. The cord may be widened where it joins the fetal skin.
Fetal ascites is common and is seen within the herniated sac. Polyhydramnios, and occasionally oligohydramnios, may be present. Other major anomalies may be apparent in as many as 70% of cases.
A number of sonographic features differentiate an omphalocele from a physiologic midgut herniation, shown in the images below.
Axial sonogram through the mid abdomen of a fetus shows exteriorized bowel in relation to the anterior abdominal wall. Multiple loops of bowel are apparent. Because the bowel loops are not covered, they have irregular edges. L indicates the liver.
Axial sonogram through the mid to upper abdomen shows free-floating exteriorized bowel in relation to the anterior abdominal wall. S indicates the stomach; V, spine.
A midgut herniation seldom exceeds 7 mm in diameter, whereas omphaloceles are much larger. Midgut herniation is invariably smaller in diameter than the abdomen, whereas the diameter of an omphalocele can be larger than that of the abdomen. The size of the omphalocele is best measured by using the ratio of the transverse diameter of the omphalocele to the transverse diameter of the abdomen. A midgut herniation seldom persists after 12 weeks of gestation or in a fetus with a crown-length measurement of more than 44 mm.
When the ratio of the diameter of the omphalocele to the transverse diameter of the abdomen is less than 60%, the lesion usually contains bowel and not liver, as described in the image below.
Diagram of the transverse section of the fetal abdomen shows gastroschisis. Note the bowel herniation in the right paramedian/paraumbilical region. The cord is inserted in the normal location to the left of the herniation. No membranous covering exists over the herniated bowel.
The relative size of the omphalocele may decrease during pregnancy.
A measurement of the ratio of the transverse area of the lung to that of the thorax may be useful in predicting associated pulmonary hypoplasia in a giant omphalocele. However, antenatal detection requires prolonged ventilation, which may be difficult and requires further study.12,38,39,40
Degree of Confidence
Ultrasonography is a sensitive technique, but it remains operator dependent. Its great advantage is that it can be quickly and frequently repeated as required. A definitive diagnosis of omphalocele is possible only beyond 12 weeks' gestation, when confusion with physiologic midgut herniation is no longer possible.
False Positives/Negatives
An anterior abdominal-wall mass has a wide differential diagnosis. It may be difficult to differentiate between a midline omphalocele and physiologic midgut herniation in early pregnancy; it may also be difficult to differentiate omphalocele from bowel herniation of gastroschisis.
Gastroschisis usually poses no problems of differentiation from an omphalocele. The only finding that the 2 share is that they both arise from the anterior abdominal wall. Gastroschisis has no membranous covering and usually presents with a ragged edge, which almost never contains liver. In addition, gastroschisis is typically right-sided, with the umbilical cord entering the abdomen to the left of the herniation (rather than on the herniation, as in an omphalocele). Spontaneous rupture of an omphalocele membrane may cause problems in differentiation, but this is so rare that it should seldom enter the differential diagnosis.
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 Imaging: Omphalocele |
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Further Reading
Keywords
exomphalos, anterior abdominal wall defect
