Abnormal Findings and the Complementary Role of US and MR Imaging

Several abnormal conditions observed on US will be discussed:

- The stomach is small or not visualized

- The stomach is too large

- The stomach is in the midline

- The digestive tract is dilated

- The bladder is dilated

- The bladder is not visible

- The kidneys are (is) not visible

- The urinary tract is dilated

- The kidneys are too echogenic

- There is a cyst on the kidneys

- There are inter-abdominal calcifications

- There is an abdominal mass.

The Stomach Is Small or Not Visualized (Second or Third Trimester)

As mentioned above, the stomach should always be visualized during sonographic examination, especially during the second and third trimester examinations.

Its absence or small size raises the suspicion of an abnormality (the first step should be a control examination of the fetus in order to confirm the anomaly).

The most common 'pathological' cause is an esophageal atresia (EA). US is not very effective for an accurate diagnosis of EA unless a pharyngeal pouch is distended. On the contrary, MR imaging can provide important information and improve the detection rate of this diagnosis by easily demonstrating the dilated pouch or by excluding the diagnosis thanks to the visualization of an entire esophagus. The other possibility is the presence of intra-uterine growth retardation with associated oligohydramnios. In this case, no abnormality of the digestive tract would be detected [9, 10].

The Stomach Is Too Big (Third Trimester)

Once again, the first step should be to confirm the anomaly.

The main cause is a duodenal atresia leading to the so-called 'double-bubble' sign. It is associated with aneu-ploidy in 30% of patients (Trisomy 21).

The differential diagnosis should include duodenal stenosis. US is sufficient for this diagnosis; no complementary imaging modality is necessary [11].

The Stomach Lies on the Midline

The midline position of the stomach could correspond to intestinal malrotation with or without obstruction and should prompt a postnatal work-up [12].

The Digestive Tract Is Dilated (Mainly Third Trimester)

The presence of dilated intestinal loops on US (over 1 cm diameter in the second trimester, over 2 cm diameter during third trimester) suggests intestinal obstruction. Although the technique is very sensitive, it is not specific for the detection of an obstruction and it cannot differentiate between small and large bowel obstructions.

With such a presentation, MR imaging can improve the diagnostic accuracy by demonstrating the colon. If the latter appears normal on T1 sequence, the obstruction most probably affects the small bowel. Conversely, if the colon is not visible, it is more difficult to assess the level of obstruction unless a specific part of it appears dilated. Noteworthy, the diagnosis of Hirschsprung disease or of isolated anal atresia is difficult or even impossible [5, 13].

The Bladder Is Too Large (Any Trimester)

During the first trimester a bladder larger than 3 cm is abnormal. If it persists, a bladder outlet obstruction (BOO) is probable and the prognosis is poor.

During the second trimester, a bladder larger than 5 cm is abnormal. The presence of an oligohydramnios suggests a BOO and again, the prognosis is poor.

During the third trimester, a large bladder can correspond to several diagnoses. The differential should include bladder obstruction related to posterior urethral valve, massive vesico-ureteric reflux and 'megacystis mi-crocolon-hypoperistalsis' syndrome.

Fetal MR imaging may clearly help in this differential diagnosis by demonstrating a normal or abnormal colon [11, 14].

The Bladder Is Not Visible

First, the examination should verify that this does not correspond to the normal cycle of filling and emptying of the fetal bladder.

If confirmed, the first suspicion should be bladder exstrophy, in which there is no normal bladder and instead a soft tissue mass is seen beneath the insertion of the umbilical cord. Also, no cystic structure is seen between the two umbilical arteries. Usually US is sufficient to reach this diagnosis [15].

One or Both Kidneys Are Not Visible

Bilateral renal agenesis is incompatible with postnatal life. The condition is associated with oligohydramnios and pulmonary hypoplasia.

In such conditions, no renal structure is visible within the lumbar areas. If necessary, fetal MR imaging can be performed in order to confirm the anomaly [14, 15]. MRI can also facilitate the diagnosis of ectopic kidneys.

Unilateral renal agenesis is common. There are usually no associated anomalies and the prognosis is good. Imaging should aim at determining whether a dysplastic kidney replaces the normal kidney or whether the kidney has developed in an ectopic position.

The Urinary Tract Is Dilated (Second and Third Trimester)

The diagnosis of a urinary tract dilatation is based upon an anterior-posterior diameter of the renal pelvis above 7 mm, a ureter above 3 mm, or a bladder above 5 cm.

The role of imaging is to determine the level of obstruction and the status of the remaining parenchyma. Assessment of associated anomalies, amniotic fluid volume and lung hypoplasia will help to determine the prognosis.

Fetal MR imaging may help in providing additional anatomical information for this evaluation [14, 15].

The Kidneys Are Hyperechoic (Second and Third Trimester)

As mentioned above, the echogenicity of the kidneys decreases with time and corticomedullary differentiation should be present by 17-18 weeks.

In some instances, the kidney parenchyma appears frankly hyperechoic and/or corticomedullary differentiation (CMD) is not present. The sonographic analysis should be as detailed as possible and should evaluate size, echogenicity, the presence of cysts, calcifications or other anomalies.

The main causes of hyperechoic kidneys are hereditary polycystic kidney diseases. Familial and genetic inquiries might help with diagnosis.

Other causes include syndromes, metabolic diseases, toxins, infections and vascular causes. A meticulous and systematic work-up will help to approach a correct diagnosis [16, 17]. To date, the role of MRI is limited for the analysis of the renal parenchyma, except for the detection of cysts.

There Are Cysts on the Kidneys (Second or Third Trimester)

As for the previous case, the approach should be meticulous, based on a systematic analysis of the kidneys.

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