Thanks to improving technology and experience, sono- graphic surveying of fetal development is now more ac- curate and precise. Further, more anomalies are detected, allowing the adaptation of postnatal management, if nec- essary.
Another striking evolution in perinatal imaging has been the increased use of fetal magnetic resonance (MR) imaging for the assessment of various fetal abdominal diseases. Faster sequences and better understanding of the normal appearances of the fetal abdomen on MR imaging have greatly facilitated its use [1-8].
Normal Sonographic Anatomy
In each trimester of the pregnancy, specific landmarks of normal fetal anatomy should be verified by obstetric ul- trasound (US) [1, 2].
During the First Trimester
The stomach appears as a cystic structure in the left hypochondrium. It should be constantly visible at around 11-12 weeks.
The small bowel herniates physiologically through the umbilical cord at around 8-9 weeks. The herniation should have resolved by 11-12 weeks.
The rest of the digestive tract is not visible at this stage.
The bladder is the other cystic structure that should be visible during the first trimester. It is located medially in the fetal pelvis and is visible around 8-9 weeks.
The kidneys are visible around the end of the first trimester and appear as hyperechoic bean-shaped struc- tures in each perilumbar area.
At this stage, the amniotic fluid is produced by the pla- centa and does not reflect the renal function.
During the Second Trimester
The stomach is globally unchanged. The small bowel gets progressively filled with meconium. It will be visible as small parallel tubular echogenic structures, mainly in the
left flank. The colon is not visible until the end of the sec- ond trimester.
The bladder should always be visible during the mid trimester examination. It should show normal cycles of filling and emptying (during a single examination).
The kidneys now appear more obvious, since their global echogenicity decreases and since cortico- medullary differentiation begins (and should be constant- ly present by 17-18 weeks).
The rest of the abdomen is ‘filled’ with the liver and the spleen, both appearing as homogenous triangular structures. The adrenals are triangular hypoechoic mass- es located on the top of the kidney. The pancreas and the abdominal vessels complete the evaluation of the fetal abdomen.
During the Third Trimester
The esophagus is visible when distended by amniotic fluid.
The stomach shows obvious peristaltic waves. It may be filled with echogenic material (gastric pseudomass) corresponding to swallowed debris.
The small bowel is now filled with swallowed amniot- ic fluid and appears as small cystic structures of variable sizes due to normal peristalsis.
The colon is progressively visible as it fills with hy- poechoic meconium and by 27-28 weeks it can be fol- lowed from the cecum to the recto-sigmoid. Its diameter widens progressively (it should be less than 2 cm).
The appearance of the bladder and kidneys should be stable. The kidneys grow progressively with gestational age. Their length measures around 3 cm at 30 weeks.
The adrenals are large and display a corticomedullary differentiation [1, 2].
The liver occupies a large part of the fetal upper ab- domen. The gallbladder is almost always visible.
Normal MR Imaging Anatomy
Fetal MR is mainly performed in the second half of the pregnancy. T1- and T2-weighted sequences can be used in order to characterize fetal abdominal anatomy.
The Fetal Abdomen: From Normal to Abnormal. From Ultrasound to MR Imaging
F.E. Avni, M. Cassart, A. Massez
Department of Medical Imaging, University Clinics of Brussels – Erasme Hospital, Brussels, Belgium
IDKD 2006
The bladder appears as a hypersignal on T2- and a hy- posignal on T1-weighted sequences. The normal kidneys are mainly visualized on T2 sequences.
The small bowel is best analyzed on T2 sequence, as the loops appear as a hypersignal, being filled with amni- otic fluid. The colon is best analyzed on T1, as it appears as a hypersignal, probably due to the proteic content of the meconium. It appears as a hyposignal on T2 sequences.
The rectum should be identified in all fetuses after 23 weeks. Its cul-de-sac should be localized 10 mm below the bladder neck on a sagittal scan. It should be possible to identify the entire colon after 30 weeks [3-8].
Abnormal Findings and the Complementary Role of US and MR Imaging
Several abnormal conditions observed on US will be dis- cussed:
– 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 visu- alized during sonographic examination, especially during the second and third trimester examinations.
Its absence or small size raises the suspicion of an ab- normality (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 diag- nosis 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 diag- nosis thanks to the visualization of an entire esophagus. The other possibility is the presence of intra-uterine growth re- tardation 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).
208
The differential diagnosis should include duodenal stenosis. US is sufficient for this diagnosis; no comple- mentary 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 diame- ter during third trimester) suggests intestinal obstruc- tion. Although the technique is very sensitive, it is not specific for the detection of an obstruction and it can- not differentiate between small and large bowel ob- structions.
With such a presentation, MR imaging can improve the diagnostic accuracy by demonstrating the colon. If the latter appears normal on T1 sequence, the obstruc- tion 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 ap- pears dilated. Noteworthy, the diagnosis of Hirschsprung disease or of isolated anal atresia is difficult or even im- possible [5, 13].
The Bladder Is Too Large (Any Trimester)
During the first trimester a bladder larger than 3 cm is ab- normal. 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 sug- gests a BOO and again, the prognosis is poor.
During the third trimester, a large bladder can corre- spond to several diagnoses. The differential should in- clude 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 cor- respond 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 in- stead a soft tissue mass is seen beneath the insertion of the umbilical cord. Also, no cystic structure is seen be- tween the two umbilical arteries. Usually US is sufficient to reach this diagnosis [15].
F.E. Avni, M. Cassart, A. Massez
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 usual- ly 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 obstruc- tion 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 de- creases with time and corticomedullary differentiation should be present by 17-18 weeks.
In some instances, the kidney parenchyma appears frankly hyperechoic and/or corticomedullary differentia- tion (CMD) is not present. The sonographic analysis should be as detailed as possible and should evaluate size, echogenicity, the presence of cysts, calcifications or oth- er 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 diag- nosis [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 meticu- lous, based on a systematic analysis of the kidneys.
MDKD Obstructive Simple Unilateral dysplasia renal cyst ADPKD
Unilateral
Bilateral ARPKD GLMCK ADPKD MCD Bilateral
MDKD obstructive
anomalies MDKD, Multicystic dysplastic kidney; ADPKD, Autosomal domi- nant polycystic kidney; ARPKD, Autosomal recessive polycystic kidney; GLMCK, Glomerulocyst kidney; MCP, Medullary cystic dysplasia
It is also important to look for associated anomalies corresponding to syndromes with renal cystic involve- ment [18].
There Are Intra-abdominal Calcifications
Intra-abdominal calcifications and echogenic masses are relatively common findings during fetal US. They may arise from any fetal part (e.g., liver, gallbladder, kidneys, spleen, adrenals).
Detection of such anomalies should prompt a detailed survey for additional findings, and a maternal history. In most cases, conservative management is sufficient.
However, for some, a transfer to a tertiary care centre will be necessary.
In case of peritoneal calcifications, an important as- sociated f inding would be dilated intestinal loops, which would orient the diagnosis towards meconium peritonitis.
Another important finding would be an underlying fe- tal mass with calcifications (see below) [19-21].
There Is an Abdominal Mass (Second and Third Trimester)
In case of an abdominal mass, the role of imaging is to determine its anatomical origin, its content and the anatomic relation with the regional structures. The most common cystic tumor is an ovarian cyst.
The most common solid-type tumor is renal or adren- al in origin.
- ovarian cyst - mesoblastic nephroma - mesenteric cyst - neuroblastoma
- lymphangioma - teratoma
- cystic teratoma - hemangioma
- cystic neuroblastoma - intra-abdominal sequestration - urethral cyst
- duplication cyst
Follow-up is by US for neonatal management and eventual surgery [22].
In conclusion, a step-by-step analysis helps to define the anomalies affecting the fetal abdomen. For selected indications, US first and MR imaging thereafter, can be used to evaluate the malformation.
Solid-type tumors Cystic tumors
Bilateral
The Fetal Abdomen: From Normal to Abnormal. From Ultrasound to MR Imaging 209
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F.E. Avni, M. Cassart, A. Massez
