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Annular Pancreas

An annular pancreas is the result of an abnormal fusion of ventral and dorsal buds during the 6th week of embryogenesis. These patients will have a "ringed" pancreas surrounding the duodenum that, with growth, may lead to narrowing of the descending duodenum. These patients may present in the newborn period with complete duodenal obstruction, often after the first feeding. Some symptoms include polyhydramnios in utero, feeding intolerance in newborns, vomiting or recurrent pancreatitis. Other anomalies such as esophageal atresia, tracheoesophageal fistula, duodenal stenosis, duodenal atresia, trisomy 21, and malrotation have been associated with annular pancreas (Gazelle et al. 1998).

In annular pancreas, the duodenum is often compressed at a point distal to the ampulla of Vater, making bilious vomiting a hallmark symptom. Abdominal distention is typically not a feature because of the proximal location of the obstruction. Patients may not pass meconium, or bowel movements may cease abruptly. A more insidious form of chronic partial duodenal obstruction may also occur.

In neonates, plain radiographs may reveal a spectrum from a "double-bubble" sign with little or no air in the distal bowel to a normal plain film abdomen. This radiographic finding correlates with a high gastrointestinal obstruction such as malrota-tion/midgut volvulus, duodenal atresia, and duodenal web (Gazelle et al. 1998) (Fig. 4.15a).

US may show an enlarged pancreatic head or a solid band of pancreatic tissue around the (possibly dilated) duodenum with gastric dilatation (Gazelle et al. 1998).

On CT, an annular pancreas (with or without pancreatitis) may be seen as enlargement of the pancreatic head, surrounding the second portion of the duodenum. Follow up CT may demonstrate calcifications limited to the annulus of the pancreas (Gazelle et al. 1998).

On ERCP, various ductal configurations may be seen. A classification of six types of annular pancreas according to the site into which the duct of the annulus drains has been reported (Gazelle et al. 1998).

On MRI, the normal pancreatic tissue has higher signal intensity compared with other tissues on T1-weighted images with fat saturation. This signal is due to the high concentration of proteins inside the acini. An annular pancreas will be seen on MRI as

Fig. 4.15. a Classic UGI image of annular pancreas. b MRCP image of an annular pancreas causing relative obstruction of the descending duodenum

a high signal intensity tissue encircling the second part of the duodenum (Fig. 4.15b). MRCP has been used to classify the annular pancreas according to the different duct subtypes and to evaluate the concomitant presence of pancreas divisum (Nijs 2005; Chevallier et al. 1999).

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