Choledochal Cyst and Cystic Dilatation of the Bile Duct

Choledochal cysts are anomalies of the biliary system characterized by dilatation of the extra- or intrahepatic bile ducts (see Chapt. 7,"Imaging of the Biliary Tree and Gallbladder Diseases", section 7.2.2.1,"Choledochal Cyst and Cystic Dilatation of the Bile Duct"). Although choledochal cysts may become evident at any age, diagnosis is made within the first ten years of age in about 60% of cases. They are more frequent in females than in males with a ratio of about 4:1.

In newborns and infants, obstructive jaundice is the most common clinical presentation, while in older children and adults the signs and symptoms are those of ascending cholangitis [3,96].

US is usually the first imaging modality to diagnose a choledochal cyst in pediatric patients. The appearance of these malformations is similar to their appearance in adult patients and the same limitations apply regarding visualization of the full extent of cystic dilatation, the relationship of the cyst to the gallbladder and pancreatic duct, and the angle and site of junction with the duodenum. In young children this may be related to the presence of gas in the bowel [50].

Although endoscopic retrograde cholan-giopancreatography (ERCP) has been reported to be safe in infants and small children suspected of having choledochal cyst, CT and MR cholangiog-raphy (MRC) are frequently used as alternative imaging techniques [50]. Of these techniques, MRC is the preferred modality in pediatric patients because it offers similar information to ERCP without the potential complications inherent in the latter procedure and without the need for ionizing radiation [57].

Bile and pancreatic secretions have high SI on MRC performed with heavily T2-weighted pulse sequences. With these sequences choledochal cysts can usually be seen as hyperintense tubular, fusiform or cystic structures (Fig. 6).

Unfortunately, the signal-to-noise ratio is reduced in small patients, and image quality is frequently sub-optimal because of respiratory motion artifacts associated with the need to acquire images using non breath-hold sequences. These limitations render imaging of non-dilated pancreatic ducts and intrahepatic ducts more difficult and explain why the quality of MRC images is often inferior to that of CT cholangiography (CTC) images in some patients [50]. The availability of respiratory triggering may overcome many of the drawbacks associated with non breath-hold sequences and permit satisfactory imaging of even non-dilated bile ducts. Similarly, single-shot fast SE sequences have been shown to be effective for imaging of the biliary tree in infants and children unable to hold their breath [44].

The use of secretin further improves bile duct visualization in pediatric subjects. This is because the secretin increases pancreatic juice secretion, which increases the pancreatic duct visualization, particularly at the distal portion. This may be important for visualization of the common channel.

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