Choledochal cysts are congenital anomalies of the bile ducts. They consist of cystic dilatations of the extrahepatic biliary tree, intrahepatic biliary radicles, or both.
Alonso-Lej et al provided the first systematic description of choledochal cysts, based on the clinical and anatomic findings in 96 cases.  The resultant system classified choledochal cysts into 3 types and outlined therapeutic strategies for each. The classification system for choledochal cysts was further refined by Todani and colleagues and currently includes 5 major types (see the images below). 
An association exists between APBJ and choledochal cysts, cholangiocarcinoma, and carcinoma of the gallbladder. The potential exists for imaging modalities to miss small cancers of the bile ducts and gallbladder; however, definitive surgery should be planned in all patients and should include resection of incidental lesions.
In a study of 394 patients with choledochal cysts, 10 (2.5%) had cholangiocarcinoma. On follow-up, 13 patients (3.3%) presented with biliary cancer. In presentation of adults versus children, adults were found to be more likely to present with abdominal pain,and children with jaundice. Type I choledochal cysts were seen more often in children, and type IV in adults. 
Radiologists, gastroenterologists, and surgeons should work as a team in treating patients with choledochal cyst. The diagnostic workup should not cease until all are satisfied that enough information is available for operative planning. Such information usually includes data derived through a combination of ultrasonography (US) and computed tomography (CT) scanning, magnetic resonance imaging (MRI)/magnetic resonance cholangiopancreatographic (MRCP), and endoscopic retrograde cholangiopancreatography (ERCP).
A number of imaging modalities can be used to detect a choledochal cyst, such as ultrasonography, CT, MR cholangiopancreatography (MRCP), endoscopic retrograde cholangiopancreatography, and percutaneous transhepatic cholangiography. MRCP, in particular, can be helpful in detecting an abnormal pancraticobiliary junction. 
According to Miyano and Yamataka, the preferred initial radiologic examination in the diagnostic workup of a choledochal cyst is an abdominal US scan.  US scanning is noninvasive and involves no radiation exposure, and its findings are sensitive and specific for the diagnosis. Clinically, these features make sense. Patients with choledochal cysts most often have symptoms referable to the hepatobiliary system, and most US operators are familiar with the anatomy of this area.
Once a preliminary diagnosis is made using US scanning, other supportive studies may be ordered, including abdominal CT scans, MRI studies, or MRCP examinations. [6, 7, 8] These studies demonstrate the cyst with more precise anatomic detail. In addition, important anatomic relationships to surrounding structures are better defined than with other modalities.
Limitations of techniques
US scanning is an excellent choice for initial imaging, but it does have limitations, including the fact that its effectiveness is dependent on operator experience, that cysts on US images may be misinterpreted as the gallbladder or other structures, and that US scanning suffers decreased sensitivity in the presence of overlying bowel gas, pancreatitis, cholangitis, or other inflammatory processes. Differentiating a choledochal cyst from a hepatic cyst, hepatic abscess, acute fluid collection, or pancreatic pseudocyst may be difficult.
Plain abdominal radiographs are of little use in the diagnosis of choledochal cysts. They offer no specific information related to this diagnosis. In patients presenting with abdominal pain, radiographs are frequently ordered as part of the standard workup. At most, radiographs may suggest displacement of an adjacent hollow viscus, such as the duodenum, by a mass.
False-positive and false-negative rates are high if the diagnosis is based on plain abdominal radiographs. Plain radiographs cannot depict choledochal cysts well. Many other masses or inflammatory processes can produce the same clinical picture, which cannot be differentiated by using plain images. Examples of these processes include an acute fluid collection associated with acute pancreatitis, a pancreatic pseudocyst, and choledocholithiasis or cholangitis.
Abdominal CT scanning is useful in the diagnostic algorithm for choledochal cysts (see the images below). CT scanning is highly accurate and offers a great deal of information that is helpful not only in confirming the diagnosis, but also in planning surgical approaches.
CT scans of a choledochal cyst demonstrate a dilatated cystic mass with clearly defined walls that is separate from the gallbladder. The fact that this mass arises from or actually is the extrahepatic bile duct usually is clear from its location and its relationships to surrounding structures. The cyst is typically filled with bile, which produces waterlike attenuation. Depending on the patient's age and clinical history, the wall of the cyst can appear thickened, especially if multiple episodes of inflammation and cholangitis have occurred.
Most patients with choledochal cysts have undergone abdominal US imaging prior to CT scanning. US findings suggest the diagnosis in most patients and may be conclusive in many.
According to Lipsett and colleagues, CT scanning confirms an unclear diagnosis and provides information concerning the relationships of the cyst to surrounding structures.  These include the portal vein, duodenum, and liver. In addition, CT scanning is superior to US scanning in defining the extent of the cyst in the extrahepatic biliary system and in detecting intrahepatic disease.
CT cholangiography is one of the CT imaging techniques that has been used in the diagnosis of choledochal cysts.
Lam and colleagues investigated the use of CT cholangiography versus MR cholangiography in the diagnosis of choledochal cysts in 14 children and had good results with both techniques. 
Satisfactory visualization of the biliary tree was achieved in 93% of patients by using CT cholangiography and in 100% of patients by using MR cholangiography. CT cholangiography adequately depicted 10 (91%) of 11 choledochal cysts, whereas MR cholangiography showed 100% of the cysts. Intrahepatic stones were detected, with sensitivities of 83% and 67% for CT scanning and MR cholangiography, respectively. Specificities were 100% for both. CT cholangiography demonstrated the pancreatic duct and the common pancreatobiliary channel in 64% of patients, whereas MR cholangiography depicted these features in 46%. Postoperatively, CT cholangiography was better for surveillance of the hepaticojejunostomy.
The authors recommended the use of MR cholangiography as the confirmatory imaging study in children with choledochal cysts because it does not require breath holding, is noninvasive, does not require the administration of contrast material, and is not associated with ionizing radiation.
Degree of confidence
CT scans can be relied on with a high degree of confidence. They provide a great deal of information concerning choledochal cysts and their relationships to surrounding structures. Typically, the only preoperative study that may be considered after a confirmatory CT scan is ERCP. ERCP often provides additional information concerning the pancreatic duct and the distal extent of the choledochal cyst. ERCP can provide more detailed information concerning the epithelium of the cyst and detect the presence of associated tumors or strictures.
False-positive and false-negative results in CT scanning are rare. If any question remains concerning the diagnosis after a CT scan, ERCP can be performed.
Magnetic Resonance Imaging
Use of MRI and MRCP techniques is increasing dramatically for the noninvasive diagnosis of biliary and pancreatic diseases.  Choledochal cysts are no exception. These cysts appear as large fusiform or saccular masses that may be extrahepatic, intrahepatic, or both, depending on the type of cyst. They produce a particularly strong signal on T2-weighted images. Associated anomalies of the pancreatic duct, its junction with the common bile duct, and the long common channel formed by the 2 are usually well demonstrated on MRI/MRCP images. [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
Lee and Lee et al compared MRCP and ERCP in 46 patients with various extrahepatic biliary diseases, including choledochal cysts, and found the overall diagnostic accuracy rates to be similar between the groups.
Irie and associates concluded in a study that MRCP is an important noninvasive diagnostic study for choledochal cysts but that it should not replace ERCP, especially in children. The authors used MRCP in the diagnosis of choledochal cysts in 16 patients.  They found that MRCP defined the proximal bile duct better than ERCP but that defects in the distal common bile duct were missed with MRCP in 2 pediatric patients. The anomalous pancreaticobiliary junction (APBJ) was delineated in all 6 adult patients but was missed in 6 of 10 pediatric patients.
Kim et al concluded that MR cholangiography is equivalent or superior to conventional cholangiography in the evaluation of choledochal cysts. The authors compared MR cholangiography with conventional cholangiography in 13 patients with choledochal cysts.  MR cholangiography was superior for complete imaging of the cyst. Detection rates for the APBJ and bile duct stones were not significantly different.
Degree of confidence
On the basis of reported results, MRCP appears to be a reliable study in the diagnosis of choledochal cysts, with diagnostic accuracy of 82-100%. In preparation for surgery, particularly when information is needed concerning the location of the APBJ and the length of the common channel, another study, such as ERCP, should be performed.
Differentiating a choledochal cyst from a pancreatic pseudocyst or other cystic lesions of the pancreas may be difficult using MRI findings.
No normal anatomic variants mimic a choledochal cyst. However, false-negative findings of the absence of the APBJ and the common pancreatobiliary channel are possible. In addition, small neoplasms of the gallbladder and bile duct may be missed.
US scanning is the initial screening examination of choice in patients with choledochal cysts. Pertinent findings include a cystic extrahepatic mass. Depending on the skill of the operator, the specific type or class of choledochal cyst may be identified. High-resolution US machines help clinicians make such diagnoses. Furthermore, advances in US technology have enabled ultrasonographers to make the diagnosis in the antenatal period. [23, 24, 25]
A diagnostic sonogram is shown in the image below.
US scan findings are diagnostic in many patients; however, in the preoperative period, complementary studies, such as ERCP, CT scans, or MRI/MRCP, may be helpful in delineating details of the surrounding anatomy, the location of an APBJ, and the length of the common pancreatobiliary channel.
Abdominal US scan findings can help in detecting conditions associated with and complications of choledochal cysts, such as choledocholithiasis, intrahepatic biliary dilatation, portal vein thrombosis, gallbladder or biliary neoplasms, pancreatitis, and hepatic abscesses.
In surgical series with a total of 140 patients, US scan findings, although sensitive, were routinely confirmed by using another study or procedure, such as CT scanning, hepatoiminodiacetic acid scanning, or ERCP. [26, 27, 28]
Sato et al reviewed their US findings in 12 patients with choledochal cysts  and found that the cyst diameter changed greatly under compression. The authors cautioned against compressing the structures, since this can lead to confusion in the diagnosis.
Degree of confidence
US scanning is sensitive in the demonstration of a cystic lesion in the region of the porta hepatis. If such a lesion is visualized, US scanning frequently may provide enough information to make the diagnosis of a choledochal cyst.
No actual normal variants are mimetic of choledochal cysts. Pathologic conditions in the differential diagnosis include pancreatic pseudocysts, hepatic cysts, and biliary atresia with associated cystic masses. At times, differentiating these lesions may be difficult using US scan findings. In this situation, other confirmatory tests are definitely indicated.
Hepatobiliary scintigraphic modalities are used commonly in the setting of acute cholecystitis and in the investigation of neonatal jaundice. In addition, these techniques are useful in the diagnosis of choledochal cysts (see the images below).
In a study, Kao and co-investigators concluded that nonvisualization of the gallbladder on cholescintigraphy in patients with choledochal cysts is not necessarily indicative of acute cholecystitis and that large choledochal cysts may compress the gallbladder, leading to nonvisualization. The study included 27 patients with choledochal cysts. Nonvisualization of the gallbladder occurred in 18 (67%) of 27 patients at 4 hours after injection of the radionuclide. Most of the patients did not have acute cholecystitis. [30, 31]
In a study, Rajnish and colleagues concluded that hepatobiliary scintigraphy was effective in helping to diagnose type I and type IV choledochal cysts. A total of 21 patients with confirmed choledochal cysts (12 type I, 9 type IV) had cholescintigraphic examinations; 2 nuclear medicine specialists were asked to review the scans in an attempt to classify the choledochal cysts according to the Todani classification. 
In the study, hepatobiliary scintigraphy correlated with ERCP or surgical diagnosis in 86% of patients. All 12 type I cysts were diagnosed correctly using hepatobiliary scintigraphy. The sensitivity for the diagnosis and classification of type IV cysts was 67% (6 of 9 cases). The difficulty here was the relative inability to diagnose intrahepatic disease in type IV cysts.
Degree of confidence
Hepatobiliary scintigraphy has reasonably good accuracy in the diagnosis of choledochal cysts. However, it probably should not be relied on as the sole study and should be complemented by ERCP, CT scanning, or MRCP.
No known normal anatomic variants mimic this disease process. However, false-negative results can derive from the relative inability to diagnose intrahepatic disease in type IV choledochal cysts.
Angiographic techniques are not part of the standard workup for choledochal cysts. However, angiographic studies and angiographic interventions can be instrumental, and even lifesaving, when major vascular complications associated with choledochal cysts occur. Fortunately, these complications are rare, and most experience has been limited to case reports. [33, 34]