Biliary atresia is a condition in which the normal extrahepatic biliary system is disrupted. Progressive damage of extrahepatic and intrahepatic bile ducts secondary to inflammation may occur, leading to fibrosis, biliary cirrhosis, and eventual liver failure. Although the exact etiology remains unknown, the primary therapy is surgical. [1, 2, 3, 4] It is thought that approximately 80-90% of currently affected infants will survive to adolescence following Kasai portoenterostomy and/or liver transplantation. 
Biliary atresia is a rare condition affecting approximately 1 in 10,000-20,000 births, with onset confined to the newborn period. Previously, the disease was described as occurring in 2 distinct clinical forms: fetal-embryonic (or syndromic) and perinatal (or acquired).
The fetal-embryonic form is characterized by early cholestasis, appears in the first 2 weeks of life, and accounts for 10-35% of all cases. In this form, the bile ducts are discontinuous at birth, and 10-20% of affected neonates have associated congenital defects, including situs inversus, polysplenia, malrotation, intestinal atresia, and cardiac anomalies, among others.
The perinatal form of biliary atresia accounts for the remaining 65-90% cases. This form is typically found in neonates and infants aged 2-8 weeks. Progressive inflammation and obliteration of the extrahepatic bile ducts occurs after birth. This form is not associated with congenital anomalies, and infants may have a short jaundice -free interval.
Several clinical variants have been defined, [5, 6] including biliary atresia with other congenital malformations, cystic biliary atresia, and isolated biliary atresia. Infants with biliary atresia may present with a spectrum of congenital malformations (syndromic or nonsyndromic), such as splenic malformations, disorders of visceral symmetry, and cardiac anomalies, among others, possibly on a genetic basis. In a small number of cases, biliary atresia may be associated with cyst formation. In the majority of cases, biliary atresia appears to be an isolated abnormality.
Biliary atresia may be classified according to whether the disease can or cannot be corrected. As the image below shows, in the correctable group (10-15% of cases), the proximal common hepatic duct is patent, allowing for primary anastomosis of the extrahepatic bile duct to the bowel.
Resection of the fibrous bile duct remnant may be done, followed by a Roux-en-Y anastomosis of the bowel to the bed of the porta hepatis, according to the Kasai portoenterostomy procedure. In the uncorrectable group, the extrahepatic bile ducts do not have the same patency as in the correctable group.
Another method of classification, the Kasai classification system, is widely used and divides cases of biliary atresia according to their location and degree of pathology. As shown in the image below, 3 main types of biliary atresia are defined.
In type I, the common bile duct is obliterated while the proximal bile ducts are patent.
In type II, atresia of the hepatic duct is seen, with cystic bile ducts found at the porta hepatis. In type IIa, the cystic and common bile ducts are patent, whereas in type IIb, the cystic, common bile duct, and hepatic ducts are obliterated.
Type III atresia refers to discontinuity of the right and the left hepatic ducts to the level of the porta hepatis. This form of biliary atresia is common, accounting for more than 90% of cases.
In current practice, children with biliary atresia are treated with the Kasai operation, liver transplantation without prior Kasai operation, or liver transplantation after Kasai operation. Although infants with biliary atresia can survive more than 20 years after Kasai portoenterostomy, most develop progressive complications in the liver.  Overall, survival has greatly improved in the era of liver transplantation. [8, 9]
Several imaging modalities have been used in the diagnosis of biliary atresia. Although some findings are highly suggestive of the disease, none is pathognomonic, and reliance on more than 1 test is common.Ultrasonography is often the initial investigation in patients with suspected biliary atresia, followed by hepatobiliary scintigraphy, a study that has been used effectively for many years.  If the diagnosis remains elusive after these studies, magnetic resonance cholangiopancreatography (MRCP) may be helpful. Hepatobiliary scintigraphy has been used in the diagnosis of biliary atresia for many years. [11, 12, 13, 14, 15, 16, 17]
Surgical and percutaneous cholangiography
Liver biopsy is often used to confirm the diagnosis of biliary atresia and may be done at the same time as surgical or percutaneous cholangiography.
Surgical cholangiography is typically performed by injecting contrast material through the gallbladder. If no communication is seen between the biliary tree and the gastrointestinal tract, biliary atresia is diagnosed.
Percutaneous transhepatic cholangiography may be technically challenging, and the results are definitive only if a normal intrahepatic and extrahepatic biliary system is seen.
Ultrasonography-guided percutaneous cholecystocholangiography is a relatively new technique in which radiographic contrast material is injected into the gallbladder under ultrasonographic guidance and the extrahepatic biliary system is viewed with fluoroscopy.  Although invasive, the technique has distinct advantages in that it is easier to perform and does not require general anesthesia.
Endoscopic retrograde cholangiopancreatography
Although it is a rarely used invasive technique, results from a study by Petersen et al led the investigators to recommended that ERCP be performed before explorative laparotomy in all patients suspected of having biliary atresia. 
In the study, ERCP was performed in cholestatic patients younger than 6 months who were suspected of having an extrahepatic cause of cholestasis, particularly biliary atresia. In this series, the sensitivity of ERCP for diagnosing biliary atresia was 92% and the specificity was 73%. The authors concluded that in preselected patients, ERCP, while not an alternative to noninvasive imaging, can be used to avoid surgery in approximately 25% of cases.
In a retrospective analysis by Shanmugam et al, ERCP had high positive and negative predictive values for biliary atresia in cholestatic infants younger than 100 days. 
ERCP allows direct visualization of the extrahepatic biliary tree with the injection of radiologic contrast agent into the extrahepatic biliary system through the papilla of Vater. It requires a general anesthetic, substantial expertise, and the availability of sufficiently small endoscopes.
This technique can show obstruction in the common bile duct and enables visualization of the extrahepatic biliary system distal to the common hepatic duct and the extrahepatic biliary system with bile lakes at the porta hepatis.
Magnetic Resonance Imaging
MRCP is a relatively new technique for neonatal imaging.
Findings in infants with biliary atresia include incomplete visualization of the extrahepatic biliary system and periportal high signal intensity on T2-weighted magnetic resonance imaging (MRI) scans (which may represent cystic dilatation of fetal bile ducts with surrounding fibrosis). [16, 17]
Degree of confidence
Complete visualization of the extrahepatic biliary system excludes biliary atresia, whereas nonvisualization of the common or hepatic bile ducts suggests the disease. Preliminary studies suggest sensitivity and specificity of 90% and 77%, respectively, for the technique.
Ultrasonography is generally the initial investigation in patients with suspected biliary atresia. It can be used to assess the neonatal hepatobiliary system and may exclude other anatomic anomalies. [23, 24] Recently, high-frequency ultrasound has been shown to provide improved sensitivity, specificity, and accuracy for the diagnosis of biliary atresia. 
Findings in infants with biliary atresia typically include an atretic gallbladder and a thin, indistinct gallbladder wall with an irregular or lobulated contour.
Although a normal (1.5 cm) or long (>4 cm) gallbladder may be seen in up to 10% of patients with biliary atresia, a length of less than 1.9 cm is most common. The constellation of findings constituting the gallbladder ghost triad, seen in babies with biliary atresia, are a gallbladder length less than 1.9 cm, a thin or indistinct gallbladder wall, and an irregular and lobular contour.  The image below shows the gallbladder ghost triad.
Ultrasonography can also be used to evaluate the hepatic parenchyma. In biliary atresia, the hepatic parenchyma is often inhomogeneous, with a marked increase in periportal echoes due to fibrosis. Sonograms in infants with biliary atresia often show a circumscribed, focal, triangular or tubular echogenic density more than 3 mm thick located cranial to the portal vein bifurcation. This is the triangular cord sign, presented in the image below; it corresponds to fibrosis of the extrahepatic biliary system. [27, 28, 29]
Although dilatation of the intrahepatic bile duct occurs infrequently, it suggests biliary atresia when present. Central biliary cysts and choledochal cysts may be associated with biliary atresia and are well depicted on sonograms, as demonstrated in the image below.
A prominent hepatic artery is often seen in children with cirrhotic changes.
The absence of gallbladder contraction is only suggestive of biliary atresia. As many as 20% of children with biliary atresia have normal gallbladder contraction. Furthermore, the absence of gallbladder contraction is seen in children with cholestasis due to other causes.
The presence of the gallbladder ghost triad is up to 97% sensitive and 100% specific for biliary atresia.
An absent common bile duct is thought to be 93% sensitive and 92% specific for the diagnosis of biliary atresia.
Reports suggest that the sensitivity of the triangular cord sign for the diagnosis of extrahepatic biliary atresia is greater than 72%, the specificity is greater than 97%, and the positive predictive value is 95%. The sensitivity may be decreased if diffusely increased periportal echogenicity from inflammation or cirrhosis obscures visualization.
It has been suggested that ultrasonography may distinguish biliary atresia from other causes of conjugated hyperbilirubinemia in over 90% of infants if multiple ultrasonographic features are carefully evaluated.
A technetium-labeled iminodiacetic acid (IDA) analogue is typically used. For example, radiopharamceuticals used include99m Tc (technetium-99m) DISIDA (diisopropyl-iminodiacetic acid) and99m Tc mebrofenin (trimethylbromo-iminodiacetic acid). Infants with biliary atresia usually have normal hepatocyte uptake of the radiotracer if they are younger than 2 months of age.
Improved sensitivity and specificity has been reported with delayed imaging, and following tracer administration, images are often acquired at 4-6 hours and 24 hours. The administration of phenobarbital 5 mg/kg/day in 2 equal doses for 3-5 days before the study may increase diagnostic accuracy. Indeed, hepatobiliary scintigraphy has been found to be up to 100% sensitive, 93% specific, and 94.6% accurate in diagnosing biliary atresia following pretreatment with phenobarbital.  The addition of single photon emission computed tomography (SPECT) may increase specificity.
If excretion of radiotracer into the bowel is seen, biliary atresia is virtually excluded. If radiotracer excretion is absent after 24 hours (as it is in the image below), biliary atresia is suspected.
Hepatobiliary scintigraphy may also be useful for the assessment of biliary excretion following surgical correction for biliary atresia.
Degree of confidence
The reported sensitivity of hepatobiliary scintigraphy is high (~100%). The specificity is variable.
Several factors, however, may limit the effectiveness of hepatobiliary scintigraphy. For example, severe neonatal hepatitis may result in decreased hepatic radiotracer uptake and therefore decreased excretion into the bowel. Also, because biliary atresia may be an evolving process, excretion of radiotracer into the gastrointestinal tract may be seen in children with biliary atresia in the early stages of the disease. Furthermore, reliability of the test diminishes with serum bilirubin levels greater than 10 mg/dL.
Patients should not have had barium studies within the 48 hours preceding hepatobiliary scintigraphy. If a barium study has been performed in this time frame, an abdominal radiograph may be indicated to make sure the bowel is clear of barium, a high density material that can result in artifacts.