Biliary Atresia or idiopathic neonatal hepatitis must be considered in any infant who presents with conjugated or mixed hyperbilirubinemia that is persistant beyond the first month of life and in whom an infectious, metabolic or toxic cause cannot be determined. It is the ligand which is chelated to the IDA which makes these agents analogues of bilirubin allowing heptocyte uptake by the same active transport mechanism as non-conjugated bilirubin itself. In biliary atresia versus severe neonatal hepatitis, there may be a discrepancy in localization of the radiotracer in the liver owing to the somewhat different mechanism for hyperbilirubinemia. Biliary atresia does not have true hepatocellular dysfunction such that tracer accumulation in the liver should be normal whereas in neonatal hepatitis there is abnormal liver function leading to depressed hepatocellular accumulation of tracer and delayed transit through the liver.
Biliary atresia is characterized by absent bile ducts most notable within the liver hilum.
Despite this atretic obstruction, the intrahepatic biliary ducts are present but small rather than dilated. The gallbladder is severely hypoplastic or absent. In neonatal hepatitis, there is inflammatory cell infiltration of the hepatocytes with associated necrosis and bile stasis although the bile ducts are normal and patent. The ducts are small and the hepatocytes have significant inflammatory changes although considerably less than in neonatal hepatitis alone. In fact, in many cases of biliary atresia, some multinucleated giant cells may be present with a component of hepatocellular necrosis.