There have been many studies on the diagnostic US findings of biliary atresia. Tan Kendrick et al. [27] suggested a “gallbladder ghost triad” for the diagnostic US findings of biliary atresiagallbladder length of <19 mm, indistinct gallbladder wall with a lack of a smooth or complete echogenic mucosal lining, and an irregular or lobular outer contour. Azuma et al. [21] reported that the absence of an extrahepatic bile duct on the US exam can further increase the diagnostic accuracy of biliary atresia. Both the triangular cord sign and the hepatic subcapsular flow are also known as one of the diagnostic US findings of biliary atresia [19,23]. However, not all these US findings are always present in patients with biliary atresia.

Mittal et al. [1] stated that CBD can potentially be visible on US in certain types of biliary atresia with distal CBD sparing. According to the proposal by Kasai et al. [37], biliary atresia with a porta hepatis obstruction and patent distal CBD is classified into type IIIa. Therefore, patients with type IIIa biliary atresia might show visible CBD on US.

There have been only a few reports on the US findings of biliary atresia on the basis of its types [33,34]. Further, no study has compared the US findings, including the visibility of CBD on US.

Among the 18 patients with type IIIa biliary atresia, CBD was not visible in 13 patients even though patent CBD was confirmed on operative cholangiography. In our study, one patient showed visible CBD at the age of 11 days and was included in group A. However, follow-up US, which was not included in this study, was performed 10 days later, and revealed that CBD was not visible. Therefore, one possible explanation for the invisible CBD is that the CBD started being affected by the disease process to become too small to be seen on US, and still insufficient to cause complete luminal obstruction. Another possible explanation is the decreased bile flow from the liver caused by the progression of the porta hepatis obstruction. This decreased bile flow subsequently causes the luminal collapse of CBD, making it difficult to be identified on US.

The triangular cord sign is defined as the triangular cord thickness of more than 4 mm [26] and has been proven to be highly specific for the diagnosis of biliary atresia by many other authors [1,17,18,22,24-26,29]. In contrast, the sensitivity of the triangular cord sign has been reported to vary from 14% to 100% [1,17,22,24-26,29]. Li et al. [28] suggested the following three possible explanations for a negative triangular cord sign and its low sensitivity: (1) no triangular cord at the porta hepatis, (2) triangular cord at the porta hepatis, but too small to be seen, and (3) insufficient experience of the US operator. In addition, Saxena et al. [38] pointed out that the triangular cord sign may initially be negative if the patient is very young and possibly become positive later. He reached this conclusion from the observation of a 22-dayold patient who showed a negative triangular cord sign at the initial US and then developed a positive triangular cord sign 2 weeks later.

In our study, group A showed a significantly smaller triangular cord thickness than group B, and two patients (2/5, 40%) in group A had a negative triangular cord sign, while all patients in group B had a positive triangular cord sign. This may be related to the suggestion of Saxena et al. [38] since the patient age at the time of US was significantly lower in group A than in group B. In particular, the two patients with a negative triangular cord sign were of the age of 20 days and 27 days, respectively, at the time of US, and were considerably younger than the others with a positive triangular cord sign.

According to Tan Kendrick et al. [27], all three findings of the “gallbladder ghost triad” were constantly positive in 97% of the infants with biliary atresia and showed no false positives. Recently, several studies have also demonstrated that the small size and the abnormal morphology of the gallbladder on US were useful in distinguishing biliary atresia from neonatal hepatitis [1,18,30-32]. According to our results, the gallbladder length was significantly greater in group A than in group B. The use of the cut-off value of the atretic gallbladder of 15 mm (length) [22] revealed that all five patients (5/5, 100%) in group A and three patients (3/13, 23%) in group B were false negative. The evaluation of the gallbladder shape was helpful in only one patient (1/5, 20%) of group A and five patients (5/13, 38%) of group B. These results are quite inconsistent with those of the previous studies on the US features of the gallbladder, particularly in group A. One possible explanation is that the gallbladder length and shape might be within normal limits at an early stage of the disease and become affected as the disease progresses, considering the progressive nature of biliary atresia and the younger age of group A.

If the gallbladder contracts after an oral feeding, biliary atresia is generally considered unlikely [1,39]. However, several authors have already reported that normal gallbladder contraction was seen after an oral feeding in patients with type IIIa biliary atresia [33,34]. In our study, three patients in group A and one patient in group B showed normal gallbladder contraction after an oral feeding. In the remaining 14 patients, whether the gallbladder contracted normally after an oral feeding was not evaluated.

Lee et al. [23] defined a hepatic subcapsular flow as a hepatic arterial flow signal continuing to the liver capsular surface on color Doppler US and suggested it to be a diagnostic US finding of biliary atresia with high sensitivity (100%) and relatively low specificity (86%). Recently, El-Guindi et al. [29] supported their findings with a sensitivity of 96.3% and a specificity of 96.3%. In our study, the subcapsular flow was positive in all patients of both groups and was the only finding suggestive of biliary atresia in one patient of group A. Therefore, a hepatic subcapsular flow can be used to predict biliary atresia even when CBD is visible on US. Furthermore, when a hepatic subcapsular flow is not evident, the possibility of biliary atresia might be greatly lowered.

It is well known that conjugated hyperbilirubinemia cannot reliably discriminate between biliary atresia and neonatal hepatitis [40]. Total bilirubin, direct bilirubin, and γ-GT were increased in all patients of both groups, and alkaline phosphatase was increased in three patients of group A and ten patients of group B. However, laboratory data in our study were not statistically different between the two groups.

Although pathological liver fibrosis grades were not statistically different between the two groups, a tendency towards the advanced grades of liver fibrosis was noted in group B. This result is consistent with the progressive nature of biliary atresia in causing hepatic fibrosis, liver cirrhosis, and eventual liver failure sequentially unless appropriate treatment is given [2].

Several limitations of our study should be acknowledged. First, the sample size was small, and the sample distribution was uneven (group A: n=5 vs. group B: n=13), lowering the reliability of our data. Further studies with larger samples are needed to support our results. Second, since it was retrospective study, the pathological correlation of the gallbladder and the CBD with their US findings could not be established. In order to understand the exact underlying mechanisms of the abnormal gallbladder morphology and the visibility of the CBD on the US, a correlation between the US images and the pathological results should be established in the future.

In conclusion, CBD can be either visible or invisible on US in patients diagnosed with type IIIa biliary atresia. When CBD was visible, other US features with respect to the triangular cord thickness and the gallbladder length and shape tended to more often show false negative results than when it was invisible. As the subcapsular flow on the color Doppler US was always noted in patients diagnosed with biliary atresia, Kasai type IIIa, a color Doppler study should be included in the routine US examination of neonatal cholestasis patients.