The first step to finding the CBD is to identify the portal vein as this is the largest of the 3 structures in the triad. You can identify these structures in either the long or short axis. It is often helpful to use the gallbladder neck as a landmark since it will point towards the triad (as seen above). Once you identify the portal vein, apply color doppler or power doppler to identify the portal vein and hepatic artery. The CBD will be the tubular structure superficial to the portal vein in long axis view, or the circular structure in short axis view, without color flow (see image above). 85% of the time the CBD will be the right "ear" on the short axis view but this emphasizes the necessity of color doppler to confirm you are measuring the proper structure.

Essentially what you are looking for in the CBD is dilation. Pathologically, this is usually due to choledocholithiasis, but pancreatic masses and autoimmune disorders can also cause this finding. As we mentioned earlier, normal internal diameter is 2-6 mm (around 1mm/decade of life), and greater than 1 cm is highly sensitive for biliary duct obstruction. However if you do find a CBD that is 7mm and the patient has clinical signs of obstruction to may want to consider consultation.

Is this cholecystitis?

Now that you have evaluated for gallstones, assessed the gallbladder wall and the CBD, you need to decided whether your patient needs to see a surgeon.

Approximately 85% of patients with cholecystitis will have gallstones (5-14% are acalculous and usually seen in the critically ill) so the lack of gallstones will make the diagnosis less likely.

Gallbladder wall thickening is specific for cholecystitis but not sensitive, so if you don't see it you cannot rule it out. On the other hand if they do have it you should be very suspicious. (caveat to follow shortly)

Pericholecystic fluid is not that helpful (similar to a coin flip) but if seen it may push you towards your diagnosis of cholecystitis

Sonographic Murphy's sign is a very helpful finding (pain when ultrasound probe pressing directly over gallbladder) and this finding with the presence of gallstones has a sensitivity that approaches 90% but has poor specificity. If you have RUQ pain, fever, and elevated WBC count, the specificity of this sign increases significantly to 87%.

If you have a + sonographic Murphy's sign & gallbladder wall thickening, acute cholecystitis should be on the top of your differential.

There are always caveats. Be sure that when you are evaluating for cholecystitis by ultrasound to not get too honed in on a single finding. Take into account the clinical picture and your patient's other co-morbidities that can skew your findings. Here are just a few conditions that can lead to gallbladder wall thickening in the absence of cholecystitis: