The bile duct carries bile from the liver to the intestine. Bile helps with digestion. The bile duct is partially imaged in ultrasound tests. The technologist who performs then ultrasound test will obtain a measurement of a segment of the bile duct. The bile duct is called the common hepatic duct above where the cystic duct or gallbladder joins it. It is called the common bile duct below.
A normal measurement is usually below 6 millimeters. The bile duct can be dilated or above 6 mm for a variety of reasons. Usually this alerts us that something may be wrong. That there may be a blockage of the duct or tube. However, this is not always the case.
Sometimes a dilated bile duct on ultrasound can be seen after a cholecystectomy or gallbladder removal. Elderly patients can also have a bile duct which is slightly more dilated than 6 mm and be normal. Especially if it is not causing any problems or lab test abnormalities. A slightly dilated bile duct can also be because of the way it was measured by the technologist. The radiologist interpreting the test may measure it himself.
A dilated bile duct can mean that there is a blockage. A blockage of the bile duct can be from a stone. Usually this passes from the gallbladder which is connected to the bile duct. This will cause the duct to dilate. Patients will have pain and blood work abnormalities in many cases. Although the ultrasound often does not show the obstructing stone in the bile duct, an MRI can be done to look at the bile duct for diagnosis. A specially trained gastroenterologist may perform an ERCP to remove the stone.
A dilated bile duct on ultrasound can be seen because of mass or narrowing of the bile duct itself. This is called cholangiocarcinoma and is a cancer. Often the bile ducts above the mass will be dilated or widened. A cancer of the pancreas can cause a dilated bile duct. The common bile duct passes through the pancreas. A mass in the head of the pancreas will obstruct it or cause the bile duct to be dilated. A cancer next to the pancreas called a periampullary tumor may also cause obstruction and dilation of the duct.
Other causes of dilation of the bile duct on ultrasound can be inflammatory conditions of the bile ducts called cholangitis which can occur from a variety of causes. Pregnancy and some drugs can cause the duct to dilate. Abnormal narrowing or function of the sphincter where the common bile duct enters the intestine can result in dilation of the common bile duct. Cysts of the ducts called choledochal cysts can cause dilation as well.
Dilated bile duct on ultrasound has many causes as detailed above. Sometimes the cause will not be apparent from the ultrasound alone. Your doctor will take a careful history and do a physical exam. Blood work is also important. Any prior imaging tests are important as well. A change or more dilation becomes more significant. Further testing is also often needed for diagnosis. This may include a special MRI called an MRCP to look at the bile ducts. Some patients will need a more invasive procedure called an ERCP done by a specialized gastroenterologist for diagnosis and treatment.