A Dilated Common Bile Duct on Ultrasound, MRI or MRCP: What Does It Mean?

When an ultrasound, MRI or MRCP report says the common bile duct is “wide” or “dilated”, it means the main bile duct has been measured wider than expected.

This finding can sometimes be due to a stone, a narrowing or another blockage in the bile duct. But particularly at an older age, or in people whose gallbladder has been removed, the bile duct can look wide without any important blockage at all.

A dilated bile duct is not on its own a diagnosis.

The most important question is not only “how many millimetres is the bile duct?” but “why is it wide?”

What is the common bile duct?

The common bile duct is the main bile passage that carries bile — produced in the liver and stored in the gallbladder — to the duodenum.

It runs close to the head of the pancreas and, where it opens into the bowel, is closely related to the pancreatic duct. So a dilated bile duct can relate not only to the gallbladder but to the bile ducts, the papilla and some conditions of the pancreas.

What should the bile duct diameter normally be?

It is not right to give a single fixed number that applies to everyone.

The bile duct diameter can vary with:

• Age
• Where the measurement is taken
• The imaging method used
• Whether the gallbladder is present
• Previous operations
• The person’s clinical features

On ultrasound in adults, diameters up to about 6 mm are often taken as normal. But some increase in bile duct diameter can be seen as age advances. In people whose gallbladder has been removed, the bile duct can be wider.

So a bile duct of 7–8 mm may be a finding that needs investigating in one person, while in another it may be consistent with their age and a previous gallbladder operation.

The bile duct diameter is not assessed on its own.

Why does the bile duct widen?

The causes can be divided roughly into two groups.

1. Causes that obstruct the flow of bile

If something is obstructing the flow of bile into the bowel, the duct can widen above it. The important causes are:

A stone in the bile duct — a stone that has dropped from the gallbladder into the main bile duct can block it. This is called choledocholithiasis. It is one of the important causes of a dilated bile duct.

Narrowing of the bile duct — a stricture can develop in the bile duct from benign or malignant causes. Previous operations, inflammation, chronic pancreatitis and some diseases of the bile ducts can cause narrowing.

Conditions in the region of the head of the pancreas — because the bile duct passes close to the head of the pancreas, some conditions in this region can press on the duct or disturb its flow. This is why, when a dilated bile duct is found together with jaundice in particular, the region of the head of the pancreas is assessed carefully.

Conditions of the papilla and ampulla — the bile duct opens into the bowel at the papilla (the papilla of Vater). Narrowing or other conditions in this region can affect the flow of bile.

Tumours of the bile ducts — some tumours of the bile ducts can cause a narrowing in the duct and widening above it.

2. Can the bile duct widen without a blockage?

Yes. This is a very important point. Not every wide bile duct has a stone, a tumour or a stricture.

Widening without a blockage can be seen particularly:

• At an older age
• In people whose gallbladder has been removed
• In people who have previously passed a stone through the bile duct
• In people taking certain medicines

So a dilated bile duct in a person with no symptoms, normal liver tests and no other abnormality on imaging is not judged in the same way as one in a person with jaundice and abnormal liver tests.

Why can the bile duct be wide after the gallbladder has been removed?

After a gallbladder operation (cholecystectomy), some people show an increase in bile duct diameter. This does not always mean the bile duct is blocked.

In a person with no symptoms, with normal bilirubin/ALP/GGT and no stone or mass on imaging, mild widening can be assessed differently in clinical terms.

But if there is newly developed marked widening or accompanying abnormal findings, the cause is looked into.

Which blood tests matter with a dilated bile duct?

The following are assessed in particular:

• Total and direct bilirubin
• ALP
• GGT
• ALT
• AST

A dilated bile duct + raised bilirubin + raised ALP/GGT more strongly suggests the likelihood of a disturbance or blockage in the flow of bile. By contrast, entirely normal laboratory tests can take the assessment in a different direction.

Which symptoms can suggest a blocked bile duct?

When the flow of bile is disturbed, there may be:

• Yellowing of the eyes and skin
• Dark urine
• Paling of the stool
• Pain in the right upper or upper abdomen
• Itching
• Nausea and vomiting

But in some people a bile duct stone or stricture may not cause obvious symptoms at first.

Why do a dilated bile duct and fever matter?

What is done if the bile duct is wide on ultrasound?

Ultrasound is a very useful first investigation for assessing the bile ducts.

But the whole of the bile duct cannot always be seen adequately on ultrasound. Gas in the bowel in particular can make the lower part of the duct and the area around the pancreas difficult to assess. So further investigation is carried out where needed.

What is MRCP and why is it requested?

MRCP (magnetic resonance cholangiopancreatography) is a special MRI method for imaging the bile and pancreatic ducts in detail.

MRCP can assess:

• Stones in the bile ducts
• Widening of the ducts
• Strictures
• The anatomy of the bile ducts
• The pancreatic duct

MRCP is a diagnostic imaging method; it is not an endoscopic treatment procedure like ERCP.

Why might EUS be done?

Endoscopic ultrasound (EUS) can be quite useful particularly for assessing the lower part of the bile duct, the papilla and the pancreas in detail.

Where a dilated bile duct has no explanation on ultrasound, CT or MRCP, EUS may be needed in some patients. It can be used particularly to assess small bile duct stones and the pancreas–ampulla region.

Not every dilated bile duct needs an EUS.

Is ERCP needed?

An ERCP is not done for every wide bile duct. This is an important distinction.

ERCP is not a method used routinely today for imaging alone; it matters particularly where treatment is needed. It can be used for:

• Removing a stone from the bile duct
• Treating a narrowing of the bile duct
• Restoring the flow of bile
• Placing a stent where needed

So the approach “the bile duct is wide → an ERCP is automatically done” is not correct.

Is a dilated bile duct a sign of cancer?

Not on its own.

Some tumours of the pancreas, the ampulla or the bile ducts can cause a dilated bile duct. But a dilated bile duct has many other causes, including stones, previous operations, age-related changes and benign strictures.

Assessment becomes more important particularly where there is:

• Newly developed jaundice
• Unexplained weight loss
• Loss of appetite
• Accompanying clinical findings such as newly diagnosed diabetes
• Rises in bilirubin and cholestasis tests
• A suspicious lesion on imaging

What does “the intrahepatic bile ducts are also dilated” mean?

The small bile ducts within the liver are called the intrahepatic bile ducts.

Widening of both the common bile duct and the intrahepatic ducts can be seen where the flow of bile is obstructed at a lower level. But again the cause has to be determined together with imaging and laboratory results.

What does it mean if both the bile duct and the pancreatic duct are wide?

Where the bile duct and the pancreatic duct are dilated together (the “double-duct sign”), the head of the pancreas and the ampullary region in particular should be assessed carefully.

This finding can be seen in some tumours; but it is not on its own a diagnosis of cancer and can be associated with harmless causes too. Depending on the clinical features, MRI/MRCP, EUS or other further investigations may be needed.

Why does comparing with old investigations matter?

Previous ultrasound, CT or MRI results are very valuable in assessing a dilated bile duct.

Mild widening that has stayed unchanged for years is not the same as widening that is new and increasing. This is why it is useful to keep your old imaging reports and, where possible, the images themselves.

When is urgent assessment needed?

What is done when a dilated bile duct is found?

1. First, do not panic. A dilated bile duct does not on its own mean cancer or a serious blockage.
2. The diameter and previous investigations are looked at. Whether it is new matters.
3. Whether the gallbladder has been removed is assessed. The bile duct can be wider after a cholecystectomy.
4. Bilirubin, ALP and GGT are assessed particularly. They give important clues about a disturbance in the flow of bile.
5. Whether there is jaundice, fever or pain is checked. If these are present, assessment is brought forward.
6. Further imaging is done where needed. Methods such as MRCP or EUS may be used.
7. ERCP is done only in patients who need it. Not every dilated bile duct needs an ERCP.

Remember

• A DILATED BILE DUCT Does not on its own mean a blocked bile duct.
• A BILE DUCT STONE Is one of the important causes.
• AGE Can affect the bile duct diameter.
• A GALLBLADDER OPERATION Can leave the bile duct looking wider afterwards.
• BILIRUBIN + ALP + GGT Matter in assessing a dilated bile duct.
• MRCP / EUS Can provide further assessment in selected patients where the cause is unexplained.
• ERCP Is not needed for every wide bile duct; it is used particularly where treatment is needed.
• JAUNDICE + FEVER/SHIVERING + RIGHT UPPER ABDOMINAL PAIN May call for urgent assessment.

The most important message

A dilated bile duct is an imaging finding; it is not on its own the name of a disease.

What it means is determined by weighing together the duct diameter, age, whether the gallbladder is present, bilirubin/ALP/GGT, the patient’s symptoms, previous imaging and any necessary further investigation such as MRCP or EUS.