# Biliary Imaging: Cholelithiasis, Choledocholithiasis, and Cholangiocarcinoma

## Overview

### Clinical Significance

Biliary disease is one of the most common reasons for abdominal imaging. Ultrasound is the first-line modality for gallbladder evaluation, while MRCP is the gold standard non-invasive modality for bile duct pathology. Accurate imaging characterization guides medical, endoscopic, or surgical management.

### Anatomy Review

The right and left hepatic ducts converge to form the common hepatic duct (CHD). The cystic duct joins the CHD to form the common bile duct (CBD), which courses through the pancreatic head and enters the duodenum at the ampulla of Vater. The normal CBD diameter measures up to 6 mm, with the accepted upper limit increasing by 1 mm per decade after age 60 and extending up to 10 mm after cholecystectomy.

## Cholelithiasis

### Ultrasound Findings

The classic ultrasound appearance of gallstones is an **echogenic focus** within the gallbladder lumen with **posterior acoustic shadowing**. Stones demonstrate **gravity-dependent mobility**, moving with patient repositioning. The **wall-echo-shadow (WES) sign** is seen when a contracted gallbladder is packed with stones and produces a sequence of an echogenic line (wall), a thin anechoic rim, another echogenic line (stones), and then shadow. Sludge appears as low-level echoes that layer dependently without shadowing.

### Pitfalls

**Gallbladder polyps** are fixed to the wall, do not move with repositioning, and typically do not shadow unless they exceed 1 cm. A **porcelain gallbladder** has a calcified wall that may mimic stones and has a debated but possible association with gallbladder carcinoma. **Adenomyomatosis** produces wall thickening with Rokitansky-Aschoff sinuses that generate comet-tail artifacts.

## Acute Cholecystitis

### Ultrasound Findings

The primary findings include gallstones impacted in the gallbladder neck or cystic duct. The **sonographic Murphy sign**, defined as maximal tenderness with transducer pressure directly over the gallbladder, is the most specific ultrasound finding. Additional findings include gallbladder wall thickening exceeding 3 mm, pericholecystic fluid, and gallbladder distension (greater than 10 cm long axis or greater than 4 cm transverse).

### Complications on CT/US

**Gangrenous cholecystitis** is characterized by irregular or absent wall enhancement, intraluminal membranes, and mural gas. **Emphysematous cholecystitis** shows gas within the gallbladder wall or lumen, is associated with diabetes and Clostridium species, and constitutes a surgical emergency. **Perforation** manifests as a pericholecystic abscess and discontinuity of the gallbladder wall.

### Acalculous Cholecystitis

Acalculous cholecystitis accounts for 5-10% of acute cholecystitis cases and occurs predominantly in ICU patients, TPN-dependent patients, and immunosuppressed individuals. On ultrasound it presents as a distended gallbladder with wall thickening and pericholecystic fluid without stones. A HIDA scan shows non-filling of the gallbladder after 4 hours or with morphine augmentation.

## Choledocholithiasis

### Ultrasound

Ultrasound may demonstrate a dilated CBD (greater than 6 mm in most adults) and an echogenic focus within the CBD with or without shadowing. However, ultrasound sensitivity for CBD stones is limited at approximately 50-70% due to distal duct obscuration by duodenal gas.

### MRCP

MRCP is the gold standard non-invasive imaging modality for CBD stones. Stones appear as **low-signal filling defects** within the high-signal bile on T2-weighted images, with sensitivity exceeding 90% and specificity exceeding 95%. Its key advantage over ERCP is that it is non-invasive and carries no risk of post-ERCP pancreatitis.

### CT

Stones may be isodense to bile and missed on CT, which has only 75-85% sensitivity for choledocholithiasis. Dual-energy CT may improve detection of isoattenuating stones. Upstream biliary dilatation serves as an indirect sign of an obstructing stone.

## Mirizzi Syndrome

Mirizzi syndrome results from extrinsic compression of the CHD or CBD by a stone impacted in the gallbladder neck or cystic duct. It causes obstructive jaundice that can mimic choledocholithiasis or cholangiocarcinoma. On MRCP, a gallstone is visible in the gallbladder neck or cystic duct with narrowing of the adjacent CHD and upstream dilatation. The Csendes classification (Types I through IV) grades the condition based on the degree of fistula formation.

## Cholangiocarcinoma

### Classification

| Type | Frequency | Location | Key Imaging Feature |
|------|-----------|----------|-------------------|
| Intrahepatic (peripheral) | 10-20% | Liver parenchyma | Mass with peripheral enhancement, capsular retraction |
| Perihilar (Klatskin tumor) | 50-60% | Hepatic duct confluence | Stricture at confluence with upstream dilatation |
| Distal extrahepatic | 20-30% | Distal CBD | Distal CBD stricture, double-duct sign |

Cholangiocarcinoma is classified into three types. **Intrahepatic (peripheral)** cholangiocarcinoma presents as a mass-forming lesion within the liver parenchyma and is the second most common primary liver malignancy after HCC. **Perihilar cholangiocarcinoma (Klatskin tumor)** is the most common type, accounting for 50-60% of cases, and arises at the hepatic duct confluence. **Distal extrahepatic** cholangiocarcinoma arises in the distal CBD.

### Bismuth-Corlette Classification (Perihilar)

| Bismuth-Corlette Type | Extent of Tumor |
|----------------------|-----------------|
| I | Below the hepatic duct confluence |
| II | Reaches confluence; does not involve secondary intrahepatic ducts |
| IIIa | Extends into right hepatic duct |
| IIIb | Extends into left hepatic duct |
| IV | Involves both right and left hepatic ducts (multicentric/bilateral) |

The Bismuth-Corlette classification categorizes perihilar cholangiocarcinoma by extent. **Type I** is located below the hepatic duct confluence. **Type II** reaches the confluence but does not involve the secondary intrahepatic ducts. **Type IIIa** extends into the right hepatic duct. **Type IIIb** extends into the left hepatic duct. **Type IV** involves both right and left hepatic ducts and may be multicentric or bilateral.

### Imaging Features

**Intrahepatic** cholangiocarcinoma appears as a hypodense mass on CT with peripheral enhancement and delayed central enhancement due to its desmoplastic stroma. Capsular retraction and satellite nodules are characteristic. **Perihilar** cholangiocarcinoma produces an abrupt stricture at the confluence with upstream biliary dilatation and may show periductal thickening and enhancement. **Distal** cholangiocarcinoma causes a stricture of the distal CBD with upstream dilatation of the biliary tree and pancreatic duct, producing the "double-duct sign."

### MRI/MRCP

On MRI, intrahepatic cholangiocarcinoma appears as a T2 hyperintense mass. The periductal type shows ductal wall thickening and enhancement with progressive narrowing. MRCP demonstrates the level and extent of obstruction and aids in surgical planning. DWI shows restricted diffusion in viable tumor.

## Primary Sclerosing Cholangitis (PSC)

### Key Features

PSC is a chronic cholestatic liver disease characterized by multifocal strictures and dilatation of both intrahepatic and extrahepatic bile ducts. It has a strong association with inflammatory bowel disease, particularly ulcerative colitis in approximately 70-80% of cases. The lifetime risk of cholangiocarcinoma is 10-15%.

### MRCP Findings

MRCP demonstrates the characteristic **beaded appearance**, consisting of alternating strictures and dilatation of intrahepatic and extrahepatic ducts. The **pruned-tree appearance** reflects peripheral duct obliteration with central duct involvement. Periductal enhancement is seen on post-contrast imaging. Progressive disease leads to cirrhosis and portal hypertension.

### Surveillance

Annual MRCP and CA 19-9 are used for cholangiocarcinoma screening, though the optimal strategy remains debated. A new dominant stricture warrants ERCP with brushings to exclude superimposed cholangiocarcinoma.

<image>An ultrasound image of the gallbladder showing multiple echogenic foci (gallstones) within the gallbladder lumen. Each stone casts a clean posterior acoustic shadow. The gallbladder wall is normal in thickness (less than 3 mm). A second panel shows a longitudinal ultrasound of the common bile duct with a small echogenic focus within the duct lumen representing a choledocholithiasis, with mild upstream ductal dilatation. Arrows label the gallstones, acoustic shadows, CBD stone, and the dilated CBD.</image>

<image>An MRCP (heavily T2-weighted coronal image) showing the biliary tree in a patient with a perihilar cholangiocarcinoma (Klatskin tumor). The intrahepatic bile ducts are markedly dilated bilaterally. There is abrupt cutoff at the hepatic duct confluence where the tumor causes obstruction. The distal CBD is decompressed and normal in caliber. An inset diagram shows the Bismuth-Corlette classification Types I through IV with schematic illustrations of tumor extent at the confluence.</image>

<image>An MRCP image demonstrating the classic beaded appearance of primary sclerosing cholangitis. Multiple short-segment strictures alternate with mild segmental dilatation throughout both intrahepatic and extrahepatic bile ducts. The peripheral intrahepatic ducts show pruning. A comparison normal MRCP is shown alongside for reference. Arrows highlight representative strictures and dilated segments.</image>

## Clinical Pearls

The sonographic Murphy sign is the single most useful ultrasound finding for acute cholecystitis; its absence should prompt consideration of gangrenous cholecystitis, where the gallbladder is denervated. Choledocholithiasis is poorly detected on CT because many stones are isodense to bile, making MRCP the non-invasive study of choice when clinical suspicion is high. The "double-duct sign" (simultaneous dilatation of the CBD and pancreatic duct) localizes obstruction to the periampullary region and should raise concern for pancreatic head mass or distal cholangiocarcinoma. In PSC, a new dominant stricture or rapid clinical deterioration should prompt evaluation for superimposed cholangiocarcinoma. The Bismuth-Corlette classification of perihilar cholangiocarcinoma is essential for surgical planning and determines resectability.

## References

- Gore RM, Levine MS. *Textbook of Gastrointestinal Radiology*. 4th ed. Elsevier, 2015
- ACR Appropriateness Criteria: Jaundice, 2019
- Blechacz B, et al. "Clinical Diagnosis and Staging of Cholangiocarcinoma." *Nature Reviews Gastroenterology & Hepatology*, 2011
- ACR Appropriateness Criteria: Right Upper Quadrant Pain, 2018