# Abdominal Ultrasound: Liver, Gallbladder, and Biliary System

## Introduction

Abdominal ultrasound is the first-line imaging modality for evaluation of the **liver, gallbladder, and biliary system**. It is widely available, portable, radiation-free, and provides real-time dynamic assessment. Competent sonographic evaluation of the right upper quadrant is a fundamental skill for radiology residents.

## Normal Sonographic Anatomy

### Liver

The liver has a homogeneous echotexture that is **isoechoic or slightly hyperechoic** relative to the renal cortex. Portal veins have echogenic walls while hepatic veins do not. The **Couinaud segmental anatomy** divides the liver into eight segments based on hepatic and portal venous anatomy. Normal liver span is less than **16 cm** in the midclavicular line.

### Gallbladder

The gallbladder is thin-walled (less than **3 mm**) with an anechoic lumen when fasting. It is located in the gallbladder fossa on the undersurface of segments IVb and V. It measures up to **10 cm** in length and **4 cm** in transverse diameter when distended.

### Biliary System

The **common bile duct (CBD)** normally measures less than **6-7 mm** in diameter (up to 8-10 mm post-cholecystectomy or in the elderly). Intrahepatic bile ducts run parallel to portal veins and are normally not visible when non-dilated. The **parallel channel sign** describes dilated intrahepatic ducts alongside portal veins, also known as the "double barrel shotgun" sign.

![Right upper quadrant ultrasound showing normal liver echotexture, gallbladder, and portal triad](images/normal-ruq-ultrasound.jpg)

## Gallbladder Pathology

### Cholelithiasis

Gallstones appear as an **echogenic focus** with posterior acoustic shadowing and gravitational dependence. Ultrasound sensitivity exceeds **95%** for stones larger than 2 mm. Stones may be mobile or impacted in the gallbladder neck (Hartmann pouch).

### Acute Cholecystitis

The **sonographic Murphy sign** is the most specific finding and refers to maximum tenderness with transducer pressure over the gallbladder. Additional findings include gallbladder wall thickening **>3 mm** and pericholecystic fluid. A gallstone impacted in the neck or cystic duct is typically present. Complicated forms include gangrenous cholecystitis (irregular wall, intraluminal membranes), emphysematous cholecystitis (intramural gas), and perforation.

### Gallbladder Polyps

Polyps appear as a fixed, non-shadowing echogenic focus projecting from the wall. Lesions **>10 mm** or showing growth warrant cholecystectomy due to malignancy risk. Cholesterol polyps are the most common type and are typically small, multiple, and pedunculated.

## Liver Pathology

### Diffuse Liver Disease

**Hepatic steatosis** manifests as increased echogenicity relative to the renal cortex with posterior beam attenuation. **Cirrhosis** shows surface nodularity, coarsened echotexture, caudate lobe hypertrophy, and signs of portal hypertension.

### Focal Liver Lesions

| Lesion | US Appearance | Key Feature |
|--------|--------------|-------------|
| Simple cyst | Anechoic, thin-walled, posterior enhancement | No further workup needed |
| Hemangioma | Well-defined, hyperechoic, homogeneous | Most common benign hepatic tumor |
| HCC | Variable echogenicity | Look for portal vein thrombus, cirrhotic liver |
| Metastases | Variable (target, hyperechoic, cystic, calcified) | Often multiple, correlate with primary |

A **simple cyst** is anechoic, thin-walled, and shows posterior enhancement; no further workup is needed. **Hemangioma** is well-defined, hyperechoic, and homogeneous, and is the most common benign hepatic tumor. **HCC** has variable echogenicity, and portal vein thrombus and a cirrhotic background should be noted. **Metastases** have variable appearance, including target, hyperechoic, cystic, or calcified patterns.

![Ultrasound demonstrating cholelithiasis with posterior acoustic shadowing and gallbladder wall thickening consistent with acute cholecystitis](images/acute-cholecystitis-us.jpg)

## Biliary Obstruction

### Findings

**Dilated intrahepatic ducts** produce the parallel channel sign and too-many-tubes sign. The **dilated CBD** is measured inner wall to inner wall at the portal vein crossing. The transition point from dilated to non-dilated duct should be identified. Common causes include choledocholithiasis, pancreatic head mass, cholangiocarcinoma, and ampullary neoplasm.

### Choledocholithiasis

An echogenic focus within the CBD with or without posterior shadowing indicates a CBD stone. Ultrasound sensitivity is limited (approximately **50-70%**) for distal CBD stones due to bowel gas. MRCP or EUS is recommended when clinical suspicion is high despite negative ultrasound.

![Longitudinal ultrasound of the common bile duct showing dilation with an echogenic calculus and posterior shadowing](images/choledocholithiasis-us.jpg)

## Technical Considerations

The patient should be **fasting for 6-8 hours** for optimal gallbladder distention. Left lateral decubitus and upright positions demonstrate stone mobility. A **curvilinear (3-5 MHz)** transducer is used for standard evaluation. Color Doppler assesses hepatic vascularity, portal vein patency, and flow direction.

## Key Clinical Pearls

The **sonographic Murphy sign** is the most specific US finding for acute cholecystitis; always report its presence or absence. Gallbladder wall thickening is nonspecific, with a differential that includes hepatitis, CHF, hypoalbuminemia, and ascites. A dilated CBD without identifiable cause on US should prompt **MRCP or EUS**. In the cirrhotic liver, every new focal lesion should be evaluated using the **LI-RADS** classification.

## References

1. ACR Appropriateness Criteria: Right Upper Quadrant Pain. American College of Radiology, 2023.
2. Rumack CM, Levine D. *Diagnostic Ultrasound*. 5th ed. Elsevier; 2017.
3. Yokoe M, et al. Tokyo Guidelines 2018: diagnostic criteria and severity grading of acute cholecystitis. *J Hepatobiliary Pancreat Sci*. 2018;25(1):41-54.
4. Harvey RT, Miller WT. Acute biliary disease: initial CT and follow-up US versus initial US and follow-up CT. *Radiology*. 1999;213(3):831-836.