Residency · Residency · Diagnostic Radiology
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.
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.
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.
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
- ACR Appropriateness Criteria: Right Upper Quadrant Pain. American College of Radiology, 2023.
- Rumack CM, Levine D. Diagnostic Ultrasound. 5th ed. Elsevier; 2017.
- Yokoe M, et al. Tokyo Guidelines 2018: diagnostic criteria and severity grading of acute cholecystitis. J Hepatobiliary Pancreat Sci. 2018;25(1):41-54.
- 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.