Hepatobiliary Ultrasound: Distinguishing Normal from Abnormal

I. Introduction

The hepatobiliary system, comprising the liver, gallbladder, and bile ducts, is a cornerstone of abdominal physiology and a frequent site of pathology. In the diagnostic imaging arsenal, ultrasound stands as the primary, first-line modality for its evaluation. Its non-invasive nature, absence of ionizing radiation, real-time imaging capability, and relative cost-effectiveness make it an indispensable tool for clinicians. The primary goal of a hepatobiliary ultrasound examination is to provide a clear, accurate distinction between normal anatomical structures and the myriad of pathological conditions that can affect this system. This differentiation is critical for guiding patient management, from confirming a benign diagnosis to prompting urgent intervention. While advanced cross-sectional imaging like thoracic spine MRI is pivotal for neurological and musculoskeletal assessments, the dynamic, bedside applicability of the ultrasound hepatobiliary system examination secures its unique and irreplaceable role in abdominal medicine. In Hong Kong, with its specific disease burdens such as high prevalence of hepatitis B and associated liver complications, the accurate performance and interpretation of these ultrasounds are of paramount public health importance.

II. Liver

A systematic evaluation of the liver is the cornerstone of any hepatobiliary ultrasound. The normal liver parenchyma exhibits a homogeneous, medium-level echogenicity, slightly more echogenic (brighter) than the renal cortex and less echogenic than the spleen. Its shape is generally triangular, with smooth contours. Normal liver size is assessed by measuring the craniocaudal length in the midclavicular line; a measurement exceeding 15-16 cm in adults is typically considered hepatomegaly, though body habitus must be considered.

Abnormal findings are diverse. Hepatomegaly can be a non-specific sign of numerous conditions, from congestion in heart failure to infiltrative disorders. Altered parenchymal echogenicity provides more specific clues. In fatty liver (hepatic steatosis), the liver becomes diffusely hyperechoic (brighter) compared to the renal cortex, with possible posterior beam attenuation. This is a significant finding in Hong Kong, where non-alcoholic fatty liver disease (NAFLD) prevalence is estimated to be around 27-30% in the general adult population, closely linked to rising rates of obesity and metabolic syndrome. Cirrhosis presents a more complex picture: the liver may be shrunken or enlarged initially, with a nodular surface, coarse and heterogeneous echotexture, and signs of portal hypertension. Focal liver masses require meticulous characterization. Simple cysts are anechoic with posterior acoustic enhancement. Hemangiomas, the most common benign liver tumor, are typically well-defined, hyperechoic masses. Malignant lesions, such as hepatocellular carcinoma (HCC)—highly prevalent in hepatitis B-endemic regions like Hong Kong—often appear as a heterogeneous, hypoechoic mass with possible vascular invasion. Doppler assessment of internal vascularity is crucial in this differentiation.

III. Gallbladder

The normal gallbladder is a pear-shaped, fluid-filled structure located in the gallbladder fossa on the inferior surface of the liver. Its lumen is anechoic (black), and its wall is thin, smooth, and measures less than 3 mm in thickness when properly distended. The sonographer must ensure the patient has fasted for at least 6-8 hours to achieve adequate distension for accurate assessment.

Pathological findings are common and often symptomatic. Gallstones appear as echogenic, mobile foci within the lumen, casting sharp acoustic shadows. They may be solitary or numerous. Sludge, a mixture of bile particulates, appears as low-level, non-shadowing echoes that layer in the dependent portion of the gallbladder. Gallbladder wall thickening (>3mm) is a non-specific sign seen in conditions ranging from acute or chronic cholecystitis (often accompanied by tenderness on transducer pressure—the sonographic Murphy's sign) to systemic conditions like heart failure, hepatitis, or hypoalbuminemia. In acute cholecystitis, additional findings like pericholecystic fluid (a rim of fluid around the gallbladder) and wall striations (alternating hyper- and hypoechoic layers) may be present. In Hong Kong, dietary patterns contribute to a significant burden of gallstone disease, making its detection a routine yet critical part of the ultrasound examination.

IV. Biliary Tree

The biliary tree, responsible for transporting bile from the liver to the duodenum, is meticulously evaluated. The normal intrahepatic bile ducts are typically not visible or are seen as tiny, thread-like structures accompanying portal venous branches. The key measurement is the common bile duct (CBD) diameter, taken in the proximal portion. A general rule is that the CBD diameter should be less than 6 mm, with an allowance of approximately 1 mm per decade after age 60, and up to 8-10 mm post-cholecystectomy.

Bile duct dilatation is the primary abnormal finding, indicating possible obstruction. The "double-barrel shotgun" sign, where dilated intrahepatic ducts run parallel to portal vein branches, is a classic indicator of intrahepatic biliary obstruction. CBD dilatation can be caused by stones (choledocholithiasis), tumors (e.g., pancreatic head carcinoma, cholangiocarcinoma), or benign strictures. Choledocholithiasis, or stones within the CBD, appear as echogenic, shadowing foci within the dilated duct. It is crucial to trace the duct to its point of caliber change to identify the cause of obstruction. While ultrasound is excellent for detecting ductal dilation, its sensitivity for distal CBD stones is limited by overlying bowel gas. In such cases, or when a complex obstruction like a tumor is suspected, further imaging with MRCP (Magnetic Resonance Cholangiopancreatography) or endoscopic ultrasound is warranted. It is worth noting that while a thoracic spine MRI is the gold standard for evaluating spinal cord compression or metastatic disease to the vertebrae, the ultrasound hepatobiliary system exam remains the first step in diagnosing biliary obstruction, a completely different clinical pathway.

V. Doppler Ultrasound of Hepatobiliary Vessels

Doppler ultrasound adds a functional dimension to the anatomical assessment, evaluating blood flow within the liver's major vessels. Normal waveforms are distinct:

• Portal Vein: Exhibits a continuous, mildly undulating hepatopetal (towards the liver) flow.
• Hepatic Artery: Shows a low-resistance waveform with continuous diastolic flow.
• Hepatic Veins: Display a triphasic waveform reflecting right atrial pressure changes during the cardiac cycle.

Abnormal Doppler findings are diagnostically powerful. Portal hypertension, a major complication of cirrhosis, manifests as dilatation of the portal vein (>13 mm), decreased flow velocity, loss of the normal undulations, and possible hepatofugal (away from the liver) flow. Collateral vessels (varices) may also be visible. Hepatic vein thrombosis (Budd-Chiari syndrome) is characterized by the absence of flow or the presence of thrombus within the hepatic veins, with dampened or monophasic waveforms. Cavernous transformation of the portal vein, where the portal vein is replaced by a tangle of collateral vessels due to chronic thrombosis, is another key Doppler finding. In Hong Kong, where chronic hepatitis B is a leading cause of cirrhosis, Doppler assessment for signs of portal hypertension is a routine and essential component of surveillance ultrasound examinations in these patients.

VI. Clinical Significance

The true value of hepatobiliary ultrasound lies in the correlation of imaging findings with the patient's clinical presentation. A finding of gallstones in an asymptomatic patient (incidentaloma) may only require watchful waiting, whereas the same finding in a patient with right upper quadrant pain and fever points directly to acute cholecystitis requiring urgent intervention. Similarly, the discovery of a simple liver cyst in a patient with abdominal fullness is reassuring, while a new, solid liver mass in a patient with known cirrhosis and rising alpha-fetoprotein levels is highly suspicious for hepatocellular carcinoma and demands immediate further action.

Ultrasound often serves as a triage tool. Its findings dictate the next steps in the diagnostic pathway. For instance, a dilated CBD with an obvious stone may lead directly to ERCP (Endoscopic Retrograde Cholangiopancreatography) for stone extraction. An indeterminate complex liver mass or a suspected pancreatic head mass causing biliary obstruction would necessitate further cross-sectional imaging. Contrast-enhanced CT or MRI of the abdomen provides superior anatomical detail, tissue characterization, and staging information. Specifically, MRI with MRCP is unparalleled for detailed biliary tree mapping. It is important to recognize the complementary roles of different modalities; a thoracic spine MRI would be ordered for back pain with neurological symptoms, while a complex hepatobiliary problem identified on the ultrasound hepatobiliary system scan would typically lead to an abdominal MRI or CT.

VII. Conclusion

Mastering the interpretation of hepatobiliary ultrasound is a fundamental skill for radiologists and sonographers. The ability to reliably distinguish normal anatomy from subtle or overt pathology directly impacts patient outcomes, enabling timely diagnosis and appropriate management. In a high-volume clinical setting like Hong Kong's healthcare system, efficiency and accuracy are crucial. This requires not only a deep understanding of sonographic principles and anatomy but also a knowledge of local epidemiological patterns, such as the high prevalence of hepatitis B and NAFLD. The field is dynamic, with ongoing advancements in ultrasound technology like elastography for liver fibrosis staging and contrast-enhanced ultrasound for lesion characterization. Therefore, continuous learning and skill development through case review, interdisciplinary collaboration, and staying abreast of guidelines are imperative for any practitioner involved in hepatobiliary imaging, ensuring the ultrasound hepatobiliary system examination remains a powerful, precise, and patient-centered diagnostic pillar.