Hepatobiliary Ultrasound: A Guid...

I. Introduction: The Importance of Understanding Normal Anatomy

In the realm of diagnostic imaging, the ability to distinguish the normal from the abnormal is the cornerstone of accurate interpretation. This principle is paramount in hepatobiliary ultrasound, a primary, non-invasive, and real-time modality for evaluating the liver, gallbladder, and biliary system. A profound understanding of standard anatomical architecture is not merely academic; it is the essential baseline against which all pathology is measured. Without this foundational knowledge, common anatomic variants can be mistaken for disease, leading to unnecessary patient anxiety, further costly investigations, or, conversely, significant pathology can be overlooked if dismissed as a normal variant. The hepatobiliary ultrasound examination is often a first-line investigation for symptoms like right upper quadrant pain, jaundice, or abnormal liver function tests. Its effectiveness hinges entirely on the sonographer's and radiologist's expertise in navigating the complex and sometimes variable landscape of the upper abdomen. For instance, a thorough knowledge of normal vascular patterns is crucial when assessing for portal hypertension or Budd-Chiari syndrome. Similarly, recognizing the typical size and contour of the gallbladder prevents misdiagnosis of conditions like adenomyomatosis or chronic cholecystitis. This guide aims to provide a detailed roadmap of normal hepatobiliary anatomy and its common variations as seen on ultrasound, empowering practitioners to make confident and accurate assessments. It is worth noting that while this article focuses on ultrasound, correlative imaging plays a vital role in complex cases. For example, a patient presenting with back pain and an incidental liver finding on a thoracic spine mri may require targeted follow-up with an ultrasound hepatobiliary system examination for a more dynamic and detailed assessment of the liver parenchyma and biliary structures.

II. Normal Anatomy of the Liver on Ultrasound

The liver is the largest solid organ in the abdomen and serves as the central subject of the hepatobiliary ultrasound. A systematic approach to its evaluation is critical.

A. Lobes and Segments

Anatomically, the liver is divided into right and left lobes by the principal plane, an imaginary line connecting the gallbladder fossa anteriorly to the inferior vena cava (IVC) posteriorly. This division is more than superficial; it is crucial for surgical planning and lesion localization. The Couinaud classification, based on hepatic venous and portal venous anatomy, further subdivides the liver into eight functionally independent segments (I-VIII), each with its own vascular inflow, outflow, and biliary drainage. On ultrasound, these segments are not demarcated by visible lines but are understood conceptually using key landmarks: the middle hepatic vein (MHV) separates the right and left lobes, the right hepatic vein (RHV) divides the right lobe into anterior and posterior sectors, and the left hepatic vein (LHV) divides the left lobe into medial and lateral segments. The caudate lobe (segment I) is a distinct area situated posteriorly, between the IVC and the fissure for the ligamentum venosum.

B. Liver size and echogenicity

Liver size is typically assessed subjectively by its span. A common measurement is the midclavicular line (MCL) span, where a normal liver measures less than 15-16 cm in length. In Hong Kong, a study on local populations suggested normal MCL ranges are consistent with these international standards. The liver parenchyma should have a homogeneous, fine-textured echogenicity. It serves as the reference standard for abdominal organ echogenicity: the liver is slightly more echogenic (brighter) than the renal cortex and significantly less echogenic than the pancreas. Increased hepatic echogenicity, often described as a "bright liver," is a key sign of fatty infiltration (steatosis), a condition with a notable prevalence in urban populations like Hong Kong due to dietary and lifestyle factors.

C. Hepatic vasculature (portal vein, hepatic veins, hepatic artery)

The hepatic vasculature is the roadmap of the liver. The portal vein is the most prominent, carrying nutrient-rich blood from the intestines to the liver. Its walls are hyperechoic (bright) due to abundant fibrous tissue. The main portal vein enters the liver at the porta hepatis and divides into right and left branches. The hepatic veins drain deoxygenated blood from the liver into the IVC. Their walls are thin and often imperceptible unless pathological. They have a characteristic phasic flow pattern on Doppler, varying with the cardiac cycle and respiration. The three major hepatic veins (right, middle, left) converge towards the IVC in a "bundle of sticks" or "crow's foot" configuration, a key landmark. The hepatic artery, a branch of the celiac axis, is much smaller in caliber and is usually seen alongside the portal vein and common bile duct in the portal triad. Its presence is confirmed with color Doppler, showing pulsatile arterial flow. Understanding these vascular relationships is fundamental, for instance, when differentiating a dilated biliary duct (which runs parallel to the portal vein) from a hepatic artery branch.

III. Normal Anatomy of the Gallbladder and Biliary Tree on Ultrasound

The gallbladder and biliary tree form the conduit system for bile, and their evaluation is a central component of the ultrasound hepatobiliary system exam.

A. Gallbladder shape, size, and wall thickness

The normal gallbladder is a pear-shaped, fluid-filled sac located in the gallbladder fossa on the inferior surface of the liver. Its size is variable depending on fasting state; a distended, fasted gallbladder is optimal for examination. Typical dimensions are up to 10 cm in length and 4-5 cm in transverse diameter. The wall should be thin, smooth, and sharply defined, measuring less than 3 mm when properly distended. It appears as a bright, continuous echogenic line. Accurate measurement requires the transducer to be perpendicular to the wall to avoid artifactual thickening. Postprandially, the gallbladder contracts, and the wall may appear thicker and more irregular, which is a normal physiologic response.

B. Common bile duct (CBD) and cystic duct

The common bile duct (CBD) is formed by the union of the common hepatic duct and the cystic duct from the gallbladder. It courses anterior to the portal vein and to the right of the hepatic artery in the free edge of the lesser omentum. On ultrasound, it is seen as a tubular anechoic structure with echogenic walls, running parallel and anterior to the main portal vein—the "shotgun" sign. Its internal diameter is a critical measurement. Normal CBD diameter increases slightly with age and after cholecystectomy. A generally accepted upper limit of normal is 6 mm, though some sources allow up to 8 mm in elderly patients. In Hong Kong clinical practice, a diameter of <7 mm is often used as a conservative cutoff for a normal, non-dilated duct in a patient with an intact gallbladder. The cystic duct is usually not visualized in its entirety on routine ultrasound due to its small size and tortuous course.

C. Intrahepatic biliary ducts

The normal intrahepatic biliary radicles accompany the branches of the portal vein in the portal triads. In a healthy state, they are either not visible or are seen as tiny, thread-like structures with walls that are less echogenic than the adjacent portal vein walls. A fundamental rule is that the diameter of a normal intrahepatic duct should be less than 40% of the diameter of its accompanying portal vein branch. Visualization of clearly tubular structures parallel to the portal veins (the "double-barrel shotgun" or "parallel channel" sign) indicates ductal dilation, a sign of possible obstruction.

IV. Common Anatomic Variants and Their Ultrasound Appearance

Recognizing common variants prevents misinterpretation and unnecessary intervention.

A. Liver: Riedel's lobe, accessory fissures

Riedel's lobe is a common variant, particularly in women, characterized by a tongue-like downward projection of the anterior edge of the right hepatic lobe. It can extend inferiorly to the level of the iliac crest and may be mistaken for hepatomegaly or a right renal or adrenal mass. On ultrasound, its key feature is continuity with the normal liver parenchyma, displaying identical echogenicity and vascular architecture. Accessory fissures are deep grooves on the hepatic surface. The most notable is the accessory fissure of the right lobe, which can create a pseudomass appearance by isolating a portion of the parenchyma. Doppler ultrasound confirming normal vascular flow through the area is diagnostic.

B. Gallbladder: Septations, Phrygian cap

Gallbladder septations are thin, incomplete membranes that project into the lumen. They are usually asymptomatic but can potentially trap bile or stones. A Phrygian cap is a specific, common variant where the fundus of the gallbladder folds back upon itself. It has no pathological significance but can mimic a mass or stone on a single image. Scanning from multiple angles demonstrates the continuity of the fold with the gallbladder wall and the absence of acoustic shadowing, differentiating it from pathology.

C. Biliary Tree: Variations in biliary duct confluence

The classic anatomy of the right and left hepatic ducts joining to form the common hepatic duct is present in only about 60% of the population. Numerous normal variations exist. A common variant is a trifurcation pattern, where the right anterior, right posterior, and left hepatic ducts all join at the same point. Another is where the right posterior duct drains directly into the left hepatic duct. These variations are of paramount importance during surgical planning for liver resection or cholecystectomy to avoid inadvertent bile duct injury. While subtle on ultrasound, careful tracing of ducts at the confluence can sometimes suggest a variant, though magnetic resonance cholangiopancreatography (MRCP) is the definitive modality for mapping biliary anatomy pre-operatively.

V. Pitfalls and Challenges in Identifying Normal Anatomy

Even with perfect knowledge of anatomy, technical and patient-related factors can obscure the view.

A. Obesity

Obesity presents a significant challenge in abdominal ultrasound. Increased subcutaneous and visceral fat attenuates the sound beam, leading to poor penetration, decreased resolution, and a generalized increase in image noise (granular appearance). The liver may appear artificially hyperechoic, mimicking steatosis, and deep structures like the CBD or the pancreas may be difficult to visualize clearly. Techniques to mitigate this include using a lower-frequency transducer (e.g., 2-5 MHz curvilinear), increasing overall gain and time-gain-compensation (TGC) settings, and applying firm, steady pressure to displace bowel gas. Patient positioning, such as left lateral decubitus or upright scanning, can also bring structures closer to the transducer.

B. Bowel gas

Gas in the stomach, duodenum, or colon is the arch-nemesis of abdominal ultrasound. It causes near-total reflection of sound waves, resulting in bright echogenic foci with posterior "dirty" shadowing that completely obscures underlying anatomy. This is particularly problematic when trying to visualize the pancreatic head, distal CBD, or gallbladder neck. Sonographers employ various maneuvers to displace gas: asking the patient to drink water to fill the stomach as an acoustic window, using the liver as a sonic window by having the patient take and hold a deep breath, or scanning in the left lateral decubitus position. Persistence and changing the angle of insonation are key.

C. Scarring and distortion from prior surgery

Previous abdominal surgery, especially in the right upper quadrant, can dramatically alter the anatomical landscape. Surgical scars can cause acoustic shadowing. More importantly, post-surgical adhesions can fix organs in abnormal positions, and anatomical relationships can be distorted. For example, after a partial hepatectomy, the remaining liver may hypertrophy and shift, changing the orientation of vessels. Post-cholecystectomy, the CBD may dilate slightly and occupy the gallbladder fossa, which can be confusing. Knowledge of the patient's surgical history is indispensable. In complex post-surgical cases, or when pain patterns are atypical (e.g., upper abdominal pain with a potential spinal component), correlation with cross-sectional imaging like a thoracic spine MRI may be necessary to rule out referred pain or other pathologies that ultrasound cannot assess.

VI. Conclusion: A thorough understanding of normal hepatobiliary anatomy and its variants is essential for accurate ultrasound interpretation.

Mastering hepatobiliary ultrasound is a continuous journey of pattern recognition, anchored firmly in the detailed knowledge of normal anatomy and its spectrum of benign variations. This guide has delineated the key structures—from the lobar segmentation of the liver and the characteristic "shotgun" sign of the portal triad to the nuances of gallbladder variants like the Phrygian cap. We have also addressed the practical challenges, from obesity to surgical changes, that test the sonographer's skill. In the diagnostic pathway, the ultrasound hepatobiliary system exam remains a powerful, accessible first step. Its findings, however, must always be integrated into the broader clinical context. When findings are equivocal, or symptoms point to multiple potential sources—such as pain that could be of hepatobiliary, pancreatic, or even spinal origin—advanced imaging like a thoracic spine MRI may provide the necessary complementary information. Ultimately, the goal is to provide patients with accurate, timely, and confident diagnoses, avoiding both the anxiety of false positives and the risk of missed pathology. This is only achievable through a disciplined, knowledgeable, and meticulous approach to every ultrasound examination.