14.1 Gallbladder & Bile Ducts

Patient Preparation and Scan Technique

A diagnostic gallbladder study begins with patient preparation: the patient should fast for 6 to 8 hours before the examination. Fasting lets the gallbladder distend with bile — a gallbladder contracted after a fatty meal has a physiologically thick wall that mimics disease and can hide small stones — and limits bowel gas that would block the acoustic window.

Scan with a curvilinear transducer (typically 2-6 MHz in adults), using the liver as an acoustic window. Standard patient positions include:

• Supine — initial survey with subcostal oblique sweeps along the right costal margin
• Left lateral decubitus (LLD) — shifts the gallbladder medially and inferiorly, away from rib shadows and bowel gas
• Upright or semi-erect — demonstrates stone mobility and moves the fundus below the ribs

Deep inspiration brings the gallbladder below the costal margin. The gallbladder rests in the gallbladder fossa on the visceral surface of the liver; the main lobar fissure, seen as a thin echogenic line extending from the right portal vein to the gallbladder neck, is the key landmark for locating a 'missing' gallbladder.

Normal Sonographic Appearance

The normal gallbladder is an anechoic, pear-shaped structure with posterior acoustic enhancement deep to it (bile attenuates the beam far less than liver, so tissue behind the gallbladder appears brighter). Normal dimensions are up to about 10 cm in length and 4-5 cm in transverse diameter; a transverse measurement over 5 cm suggests hydrops. The gallbladder wall must measure less than 3 mm — measure the anterior wall perpendicular to the beam, because the posterior wall is artificially thickened by through-transmission.

Cholelithiasis

Cholelithiasis (gallstones) is diagnosed by three classic sonographic criteria:

1. An echogenic focus within the gallbladder lumen
2. Posterior acoustic shadowing — a clean, anechoic shadow, best produced when the stone is at least 2-3 mm and the beam strikes it perpendicularly
3. Mobility — the stone rolls to the dependent wall when the patient is repositioned

A focus that is echogenic and mobile but does not shadow is still likely a small stone; shadowing improves with higher-frequency transducers, focal zones placed at the stone's depth, and perpendicular insonation.

When the gallbladder is contracted and completely packed with stones, the lumen disappears and the WES sign (wall-echo-shadow) appears, also called the double-arc-shadow sign: the echogenic wall, then the echo from the surface of the stones separated by a thin anechoic bile rim, then a clean shadow. Distinguish this from a gas-filled duodenum, which produces a 'dirty' reverberation shadow instead.

Acute Cholecystitis and Its Variants

Acute cholecystitis is inflammation of the gallbladder, usually caused by a stone impacted in the cystic duct. The major sonographic findings are:

The sonographic Murphy's sign is the most specific finding, but it can be blunted by pain medication or gangrene. Acalculous cholecystitis accounts for 5-10% of acute cases and occurs in critically ill patients — those on total parenteral nutrition, or after major surgery, trauma, or burns — showing wall thickening, pericholecystic fluid, and a positive Murphy's sign without stones.

Two complicated forms carry much higher mortality and are worth recognising on sight:

• Gangrenous cholecystitis — mural necrosis. Look for striated or asymmetric wall thickening, sloughed intraluminal membranes floating in the lumen, complex pericholecystic fluid, and irregular or absent wall flow on Doppler. The catch is that the sonographic Murphy's sign is frequently absent, because mural denervation removes the tenderness; a negative Murphy's sign therefore does not reassure in this setting.
• Emphysematous cholecystitis — gas-forming infection of the wall or lumen, classically in elderly or diabetic men. Intramural or intraluminal gas produces dirty shadowing or ring-down/reverberation arising from the non-dependent wall, and the gas may shift with patient position. Perforation risk is high, so it is a surgical emergency rather than a finding to follow.

Sludge and Its Mimics

Biliary sludge is thickened bile that layers in the dependent gallbladder as low-level, non-shadowing echoes with a fluid-fluid level; it shifts slowly with repositioning. Tumefactive sludge (a sludge ball) forms a rounded mass-like collection that can mimic a tumor — the keys to recognizing it are mobility, absence of shadowing, and no internal blood flow on Doppler. Beware side-lobe pseudo-sludge: side-lobe beams reflecting off strong adjacent reflectors paint a false layer of echoes across the lumen. Unlike real sludge, it does not layer dependently and vanishes when you change the scan angle or patient position.

Wall Lesions: Polyps, Adenomyomatosis, Porcelain Gallbladder

• Cholesterol polyps — small, echogenic, fixed (non-mobile), non-shadowing projections from the wall; those under 10 mm are almost always benign, while lesions over 10 mm raise concern for gallbladder carcinoma
• Adenomyomatosis — benign hyperplastic wall thickening in which cholesterol crystals trapped in Rokitansky-Aschoff sinuses (mucosal diverticula) produce the characteristic comet-tail artifact: a bright, V-shaped reverberation tapering away from the wall
• Porcelain gallbladder — dystrophic calcification of the wall, seen as a bright echogenic wall with posterior shadowing; it is associated with gallbladder carcinoma and is an indication for cholecystectomy

The Bile Ducts

The common bile duct (CBD) is measured inner wall to inner wall in the porta hepatis, where it runs anterolateral to the portal vein alongside the hepatic artery — the portal triad, seen in transverse as the 'Mickey Mouse' view. The accepted upper limit of normal is 6 mm in adults under about 60 (roughly 4 mm is the reported mean, not the ceiling); beyond age 60 add about 1 mm per decade, so ~7 mm at 70 and ~8 mm at 80 can be normal, and up to 10 mm is acceptable after cholecystectomy. Normal intrahepatic ducts are barely visible (under 2 mm); dilated ducts running beside portal veins create the 'parallel channel' or 'double-barreled shotgun' sign.

Choledocholithiasis — a stone in the CBD — appears as an echogenic intraductal focus, ideally with shadowing and proximal dilation; distal stones are often hidden by duodenal gas. Ascending cholangitis (infected, obstructed bile) presents clinically with Charcot's triad of fever, right upper quadrant pain, and jaundice, with duct wall thickening and intraductal debris on ultrasound. Obstruction patterns matter: extrahepatic obstruction dilates the CBD first and the intrahepatic ducts later, whereas intrahepatic obstruction dilates only the ducts upstream while the CBD stays normal. Courvoisier's law states that painless jaundice with a distended gallbladder suggests malignant obstruction (pancreatic or periampullary carcinoma) rather than stones, because a stone-scarred gallbladder is too fibrotic to distend. Pneumobilia — air in the biliary tree after sphincterotomy, biliary surgery, or a biliary-enteric fistula — appears as echogenic foci with dirty shadowing that collect centrally within the ducts; portal venous gas, in contrast, extends out to the liver periphery.