Diagnostic Ultrasound - Abdomen and Pelvis

Biliary Ductal Gas
TERMINOLOGY
Synonyms
• Pneumobilia, aerobilia
Definitions
• Gas within biliary tree including bile ducts or gallbladder
IMAGING
General Features
• Best diagnostic clue
○ Bright echogenic foci in linear/branching configuration
following portal triads associated with reverberation
artifact/dirty shadowing
• Location
○ Most commonly seen within intrahepatic bile ducts,
though may also involve extrahepatic bile ducts and
gallbladder
Ultrasonographic Findings
• Grayscale ultrasound
○ Gas within intrahepatic bile duct
– Bright echogenic foci in linear configuration following
portal triads
– In nondependent position: Left > right lobe biliary
ducts with patient in supine position
– Associated with "dirty" shadowing: Shadows filled
with acoustic noise caused by sound-reflecting objects
(gas)
– Reverberation artifacts with large quantities of gas
– Movement of gas, best demonstrated following
change in patient's position
○ Gas within extrahepatic bile duct
– Linear echogenic foci with "dirty" shadowing
– Within extrahepatic bile ducts adjacent to major
structures in porta hepatis
○ Gas within gallbladder
– Band-like echogenic layer in nondependent portion of
gallbladder
– Prominent reverberation artifacts obscures lumen
CT Findings
• CECT
○ Linear/tubular gas density adjacent to well-opacified
portal venous radicles and portal veins
– May be found within bile ducts (intrahepatic &/or
extrahepatic) or within gallbladder
○ Biliary ductal gas tends to be more central in location as
opposed to portal venous gas, which tends to be more
peripheral
Radiographic Findings
• "Saber" sign: Sword-shaped lucency in right paraspinal
region
Fluoroscopic Findings
• Filling defects: May be rounded, whereas calculi may be
angular/faceted
MR Findings
• Hypointense filling defect on T2-weighted MRCP (similar to
stones)
• Signal void structures producing susceptibility artifact on
T1-weighted dual echo gradient sequence
Imaging Recommendations
• Best imaging tool
○ Ultrasound, CT
• Protocol advice
○ Examine patient in supine and oblique positions to
demonstrate movement of gas
○ Set appropriate focus level to optimize visualization of
reverberation artifacts or posterior acoustic shadowing
DIFFERENTIAL DIAGNOSIS
Portal Venous Gas
• Branching echogenic foci in periphery of liver parenchyma
within portal venous radicle
• Sharp bidirectional spikes on Color Doppler, superimposed
upon usual Doppler tracing of portal vein
Intrahepatic Ductal Stones/Sludge
• Echogenic foci casting dense posterior acoustic shadowing
• In region of portal triad or within dilated intrahepatic ducts
Hepatic Arterial Calcification
• Hyperechoic double lines representing calcified arterial wall
accompanying portal veins
• Acoustic shadows of various sizes throughout liver
PATHOLOGY
General Features
• Etiology
○ Previous biliary intervention, iatrogenic causes
– ERCP ± sphincterotomy
– Biliary-enteric anastomosis
– Presence of internal biliary stent or external biliary
drainage catheter
○ Cholecysto-enteric/choledocho-enteric fistula
– Prolonged acute cholecystitis ± superimposed
gallstone ileus
– Perforated duodenal ulcer
– Erosion by biliary malignancy (e.g., carcinoma of
gallbladder)
○ Biliary infection with gas-forming organism
– Emphysematous cholecystitis
– Acute bacterial cholangitis
○ Recurrent pyogenic cholangitis
CLINICAL ISSUES
Natural History & Prognosis
• Majority will resolve spontaneously
• Prognosis depends on underlying etiology
SELECTED REFERENCES
1. Shah PA et al: Hepatic gas: widening spectrum of causes detected at CT and
US in the interventional era. Radiographics. 31(5):1403-13, 2011
2. Sherman SC et al: Pneumobilia: benign or life-threatening. J Emerg Med.
30(2):147-53, 2006
3. Okuda K et al: Sonographic features of hepatic artery calcification in chronic
renal failure. Acta Radiol. 44(2):151-3, 2003
4. Rubin JM et al: Clean and dirty shadowing at US: a reappraisal. Radiology.
181(1):231-6, 1991
Diagnoses: Biliary System
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