Double-arc shadow sign | Radiology Signs

Answer:

The double-arc shadow sign is the appearance of two curved echogenic arcs separated by a thin anechoic line, with dense acoustic shadowing behind, in the gallbladder fossa. It arises when a contracted gallbladder contains stones: the near arc is the gallbladder wall, the thin dark line is residual bile and the far arc is the surface of the stones. It is closely related to the wall-echo-shadow sign and the two terms are often used for the same finding. The important discrimination is from a porcelain gallbladder, in which the wall itself is calcified and produces a single strongly echogenic arc with shadowing, and from bowel gas, which produces dirty shadowing with reverberation and comet-tail artefacts and changes with peristalsis. CT is the arbiter, showing mural calcification in porcelain gallbladder, calculi within a normal wall in cholelithiasis and gas in emphysematous cholecystitis. Porcelain gallbladder carries an increased but lower than historically quoted risk of gallbladder carcinoma, with selective rather than universal cholecystectomy now recommended, and the pattern of calcification matters, since a selective mucosal or stippled pattern is regarded as higher risk than diffuse intramural calcification. Any focal wall thickening, a mural mass or loss of the interface with the liver in this setting should prompt cross-sectional imaging for malignancy.

Why is it called so?

Two curved bright lines, the wall and the stone surface, appear as a double arc with shadowing beneath.

Pathophysiology

The appearance is generated by two successive strong acoustic interfaces. The first is between soft tissue and the gallbladder wall, the second between the small residual volume of bile and the calcium-containing calculi. Because the impedance mismatch at the stone surface is large, almost all remaining energy is reflected there and a clean shadow is cast beyond it. In a porcelain gallbladder the calcium lies within the wall itself, so the two reflectors coincide and only one arc is produced, with the shadow beginning immediately. Chronic inflammation with fibrosis and dystrophic calcification of the wall underlies the porcelain gallbladder, and it is almost always associated with gallstones and a non-functioning gallbladder.

Alternative names: Double arc sign; wall-echo-shadow complex

Other associated named signs: WES sign, the effervescent gallbladder of emphysematous cholecystitis and comet-tail artefact of adenomyomatosis

References

1. Raptopoulos V, D’Orsi C, Smith E, Reuter K, Moss L, Kleinman P. Dynamic cholecystosonography of the contracted gallbladder: the double-arc-shadow sign. AJR Am J Roentgenol. 1982;138(2):275-8. PMID: 6976730.