Imaging of Ruptured Hydatids

Contained rupture makes the detached endocyst visible by all cross-sectional imaging methods. The lesion does not become smaller (Fig. 3.27).

Communicating rupture also produces a characteristic separation of endocyst from pericyst, and the cyst becomes smaller (Figs. 3.28 and 3.38). Biliary obstruction, if it occurs, may result in dilatation of downstream ducts. Occasionally daughter cysts or fragments of endocyst may be seen in the biliary tree by US, CT or MRI. Endoscopic retrograde cholangiopancreatography (ERCP) shows filling defects within the biliary tree and may also reveal retrograde communication with the cyst cavity (Fig. 3.29). If air replaces some of the fluid in the pericyst cavity of lung lesions, the air meniscus and water lily signs are produced on upright chest radiographs (Figs.3.39, 3.87-3.93).

Fig. 3.38 Communicating rupture of a type I liver hydatid. CT scan in this 22-year-old Tunisian man shows partial rupture through the slightly calcified anterior wall of the cyst into the surrounding hepatic parenchyma. The "floating membrane sign" produced by detachment of the germinal membranes of the endocyst is well demonstrated. (Courtesy of Dr. Gharbi, Tunis).

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Fig. 3.39 Air meniscus and double arch signs in ruptured hydatid cysts of the lungs in four different patients. (A) A thin crescent of air is seen in a large hydatid cyst in the right lower lung of a Spaniard. The air has dissected the two walls of the cyst, one wall being the fibrous wall produced by the lung (pericyst) and the other being the wall produced by the parasite itself (endocyst). (B) Air meniscus in another pulmonary hydatid seen on AP tomographic cut. There is also air in the endocyst itself producing a "double arch sign" as well. (C and D) The "sign of the double arch" or "cyst-within-a-cyst" appearance (arrows) in a ruptured pulmonary hydatid. Once rupture of the cyst begins, a large opening may develop in the membranes and the shrinking cyst expels part or all of its fluid into the communicating bronchus. At this stage, air may enter the endocyst, producing an air-fluid level within the cyst that may itself be capped with a crescent of air between the pericyst and endocyst. The wall of the endocyst appears as an arch above the air-fluid level and the surrounding pericyst is visible as a larger arch.