Computed Tomography (CT)

Computed tomography, particularly spiral (helical) CT with reconstruction, is a very accurate way to demonstrate hydatid cysts and show their regression after treatment. Nonenhanced CT may show minimal or partial calcification of the rim of hydatid cysts (Fig. 3.45A). This is important, because a segmentally calcified cyst may still harbor a living parasite and therefore may be infective. Total rim calcification suggests degeneration and the parasite is usually, but not invariably, dead (Fig. 3.45C); clinically, it can be observed rather than actively treated. CT can also demonstrate rupture of a cyst with detachment of the endocyst (Fig. 3.45D), allowing the contents to leak into the surrounding tissues or cavities (direct rupture). Hydatid fluid has similar Hounsfield CT values to water and, because the cyst is avascular, these values do not change after intravenous contrast enhancement.

Contrast-enhanced CT (Fig. 3.46) may show ring enhancement around a cyst as a sign of superimposed infection, especially if there are air bubbles. Abscesses and fistulous tracts may develop. The contents of an infected cyst usually have higher attenuation values than a cyst that is not infected. Old hydatid abscesses may not be recognized as such if all the parasitic material has been resorbed: they may contain only amorphous calcification. However, hydatid hooklets are very resistant and often persist; they should be sought histopathologically if no other parasitic material can be found. Even a single hooklet found in pus from an abscess is convincing evidence of a disintegrated hydatid cyst.

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Fig. 3.45 Nonenhanced CT scans of the liver in 4 different patients with various types of hydatid cysts. (A) Large hypermature type II hydatid in the right lobe of the liver causing bulging of the liver capsule. The cyst wall is faintly calcified and there is abundant matrix and several daughter cysts (arrows) within the endocyst. (Courtesy of Dr. von Sinner and Seminars in Roentgenology, 1997.) (B) Large type II hydatid in the superior aspect of the right lobe of the liver protruding into the right lower hemithorax. There are numerous daughter cysts present within the mother cyst. (Courtesy of Dr. R. Dick, Royal Free Hospital, London.) (C) Thick calcified wall of an old hypermature liver hydatid with daughter cysts. (D) Ruptured type I liver hydatid with detached internal membranes appearing as a "dancing hand" or a "serpent" within the cyst fluid. (C and D courtesy of Prof. Xing-Rong Chen, Shanghai, China).

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