Gastrointestinal Imaging

Hydatid disease, hepatic

For a general description, see hydatid disease. The liver is the most frequent localization of hydatid cysts. Most hydatid liver cysts caused by E. granulosus are multiple and localized in the right lobe. They can rupture spontaneously into the bile duct system, expelling the contents of the cyst into the bile ducts, and causing obstructive jaundice. They can also rupture into the free peritoneal space following trauma. Large cysts, situated close to the liver hilum can compress the main bile ducts and the vessels, causing obstructive jaundice or lobar atrophy or portal hypertension. Cysts caused by E. multilocularis are usually solitary masses, sometimes with peripheral satellite extension. The infiltrative process can involve large portions of the liver and cause stenosis of intrahepatic bile ducts and hepatic and portal veins.

Hydatid cysts of the liver are revealed on plain films of the abdomen if calcifications are present. Typical features of E. granulosus are lobulated rim calcifications in the wall of both primary and daughter cysts or crushed egg-shell calcifications. Calcifications in E. multilocularis are present in 70% of cases and consist of multiple small spherical densities with a radiolucent centre, ranging from 24 mm in diameter.

The ultrasound appearance of E. granulosus cyst depends on the evolution phase. Single unilocular cysts are visualized as anechoic masses with well-defined margins, marked posterior enhancement and containing mobile echogenic debris. Free floating membrane within the cyst is observed during a later phase. A cyst containing daughter cysts is visible as multiple smaller fluid containing multiseptated masses. Calcifications of the cyst wall appear as curvilinear hyperechoic structures with acoustic shadowing. In patients infected by E. multilocularis, the lesion may present on ultrasound as an anechoic ill-defined mass with internal echoes due to debris. But in other instances the lesion presents as a predominantly solid mass with echogenic and hyperechogenic areas.

Crescentic or ring-shaped calcification of the walls or linear calcifications of the septa are more frequently visualized on CT (Fig.1) than on the plain film of the abdomen. On CT (Fig. 2a) (Fig. 2b) (Fig. 2c) E. granulosus cysts appear as well-demarcated thin- or thick walled hypoattenuating structures which can be uni- or multilocular. Daughter cysts are usually visible as small structures with their own walls in the interior of the mother cyst. Sometimes the daughter cysts are confined only the peripheral area of the mother cyst. Typically, the fluid in the daughter cysts has a lower attenuation value than that in the mother cyst. Thin linear structures within the fluid represent free floating membranes following wall rupture. Dilatation of the bile ducts either due to external compression or to intrabiliary rupture with obstruction (Fig.3) may be present. In case of intrabiliary rupture, small daughter cysts or high attenuation material can be observed within the larger bile ducts. Septa and cyst walls enhance following intravenous contrast medium administration. The CT appearance of E. multilocularis is different from that produced by E. granulosus. Lesions appear hypoattenuating but inhomogeneous and with unsharp borders. There is no evidence of septation. Calcifications are punctiform or amorphous. The masses enhance poorly or not at all. Distinction between hydatid cysts and primary or secondary malignant neoplasms may be difficult on CT alone.

Most hydatid cysts are hypointense on T1-weighted and hyperintense on T2-weighted images. The visualization of a peripheral rim of low signal intensity both on T1-weighted and on T2-weighted images is considered to be typical. The internal structure of the cyst can be displayed well on MRI (Fig. 2d).

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Fig.3

Hydatid cyst liver with biliary extension. a. and b. CT of the upper abdomen following intravenous contrast medium administration. There is a large hydatid cyst in left lobe of the liver with typical multilocular appearance. Note the marked dilatation of the left hepatic duct (arrow) due to obstruction of the bile duct by intrabiliary dissemination of daughter hydatid cysts.
Hydatid disease, hepatic, Fig.1
Hydatid disease, hepatic, Fig.2 (a)
Hydatid disease, hepatic, Fig.2 (b)
Hydatid disease, hepatic, Fig.2 (c)
Hydatid disease, hepatic, Fig.2 (d)
Hydatid disease, hepatic, Fig.3 (a)
Hydatid disease, hepatic, Fig.3 (b)