Hepatic Hydatid Disease.
Hepatic hydatid disease (HHD) is a major endemic problem in sheep-rearing regions of the world. Man is the incidental host affected accidentally when he comes into contact with food contaminated with dog feces or when he comes in close contact with sheep. The liver acts as a filter for hydatid larvae, making it the most commonly affected organ. Up to one-third of patients with HHD present with complications such as rupture (into the biliary tree, thorax or peritoneum), secondary infection, anaphylactic shock and sepsis.
HHD disease is the commonest form of echinococcosis. The right lobe of the liver is affected in 80% of cases and the left lobe in 20% of cases. It occurs into the right duct in 60% of cases, into the left duct in 30%.Intrabiliary rupture can lead to obstructive, septic or allergic manifestations. Patients commonly present with right upper abdominal pain (82%), obstructive jaundice (57–100%), fever (70–90%), acute cholangitis (20–37%), abdominal lump (22–39%), and rarely with acute pancreatitis, liver abscess or septicaemia, or it may be asymptomatic (5–6%) .
Lewall and McCorkell have classified rupture of echinococcal cysts into three types: contained, communicating and direct.
Imaging findings in hepatic hydatid disease depend on the stage of cyst growth (ie, whether the cyst is unilocular, contains daughter vesicles, contains daughter cysts, is partially calcified, or is completely calcified [dead]).
Cyst calcification usually occurs in the cyst wall. US demonstrate a hyperechoic contour with a cone-shaped acoustic shadow. When the cyst wall is heavily calcified, only the anterior portion of the wall is visualized and appears as a thick arch with a posterior concavity.
Multivesicular cysts manifest as well-defined fluid collections in a honeycomb pattern with multiple septa representing the walls of the daughter cysts. Daughter cysts appear as cysts within a cyst. When daughter cysts are separated by the hydatid matrix (a material with mixed echogenicity), they demonstrate a \"spoke wheel\" pattern . The matrix represents hydatid fluid containing membranes of broken daughter vesicles, scolices, and hydatid sand. Membranes may appear within the matrix as serpentine linear structures, a finding that is highly specific for hydatid disease (WATER LILY SIGN).
Computed Tomography may display the same findings as Ultrasound.Cyst fluid usually demonstrates water attenuation (3-30 HU). Calcification of the cyst wall or internal septa is easily detected at CT . A hydatid cyst typically demonstrates a high-attenuation wall on unenhanced CT even without calcification. Detachment of the laminated membrane from the pericyst can be visualized as linear areas of increased attenuation within the cyst. Daughter vesicles manifest as round structures located peripherally within the mother cyst.
Cyst calcification usually occurs in the cyst wall. US demonstrate a hyperechoic contour with a cone-shaped acoustic shadow. When the cyst wall is heavily calcified, only the anterior portion of the wall is visualized and appears as a thick arch with a posterior concavity.
Multivesicular cysts manifest as well-defined fluid collections in a honeycomb pattern with multiple septa representing the walls of the daughter cysts. Daughter cysts appear as cysts within a cyst. When daughter cysts are separated by the hydatid matrix (a material with mixed echogenicity), they demonstrate a \"spoke wheel\" pattern . The matrix represents hydatid fluid containing membranes of broken daughter vesicles, scolices, and hydatid sand. Membranes may appear within the matrix as serpentine linear structures, a finding that is highly specific for hydatid disease (WATER LILY SIGN).
Computed Tomography may display the same findings as Ultrasound.Cyst fluid usually demonstrates water attenuation (3-30 HU). Calcification of the cyst wall or internal septa is easily detected at CT . A hydatid cyst typically demonstrates a high-attenuation wall on unenhanced CT even without calcification. Detachment of the laminated membrane from the pericyst can be visualized as linear areas of increased attenuation within the cyst. Daughter vesicles manifest as round structures located peripherally within the mother cyst.
MR imaging in hydatid:
Simple viable hydatid cyst:
The wall of the intact viable hydatid cyst consist of two layers 1) Parasitic membrane consisting of gelatin and proteinaceous material 2) An outer layer consisting of dense fibrous capsule. In intact cyst these layers cannot be separated on MR images. On T1W images the cyst wall is isointense and on T2W images it appears as low intensity rim surrounding the homogeneous high signal cyst contents.
Hydatid cyst with daughter cyst:
On T1W images the viable daughter cysts may be isointense or slightly hypointense compared with the contents (hydatid sand) of the mother cyst. Daughter cyst and mother cyst are isointense on T2W images.
Hydatid cyst with detached parasitic membranes:
These membranes are seen floating within the cyst and appear dark on T1W and T2W images.
Intrahepatic rupture of hydatid cyst:
The next stage in the process of degeneration of hydatid cyst is rupture of pericyst.On MR images rupture of cyst is recognised as an area of discontinuity of the low intensity cyst wall.
End stage calcified cyst:
During the course of disease spontaneous collapse and calcification of the cyst may occur leaving an area of calcification in host tissue. On MR images calcification is seen as an area of signal dropout.