HYDATID CYST OF LIVER-MRI APPEARANCE

FIGURE 1:T2 HASTE-AXIAL

FIGURE 2:T1 FS-AXIAL

FIGURE 3:T2 HASTE- CORONAL

Figures above show multiple twisted linear structures within a large SOL in the right lobe of liver s/o collapsed membranes in a hydatid cyst.

HYDATID CYST OF LIVER:
Hydatid disease is a worldwide zoonosis and is cause by the larval stage of Echinoccoccus tapeworm.
There are two main types of hydatid disease cause by E granulosis and E multilocularis. Disease cause by E granulosus is the more frequently encounterd  hydatid disease in humans.
In the life cycle of E granulosus, Dog or other canine animals are the definitive hist while sheep is the most common intermediate host. Man can sometimes be the intermediate host by acquiring the disease either by contact with definitive host or by consumption of contaminated water or vegetables.
Liver is the most commonly involved organ in man( 75%) followed by lungs( 15%) and other viscera( 10%).
Right lobe is the most frequently affected lobe of liver.

STRUCTURE OF HYDATID CYST:
There are three layers: a)pericyst: composed of modified host cells forming a thick fibrous protective layer; b) middle laminated membrane: which is acellular and aloows the passage of nutrients, alos called the ectocyst; c)inner germinal layer or endocyst which forms the scolices and laminated membranes.

IMAGING: depends on the stage of cyst growth. It may be unilocular, may have daughter vesicles, daughter cysts, partial or complete calcification.
X-rays, Ultrasound, CT and MRI all show the hydatid cyst with variable appearances. Complications of hydatid cyst are also seen on imaging.

On MRI, low signal intensity rim on T2WI is characteristic and is likely to represnt the pericyst.
Daughted cysts are seen as cystic structures attached to the germinal layer and appear hypointense as compared to the intracystic fluid on T1 and hyperintense on T2.
Collapsed parasitic membranes are seen as twisted linear structures.
Rim irregularities are also seen on MRI.
MRI also shows complications like migration through diaphragm( on sagittal sequences)

REFERENCES:
Pedrosa I, Saiz A, Arrazola J et al. Hydatid disease: Radiologic and Pathologic Features and Complications. Radiographics 2000;20:795-817