Imaging findings
Ultrasound is accepted as the first choice imaging modality in the diagnosis and follow-up. It is an adequate and efficient imaging modality in screening AE lesions. Since it is cheap, readily available, and does not contain radiation, US can be used in endemic regions as a fast diagnostic tool in large populations (6,7). But US have lower specificity and is highly operator dependent modality.
CT remains the most widely used imaging techniques for detecting AE liver lesions: the anatomic location, dissemination and characterization of the lesions.
While CT best depicts the characteristic calcification pattern of these lesions, the assessment of other lesion components is limited to this method. However, due to their ill-defined contours and infiltrative growth pattern, AE lesions without internal calcification may mimic primary or secondary liver malignant neoplasms.
MRI is the best modality for characterizing the components of parasitic lesions and demonstrating vascular, biliary and extrahepatic involvement. MRI should be performed preoperatively, especially in patients undergoing extensive liver resection or transplantation (8), and MR imaging should include Diffusion-weighted MR (DW-MR) imaging and performed with contrast administration.
AE lesions consist of cystic and solid components. Small smooth cysts reflect metacestodal vesicles whereas large irregular cystic areas reflect liquefaction necrosis. The cystic areas within the masses are best shown by T2-weighted (T2w) images and are hyperintense. The solid component is formed by coagulation necrosis, granuloma, and/or calcification. These areas are iso- or hypointense on T2w images (8,9). Low signal on T2w images can be interpreted as the result of very small vesicles embedded in fibrous tissue.
Hepatic AE is classified into 5 types according to MR findings in Kodama (9) classification (Fig. 6):
Type 1, multiple small cysts without a solid component
Type 2, a solid component associated with small cysts
Type 3, a solid component associated with irregular, large cysts
Type 4, a solid component with no cysts
Type 5, a large cyst without a solid component
It is difficult to differentiate AE from malignant liver lesions with conventional MR sequences, especially because type 4 is completely solid. Contrast administration and DWI MRI have critical role for distinguish it from liver malignancies.
DW MRI uses the microscopic random movement of water and is often used to distinguish between benign and malignant lesions. The diffusion of malignant tumors, unlike that in AE lesions, is restricted because of increased cellularity and reduced extracellular space (10). Although it is completely solid, type 4 lesions have high ADC values due to the multilocular cystic nature of the lesion. The lack of general restriction of diffusion in AE lesions is an important finding in distinguishing them from other liver malignancies (4). The restriction of diffusion can be secondary to abscess formation of the infected lesion (4).
The absence of contrast uptake in a large proportion of the mass following intravenous contrast administration is an important diagnostic feature of these lesions. However, mild peripheral contrast enhancement is seen on delayed phase of gadolinium-enhanced T1w images, perilesional fibroinflammatory considered to be responsible for such contrast enhancement.