In diagnosing CE, MRI is of more value than CT. On MRI, hepatic cysts show homogeneous very low signal intensity on T1WI and homogeneous very high signal intensity on T2WI. Due to their fluid content, there is a notable increase in signal intensity on heavily T2WI, which helps differentiate these cysts from metastatic lesions . No enhancement is observed after the administration of gadolinium contrast. In rare cases of intracystic hemorrhage, when blood products are mixed within the cyst, the signal intensity becomes high, and a fluid level is seen both on T1WI and T2WI . These features are observed within hydatid cysts as well, although there are differences regarding their histopathological features. A hydatid cyst has three distinct layers; the outer adventitial layer formed by the host liver tissue (pericyst), middle laminated layer and inner germinal layer both derived from the parasite [4].
MR imaging clearly visualizes pericyst, matrix and daughter cysts. The general depiction of the pericyst appears as a hypointense rim on both T1WI and T2WI due to its fibrous structure and the presence of calcifications. This is a specific feature of hydatid cysts, called the rim sign, mostly better visualized on T2WI. The matrix represents hydatid fluid containing membranes of broken daughter vesicles, scolices, and hydatid sand. The hydatid matrix appears hypointense on T1WI and significantly hyperintense on T2WI. When daughter cysts are present, they are typically more hypointense than the matrix on T2WI [5].
If the membrane is separated it can shift with movement, resembling a water lily floating on the surface of a pond, previously described as water lily sign. Diffusion weighted imaging (DWI) is an MRI technique that allows quantitative and non-invasive measurement of water molecule diffusion in biological tissues. DWI enables to calculate apparent diffusion coefficients using different b-values. B-value measures the degree of diffusion weighting applied Recent tudies found significant difference between hydatid cysts type CE 1 and CE 2 and simple cyst based on significantly lower mean ADCs of hydatid cysts. Distinction between simple cysts and hydatid cysts (CE 1 and CE 2) can be identified using ADC measurements at b600 and b1000 values. Both simple and hydatid cysts (CE 1 and CE 2) appear isointense on DWI with b1000, but simple cysts exhibit a peripheral moderate hyperintensity when compared to hydatid cysts [6].
The hydatid cyst can be staged with six types according to the WHO-IWGE classification, a fusion of the former WHO classification and the Gharbi classification. Originally based on ultrasound imaging, it is commonly applied to MRI and CT modalities.
CE1 cysts on MRI show homogeneous very low signal intensity on T1WI and high signal intensity on T2WI, Furthermore, there is an increase in signal intensity on heavily T2WI, differentiating them from metastatic lesions. The pericyst appears as a hypointense rim on both T1WI and T2WI due to its fibrous structure, most clearly seen on T2WI giving a possible rim sign. A hyperintense hydatid matrix on DWI and ADC can be observerd, whereas simple cysts show a peripheral moderate hyperintensity compared to hydatid cysts (Figure 1).
CE2 presents with a multicystic mass with septa, a so-called wheel-spoke pattern, daughter cysts are hypointense or isointense compared to the mother cyst on both T1WI and T2WI (Figure 2).
CE3A is characterized by detachment of laminated membranes with floating membranes as a possible water lily sign or serpent sign. Daughter cysts may be visible in the matrix, appearing hypointense or isointense compared to the mother cyst on both T1WI and T2WI (Figure 3).
CE3B features a solid matrix with daughter cysts; signal intensity varies depending on the proteinaceous content of the cyst. Beginning calcified areas may be obeserved (Figure 4).
In CE4 a heterogeneous mass with hypointense and hyperintense areas, often calcified as well as potential rupture can be seem. This stage can show signs of complication like rupture (e.g. biliary communication) or infection (Figure 5).
CE5 presents a stage of complete calcification, appearing as hypointense on all MRI sequences due to lack of fluid. This stage contains no residual daughter cysts (Figure 6).
The imaging and appropriate classification have a key role in therapeutic approach of hydatid disease. The cancer-like nature of the disease conditioned the management to become multidisciplinary, to combine drug and surgery along with a long-term follow up. Therapeutic options are largely influenced by the environmental context in which the patient resides. The use of only albendazole is reserved for early stages where the cysts are smaller than 5 cm. In the later stages, as the cysts grow, the implementation of PAIR, MoCAT, and surgery becomes necessary. In contrast, for inactive cysts (CE4 and CE5), the watch-and-wait approach is advised. The only curative treatment is total cystectomy. Other invasive methods include sub-total and partial cystectomy, Puncture-Aspiration of cyst contents-Injection of protoscolecidal agents-Reaspiration (PAIR) and modified catheterization technique (MoCAT) [7].
When the cyst is adjacent to major vessels, sub-total cystectomy, which avoids dissection of these vessels, is encouraged. Sub-total cystectomy includes partial resection of adventital layer while completely resecting other layers. Partial cystectomy includes opening of the cyst and may leave all part of middle and inner layer. It depends on use of protoscolecidal agents but still has a high risk of recurrence, therefore it is not suggested [8].