serpent sign
The serpent sign, or snake sign, is described in hydatid disease. The WHO classification (2001) or Gharbi classification (1985) of the hydatid disease describe several stages on ultrasound . During the active stage, the cyst is composed of three layers: the outer (pericyst), the middle (ectocyst), and the inner (endocyst) . When the pressure increases within the cyst, there may be rupture with dissection of the ectocyst from the pericyst . This collapsed ectocyst, within an intact pericyst, can be visible on ultrasound as well as on CT and MRI and looks like a snake or serpent, and thus is termed the serpent or snake sign .
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