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 1. During the active stage, the cyst is composed of three layers: the outer (pericyst), the middle (ectocyst), and the inner (endocyst) 1. When the pressure increases within the cyst, there may be rupture with dissection of the ectocyst from the pericyst 1. 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 1.
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