Ball of wool sign (hydatid cyst)

Last revised by Dr Ayla Al Kabbani on 01 Feb 2020

The ball of wool sign, also referred to as the yarn sign or congealed water lily sign, is an ultrasound appearance, representing degeneration of hydatid cysts (WHO class CE 4). The inner side of the cyst detaches from the cyst wall and folds on itself, causing a change from anechoic (fluid) to a solid, hyperechoic appearance without internal septations or daughter cysts 4.

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Cases and figures

  • Case 1
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