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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- 4. Ilter T, Ozgüven O, Mentes NK. "Ball of wool" or "yarn" sign: a new ultrasonographic sign for the diagnosis of hydatid cysts. A preliminary report. The British journal of radiology. 58 (695): 1141. Pubmed