What causes the sack of marbles sign on ultrasound, CT and MRI?
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Answer:
The sack of marbles sign is multiple rounded, mobile globules of fat or sebaceous material lying free within a cyst, so the lesion resembles a bag of marbles. The globules are hyperechoic on ultrasound, of fat attenuation on CT, and hyperintense on both T1- and T2-weighted MRI with complete signal loss on fat-saturated sequences, which confirms their nature. Two settings are recognised: dermoid cysts of the head and neck and orbit, where the term is most used, and mature cystic teratoma of the ovary, where the same appearance is often called the floating balls sign. It is uncommon but essentially diagnostic of a fat-containing cystic lesion, and it should be reported because it removes any need for further characterisation. In the ovary it is found alongside the other features of a dermoid: the Rokitansky nodule, a mural protuberance that does not float; the tip of the iceberg sign, in which an echogenic anterior interface obscures everything behind it; and echogenic hair-fluid levels.
Why is it called so?
The multiple rounded globules moving freely inside the cyst look like marbles loose in a sack.
Pathophysiology
Mature cystic teratomas and dermoid cysts are lined by keratinising squamous epithelium with adnexal structures, and they fill with desquamated keratin and sebaceous secretions. At body temperature sebum is liquid and immiscible with the aqueous cyst fluid, so it forms discrete droplets. Keratin debris and calcific material act as nuclei about which the sebum congeals into small spheres, and because these spheres are less dense than the surrounding fluid they float and move as the patient changes position. Their fat content is what produces the hyperechogenicity, the low CT attenuation and the fat-suppressible high T1 signal.
Alternative names: Floating balls sign, floating globules sign
Other associated named signs: Rokitansky nodule, tip of the iceberg sign, dermoid mesh or dot-dash sign, and the fat-fluid level
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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