Medically reviewed by Mary Choy, PharmD A pilonidal cyst, sometimes called pilonidal disease, is a fluid-filled sac that usually occurs on the tailbone at the top of the buttocks. It usually starts as ...

not seen with abscesses. Hydatid cyst with detached membranes floating in the cyst fluid (a) are shown on T2-weighted SS-ETSE image (arrow). (b) Hydatid cyst with multiple daughter cysts.