Calcaneal aneurysmal bone cyst
Fluid-fluid levels within a bone or soft tissue lesion appear due to presence of fluids of differing density/signal, like blood and clear fluid in the same cavity/compartment. Their demonstration needs the fluid to settle by gravity and the plane of imaging to be gravity-dependent and perpendicular to the fluid level. This can be separation of blood and serum in a cavernous space containing blood, hemorrhage and separation of serum and blood products or separation of cells and fluid in necrosis.
They occur in a wide variety of lesions. These include aneurysmal bone cyst, simple bone cyst with hemorrhage, telangiectatic osteosarcoma, chondromyxoid fibroma. Amongst these, the possibilities can be narrowed down, as in this case, by other factors such as age, history, location of lesion and other imaging features.