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Background
Fat-fluid level is an interface formed by layers of fat and blood. The fat represents intramedullary fat released by pathology within the bone. This educational exhibit provides a review of this radiologic sign on different imaging techniques and discusses the significance of its presence.

Educational Goals / Teaching Points
Define the locations of fat-fluid level within the musculoskeletal system. Discuss the appearance of fat-fluid level on different imaging techniques. Discuss the differential diagnosis of a fat-containing extramedullary soft tissue mass.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Fat-fluid level may be seen within a joint (intraarticular), within a bone (intramedullary), or outside a bone (extramedullary). In the musculoskeletal system, fat-fluid level is most frequently seen within the joints. Fat-blood interface is seen on lateral knee radiograph obtained with a horizontal beam technique, with a layer of fat density seen in the nondependent portion of the knee joint. With time, lipohemarthrosis may evolve into three layers on sonography: nondependent hyperechoic layer, anechoic middle serum layer, and dependent hypoechoic red blood cell precipitate layer. In acute knee trauma, CT aids in the detection of lipohemarthrosis and intraarticular fracture. On MRI, the fat in a nondependent position follows the signal intensity of subcutaneous fat on all sequences. There is a thin zone of chemical shift artifact separating the fat layer from the dependent layer of hemarthrosis, which follows the signal of fluid. Intramedullary fat globules can be seen with fracture or osteomyelitis. With cortical destruction, the intramedullary fat globules are extruded outside the bone, forming an extramedullary fat-fluid level. The literature stated that an extramedullary fat-fluid level is specific for osteomyelitis. However, in our clinical experience, it is found more commonly with trauma. Differential diagnoses of a lesion with an extramedullary fat-fluid level are Morel-Lavallee lesion, lipoma arborescens, intramuscular myxoma, lipomatous neoplasm, and synovial sarcoma. Morel-Lavallee lesion contains fluid-fluid level and fat globules but not fat-fluid level. Intramuscular myxoma is a cystic mass within muscle with a peripheral rind of fat. Lipoma arborescens has multiple finger-like and globular synovial masses isointense to fat on all MR sequences but without fat-fluid level. Fat-fluid level has been reported in well-differentiated liposarcoma of the retroperitoneum but not in lipomatous neoplasm in the extremity. Synovial sarcomas have a bowl of fruit appearance but do not have fat-fluid level.

Conclusion
Fat-fluid level is frequently seen in clinical practice, most commonly in the knee joint. Precise localization of fat-fluid level aids in the correct diagnosis and guides the diagnostic evaluation. The most common cause for fat-fluid level is fracture. Therefore, the presence of fat-fluid level either within or outside the joint should prompt close inspection for occult fracture. In the absence of a fracture on any imaging technique, osteomyelitis must be considered.