Musculoskeletal lesions with fluid-fluid level: a pictorial essay

A fluid-fluid level was identified in 11.2% of focal lesions of bone. Fluid-fluid levels are observed on cross-sectional imaging with either computed tomography (CT) or magnetic resonance (MR) imaging. They become apparent when a fluid collection containing substances of different density is allowed to settle, and when the plane of imaging is perpendicular to the fluid level. For instance, within a collection of blood, the cellular components will settle dependently, with the lower density plasma forming a layer superiorly. The difference in the density of these layers can be observed on CT imaging. With MR imaging, these layers will have different signal characteristics, allowing for visualization of the fluid-fluid level. The presence of fluid-fluid levels within a musculoskeletal lesion is an important finding, which can significantly aid in the differential diagnosis. This finding can be observed in a wide variety of lesions: osseous and soft tissue masses, neoplastic or nonneoplastic lesions, malignant or benign neoplasms, and primary or metastatic malignancies. When a fluid-fluid level is detected, in conjunction with clinical history, the differential diagnosis for a lesion can often be limited to a few choices. For this reason, it is important to be aware of the multiple lesions that can produce fluid-fluid levels, as well as their differentiating characteristics and typical presentations. We will review both osseous and soft tissue lesions, focusing on their imaging characteristics and other important findings.