MR imaging in the evaluation of cystic-appearing soft-tissue masses of the extremities.

Cystic-appearing lesions are commonly seen in clinical practice at imaging of the extremities. However, only some of these lesions are truly cystic lesions (eg, ganglia or synovial cysts, bursae) and may be managed conservatively. Fluid-filled lesions usually have homogeneous high T2 signal at magnetic resonance (MR) imaging. A broad array of solid benign masses (eg, myxomas, peripheral nerve sheath tumors [PNSTs], certain vascular lesions, glomus tumors) and malignant solid masses (including undifferentiated pleomorphic sarcomas, myxofibrosarcomas, myxoid liposarcomas, synovial sarcomas, extraskeletal myxoid chondrosarcomas, and, less frequently, soft-tissue metastases) may also exhibit bright T2 signal at MR imaging, thereby simulating a cyst. On the other hand, fluid-filled lesions with associated complications (eg, bleeding or inflammatory changes) may have a more complex appearance. MR imaging plays a major role in distinguishing truly cystic lesions from solid lesions. If a cystic-appearing lesion demonstrates wall thickening or internal complexity (heterogeneous signal, nodules, or thick septa), evaluation with contrast material enhancement is mandatory, and a solid lesion must be suspected if any internal enhancement is present. In addition to categorizing the lesions as truly cystic or solid, the differential diagnosis may be further narrowed by considering the anatomic location of the lesion or characteristic imaging features (eg, internal linear or patchy enhancement at contrast-enhanced MR imaging and an intramuscular location in myxomas; the "split fat sign," "string sign," and "target sign" in PNSTs; tiny foci of fat in myxoid liposarcomas). In most cases, however, histologic analysis is required to achieve a definitive diagnosis. © RSNA, 2013.