Aneurysmal bone cyst.

Key findings that aid in the diagnosis of an aneurysmal bone cyst are: 1) typically the patients are young individuals less than 20 years old; 2) an expansile bone lesion is bordered by a thin, low signal intensity rim mass; 3) the lesion is inhomogenously increased in signal on T2-weighted images; and 4) multiple fluid-fluid levels are seen with the mass on T2-weighted magnetic resonance imaging scans.