MR imaging of simple bone cysts in children: not so simple

Objective. The unicameral bone cyst (UBC) is a common cystic bone lesion seen in children. We review and summarize its MR findings, focusing on their appearance following contrast enhancement, and correlating them to known histologic features of UBC. Subjects and methods. A retrospective review of 20 cases (13 boys, 7 girls; age range, 1-17 years; mean age, 8.9) diagnosed as UBC was conducted. Clinical histories, radiographic and MR features, follow-up data, and available pathologic findings were noted. Results. At initial presentation 18 (90 %) of the patients with UBC had a history of acute or remote pathologic fracture. Cysts with history of fracture displayed heterogeneous fluid signals on T1- (n = 9, 50 %), and T2-weighted (n = 15, 83 %) MR images. Gadolinium-enhanced images were obtained in 15 of the 18 UBC cases with history of previous fracture. All showed enhancement with focal, thick peripheral, heterogeneous, or subcortical patterns. Focal nodules of homogeneous enhancement (diameter > 1 cm) within the UBC (n = 5) correlated with areas of ground-glass opacification on plain film. Other interesting MR features were fluid-fluid levels (n = 11), "fallen-leaf" sign (n = 1), soft-tissue changes (n = 2), and detection of septations not seen on plain film (n = 2). Conclusion. UBCs frequently appear complicated on MR imaging, with heterogeneous fluid signals and regions of nodular and thick peripheral enhancement related to previous pathologic fracture and early healing.