Osteoid osteoma. A, T1-weighted axial MR image shows medial femoral cortical thickening. B, Fluid sensitive axial MR image demonstrates bone marrow edema without a discrete lesion. C, Axial CT image reveals a radiolucent nidus within the thickened cortical bone consistent with osteoid osteoma. D, Axial CT image acquired during radiofrequency ablation confirms coaxial placement of the RF probe into the nidus.