A 23-year-old man presented after 18 months of knee pain due to medullary osteoid osteoma. Axial computed tomography scan from a treatment planning study showing the lucent osteoid osteoma, subtle central calcification, and surrounding medullary sclerosis. Note the vascular channel entering the medial aspect of the lesion (A). Coronal proton-density fat-saturated magnetic resonance image showing a zone of marrow edema centered on a high-signal nidus adjacent to the physeal scar (B).