MRI showed a T1- and T2-hyperintense lesion in the right interarticular portion and upper articular process of the 9th thoracic vertebra with intense perifocal bone and soft-tissue edema. Contrast enhancement after Gd was present not only in the nidus but also in the adjacent soft tissues and bones. CT showed periosteal bone appositions at the right posterior arch, bulging into the spinal canal. Interestingly, there was also new periosteal bone formation at the vertebral end of the 9th rib, that wasn't directly affected by the lesion.
Histologically, there is no clear difference between osteoid osteoma and osteoblastoma of the spine, though the latter tend to be larger and more aggressive.