Background: The radiographic appearance of osteoid osteoma depends on its site within the involved bone. There are three subtypes based on site: cortical, medullary, and sub-periosteal. Cortical lesions represent approximately 75% of osteoid osteomas, medullary lesions represent approximately 20% of tumors, and subperiosteal osteoid osteomas represent fewer than 5% of lesions. The sclerosis is reactive and does not represent the lesion itself. Radiographs characteristically show a circular ovoid cortical lucency representing the nidus (usually less than 1.5 cm in diameter) with a variable degree of surrounding sclerosis. sclerosis is extensive, it may interfere with visualization of the radiolucent nidus. Intramedullary and subperiosteal lesions may not demonstrate significant sclerosis, and the cortex overlying the site may appear normal, making intraoperative surgical localization difficult. Plain radiography is the first line of bone imaging. May appear normal and may show cortical thickening surrounding a solid periosteal reaction. The nidus may appear as a well-defined lucent region with a central dot of sclerosis. CT is the modality of choice and is excellent at characterizing the lesion. The most common appearance is a cortical-based lucency measuring less than 2 cm. Technetium-99–labeled MDP bone scintigraphy may be useful for confirming an osteoid osteoma diagnosis. The sensitivity of skeletal scintigraphy for detection is virtually 100%. Surgeons have used intraoperative gamma cameras both to locate tumors and to confirm resection. Pathologists have also used radionuclide studies to locate the nidus, thus facilitating histopathologic evaluation. On ultrasound, at intra-articular lesions, there will be a focal area of cortical irregularity and adjacent hypoechoic synovitis. The nidus can show hypoechogenicity with posterior acoustic enhancement. Ultrasound can detect hypervascularity of the nidus with Doppler assessment. Magnetic Resonance Imaging (MRI) is non-specific although being sensitive to marrow changes, it is usually unable to identify the nidus. The hyperemia and resultant bone marrow edema pattern may result in the scans being misdiagnosed as representing aggressive pathology. The MRI appearance of the nidus is variable. Most commonly, the tumor exhibits low to intermediate T1-weighted signals and heterogeneously high signal on T2-weighted and STIR sequences.