Osteoid osteoma is a benign skeletal neoplasm of unknown etiology that is composed of osteoid and woven bone. The tumor is usually smaller than 1.5 cm in diameter. Osteoid osteoma can occur in any bone, but in approximately two thirds of patients, the appendicular skeleton is involved. The skull and facial bones are involved exceptionally.
Most patients with osteoid osteoma (demonstrated in the images below) are young. Rarely, an ossification center is affected. The classic presentation is that of focal bone pain at the site of the tumor. The pain worsens at night and increases with activity; it is dramatically relieved with small doses of aspirin.
The lesion initially appears as a small sclerotic bone island within a circular lucent defect. This central nidus is seldom larger than 1.5 cm in diameter, and it may be associated with considerable overlying cortical and endosteal bone sclerosis. The tumors may regress spontaneously. The mechanism of this involution is not known, but tumor infarction is a possibility.
Shukla et al performed a retrospective review of radiographic, computed tomography (CT), and magnetic resonance imaging (MRI) scans in 9 patients with osteoid osteoma of the foot and found that in young patients with chronic hindfoot pain and a normal radiograph, MRI features suggestive of possible osteoid osteoma include extensive bone marrow edema limited to a single bone, with a possible nidus demonstrated in two thirds of cases. The authors in the Shukla study noted that the presence or absence of a nidus should be confirmed with high-resolution CT. 
In the Shukla study, classic symptoms (ie, night pain, relief with aspirin) were identified in 5 of 8 cases. In 5 patients, the lesion was located in the hindfoot (4 calcaneus, 1 talus), while 4 were in the midfoot or forefoot (2 metatarsal, 2 phalangeal). Radiographs were normal in all patients with hindfoot lesions, and CT identified the nidus in all cases except in a case of terminal phalanx lesion; MRI demonstrated a nidus in 6 of 9 cases.The nidus was of predominantly intermediate signal intensity on T1-weighted images and of intermediate to high signal intensity on T2-weighted images. High-grade bone marrow edema limited to the affected bone and adjacent soft-tissue edema was identified in all cases. 
According to von Kalle et al, tailored high-resolution MRI with dynamic contrast enhancement can reliably diagnose osteoid osteomas and pinpoint the nidus. The authors retrospectively correlated the results of dynamic contrast-enhanced MRI with histologic and clinical diagnoses in 54 patients with osteoid osteoma who were diagnosed by MRI. In 49 of 54 patients (90.7%), the diagnosis of osteoid osteoma was determined to be certain or highly probable, and 38 of 54 osteoid osteomas were histologically proven. Five MRI diagnoses were regarded as false positives.
The authors proposed a stepwise approach to the diagnosis of osteoid osteosarcoma, with short-tau inversion recovery (STIR) sequences, dynamic contrast-enhanced scanning, and high-resolution, postcontrast T1 spin echo sequences with fat saturation. 
Radiography is the initial examination of choice and may be the only examination required. CT is used for precise localization of the nidus and may be used for guiding percutaneous ablation. [3, 4, 5] MRI is a useful imaging technique, but CT appears superior for precise localization. The roles of conventional and Doppler ultrasonography have not been established. Angiography may be useful in differentiating the tumor from a Brodie abscess. Single-photon emission computed tomography (SPECT) scanning is useful in the localization of the tumor when the spinal arch or spinous process is involved. [6, 7, 8, 9]
Radionuclide scanning for technetium-99m diphosphonate uptake shows fairly intense activity at the tumor site. This examination may also be used to localize the tumor preoperatively and to establish complete removal of the nidus by using a hand-held radioactivity detector. Radionuclide scanning is a sensitive technique, and findings may be positive before radiographic changes are apparent. [10, 11]
The nidus in spinal involvement may be difficult to detect by using plain radiographs. Intra-articular tumors are difficult to detect on plain radiographs because of the absent or limited sclerosis around the nidus.
CT has the disadvantage of ionizing radiation. On MRIs, tumors are not as conspicuous as they are on CT scans. Angiography is an invasive procedure, and a minor overlap of angiographic features occurs with a Brodie abscess. The specificity of radionuclide bone scanning is low.
Radiographic features of osteoid osteoma (seen in the images below) depend on the site of involvement, the duration of symptoms, and the age of the patient.
A circular or ovoid lucent defect is seen in 75% of patients. This defect is usually smaller than 1.5 cm in diameter and is associated with a variable degree of cortical and endosteal sclerosis.
The site of the tumor determines the degree of bone sclerosis. In medullary tumors, sclerosis is minimal or absent. Cortical and subperiosteal tumors provoke considerable sclerosis. Long-standing tumors demonstrate more sclerosis. Children also mount more of a sclerotic response than do adults.
In subarticular and intracapsular tumors, reactive sclerosis may be absent or minimal, or it may occur relatively distant to the lesion. This sclerosis usually occurs in tumors of the femoral neck because no periosteum covered by articular cartilage is present on the surface. However, whatever periosteum exists beyond the areas covered by the articular cartilage cannot be elevated because it is bound down by enhancing Sharpey fibers.
Intra-articular tumors may show joint effusion associated with the premature loss of cartilage. Osteoarthrosis affects approximately one half of patients with intra-articular tumors. Rarely, patients experience regional osteoporosis, presumably as a result of disuse. Regional osteoporosis may appear as an area of osteopenia around a joint.
With spinal involvement, alignment abnormalities may be obvious, such as scoliosis, kyphosis, or hyperlordosis. In children with a long-standing tumor, the involved bone may demonstrate overgrowth.
Supplementary radiographic methods, such as overpenetrated radiographs or thin-section planning radiographs, may be useful in locating tumors in portions of the skeleton with complex anatomy.
Degree of confidence
Radiographs remain the mainstay of imaging in an orthopedic workup. Usually, radiography is the first examination performed in patients with bone pain. In three quarters of patients, a diagnosis may be suggested on the basis of the plain radiographic findings. However, some areas of the skeleton are difficult to assess by using plain radiographs in patients with a suspected osteoid osteoma. These areas include the spine, the femoral neck, and the small bones of the hands and feet. In the spine, overlapping shadows of the vertebral column can easily obscure the tumor.
A long list of conditions may mimic an osteoid osteoma. If excessive, new bone formation can mask the nidus, resulting in a false-negative diagnosis. When the tumor is in a long bone, a periosteal reaction may occur distant to the lesion or in an adjacent bone; these may cause diagnostic problems. However, this difficulty should not deter the radiologist from making the diagnosis.
Particular pitfalls include a Brodie abscess and a tumor in the long bones of children where overgrowth may occur. Osteoblastoma and osteoid osteoma have a propensity for the posterior elements of the spine. Both are osteoblastic tumors; they are differentiated primarily by their sizes. Osteoblastomas become considerably larger than osteoid osteomas and are better depicted on plain radiographs.
CT scanning is the ultimate diagnostic tool for the precise localization of the nidus. The nidus enhances after the intravenous administration of contrast medium. The nidus shows a variable degree of mineralization, which may be amorphous, punctate, ringlike, or, in rare cases, uniformly dense. Reactive sclerosis around the nidus varies from being extremely dense to manifesting no reaction at all. The CT scan features of osteoid osteoma are demonstrated in the images below.
Several techniques are available for ablation of osteoid osteoma. The tumor can be percutaneously ablated by using radiofrequency (RF), ethanol, laser, or thermocoagulation therapy under CT guidance. In spinal tumors, complete ablation or resection of the tumor is desirable but not always feasible. [12, 13, 14, 15, 16, 17, 18, 19, 20]
Percutaneous RF ablation is performed under CT guidance by using general or spinal anesthesia.  After localization of the nidus with 1- to 3-mm CT sections, an osseous access is established with either a 2-mm coaxial drill system or an 11-gauge Jamshidi needle.
Degree of confidence
CT is the ultimate tool for the detection and the precise localization of the nidus. It is particularly effective in areas with complex anatomy, such as the spinal pedicles, laminae, and femoral neck.
Excellent long-term results have been reported following CT-guided radiofrequency ablation of osteoid osteomas. There is no correlation of the CT and MRI patterns with the clinical outcome. Rehnitz et al suggests that treatment decisions should not be solely based on the imaging findings and investigators should also be familiar with the variety of imaging patterns after radiofrequency ablation. 
As a result of the partial volume effect in small lesions, problems can occur with CT scanning. Rarely, osteoid osteoma may be confused with a Brodie abscess. False-negative CT results may occur with extracortical tumors. CT scans may not help in diagnosing osteoid osteoma when the nidus is in a cancellous location because of a lack of changes in attenuation around the nidus.
Magnetic Resonance Imaging
Bone marrow edema is depicted around the nidus in approximately 60% of patients. Soft tissue edema is depicted adjacent to the tumor in slightly fewer than one half of patients. Perinidal edema is more pronounced in young patients.
Intra-articular lesions cause synovial thickening or inflammation and joint effusion, which may be readily apparent on MRIs. 
Degree of confidence
MRI reliably demonstrates the nidus, which has a variable appearance related to its position relative to the cortex of the bone. Compared with other techniques, MRI is better in the diagnosis of cancellous osteoid osteomas, whereas difficulty may be encountered by using plain radiography and CT.
MRI is not as effective in the diagnosis of intracortical tumors as it is in the diagnosis of cancellous bone tumors.
Differences in MR perfusion parameters have been reported in patients who have had successful treatment and those patients with recurrent osteoid osteoma. Teixeira et al identified an early and steep enhancement with short time to peak and a short delay between the arterial and nidus peaks on MR perfusion in the postoperative setting as highly indicative of an osteoid osteoma recurrence. 
Duplex color Doppler ultrasonography has been used to guide percutaneous localization and biopsy.
Ultrasonography has also been used to aid in the diagnosis of intra-articular osteoid osteomas. Some have suggested that the sonographic findings of a cortical irregularity and focal synovitis indicate the possibility of intra-articular osteoid osteoma, prompting the search for characteristic findings on correlative imaging studies. [25, 26]
Radionuclide bone scanning of uptake of technetium-99m phosphonates shows intense activity at the site of the tumor. Occasionally, a double-density sign is seen in which a small focus of radioactivity in the nidus is superimposed on a larger area of radioactivity. (The radionuclide scan features of osteoid osteoma are seen in the images below.) [6, 8]
SPECT scanning may be useful in areas with complex anatomy, such as the posterior elements of the spine. Radionuclide imaging may be used preoperatively and intraoperatively as well, to localize the tumor and to establish complete removal of the nidus by using a hand-held radioactivity detector.
Positron-emission tomography (PET) scanning with 18-fluorodeoxyglucose has also been used in the diagnosis of osteoid osteoma. 
Degree of confidence
The average time from the onset of symptoms to diagnosis is reported to be 28 months with spinal tumors. Radionuclide bone scanning has been found to reduce the time to diagnosis in 66% of patients. The sensitivity of radionuclide bone scans is extremely high. A radionuclide bone scan is considered mandatory in patients with painful scoliosis. A radionuclide bone scan can demonstrate the tumor before abnormal radiographic findings are apparent.
No false-negative results have been recorded in osteoid osteoma. However, the specificity of isotope bone scans does not match its sensitivity, and a variety of infective, neoplastic, metabolic, and traumatic lesions show increased activity.
An osteoid osteoma is highly vascular, a feature that can be demonstrated angiographically. The nidus is the most vascular part of the tumor, with an intense circumscribed blush that appears in the early arterial phase and that persists into the venous phase.
Degree of confidence
Blush persisting into the venous phase during angiography is believed to be diagnostic of osteoid osteoma.
A Brodie abscess may be difficult to distinguish from osteoid osteoma radiologically. Hypervascularity may also be observed with a Brodie abscess, but the characteristic blush seen in the venous phase in osteoid osteoma usually does not occur in a Brodie abscess.