Fibrous dysplasia is a skeletal developmental anomaly of the bone-forming mesenchyme that manifests as a defect in osteoblastic differentiation and maturation. Virtually any bone in the body can be affected. It is a nonhereditary disorder of unknown cause. (See the images below.) [1, 2, 3, 4, 5, 6, 7]
Plain radiography is the first-line study. Usually, the diagnosis is straightforward when typical features are present. Computed tomography (CT) scanning may be required to assess complex regions such as the spine, pelvis, chest, and facial skeleton. [8, 9, 10, 11] Bone scintigraphy has a limited role in the detection of subtle pathologic fractures. In fibrous dysplasia, the features on a bone scan are nonspecific for diagnostic purposes. Magnetic resonance imaging (MRI) may be necessary to assess cord compression when the spine is involved. [12, 13, 14]
Common locations for lesions are the ribs, craniofacial bones, femoral neck, tibia, and pelvis. Radiographic findings in these and other structures are discussed below. 
Long and short tubular bones
The usual appearance of fibrous dysplasia includes a lucent lesion in the diaphysis or metaphysis, with endosteal scalloping and with or without bone expansion and the absence of periosteal reaction. Usually, the matrix of the lucency is smooth and relatively homogeneous; classically, this finding is described as a ground-glass appearance. Irregular areas of sclerosis may be present with or without calcification. The lucent lesion has a thick sclerotic border and is called the rind sign. (See the image below.)
The lesion may extend into the epiphysis only after fusion. Premature fusion of the ossification centers may occur, resulting in adult dwarfism. The dysplastic bone may undergo calcification and enchondral bone formation.
Skull and facial bones
The frontal bone is involved more frequently than the sphenoid, with obliteration of the sphenoid and frontal sinuses. The skull base may be sclerotic. Single or multiple, symmetrical or asymmetrical, radiolucent or sclerotic lesions in the skull or facial bones may be present. The external occipital protuberance may be prominent; however, these features are less common in Paget disease, neurofibromatosis, and meningioma.
Most commonly, maxillary and mandibular involvement has a mixed radiolucent and radiopaque pattern, with displacement of the teeth and distortion of the nasal cavities. The diploic space is widened, with displacement of the outer table. The inner table of the skull is spared in fibrous dysplasia, unlike in Paget disease. Cystic calvarial lucencies, which commonly cross the sutures with sclerotic margins, may have a doughnut configuration.
Pelvis and ribs
These bones have lucencies, with a diffuse ground-glass appearance and rind lesions. Cystic lesions are common. Protrusio acetabuli is a feature on the pelvic radiograph. (See the image below.)
Spinal involvement is common in polyostotic disease and rare in monostotic disease. Well-defined, expansile, radiolucent lesions with multiple internal septa or striations involve the vertebral body and, occasionally, the pedicles and arches. Paraspinal soft-tissue extension and vertebral collapse are rare. Kyphotic deformity and spinal cord compression may occur.
Degree of confidence
Plain radiographs are highly specific when characteristic features are present in a lesion. However, the specificity decreases when the lesion occurs at more complex sites, such as the spine, the skull, and, sometimes, the pelvis. The identification of malignant change and soft-tissue extension on plain radiographs may be difficult; cross-sectional imaging may be required.
Radiographic features suggestive of malignant degeneration include a rapid increase in the size of the lesion and a change from a previously mineralized bony lesion to a lytic lesion.
CT scanning is not often required for diagnosis. The modality demonstrates the nature of the lesion better by characterizing the matrix of the lesion. It also depicts expansion of the affected bone and its subtle mineral contents. It can demonstrate subtle nondisplaced pathologic fractures. CT is extremely useful in evaluating the extent of disease in complex locations, such as the facial bones, pelvis, chest wall, and spine. Usually, attenuation is in the range of 70-130 HU (Hounsfield unit). [8, 9, 10, 11]
In the skull, the outer table always expands outward. Therefore, the lesion is invariably convex; both tables are intact, although they are thinner. In the spine, CT can demonstrate the extent of bony disease and compromise of the spinal canal space. Paraspinal soft-tissue extension can be demonstrated at CT. CT scans may suggest malignant transformation, with the definition of an extraosseous soft-tissue mass and bone destruction. (See the images below.)
Degree of confidence
CT scanning is not optimal for the differentiation of fibrous dysplasia from other lesions that mimic it. CT findings complement plain radiographic findings.
Magnetic Resonance Imaging
On T1-weighted MRIs, the lesion has low-to-intermediate signal intensity equal to that of muscle. T2-weighted images also show low signal intensity owing to the high content of collagen and bone. Cartilaginous islands may be present in some lesions, and they appear as areas of high signal intensity on T2-weighted images. In children, T2-weighted images show hyperintense signal greater than that of subcutaneous fat; this finding is characteristic of fibrous dysplasia. (See the images below.) 
Also, fluid-fluid levels are reported in fibrous dysplasia. On short–inversion time inversion-recovery (STIR) images, the signal intensity of the lesion may be very high. MRI may be useful in assessing malignant change and demonstrating extension of the tumor into the surrounding soft tissues.
For postoperative follow-up, gadolinium-enhanced MRI is useful in demonstrating the proliferation of fibrocellular tissue.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.
In fibrous dysplasia, accumulation of isotope increases because of the lesion's hypervascularity. Hot spots or increased uptake of the radioisotope tracer technetium-99m methylene diphosphonate (99m Tc MDP) occurs in the spine, pelvis, ribs, and appendicular skeleton. Pathologic or stress fractures also can increase isotopic activity in the lesions. The features on the bone scan are nonspecific for a conclusive diagnosis based solely on the distribution of the isotope. [12, 13]
Degree of confidence
The technique is not specific for a firm diagnosis based on the imaging characteristics. The specificity is relatively poor.