Aneurysmal Bone Cyst
Aneurysmal bone cysts are expansile lytic lesions that contain thin-walled, cystic cavities; their cause is unknown.
They can be seen at any age, although 75% are seen in patients younger than 20 years of age and they are rare after 30 years of age. There is a slight female predominance, with a male-to-female ratio of 1:1.5.
Aneurysmal bone cysts can occur in any bone but arise predominantly in the long bones and the spine. In the long bones, they are usually metaphyseal or metadiaphyseal. In the spine, they involve the posterior elements and may extend into the body, but involvement of the vertebral body alone is rare.
On CT scans, the cyst appears as an expansile lesion with a thin cortical shell. Fluid-fluid levels may be seen and are thought to represent sedimentation of degraded blood cell products within the cystic cavity.45 Fluid-fluid levels are best visualized if the patient is imrnobilized for some time before scanning. Fluid-fluid levels are a nonspecific finding and have also been described in giant cell tumors, chondroblastoma, and telangiectatic osteosarcomas.
On both TI-weighted and T2-weighted MR images, an aneurysmal bone cyst appears as a well-defined, expansile mass with multiple internal septations, surrounded by a well-defined, low-signal-intensity rim of variable thicknes. The rim represents a bony shell. Fluidfluid levels are often seen within well-defined cystic cavities.* On TI-weighted images, signal intensity in the cyst may be higher than that of muscle because of intracystic hemorrhage. One cavity may have signal intensity characteristics that are markedly different from those of an adjacent cavity.
The different signal characteristics probably reflect intracystic hemorrhage of different ages. Edema may be present in the surrounding muscles. Septations within the lesion enhance after administration of gadolinium diethylenetriamine-penta-acetic acid, or pentetic acid (Gd-DTPA), but the cystic component does not enhance.