Figure 8a. Aneurysmal bone cyst in a 54-year-old woman. (a) Nonenhanced CT scan at the level of the clavicle (C) shows an expansile lytic mass (M) in the medial left area of the clavicle. The mass is not well differentiated from the overlying muscle. (b) Axial T2-weighted (6,000/112) MR image shows a mass with overall signal intensity higher than that of fat. The mass contains regions with the signal intensity of fluid (arrow), as well as multiple septa with low signal intensity (arrowheads). (c) Photomicrograph (original magnification, x100; hematoxylin-eosin stain) shows osteoclast-like giant cells and fibroblasts with blood-filled spaces (arrows) that account for the fluid-fluid level seen on MR images.