Imaging findings of aneurysmal bone cyst closely mirror the macroscopic structure of the lesion (i.e., multiple cavernous blood-filled spaces expanding and remodeling the host bone) (Som et al. 1991).
Conventional X-ray films demonstrate a well-defined lytic lesion, demarcated by a "ballooned" bony contour, probably reflecting new bone formation by the periosteum. In about 30% of cases, irregular densities can be seen within the lesion corresponding to calcified chondroid-like material (KrANSdORF and Sweet 1995).
CT scan allows additional information such as focal cortical breaches and fluid-on-fluid levels (in up to 35% of cases) (Senol et al. 2002). Their identification requires, in some cases, a few minutes delay before scanning (to allow them to reform in the supine decubitus) and to display images with narrow window settings.
MR shows a well-defined expansile lesion demarcated by a thin rim and crossed by multiple internal septa. Both these display hypointense signal related to the presence of fibrous tissue. Fluid material retained within the cyst exhibits non-homogeneous signal, generally hypointense on T1 and hyperin-tense on T2. Nonetheless, focal areas of spontaneously hyperintense T1 signal are also described, reflecting the presence of different byproducts of hemoglobin (HriShikESh et al. 2002). Fluid-on-fluid levels are much more commonly identified than on CT; they are thought to represent the layering of blood-tinged serous material above the un-clotted liquid blood, laying in the dependent part of the cyst (Fig. 8.7).
After contrast administration, bright enhancement is observed exclusively along both peripheral rim and internal septa (De MiNTEguiAgA et al. 2001).
Digital subtraction angiography also demonstrates the hypervascularity of the peripheral part of the lesion; however, this technique may play a more relevant role in the pre-treatment work-up. As significant blood loss may occur during surgery, preoperative embolization may grant easier resection of the lesion. Direct intraoperative sclerother-apy has also been reported (ChARTRANd-LEFEBvrE et al. 1996).
The differential diagnosis of aneurysmal bone cyst is rather complex, reflecting the controversy still existing about the real nature of this lesion. In fact, the propensity to create blood-filled spaces is shared by a list of extremely different lesions including fibrous dysplasia, chondromyxoid fibroma, non-ossifying fibroma, osteoblastoma, angioma, chondroblastoma, telangiectatic osteosarcoma. This observation has led several authors to consider aneurysmal bone cyst as a pathophysiologic change in a preexisting lesion, rather than a distinct entity (KrANSdORF and Sweet 1995; CiTARdi et al. 1996). The main weak point of this theory is represented by the low rate of associated lesions ranging between 29% and 35%.
This could be interpreted as a progressive involution of the primary lesion that gradually looses its characteristic structure. Anyway, whenever an aneurysmal bone cyst is suspected, the main task of imaging is to accurately detect any coexisting bone change: in this regard, CT may be preferred to MR, particularly when areas with complex bone anatomy are assessed.
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