Imaging Findings

Imaging findings of fibrous dysplasia reflect the pathophysiology of this lesion, (i.e., focal or complete replacement of medullary bone by woven fibroosse-ous tissue.) As a general rule, the cortex is unaffected by this process (Kransdorf et al. 1990; Som and Lidov 1992).

The degree of mineralization of the tissue will determine radiographic and CT density of fibrous dys-plasia. This may range from radiolucent, difficult to differentiate from a simple bone cyst (particularly in the monostotic form of the disease), to ground glass (equal proportions of fibrous and osseous tissue), or even sclerotic (predominance of dense osseous tissue) (Kransdorf et al. 1990; Wenig et al. 1995) (Figs. 8.3, 8.4).

MR signal on SE T2 sequence is rather variable. Fibrous dysplasia has been reported as having overall hypointense signal and cystic/necrotic hyperintense areas (Casselman et al. 1993; Som and Lidov 1992), but also hyper- or isointense signal as compared to subcutaneous fat tissue (Utz et al. 1989). SE T1 signal is unanimously reported as hypointense (Som and Lidov 1992; Utz et al. 1989; Kransdorf et al. 1990); non-homogeneous enhancement may be obtained after contrast administration.

As the amount of fibrous tissue within the lesion progressively increases, expansile remodeling of the affected bone can occur (Fig. 8.5). In the maxillofacial area, this may result in encroachment of optic canal and skull base foramina and fissures (Sterling et al. 1993), entailing the risk of potentially severe neurologic complications (Commins et al. 1998).

Imaging findings are usually insufficient to precisely discriminate between fibrous dysplasia and ossifying fibroma. As for fibrous dysplasia, CT density ranges between radiolucency (Engelbrecht et al. 1999), ground glass (Sterling et al. 1993), and sclerosis (Marvel et al. 1991). Nonetheless, in most

Fibrous Dysplasia Etmoidal

Fig. 8.3a,b. Fibrous dysplasia. Plain CT on coronal plane, in two different patients. a Lesion involves both the right frontal and ethmoid bone. Note expansion of crista galli and middle turbinate, the latter contralaterally displacing the nasal septum. b Monostotic form involves the sphenoid bone, expanding the left pterygoid root and laminae. Note initial narrowing of both foramen rotundum (white arrow) and vidian canal (black arrowheads), at its anterior opening

Fig. 8.3a,b. Fibrous dysplasia. Plain CT on coronal plane, in two different patients. a Lesion involves both the right frontal and ethmoid bone. Note expansion of crista galli and middle turbinate, the latter contralaterally displacing the nasal septum. b Monostotic form involves the sphenoid bone, expanding the left pterygoid root and laminae. Note initial narrowing of both foramen rotundum (white arrow) and vidian canal (black arrowheads), at its anterior opening

Pterygoid Root

Fig. 8.4a-c. Fibrous dysplasia. Plain CT on axial (a) and coronal plane (b,c). Polyostotic form of the disease involves the temporal, sphenoid, frontal, maxillary, and palatine bone. Note the different degrees of mineralization of the expanded medullary cavities, ranging from ivory (temporal bone), to ground glass (sphenoid) and radiolucent (maxillary) appearance. Vidian canal, indicated by black arrow on the left, is not detected on the affected site (white arrow)

Fig. 8.4a-c. Fibrous dysplasia. Plain CT on axial (a) and coronal plane (b,c). Polyostotic form of the disease involves the temporal, sphenoid, frontal, maxillary, and palatine bone. Note the different degrees of mineralization of the expanded medullary cavities, ranging from ivory (temporal bone), to ground glass (sphenoid) and radiolucent (maxillary) appearance. Vidian canal, indicated by black arrow on the left, is not detected on the affected site (white arrow)

Sinonasal Fibrous Dys

Fig. 8.5a,b. Fibrous dysplasia. Plain CT (a), SE T2 (b) on axial plane. a At CT, a high density ground glass lesion is detected in an 8-year-old boy with right proptosis. The right ethmoidal mass appears non-homogeneously hypointense on T2. The lesion abuts the right orbit, with no signs of invasion present (arrows)

Fig. 8.5a,b. Fibrous dysplasia. Plain CT (a), SE T2 (b) on axial plane. a At CT, a high density ground glass lesion is detected in an 8-year-old boy with right proptosis. The right ethmoidal mass appears non-homogeneously hypointense on T2. The lesion abuts the right orbit, with no signs of invasion present (arrows)

cases ossifying fibroma is reported as a multilocu-lated lesion, bordered by a peripheral eggshell-like dense rim (Han et al. 1991). Whether the outer rim is a proliferating part of the tumor or just reactive hyperostosis is a matter of debate in the literature. At MR, ossifying fibroma shows hyperintensity on T2 sequence, whereas its T1 pattern consists of inter-mediate-to-hyperintense signal in the central part combined with hypointensity of the outer shell. The latter strongly enhances after paramagnetic contrast administration (Fig. 8.6), a finding impossible to appreciate at CT due to the bone-like density of this component of the lesion.

Better than a density/signal pattern, the site of the lesion may sometimes help to rule out a differential diagnosis. Isolated sphenoid or temporal bone lesions as well as diffuse craniofacial involvement better apply to fibrous dysplasia, whereas the suspicion of ossifying fibroma is raised in the presence of zygo-matic or mandibular tumor. Fronto-ethmoid lesions are, conversely, rather unpredictable (Som and Lidov 1992).

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