To achieve differential diagnosis of orbital masses the following points should be considered: the age or the patient, the duration of the symptoms, the location of the lesion, the morphology of the mass (is it well defined, lobulated, or infiltrative?), the internal structure of the mass (are there septations? prominent vessels?), possible evidence of associated bone changes, and if there are such changes, whether they are due to pressure erosion, infiltration, or destruction.

Inflammatory Lesions

INFECTION

Bacterial orbital infection is most often due to paranasal sinusitis. Other causes of orbital infection include foreign bodies, skin infections, infected insect bites, and bacteremia. Once infection extends into the orbit, it can produce a diffuse inflammatory reaction that in some cases can evolve into an abscess. Management of a patient depends on the extent of infection and on the size of an abscess, if such is present. Clinical assessment may be very difficult because of the marked swelling and inflammation of the eyelids. Imaging is an excellent method to "look" inside the orbit. Following intravenous injection of a contrast material, both CT and MRI (Fig. 9.2) can demonstrate location, extent, and size of the inflammatory process and of the abscess. CT additionally provides information regarding the integrity of adjacent bone, if there is dehiscence or osteomyelitis. If there is diffuse orbital inflammation, with or without a small subperiosteal abscess, then treatment with antibiotics alone usually suffices. A large subperiosteal abscess almost always requires drainage. Young children most frequently develop infections in the medial aspect or the orbit secondary to ethmoid sinus disease. Adolescents and adults can develop subperiosteal abscesses in the superior portion of the orbit in connection with frontal sinusitis. On scans after contrast enhancement, the abscesses appear as elliptical, ovoid, or rounded regions of low density on CT and low intensity on T1WI MRI (Fig. 9.2), surrounded by enhancing tissues. Muscles adjacent to the abscess are swollen and displaced and show contrast enhancement. Rarely, there can be diffuse infection and inflammation of fat (Fig. 9.6). Extraconic fat, and at times intraconic fat, shows evidence of edema and inflammation and has the so-called dirty fat appearance. The connective tissue septa within the fat are accentuated, and their density is increased on CT; in an MR study, the intensity is decreased on T1WI and increased on T2WI. Both CT and MRI show the intracranial complications, such as epidural and subdural empyemas, but MRI is superior to CT. Thrombosis of venous sinuses or arteries is best demonstrated by MRI that includes MR venography and MR ungiography. Fungal infections, such as aspergillosis or mucormycosis, which tend to occur in immunocompromised or diabetic patients, can progress rapidly with osteomyelitis, thrombosis, and infarction.

Figure 9.6. Extensive infection and inflammation of the fatty tissues of the orbits and cheeks in a patient with rejection of a trans-planted liver. A. Coronal T1-weighted MR image shows irregular infiltrative process throughout the fat of both of the orbits, obscuring the anatomic detail of structures within the orbits. B. Coronal MRI after gadolinium injection and fat suppression shows slight-to-moderate enhancement of the involved fatty tissues (arrows) of both orbits and cheeks. This results in a grayish smeared appearance to the fat.

IDIOPATHIC ORBITAL INFLAMMATION

Idiopathic orbital inflammation, also commonly referred to as pseudotumor, is the most common cause of an intraorbital mass in patients aged 10 to 40 years. It often is unilateral, but it can also be bilateral. The idiopathic inflammation can be acute, subacute, or chronic. In the acute form, there is diffuse infiltration of tissues by lymphocytes, plasma cells, macrophages, and, at times, eosinophils. In the subacute and chronic forms, there is a variable degree of fibrosis, with some cases being primarily sclerotic, manifested by fixation and immobility of structures. On imaging, idiopathic inflammation shows a wide spectrum of changes, ranging from single muscle involvement (Fig. 9.7) to diffuse infiltration of the orbit (Figs. 9.8, 9.9, and 9.12).

Figure 9.7. Idiopathic orbital inflammation of the myositis type. Axial CT after contrast enhancement reveals enlargement of the left lateral rectus (black arrow), including its insertion (white arrow).

Figure 9.8. Idiopathic orbital inflammation. Irregular infiltrative process present around the globe and the optic nerve sheath complex (open arrow) is demonstrated on an axial contrast-enhanced CT scan. There is enlargement of rectus muscles, particularly the lateral rectus and especially at the insertion of the muscles on the globe (black arrows). There is also involvement of the extraconic fat lateral to the lateral rectus muscle.

Figure 9.9. Bilateral idiopathic orbital inflammation (pseudotumor), much more extensive on the left. Coronal MR image after gadolinium enhancement and fat suppression reveals diffuse enhancement of the left orbital fat and enlargement of the rectus muscles. In addition, there is a perioptic lobulated mass (black arrow). There is slight enhancement of the extraconic fat (open arrow) and some fullness and irregularity of the lacrimal gland (solid arrow) on the right side.

One of the more typical imaging findings of idiopathic inflammation is that of contrast-enhancing uveal-scleral thickening. However, any structure can be involved, with the lacrimal gland being most commonly affected. Often there are multiple sites and structures involved (i.e., lacrimal glands, muscles, fat) (Figs. 9.8, 9.9, and 9.12). Infrequently, the idiopathic inflammation may mimic a well-defined mass. When the process extends into the cavernous sinus (Figs. 9.10 and 9.12) or is primarily in the cavernous sinus (Fig. 9.11), then the constellation of findings represent Tolosa-Hunt syndrome.

	Figure 9.10. Idiopathic orbital inflammation on the right, with extension beyond the orbit. A. Axial T1-weighted MR image reveals markedly thickened right lateral rectus (straight arrow), and infiltrative process that envelops the posterior half of the optic nerve sheath complex and extends posteriorly (curved arrow) through the su-perior orbital fissure. B. Coronal T1-weighted image shows an expanded superior orbital fissure (open arrows) by the inflammatory mass. In addition, there is evidence of extension of the inflammatory process through the inferior orbital fissure into the pterygopalatine fossa (curved arrows). Compare with the normal high intensity of fat, in both the inferior orbital fissure and the pterygopalatine fossa. C. Axial image after gadolinium enhancement and fat suppression reveals fairly diffuse enhancement throughout the right orbit and of that portion that extends through the superior orbital fissure (arrow). D. Inversion recovery image shows the mass (white arrow) in the apex of the orbit as well as enlargement of the anterior portion of the cavernous sinus (black arrow). E. Axial T2-weighted image shows that the idiopathic inflammation is hypointense and therefore difficult to delineate from the also hypointense orbital fat.

Figure 9.11. Sphenoid sinusitis and right cavernous sinus inflammatory mass. A. Coronal T1-weighted image through the cavernous sinuses reveals diffusely opacified sphenoid sinus (thick vertical black arrow) and diffuse enlargement of the right cavernous sinus by a mass that encircles and constricts the intracavernous internal carotid artery (thin horizontal arrow). Compare with the normal size of the cavernous sinus and normal size of the internal carotid artery on the left side. B. Coronal T1-weighted MR image after gadolinium enhancement and fat suppression reveals marked enhancement of the inflammatory process in the sphenoid sinus and moderate enhancement of the inflammatory mass involving the right cavernous sinus. The straight arrow indicates the encased internal carotid artery, and the curved arrow points to the involved enlarged maxillary portion of the fifth cranial nerve. C. Axial T2-weighted MRI shows increased signal in the ethmoid and sphenoid sinuses involved by inflammation. There is decreased signal in the involved enlarged right cavernous sinus (open arrows).

Figure 9.12. Idiopathic orbital inflammation with extension into the right cavernous sinus. A. Coronal T1-weighted MRI shows a poorly defined hypointense mass involving a large portion of the right orbit. B. Coronal Tl-weighted MRI through the superior orbital fissure shows expansion and infiltration of the right superior orbital fissure (curved arrow) as well as sclerosis of the right pterygoid bone (short arrows). There is also infiltration beyond the bone into the adjacent soft tissues. C. A slightly more posterior coronal section shows infiltration and enlargement of the cavernous sinus (short arrows) and tilting of the chiasm (curved arrow) due to the superior extension of the mass. Solid arrow indicates involvement of the mandibular branch of the trigeminal nerve. D. Following contrast enhancement and fat suppression, there is diffuse prominent enhancement of the contents of the right orbit. Arrow points to the optic nerve sheath complex. E. Coronal T2-weighted MRI obtained using a HASTE sequence that takes 1.2 seconds per scan shows that Ihe mass is hypointense. Arrow indicates normal subarachnoid space, which surrounds the left optic nerve. Due to compression and infiltration by the mass, the subarachnoid space cannot be seen on the right. F. Axial HASTE Mitt shows the hypointense mass infiltrating the posterior orbit and orbital apex. Note the normal optic nerve sheath complex (arrow) demonstrated in the left orbit.

On CT, the density of the idiopathic inflammation is nonspecific, and enhancement varies with the type of process, being most prominent in acute forms (Fig. 9.7) and much less so in sclerotic types. On MRI, the idiopathic inflammation most typically is of low signal intensity, both on T1WI and on T2WI (Figs. 10E and 11 C). The acute myositic form shows low intensity on T1 but increased intensity on T2WI. As on CT, the degree of enhancement on MRI varies with the type of idiopathic inflammation.

The imaging findings of idiopathic orbital inflammation, although typical, are not specific and therefore need to be correlated with the clinical picture and response to steroid therapy. The differential diagnoses of idiopathic orbital inflammation include lymphoproliferative diseases, bacterial and fungus

Neoplasms

LYMPHOPROLIFERATIVE DISORDERS

Lymphoproliferative disorders in the orbit range from benign to malignant, and distinction among them is difficult clinically and by imaging. On imaging, they show a wide spectrum of patterns, and typically they tend to be extensive and lobulated as well as infiltrative (Fig. 9.13). There can be lobules of tumor that have both an extraconic and intraconic component. The lymphoproliferative disease can be unilateral or bilateral and can involve various structures: lacrimal glands, conjunctiva and eyelids, muscles, and perioptic or periglobar regions. The globes often are encircled and displaced but not deformed by the lobulated masses. Both CT and MRI demonstrate the morphology and extent of the lymphoproliferative le-sions. The rapidity of CT scanning is an advantage in elderly patients, who tend to develop the lymphoproliferative disorders. These patients often have difficulty lying flat and difficulty cooperating in the more claus-trophobic environment of MRI. However, the elderly patients have difficulty getting into the uncomfortable position required for coronal scanning, so in this regard, the multiplanar capability of MRI is an advantage. This becomes even more important when the process is at the apex of the orbit or has extraorbital extent through superior or infe-rior orbital fissures (Fig. 9.13). On CT, the lymphoproliferative masses are homogeneous, are similar in density to muscle, and. do not show significant contrast enhancement. On MRI, the lymphoproliferative masses are df low signal intensity on T1WI and show variable signal intensity on T2-weighted images, ranging from low to somewhat increased intensity. After injection of contrast material, there is usually fairly prominent enhancement noted, particularly with use of fat suppression (Fig. 9.13). The results of imaging, CT or MRI, combined with the clinical information in the older patient with slow development of proptosis or mass, often lead to the correct diagnosis of lymphoproliferative disorder.

Figure 9.13. Lymphoma. Axial MR scan after gadolinium enhancement and fat suppression reveals a lobulated mass (white arrow) at the apex of the left orbit. It causes displacement of the optic nerve sheath complex (open arrow).

NEUROGENIC TUMORS

Meningioma. Meningioma can originate in the orbit or can secondarily involve the orbit by extending from its primary intracranial location. Those meninglomas that secondarily involve the orbit, either by growing through bone or extending perioptically (Fig. 9.14), or through superior orbital fissure are more common than those that originate within the orbit. When the meningiomas originate in the orbit, they most commonly arise from the porloptic dural sheath and only rarely front ectopic rests elsewhere in the orbit. Meningiomas are well-delineated but lobulated tumors that show great propensity to grow through dura; along dura, and through bone. On CT, the meningiomas have a density similar to or greater than brain, produce bone sclerosis and bone expansion (Fig. 9.15. A and B), and contrast enhance markedly. On MRI, on T1-weighted images, meningiomas have similar intensity to that of brain; on proton density or on T2WI, they show variable intensity, ranging from low to high intensity (Fig. 9.15C). The contrast-enhanced MRI with fat suppression is most valuable in detection and delineation of orbital meningiomas (Figs. 9.14 and 9.15D). When there is early involvement of the orbital apex by a meniongioma, unless contrast material is used with thin contiguous sections, a meningioma may be missed. To demonstrate bony changes due to meningioma, it is necessary to use wide-window photography with CT (Fig. 9.15B). MRI can effectively demonstrate bony irregularity, sclerosis, bone marrow replacement, and bone scalloping produced by a meningioma.

Figure 9.14. Perioptic extent of meningioma. A. Axial MRI after gadolinium enhancement and fat suppression shows prominent enhancement of the meningioma including the perioptic components (open arrows). B. Coronal MRI after gadolinium enhancement and fat suppression demonstrates the encircling meningioma (arrows) around each optic nerve sheath complex.

Figure 9.15. Meningioma of the right orbit, right nasal cavity, and right anterior intracranial fossa. A. Coronal CT shows a mass (open short white arrows) in the superomedial aspect of the orbit, which has expanded that portion of the bony orbit. In addition, there is a faint calcification (while solid arrow) noted intracranially. An elongated mass (curved arrow) fills the right nasal cavity. B. A bone-window image of the same scan shown in A better reveals the expanded squared-off right orbit and also expanded right nasal cavity. In addition, there is bony sclerosis (arrows) involving the right ethmoid sinus and the crista galli. C. Coronal proton-density MRI shows a hypointense mass (white in the right orbit and a hyperintense mass (black arrows) intracranially. D. Coronal MRI after gadolinium injection and fat suppression shows slight enhancement of the intraorbital mass component of the meningioma (curved arrow) and marked enhancement of the intracranial component (straight arrow).

Peripheral Nerve Sheath Tumors

Plexiform neurofibroma. Plexiform neurofibroma is the most frequent orbital manifestation of neurofibromatosis. This tumor represents an overgrowth of the components of a peripheral nerve, is not encapsulated, and grows centripetally along the nerve from the periphery toward the center. Plexiform neurofibromas are highly vascular and diffusely infiltrating, producing irregularity and enlargement of muscles, lacrimal gland, and optic nerve sheath complex as well as expanding the bony orbit and its fissures. The infiltrating pattern is well demonstrated on CT and MRI; however, more definition and more precise delineation is provided by MRI (Figs. 9.16 and 9.17). While on CT the optic nerve sheath complex may be demonstrated to be enlarged and irregular, MRI shows separately the optic nerve and the surrounding plexiform neurofibroma (Fig. 9.16, B and C). Often the plexiform neurofibroma extends into the cavernous sinus and into the pterygopalatine fissure (Figs. 9.16 and 9.17). These tumors show low density on CT, low intensity on T1WI (Fig. 9.16A), and variable intensity on T2WI, ranging from low to moderately increased (Fig. 9.17A). They show prominent contrast enhancement both on CT and MRI (Figs. 16, B and C, and 17, B and C). Differential diagnosis includes extensive capillary lymphangiomas, which show a similar infiltrative pattern on imaging. The lymphangiomas generally show greater signal intensity on T2WI.

Figure 9.16. Plexiform neurofibroma of the right orbit and face. A. Coronal T1-weighled MRI shows a mass with infiltrative reticular pattern throughout the right orbit, causing obliteration and distortion of the normal tissue planes. The optic nerve sheath complex is enlarged and irregular. B. Following contrast enhancement and fat suppression, there is enhancement of the extensive plexiform neurofibroma, which is noted to encircle the optic nerve sheath complex (solid arrow). The plexiform neurofibroma also extends through the inferior orbital fissure and infiltrates muscles and fat (open arrows) below the orbit. C. Axial MRI after gadolinium enhancement and fat suppression shows well the optic nerve (open arrows.), which itself is normal but is surrounded by the extensive contrast-enhancing plexiform neurofibroma which also extends posteriorly into the cavernous sinus (solid arrow).

Schwannoma. Schwannomas are encapsulated tumors that grow eccentrically from peripheral nerves and occur infrequently in the orbital and periorbital regions. However, when there is a well-defined mass or elongated mass encountered in the orbit, the diagnosis of schwannoma should be considered. If the muscles supplied by the oculomotor nerve are atrophic, then the cause may be an intracavernous schwannoma of the oculomotor nerve (Fig. 9.18). Because schwannomas grow slowly and may be in an extraconic location, they can produce bone scalloping, or widening of fissures. If the tumors are round and homogeneous, they may be difficult to distinguish from such lesions as isolated neurofibromas or cavernous hemangioma. But if they show internal heterogeneity due to cystic or fatty regions, such a finding favors schwannoma. On CT, they are of low density and show contrast enhancement. On MRI, they are hypointense on T1WI (Fig. 9.18B) and hyperintense on T2WI and show prominent contrast enhancement.

Figure 9.18. Third-nerve schwannoma resulting in atrophy of the right rectus muscles. A. Coronal T1-weighted MRI shows marked asymmetry in the size of the rectus muscles when the two orbits are compared. The right inferior rectus, medial rectus, and superior rectus muscles are markedly atrophic. B. Coronal T1-weighted MRI through the cavernous sinuses shows a mass (arrows) of the intracavernous portion of the right oculomotor nerve. It displaces the intracavernous carotid inferiorly. C. Coronal T1-weighted MRI slightly more posterior to the section shown in B reveals marked asymmetry in the size of the cisternal portion of the oculomotor nerves. The right one (open arrows) is atrophic compared with the left one (solid arrow).

VASCULAR TUMORS

Capillary Hemangioma. Capillary hem-angiomas that consist of abnormal blood vessels with varying degrees of endothelial proliferation are considered by some to represent hamartomas. These tumors often present at birth or during the first few weeks of life, show exuberant growth during the first 6 months, reach a stationary phase by the first or second year of life, and finally slowly regress during the next 3 to 4 years. MRI is the preferable procedure for evaluation of these lesions because it shows their internal structure much better than CT does and thus leads to the correct diagnosis and differentiation from such lesions as lymphangiomas or rhabdomyosarcomas. On MRI, these lesions are poorly marginated and heterogeneous because of internal lobulations and vascularity (Fig. 9.19). On T1WI images, these lesions are of slightly higher signal intensity than extraocular muscles; on T2WI, they are hyperintense and show prominent enhancement (Fig. 9.19, B and C) after injection of contrast material. Rhabdomyosarcomas (Fig. 9.20) do not have internal lobulations and such vacularity as the capillary hemangiomas. Also, in contrast to the hemangiomas, the rhabdomyosarcomas often are hypointense on T2WI (Fig. 9.20C).

Cavernous Hemangioma. Cavernous hemangiomas are the most frequent orbital tumors of young adults. These lesions are encapsulated and therefore are well defined. They typically are located intraconically and are generally round or oval (Figs. 9.21 and 9.22). Again MRI has an advantage over CT by showing delicate septation within them (Fig. 9.22), indicating their internal lobular structure; also, scanning during or immediately after injection of contrast material shows heterogeneity caused by the opacification of the large vascular channels within the tumor. If there is an interval between injection of contrast material and scanning, then the lesion will appear homangiomas appear as well-defined homogenous masses that contrast enhance. On T1WI MRI, they are hypointense 10 fat and isointense to muscle; on T2WI, they become hyperintense (Fig. 9.22).

Figure 9.21. Cavernous hemangioma. A. Axial MRI after gadolinium enhancement and fat suppression shows a markedly enhancing oval mass (arrow) in the medial intraconal space of the left orbit. B. Coronal MRI after gadolinium enhancement and fat suppression localizes the mass (arrow) to the inferomedial aspect of the left orbit.

Figure 9.22. Cavernous hemangioma. Axial T2-weighted MRI shows a well-defined intraconic mass (arrow) displacing the left globe anteriorly. A septum can be seen within the hemangioma.

Lymphangiomas. Lymphangiomas occur in the orbit, even though the normal post-septal orbit does not contain lymphatic tissue. They are considered benign tumors of congenital origin that probably arise from misdirected vascular precursors. Most of the lymphangiomas present during childhood. They are histologically heterogeneous, con-sisting of dilated lymphatic vessels, dysplastic blood vessels, blood products in various stages of evolution, lymphocytic aggregates, bundles of smooth muscle fibers, and loose connective tissue septa. The lesions lack a capsule and often insinuate them-selves around structures or infiltrate tissues, thus crossing anatomic compartments. The lymphangiomas show great propensity for hemorrhage; therefore, they can present with sudden proptosis, which can be recurrent because of rebleeding or change in osmotic pressure within a resolving hematoma in the lymphangioma. The heterogeneity of the lymphangiomas is well demonstrated on CT and MRI images, but particularly with MRI because of its greater sensitivity to various tissues and blood products (Fig. 9.23). The infiltrating serpiginous and cystic character of the lesions is shown on CT, and if there is acute hemorrhage, it is specifically identified. However, when blood products break down, CT becomes less specific because the blood no longer appears as high density and mimics other fluids and tissues. On MRI, the lymphangiomas show a hetero-geneous intensity pattern both on T1 and T2WI (Fig. 9.23). Because of the frequent presence of blood products within lymphangiomas, it is important to perform a T2 sequence in which fat is suppressed as well as to perform contrast enhancement with fat suppression to differentiate the cystichemorrhagic components from interspersed fat lobules and contrast-enhancing venous channels. The structure of the lymphangioma depends on which components predominate: capillary, cavernous, or cystic. When the lymphangioma is primarily of the capillary type and has not hemorrhaged, it may be difficult to differentiate from other diffusely infiltrating lesions such as plexiform neurofibroma.

Figure 9.23. Extraconic lymphangioma. A. Axial T1-weighted MR scan shows an elongated hyperintense mass (arrows) in the medial extraconic space of the left orbit. The increased intensity of the mass is consistent with hemorrhage. B. T2-weighted axial MRI shows marked a decrease in the portion of the mass that was hyperintense on the T1-weighted image, indicating intracellular methemoglobin. Anterior and posterior to this hemorrhagic component are other cystic components (curved arrows) of the lymphangioma.

METASTATIC LESIONS

Orbital metastases are reported to be relatively infrequent; however, they are increasing because of longer survival of cancer patients. In adults, carcinoma of the breast is the most frequent orbital metastatic tumor, followed by carcinoma of the lung, prostate, gastrointestinal tract, kidney, and thyroid. In adults, 70% of metastases are related to the globe (Fig. 9.24); only 30% are orbital. In contrast, in children, the orbit is more involved than the globe. The pediatric orbital metastases are often due to embryonal tumors, neuroblastoma, Ewing's sarcoma, and leukemia. On imaging, most of the metastatic tumors are poorly defined and infiltrative.

Figure 9.24. Lymphoma metastatic to the anterior chamber of the left eye. A. Axial T1-weighted MRI shows increased intensity in the anterior chamber (arrows). Compare with the normal lower intensity of the anterior chamber of the right eye. B. Axial T1-weighted MRI following gadolinium injection reveals enhancement of the lymphoma in the anterior chamber.

Metastic breast carcinoma can mimic idiopathic orbital inflammation by infiltrating the uveoscleral junction, around the optic nerve sheath complex and the rectus muscles. On CT, these are of low density and show contrast enhancement. On MRI, such metastases are hypointense on T1WI but become hyperintense on T2WI, unlike most orbital idiopathic inflammations, which become hypointense. Metastatic scirrhous carcinoma produces enophthalmos and will not show hyperintensity on T2WI because of its fibrous content. Metastases to the bony walls of the orbit can produce a lytic or sclerotic pattern (Fig. 9.25 and 9.26). Carcinoma of the breast typically produces a lytic permative pattern (Fig. 9.25), while carcinoma of the prostate or neuroblastoma produces bony thickening, sclerosis, and soft-tissue mass. On images, metastatic prostate carcinoma, neuroblastoma, and meningioma may have a very similar appearance.

Figure 9.25. Metastatic breast carcinoma to the walls of the left orbit. A. Coronal CT after contrast enhancement reveals enhancing soft tissue mass extending from the involved bones (black arrowheads and white arrows). The mass has actually broken through the roof of the orbit (long black arrow). B. Axial CT after contrast enhancement reveals the abnormal, irregularity thickened bone, with tumor extending beyond the margin of the bone (open arrows). C. Axial CT photographed with a wide window to bring out bone detail shows extensive bony destruction (arrows) by the metastatic tumor.

Figure 9.26. Metastatic retinoblastoma to the left maxillary sinus and orbit. Coronal CT reveals a very large mass involving the left maxillary sinus, destroying its walls and extending into the adjacent structures, including the inferior portion of the left orbit.

ORBITAL WALL AND PARANASAL SINUS LESIONS

Langerhans Cell Histiocytosis. Disease entities formerly referred to as eosinophilic granuloma, Hand-Schuller-Christian disease, and Letterer-Siwe disease are now all designated as Langerhans cell histiocytosis, because all these disorders have the same histopathology. However, Langerhans cell histiocytosis represents a spectrum in terms of sites, extent of involvement, aggressiveness, and prognosis. The etiology is unknown but is thought to be related to an abnormality of the immune system. Orbital involvement may be focal and lytic or diffusely infiltrative (Fig. 9.27). When it is diffuse, the bony lesions can mimic metastatic neuroblastoma or leukemic infiltration.

Figure 9.27. Extensive Langerhans cell histiocytosis of both orbits and facial structures. A. Coronal T1-weighted MRI shows irregularity of the walls of the orbits, with prominent soft tissue, which is minimally hyperintense, encroaching on and widening the extraconic space of both orbits (open arrows). B. Axial MRI after gadolinium enhancement and fat suppression reveals a moderately enhancing soft-tissue mass that has extended beyond the confines of the involved irregular bones of the orbit. The open arrows indicate the soft-tissue components of the histiocytosis.

Fibrous Dysplasia. Fibrous dysplasia, which probably represents a developmental mesodermal disorder, typically presents during the first two decades of life. Normal bone is replaced by immature bone within a fibrous stroma, which causes expansion of bone and encroachment on or upon adjacent structures. The monostatic facial form can occur in the orbit, resulting in proptosis and visual loss if there is involvement of the optic canal. While CT is better in characterizing and delineating the bony changes of fibrous dysplasia (Fig. 9.28A), MRI provides better information about the soft tissues affected by the expansile bone (Fig. 9.28B). On MRI, regions of fibrous dysplasia may be so hypointense (Fig. 9.28B) that they appear similar to air and, therefore, may be difficult to delineate within the paranasal sinus and If small may not be detected. In addition to fibrous dysplasia, other fibro-osseous lesions, such as osteomas, ossifying fibromas, and osteoblastomas may involve the facial bones and paranasal sinuses and thus encroach on the orbits (Fig. 9.29). On CT, they are of increased density; on MRI, they are hypointense on both T1- and T2-weighted images (Fig. 9.29).

Paranasal Sinus Diseases. The orbit and paranasal sinuses have many walls in common; therefore, it is not unusual to see intraorbital extension of various paranasal diseases. Infections of paranasal sinuses, particularly, have a tendency to involve the orbits. Both acute (Fig. 9.2) and chronic (Fig. 9.30) infections can affect the orbits. MRI can be especially helpful in longstanding cases when the content of the sinuses has become viscous and desiccated or even calcified, and when the sinuses are expanded and their walls demineralized. On CT, such processes can mimic tumors such as chondrosarcoma MRI leads to the correct diagnosis by show. ing that the internal architecture of the sinuses is preserved, although the air cells are expanded. Most importantly, MRI reveals that the contents of the sinuses have such a low signal (on T1WI, but particularly on proton density and T2WI) that it represents inspirated material rather than tumor. In many such cases, the underlying Cause is chronic fungal infection. However, Nom and coworkers have shown that the signal of the sinus contents depends on the concentration of macromolecular protein, the amount of free water, and the viscosity Of contents and is not due to the fungal infection itself, blood, or paramagnetic ions.

Figure 9.28. Vibrous dysplasia. A. Axial CT reveals diffuse expansion and increased density (solid arrows) involving the right floor of the anterior cranial fossa and extending slightly to the left. The roof of the right orbit, the planum sphenoidale, and the right anterior clinoid process are extensively involved. Open arrows indicate narrowed optic canals. B. Coronal MRI on another patient with fibrous dysplasia reveals, on a T1-weighted image, expansion and a prominent decrease in signal in the involved portions of the sphenoid bone (black arrows). There is marked narrowing of the left optic canal (open arrow), constricting the left optic nerve. Another open arrow points to the normal right optic nerve.

Figure 9.29. Expansile fibro-osseous lesion of the right maxillary sinus. A. Coronal T1-weighted MRI shows a large, expansile, fairly homogeneous mass that has encroached on the nasal cavity and right ethmoid sinuses as well as the inferior aspect of the right orbit, elevating the inferior mews (arrow). B. Following gadolinium enhancement and fat suppression, there is faint and mottled enhancement of the mass, which is more prominent on the lateral aspect of the mass. The mass remains mostly hypointense. C. Axial T2-weighted image through the floors of the orbits reveals heterogeneous, primarily hypointense mass. Open arrows indicate the expansion of the mass into the adjacent structures. Note that the mass is almost as hypointense as the normal aerated. maxillary sinus on the left.

Figure 9.30. Allergic aspergillosis of the paranasal sinuses. A. T1-weighted axial MRI reveals a markedly expansile process involving the ethmoid and sphenoid sinuses. Due to the marked expansion of the paranasal sinuses, there is encroachment onto the orbits and displacement of the medial rectos muscles. Portions of the mass are similar to soft tissues, and other portions are markedly hypointense (arrows). B. Coronal T1-weighted MRI shows the heterogeneous expansile mass, which causes elevation and thickening of the roof of the left ethmoid (open arrow). A portion of the mass is markedly hypointense (straight highlighted arrow). Note that the hypointense component of the mass is almost as dark as the air in the maxillary sinus (curved arrows). C. Coronal MRI after gadolinium enhancement and fat suppression reveals that the mass contains lobules of hypointensity that enhance on the periphery. Because of the expansion and its effect on the bone of the anterior cranial fossa, there is enhancement of the chilli (open arrows) adjacent to the affected bone. D. Coronal T2-weighted MRI through the sphenoid sinus reveals it to be expanded by the hypointense mass, and the planum sphenoidale is lifted and thickened (open arrows). There is narrowing of the left optic canal, resulting in a smaller left optic nerve (curved arrow), compared with the normal right optic nerve (curved arrow). The straight arrow indicates the high intensity of fluid trapped in the left maxillary sinus.