Imaging of the Head and Neck II

Edited by Michael Forsting

Ch011-A05375.indd 469 8/13/2008 3:33:56 PM

Orbit 11Colin S. Poon Michael Abrahams James Abrahams

Anatomy

The soft tissue structures of the orbit are contained within a bony cavity. These soft tissue structures include the globe, the extraocular muscles, the optic nerve–sheath complex, the lacrimal apparatus, and various vascular and nerve structures.

• Bony Anatomy

The bony orbit is a conical structure with the apex pointing posteriorly. The orbital roof is composed of the frontal bone and is thinner anteriorly. The medial wall is com-posed of the frontal process of the maxillary bone anteri-orly, the lamina papyracea of the ethmoid air cells at the midportion, and the sphenoid bone posteriorly. The lamina papyracea is very thin, and not surprisingly it is a common site of orbital blowout fracture and spontaneous dehiscence of orbital fat. The lateral orbital wall is formed by the orbital surface of the zygomatic bone. The orbital floor is formed by the orbital plate of the maxilla, the orbital process of the palatine bone, and the orbital surface of the zygomatic bone. The orbital plate of the maxilla is thin and a common site of inferior blowout fracture.

Multiple foramina and canals go through the bony orbits (Box 11-1). The optic canal (also called optic foramen) is located at the orbital apex. It is bordered by two bony spikes of the lesser wing of the sphenoid bone, commonly referred as the optic struts. The canal contains the optic nerve and the ophthalmic artery, both of which are con-tained within a dural sheath.

The superior orbital fissure is located at the margin between the lateral wall and the orbital roof. The greater wing of the sphenoid bone forms its lateral boundary, while the lesser wing forms its medial boundary. The supe-rior orbital fissure contains the superior ophthalmic vein; the oculomotor (III), trochlear (IV), and abducens (VI) nerves; and the ophthalmic division of trigeminal nerve (V1). The superior orbital fissure forms the largest com-munication between the orbit and intracranial structures and therefore forms a conduit for infectious or neoplastic processes between the orbital apex and the cavernous sinus.

The inferior orbital fissure is located at the margin between the lateral wall and the orbital floor. It contains the infraorbital (branch of V2) and zygomatic nerves, the nerve branches from the pterygopalatine ganglion, and venous connection between the inferior ophthalmic vein and the pterygoid plexus. The inferior orbital fissure con-nects with the pterygopalatine fossa and the masticator space/infratemporal fossa, allowing the spread of deep facial infection and neoplasm to the orbital apex.

The globe is essentially a spherical structure, with the wall consisting of three layers: retina (innermost), choroids (middle), and sclera (outermost). These layers cannot be resolved with current clinical imaging technology, unless they are separated by pathologic processes (e.g., retinal detachment). The globe is divided into three fluid-filled cavities: anterior chamber, posterior chamber, and vitreous cavity.4,40 The anterior chamber and posterior chamber constitute the anterior segment, while the vitreous cavity constitutes the posterior segment. The anterior chamber extends from the cornea to the iris. The posterior chamber extends from the posterior surface of the iris to the anterior surface of the vitreous. The vitreous cavity is posterior to the posterior chamber.

The anterior border of the orbit is formed by the orbital septum, a fibrous structure adherent to the inner margin of the orbital rim with central portions that extend

471

Box 11-1. Major Foramina of the Orbit and Their Neurovascular Contents

Optic CanalOptic nerveOphthalmic artery

Superior Orbital FissureCranial nerves: III, IV, VI, V1

Lacrimal and frontal nervesSuperior and inferior ophthalmic veins

Inferior Orbital FissureCranial nerve: V2

Zygomatic nerveInfraorbital vessels

Ch011-A05375.indd 471 8/13/2008 3:33:57 PM

472  II  Imaging of the Head and Neck

into the tarsus of the eyelids. Although there are a few ori-fices for passage of vessels, nerves, and ducts, the septum forms an effective barrier to prevent superficial processes from extending into the orbit proper. A pathologic process such as cellulitis may be designated as preseptal versus post-septal. A postseptal process signals the involvement of more critical structures of the orbit, the possibility of extension into the cavernous sinus and intracranial structures.

• Soft Tissue Anatomy

There are seven extraocular muscles: the superior, inferior, medial and lateral rectus; the superior and inferior oblique; and the levator palpebrae superioris muscles. The levator palpebrae muscle can be seen immediately above the supe-rior rectus muscle. With the exception of the inferior oblique muscle, all extraocular muscles originate from the annulus of Zinn, a tendinous ring in the orbital apex. They pass anteriorly and insert on the globe just behind the corneoscleral border. The four rectus muscles and the fibrous septa connecting between them form the muscle cone of the orbit. The intraconal space is filled with orbital fat. Orbital vessels, sensory and motor nerves to the extra-conal muscles, and the optic nerve–sheath complex also traverse the intraconal space.

The optic nerve may appear straight or slightly tortu-ous depending on the eye position. It consists of three segments: orbital, canalicular, and intracranial. The orbital segment is covered by the same meningeal sheaths as the brain. The normal diameter of the optic nerve is up to 4 mm. A layer of cerebrospinal fluid can be seen between the meningeal sheath and the optic nerve.

The extraconal space represents the area between the muscle cone and the bony orbit. This space contains orbital fat and the lacrimal gland. The lacrimal gland is located superolateral to the globe. The upper margin of the gland is convex. The lower margin is concave and lies on the levator palpebrae and lateral rectus muscles. The lacrimal system drains through the lacrimal ductal system near the medial canthus. It consists of the superior and inferior puncta, their associated ducts, the lacrimal sac, lacrimal duct, and the valve of Hasner, which is a draining orifice inferolateral to the inferior nasal turbinate.

The vascular anatomy of the orbits can be well dem-onstrated on high-resolution magnetic resonance imaging (MRI)15 and computed tomography (CT) angiography. The primary arterial supply to the orbit is the ophthalmic artery. It is superior to the optic nerve and can be seen crossing the optic nerve almost perpendicularly (see Fig. 11-4). The ophthalmic artery most often originates from the internal carotid artery. The origin is usually at the anteromedial aspect of the internal carotid artery as it exits the cavernous sinus. Variants of its origin include the cav-ernous segment of the internal carotid artery and the middle meningeal artery (i.e., external carotid artery branch). Secondary arterial supply to the orbits comes from the external carotid artery. Because the orbits receive

blood supply from both the internal and external carotid arteries, orbital arteries may serve as anastomosis between the two arterial systems.

The largest orbital vein visualized on CT or MRI is the superior ophthalmic vein. It can be seen arising near the base of the nose, coursing anteromedially to postero-laterally, and draining into the cavernous sinus. It crosses over the optic nerve in its mid course, at approximately 20 degrees (see Fig. 11-4). The midportion of the superior ophthalmic vein is an intraconal structure that lies between the superior rectus muscle and the ophthalmic artery. The inferior ophthalmic vein is much smaller than the superior ophthalmic vein. It is usually not well visualized on CT or MRI studies. Both the superior ophthalmic vein and infe-rior ophthalmic vein receive tributaries from the veins of face and nose.

Imaging Techniques

The major modalities for imaging of the orbits include CT and MRI. The abundance of intraorbital fat provides good intrinsic soft tissue contrast on CT for most clinical applica-tions. The advances of multidetector CT technology now make high-resolution CT imaging possible. The source images can be reformatted in different planes, providing high-resolution isotropic imaging. This renders the previ-ous advantage of multiplanar capability of MRI obsolete. CT is superior to MRI for delineation of osseous structures and calcifications. It requires short imaging time and is therefore less sensitive to motion of the globe and eyelid. CT imaging can be completed quickly and requires less patient cooperation, making it ideal for imaging orbital trauma.

Compared to CT, MRI provides superior soft tissue contrast. It also provides better imaging details of the intra-cranial structures. When it is important to assess intracra-nial abnormalities, either as direct extension of orbital lesions or as associated lesions in certain diseases (e.g., in multiple sclerosis), MRI is superior to CT.

In the past, evaluation of suspected vascular lesions of the orbits required conventional angiography. The advances in CT angiography and MR angiography now allow many vascular lesions to be evaluated noninvasively. In some cases, conventional angiography can be foregone.

CT and MRI often provide complementary roles in orbital imaging. The choice of CT versus MRI for initial imaging of the orbits depends on the clinical problem. CT is usually preferred for trauma, for evaluation of the bony orbits or calcified lesions, and when MRI is contraindi-cated. For other applications, MRI is generally preferred because of the absence of radiation risks and its high soft tissue contrast. MRI is the initial imaging of choice for evaluation of the optic nerve, other cranial nerves, and intracranial lesions. Exceptions can be found in a small number of optic nerve meningiomas, which are very small

Ch011-A05375.indd 472 8/13/2008 3:33:58 PM

11  Orbit  473 11

and mostly calcified. These lesions may be missed by MRI and are better detected by CT.

• Computed Tomography

The orbits are often included in routine CT head or maxil-lofacial CT examinations. These screening examinations are usually performed according to the standard head or maxillofacial CT protocols.

When dedicated orbital CT is performed, thin sections (usually less than 3 mm and preferably less than 1.5 mm) are acquired. Coronal images are especially important in that cross-sectional evaluation of all of the intraorbital structures is optimal (e.g., extraocular muscles, optic nerve–sheath–nasal complex, vessels, and globe) (Fig. 11-1). This plane is also imperative for assessing spread of

processes from surrounding structures (e.g., paranasal sinuses, trauma, tumor). Before the era of multidetector CT, direct coronal scanning was often performed to provide the best spatial resolution in this plane. Multidetector CT, however, allows high-resolution reformation in any plane. Axial scanning (Fig. 11-2) also has the advantage that it can minimize the problem of streak artifacts from dental hardware, a problem often encountered previously with direct coronal imaging (Fig. 11-3).

A typical orbital CT protocol can be performed with scanning in the axial plane. This plane is usually chosen to be parallel to the orbital long axis. In practice, imaging is performed in the plane parallel to the infraorbital-meatal line. Coronal reformation should be included in the routine protocol. This can be performed in the plane per-pendicular to the axial plane. Parasagittal reformation, in

C

Laminapapyracea

Infraorbital foramen

Frontal bone Frontozygomatic suture

Zygomaticbone

Maxillary bone

Superior orbitalfissure

Inferior orbital fissure Optic strut

Optic canalAnterior

clinoid process

D

A g

a

b

c d

e

f

B

g

h

i

j

b

a

c d e f

Figure 11-1. A, Coronal CT scan: normal anatomy. Lateral rectus (a), superior rectus (b), medial rectus (c), superior oblique (d), levator palpebrae superioris (e), lacrimal gland (f), inferior oblique (g). B, Medial rectus (a), superior oblique (b), ophthalmic artery (c), superior rectus/levator palpebrae superioris complex (d), dural sheath (e), superior ophthalmic vein (f), subarachnoid space (g), optic nerve (h), inferior rectus (i), lateral rectus (j). C, Bony orbits at mid anterior level. D, Bony orbits at orbital apex.

Ch011-A05375.indd 473 8/13/2008 3:34:01 PM

474  II  Imaging of the Head and Neck

C

Lacrimal gland

Lacrimalvein

Superiorophthalmic vein

Superior orbital fissure

B

Lateralrectus m.

Ciliarybodies Medial rectus m. Anterior chamber

Lens

Vitreouscavity

Ophthalmic artery

Optic nerveand sheath

Superior orbital fissure

AInferior

orbital fissureInferior rectus m.

Lacrimal sacand duct

Figure 11-2. Normal orbital anatomy. Direct axial CT scanning from inferior to superior. A to D, Soft tissue window. E, Bone window.

a plane parallel to the long axis of the optic nerve, may also be added.

Intravenous contrast is often used in the evaluation of inflammatory, infectious, neoplastic, and vascular orbital diseases. For evaluation of vascular lesions, a bolus injec-tion may be used for better depiction of its arterial blood supply.

When orbital varix is suspected, the CT study should be repeated without and with the Valsalva maneuver.

Enlargement of a lesion with the Valsalva maneuver is indicative of an orbital varix. Less commonly, cavernous hemangiomas may enlarge with the Valsalva maneuver.17 In patients unable to cooperate, similar effects can be obtained by positioning the patient prone during scanning.

CT angiography can provide good depiction of the major vascular anatomy in the orbits. In addition to the ophthalmic artery and superior ophthalmic vein, their

Ch011-A05375.indd 474 8/13/2008 3:34:02 PM

11  Orbit  475 11

D

Trochlear m.

Superior rectus m.

Superioroblique m.

Orbitalseptum

Lacrimal gland

ESuperior orbital

fissureOptic canal Optic

strut

Sphenotemporalsuture

Greater wingof sphenoid

Sphenozygomaticsuture

Zygomaticbone

Laminapapyracea

Nasomaxillarysuture

A B

Figure 11-2, cont’d

Figure 11-3. A, Coronal CT scan in a patient whose extensive dental hardware obscures detail in the orbits. B, Axial scanning with coronal reformation avoids this problem, showing enlargement of the extraocular muscles on the left side. In view of the patient’s known hyperthyroidism, this finding was thought to represent Graves’ disease.

Ch011-A05375.indd 475 8/13/2008 3:34:04 PM

476  II  Imaging of the Head and Neck

branches, tributaries, and many other smaller vessels can often be seen and traced. The study can be performed as part of a CT angiographic study of the head and neck (Fig. 11-4).66 Bolus injection of iodinated contrast is required. It is important to use a field-of-view sufficiently wide to include extraocular pathology that may be associated with the vascular orbital lesions, such as carotid-cavernous fistula.

CT dacryocystograms can be performed by administra-tion of contrast material into the nasolacrimal duct to evaluate for patency (Fig. 11-5). This requires cannulation of the lacrimal duct, usually by an ophthalmologist.

• Magnetic Resonance Imaging

MRI of the orbits can be performed with the head coil. For high-spatial-resolution imaging of the anterior orbital structures, special orbital surface coils may be advanta-geous. However, the sensitivity of surface coils decreases

rapidly with distance from the coils, leading to rapid signal falloff and inadequate coverage of deeper structures.

For routine imaging, the field-of-view should include the cavernous sinus, the optic chiasm, the optic tracts and radiations, and the nuclei of the oculomotor, abducens, and trochlear nerves in the midbrain and pons.

The protocol should include T1-weighted and T2-weighted imaging in axial and coronal planes (Fig. 11-6). Intravenous gadolinium contrast is routinely used. For dedicated orbital imaging, fat suppression is usually per-formed for T2-weighted imaging and postgadolinium imaging to prevent the obscuration of enhancing lesions by the high intraorbital fat signal (Figs. 11-7 and 11-8). The fat suppression for fluid-sensitive imaging (i.e., T2-weighting) can also be performed effectively using inver-sion recovery (Fig. 11-9).30

Orbital MRI is susceptible to image artifacts because of several factors.25 First, chemical shift artifacts may be seen at the interface of the orbital fat and the globe. Similar

aa

b

B

a

A

C

a

b

a

Figure 11-4. CT angiography of the orbits, superior to inferior (A to C). The ophthalmic artery (a) arises from the internal carotid artery as it exits the cavernous sinus. It enters the orbit through the optic canal and crosses the optic nerve underneath the superior rectus muscle. The superior ophthalmic veins (b) cross the optic nerve more distally and at a more obtuse angle.

Ch011-A05375.indd 476 8/13/2008 3:34:05 PM

11  Orbit  477 11

artifacts may also be present if silicone oil is used to fill the globe in treatment of retinal detachment. These chemical shift artifacts can be reduced by using fat or silicone satura-tion, using a higher gradient strength, or narrowing the bandwidth. Second, the proximity of orbital structures to

the air cavities of paranasal sinuses makes orbital imaging susceptible to image artifacts. Exogenous metallic materi-als, such as cosmetics, can also lead to susceptibility arti-facts. Third, motion artifacts may be present. To minimize motion of the globe, a patient can be asked to fixate his or

A B

Figure 11-5. A, Coronal reformatted image from a CT dacryocystogram shows dilation of the lacrimal duct (arrow). The nasal septum and inferior turbinate are deviated leftward and cause obstruction at the valve of Hasner. B, The obstruction is only partial, as evidenced by the presence of contrast material in the posterior nasopharynx (arrow).

A

e

b

c

d

a

f

B Ca

b

c

ed f

g

h

Figure 11-6. A, Coronal T1-weighted MRI study: normal anatomy. Superior ophthalmic vein (a), lateral rectus (b), inferior rectus (c), medial rectus (d), superior oblique (e), superior rectus-levator palpebrae superioris complex (f). B, Axial T1-weighted MRI study: normal anatomy. Optic nerve in the optic canal (a), optic nerve sheath complex (b), medial rectus (c), anterior chamber (d), lens (e), lid (f), medial and lateral aspects of the orbital septum (arrow; g), lateral rectus (h). C, Axial T2-weighted MRI study: posterior visual apparatus. Position of lateral geniculate body (arrows), path of optic radiations (arrowheads).

Ch011-A05375.indd 477 8/13/2008 3:34:10 PM

478  II  Imaging of the Head and Neck

ab

c

d

e

f

g

h

Figure 11-7. Coronal T1-weighted MRI study with fat saturation: normal anatomy. a, superior rectus/levator palpebrae superioris complex; b, superior ophthalmic vein; c, optic nerve; d, lateral rectus; e, dural sheath; f, inferior rectus; g, medial rectus; h, superior oblique.

A B

Figure 11-8. A, Precontrast axial T1-weighted image performed without fat suppression demonstrates a mass at the left orbital apex in a patient with known cutaneous lymphoma. B, Postcontrast T1-weighted image of the same patient in which the fat saturation pulse failed to suppress the orbital fat (this may be due to dental artifact). The lesion demonstrates marked contrast enhancement and is now indistinguishable from the high signal of the orbital fat. This case illustrates the importance of performing nonsuppressed precontrast images and fat-suppressed postcontrast studies.

a c

b

her vision at a certain object when the eyes are open. Tem-poral averaging can also be performed.

Approach to Differential Diagnosis

A large number of disease processes can involve the orbits, and orbital complaints such as proptosis, orbital pain, visual loss, and ophthalmoplegia are nonspecific. Propto-sis is the abnormal protrusion of the globe; exophthalmos is the abnormal prominence of the globe. On imaging, proptosis is best evaluated at a level of the lens on axial images. A line connecting the most distal tips of the lateral

Figure 11-9. Coronal inversion recovery, fast-spin echo MRI study: normal anatomy. a, optic nerve; b, subarachnoid space with cerebrospinal fluid; c, optic nerve sheath.

orbital walls is drawn. The distance from the anterior margin of the globe to this line should not exceed 21 mm.28

• General Principle

Various characteristics of an orbital lesion can be used to help construct a differential diagnosis. These include its location, anatomic structure, and imaging features and the

Ch011-A05375.indd 478 8/13/2008 3:34:12 PM

11  Orbit  479 11clinical presentation of the patient.21,34,36,58 Using a com-partmental approach, a lesion is first localized to one of the four compartments: globe, optic nerve–sheath complex, intraconal space, or extraconal space. Differential diagnosis of an extraconal lesion can be further refined if it can be determined to be associated with the lacrimal gland and apparatus. Once the primary location of a lesion is deter-mined, other parameters including imaging features (e.g., characteristics of margin, associated bony changes, enhance-ment patterns), pathophysiologic basis, age of presenta-tion, and chronicity can be considered to further reduce the differential diagnosis. The presence of calcification may also be helpful in refining the differential diagnosis, espe-cially for globe lesions.

Obviously, some lesions may extend over more than one compartment. Nevertheless, this compartmental approach helps simplify the diagnostic thought process. The optic nerve–sheath complex, strictly speaking, is also an intraconal structure. However, because of its unique significance, it can be considered a separate compartment to improve the specificity of the differential diagnosis. Dif-ferential diagnosis of orbital lesions is summarized in Boxes 11-2 to 11-7.

Apart from aiding differential diagnosis, the localiza-tion of a lesion in the extraclonal space versus the intra-clonal space may also have management implications. In general, intraconal lesions may require surgical attention, whereas extraconal lesions may be amenable to medical management.

Box 11-2. Differential Diagnosis of Globe Lesions21,34

CongenitalPersistent hyperplastic primary vitreousCoat’s diseaseColobomaGlobe hypoplasia/aplasia

DegenerativeOptic nerve drusenPhthisis bulbiStaphyloma

TraumaVitreous hemorrhageChoroidal hematomaChoroidal effusionForeign body

InflammatoryOrbital pseudotumor (uveal/scleral thickening)Sclerosing endophthalmitis (Toxocara canis)

NeoplasmUveal melanoma (adults)Retinoblastoma (children)MetastasisChoroidal hemangiomaMedulloepithelioma

Box 11-3. Differential Diagnosis of Globe Lesions Associated with Calcification21,34

CongenitalDegenerativeCataractsOptic nerve drusenPhthisis bulbiRetinal detachment (chronic)Retrolental fibroplasiaCalcification of ciliary muscle insertionIatrogenic (e.g., scleral banding)

TraumaForeign body

InflammatoryInfection (cytomegalovirus, herpes simplex, rubella, syphilis,

toxoplasmosis, tuberculosis)

NeoplasmAstrocytic hamartoma (neurofibromatosis, tuberous sclerosis, von

Hippel-Lindau syndrome)Retinoblastoma (children)Choroidal osteoma

MetabolicHypercalcemiaSarcoidosis

Box 11-4. Differential Diagnosis of Optic Nerve Sheath Lesions21,34

TraumaContusionHematomaOptic nerve avulsion

InfectionToxoplasmosisTuberculosisSyphilis

Noninfectious InflammatoryThyroid ophthalmopathyOptic neuritisPseudotumorSarcoidosis

VascularCentral retinal vein occlusionNeoplasmOptic nerve gliomaMeningiomaNeurofibromaSchwannomaLymphoma/leukemiaMetastasisHemangioblastomaHemangiopericytoma

MiscellaneousIncreased intracranial pressureOptic hydrops

Ch011-A05375.indd 479 8/13/2008 3:34:12 PM

480  II  Imaging of the Head and Neck

Box 11-5. Differential Diagnosis of Conal/Intraconal Lesions21,34

TraumaHematomaForeign body

InfectionCellulitisAbscess

Noninfectious InflammatoryThyroid ophthalmopathyPseudotumorSarcoidosisWegener’s granulomatosis

VascularCarotid-cavernous fistulaVenous varixSuperior ophthalmic vein thrombosisVenous angiomaArteriovenous malformationCavernous hemangioma (adults)Capillary hemangioma (children)Lymphangioma

NeoplasmLymphomaMetastasisRhabdomyosarcoma (children)HemangiopericytomaNeurofibroma/schwannoma (cranial nerve III, IV, VI)Ectopic meningioma

Box 11-6. Differential Diagnosis of Extraconal Lesions21,34

TraumaFractureHematomaInfectionCellulitisAbscess

Noninfectious InflammatoryPseudotumorPostviral syndrome (lacrimal gland)Sjögren’s syndrome (lacrimal gland)Mikulicz’s syndrome (lacrimal gland)

NeoplasmMetastasisPrimary malignancy from adjacent structuresBenign mixed tumor (lacrimal gland)Adenoid cystic carcinoma (lacrimal gland)Non-Hodgkin’s lymphomaRhabdomyosarcoma (children)

CongenitalCephaloceleDermoid/epidermoid

Box 11-7. Differential Diagnosis of Lacrimal Gland and Apparatus Lesions1,21,34

TraumaHematoma

InfectionDacryoadenitis

Noninfectious InflammatoryPseudotumorPostviral syndromeSarcoidosisSjögren’s syndromeMikulicz’s syndromeWegener’s granulomatosis

NeoplasmPapillomaBenign mixed tumor (pleomorphic adenoma)Adenoid cystic carcinomaMucoepidermoid carcinomaAdenocarcinomaMalignant mixed tumorUndifferentiated carcinomaSquamous cell carcinomaSebaceous carcinomaPrimary malignancy from adjacent structuresNon-Hodgkin’s lymphomaMetastasisDermoid/epidermoid

CongenitalDacryoceleDacryocystocele

• Specific Clinical Scenarios

There are a few clinical scenarios that may be helpful to have their own differential consideration. The first is lesions of the lacrimal gland and apparatus.1 Lacrimal lesions are most often benign inflammatory processes, with tumors being less common. Viral adenitis is the most common acute process. More chronic inflammatory processes include sarcoidosis, Wegener’s granulomatosis, and Sjögren’s syndrome. Histologically, the lacrimal gland is analogous to the minor salivary gland in other regions of the head and neck. They therefore share many common pathologic processes. Most lacrimal gland tumors are epi-thelial cell tumors, with half of these being benign mixed tumors and half carcinomas. Lymphoma also occurs com-monly at the lacrimal gland fossa.

In a young patient presenting with leukokoria, one will need to exclude retinoblastoma. Other differential considerations include developmental and congenital con-ditions such as retinopathy of prematurity, Coats’ disease, persistent hyperplastic primary vitreous, toxocariasis, retina dysplasia, and congenital retinal fold.

Ch011-A05375.indd 480 8/13/2008 3:34:13 PM

11  Orbit  481 11

Pathophysiology

Orbital diseases can be categorized based on their patho-physiology: trauma, infection, noninfectious inflamma-tion, neoplasm, vascular lesions, congenital and developmental abnormalities, and degenerative condi-tions. A more popular approach for differential diagnosis is based on a compartmental approach (see above).

• Trauma

CT is the imaging method of choice for evaluation of orbital trauma. The most common traumatic injury is fracture of the orbital walls. Less commonly, hemorrhage in the globe, globe rupture, perforation and penetrating injury, and contusion or avulsion of the optic nerve sheath may occur. A common type of orbital fracture is “blowout” fracture, which results from increased intraorbital pressure transmitted to the orbital walls secondary to blunt trauma (Fig. 11-10). Blowout fractures most often involve the inferior and medial walls because they are the thinnest. Intraorbital soft tissue contents may herniate through the fracture. Muscle entrapment is a potential complication of orbital fractures. Because the extraocular muscles are teth-ered to the orbital walls by tiny fibrous strands that are too small to image on CT or MRI, muscle entrapment may occur even without herniation of the muscle itself.35

Evaluation for foreign bodies is best performed with thin-section CT (Fig. 11-11). Wood fragments pose a chal-lenge to CT evaluation because they may have variable densities owing to differences in hydration. Wood may appear hypodense, isodense, or hyperdense. Air may be present within a wood fragment.55 Therefore, unusual air pockets should be evaluated carefully.

• Infection

Orbital infection is most often caused by direct extension from adjacent structures; hematogenous infection is less common. It is important to localize orbital infection to the following compartments: (1) preseptal versus postseptal and (2) extraconal versus intraconal. Preseptal or extra-conal infection can usually be treated by standard antimi-crobial therapy. Postseptal or intraconal infection requires more aggressive management because of the risk of neuro-vascular injury and further intracranial spread.20 Identifica-tion of orbital abscesses is also crucial because they may require surgical intervention.

Orbital infection is most commonly caused by con-tiguous spread of sinusitis or a superficial periorbital cel-lulitis of the face. In children, infection is most commonly secondary to extension from ethmoid air cells (Fig. 11-12), whereas in adults, extension from the frontal sinus is most common (Fig. 11-13).63 Common organisms include Strep-tococcus pneumoniae and beta-hemolytic streptococci. Hae-mophilus influenzae, staphylococci, and anaerobes are less common.

mm

Figure 11-10. CT scan shows blowout fracture of the left orbital floor with herniation of extraconal fat and inferior rectus muscle (arrow). m, maxillary sinus.

Figure 11-11. CT scan shows intraconal metallic foreign bodies just medial to the medial rectus (large arrow) and intraocular (small 

arrow). Scleral band in place in right globe (arrowheads).

Figure 11-12. CT scan shows an extraconal subperiosteal abscess (small arrowheads). This is a complication of ethmoid sinusitis. A thickened, displaced medial rectus (large arrowhead) and preseptal soft tissue swelling (arrows) are seen.

Ch011-A05375.indd 481 8/13/2008 3:34:14 PM

482  II  Imaging of the Head and Neck

muscle enlargement is severe), and inflammatory changes of the periorbital fat (Fig. 11-14; see Fig. 11-3). There may be increase of the intraorbital fat. The lacrimal glands may also be affected. The extraocular muscles most often affected, in descending order of frequency, are the inferior rectus, medial rectus, and superior rectus–levator palpe-brae muscle complex.49 Periorbital soft tissue swelling and proptosis may be seen. Involvement is usually bilateral but may be asymmetrical.

Direct involvement of the globe and optic nerve sheath is uncommon. However, secondary compression of the optic nerve sheath may occur and can lead to irreversible visual loss. It is important to assess for this possibility on imaging.

• Orbital Pseudotumor

Orbital pseudotumor is also known as idiopathic orbital inflammatory disease. It is an idiopathic nongranulomatous inflammatory process that often involves the extraocular muscles and orbital fat. Less frequently, other intraorbital structures including the uveal tract, sclera, optic nerve, and lacrimal glands may also be involved.13,16,44 The muscular involvement is usually diffuse. As opposed to thyroid- associated ophthalmopathy, involvement is often unilat-eral and there is usually extension to the muscular tendon attachments (Figs. 11-15 and 11-16). Orbital pseudotumor is usually painful, which helps distinguish it from thyroid-associated ophthalmopathy.

Orbital pseudotumor may be difficult to differentiate from other tumefactive inflammatory processes and neo-plasms. However, a quick response to a trial steroid therapy may help establish the diagnosis.45

• Sarcoidosis

Sarcoidosis is a noninfectious granulomatous disease that may affect any part of the optic pathway, from the globe to the optic radiations.8 The lacrimal gland, anterior layer of the globe, and eyelids are commonly involved. The imaging findings can simulate pseudotumor (Figs. 11-17 and 11-18).

Figure 11-13. CT scan shows subperiosteal abscess of the superior orbit (arrowheads) from ethmoid or frontal sinus disease.

A B

Figure 11-14. A, Axial contrast-enhanced CT scan shows Graves’ ophthalmopathy characterized by enlarged superior, medial, and inferior recti with compromise of the orbital apex. Note the sparing of the muscle tendon insertions. B, Coronal contrast-enhanced CT scan in the same patient.

CT is the imaging modality of choice because it can demonstrate inflammatory soft tissue changes, fluid collections/abscesses, and bone changes (e.g., osteomyelitis).

Imaging findings vary from mild mucoperiosteal thick-ening or elevation to frank intraorbital abscesses.

• Inflammation

• Thyroid-Associated Ophthalmopathy

Thyroid-associated ophthalmopathy is an autoimmune-mediated inflammation of the extraocular muscles and periorbital connective tissues. It is most often associated with Graves’ disease, although association with other thyroid diseases such as Hashimoto’s thyroiditis, thyroid carcinoma, and neck irradiation has also been reported. In approximately 10% to 20% of patients, thyroid-associated ophthalmopathy may present before any other clinical symptoms or signs. Clinical presentation may include eyelid retraction, proptosis, chemosis, periorbital edema, and impaired ocular motility.

The classic imaging findings are fusiform enlargement of the extraocular muscles with sparing of the tendinous attachments (which may be difficult to appreciate when

Ch011-A05375.indd 482 8/13/2008 3:34:15 PM

11  Orbit  483 11

nodules or infiltrates in the retrobulbar space.52,62 Enhance-ment is generally present.

On MRI, retrobulbar masses may demonstrate marked hypointensity. Although this finding is not unique, it strongly suggests Wegener’s granulomatosis.12,52

• Optic Neuritis

Optic neuritis represents nonspecific inflammation of the optic nerve that can be associated with infection, granulo-matous diseases, pseudotumor, postradiation, or demye-linating diseases. A large proportion of cases are idiopathic. Association with multiple sclerosis is established in approx-imately 50% of patients.3 Imaging findings are best dem-onstrated on MRI, which may include enhancement and T2 prolongation (Fig. 11-19). These findings can be subtle. It is important to include the whole brain in image

A B

Figure 11-15. A, Axial CT scan shows pseudotumor of the orbit with swollen bilateral medial rectus (arrows), which includes tendinous insertion on the globe. Thickening and enhancement of the globe (arrowheads) are also shown. B, Coronal CT scan shows pseudotumor of the orbit in the same patient.

Figure 11-16. CT scan of orbital pseudotumor with bilateral medial rectus and left lacrimal involvement.

Figure 11-17. Axial T1-weighted MRI study with gadolinium shows sarcoid of the anterior left globe (arrowheads).

Figure 11-18. Axial T1-weighted MRI study with gadolinium shows sarcoid of the chiasm (long arrow), left cerebral peduncle (large 

arrowhead), lateral geniculate body (short arrow), and superior colliculus (small arrowhead).

• Wegener’s Granulomatosis

This is a form of necrotizing granulomatous vasculitis. Orbital involvement is common and is seen in slightly more than 50% of patients.23,29,47 Any orbital structures can be involved. Findings may include conjunctivitis, episcleri-tis, scleritis, uveitis, optic nerve vasculitis, retinal artery occlusion, nasolacrimal duct obstruction, and retrobulbar diseases. CT examination may demonstrate nonspecific

Ch011-A05375.indd 483 8/13/2008 3:34:17 PM

484  II  Imaging of the Head and Neck

The differential diagnosis includes pseudotumor and metastasis. Lacrimal gland involvement can be seen either in isolation or in combination with the other manifesta-tions of lymphoma within the orbit.

• Optic Glioma

Optic gliomas most often occur in children, especially between the ages of 2 and 6 years. They are usually benign, but a small number of lesions may develop aggressive behavior.68 This lesion usually involves the anterior optic apparatus (e.g., optic nerves, chiasm, and optic tracts) and causes enlargement and often tortuosity of these structures. About half of all optic gliomas occur in patients with neurofibromatosis type I, and 10% to 15% of neurofibro-matosis type 1 patients develop optic gliomas (Fig. 11-22).14 These lesions do not calcify.10

MRI has become the modality of choice, given the necessity of evaluating the intracranial extent of the tumor (Fig. 11-23). Optic gliomas are typically either nonenhanc-ing or weakly enhancing. The lesions are generally isoin-tense to slightly hypointense on T1-weighted images and hyperintense on T2-weighted images.19

CT can help in assessing bony changes and is espe-cially valuable in detecting expansion of the optic canal. CT thus complements MRI in evaluation of these lesions.

• Optic Nerve Sheath Meningioma

Optic nerve sheath meningiomas (ONSMs) are meningio-mas that arise from the meninges surrounding the optic

Figure 11-19. Coronal MRI study, inversion recovery fast-spin echo, shows left optic neuritis. Contrast the increased signal at the left optic nerve (long arrow) with the low signal of the normal right optic nerve (short arrow).

A B

C

Figure 11-20. A, T1-weighted axial MRI study shows a large mass replacing the intraconal fat in the right orbit (arrow). B, T2-weighted axial image of the same patient. C, Gadolinium-enhanced axial T1-weighted image of the same patient demonstrates marked enhancement (long arrow). Fat suppression allows the smaller lesion to be visible at the apex of the left orbit (short arrow).

evaluation to exclude intracranial lesions, particularly the presence of demyelinating lesions.

• Neoplasms

• Lymphoma

Lymphoma is the most common neoplasm in the orbit, accounting for just more than half of all cases.65 B-cell lymphomas of the non-Hodgkin’s type are by far the most common, although T-cell lineages have also been described.11 Usually, orbital lymphomas are primary to the orbit, but occasionally orbital manifestation of a systemic lymphoproliferative process is seen. The usual appearance is a well-defined mass within the muscle cone (Figs. 11-20 and 11-21; see Fig. 11-8). Less frequently, extraconal masses or diffuse infiltration of the orbital fat can be seen.

Ch011-A05375.indd 484 8/13/2008 3:34:18 PM

11  Orbit  485 11

nerve. It is not an uncommon tumor, making up between 5% and 7% of primary orbital tumors.31 The onset occurs at a median age of 38 years and is seen four times more frequently in females than males.31 Because meningiomas, in general, occur more frequently in patients with neurofi-bromatosis type 2, ONSM also occurs more frequently in these patients. The presenting symptom with ONSM is

usually diminished visual acuity from optic nerve com-pression or proptosis. If there is no evidence of visual loss or intracranial extension, these lesions are often treated by close observation. In the setting of visual loss, radiation treatment is frequently used. Surgery is usually reserved for intracranial extension and larger tumors.

On axial imaging, the most common presentation is the well-known tram-track appearance, caused by the enhancing tumor wrapping around the sheath (Fig. 11-24). Inflammation of the dura from other causes may occa-sionally have a similar appearance. ONSM can also present a fusiform enlargement of the sheath on one side (Fig. 11-25). As with all meningiomas, they enhance vividly with contrast and often demonstrate calcification. Hyperostosis may occasionally be seen when the lesion is at the orbital apex or in the optic canal. Optic nerve glioma may initially have the appearance of a meningioma on the axial images, but on the coronal fat-suppressed enhanced MR image, the nerve should be seen separate from the surrounding enhancing meningioma.

Although MRI is the imaging modality of choice, thin-section CT is often helpful because it demonstrates the calcifications or hyperostosis that may be present, thus aiding in the differential diagnosis. A noncontrast-enhanced CT scan should be performed first so the enhancing tumor does not hide the calcifications.

B

M

MM

M

C

A

Figure 11-21. Orbital lymphoma. A, Contrast-enhanced CT scan demonstrates a homogeneously enhancing intraconal mass (black arrow) adjacent to the left optic nerve, causing medial deviation of the nerve (white arrow). B, Axial postgadolinium fat-suppressed T1-weighted image confirms the CT findings (long arrow shows the enhancing mass; short arrows show the optic nerve). C, Coronal postgadolinium fat-suppressed images more clearly demonstrate the enhancing mass (long arrow) separate from the nonenhancing left optic nerve (short arrow). M, extraocular muscles.

Figure 11-22. Axial T1-weighted MRI study shows optic glioma of bilateral optic nerves, with involvement of the chiasm (arrows) in a patient with neurofibromatosis.

Ch011-A05375.indd 485 8/13/2008 3:34:19 PM

486  II  Imaging of the Head and Neck

A B

Figure 11-23. A, T1-weighted axial MRI study with gadolinium enhancement shows optic glioma of the optic chiasm (arrows). B, T1-weighted coronal MRI study with gadolinium.

A B

Figure 11-24. A, Coronal contrast-enhanced CT scan shows optic nerve sheath meningioma (arrow). B, Axial contrast-enhanced CT scan shows same patient as in A with tram-track appearance (arrow).

ONSMs, particularly when in the optic canal, can be quite small and yet cause significant symptoms. As a result, they can be easily missed, unless there is a high degree of suspicion and careful inspection. MRI can therefore be extremely useful for finding these lesions. Coronal and

g

m

Figure 11-25. Axial contrast-enhanced CT scan shows optic nerve sheath meningioma (m). g, globe; arrow, displaced optic nerve sheath complex emerging from mass.

axial contrast-enhanced MRI with fat suppression allows the enhancing lesion to be seen against the fat and bone, which turn dark.

• Melanoma

Primary orbital melanoma usually presents as an ocular lesion. It originates in the uveal tract (iris, choroid, and ciliary bodies) and may extend posteriorly to the rest of the orbit. On CT imaging, melanomas appear as focal soft tissue masses with mild to moderate enhancement (Fig. 11-26).42

MRI studies may help differentiate melanomas from other ocular lesions, evaluate its intraorbital extent, and search for metastatic disease.51 On MRI studies, the amount of melanin contained in melanoma determines the signal characteristics. Melanin shortens T1 and T2, thereby causing increased signal on T1-weighted images and mildly decreased signal on T2-weighted images (Fig. 11-27).

Ch011-A05375.indd 486 8/13/2008 3:34:21 PM

11  Orbit  487 11

mas and helps differentiate the tumor from retinal detachment.6,41

While melanotic lesions have characteristic appear-ances, nonpigmented melanomas cannot be reliably dif-ferentiated from other masses.54

• Metastatic Disease

In adults, the most common tumor to metastasize to the orbit is carcinoma of breast. Other primary sites include lung, colon, and prostate (Fig. 11-29). In children, most common primary lesions include neuroblastoma,

Figure 11-26. Coronal contrast-enhanced CT scan shows melanoma of the left ciliary body.

Figure 11-27. Ocular melanoma of the inferior aspect of the globe. Top, T1-weighted coronal MRI study. Bottom, T2-weighted coronal MRI study.

A

B

C

Figure 11-28. A, T1-weighted sagittal MRI study shows retinal detachment with hemorrhage. B, T2-weighted axial MRI study of the same patient as in A. C, Coronal CT scan shows retinal detachment (different patient). (A and B, Courtesy of Guy Wilms, MD, Universitaire Ziekenhuizer, Leuven, Belgium.)

MR signal is also affected by hemorrhage, which is not uncommon in patients with melanotic lesions. With hemorrhage, the differential diagnosis includes retinal and choroidal detachment from other causes (Fig. 11-28). The presence of gadolinium enhancement favors melano-

Ch011-A05375.indd 487 8/13/2008 3:34:22 PM

488  II  Imaging of the Head and Neck

LR

Figure 11-29. Axial contrast-enhanced CT scan shows prostate metastasis to the left orbit roof.

Figure 11-30. Axial contrast-enhanced CT scan shows bilateral neuroblastoma metastasis.

A B

Figure 11-31. A, Axial contrast-enhanced CT scan shows extension of squamous cell carcinoma of the maxillary sinus to the orbit. B, Coronal contrast-enhanced CT scan in the same patient.

leukemia, and Ewing’s sarcoma. Metastatic lesions may affect any of the intraorbital structures as well as the bony orbit itself (Fig. 11-30).27,60 None of the available imaging techniques offers specificity to differentiate metastases from the many other orbital lesions. The findings may be subtle, with small areas of focal thickening of the globe, or large destructive lesions.21 In addition, extension of tumor from an adjacent structure (e.g., the paranasal sinuses) may occur (Fig. 11-31). Enophthalmos may be present in primary disease that is often associated with extensive fibrous response, such as scirrhous carcinoma of the breast.

• Retinoblastoma

Retinoblastoma is seen primarily in infants and has an occurrence of 1 in every 18,000 to 30,000 live births.50 It is responsible for 1% of all childhood cancer-related deaths in the United States.59 Early diagnosis extends the 5-year survival rate to more than 90%; however, if the tumor extends beyond the globe, the mortality rate approaches 100%.33 Retinoblastoma has been strongly linked to muta-tions on the RB1 allele of chromosome 13. Whereas about 10% of cases are said to be inherited, most retinoblastoma cases are not inherited. Hence, there is both a familial

hereditary form of retinoblastoma and a nonfamilial sporadic form. Aside from the hereditary differences, the tumors are the same. Patients with nonfamilial retinoblas-toma have unilateral solitary tumors, whereas patients with the familial form have a much higher rate of bilateral than unilateral disease. Patients with the familial form of reti-noblastoma have a highly incidence of nonocular cancers as well.

The “trilateral retinoblastoma” refers to a patient who has bilateral retinoblastomas and a third midline tumor. The midline tumor is histologically the same as the intra-ocular tumor and may occur in the pineal region, suprasel-lar region, or fourth ventricle. It is important to remember that the midline tumor may not be seen at the same time as the ocular tumors but may be discovered several years later.

Because most of these lesions, which arise from the retina, are calcified, CT is extremely important in their diagnosis (Fig. 11-32). The lesions will also enhance with intravenous contrast. The tumor may spread in the lym-phatics or along the optic nerve to gain intracranial access. If a tumor is discovered in one globe, very close inspection of the other globe is necessary to exclude bilateral disease. On initial evaluation and on follow-up examinations, close inspection of the pineal region, suprasellar region, and fourth ventricle is important to seek out trilateral disease.

Because these tumors enhance, MRI with contrast and fat suppression is excellent for identifying the lesion; however, CT is better at identifying the calcification.

Ch011-A05375.indd 488 8/13/2008 3:34:23 PM

11  Orbit  489 11

With bilateral disease the diagnosis is easy, but with a unilateral tumor it may be more difficult. Entities such as Coats’ disease and Toxocaris canis infection can be con-fused with retinoblastoma, but these typically lack contrast enhancement.

• Rhabdomyosarcoma

Even though the most common malignant ocular tumors in children are retinoblastoma, the most common malig-nant orbital tumors in children are rhabdomyosarcomas.9 They may arise primarily or secondarily in the orbits. They are very aggressive tumors and may grow rapidly. On CT imaging, they are seen as enhancing soft tissue masses with associated permeative or lytic bone destruction (Fig. 11-33). On MRI, they are hypointense to isointense on T1-weighted images and isointense to hyperintense on T2-weighted images. Enhancement is variable.2

• Langerhans Cell Histiocytosis

Langerhans cell histiocytosis is not a true neoplasm but a reticuloendothelial disorder of unknown origin. Like rhab-domyosarcoma, it occurs most often in children. Because its clinical presentation and imaging features are often

similar to neoplastic processes, it is often included in the differential consideration of a soft tissue mass. The imaging appearance can simulate rhabdomyosarcoma.9 On CT imaging, an isodense to hyperdense soft tissue mass is seen. Enhancement of the lesion and associated lytic bone changes are usually present.2 On MRI study, the mass is isointense to hypointense on T1-weighted images, and isointense to hyperintense on T2-weighted images.

• Teratoma

The teratoma is a rare benign lesion that contains mixed endodermal, mesodermal, and ectodermal elements. It usually calcifies. Because the teratoma is usually seen in neonates, knowing a patient’s age can help one decide whether to include this entity in a differential diagnosis (Fig. 11-34).24,69

• Vascular Abnormalities

• Carotid Cavernous Fistula

Carotid cavernous fistula is an abnormal high-flow com-munication between the arterial and venous circulations. This results in transmission of arterial flow into the cavern-ous sinuses, consequently leading to reversal of flow in venous structures draining into the cavernous sinus. Two types of carotid cavernous fistulas have been described. The more common type is direct fistula formed by an abnormal communication between the internal carotid artery and the cavernous sinus. The less common type, indirect fistula, is

Figure 11-32. Axial contrast-enhanced CT scan shows calcified retinoblastoma of the left eye.

R

Figure 11-33. Rhabdomyosarcoma of the orbit in a young child. CT demonstrates enhancing soft tissue mass (R). There is aggressive bone destruction, with the tumor extending into the ethmoid sinus (arrow).

Figure 11-34. Axial contrast-enhanced CT scan shows calcified teratoma with areas of enhancement.

Ch011-A05375.indd 489 8/13/2008 3:34:24 PM