Radiologic Evaluation of the Orbit: Computed Tomography and Magnetic Resonance Imaging

Osseous Anatomy

The orbital region is a complex coalescence of seven craniofacial bones—namely, the frontal, maxillary, zygomatic, sphenoid, ethmoid, lacrimal, and palatine bones, which form the conical boundaries of the orbit whose apex is directed dorsally and medially. The orbital roof is also the floor of the frontal fossa and frontal sinus and is composed of the orbital plate of the frontal bone and a portion of the lesser wing of the sphenoid bone. The orbital roof is quite thin and tends to become thinner with age. The orbital apex is primarily composed of the sphenoid and ethmoid bones and the orbital process of the palatine bone. Importantly, these bones form the optic canal along the medial superior margin, with the obliquely oriented superior orbital fissure (SOF) lateral to this, and inferolaterally is the inferior orbital fissure. The medial orbital wall serves as the lateral boundary of the ethmoid sinus, slightly angles laterally and inferiorly, and is largely formed by the delicate lamina papyracea of the ethmoid bone, with a portion of the body of the sphenoid bone dorsally and the lacrimal plate of the lacrimal bone anteriorly. The orbital floor or roof of the maxillary sinus is also very thin, slopes anteriorly and inferiorly, and is formed by the orbital portion of the maxillary bone and the orbital processes of the zygomatic and palatine bones. The orbital floor contains the infraorbital canal, which follows an anteroposterior course in the floor from the inferior orbital fissure to the infraorbital foramen. The lateral wall is considerably thicker and is formed by a portion of the greater wing of the sphenoid bone and the orbital plate of the zygomatic bone. The Whitnall tubercle (lateral orbital tubercle) is an important small bony protuberance along the lateral wall, caudal to the zygomaticofrontal suture and 1 cm dorsal to the orbital rim, which serves as a point of attachment for the levator aponeurosis, a suspensory ligament for the globe, and the lateral palpebral ligament. The sutural distinction between these bones within the orbit is not always possible with standard CT imaging. In the mid orbit, the relatively thin caliber of the bones can result in poor visibility, which reinforces the necessity of high-resolution imaging.

The bony orbit is best evaluated with CT. The bony orbit contains several important foramina and canals, which demonstrate variability in anatomic shape but consistent relationships. The superior orbital fissure is formed by the greater and lesser sphenoid wings and the ethmoid and palatine bones, located at the orbital apex ( Fig. 9.1 ). The superior orbital fissure transmits cranial nerves (CN) III (oculomotor), IV (trochlear), V1 (ophthalmic), and VI (abducens), in addition to vascular structures, such as the superior ophthalmic vein and branches of the meningeal and lacrimal arteries. The optic foramen is the ventral termination of the optic canal, situated at the medial margin of the superior orbital fissure ( Figs. 9.1 and 9.2 ) and transmits the optic nerve and the ophthalmic artery. The inferior orbital fissure is formed primarily by the maxillary, sphenoid, and zygomatic bones and is contiguous with the foramen rotundum and pterygopalatine fossa ( Figs. 9.1 and 9.3 ). The inferior orbital fissure receives CN V2 (maxillary) from the foramen rotundum and transmits V2 fibers and the inferior ophthalmic vein. Along the dorsal margin of the floor, the inferior orbital fissure communicates with pterygopalatine fossa and the temporal fossa. Continuing ventrally from the inferior orbital fissure and pterygopalatine fossa, the infraorbital canal ( Fig. 9.4 ) carries the infraorbital nerve (V2) through the orbital floor to the maxilla, terminating at the ventral margin of the maxilla as the infraorbital foramen (see Fig. 9.1 ). The supraorbital foramen is visualized as a small notch along the superior orbital rim (see Fig. 9.1 ) and contains the supraorbital nerve (V1). The nasolacrimal canal extends caudally from the lacrimal sac in the lacrimal groove of the lacrimal plate and transmits the nasolacrimal duct ( Fig. 9.5 ), draining into the inferior meatus of the nasal cavity below the inferior turbinate.

Multiple fracture patterns involving the orbital walls are encountered in the setting of trauma, including orbital blowout fractures, nasoorbitoethmoidal, LeFort II/III, and zygomaticomaxillary complex fractures. Potential sequelae of orbital trauma include diplopia as the result of extraocular muscle impingement or entrapment, or hypoesthesia in the maxillary sensory distribution as the result of fracture involving the infraorbital canal ( Fig. 9.6 ). Only the resolution and contrast of CT can effectively characterize such fractures for clinical decision making. Similarly, only the bony detail of CT can distinguish bony remodeling and attenuation from frank bony destruction in the setting of infection or neoplasm within or adjacent to the bony orbit.

Soft-Tissue Anatomy

The soft-tissue structures of the orbit typically assessed on imaging consist of the globe, extraocular muscles, optic nerve, intraorbital fat, lacrimal gland, periorbita or orbital fascia, orbital septum, and neurovascular structures.

The orbital septum is an important imaging landmark, although it is not typically visible on CT and is infrequently evident on MRI. It is composed of a fibrous septum contiguous with the aponeurosis of levator palpebrae superioris, the capsulopalpebral fascia, and the tarsal plates and extends to the orbital rims to blend with the periorbita. The orbital septum effectively serves as the anterior border of the orbit and plays a significant role as a barrier to intraorbital extension of infection. Thus assessment of preseptal and/or postseptal involvement is an important distinction on imaging because patients present very differently and the distinction has a considerable impact on the type and duration of therapy ( Fig. 9.7 ).

The striated extraocular muscles involved in movement of the globe include the medial, superior, lateral and inferior rectus muscles, and the superior and inferior oblique muscles (see Fig. 9.4 B). The four rectus muscles and the levator palpebrae superioris all arise from a thickened, conical tendinous ring that surrounds the optic foramen and medial aspect of the superior orbital fissure and is contiguous with the periorbita, known as the annulus of Zinn. The superior oblique muscle lies in the upper medial quadrant of the orbit, arises from the sphenoid bone periosteum, passes anteriorly through a fibrocartilaginous ring (trochlea), and then courses dorsally, medially, and inferiorly, subjacent to the superior rectus, to insert onto the sclera of the dorsal superior globe. The inferior oblique arises from the orbital floor, dorsal and lateral to the lacrimal sac, and then follows a dorsal lateral superior course below the inferior rectus to insert on the dorsal lateral sclera of the globe. The levator palpebrae superioris functions to elevate the eyelid, running parallel and cephalad to the superior rectus muscle. The levator palpebrae superioris and superior rectus demonstrate variable separation on imaging and are sometimes apposed in the coronal plane. The extraocular muscles demonstrate hypointense T1 and T2 MRI signal relative to intraorbital fat, with normal mild, uniform postcontrast enhancement. The muscles normally demonstrate a tapered caliber at their ventral and dorsal tendinous margins, whereas each muscle belly has a larger, flattened ovoid configuration in the coronal plane. The extraocular muscles are surrounded by orbital fat and form the boundaries of the intraconal and extraconal spaces within the orbit ( Fig. 9.8 ), which are useful aids to predict the nature of orbital pathology on imaging studies.

The globe is encased by the elastic tissue of the sclera from the periphery of the cornea to the optic nerve, where it is fused with its dural sheath. The choroid and retina are not typically distinguishable from the scleral margin on standard imaging. The sclera is surrounded by the capsule of Tenon, which is a fibrous sheath that envelops the Tenon (episcleral) space, inserts in the sclera anteriorly, is pierced dorsally by the optic nerve and its sheath, and separates the globe from the surrounding orbital fat. The optic nerve runs through the optic canal along with the ophthalmic artery. A dural sheath surrounds the optic nerve as it traverses the canal, after which the dura inserts to the periosteum of the bony orbit. The optic nerve normally demonstrates uniform hypointense T1 and T2 signal without postcontrast enhancement. There is normal hyperintense T2 signal in the CSF space within the optic nerve sheath ( Fig. 9.9 ) that is contiguous with the intracranial subarachnoid space. On CT, it is not typically possible to distinguish the nerve from the dural sheath and the interposed subarachnoid space; however, this is possible with MRI.

The periorbita is the periosteum of the bony orbit but is more loosely adherent to the bone than the periosteum elsewhere. The periorbita is continuous with the dura in the orbital apex and with the orbital septum anteriorly. It is not normally discernible with imaging but is important to consider in the setting of neoplasm or infection because direct involvement or extension through the periorbita is an important distinction that alters prognosis and clinical management in such settings ( Fig. 9.10 ). Making this distinction can be difficult on imaging, but subperiosteal tissue is generally sharply defined and crescentic or lentiform in configuration confined to the extraconal space, whereas tissue extending through the periosteum is generally larger, focal, lobulated, or irregular and ill defined.

The lacrimal gland is a compound tuboloacinar gland located in the lacrimal recess in the ventral superolateral orbit, contained within periorbita and supported inferiorly by the Whitnall capsule. Frequently overlooked on imaging, assessment of the lacrimal gland is important as it may be involved with infection, primary neoplasm (such as adenoid cystic, adenocarcinoma, squamous cell or mucoepidermoid carcinoma), lymphoma, pseudotumor, and granulomatous disease among other etiologies. The lacrimal gland normally demonstrates hyperintense T2 signal, isointense T1 signal, and uniform postcontrast enhancement ( Fig. 9.11 ). Keys to pathology include atypical signal intensity characteristics, restricted diffusion, and irregular enhancement or enlargement, noting that pathology may be bilateral.

The orbits contain many vascular structures, the most important of which are the ophthalmic arteries, which arise directly from the internal carotid arteries and traverse the optic canals along with the optic nerves, providing the main arterial supply to the globe. The extraocular muscles are supplied by muscular branches of the ophthalmic artery as well as the lacrimal and infraorbital arteries. The superior ophthalmic veins (SOVs) are readily visualized on imaging and follow a circuitous course: initially along a posterolateral course medial to the superior rectus muscle, then the veins pass caudal to the superior rectus but superior and lateral to the nerve sheath, and then posteriorly and medially to the superior orbital fissure to the cavernous sinus but lateral to the superior rectus. The SOVs should be assessed on imaging for enlargement and/or thrombosis, which can portend cavernous sinus thrombosis or cavernous-carotid fistula ( Fig. 9.12 ). The inferior ophthalmic veins are considerably smaller and arise inferiorly and laterally, course dorsally along the inferior rectus muscle, and drain into the pterygoid plexus through the inferior orbital fissure but also communicate with the SOVs.