Imaging plays an important role in the evaluation of patients with suspected orbital disease. Radiography, ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) are commonly used in clinical practice. Each modality has its advantages and disadvantages in terms of diagnostic value, accessibility, speed of acquisition, and radiation exposure. This chapter focuses on the cross-sectional modalities of CT and MRI, as these methods have evolved to occupy a central role in the assessment, preoperative planning, and intraoperative guidance of orbital pathology.
Anatomy
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.
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