Constantly improving MR hardware, new pulse sequences, enhanced coil designs, and the option of scanning at 3.0 T or higher contribute to ever-improving orbital imaging. In the past, MR evaluation of the eye and orbit was best performed using surface coils placed over the orbital regions, as discussed in previous editions of this text (1). Ongoing refinements in MR technology permit excellent evaluation of the retrobulbar orbital structures with commercially available head coils. While providing superb signal-to-noise ratio (SNR) within the orbit, orbital surface coils were limited in their depth of signal penetration such that the optic chiasm and brain were not evaluated. Switching from a surface coil to a head coil mid-examination in order to image the entire visual axis would add considerably to study duration.

Clinical imaging at 3 T is now commonplace. Imaging at a higher field strength increases SNR; this can then be translated into higher spatial resolution at a given scan duration or for maintaining resolution and decreasing imaging time. However, the increased field strength brings with it several disadvantages and challenges, particularly for orbital imaging. There is increased sensitivity to magnetic field inhomogeneities and increased susceptibility artifacts at 3 T when compared to 1.5 T. Given that the orbits are surrounded by pneumatized paranasal sinuses, the resultant distortion artifacts at air–bone and air–brain interfaces are an important issue. These artifacts along the planum sphenoidale may severely limit evaluation of the intracanalicular and intracranial portions of the optic nerve (2). The artifacts worsen with frequency-selective fat suppression pulses used in conventional chemical shift selection suppression (CHESS) imaging. If parallel imaging (PI) is used, it reduces these susceptibility artifacts for an improvement in image quality, an important advantage of this technique at 3 T. PI also reduces the amount of energy deposited into the patient’s tissues, known as specific absorption rate (SAR). Maximum SAR levels are reached more quickly at 3 T than at 1.5 T, and the decreased imaging time afforded by PI keeps the SAR lower (3). Orbital images are also more susceptible to motion degradation at 3 T than at 1.5 T. One other disadvantage of high field is lengthening of the T1 relaxation time, but this is not as limiting for scanning the orbit as for the brain.

Small-diameter surface coils provide the highest-resolution images of ocular and orbital anatomy and pathology by virtue of an inherently greater SNR. This is because the receive coil (the surface coil) is closer to the area of interest than is the head coil, and unwanted patient-generated motion from areas outside the orbital region does not compromise the images. As mentioned previously, surface coils are often unnecessary when a modern high-quality phased array head coil is employed at 3 T. However, a concentric coil technique may be advantageous (4).

Even with faster and more efficient scanning techniques, MR of the orbits continues to be adversely affected by eye and lid motion (Fig. 18.1). The patient should be asked to relax
and keep the eyes open or closed but to try to avoid repeatedly shifting gaze. For shorter sequences, the patient may be asked to keep the eyes open and fix their gaze on a point within the bore of the magnet. For imaging children and claustrophobic patients, appropriate sedation is important for high-quality orbital imaging. Patients wearing eye makeup should be instructed to remove it as metallic oxides are often present in these cosmetics and cause susceptibility artifact (Fig. 18.2). These artifacts are more pronounced at 3.0 T (5).

The most abundant tissue in the orbit is fat, and differences in the resonant frequencies of protons bound to water and those bound to fat result in chemical shift misregistration, a troublesome artifact causing image degradation. The near-universal use of fast spin-echo (FSE) sequences in modern practice has resulted in some reduction in these artifacts when compared with early spin-echo (SE) imaging. Conspicuity of enhancing tumor margins and localization of inflammatory and infectious diseases with respect to the various structures of the orbit has been further enhanced by improved fat suppression techniques. The short-tau inversion recovery (STIR) sequence offers nulling of the fat signal and excellent display of structures within the fat. Contemporary fat suppression methods include Dixon techniques where in- and out-of-phase SE acquisitions are performed with (3-point) or without (2-point) a phase correction acquisition. The resultant images with pure water and pure fat signal can produce excellent T1-weighted postcontrast or T2-weighted clinical sequences (5). Dixon fat suppression techniques have improved SNR over CHESS and also result in more uniform fat suppression over the imaging volume, a significant advantage in orbital imaging (Fig. 18.3).

Diffusion-weighted imaging (DWI) of the orbit may provide helpful diagnostic information, particularly when infection is suspected (6). Apparent diffusion coefficient (ADC) values may also help differentiate benign from malignant orbital lesions (7). In particular, DWI has shown promise in distinguishing orbital lymphoma from benign lymphoproliferative lesions (8). Standard whole-brain echo-planar diffusion may suffer from significant susceptibility artifact in the orbit, limiting clinical utility. Nonecho-planar DWI appears to successfully reduce susceptibility and motion artifact at the expense of SNR. This technique has been validated in retinoblastoma detection and appears to hold widespread promise in orbital imaging (9).

A complete orbital MRI protocol should include T1-weighted and fat-suppressed (FS) T2-weighted sequences for lesion characterization, and images should be obtained in the axial and coronal planes. For optimal demonstration of optic nerve lesions, orienting the axial images slightly inferior to the orbitomeatal line will result in most of the optic nerve visualized on a single slice. The coronal plane best demonstrates the entire course of the optic nerve in cross section. Obliquely oriented sagittal images parallel to the plane of the optic nerve may also be obtained. A small field of view (FOV), on the order of 12 to 18 cm, is recommended, as well as minimal slice thickness, typically 3 mm with 10% gap (Table 18.1). Many vendors offer three-dimensional (3D) sequences with good SNR and the volume acquisition negates consideration of a slice gap. Variable matrix sizes of 192 to 512 are generally used depending on FOV and SNR. Submillimeter voxels are usually achieved. Administration of intravenous (i.v.) gadolinium-based contrast is indicated for virtually all orbital MR studies and postcontrast axial and coronal T1-weighted FS sequences are then obtained. There are several reasons for having precontrast and postcontrast images in the orbit. For most pathology, the lesion is very conspicuous against the background of hyperintense orbital fat on unenhanced T1-weighted images. A minority of lesions, including calcified, hemorrhagic, or proteinaceous abnormalities, have intrinsic T1 hyperintensity, which could be mistaken for enhancement if only postcontrast images were available. Furthermore, fat suppression may be imperfect in superficial regions like the orbit, and confusing T1 hyperintense artifacts caused by the fat suppression pulse can masquerade as enhancement (Fig. 18.4). This artifact is also problematic in the presence of ferromagnetic dental work and aerated paranasal sinuses, where fat signal suppression is difficult. Coronal orbital sequences should extend posteriorly through the optic chiasm, and axial fluid-attenuated inversion recovery (FLAIR) and/or T2-weighted images are typically obtained through the entire brain. Prior to imaging at any field strength, all patients should be questioned for the possibility of metallic material in or around the orbit. Ferromagnetic material may move in response to the magnetic field and cause injury to the eye or optic nerve. If there is uncertainty with regard to presence or composition of an orbital foreign body, a noncontrast CT study should be obtained prior to MR scanning.

The anatomy of the orbit and its contents has been thoroughly reviewed by many authors (10,11,12). The ease of imaging in multiple planes, coupled with the high inherent contrast of intraorbital structures as visualized by MRI, makes the anatomy of the orbit ideal for evaluation by this modality.

On MR, the characteristic signal intensity of retrobulbar fat (hyperintensity on T1-weighted sequences and hypointensity on FS T2-weighted sequences) as well as the absence of signal from cortical bone and, in most cases, from flowing blood provide an excellent background for depiction of intraorbital anatomy and pathology using conventional pulse sequences. As a foundation to recognizing ocular and orbital pathologies, the normal anatomy of these regions is reviewed.

Six skeletal extraocular muscles insert on the sclera and control motion of the eyeball (Figs. 18.6–18.12). The four rectus muscles (superior, inferior, lateral, and medial) arise from a common tendinous ring at the orbital apex, the annulus of Zinn, and form a muscle cone that runs forward and inserts onto the front of the sclera. The superior rectus is the longest muscle, being approximately 40 mm in length, and the medial rectus has the largest diameter in the normal orbit. The medial and lateral recti lie in the same axial plane, and the superior and inferior recti lie in the same vertical plane (11). The inferior
oblique muscles insert into the posterior portion of the sclera. The superior oblique is the thinnest extraocular muscle and lies in the superomedial aspect of the orbit, coursing forward above the medial rectus muscle. Its tendon passes through the cartilaginous trochlea; turns laterally, posteriorly, and downward; and inserts into the posterolateral aspect of the sclera. The course of this muscle is well delineated by MR. The inferior oblique muscle, the only muscle to arise from the front of the orbit (11), originates from a depression in the upper surface of the maxilla, lateral to the nasolacrimal canal. This muscle passes backward and laterally, inferior to the inferior rectus, to insert into the posterolateral sclera. Its course is best seen on coronal images. The superior oblique muscle is supplied by the trochlear nerve, and the lateral rectus is supplied by the abducens nerve, whereas all other extraocular muscles are supplied by the oculomotor nerve.

The levator palpebrae superioris muscle lies just under the roof of the orbit, above the superior rectus muscle. It is also supplied by the oculomotor nerve and controls the upper eyelid. Below the superior rectus muscle lies the SOV, and below it lies the optic nerve, all clearly seen on coronal images.

The normal extraocular muscle has intermediate intensity on T1-weighted images and stands out as hypointense against the background of high-intensity orbital fat. It is similarly relatively hypointense on FSE T2-weighted images without fat suppression. It should be noted that normal muscles enhance significantly after i.v. contrast; this enhancement is dramatic on FS images.

The optic nerve (the second cranial nerve) connects the retina with the brain and extends between the posterior globe and the optic chiasm (Figs. 18.6–18.12). Approximately 90% of its fibers are afferent (14), arising in the retinal ganglion cell layer. In the orbit, the optic nerve lies within the muscle cone and proceeds posteriorly and medially from the globe in an oblique sinuous course to gain entry into the middle cranial fossa via the optic canal and ending in the optic chiasm. The optic nerve is fully myelinated by 7 months of age, but it continues to increase in size and thickness for the first 8 years of life (15). The intracanalicular portion of the nerve enters the optic canal superior to the ophthalmic artery. The intraorbital optic nerve is surrounded by continuations of the three layers of meninges, as well as by the cerebrospinal fluid (CSF)-filled subarachnoid space. The outermost dura fuses with orbital periosteum and with the annulus of Zinn, which perhaps explains the finding of pain associated with eye movement in optic neuritis (ON) (15). The perioptic arachnoid is adherent to the pia and dura
within the optic canal except along its inferior side, which is the only site of free communication between perioptic and intracranial subarachnoid spaces (15). The optic nerve is easily distinguished from CSF in the optic nerve sheath by MR, especially on T2-weighted images, whereas with CT, intrathecal contrast administration is generally required (16).

The third cranial nerve, or oculomotor nerve, is the major motor supply for eye movements, enervating extraocular muscles except the superior oblique and lateral rectus muscles. After emerging from the midbrain just medial to the cerebral peduncle, it runs just lateral to the posterior communicating artery and then traverses the upper lateral wall of the cavernous sinus. The nerve divides into superior and inferior divisions, which pass through the superior orbital fissure and enters the muscle cone of the orbit, alongside the nasociliary nerve, a branch of the first division of the fifth nerve.

The trochlear nerve (fourth cranial nerve) is the only cranial nerve that arises from the dorsal aspect of the brainstem, emerging just below the inferior colliculus. It passes from the brainstem through the lateral wall of the cavernous and the superior orbital fissure and enters the extraconal orbit to supply the superior oblique muscle. Because of its unique intracranial course, it is commonly contused after significant head trauma.

The ophthalmic nerve (the first division of the fifth cranial nerve) is a sensory nerve that receives input from the globe and its conjunctivae, the lacrimal gland, the nose and nasal mucosa, the upper lid, frontal sinus, scalp, and forehead (10,11). Arising from

the trigeminal ganglion, it traverses the dura in the lateral aspect of the cavernous sinus to divide near the superior orbital fissure into its three branches: lacrimal, frontal, and nasociliary. These small branches subsequently pass through the superior orbital fissure into the orbit and can often be seen on T1-weighted MR images as they course through the orbital fat. The lacrimal nerve enters the orbit superior to the annulus of Zinn (a tendinous ring from which the rectus muscles arise) and proceeds along the lateral aspect of the extraconal space (with the lacrimal artery), above the superior border of the lateral rectus muscle, to give branches to the lacrimal gland, upper eyelid, and conjunctiva. The frontal nerve is also extraconal in location and courses anteriorly on the levator palpebrae superioris muscle (superior to the superior rectus muscle). The nasociliary nerve—the sensory nerve for the globe—enters the orbit via the superior orbital fissure to its intraconal location, between the two divisions of the oculomotor nerve. It runs anteriorly under the superior rectus muscle and courses medially to lie between the superior oblique and the medial rectus muscles. The nasociliary nerve forms part of the afferent limb of the corneal reflex (11).

The abducens nerve is the sixth cranial nerve and supplies only the lateral rectus muscle. Exiting the pontomedullary junction, it traverses the central aspect of the cavernous sinus just below the internal carotid artery. After passing through the superior orbital fissure and into the intraconal orbit, it enters the medial aspect of the lateral rectus muscle.

Vascular structures containing flowing blood emit variable signal intensities on MR, depending on the pulse sequence parameters, the velocity of the flow, and physical properties of the state of flow (i.e., laminar, turbulent, etc.) (Figs. 18.8–18.11). In the orbital vessels detected by MR, relevant normal flow phenomena (17,18,19) include (a) signal void, identified on most SE pulse sequences when flow is rapid or sufficiently complex (e.g., turbulent); (b) flow-related enhancement, seen as high intensity in patent but relatively slow flow states, at entry level slices (i.e., at one end of a stack of acquired slices), or on gradient-echo images (20); and (c) second-echo rephasing (17), identified as high signal intensity within patent but again relatively slowly flowing systems whereby early echo images demonstrate low (or absent) signal and delayed second-echo images show a marked increase in signal.

The OA is the first major branch of the internal carotid artery (ICA) and the chief artery of the orbit. In the vast majority of individuals, it arises from the anteromedial aspect of the ICA just distal to the point where the artery has penetrated the dura. It passes anterolaterally through the optic canal, below the optic nerve. Once within the orbit, it crosses over or under the optic nerve and proceeds medially. Within the orbit it gives off numerous branches, supplying the retina, ciliary body, extraocular muscles, lacrimal glands, and ethmoid sinus, among other structures (21). It is usually visible on thin-section MR images as a very thin serpentine structure with internal signal void.

The orbit is drained by the superior and inferior ophthalmic veins. The SOV, the largest and most consistently visualized intraorbital vessel on MR images, forms at the root of the nose from the union of the supraorbital and facial veins and enters the intraconal space of the orbit to lie adjacent to the inferior aspect of the superior rectus muscle, above the optic nerve. It courses posteriorly and laterally within the muscle cone and then turns medially and enters the superior orbital fissure at the orbital apex to drain into the cavernous sinus. The diameter of the SOV varies from 2 to 3.5 mm and can change somewhat with head position or Valsalva maneuver. Asymmetry of the SOVs may be present in normal patients (22). The inferior ophthalmic vein originates as a plexus on the floor of the orbit and drains either directly or indirectly into the cavernous sinus (10,11). It is not easily seen on MR unless it is pathologically enlarged.

The lacrimal gland lies in the lacrimal fossa, a postseptal extraconal space in the superolateral aspect of the anterior orbit (Figs. 18.7–18.10). The gland abuts the lateral rectus and levator superioris muscles. It consists of a smaller and more superficial palpebral portion and a larger orbital portion; the palpebral portion may be visualized when the upper lid is everted. Glandular tissue may normally be somewhat heterogeneous on MR, but it is generally isointense to muscle on T1- and T2-weighted images. Note that if one uses a surface coil, the lacrimal gland, because of its proximity to the coil, may be hyperintense without true pathologic involvement, particularly when fat suppression is also used. The gland normally enhances. Tears drain medially into superior and inferior canaliculi, which empty into the lacrimal sac in a small fossa at the inferomedial margin of the orbit, usually visualized only if it is pathologically enlarged due to nasolacrimal duct obstruction or a neoplastic or inflammatory process. The nasolacrimal duct measures 2 cm in length and empties into the inferior nasal meatus (11).

The globe is unique in that it contains both the most and the least water-laden soft tissues in the body—the vitreous and the lens, respectively (Figs. 18.8–18.14). The vitreous is 98% to 99% water, the lens is 65% to 69% water, and the cornea is 80% water; these differences alone produce highly contrasting MRI characteristics. The eye occupies approximately one-third (or less) of the volume of the orbit (11). The vitreous represents about two-thirds of the volume of the eye, approximately 4 mL, and is a gel-like, transparent extracellular matrix composed of a meshwork of 0.2% collagen fibrils interspersed with 0.2% hyaluronic acid, polymers, water, and a small amount of soluble proteins.

The wall of the globe consists of three layers. The outermost layer is a fibrous protective coat that constitutes the sclera and, anteriorly, the transparent cornea. The cornea forms the anterior one-sixth of the outer circumference of the globe. Centrally, the cornea is only 0.5 mm thick, where the anterior and posterior surfaces are parallel. While the cornea is composed of five distinct layers, the stroma (or substantia propria) forms 90% of its thickness. This layer is mainly interlacing parallel bundles of avascular collagen fibrils surrounded by mucopolysaccharide ground substance (23). The functions of the cornea are to protect the anterior segment of the eye and to be a component of the eye’s refractive system. The curvature of the cornea is stable throughout adult life in the absence of pathology, but it is significantly greater in newborns and young infants. On MRI, the cornea appears mainly as a low-intensity structure because it is fibrocollagenous and very low in mobile water proton density. The cornea is highlighted by a thin rim of slight hyperintensity on T1-weighted images, which may represent the coat of adherent tear film rather than corneal tissue itself. The rigid sclera comprises the remaining outer protective coat of the globe, merging with the cornea at the limbus anteriorly. The sclera is also mainly composed of collagen bundles, and functions to protect the globe, maintain intraocular pressure, and serve as a site of attachment for extraocular muscles (15). The outer layer of sclera is continuous posteriorly with the optic nerve dura. The sclera changes shape and thickness throughout life. During early childhood, it thickens and becomes opaque, enlarging with the growth of the eye. With a sustained increase in intraocular pressure during this time, the sclera may stretch and result in an enlarged globe. In contrast, the adult sclera is less likely to stretch diffusely, but focal thinning with ectasia can result in staphylomas or focal protrusions in areas of thinned sclera (15). In elderly patients, the sclera becomes even more rigid due to a decrease in hydration; scleral calcification can occur, commonly at sites of rectus muscle insertion (15). On MR, the sclera is similar to the cornea and other fibrocollagenous structures and is seen as a thin, low-intensity region at the periphery of the globe.

The sclera is covered by Tenon’s capsule, which is a fibroelastic membrane that envelops the eye from the optic nerve to the level of ciliary muscle. Tenon’s capsule is also called the bulbar fascia of the globe. This fibroelastic socket encloses the posterior four-fifths of the globe and separates the globe from the central orbital fat. Anteriorly, Tenon’s capsule blends with the sclera just behind the corneoscleral junction and fuses with the bulbar conjunctiva. It is perforated behind by the optic nerve and its sheath and the ciliary nerves and vessels. The inner surface of Tenon’s capsule is smooth and is separated from the outer surface of the sclera by the episcleral (Tenon’s) space. This is a potential space that is traversed by fibers of loose connective tissue, which extend between the fascia and the sclera. Orbital inflammatory and intraocular neoplastic processes (e.g., retinoblastoma, melanoma) are the most common lesions to involve Tenon’s space.

The uveal tract consists of the choroid, ciliary body, and iris, a set of structures having vascular and nutritive function. In addition to blood vessels, it contains nerves, connective tissue, and pigmented melanocytes. The vascular supply of the uveal tract includes “blood–ocular barriers,” analogous to the blood–brain barrier at several points, including the retinal pigment

epithelium (for the choroid), the retinal blood vessel endothelium (for the retina), the vessel endothelium of the iris, and the pial and retinal vessels at the optic nerve head (15). The iris is a pigmented diaphragm that separates the region between the peripheral aspects of the lens and the superficial cornea into an anterior and a posterior chamber. Both these chambers contain aqueous humor and are depicted as fluid-containing spaces on MR. The peripheral part of the iris attaches to the ciliary body. Two major parts of the ciliary body are the pars plicata, which forms the most anterior 2 mm, and the pars plana, comprising the more posterior flat portion. The main source of aqueous is the pars plicata (15). The ciliary body extends from the iris to merge with the choroid at the ora serrata, which also marks the point of tight fusion between sensory retina and retinal pigment epithelium. The ciliary body connects to the lens via the zonule, a suspensory ligament. These structures are involved in lens accommodation. The choroid extends from the ora serrata to the optic nerve head and is the major vascular and pigmented tissue of the uvea “middle coat” of the globe (15). The entire uveal tract is hyperintense on T1-weighted MR images. These structures stand out on T2-weighted images as low intensity against the hyperintense vitreous and aqueous. The retina consists of a thin outer retinal pigment epithelium layer and an innermost sensory retina, which contains neural elements for visual perception. These two retinal layers are bound tightly only at the ora serrata and at the optic disk; otherwise, they are maintained in close apposition by the intraocular pressure and weak contact points. In young patients, the vitreous is rather firmly held to the inner sensory retina, but this bonding becomes tenuous with aging. MR can routinely separate the sclera from the choroid and retina because the fibrous sclera is low intensity on T1-weighted images, in contrast to the hyperintense choroid/retina. High-resolution conventional MR can often delineate the iris and ciliary body in cooperative patients. Ultrahigh-resolution imaging (23) can separate the pars plana from the pars plicata of the ciliary apparatus. It has been hypothesized that the high intensity of portions of the choroid, iris, and ciliary apparatus on T1-weighted images is related to their pigmented (i.e., melanin-containing) epithelium. Furthermore, it has been shown by electron microscopy that the retinal pigment epithelium may contain other naturally occurring metallic (paramagnetic) ions, including iron and copper (24).

The lens is a normally transparent, biconvex crystalline structure approximately 1 cm in greatest diameter that transmits light and separates the aqueous from the vitreous. It forms part of the posterior boundary of the anterior chamber. Zonular fibers of the suspensory ligaments hold the lens in place by attaching it to the ciliary body. During fetal development, the lens is nearly spherical in shape, but eventually it assumes an elliptical configuration because of selective growth in the equatorial diameter into adult life. The infantile lens is usually seen as a globular and spherical shape. The adult lens, measuring approximately 4 μm in thickness, consists of multiple layers of cells arranged in a concentric pattern (10). These zones are acquired in a circumferential pattern with age, and their formation may depend on periods of slow growth (25). Cells forming the adult lens nucleus grow during childhood, whereas the lens cortex surrounding the nucleus continues to develop throughout life (25). The lens cortex is surrounded by a 2- to 20-mm thick lens capsule, essentially an acellular insoluble protein-polysaccharide material. The lens itself contains approximately two-thirds water and one-third structural protein. These proteins are synthesized in the cortex and precipitate in the nuclear region (26,27). Although most lens protein is soluble, the amount of insoluble protein increases in concentric layers toward the central part of the lens (15). The development of lens opacification (i.e., cataract), although of several diverse etiologies, seems to be related more to conformational changes in protein structure than to an increase in amount of insoluble protein (15). MR appears to be able to distinguish the low-intensity lens nucleus from the external cortex, a more hyperintense structure on T1-weighted images. The lens nucleus is markedly hypointense on T2-weighted images, which is probably related to a low density of mobile water protons in combination with markedly restricted motion of water by macromolecules (i.e., lens proteins). The highly structured nature of lens proteins, which would significantly affect nearby water, is a striking example of the importance of water motion, in addition to water content, to the signal intensity on MR images (28).

Another rare enhancing ocular lesion is the retinal astrocytoma, an entity seen almost exclusively in the setting of tuberous sclerosis or neurofibromatosis type 1 (NF1) (Fig. 18.31). These lesions may calcify and thus be difficult to distinguish from Rb; however, they are typically limited to the optic disk and generally lack hemorrhage or necrosis (51). Papillitis reflects focal swelling of the optic disc. It typically occurs in children and is described most frequently in the setting of multiple sclerosis (MS) (Fig. 18.32) (61). Papilledema may be seen with increased intracranial pressure from a variety of causes, and is typically a bilateral process, unlike papillitis (Fig. 18.33) (62).

Axial myopia is characterized by elongation of the globe in the anteroposterior dimension. The shape of the globe is smooth, without any focal bulge. A staphyloma is an acquired defect in the globe wall resulting in focal protrusion of a thinned cornea or sclera, usually posterotemporal to the disk (Fig. 18.40).
Staphylomas are lined with iris or choroidal tissue. Retinal detachment and/or staphyloma commonly accompany a severely myopic globe (Table 18.5) (68).

Colobomas (Fig. 18.41) are congenital defects in the retina, choroid, iris, lens, and/or optic nerve situated inferonasally and are often seen as excavations into the optic nerve head that result from failure of closure of the fetal optic fissure (15). These lesions are bilateral in up to 60% of cases. Colobomas are transmitted as an autosomal dominant trait with variable penetrance. The utility of MRI in the workup of these lesions lies in its ability to reveal any additional associated CNS anomalies that are frequently seen with colobomatous lesions, such as encephalocele or callosal agenesis and its spectrum of concurrent malformations (69,70).

Microphthalmia is classified as either simple or complex, depending on whether the small eye is anatomically intact or is malformed (71). Microphthalmos and retinal cysts may occur in conjunction with coloboma (colobomatous cyst) and are clearly detected with MR (72). If a colobomatous cyst is present, the cyst lies inferior to the site of exit of the nerve from the globe and is often the cause of superior displacement and proptosis of a microphthalmic eye (15). The major differential diagnoses in that case are meningocele and congenitally cystic globe. Coloboma may also occur in association with choristoma, a malformation composed of heterotopic mesodermal elements usually within or adjacent to the coloboma (15). Microphthalmia may also occur secondary to intrauterine infection or as an isolated anomaly, in which case it is often bilateral.

Macrophthalmos (buphthalmos), or congenital glaucoma, is a result of maldevelopment of the outflow channels in the anterior chamber angle causing increased intraocular pressure. Glaucoma in a young child results in an enlarged globe due to distensibility of the sclera. Buphthalmos (Fig. 18.42) may be unrelated to intraocular pressure, however, and may be seen as an isolated entity, secondary to massive intraocular tumor, or in association with neurofibromatosis type 1.

Marfan syndrome is a common cause of ectopica lentis which may manifest with subtle lens subluxation or complete bilateral lens dislocation; bilateral globe enlargement is also seen (Fig. 18.43) (73).

Phthisis bulbi represents an end-stage atrophic globe that has undergone extensive degenerative changes. This condition can result from any of a variety of insults, including prior inflammation and trauma. In phthisic eyes, the sclera becomes markedly thickened, irregular, and calcified. These globes are easily recognized on MR by their characteristic morphology and by their signal intensity patterns: peripheral hyperintensity on T1-weighted images and hypointensity on T2-weighted images (Fig. 18.44), reflecting iron and other paramagnetic ion deposition along with the dystrophic calcification. Finally, a characteristic distortion of globe shape is seen in patients who have had prior scleral banding procedures as treatment for retinal detachment, often in patients with high myopia (Fig. 18.34).

CT is the first-line imaging modality in the acute presentation of ocular trauma. CT benefits from rapid acquisition, thin multiplanar reformations, and sensitivity for orbital fracture approaching 100% (74). MR shows significant benefit for characterization of ocular detachments and nonradiopaque foreign bodies including wood, which may be particularly problematic on CT. MR should not be performed if there is a chance of an intraocular metallic foreign body (e.g., bullet fragments or shrapnel) (75).