Anatomy of the Orbital Apex and Cavernous Sinus on High-Resolution Magnetic Resonance Images

SURVEY OF OPHTHALMOLOGY VOLUME 44 • NUMBER 4 • JANUARY–FEBRUARY 2000

DIAGNOSTIC AND SURGICAL TECHNIQUES MARCO ZARBIN AND PETER HERSH, EDITORS

Anatomy of the Orbital Apex and Cavernous Sinus on High-Resolution Magnetic Resonance Images Armin Ettl, MD,1 Karin Zwrtek, MD,2 Albert Daxer, MD,3 and Erich Salomonowitz, MD2 1 Departments of Neuro-Ophthalmology, Oculoplastic and Orbital Surgery, 2Radiology, and 3Ophthalmology, General Hospital, St. Pölten, Austria

Abstract. Diseases of the orbital apex and cavernous sinus usually present with involvement of multiple cranial nerves, corresponding to the complex anatomy of the region. In nontraumatic disorders, magnetic resonance imaging is the diagnostic modality of choice. However, its capabilities can be fully used only with thorough knowledge of the complicated topographic relationships in this region. This article describes the imaging anatomy of the cranio-orbital junction and adjacent subarachnoid spaces. High-resolution magnetic resonance images of normal subjects are presented, and the results are compared with findings reported in the literature. The following anatomic structures can be visualized on high-resolution magnetic resonance images: extraocular muscles and corresponding connective tissue, major orbital and cerebral arteries, ophthalmic veins, cavernous sinus, and all sensory and motor cranial nerves of the eye along their intraorbital and intracranial course. (Surv Ophthalmol 44:303–323, 2000. © 2000 by Elsevier Science Inc. All rights reserved.) Key words. anatomy • brainstem • cavernous sinus • magnetic resonance imaging • orbital apex • orbit • parasellar region

The orbital and retro-orbital regions are involved in various injuries and diseases. The intracanalicular or intracranial optic nerve may be damaged by sphenoid fractures.31,71 Visual and neurologic deficits may arise from inflammatory processes (e.g., infectious lesions, orbital pseudotumor,58,79 Tolosa-Hunt syndrome40,43,79), vascular lesions (e.g., aneurysms, carotid-cavernous shunts, thrombosis of orbital veins or cavernous sinus), and neoplastic lesions (e.g., pituitary adenoma, meningioma,22,23 craniopharyngioma, etc.). Lesions at the cranio-orbital junction present with different neuro-ophthalmic symptoms and signs, depending on their size and location.60

Because of the crowding of critical anatomic structures at the orbital apex, patients often suffer from dysfunction of more than one cranial nerve. Previously, various syndromes such as the “orbital apex syndrome” (involvement of cranial nerves II, III, IV, V.1, and VI ), the “superior orbital fissure syndrome” (nerves III, IV, V.1, and VI), and the “cavernous sinus syndrome” (nerves III, IV, V.1, V.2, VI, and periarterial sympathetic plexus) have been described. In the past, physicians had to rely solely on clinical differences to estimate size and location of a lesion. Modern imaging techniques have proven these subtle differences in presentation to be unreliable indicators of the location and size of lesions. Therefore, 303

© 2000 by Elsevier Science Inc. All rights reserved.

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Key to Abbreviations ACA ACP AICA BA C CC CLT COI COS CP CRA CSF DS ES FLB FN FV G GWS H ICA IH II III III.i III.s IOF IOM ION IOV IPC IRM IV LN LPCA LPP LPS LRM LWS MA MEA MB MCA MO MOM MPCA MPP MRM MS NCN NPC OA OCA OCH

anterior cerebral artery anterior clinoid process anterior inferior cerebellar artery basilar artery clivus chiasmatic cistern cisterna laminae tecti colliculus inferior laminae tecti colliculus superior laminae tecti cerebral peduncle central retinal artery cerebrospinal fluid diaphragm sellae ethmoidal sinus frontal lobe of brain frontal nerve fourth ventricle globe greater wing of sphenoid bone hypophysis internal carotid artery infundibulum of hypophysis optic nerve oculomotor nerve inferior division of oculomotor nerve superior division of oculomotor nerve inferior orbital fissure inferior oblique muscle infraorbital nerve inferior ophthalmic vein interpeduncular cistern inferior rectus muscle trochlear nerve lacrimal nerve lateral posterior ciliary artery lateral plate of pterygoid process levator palpebrae superioris muscle lateral rectus muscle lesser wing of sphenoid bone maxillary artery mesencephalic aqueduct midbrain middle cerebral artery medulla oblongata Müller’s orbital muscle medial posterior ciliary artery medial plate of pterygoid process medial rectus muscle maxillary sinus nasociliary nerve nerve of pterygoid canal ophthalmic artery optic canal optic chiasm

ETTL ET AL

OPF OT P PCA PCL PCOA PCP PD PPF PPG RBF SCA SOF SOM SOV SP SRM SS TC TG TL TLB TV TZ V V.1 V.2 V.3 VI

orbital plate of frontal bone optic tract pons osterior cerebral artery petroclinoid ligament posterior communicating artery posterior clinoid process perioptic dura pterygopalatine fossa pterygopalatine ganglion retrobulbar fat superior cerebellar artery superior orbital fissure superior oblique muscle superior ophthalmic vein sympathetic plexus superior rectus muscle sphenoidal sinus tentorium cerebelli trigeminal ganglion tendon of Lockwood temporal lobe of brain third ventricle tendon of Zinn trigeminal nerve ophthalmic nerve maxillary nerve mandibular nerve abducens nerve

Miller more practically described the clinical picture of mass lesions at the cranio-orbital junction as “sphenocavernous syndrome.” 57 Imaging of the cranio-orbital junction necessitates sophisticated techniques because of its anatomic complexity. Additionally, good contrast resolution is required, because lesions such as cranial nerve tumors have little contrast to surrounding tissues. Standardized echography 12 and color Doppler ultrasonography are useful diagnostic techniques for the anterior and middle thirds of the orbit, but they lead to image distortion and loss of resolution when applied to the orbital apex. Computed tomography (CT) depicts not only orbital soft tissue details14 but also the complex bony anatomy of the orbital apex.31 However, CT scans of the orbital apex are distorted by beam hardening and dental filling artifacts.82 Computed tomography cannot identify individual cranial nerves within the superior orbital fissure,16,18 although contrastenhanced CT may demonstrate cranial nerves III, V.1, and V.2 within the cavernous sinus.17,44,54 In contrast to magnetic resonance imaging (MRI), plain CT scans do not allow differentiation of the intracavernous carotid artery from the surrounding sinus tissue.

ANATOMY OF ORBITAL APEX ON MRI

305 Fig. 1. Schematic representation of the superior orbital fissure and optic foramen. The superior orbital fissure is divided by the annulus of Zinn and contains frontal nerve (FN), lacrimal nerve (LN), trochlear nerve (IV) above the annulus; superior (III.s) and inferior (III.i) oculomotor nerve branches, abducens nerve (VI), nasociliary nerve (NCN), superior ophthalmic vein (SOV) within the annulus (“oculomotor foramen”); and inferior ophthalmic vein (IOV) below the annulus. II ⫽ optic nerve, IRM ⫽ inferior rectus muscle, LPS ⫽ levator palpebrae superioris muscle, MRM ⫽ medial rectus muscle, OA ⫽ ophthalmic artery, SOM ⫽ superior oblique muscle, SOF ⫽ superior orbital fissure, SRM ⫽ superior rectus muscle.

The enthusiasm for high-resolution MRI is based on the progress that has been demonstrated in both structural and vascular imaging. Precise maps of anatomic information allow comprehensive evaluation at high spatial, temporal, and contrast resolution. Magnetic resonance imaging is now the diagnostic modality of choice for imaging the orbital apex and retro-orbital region. Many of the important anatomic details of the anterior and posterior orbit can be visualized by MRI.7,8,19,25–29, 53 The interpretation of clinical MRIs of the cranioorbital junction requires a profound knowledge of

anatomy,24,41,54,81 especially sectional anatomy.38,47–49,67,74,83 A schematic overview of the anatomic structures is provided in Figs. 1–3. Detailed knowledge of the anatomic relationships in the orbital apex region is crucial not only for diagnostic purposes, but also for successful surgical intervention in a region where critical structures are only a few millimeters apart. The purpose of this article is to update the physician on current possibilities for imaging deep structures of the orbit and retro-orbital region, focusing on standardized and widely used MRI techniques.

Fig. 2. Schematic side view of the cavernous sinus. ICA ⫽ internal carotid artery, III ⫽ oculomotor nerve, IV ⫽ trochlear nerve, SP ⫽ sympathetic plexus, TG ⫽ trigeminal ganglion, V.1 ⫽ ophthalmic nerve, V.2 ⫽ maxillary nerve, V.3 ⫽ mandibular nerve, VI ⫽ abducens nerve. (Reprinted with permission from Kline.43 )

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ETTL ET AL Fig. 3. Schematic cross-section through the cavernous sinus. H ⫽ hypophysis, ICA ⫽ internal carotid artery, SP ⫽ sympathetic plexus, III ⫽ oculomotor nerve, IV ⫽ trochlear nerve, V.1 ⫽ ophthalmic nerve, V.2 ⫽ maxillary nerve, VI ⫽ abducens nerve. (Reprinted with permission Kline.43)

We describe the normal anatomy of the cranioorbital junction on MRIs, grouping the figures according to the view scanned: coronal (Figs. 4–12), axial (Figs. 13–23), and oblique-sagittal (Figs. 24–29). Our findings are related to those of others, as reported in the literature.

I. Examination Protocols Magnetic resonance imaging examinations were performed in eight normal volunteers on a 1.5-T

Fig. 4. T1-weighted coronal enhanced MRI of the posterior cavernous sinus at the level of Meckel’s cave. ACA ⫽ anterior cerebral artery, CC ⫽ chiasmatic cistern, DS ⫽ diaphragm sellae, H ⫽ hypophysis, ICA ⫽ internal carotid artery, IH ⫽ infundibulum of hypophysis, III ⫽ oculomotor nerve, IV ⫽ trochlear nerve, MCA ⫽ middle cerebral artery, OCH ⫽ optic chiasm, SS ⫽ sphenoidal sinus, TG ⫽ trigeminal ganglion, TLB ⫽ temporal lobe of brain, VI ⫽ abducens nerve.

MRI unit (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands). A circular polarized head coil was used for the cavernous sinus and brain stem, and a surface coil with a diameter of 17 cm was used for the orbital apex. Scans 1.2 to 3 mm thick with ⫺0.6- to 0.3-mm interslice gap were obtained in axial, oblique-axial (along the neuro-ophthalmic plane82,83), coronal, oblique-coronal (along the long axis of the petrous portion of the temporal bone), and oblique-sagittal (along cranial nerves III, V, and VI) planes by means of T1- weighted (T1w) se-

Fig. 5. T1-weighted enhanced coronal MRI of the posterior cavernous sinus at the level of the foramen ovale. DS ⫽ diaphragm sellae, H ⫽ hypophysis, ICA ⫽ internal carotid artery, IH ⫽ infundibulum of hypophysis, III ⫽ oculomotor nerve, OCH ⫽ optic chiasm, SS ⫽ sphenoidal sinus, TLB ⫽ temporal lobe of brain, IV ⫽ trochlear nerve, VI ⫽ abducens nerve.

ANATOMY OF ORBITAL APEX ON MRI

Fig. 6. T1-weighted coronal contrast-enhanced MRI of the anterior cavernous sinus. ACA ⫽ anterior cerebral artery, H ⫽ hypophysis, ICA ⫽ internal carotid artery, II ⫽ optic nerve, III ⫽ oculomotor nerve, IV ⫽ trochlear nerve, SS ⫽ sphenoidal sinus, TLB ⫽ temporal lobe of brain, V.1 ⫽ ophthalmic nerve, V.2 ⫽ maxillary nerve, VI ⫽ abducens nerve.

quences (repetition time [TR] ⫽ 550–620 ms, echo time [TE] ⫽ 14–18 ms) spin-echo (SE) sequences with and without intravenous injection of gadolinium-DTPA, and T2-weighted (T2w [TR ⫽ 2,800 ms, TE ⫽ 120 ms]) turbo-spin echo (TSE) sequences. Also, T2w three-dimensional TSE acquisitions (TR ⫽ 4,000 ms, TE ⫽ 250 ms, turbo factor (TF) ⫽ 48) were used for detailed demonstration of the cranial nerves in the subarachnoid cisterns. The fields of view (FOV) ranged between 120 and 140, and the images were usually obtained in a 256 ⫻ 256 matrix. This technique required a total examination time of 2.5 to 9 minutes.

II. Imaging A. BONY ANATOMY

Cortical bone does not produce a perceptible signal and is seen only indirectly by contrast demarcation to adjacent signal-generating tissue (e.g., brain, cerebrospinal fluid [CSF], muscle, fat, sinus mucosa, etc.). Cancellous bone is visualized indirectly by its fatty tissue content. The orbital apex is defined as the region between the posterior ethmoidal foramen and the openings of the optic canal and the superior orbital fissure. The posterior ethmoidal foramen is visible on axial and coronal MRIs.26 It transmits the corresponding neurovascular bundle at an average distance of 5 mm

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Fig. 7. T1-weighted coronal MRI of the anterior end of the cavernous sinus (CS) at the level of the foramen rotundum (FR). Cranial nerves III, IV, and VI are not visible on this nonenhanced image. ACP ⫽ anterior clinoid process, FLB ⫽ frontal lobe of brain, ICA ⫽ internal carotid artery, II ⫽ optic nerve, LPP ⫽ lateral plate of pterygoid process, MPP ⫽ medial plate of pterygoid process, NPC ⫽ nerve of pterygoid canal (Vidian’s nerve), SS ⫽ sphenoidal sinus, TLB ⫽ temporal lobe of brain, V.2 ⫽ maxillary nerve.

from the optic canal13 and represents an important landmark during orbital decompression surgery. The bony walls of the orbital apex can be demonstrated relatively well because of the contrast between orbital fat and tissue surrounding the orbit. The orbital roof of the apex consists of the lesser wing of the sphenoid bone (Figs. 9 and 10). Its medial wall is the lateral wall of the ethmoidal sinus (Figs. 10–12), its lateral wall is the greater wing of the sphenoid (Figs. 10–12), and its floor is the orbital plate of the palatine bone. The lesser wings of the sphenoid bone terminate at the anterior clinoid processes (Figs. 7 and 8), symmetric bony spines between the optic canal and superior orbital fissure, onto which the tentorium cerebelli (Figs. 17 and 24) is attached. The optic canal is 5- to 6-mm wide and 8- to 12mm long, and it forms an angle of approximately 35⬚ with the sagittal plane. It transmits the optic nerve and the ophthalmic artery (within a dural slit inferior to the nerve) and is bounded by the sphenoid bone medially, its lesser wing superiorly, the anterior clinoid process laterally, and the optic strut inferiorly (Figs. 8, 9, 15, and 16). The bony medial wall of the optic canal may be absent in about 4% of the population, which should be considered during transsphenoidal optic nerve decompression procedures.30 With extensive pneumatization, the optic ca-

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Fig. 8. T1-weighted coronal MRI of the orbital apex at the level of the cranial openings of the superior orbital fissures (SOF). Cranial nerves VI (abducens nerve) and V.1 (ophthalmic nerve) cannot be separated from each other. ACP ⫽ anterior clinoid process, FLB ⫽ frontal lobe of brain, II ⫽ optic nerve, III ⫽ oculomotor nerve, IV ⫽ presumed trochlear nerve, OA ⫽ ophthalmic artery, SOF ⫽ superior orbital fissure, SS ⫽ sphenoidal sinus, TLB ⫽ temporal lobe of brain, V.1 ⫽ presumed ophthalmic nerve, V.2 ⫽ maxillary nerve, VI ⫽ abducens nerve.

nal may become entirely surrounded by a (posterior) ethmoidal “Onodi” air cell or the sphenoid sinus proper or an aerated anterior clinoid process. On MRI, this would result in signal loss around the optic nerve.15,76 The various relationships between the paranasal sinuses and the optic canal and their clinical implications have been described in a comprehensive review on optic nerve trauma.31,71 Each optic canal opens into the chiasmatic groove, which terminates posteriorly at the tuberculum sellae. Located further posteriorly is the sella turcica, which contains the pituitary gland (Figs. 4–6). The dorsum sellae carries the posterior clinoid processes (Fig. 28), onto which the tentorium cerebelli (Figs. 17 and 21) is attached. On either side of the sella turcica lies the carotid groove for the intracavernous part of the internal carotid artery. Gruber’s petroclinoideal ligament, a dural fold extending from the posterior clinoid process to the apex of the petrous portion of the temporal bone, forms a passage for the abducens nerves (Figs. 26 and 28).60 The superior orbital fissure is situated between the lesser and greater sphenoid wings and usually contains a small amount of adipose tissue (Figs. 8, 9, and 14). It transmits (1) the superior ophthalmic vein and the frontal, lacrimal, and trochlear nerves above the common tendinous annulus, (2) the supe-

ETTL ET AL

Fig. 9. T1-weighted coronal MRI of the orbital apex at the level of the orbital opening of the superior orbital fissure. At this level, the superior orbital fissure is continous with the inferior orbital fissure (IOF). The oculomotor, trochlear, abducens, nasociliary, frontal, and lacrimal cranial nerves (CN) are not visualized as individual structures. ES ⫽ ethmoid sinus, FLB ⫽ frontal lobe of brain, GWS ⫽ greater wing of sphenoid bone, II ⫽ optic nerve, IOF ⫽ inferior orbital fissure, LWS ⫽ lesser wing of sphenoid bone, MA ⫽ maxillary artery, MOM ⫽ Müller’s orbital muscle, OA ⫽ ophthalmic artery, PPF ⫽ pterygopalatine fossa, PPG ⫽ pterygopalatine ganglion, TLB ⫽ temporal lobe of brain, TZ ⫽ tendon of Zinn (origin of medial, inferior, and lateral rectus muscles), V.2 ⫽ maxillary nerve.

rior and inferior branch of the oculomotor nerve, the abducens nerve, and the nasociliary nerve within the annulus, and, occasionally, (3) the inferior ophthalmic vein below the annulus. The inferior orbital fissure (Figs. 9, 10, 13, and 23) lies between the orbital floor and the lateral orbital wall and communicates with the pterygopalatine (Figs. 9–11) and infratemporal fossae. It is bridged by the orbital muscle of Müller (Figs. 10–12) and transmits the infraorbital artery, venous collaterals between the inferior ophthalmic vein and the pterygoid plexus, and the maxillary nerve, which exits the middle cranial fossa via the foramen rotundum (Figs. 8 and 23). B. EXTRAOCULAR MUSCLE AND CONNECTIVE TISSUE SYSTEM ANATOMY

The extraocular muscles demonstrate intermediate signal on both T1w and T2w MRIs.27 The four rectus muscles originate from the common tendinous annulus of Zinn, which spans across the superior orbital fissure and encloses the optic foramen (containing optic nerve and ophthalmic artery) and

ANATOMY OF ORBITAL APEX ON MRI

Fig. 10. T1-weighted coronal MRI of the orbital apex at the level of Zinn’s tendinous annulus. ES ⫽ ethmoidal sinus, FLB ⫽ frontal lobe of brain, GWS ⫽ greater wing of sphenoid bone, II ⫽ optic nerve, III.s ⫽ presumed superior division of oculomotor nerve, IV, FN, LN ⫽ presumed trochlear frontal and lacrimal nerves, LRM ⫽ lateral rectus muscle, LWS ⫽ lesser wing of sphenoid bone, MOM (IOF) ⫽ Müller’s orbital muscle (inferior orbital fissure), MS ⫽ maxillary sinus, NCN ⫽ nasociliary nerve, OA ⫽ main trunk of ophthalmic artery (inferior to optic nerve), PPF ⫽ pterygopalatine fossa, SOF ⫽ superior orbital fissure, SOV ⫽ presumed superior ophthalmic vein, TL ⫽ tendon of Lockwood (origin of superior rectus muscle), TLB ⫽ temporal lobe of brain, TZ (IRM) ⫽ tendon of Zinn (origin of inferior rectus muscle), TZ (LRM) ⫽ tendon of Zinn (origin of lateral rectus muscle), TZ (MRM) ⫽ origin of medial rectus muscle), V.2 ⫽ maxillary nerve, VI ⫽ abducens nerve.

the oculomotor foramen (Fig. 10). Zinn’s annulus consists of two half-circles. Superiorly, the tendon of Lockwood serves as origin for the superior rectus muscle (Fig. 10), and inferiorly the tendon of Zinn for the medial, inferior, and lateral rectus muscles (Figs. 9 and 10). The annulus divides the superior orbital fissure into three spaces, the superolateral, central (“oculomotor foramen”), and inferior spaces. The superolateral portion of the superior orbital fissure contains the frontal, lacrimal, and trochlear nerves; the central portion contains the nasociliary nerve, superior and inferior branch of the oculomotor nerve, abducens nerve, and the superior ophthalmic vein; the inferior portion of the superior orbital fissure contains the inferior ophthalmic vein (Fig. 1). For the first 5 mm of their lengths, the rectus muscles do not appear as individual structures but are firmly embedded within Zinn’s annulus. This anatomic relationship explains that enlargement of the rectus muscle origins associated with thyroid

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Fig. 11. T1-weighted coronal MRI of the orbital apex (scan level between Figs. 10 and 12). The cranial nerves III and VI appear in close contact to the corresponding extraocular muscles. The trochlear nerve lies on top of the superior rectus muscle, from which it cannot be differentiated. CRA ⫽ central retinal artery, ES ⫽ ethmoidal sinus, FLB ⫽ frontal lobe of brain, FN, LN ⫽ presumed frontal and lacrimal nerves, GWS ⫽ greater wing of sphenoid bone, II ⫽ optic nerve, III.i ⫽ inferior division of oculomotor nerve, III.s ⫽ superior division of oculomotor nerve, IOV ⫽ inferior ophthalmic vein, IRM ⫽ inferior rectus muscle, LRM ⫽ lateral rectus muscle, MOM ⫽ Müller’s orbital muscle, MRM ⫽ medial rectus muscle, MS ⫽ maxillary sinus, NCN ⫽ nasociliary nerve, OA ⫽ bend of ophthalmic artery (lateral to optic nerve), PPF ⫽ pterygopalatine fossa, SOV ⫽ presumed superior ophthalmic vein, SRM ⫽ superior rectus muscle, V.2 ⫽ maxillary nerve, VI ⫽ abducens nerve.

orbitopathy may cause compression of the optic nerve. Histologically, the origins of the rectus muscles are separated from each other by thin connective tissue septa. On MRI, the entire inferior annulus appears as one single unit of muscle masses (Fig. 10). Approximately 8 mm anterior to the optic strut, the rectus muscles separate and appear as individual structures (Figs. 11 and 12). The superior oblique muscle originates superiorly and medially to Zinn’s annulus from the lesser sphenoid wing with a short tendon (Figs. 11 and 12). The levator palpebrae superioris muscle originates from the lesser sphenoid wing and the annulus of Zinn, where it blends with the origin of the superior rectus muscle (Figs. 10–12).28 In the orbital apex, the system of connective tissue septa46–49 is less well developed. The superolateral intermuscular septum25,27 between the superior and the lateral rectus muscles starts at a distance of about 5 mm from the posterior end of the orbit and thin

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Fig. 12. T1-weighted coronal MRI of the orbital apex just posterior to the posterior ethmoidal foramen. The cranial nerves III, IV, and VI appear in close contact to the corresponding extraocular muscles. CG ⫽ presumed ciliary ganglion, CRA ⫽ central retinal artery, ES ⫽ ethmoidal sinus, FLB ⫽ frontal lobe of brain, FN ⫽ frontal nerve, GWS ⫽ greater wing of sphenoid bone, II ⫽ optic nerve, III.i ⫽ inferior division of oculomotor nerve, III.s ⫽ presumed superior division of oculomotor nerve, ION ⫽ infraorbital nerve, IOV ⫽ inferior ophthalmic vein, IRM ⫽ inferior rectus muscle, IV ⫽ presumed trochlear nerve, LN ⫽ presumed lacrimal nerve, LPS ⫽ levator palpebrae superioris muscle, LRM ⫽ lateral rectus muscle, MOM ⫽ Müller’s orbital muscle, MRM ⫽ medial rectus muscle, MS ⫽ maxillary sinus, NCN ⫽ nasociliary nerve, OA ⫽ ophthalmic artery, OPF ⫽ orbital plate of frontal bone, SOM ⫽ superior oblique muscle, SOV ⫽ superior ophthalmic vein, SRM ⫽ superior rectus muscle.

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Fig. 13. T1-weighted axial MRI at the level of the inferior orbital fissure (IOF) showing parts of Müller’s orbital muscle (MOM). The bony borders of the IOF are indicated by white dots. GWS ⫽ greater wing of sphenoid bone, IOM ⫽ inferior oblique muscle, IOV ⫽ inferior ophthalmic vein, IRM ⫽ inferior rectus muscle, MS ⫽ maxillary sinus, SS ⫽ sphenoidal sinus.

the intracranial dura. The subarachnoid space of the intraorbital optic nerve appears hypointense on T1w and hyperintense on T2w (Fig. 21). The intracranial optic nerve is covered only by pia and is surrounded by the CSF of the suprasellar cistern (Figs. 6–8).

radial septa course from the rectus muscles to the orbital walls.47 C. CRANIAL NERVE ANATOMY

1. Optic Nerve and Chiasm The entire optic nerve (II) can be visualized on appropriate MRIs. The optic nerve measures 45–50 mm in length and 3–5 mm in diameter, including its nerve sheath. It consists of a 3- to 16-mm-long intracranial portion, a 6- to 10-mm-long intracanalicular portion, a 21- to 34-mm-long intraorbital portion, and an approximately 1-mm-long intraocular portion, i.e., the papilla.7,22,55 The optic nerve consists of myelinated nerve fibers and exhibits MRI signal characteristics similar to those of white matter of the brain. The intracanalicular and intraorbital portions of the optic nerve are surrounded by pia, arachnoidea, and dura. At the orbital opening of the optic canal, the dura of the intracanalicular optic nerve splits into periorbita and perioptic dura. At the intracranial opening of the canal, it is continous with

Fig. 14. T1-weighted axial MRI at the level of the superior orbital fissure (SOF). The bony borders of the SOF are indicated by white dots. ES ⫽ ethmoidal sinus, G ⫽ globe, GWS ⫽ greater wing of sphenoid bone, ICA ⫽ internal carotid artery, LRM ⫽ lateral rectus muscle, MRM ⫽ medial rectus muscle, OA ⫽ ophthalmic artery, SS ⫽ sphenoidal sinus.

ANATOMY OF ORBITAL APEX ON MRI

Fig. 15. T1-weighted axial MRI in the neuroophthalmic plane82,83 at the level of the optic canal (OCA) showing the intraorbital and intracanalicular portion of the optic nerve (II). The bony borders of the OCA and SOF are indicated by white dots. ACP ⫽ anterior clinoid process, ES ⫽ ethmoidal sinus, G ⫽ globe, GWS ⫽ greater wing of sphenoid bone, LPCA ⫽ lateral posterior ciliary artery, LRM ⫽ lateral rectus muscle, MPCA ⫽ medial posterior ciliary artery, MRM ⫽ medial rectus muscle, OA ⫽ ophthalmic artery (before crossing over the optic nerve), SOF ⫽ superior orbital fissure, SS ⫽ sphenoidal sinus.

In the primary gaze position, the intraorbital portion of the optic nerve describes an S-shaped path from the globe inferomedially, and then superiorly, to the optic foramen.29 The redundant length of the optic nerve allows the globe to move freely and protects the nerve in case of proptosis.29 The intracanalicular portion passes above the ophthalmic artery through the optic canal (Figs. 8, 9, 15, and 16). On axial scans, the intracanalicular optic nerve can be identified medial to the anterior clinoid process (signal characteristics: see above) and lateral to the sphenoid sinus (signal void of air) (Figs. 15 and 16).29,31 Coronal scans show the optic nerve in the chiasmatic cistern (Fig. 6), in its canal (Figs. 8 and 9), in the orbital apex inside the annulus tendineus (Fig. 10), and in the orbit surrounded by the rectus muscles (Figs. 11 and 12). The optic chiasm lies within the floor of the third ventricle and superior to the diaphragm sellae (Figs. 4, 5, and 16). Here, nasal retinal ganglion cell axons cross from the optic nerve to the contralateral optic tract. The chiasm normally measures 10 to 20 mm in transverse, 4 to 13 mm in anteroposterior, and 3 to 5 mm in craniocaudal diameter.24 2. Motor Nerves The motor nerves (with the exception of the inferior division branches of the oculomotor nerve to

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Fig. 16. T2-weighted oblique-axial MRI at the level of the optic chiasm (OCH). ACP ⫽ anterior clinoid process, CC ⫽ chiasmatic cistern, GWS ⫽ greater wing of sphenoid bone, ICA ⫽ internal carotid artery, II ⫽ optic nerve, LRM ⫽ lateral rectus muscle, MCA ⫽ middle cerebral artery, MRM ⫽ medial rectus muscle, OCA ⫽ optic canal, OT ⫽ optic tract, RBF ⫽ retrobulbar fat.

the inferior oblique muscle and the trochlear nerve) ramify far posteriorly in the orbital apex into numerous fascicles, which travel anteriorly embedded between muscle fibers to innervate the extraocular muscles in their posterior third and from their intraconal surface.68 The tiny ramifications and the fasci-

Fig. 17. T2-weighted axial MRI at the level of the midbrain (MB) showing the oculomotor nerve (III) in the interpeduncular cistern. MEA ⫽ mesencephalic aqueduct, PCA ⫽ posterior cerebral artery, PCP ⫽ posterior clinoid process, SOF ⫽ superior orbital fissure, TC ⫽ tentorium cerebelli.

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Fig. 18. T2-weighted axial MRI at the level of the midbrain (MB) showing the origin of the oculomotor nerve. III ⫽ oculomotor nerve with partial volume effect (see paragraph 4.5 of text) BA ⫽ basilar artery, CLT ⫽ cisterna laminae tecti, COS ⫽ colliculus superior laminae tecti, CP ⫽ cerebral peduncles, CSF ⫽ cerebrospinal fluid, ICA ⫽ internal carotid artery, II ⫽ optic nerve, IPC ⫽ interpeduncular cistern, MEA ⫽ mesencephalic aqueduct, PCP ⫽ posterior clinoid process, PD ⫽ perioptic dura, SCA superior cerebellar artery.

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Fig. 19. T2-weighted axial MRI showing the origin of the ophthalmic artery (OA) from the internal carotid artery (ICA). IRM ⫽ inferior rectus muscle, LRM ⫽ lateral rectus muscle, MRM ⫽ medial rectus muscle, PCOA ⫽ posterior communicating artery.

cles of the motor nerves cannot be visualized on MRI. Only the nerve trunks in the orbital apex and also the oculomotor nerve branch to the inferior oblique muscle are visualized on high-resolution MRI.25,26

The small parasympathetic twig of the inferior division of the oculomotor nerve (usually the branch to the inferior oblique muscle) that joins the ciliary ganglion cannot be seen on MRI. The ciliary ganglion that transmits afferent sen-

Fig. 20. T2-weighted oblique-axial MRI at the level of the dorsal caudal midbrain (MB) and ventral rostral pons showing the presumed trochlear nerve (IV). BA ⫽ basilar artery, MEA ⫽ mesencephalic aqueduct, SCA ⫽ superior cerebellar artery.

Fig. 21. T2-weighted axial MRI at the level of the pons (P) showing the trigeminal nerve (V) in the prepontine cistern. V ⫽ trigeminal nerve with partial volume effect (see paragraph 4.5 of text), BA ⫽ basilar artery, FV ⫽ fourth ventricle, TG ⫽ trigeminal ganglion inside Meckel’s cave.

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Fig. 22. T2-weighted oblique-axial MRI at the level of the caudal pons (P) showing the abducens nerve (VI) in the subarachnoid space. AICA ⫽ anterior inferior cerebellar artery, BA ⫽ basilar artery, C ⫽ clivus, FV ⫽ fourth ventricle.

sory fibers and efferent parasympathetic fibers for pupillary constriction and accommodation is situated in the orbital apex very close to the lateral aspect of the optic nerve.68 It may be seen on high-resolution MRI just anterior to the knee of the ophthalmic artery between the optic nerve and the lateral rectus muscle (Fig. 12).26 a. Oculomotor Nerve The neurons of the third cranial nerve arise in the oculomotor nuclei, which lie in the ventral periaqueductal gray matter of the midbrain. The third nerve exits the brain medially to the cerebral peduncles (Fig. 18) and passes between the superior cerebellar and posterior cerebral arteries (Fig. 29) through the interpeduncular cistern (Figs. 17 and 27), where it lies adjacent to the posterior communicating artery (Fig. 19). Then, the third nerve pierces the dura at the top of the clivus (Fig. 27). Embedded within the dural border of the cavernous sinus, the third nerve courses forward just superior to the trochlear nerve (Figs. 4 and 6). Just posterior to the orbital opening of the superior orbital fissure, the third nerve crosses under the trochlear nerve and divides into a superior and inferior division, both of which pass the central portion (oculomotor foramen) of the superior orbital fissure to enter the orbit (Fig. 9). The superior division of the third nerve courses superiorly to innervate the superior rectus and the levator palpebrae muscles (Figs. 10–12). The inferior division of the third nerve courses medially and inferiorly to innervate the medial and inferior rectus muscles (Figs. 10–12). A long nerve branch courses anteriorly to the inferior oblique muscle.25,26

Fig. 23. T1-weighted oblique-axial MRI at the level of the inferior orbital fissure (IOF) and foramen rotundum (FR). C ⫽ clivus, GWS ⫽ greater wing of sphenoid bone, ICA ⫽ internal carotid artery, MS ⫽ maxillary sinus, P ⫽ pons, SS ⫽ sphenoidal sinus, V.2, FR ⫽ maxillary nerve inside foramen rotundum.

b. Trochlear Nerve The neurons of the fourth cranial nerve originate from the trochlear nucleus in the ventral periaqueductal gray matter of the midbrain (rostral to the oculomotor nuclei), decussate, and emerge on the dorsal surface of the midbrain, below the level of the inferior colliculus. Running along the free border of the tentorium cerebelli, the fourth nerve passes between the superior cerebellar and posterior cerebral arteries around the cerebral peduncles (Fig. 20) to enter the lateral border of the cavernous sinus. Because of its long intracranial course, the fourth nerve is predisposed to injury from blunt head trauma. Inside the lateral dural border of the cavernous sinus, the fourth nerve courses forward, first inferior to the third nerve (Fig. 4) and finally superior to the third nerve. Then, the fourth nerve exits the cavernous sinus and traverses the superolateral portion of the superior orbital fissure. In the orbital apex, the fourth nerve crosses over the origin of the superior rectus and levator palpebrae muscles (Fig. 12) to innervate the superior oblique muscle from its lateral surface about 10 mm from the orbital apex.29 c. Abducens Nerve The neurons of the fourth nerve arise in the abducens nucleus of the pons just inferior to the rostral part of the floor of the fourth ventricle. The

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Fig. 24. T2-weighted oblique-sagittal, reconstructed MRI in a plane along the trigeminal nerve (V). BA ⫽ basilar artery, C ⫽ clivus, P ⫽ pons, RBF ⫽ retrobulbar fat, TC ⫽ tentorium cerebelli, TLB ⫽ temporal lobe of brain.

nerve emerges in the ventral sulcus between pons and medulla oblongata (Fig. 25). It ascends along the ventral surface of the pons, pierces the dura, and passes over the petrous apex through Dorello’s canal, an osteofibrous canal underneath Gruber’s petroclinoidal ligament (Figs. 26 and 28). Then, the fourth nerve enters the cavernous sinus and proceeds inside this venous plexus, first laterally and then inferolaterally, to the internal carotid artery (Figs. 4–6). The fourth nerve enters the orbit via the central portion of the superior orbital fissure (Figs. 8 and 9) to innervate the lateral rectus muscle from its intraconal surface (Figs. 10–12).

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Fig. 25. T2-weighted oblique-sagittal reconstructed MRI in a plane along the origin of the abducens nerve (VI) from the medullopontine sulcus. BA ⫽ basilar artery, COI ⫽ colliculus inferior laminae tecti, COS ⫽ colliculus superior laminae tecti, FV ⫽ fourth ventricle, MB ⫽ midbrain, MO ⫽ medulla oblongata, P ⫽ pons, TV ⫽ third ventricle.

The sensory neurons of the trigeminal nerve (fifth nerve) terminate in the main sensory nucleus in the pons and the spinal tract of the fifth nerve. The sensory root (together with the motor root) emerges from the pons just above the middle cerebellar peduncle (Figs. 21 and 24). The neurons synapse in the trigeminal ganglion Gasseri, which is situated in Meckel’s cave, a dural split over the apex of the petrous portion of the temporal bone (Figs. 4 and 21). Three main divisions of the fifth nerve arise from the trigeminal ganglion: the ophthalmic (V.1), maxillary (V.2), and mandibular (V.3) divisions.

rior cavernous sinus, the ophthalmic nerve divides into the lacrimal, frontal, and nasociliary nerves, which exit the cavernous sinus as individual nerve branches. The small lacrimal nerve enters the orbit through the superolateral portion of the superior orbital fissure. It courses anteriorly along the superior border of the lateral rectus muscle toward the lacrimal gland. It may be visualized on MRIs of the anterior orbit,25,26 but it cannot be differentiated from the frontal nerve in the orbital apex (Figs. 10–12). The frontal nerve also enters the orbit via the superolateral portion of the superior orbital fissure (Fig. 9). It passes forward between levator palpebrae superioris muscle and orbital roof (Figs. 10–12) and divides into the supratrochlear nerve, the medial branch of the supraorbital nerve, and the lateral branch of the supraorbital nerve.25,26 The nasociliary nerve (Figs. 10–12) enters the orbit through the central portion of the superior orbital fissure, crosses over the optic nerve about 10 mm from the orbital apex, and courses anteriorly at the lateral side of the medial rectus muscle.26

a. Ophthalmic Nerve (V.1)

b. Maxillary Nerve (V.2)

The ophthalmic nerve courses forward inside the lateral dural border of the cavernous sinus just inferior to the fourth nerve (Figs. 6 and 8). In the ante-

The origin of the maxillary nerve lies inside the inferolateral dural border of the cavernous sinus (Figs. 5 and 6). It exits the middle cranial fossa via the fora-

3. Sensory Nerves

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Fig. 26. T2-weighted oblique-sagittal reconstructed MRI in a plane along the abducens nerve (VI) inside the prepontine cistern and underneath the petroclinoidal ligament (PCL). ACA ⫽ anterior cerebral artery, BA ⫽ basilar artery, COI ⫽ colliculus inferior laminae tecti, COS ⫽ colliculus superior laminae tecti, FV ⫽ fourth ventricle, III ⫽ oculomotor nerve, MB ⫽ midbrain, MCA ⫽ middle cerebral artery, MO ⫽ medulla oblongata, OT ⫽ optic tract, P ⫽ pons, PCA ⫽ posterior cerebral artery, SCA ⫽ superior cerebellar artery.

Fig. 27. T2-weighted oblique-sagittal reconstructed MRI in a plane along the intracisternal portion of the oculomotor nerve (III). BA ⫽ basilar artery, COI ⫽ colliculus inferior laminae tecti, COS ⫽ colliculus superior laminae tecti, FLB ⫽ frontal lobe of brain, FV ⫽ fourth ventricle, G ⫽ globe, IPC ⫽ interpeduncular cistern, MB ⫽ midbrain, MCA ⫽ middle cerebral artery, MO ⫽ medulla oblongata, MS ⫽ maxillary sinus, OT ⫽ optic tract, P ⫽ pons, PCA ⫽ posterior cerebral artery, RBF ⫽ retrobulbar fat, SCA ⫽ superior cerebellar artery.

men rotundum7,15 (Figs. 7 and 8) to enter the inferior orbital fissure (Figs. 9 and 10). Inside the inferior orbital fissure, it divides into the zygomatic nerve and the infraorbital nerve, which travels anteriorly inside the infraorbital canal (Fig. 12).25,26

Occasionally, the ophthalmic artery may arise from the ICA within the cavernous sinus.35,37,52 While coursing upward, the ICA gives off the posterior communicating artery (Fig. 19) to the posterior cerebral artery (Fig. 17), and then divides into its two terminal branches, the middle cerebral artery (Fig. 4) and the anterior cerebral artery (Figs. 4 and 6) thus forming the circle of Willis.

D. VASCULAR ANATOMY

1. Arterial System a. Internal Carotid Artery

b. Ophthalmic Artery

The arterial supply to the orbit mainly derives from the internal carotid artery (ICA), which courses through the petrous portion of the temporal bone in the carotid canal and enters the middle cranial fossa by passing over the foramen lacerum. The ICA courses superiorly to the posterior clinoid process (Fig. 23) and then turns anteriorly to enter the venous plexus of the cavernous sinus (CS). Within the cavernous sinus, the ICA proceeds anteriorly with the abducens nerve along its lateral side (Figs. 4–6), giving off several small branches to supply the surrounding cranial nerves and the hypophysis. Then, the ICA makes an upward S-shaped turn to form the carotid siphon (Figs. 7, 18, and 19). In most individuals, the ICA gives off the ophthalmic artery (OA) outside the cavernous sinus at the medial side of the anterior clinoid process (Fig. 19).

The approximately 2- to 3-mm-long intracranial portion of the OA originates from the ICA below the intracranial optic nerve (Fig. 19) and enters the optic canal within a split of the perioptic dura. Inside the canal, the vessel courses inferiorly to the optic nerve (Figs. 8 and 9). The ophthalmic artery usually emerges in the inferolateral portion of the optic foramen and then proceeds nasally (Fig. 10). The ophthalmic artery crosses over the optic nerve (Figs. 11, 12, and 15) in 72% to 95% of individuals or crosses under it in 5% to 28%.24,35,52 The first branch of the ophthalmic artery is usually the 0.3- to 0.4-mm-thick central retinal artery (CRA), which courses inferiorly to the optic nerve (Fig. 12) and enters its dural sheath about 10 mm (range, 5–16 mm) behind the globe.5,6,36 Occasionally, the CRA branches off the ophthalmic artery together with or following the lat-

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Fig. 28. T2-weighted oblique-coronal MRI in a plane along the long axis of the petrous portion of the temporal bone (PPT) showing the abducens nerve (VI) underneath the petroclinoidal ligament (PCL). III ⫽ oculomotor nerve , PCP ⫽ posterior clinoid process, TG ⫽ presumed posterior extension of trigeminal ganglion, TLB ⫽ temporal lobe of brain.

eral posterior ciliary artery.24 The order of branching along the OA varies considerably. Other major branches that may be visualized on MRI26,29 include the posterior ciliary arteries (Fig. 15), the lacrimal artery, the posterior and anterior ethmoidal arteries, and the supraorbital and supratrochlear arteries. 2. Venous System

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Fig. 29. T2-weighted oblique-coronal MRI in a plane posterior and parallel to the scan of Fig. 28 showing the oculomotor nerve (III) between posterior cerebral artery (PCA) and superior cerebellar artery (SCA). AICA ⫽ anterior inferior cerebellar artery, BA ⫽ basilar artery, P ⫽ pons, V ⫽ trigeminal nerve.

rior and lateral rectus muscles (Figs. 11 and 13), and leaves the orbit via the inferior portion of the superior orbital fissure. Occasionally, the inferior ophthalmic vein may pass superiorly to join the superior ophthalmic vein as it drains into the cavernous sinus. A medial ophthalmic vein that courses in the nasal extraconal orbit26 may occur in about 40% of individuals. When present, it joins the superior ophthalmic vein near the superior orbital fissure.24

a. Veins The central retinal vein (not visualized in the present study) drains the blood from the retina. It traverses the optic nerve, courses in its subarachnoid space, and exits the perioptic dura to either join the superior ophthalmic vein or drain into the cavernous sinus. Drainage from the choroid of the eye is provided by the vortex veins, which subsequently drain into the superior and inferior ophthalmic veins.26 The superior ophthalmic vein crosses under the superior rectus muscle (Fig. 12), passes between the origins of the superior and lateral rectus muscles (Fig. 10), and leaves the orbit through the superolateral portion of the superior orbital fissure. In axial scans below the level of the superior rectus muscle, it is usually possible to visualize the entire superior ophthalmic vein.26,29 The inferior ophthalmic vein (IOV) communicates with the pterygoid plexus via the inferior orbital fissure, passes between the origins of the infe-

b. Parasellar Venous Plexus (Cavernous Sinus) The cavernous sinuses are situated on either side of the body of the sphenoid bone and sphenoid air sinuses (Figs. 4–7). They extend from the apex of the petrous portion of the temporal bone posteriorly to the superior orbital fissure anteriorly. The cavernous sinuses are not trabeculated sinuses, as originally described by Winslow in 1732,4,33 but rather represent extradural venous plexuses surrounded by a dural fold.20,64,72,73,80 The concept of a venous plexus, as opposed to a true venous sinus, has been verified by means of contrast-enhanced, dynamic CT scanning.9 Contrast-enhanced MRI studies have demonstrated that venous flow in the cavernous sinuses can be divided into rapid-flow channels, characterized by marked enhancement, and low-flow channels, which have less enhancement.45 Orbital tributaries to the cavernous sinus are the superior ophthalmic vein, the inferior ophthalmic

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vein, and the central retinal vein.20 The two cavernous sinuses communicate with each other via the anterior and posterior intercavernous sinuses, which are located anterior and posterior to the pituitary stalk in the diaphragm sellae.42 The cavernous sinus mainly drains into the internal jugular vein via the petrosal sinuses. The intracavernous internal carotid artery (ICA), with its periarterial sympathetic plexus, was originally described to pass the sinus in direct contact with the venous blood.4 However, in reality, it runs between the venules of the parasellar venous plexus.4 The abducens nerve may run very close to the ICA (Figs. 4–6), either separated from it only by its nerve sheath79 or embedded inside the lateral dural border of the sinus.75 Within the lateral dural border of the cavernous sinus (“lateral wall”), from superior to inferior, run the oculomotor and trochlear nerves and ophthalmic and maxillary divisions of the trigeminal nerve (Figs. 4–6).17 Occasionally, these nerves are separated from the parasellar venous plexus only by a thin connective tissue layer.75 The trigeminal ganglion is situated in a dural fold (Meckel’s cave) in continuation to the lateral wall of the cavernous sinus (Figs. 4 and 21).

III. Discussion of Techniques

Thicker sections may catch even longer segments of neurovascular structures, but partial volume averaging affects the image quality. Those parts of anatomic structures that are partially out of the imaging plane are not represented in focus and show an altered signal intensity (partial volume effect). B. SIGNAL VOID OF FLOWING BLOOD

Fast and turbulent flow in blood vessels results in loss of intravascular signal intensity (signal void) because of dephasing inside the measured volume element (voxel). Intravoxel dephasing can be minimized by using a short echo time and small voxels. Slow flow or vortex flow results in reduced intravascular signal intensity because of saturation effects. Saturation effects can be minimized by reducing the flip angle or lengthening the TR. Therefore, the lumen of blood vessels appears dark or hypointense depending on blood flow velocity and imaging parameters.21 The internal carotid artery inside the cavernous sinus appears as signal void in both T1w (Figs. 7 and 20) and T2w images. On T2w images, this flowing blood is clearly differentiated from hyperintense CSF (Fig. 18). The hypointense lateral border of the cavernous sinus is also sharply demarcated from adjacent CSF on T2w images (Fig. 17).

A. SEQUENCES AND COILS

C. CONTRAST ENHANCEMENT

Magnetic resonance imaging demonstrates the anatomy of the orbit and retro-orbital region with superb detail. The best resolution of orbital structures is presently obtained by using standard T1w SE2,3 or T2w TSE pulse sequences. Conventional T2w and proton density images need too long an acquisition time leading to motion artifacts. For the orbits, local surface coils2,3 and, whenever possible, phased assay coils should be used. However, since the signal decay in the orbital apex depends on the diameter of the coil, imaging of this region requires a larger surface coil (Fig. 30). When additional imaging of the middle cranial fossa is required, the use of a head coil is recommended. The cranial nerves within the subarachnoid cisterns are best visualized on T2w images, where they appear hypointense against the bright background of the CSF. The high fat content of the orbit is responsible for the excellent contrast of orbital MRIs, enhancing the detection of tiny anatomic structures. Fat appears hyperintense (bright) on T1w and T2w images, and other structures, such as vessels, nerves, and muscles, appear darker (hypointense) than orbital fat. Even parts of the orbital connective tissue system can be visualized.25 Thin slices, such as in the present study (0.5–2 mm), enable visualization of relatively long segments of delicate blood vessels and nerves.

The paramagnetic contrast medium gadoliniumdiethylenetriaminepenta-acetic acid (Gd-DTPA) enhances vascular structures, such as cavernous sinus or the venous plexus surrounding Meckel’s cave and the hypophysis (Figs. 4–6). The cranial nerves passing the sinus can be visualized as individual struc-

Fig. 30. Surface coil for orbital imaging. A surface coil is a radiofrequency receiver that is placed upon the body surface over the region of interest. In the present study, a surface coil was placed upon the face and centered over both orbits.

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tures only in contrast-enhanced coronal MRIs (Figs. 4–6). Inside the orbit, the enhancing effect of GdDTPA may result in decreased contrast because of the intense signal of orbital fat on T1w images. Various fat suppression techniques are available to permit the evaluation of gadolinium-enhanced tissue within the retrobulbar fat.50 Fat suppression techniques with or without contrast enhancement are especially useful for the diagnosis of retrobulbar optic neuritis56 and intraorbital meningiomas.50,55 One limitation of fat suppression techniques is that they create magnetic field inhomogeneities, which cause alterations of signal intensities. Contrast enhancement is not necessary for depicting the cranial nerves inside the subarachnoid cisterns. They are also delineated on nonenhanced T2w images because of the excellent contrast between hypointense nerves and hyperintense CSF (Fig. 16–18, 20–22, 24–29). D. ADVANTAGES AND DISADVANTAGES OF MRI

The advantages of MRI are related not only to its ability to obtain a high degree of spatial resolution, but also to the ability to characterize tissues based on differences in their water content. Some experience in MRI techniques and familiarity with both data acquisition and contrast-to-noise relations are necessary to achieve this goal. There are pitfalls in the interpretation of fast-acquisition MR images, and certain caveats have to be heeded. However, this recognition becomes less problematic as the radiologist gains experience. Another major advantage of MRI is that it can perform multiplanar imaging without the need to reposition the patient. The selection of imaging planes along specific anatomic structures of interest enables visualization of long sections of these structures, which is important for depicting the relatively thin cranial nerves. Magnetic resonance imaging allows visualization of the intracanalicular optic nerve and details of the cavernous sinus and avoids the beam-hardening artifacts known from CT at the orbital apex.82 For these reasons, MRI is generally superior to CT in imaging soft tissue lesions of the cranio-orbital junction. However, in comparison with MRI, CT allows much better delineation of bones. Especially the complicated bony anatomy of the optic canal and superior orbital fissure is better visualized on CT scans31 than on MRI. Therefore, CT scans with bone window algorithms are the first-choice imaging modality in patients with orbital trauma,82 bone lesions, or craniofacial deformities.84 In soft tissue lesions, CT scans are useful for detecting secondary bony abnormalities (e.g., hyperostosis in meningiomas or bone erosion by malignancies) or calcifications within tumors (e.g., meningiomas or gliomas). When a sur-

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gical procedure is planned, we recommend ordering CT scans in addition to MRI in all soft tissue lesions of the cranio-orbital junction to detect bone involvement and show the topographic relationship of the lesion to bony structures. Computed tomography must also be ordered for uncooperative or claustrophobic patients and patients with contraindications to MRI (see section F). Although MRI and magnetic resonance angiography 32 may be helpful in diagnosing intracranial aneurysms or shunts at the cavernous sinus, the “gold standard” for intracranial vascular disease is catheter angiography and superselective vessel exploration. In this article, we review neither magnetic resonance angiography nor invasive carotid angiography. Instead, emphasis is placed on the discussion of vascular pathways as depicted by standard and widely accessible MRI equipment, without the use of contrast material. We point out that anatomic knowledge remains the basic denominator of our diagnostic capabilities in face of emerging advancedlevel spatial encoding techniques. E. IMAGING ARTIFACTS

It is important to recognize imaging artifacts in order to avoid misinterpretation. The following artifacts may present considerable problems in high-resolution MRI of the orbit. 1. Motion Artifacts Eye motion and eyelid blinking result in creation of “ghost images.” This can be minimized by having patients keep their lids open and their view fixed on a point inside the MRI gantry.3 Reflex blinking may be reduced by instillation of local anesthetic eye drops and artificial tears. However, if a longer acquisition time is needed, good results may also be obtained by having patients close their lids. Because of differences in voxel size and signal-to-noise ratio, surface coils are more sensitive to motion artifacts than volume coils.2 Therefore, high-resolution MRI of the orbit should be restricted to cooperative subjects who are able to lie still in the scanner for at least 2 to 3 minutes. Motion artifacts can occur with all pulse sequences but increase with scanning time depending on repetition time, number of excitations, and matrix size.50 2. Partial Volume Artifacts The signal intensity value measured is an average value for the signal intensities of the different tissue strucutures in the measured volume element (voxel). Different tissue structures inside one voxel contribute to the total signal intensity of that voxel. This partial volume effect can be reduced, for example, by using thinner slices and imaging matrices with

ANATOMY OF ORBITAL APEX ON MRI

higher resolution. When segments of the examined structure are partially out of the imaging slice, they appear hypointense or thinner (Figs. 18 and 21).77,78 Thus, partial volume averaging is a potential source of error during the identification of anatomic structures in MRIs. In order to circumvent this problem, it is always necessary to analyze the whole series of adjacent imaging slices (e.g., stacked on a workstation) and corresponding perpendicular orientations. 3. Chemical Shift Artifact This artifact occurs because of a difference in the resonant frequency of protons within tissues containing water and fat. In the orbit, a black and white band may be seen at the interface of orbital fat and adjacent tissues (Fig. 31). In T1w images, the chemical shift artifact may cause hypointense borders of the optic nerve, which may be confused with the T1hypointense subarachnoid space of the nerve.15 With alteration of signal encoding directions, use of the smallest possible pixel size, keeping the echo time in phase, and application of suppression techniques, this artifact can be reduced or eliminated.50 Chemical shift artifacts in T2w MRIs can especially be minimized by use of TSE sequences instead of conventional SE sequences. 4. Metal Artifacts Ferromagnetic materials, such as steel wire, mascara, eyelining tattoos, and palpebral springs (for facial nerve palsy), cause artifacts in all imaging sequences, whereas titanium orbital implants, miniplates, and gold eyelid weights (for facial nerve palsy) are seen as signal voids.50

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It is also possible to delineate the intraorbital and intracranial course of sensory and motor cranial nerves of the orbit on MRI.18,29 In the present study, nerves were traced from the brain stem via their passage through the cavernous sinus to the orbital apex. Most of the cranial nerves within the cavernous sinus and orbital apex that were delineated in this article have been visualized in previous studies,7,15–18,53 although earlier MRI technology has not provided the resolution necessary for discriminating individual nerves (except for the third nerve and maxillary nerve). With present MRI technology, it is feasible to visualize individual nerves within the cavernous sinus and the orbit. Only the thin trochlear nerve is difficult to visualize. On appropriate sections, it may be depicted as it courses around the midbrain (Fig. 20), inside the dural border of the cavernous sinus (Fig. 4–6), and within the orbit (Fig. 12).29 The imaging appearance of the optic nerve is influenced by its sinuous course,69,76–78 as well as the plane and thickness of sectioning. Thin axial slices at the level of the optic canal show the intracanalicular portion of the optic nerve (Fig. 16), but not the intraorbital part and vice versa (Fig. 18). However, thicker (about 3 mm), either oblique-axial slices along the neuro-ophthalmic plane82 (Fig. 15) or oblique sagittal sections (parallel to the course of the nerve) can depict the entire optic nerve.7,26,29,53 If the entire optic nerve is to be visualized in one single image, it may be helpful to obtain scans in upgaze to stretch the nerve, as first proposed for CT scanning.77,78

F. CONTRAINDICATIONS TO MRI

Ferromagnetic particles can move in a strong magnetic field and electrical devices may suffer dysfunction because of the potential induction of currents or heat. Therefore, MRI is contraindicated in patients with cardiac pacemakers, ferromagnetic implants, especially those located near vital structures (e.g., aneurysm clips), and ferromagnetic foreign bodies, especially if they are located close to important structures (e.g., intraorbital or intraocular foreign bodies).50

IV. Anatomic Features Demonstrated by MRI All major anatomic structures, including the origin of the extraocular muscles27 and the apical orbital connective tissue system, can be demonstrated by MRI. In the selected pulse sequences, blood vessels usually appear dark (“signal void”), as discussed earlier.52 All important arterial and venous vessels of the orbit can be identified without contrast enhancement.26

Fig. 31. T1-weighted axial MRI demonstrating chemical shift artifacts. Black bands are noted at the interface of orbital fat and adjacent tissue. The arrow indicates the chemical shift artifact between lateral rectus muscle and orbital fat.

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The cross-sectional diameter of the optic nerve can be measured in fat-supressed, T2w MRIs in an oblique-coronal plane, perpendicular to its course. The mean (⫾ SD) pial diameter of the intraorbital portion of a normal optic nerve ranges between 3.2 ⫾ 0.4 mm anteriorly and 2.6 ⫾ 0.4 mm posteriorly, whereas the mean (⫾ SD) dural diameter measures between 5.2 ⫾ 0.9 mm (anteriorly) and 3.9 ⫾ 0.4 mm (posteriorly).51

V. Clinical Implications High-resolution MRI enables exact delineation of space occupying orbital processes in relation to surrounding anatomical structures, thus facilitating planning of surgical procedures. This feature will be essential for computer-assisted surgery using neuronavigation.59 MRI reveals information on blood flow and may differentiate between flowing and stagnant blood in orbital vascular lesions. This is extremely important for treatment planning. Magnetic resonance angiography may provide further noninvasive diagnostic insights, depicting the entire course of vessels, including the circle of Willis. The closeness of vessels to oculomotor nerves and the chiasm explains the occurrence of eye muscle palsies and, occasionally, visual field defects caused by aneurysms. For instance, the abducens nerve courses in close contact to the internal carotid artery through the cavernous sinus (Figs. 4–6). For this reason, the sixth nerve is usually the first nerve affected by intracavernous carotid aneurysms or arteriovenous shunts. It must be noted that small aneurysms may only be detected by means of catheter selective angiography techniques. Increased pressure within the cavernous sinus, as in high-flow carotid-cavernous or low-flow dural-cavernous fistulae, results in enlargement of the ophthalmic veins. Low-flow fistulae may be supplied by small branches of the ICA to the oculomotor nerves (see above) or dural arterioles, neither of which is visible on MRI.24 Enlarged ophthalmic veins may also be encountered in other disorders, such as arteriovenous malformation, cavernous sinus thrombosis, and Graves’ ophthalmopathy.39 Septic cavernous sinus thrombosis is a rare but life-threatening disease. It always occurs bilaterally, because the intercavernous sinuses contain no valves. Enlargement of superior or inferior ophthalmic veins is best recognized on axial T1w scans. Care should be taken not to misinterpret enlarged veins at the orbital apex as a space-occupying lesion. Tumors of the oculomotor cranial nerves, causing progressive eye muscle palsies, usually cannot be reliably differentiated from other tumors because of their nonspecific imaging characteristics. High-resolution MRI might help to demonstrate an anatomic

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relation between tumor and nerve, thus suggesting the diagnosis of a neurogenic tumor.70 Similarly, a space-occupying lesion with extension to the orbital venous system (and enlargement during Valsalva maneuver) is suggestive of an orbital varix. Using contrast-enhanced MRI and fat supression, it is possible to localize inflammatory lesions of cranial nerves along their course.61 It has been demonstrated that high-resolution MRI may disclose compressive nerve lesions in patients with eye muscle palsies in whom routine brain MRI studies were unremarkable. In recent studies, spoiled gradient recalled steady-state acquisitions revealed vascular or neoplastic compression of the subarachnoid portions of the third and sixth nerves in patients with corresponding palsies.34,62,70 The intracranial portion of the abducens nerve passes through Dorello’s canal underneath the petroclinoideal ligament (Figs. 26 and 28). Here, the sixth nerve is predisposed to injury from compressive lesions or head trauma. In some individuals, the petroclinoidal ligaments may be ossified, causing compression of the sixth cranial nerve.66 For these reasons, it is advisable to individually check this structure when interpreting MRIs of patients with sixth nerve palsies. The trigeminal ganglion is located close to the posterolateral wall of the cavernous sinus (Figs. 4 and 11). This topographic relation explains the potential involvement of the entire fifth nerve (including the mandibular nerve, clinically apparent as loss of masticatory function) in lesions of the posterior cavernous sinus. A new neuro-ophthalmologic application of highresolution MRI is to demonstrate anatomic abnormalities in congenital motility disorders. Recently, absence of the abducens nerve in Duane’s syndrome, previously described only in postmortem studies, has been verified in vivo.65 The dural optic nerve diameter can be measured on oblique-coronal MRIs. An increased dural diameter with synchronous flattening of the posterior sclera, excessive tortuosity of the optic nerve, intravitreous protrusion, enhancement of the papillae, and an empty sella are suggestive of pseudotumor cerebri.11 The subarachnoid space of the optic nerve is 0.5 to 1 mm wide and contains CSF that is freely exchanged with the subarachnoid space of the brain. Clinically, this communication provides a route for spread of infection, neoplastic cells, or hemorrhage. The subarachnoid perioptic space may be enlarged in optic nerve glioma, in which it is filled with watercontaining T2-hyperintense gliomatous tissue.10 Echographic A-scan measurements of the optic nerve (“30⬚ test”) imply a redistribution of CSF after abduction and have been claimed to enable a differ-

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entiation between optic nerve thickening caused by fluid distension and optic nerve tumors.63 However, MRI studies could not verify a significant displacement of CSF after gaze changes.51 Further comparative studies on the validity of MRI and echography for the differential diagnosis of optic nerve thickening are warranted. The optic chiasm is separated from the pituitary gland by the diaphragm sellae and the up to 10-mmwide suprasellar cistern. Pituitary adenomas must, therefore, be quite large to produce visual field defects. The chiasm is normally situated on top of the diaphragm sellae. In some individuals, it may extend onto the dorsum sellae (postfixed chiasm) or close to the planum sphenoidale (prefixed chiasm), which accounts for variations in visual field defects associated with tumors in this region (e.g., pituitary adenoma).1 The superior orbital fissure represents a communication channel that transmits important neurovascular structures between the intracranial space and the orbit. This explains the spread of inflammatory or neoplastic lesions between the two compartments. Small lesions at the superior orbital fissure and retroorbital region (e.g., Tolosa-Hunt-syndrome79) may easily be overlooked. Therefore, one should always pay careful attention to this region when interpreting cranio-orbital MRIs. These clinical examples show that MRI may contribute to a specific diagnosis in space-occupying lesions at the cranio-orbital junction. The present article has provided the basic morphologic knowledge that is essential for a successful application of this noninvasive diagnostic technique.

Method of Literature Search An on-line search of the international literature was conducted by MEDLINE, covering all years from 1966 to 1999, and using the following search words: magnetic resonance imaging (MRI), cavernous sinus, orbital apex. Other sources included original and review articles from our own files, articles cited in the reference lists of other articles, and standard textbooks on orbital imaging and anatomy. An important inclusion criterion was the coverage of anatomic and radiologic rather than purely clinical questions. Clinical articles referring to specific anatomic and radiologic topics were also included. The four non-English articles were in German and read in the original language.

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Outline I. Examination protocols II. Imaging A. Bony anatomy B. Extraocular muscle and connective tissue system anatomy C. Cranial nerve anatomy 1. Optic nerve and chiasm 2. Motor nerves a. Oculomotor nerve b. Trochlear nerve c. Abducens nerve 3. Sensory nerves a. Ophthalmic nerve (V.1) b. Maxillary nerve (V.2) D. Vascular anatomy 1. Arterial system

323 a. Internal carotid artery b. Ophthalmic artery 2. Venous system a. Veins b. Parasellar venous plexus (cavernous sinus) III. Discussion of techniques A. Sequences and coils B. Signal void of flowing blood C. Contrast enhancement D. Advantages and disadvantages of MRI E. Imaging artifacts IV. Anatomic features demonstrated by MRI V. Clinical implications

We thank our radiologic technicians for collecting the imaging data, Erika Just for her secretarial assistance, and Peter Mentil and Anton Jaeger for preparing the figures. The authors have no proprietary or commercial interest in any products or concepts discussed in this article. This study was supported by the Ludwig Boltzmann-Institute for Interventional Magnetic Resonance, St. Pölten, Austria. Reprint address: Armin Ettl, MD, Abteilung für Neuro-Ophthalmologie, Okuloplastische-u. Orbitachirurgie, Allgemeines Krankenhaus, Postfach 176, A-3100 St. Pölten, Austria (fax: 01143-2742-300-3285; e-mail: [email protected]).