Practical Anatomy of the Central Skull Base Region Philip R. Chapman, MD*, Asim K. Bag, MD*, R. Shane Tubbs, PhD†, and Paul Gohlke, MD, PhD* The central skull base region represents a complex intersection between the intracranial compartment, the osseous foundation of the skull base, the orbits, the paranasal sinuses, and the suprahyoid neck. A modern radiologic approach to this region should take into account the 3-dimensional complexity of the region as well as the cross-sectional anatomical detail available to today's radiologist. This analytical approach should permit identification of lesional anatomical subsites, establishment of lesional origins, and allow for an anatomy-based differential diagnosis. In this article, we define a practical central skull base region that includes structures that directly affect neuroimaging of this dense landscape. By reframing the boundaries, the central skull base region becomes comprehensive, emphasizing the natural tendency for pathologic processes to involve contiguous anatomical subunits, and underscores the complexity and challenges of this region for neuroimaging specialists. Semin Ultrasound CT MRI 34:381-392 C 2013 Elsevier Inc. All rights reserved.

Overview

T

he skull base has been traditionally divided into anterior, central, and posterior components based principally upon the intracranial appearance of the skull base as viewed from above. This approach is widely accepted and supports the general compartmentalization of the intracranial space into the anterior, middle, and posterior fossae. Previous descriptions of the central skull base include relatively broad lateral boundaries, extending from the sella turcica centrally to the squamous temporal bones laterally. By tradition, the central skull base has been separated from the anterior skull base by an arbitrary line drawn along the anterior margin of the sella and extending along the posterior margins of the anterior clinoid processes and the lesser wings of the sphenoid bones. From a practical radiologic perspective, such approaches ignore the 3-dimensional complexity of the central skull base and

Project Editor: Suzanne Byan-Parker. Tel.: þ1-205-934-4274; þ1-205-4823229 (mobile). E-mail: [email protected] The author thank medical illustrator David Fisher for providing the conceptual, anatomic illustrations. *Department of Radiology, Section of Neuroradiology, University of Alabama at Birmingham, Birmingham, AL. †Department of Neurosurgery, Section of Pediatric Neurosurgery, University of Alabama at Birmingham, Birmingham, AL. Address reprint requests to Philip R. Chapman, MD, Jefferson Towers N424, 619 19th St South, Birmingham, AL 35249-6830. E-mail: [email protected] E-mail: [email protected]

0887-2171/$-see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.sult.2013.08.001

are unsatisfactory, given the advances in cross-sectional imaging as well as focused neurosurgical and radiation treatment options.1 The central skull base region represents a complex intersection between the intracranial compartment, paranasal sinuses, and suprahyoid neck. There are numerous unique tissue types in this region capable of giving rise to a variety of pathologies. A current or modern approach should take into account the level of anatomical detail available to today's radiologist with access to multiplanar, high-resolution computed tomography (CT) and magnetic resonance imaging (MRI).2 Such an analysis should allow the radiologist to identify specific anatomical subsites of a particular lesion, establish a potential origin for the lesion, and allow for an anatomy-based differential diagnosis. From a practical standpoint, the central skull base represents a 3-dimensional region roughly spherical in shape (Fig. 1). The superior pole is positioned at the optic chiasm and the inferior pole is positioned at the level of the nasopharynx. The equator of the sphere encompasses the pterygopalatine fossa (PPF) anteriorly, the foramen ovale laterally, and the prepontine cistern posteriorly. These boundaries include all pertinent structures that can give rise to or be affected by lesions involving the central skull base region that are seen in daily radiologic practice. With these proposed boundaries, the central skull base region is more inclusive, emphasizes the natural tendency for pathologic processes to involve contiguous anatomical subunits, and underscores the true 381

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Figure 1 Three-dimensional illustrations of the superior, anterior, and lateral views of the sphenoid bone, the osseous foundation of the central skull base. The outlined spherical region denotes the central skull base region and includes the adjacent endocranial structures including the pituitary gland and the exocranial structures of the neck, including the nasopharynx.

complexity and challenge of this region for the neuroimaging specialist. Therefore, the central skull base region practically includes the orbital apex, optic nerve and canal, optic chiasm, superior orbital fissure (SOF), PPF, pituitary gland and stalk, the cavernous sinus, significant portions of the internal carotid artery (ICA), numerous cranial nerves (CN), the local pachymeninges, multiple important skull base foramina, the sphenoid sinus, the clivus (basisphenoid and basiocciput), the foramen lacerum, the nasopharynx, the petro-occipital fissure, and the petrous apex. Reframing the central skull base boundaries de-emphasizes the lateral components of traditional central skull base (lesions lateral to the foramen ovale) and middle cranial fossa. This approach seems more reasonable, as pathologies affecting the more lateral structures are typically more straightforward in daily radiologic practice.

clinoid processes are situated superolateral to the dorsum sella. The dorsum sella is contiguous posteriorly with the clivus. The sella turcica is covered superficially by a layer of dura. The pituitary gland, composed of anterior and posterior lobes, resides within the sella, intimately associated with the sella floor. The diaphragma sellae represents a thin dural sheet that stretches across the top of the sella and is perforated centrally by the pituitary stalk.3 Typically, the diaphragma sellae itself is mildly convex inferiorly. There is

Endocranial Relationships Sella Turcica The sella turcica, a saddle-shaped depression within the sphenoid bone, represents the most conspicuous landmark of the central skull base in the superior view (Fig. 2).3 The sella itself is spherical in shape and contains the pituitary gland. The sella is bound anteriorly by the tuberculum sella and posteriorly by the dorsum sella. Just anterior to the tuberculum sella is a shallow shelf, the chiasmatic sulcus. Above and anterior to the chiasmatic sulcus is the planum sphenoidale. The sella is surrounded by 4 bony projections, the anterior and posterior clinoid processes. The anterior clinoid processes are located along the medial margins of the lesser wings. The posterior

Figure 2 Sagittal CT reformation through the central skull base demonstrates the bony anatomy of the sella turcica (ST). The anterior wall of the sella is very thin and separates the sella from the sphenoid sinus (Sph). The clivus is formed by the basisphenoid (BS) superiorly and the basiocciput (BO) inferiorly.

Practical anatomy of the central skull base region significant variability in the appearance of the diaphragma sellae as well as the size of the central opening.4

Cavernous Sinus The cavernous sinuses are complex venous structures of the central skull base with multiple venous tributaries including intercavernous sinuses, basilar plexus, and superior and inferior petrosal sinuses (Fig. 3).5 There are 5 sides of the cavernous sinus.6 The medial wall of the cavernous sinus consists of an upper sellar component and a lower sphenoid component. The upper sellar component of the medial wall is a thin dural membrane, typically a single cell layer in thickness. This separates the venous compartment from the lateral margin of the pituitary gland. More inferiorly, the medial wall of the cavernous sinus is thicker and adherent to the carotid sulcus. The lateral wall of the undissected cavernous sinus is seen from the side as a sail-shaped dural sheet that extends from the SOF and the anterior clinoid process anteriorly to the petrous apex posteriorly.7 The lateral wall faces the medial temporal lobe and consists of a thick dural membrane that is typically easily dissected into 2 distinct layers: a thin outer (meningeal) layer and a thick inner (dural) layer. This bilayered membrane is important because it envelops CN III, IV, and the ophthalmic segment of cranial nerve V. The lateral and medial walls of the cavernous sinus merge inferiorly along the lateral margin of the sphenoid body, just above the maxillary division of CN V. Although the second and third divisions of the trigeminal nerve are invested by the contiguous dura, they are not considered part of the cavernous sinus.7,8 The posterior wall of the cavernous sinus extends from the lateral margin of the dorsum sella to the medial aspect of the trigeminal impression and superior medial aspect of Meckel cave. The posterior wall is limited inferiorly by the junction of the petrous apex and body of the sphenoid bone at the superior

Figure 3 Posterior view through the cavernous sinuses. The central location of the cavernous internal carotid artery is noted. Cranial nerve VI, the abducens, is the only nerve that is truly intracavernous. From superior to inferior, the oculomotor, trochlear, opthalmic, and maxillary nerves are seen along the lateral margin of the cavernous sinus. The lateral wall is divided into 2 layers, an outer meningeal layer, and an inner dural layer. The inner dural layer envelops the oculomotor, trochlear, and ophthalmic nerves.

383 medial aspect of the petroclival fissure. Here the cavernous sinus is contiguous with inferior petrosal sinus. CN VI enters the posterior wall of the cavernous sinus, beneath the petrosphenoid ligament, through the Dorello canal, and travels within the cavernous sinus, lateral to the cavernous ICA (Fig. 4).5 The sixth CN is the only CN that is truly intercavernous.5 The anterior wall of the cavernous sinus is essentially rectangular and extends from the optic strut beneath the anterior clinoid process laterally to include the SOF. The inferior margin is considered the superior end of the foramen rotundum. The roof of the cavernous sinus extends from the optic strut and SOF anteriorly, to the petrous apex and tentorial edge posteriorly. The medial margin of the cavernous sinus roof is contiguous with the diaphragma sellae. The lateral margin of the roof is separated from the lateral wall of the cavernous sinus by a cord-like thickening of the dura, the anterior petroclinoid fold, which extends from the tentorial edge to the anterior clinoid process. A separate fold extends from the tentorial edge to the posterior clinoid process, the posterior petroclinoid fold. Finally, there is a thin band of dura that extends from the anterior clinoid process to the posterior clinoid process, the interclinoid fold. These 3 folds form a triangle, the oculomotor

Figure 4 The right CN VI, abducens nerve, enters the posterior wall of the cavernous sinus, beneath the petrosphenoid ligament, through the Dorello canal, and travels within the cavernous sinus, lateral to the cavernous internal carotid artery.

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Meckel Cave

Figure 5 Cranial nerve III, the oculomotor nerve, pierces the roof of the cavernous sinus on the left. Marginal thickening of the dura forms 3 distinct folds, the anterior petroclinoid fold (APcF), the posterior petroclinoid fold (PPcF), and the interclinoid (IcF) fold, that form the oculomotor triangle.

triangle (Fig. 5), that represents a principal anatomical landmark of the cavernous sinus roof. The oculomotor nerve pierces the oculomotor triangle from posterosuperior approach, travels within a short oculomotor cistern, and enters the lateral wall of the cavernous sinus near the anterior clinoid process. The trochlear nerve enters the posterolateral aspect of the oculomotor triangle, just posterior to the oculomotor nerve.9

The Meckel cave is a localized dural outpouching that extends from the posterior fossa over the petrosphenoid junction into the medial and posterior aspect of the middle cranial fossa. The Meckel cave contains the trigeminal nerve including the trigeminal ganglion.10-12 Just lateral to the petrosphenoid junction there is a small depression in the superior aspect of the petrous apex, the trigeminal impression, above which the trigeminal nerve is located (Fig. 6). The superior, anterior, and medial portion of the Meckel cave is immediately adjacent to the posterior and lateral aspect of the cavernous sinus. The medial and inferior aspect of the Meckel cave is just lateral to the ICA as it arises from the medial opening of the carotid canal and begins to turn vertically and anteriorly into the cavernous sinus. The trigeminal ganglion is positioned within the anterior and inferior aspect of the Meckel cave and divides into 3 major trunks of the trigeminal nerve: the ophthalmic division (V1), maxillary division (V2), and mandibular division (V3).13 The ophthalmic division extends medially and anteriorly, entering the lateral wall of the cavernous sinus and traveling to the SOF. The maxillary division of the trigeminal nerve extends anteriorly, along the inferior margin of the cavernous sinus and exits the foramen rotundum. The mandibular division extends inferiorly and laterally through the foramen ovale within the greater wing of the sphenoid.13

Intrinsic Anatomy of the Central Skull Base Sphenoid Bone and Sphenoid Sinus The sphenoid bone is the principal skeletal underpinning of the central skull base and is architecturally complex.14 This centrally located body contains the sella turcica above and sphenoid sinus below. There are lateral extensions, the lesser and greater

Figure 6 Petroclival meningioma with obliteration of the Meckel cave. A 61-year-old man presents with progressive rightsided sixth nerve palsy. (A) Coronal, T2-weighted image demonstrates normal Meckel cave on the left (small arrow), containing normal CSF fluid. Components of the trigeminal nerve are easily identified. On the right, there is a large mass (large arrow), a petroclival meningioma completely infiltrating and obscuring the Meckel cave. Notice the relationship of the normal Meckel cave on the left and the mass on the right to the internal carotid artery as it leaves the petrous canal (arrowheads). CSF, cerebrospinal fluid.

Practical anatomy of the central skull base region sphenoid wings, and inferior projections, the pterygoid processes, that give rise to the medial and lateral pterygoid plates. The body of the sphenoid occupies the upper portion of the clivus and is joined to the basilar occipital bone to form the complete clivus. The clivus contains significant trabecular bone and thus possesses one of the most conspicuous localized areas of vascularized marrow space in the skull.15 The clivus is susceptible to the development of pathologic processes ranging from fibrous dysplasia to metastatic disease and myeloma.16 The clivus may harbor notochordal remnants near the spheno-occipital synchondrosis that can give rise to chordomas.17 The sphenoid sinus, for practical purposes, is considered intrinsic to the sphenoid bone. The sphenoid sinus is a mucosal-lined, variably pneumatized posterior extension of the paranasal sinuses. The sphenoid sinus can be described in terms of the degree of pneumatization and proximity of the aerated cavity to the sellar floor.3 In conchal pneumatization, the region below the sella is completely ossified. In presellar pneumatization, the sphenoid cavity does not extend posteriorly beyond a coronal plane aligned with the anterior sellar wall. Sellar pneumatization is the most common type in the adult, occurring approximately 75% of the time. In this type, the cavity at the sphenoid sinus extends posteriorly, below the sella, often to the posterior clival margin. In this configuration, the anterior sellar wall and sellar floor typically measures o1 mm in thickness. In addition to the variability of pneumatization, there is significant variation in terms of the presence and location of septae within the sphenoid sinus.3,18 The sphenoid sinus is intimately related to the cavernous carotid arteries as they course along the lateral margins of the sphenoid bone as well as the cavernous sinuses themselves.3 The sphenoid sinus may communicate with the optic strut and anterior clinoid process, bringing components of the sphenoid sinus in close proximity to the optic nerves and adjacent to the superior and inferior orbital fissures.19-22 Therefore, pathologic processes of the sphenoid sinus can affect cavernous sinuses, the cavernous ICAs, the optic nerves, and orbital apex.

385 The superior and medial margin of the petrous apex is fused to the posterior lateral sphenoid body, below the posterior clinoid process. This relatively small bony junction forms the floor of the Dorello canal. The petrous apex junction represents the superior and medial aspect of the petro-occipital fissure, the variably ossified, obliquely oriented, cartilaginous junction between the inferior medial petrous bone and the inferolateral basisphenoid portion of the clivus.25,26 It is readily identified on most CT scans as an irregular lucency passing along the lateral margins of the clivus. The petro-occipital junction is an important structure because of its relationship to skull base chondrosarcomas, the majority of which appear to originate from chondroid tissue within the petro-occipital fissure.1,27 The petrous bone contains the obliquely oriented carotid canal, which contains the petrous (C2) segment of ICA as it passes medially into the central skull base region.28,29 The ICA emerges from the endocranial carotid canal through an irregular opening, just above a horizontal layer of cartilage that bridges the foramen lacerum. The bony foramen lacerum is a gap separating the exocranial surface of the petrous apex from the basisphenoid bone medially and the basioccipital bone posteriorly. At this point, the ICA is referred to the lacerum segment (C3), before entering the cavernous sinus.25 The perolingual ligament (slightly inferior to the petrosphenoid ligament) is a fibrous band that extends from the lingula of the carotid sulcus along the sphenoid body to the petrous apex and is anterolateral to the C3 ICA at this point.30,31 The Eustachian tube is a narrow tube connecting the middle ear to the nasopharynx (Fig. 7). The proximal or lateral bony component begins along the anterior wall of the

Petrous Apex and Petroclival Junction The petrous apex is the pyramidal-shaped, medial projection of the petrous portion of the temporal bone. Because of its medial location and intimate relationship to the petro-occipital fissure, clivus, cavernous sinus and Meckel cave, the petrous apex is considered part of the central skull base region. In general, the petrous apex is composed of dense bone and bone marrow. Pneumatization of the petrous apex occurs when epithelial-lined air cells develop as medial communications from the mastoid air cells. This occurs in 9%-30% of patients and in general, there is a positive correlation between the degree of mastoid segment pneumatization and aeration of the petrous apex.23,24 Pneumatization can be highly variable, involving a large portion of the petrous temporal bone, or only a small posterolateral segment.23 The air cells of the petrous apex are susceptible to similar pathologic processes that occur in the mastoid segment, including obstruction, opacification, inflammation, and infection.

Figure 7 Axial, CT image through the skull base demonstrates the relationships between various foramina, fissures, and canals along the lateral margin of the central skull base region. The internal carotid artery passes obliquely through the carotid canal (CC) and exits the canal just above cartilage-filled gap, the foramen lacerum (FL). The Eustachian tube (ET) consists of bony and cartilaginous segments and passes both medially and inferiorly from the middle ear to the nasopharynx. Notice the intimate relationship between the Eustachian tube, the carotid canal, and the foramen spinosum (FS).

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386 tympanic cavity and passes anteromedially and inferiorly toward the nasopharynx. This bony canal passes just inferolateral to the proximal petrous carotid canal. The bony Eustachian tube emerges inferiorly into the gap between the posterior margin of the greater wing of the sphenoid and the anterior margin of the petrous bone (sphenopetrosal groove).25,32 At this point, it becomes a cartilaginous structure and can be seen as soft tissue density, just posterior to the foramen ovale and foramen spinosum on axial CT scans. The cartilaginous tube then continues into the nasopharynx.

Foramina, Fissures, and Spaces Several canals, foramina, fissures, and spaces are found within or between the bony structures of the central skull base (Figs. 8 and 9). Many of these structures contain important neurovascular structures, particularly CN (Table). These openings are important pathways of communication between the central skull base and extracranial compartments including the orbit, PPF, and suprahyoid neck.2,14 Therefore, they provide important routes of spread for invasive lesions, including aggressive infections or neoplasms. Depending on the pathology and mode of infectious or metastatic spread, such foramina may be relatively undisturbed as a lesion passes through it, as in perineural tumor spread. The foramen or canal may be smoothly scalloped, well corticated, and widened by a slow-growing lesion such as a schwannoma, or the foramen may demonstrate permeative erosion and destruction as with a squamous cell carcinoma.

Extracranial Relationships PPF The PPF is an important anatomical and radiologic landmark because of its unique location and intimate relationships with the central skull base, orbit, and sinonasal cavity (Fig. 10). Aggressive neoplasms and infections can enter this region and extend to the central skull base, either by direct bony or soft tissue invasion, or by perineural tumor spread. Intracranial or intrinsic skull base lesions including meningiomas can also extend into this region from above. The PPF contains a generous amount of fat, the vidian nerve as it arises from the vidian canal, the maxillary nerve as it arises from the foramen rotundum, branches of the maxillary artery, and the pterygopalatine ganglion (PPG).25 In general, the PPG is considered to be a single 2-3-mm ovoid structure of the parasympathetic nervous system associated with the maxillary nerve and residing within the medial aspect of the PPF. However, microscopic dissections have shown variations in the PPG and its associated neural scaffolding, often showing a bipartite structure with additional, more diffuse microscopic clusters of ganglion cells.33 In addition, the ganglion contains contributions from sympathetic nerve fibers as well.34 Even with high-resolution imaging, it is difficult to define the PPG within the PPF and separate it from adjacent vessels and larger nerve branches.

The PPF is best identified by means of axial CT or MRI scans, with demonstration of a roughly rectangular region of fat sandwiched between the dorsal wall of the maxillary sinus anteriorly and the base of the pterygoid process posteriorly. A small but complex bone, the palatine bone, intervenes between the maxilla and sphenoid bone and contributes to the medial boundaries of the PPF.35,36 Inferiorly, the PPF tapers into a common bony palatine canal that carries the palatine nerves. This canal ultimately divides into separate greater and lesser palatine canals, transmitting the nerves to the posterior hard palate. Inferiorly and anteriorly, the PPF is contiguous with the inferior orbital fissure. The maxillary nerve continues anteriorly from the PPF through the inferior orbital fissure, becoming the inferior orbital nerve. Laterally, the PPF communicates with the infratemporal fossa through the pterygomaxillary fissure. Medially, the PPF is contiguous with the submucosa of the posterior nasal cavity through a gap or notch in the perpendicular plate of the palatine bone. This region is referred to as the sphenopalatine foramen. Superiorly, the PPF is contiguous with the SOF.35,36 The PPF represents an intersection of pathways that allow infection or neoplasm to spread from various extracranial locations including the orbital apex, sinonasal cavity, the masticator space, and even the oral cavity to gain entrance to the central skull base.

The Orbital Apex The orbital apex is intimately related to the central skull base by the lesser and greater wings of the sphenoid bone, which contribute to the superior and lateral walls of the apex (Fig. 11).37,38 The orbital apex provides a route of communication between the orbit and central skull base or cavernous sinus region through the optic canal and SOF. Orbital processes can also approach the PPF through the inferior orbital fissure. Lesions in or of the orbital apex can lead to orbital apex syndrome with vision loss and ophthalmoplegia related to multiple cranial neuropathies. Apical lesions extending to the skull base include optic nerve meningiomas, orbital pseudotumor (Tolosa-Hunt syndrome), or invasive infections. Thrombophlebitis of the superior ophthalmic vein can extend posteriorly to the cavernous sinus, resulting in cavernous sinus thrombosis.39 The optic canal is essentially a channel through the lesser wing of the sphenoid bone. The medial wall is bounded by the anterolateral body of the sphenoid body itself. The roof of the optic canal is formed by a thin bony bridge called the superior root of the lesser wing. The lateral wall is formed by the optic strut (or inferior root of the lesser wing) inferolaterally and the anterior clinoid process laterally.37,38,40 The optic strut is a useful landmark that separates the optic canal from the medial margin of the SOF.37,38 This oblique gap is formed between the greater and lesser wings of the sphenoid bone. The medial SOF is wider and lies directly anterior to the cavernous sinus. The inferior orbital fissure is an oblique gap located between the lateral wall and floor of the orbit. It communicates with the infratemporal fossa inferolaterally and the PPF posteriorly.37,38

Practical anatomy of the central skull base region

Figure 8 Axial, CT anatomy. Axial, unenhanced CT images through the skull base, superior to inferior. (A) The posterolateral margin of the sella is marked by the posterior clinoid process (PC). The anterior clinoid (AC) is a bony projection along the anterolateral sella that is contiguous with the lesser wing of the sphenoid bone (LWS). Medial to the anterior clinoid process is the optic nerve canal (ONC) that transmits the optic nerve and ophthalmic artery. The planum sphenoidale (PS) is a horizontal segment of bone just anterior to the sella that forms part of the roof of the sphenoid sinus. (B) As the axial images move inferiorly, the superior orbital fissure (SOF) becomes more conspicuous. The optic nerve canal (ONC) is intimately related to the sphenoid sinus (SS) medially and bony strut of the anterior clinoid process (AC) laterally. (C) The posterior vertical margin of the sella is the dorsum sella (DS). At this level, there is contiguous soft tissue density between the middle cranial fossa (specifically, the cavernous sinus) and the orbital apex through the superior orbital fissure. The SOF transmits the ophthalmic vein, V1 segment of the trigeminal nerve, and cranial nerves III, IV, and VI. Please note greater wing of sphenoid (GWS). (D) In this example, there is a single sagittal septum in the sphenoid sinus. The natural sphenoid ostia are identified on either side of the septum, allowing mucus to drain into the posterior nasal passage. The superior orbital fissure (SOF) is noted (arrow). (E) At this level, the petrous apex (PA) can be seen as a pyramidal-shaped, medial extension of the temporal bone. Along the superior and medial margin of the petrous apex is a shallow concavity, the trigeminal impression (TI). The medial opening of the carotid canal (CC) is visualized, separated from the sphenoid sinus by thin cortical bone. The foramen rotundum (FR) opens into the upper recess of the pterygopalatine fossa (PPF). (F) The foramen lacerum (FL) is a triangular-shaped, horizontal layer of cartilage between the clivus and petrous apex. The Eustachian tube is seen just lateral to the carotid canal, extending lateral to medial and superior to inferior. The foramen ovale (FO) and foramen spinosum (FS) are seen in the lateral sphenoid bone. The vidian canal (VC) contains the vidian nerve and travels from a point near the foramen lacerum forward to the pterygopalatine fossa (PPF). The PPF connects with the masticator space laterally through the pterygomaxillary fissure (PMF, large oval) and medially through the sphenopalatine foramen (SPF, small oval) with the nasal cavity. The infraorbital nerve passes from the PPF into the inferior orbital fissure (IOF) on its way to the cheek. (G) At this level, soft tissues that form the roof of the nasopharynx begin to show up ventral to the clivus. Notice that these soft tissues are directly contiguous with the region of the Eustachian tube and foramen lacerum.

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Figure 9 Coronal CT anatomy, anterior to posterior. Coronal, unenhanced CT images through the central skull base. (A) Coronal image through orbital apices at the level of the pterygoid process (PP) demonstrates the relationship between the pterygopalatine fossa and the orbital apex. (B) The pterygopalatine fossa (PPF) contains fat, the distal branches of the internal maxillary artery, veins, the pterygopalatine ganglion and its connections. The PPF is contiguous with the inferior orbital fissure (IOF) and ultimately, the orbital apex. (C) Near the apex, the orbital roof is formed by the lesser wing of the sphenoid bone (LWS). The medial pterygoid plate (MPP) and lateral pterygoid plate (LPP) project posteriorly from the pterygoid process (PP). (D) Near the apex, there is an obliquely oriented superolateral fissure, the superior orbital fissure (SOF), that transmits the ophthalmic vein, V1 segment of the trigeminal nerve, and cranial nerves III, IV, and VI. (E) The foramen rotundum (FR) opens into the upper recess of the pterygopalatine fossa (PPF). In the coronal plane, the foramen rotundum is seen superolateral to the vidian canal (VC). (F) Coronal image through the sphenoid sinus demonstrates the relationship between the anterior clinoid process (AC) and the optic nerve canal (ONC). The roof of the sphenoid sinus is flat and referred to as the planum sphenoidale (PS). The optic strut (OS) is a thin bridge of bone that defines the lateral margin of the optic nerve canal. At this level, the foramen rotundum opens into the middle cranial fossa as V2 travels toward the lateral wall of the cavernous sinus. (G) Coronal image through sphenoid sinus demonstrates foramen ovale (FO) laterally, which transmits V3 into the masticator space and more poorly defined foramen lacerum (FL) medially. (H) Coronal image through the posterior aspect of the sella. The posterior clinoid processes (PC) can be seen as bilateral superolateral projections. The posterior wall of the sella is the dorsum sella (DS). The upper one-half of the clivus is formed from the sphenoid bone and referred to as the basisphenoid (BS).

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Table Foramina, Fissures, Canals, Spaces, and Relevant Contents and Relationships Skull Base Structure

Contents or Relationships or both

Optic nerve canal Superior orbital fissure

Optic nerve and ophthalmic artery CN III, oculomotor nerve; CN IV, trochlear nerve; CN V (V1), ophthalmic nerve; CN VI, abducens; and superior ophthalmic vein CN V2, infraorbital nerve; CN V2, zygomatic nerve; and inferior ophthalmic vein Pterygopalatine ganglion; CN V2, maxillary nerve; vidian nerve; distal branches of internal maxillary artery and veins; and greater and lesser palatine nerves Communication between lateral pterygopalatine fossa and infratemporal fossa Communication between medial pterygopalatine fossa and nasal cavity CN V (V2) and maxillary division Vidian artery and vidian nerve Internal carotid artery and sympathetic plexus Fibrocartilage and meningeal branches of ascending pharyngeal artery CN V (V3) and mandibular division Middle meningeal artery and recurrent meningeal branch of the mandibular nerve CN VI and abducens nerve

Inferior orbital fissure Pterygopalatine fossa Pterygomaxillary fissure Sphenopalatine foramen Foramen rotundum Vidian canal Carotid canal Foramen lacerum Foramen ovale Foramen spinosum Dorello canal

Critical neural and vascular structures converge upon the funnel-shaped orbital apex and share the space with extraocular muscles that arise from the margins of the apex.41 There is a tough fibrous ring encircling the orbital apex, the annulus of Zinn that serves to anchor most of the extraocular muscles (the 4 recti muscles and superior oblique muscles) and separates the intraconal space of the apex from the extraconal space. The annulus of Zinn encompasses the optic canal superomedially, allowing passage of the optic nerve and ophthalmic artery into the intraconal space. Laterally, the annulus crosses the medial portion of the SOF so that the medial components (nasociliary nerve, CN III, and CN VI) enter the intraconal space through the annulus and the lateral components (CN IV, superior ophthalmic vein, and frontal

Figure 10 Pterygopalatine fossa (PPF). The posterior aspect of the lateral nasal wall has been dissected, exposing the sphenopalatine foramen and pterygopalatine fossa. The pterygopalatine ganglion (PPG) has multiple neural connections. The maxillary nerve (V2) traverses the upper aspect of the PPF. The vidian nerve (VN) is seen passing from the ganglion into the vidian canal. The greater and lesser palatine nerves (Pn) descend into the palatine canal and into the submucosa of the palate.

and lacrimal nerves) enter the orbit in the posterior extraconal space.41,42

Suprahyoid Neck In addition to the orbit and PFF, the exocranial central skull base is related to the nasopharynx, the masticator space, and preclival or prevertebral space.

Nasopharynx The nasopharynx represents the uppermost aspect of the pharyngeal airway and extends from the nasal choanae above to the level of the soft palate below, where the pharynx becomes the oropharynx (Fig. 12). The superficial mucosa lining the nasopharyngeal airway is part of the pharyngeal mucosal space (PMS). The superficial mucosa of the PMS is critically important because squamous cell carcinoma and nasopharyngeal carcinoma can originate from this space.16,43 The PMS also includes the lymphoid tissue of the Waldeyer ring (adenoids, tonsils, etc.), minor salivary glands, and supporting soft tissues, including the middle layer of deep cervical fascia, the pharyngeal constrictor muscles, the cartilaginous end of the Eustachian tube, and the pharyngobasilar fascia.44 The inferior exocranial skull base (the floor of the sphenoid sinus, the clivus, and medial portions of the petrous apices) forms the bony roof and posterior wall of the nasopharynx. The midline pharyngeal raphe attaches to the pharyngeal tubercle of the occipital bone. The paired superior constrictor muscles also attach to the tubercle by a shared aponeurosis. Each superior constrictor muscle drapes forward, attaching to the lower portion of the medial pterygoid plate, and exposing a gap between the muscle and the skull base. This gap is largely filled by a thick fascial or aponeurotic band, the pharyngobasilar fascia, which extends from the upper margin of the muscle and attaches to the skull base. From anterior to posterior, the pharyngobasilar fascia attaches to the medial pterygoid plate, the body of the sphenoid bone, the petrous apex, and foramen lacerum, and finally the basiocciput.44

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middle layer from the deep layer of deep cervical fascia. The superior RPS extends to the basiocciput. The medial aspect of the nasopharyngeal RPS is not normally radiologically apparent, may contain minimal fat, and yet, this unobtrusive lair offers opportunistic invaders a safe haven for growth. The lateral RPS at this level may contain the nodes of Rouvière, the most cephalad of the lateral retropharyngeal lymph nodes.46

Prevertebral Space At the level of the nasopharynx, there are paired midline muscles identified extending from the anterior and lateral aspects of the C1 to the basiocciput: the longus capitis and medial rectus capitis muscles. The longus capitis muscles are more conspicuous and easily identified as midline muscles interposed between the posterior wall of the mucosa and the clivus. These muscles are bound anteriorly by the deep layer of deep cervical fascia and are technically within the perivertebral (or prevertebral) space.44 The muscles attach to the periosteum of the clivus. Figure 11 Orbital Apex. The superior, medial, inferior, and lateral rectus muscles converge posteriorly and attach to a dense fibrous ring, the annulus of Zinn. The ring circumscribes the optic canal and the inferomedial aspect of the superior orbital fissure. The optic canal contains the optic nerve and ophthalmic artery. Cranial nerve III, the oculomotor nerve, enters the medial superior orbital fissure and divides into superior and inferior divisions. The nasociliary and abducens (CN VI) nerves also enter through the medial compartment. Laterally, the superior orbital fissure contains the trochlear nerve (CN IV), the lacrimal nerve, the frontal nerve, and the superior ophthalmic vein (SOV).

Masticator Space The masticator space contains the muscles of mastication along with the mandibular nerve and branches of the internal maxillary artery.46 The superomedial margin of the masticator space is directly associated with the exocranial surface of the greater wing of the sphenoid bone, the foramen ovale, the mandibular division of the trigeminal nerve (V3), and the

The pharyngobasilar fascia is partially deficient posteriorly, allowing passage of the Eustachian tube and levator veli palatini muscle from the skull base to the nasopharyngeal mucosal space. The region or space of deficiency is known as the sinus of Morgagni.44 Most authors believe that the sinus of Morgagni and its perforating structures are partly responsible for the spread of nasopharyngeal mucosal tumor through the pharyngobasilar fascia to the parapharyngeal soft tissues and skull base.45 The most recognizable anatomical and radiologic feature of the lateral wall of the nasopharynx is the torus tubarius. The torus tubarius is essentially the medial, mushroom-shaped end of the cartilaginous Eustachian tube that protrudes into the nasopharynx, elevating the surrounding mucosa. The mucosa along the posteromedial margin of the torus tubarius juxtaposes against the mucosa of the posterior wall of the nasopharynx, creating a small cleft, the fossa of Rosenmüller. The fossa of Rosenmüller is important because this area is suspected to be the site of origin of nasopharyngeal carcinoma.43

Retropharyngeal Space (RPS) The pharyngobasilar fascia and associated middle layer of deep cervical fascia form the posterior margin of the nasopharyngeal mucosal space. Immediately posterior to the middle layer of deep cervical fascia is a space, the RPS, that separates the

Figure 12 The submucosal nasopharynx. The superior constrictor muscle (SCM) attaches to the skull base via the pharyngobasilar fascia (PBF). The Eustachian tube (ET) passes through a defect in the PBF referred to as the sinus of Morgagni. The tensor veli palatini (TVP) muscle is seen arising from the cartilaginous Eustachian tube and extending inferiorly to the lateral margin of the soft palate.

Practical anatomy of the central skull base region foramen spinosum. The medial masticator space is bound by a superficial layer of deep cervical fascia that extends from the inferior margin of the mandible, over the medial pterygoid muscle, and then cephalad to the skull base.47 This attaches to the sphenoid bone just lateral to the foramen ovale and foramen spinosum. The mandibular nerve exits the foramen ovale and enters the top of the masticator space. The middle meningeal artery, a branch of the maxillary artery, leaves the masticator space and enters the middle cranial fossa through the foramen spinosum. Ultimately, lesions of the masticator space approach the central skull base indirectly through the PPF, perineurally through the foramen ovale, or directly where the masticator space contacts the skull base.48

Neuroimaging of the Central Skull Base A multidisciplinary approach is used at our institution in diagnosing, treating, and managing central skull base lesions. Given the diversity of pathologic findings of the central skull base, a combination of imaging studies may be required to assess a particular lesion fully. CT and MRI play such complementary roles in evaluation of the central skull base pathology that both are considered essential for an initial a radiologic workup. High-resolution nonenhanced helical CT scan performed with thin collimation (o1.5 mm) and bone algorithm yields the best overall evaluation of the bony architecture of the central skull base. Cortical margins, neuroforamina, trabecular bone, and tumoral or soft tissue calcifications can be evaluated with fine detail. Additionally, this technique allows for highresolution multiplanar reformations. Other CT techniques are also commonly performed and may provide additional information. Protocols for contrastenhanced CT for soft tissue of the neck are generally performed with larger collimation, but allow for better soft tissue evaluation, and are typically performed to include the entire neck for evaluation of the cervical lymph nodes. Contrastenhanced CT angiography of the neck or head may be necessary to evaluate the vessels at the skull base. MRI is considered the best imaging study to evaluate the soft tissues of the skull base.1,2,49 Multiple sequences in multiple planes are employed to determine the true extent of a pathologic process of the skull base. Our basic institutional protocol begins with unenhanced T1-weighted nonfat-saturated images in the axial and coronal planes, using 3-4-mm collimation and relatively small field of view (16-18 cm). The presence of fat in the soft tissues and in the marrow spaces is extremely useful in defining the anatomy. Coronal and axial short inversion-time inversion-recovery is used to suppress signal related to fat and maximize T2 signal related to infection, inflammation, fluid, or neoplasm. Postcontrast gadolinium-enhanced images are performed in at least 2 planes and are essential for detecting disease of the suprahyoid soft tissues, the bone marrow, and the meninges, as well as perineural tumor spread. One or both of these enhanced sequences are typically performed with fat

391 saturation. However, susceptibility artifacts are common in the skull base because of field inhomogeneity and multiple airbone-soft tissue interfaces. Ultimately, imaging protocols may be individualized depending on the particular lesion or clinical question. Pituitary adenoma evaluation for instance, often requires dynamic contrast-enhanced imaging. Magnetic resonance angiography, like CT angiography can be used to evaluate the vessels of the skull base. High-resolution heavily T2-weighted sequences, such as fast imaging employing steady-state acquisition or constructive interference into steady state, are excellent at depicting CNs and vessels as they course through the basilar cisterns. Diffusion-weighted imaging of the skull base may be useful if osteomyelitis of the skull base is suspected. Other modalities can be used to evaluate the central skull base. Fluorine-18 fluorodeoxyglucose positron emission tomography can be helpful in evaluation of some skull base tumors but is not routinely used at our institution to evaluate central skull base tumors. Routine nuclear medicine bone scanning as well as scanning with Gallium-67 have been advocated to evaluate skull base osteomyelitis.50,51 Conventional angiography may be necessary for evaluation and treatment of aneurysms, arteriovenous malformations, or carotid cavernous fistulas. Preoperative embolization of highly vascular lesions, such as juvenile nasopharyngeal angiofibromas or paragangliomas, may facilitate surgical resection and reduce intraoperative hemorrhage.

Conclusions The central skull base is one of the most difficult anatomical regions for the neuroimaging specialist, the skull base surgeon, and the radiation therapist. It represents a convergence between the intracranial compartment, the orbit, the deep face, and the suprahyoid neck. Lesions can have endocranial origins and invade the skull base from above, arise from the intrinsic skull base, or invade the skull base from adjacent exocranial compartments. The numerous tissue types give rise to multiple pathologies in this region. Modern neuroimaging with multiplanar, high-resolution CT and MRI images now allows for detailed evaluation of this region. Knowledge of this region should allow the radiologist to identify anatomical subsites of a particular lesion, establish a potential origin for the lesion, and develop an anatomy-based differential diagnosis.

References 1. Borges A: Imaging of the central skull base. Neuroimaging Clin N Am 19:441-468, 2009 2. Morani AC, Ramani NS, Wesolowski JR: Skull base, orbits, temporal bone, and cranial nerves: Anatomy on MR imaging. Magn Reson Imaging Clin N Am 19:439-456, 2011 3. Rhoton AL Jr.: The sellar region. Neurosurgery 51(suppl 4):S335-S374, 2002 4. Sage MR, Blumbergs PC, Mulligan BP, et al: The diaphragma sellae: Its relationship to the configuration of the pituitary gland. Radiology 145:703-708, 1982 5. Yasuda A, Campero A, Martins C, et al: Microsurgical anatomy and approaches to the cavernous sinus. Neurosurgery 56(suppl 1):4-27, 2005; [discussion 24-27]

392 6. Miyazaki Y, Yamamoto I, Shinozuka S, et al: Microsurgical anatomy of the cavernous sinus. Neurol Med Chir (Tokyo) 34:150-163, 1994 7. Dalgic A, Boyaci S, Aksoy K: Anatomical study of the cavernous sinus emphasizing operative approaches. Turk Neurosurg 20:186-204, 2010 8. Tubbs RS, Hill M, May WR, et al: Does the maxillary division of the trigeminal nerve traverse the cavernous sinus? An anatomical study and review of the literature. Surg Radiol Anat 30:37-40, 2008 9. Isolan GR, Krayenbuhl N, de Oliveira E, et al: Microsurgical anatomy of the cavernous sinus: Measurements of the triangles in and around it. Skull Base 17:357-367, 2007 10. Downs DM, Damiano TR, Rubinstein D: Gasserian ganglion: Appearance on contrast-enhanced MR. Am J Neuroradiol 17:237-241, 1996 11. Woolfall P, Coulthard A: Pictorial review: Trigeminal nerve: Anatomy and pathology. Br J Radiol 74:458-467, 2001 12. Arani KN KA, Dulai MP, Silbergleit R. The Meckel's Cave: What Hides Within? Paper presented at: American Society of Neuroradiology 49th Annual Meeting. 2011; Seattle, WA. 13. Kamel HA, Toland J: Trigeminal nerve anatomy: Illustrated using examples of abnormalities. Am J Roentgenol 176:247-251, 2001 14. Laine FJ, Nadel L, Braun IF: CT and MR imaging of the central skull base. Part 1: Techniques, embryologic development, and anatomy. Radiographics 10:591-602, 1990 15. Kimura F, Kim KS, Friedman H, et al: MR imaging of the normal and abnormal clivus. Am J Roentgenol 155:1285-1291, 1990 16. Laine FJ, Nadel L, Braun IF: CT and MR imaging of the central skull base. Part 2. Pathologic spectrum. Radiographics 10:797-821, 1990 17. Gehanne C, Delpierre I, Damry N, et al: Skull base chordoma: CT and MRI features. JBR-BTR 88:325-327, 2005 18. Renn WH, Rhoton AL Jr.: Microsurgical anatomy of the sellar region. J Neurosurg 43:288-298, 1975 19. Kazkayasi M, Karadeniz Y, Arikan OK: Anatomic variations of the sphenoid sinus on computed tomography. Rhinology 43:109-114, 2005 20. Unal B, Bademci G, Bilgili YK, et al: Risky anatomic variations of sphenoid sinus for surgery. Surg Radiol Anat 28:195-201, 2006 21. Mikami T, Minamida Y, Koyanagi I, et al: Anatomical variations in pneumatization of the anterior clinoid process. J Neurosurg 106:170-174, 2007 22. Kulwin C, Tubbs RS, Cohen-Gadol AA: Anterior clinoidectomy: Description of an alternative hybrid method and a review of the current techniques with an emphasis on complication avoidance. Surg Neurol Int 2:140, 2011 23. Jen A, Sanelli PC, Banthia V, et al: Relationship of petrous temporal bone pneumatization to the eustachian tube lumen. Laryngoscope 114:656-660, 2004 24. Virapongse C, Sarwar M, Bhimani S, et al: Computed tomography of temporal bone pneumatization: 1. Normal pattern and morphology. Am J Roentgenol 145:473-481, 1985 25. Rhoton AL Jr.: The temporal bone and transtemporal approaches. Neurosurgery 47(suppl 3):S211-S265, 2000 26. Tedeschi H, Rhoton AL Jr.: Lateral approaches to the petroclival region. Surg Neurol 41:180-216, 1994 27. Balboni AL, Estenson TL, Reidenberg JS, et al: Assessing age-related ossification of the petro-occipital fissure: Laying the foundation for understanding the clinicopathologies of the cranial base. Anat Rec A Discov Mol Cell Evol Biol 282:38-48, 2005

P.R. Chapman et al. 28. Keshelava GM, Abzianidze I, Kikalishvili G, et al: Surgical anatomy of petrous part of the internal carotid artery. Neuroanatomy 8:46-48, 2009 29. Osawa S, Rhoton AL Jr., Tanriover N, et al: Microsurgical anatomy and surgical exposure of the petrous segment of the internal carotid artery. Neurosurgery 63(4 suppl 2):210-238, 2008; [discussion 239] 30. Icke C, Ozer E, Arda N: Microanatomical characteristics of the petrosphenoidal ligament of Gruber. Turk Neurosurg 20:323-327, 2010 31. Liu XD, Xu QW, Che XM, et al: Anatomy of the petrosphenoidal and petrolingual ligaments at the petrous apex. Clin Anat 22:302-306, 2009 32. Falcon RT, Rivera-Serrano CM, Miranda JF, et al: Endoscopic endonasal dissection of the infratemporal fossa: Anatomic relationships and importance of eustachian tube in the endoscopic skull base surgery. Laryngoscope 121:31-41, 2011 33. Rusu MC, Pop F, Curca GC, et al: The pterygopalatine ganglion in humans: A morphological study. Ann Anat 191:196-202, 2009 34. Rusu MC, Pop F: The anatomy of the sympathetic pathway through the pterygopalatine fossa in humans. Ann Anat 192:17-22, 2010 35. Daniels DL, Mark LP, Ulmer JL, et al: Osseous anatomy of the pterygopalatine fossa. Am J Neuroradiol 19:1423-1432, 1998 36. Curtin HD, Williams R: Computed tomographic anatomy of the pterygopalatine fossa. Radiographics 5:429-440, 1985 37. Daniels DL, Mark LP, Mafee MF, et al: Osseous anatomy of the orbital apex. Am J Neuroradiol 16:1929-1935, 1995 38. Rene C: Update on orbital anatomy. Eye (Lond) 20:1119-1129, 2006 39. Aviv RI, Miszkiel K: Orbital imaging: Part 2. Intraorbital pathology. Clin Radiol 60:288-307, 2005 40. Slavin KV, Dujovny M, Soeira G, et al: Optic canal: Microanatomic study. Skull Base Surg 4:136-144, 1994 41. Aviv RI, Casselman J: Orbital imaging: Part 1. Normal anatomy. Clin Radiol 60:279-287, 2005 42. Martins C, Costa e Silva I, Campero A, et al: Microsurgical anatomy of the orbit: The rule of seven. Anat Res Int 2011:1-14, 2011 43. Goh J, Lim K: Imaging of nasopharyngeal carcinoma. Ann Acad Med Singapore 38:809-816, 2009 44. Som PM, Curtin HD: Fascia and spaces of the neck. In: Som PM, Curtin HD (eds): Head and Neck Imaging, Vol 2 (ed 3). St. Louis, Mosby, 1805-1827, 2003 45. Tubbs RS, Jones V, Loukas M, et al: Quantification and anatomy of the sinus of morgagni at the skull base. Biomed Int 1:16-18, 2010 46. Mukherji SK, Castillo M: A simplified approach to the spaces of the suprahyoid neck. Radiol Clin North Am 36:761-780, 1998; (v) 47. Curtin HD: Separation of the masticator space from the parapharyngeal space. Radiology 163:195-204, 1987 48. Wei Y, Xiao J, Zou L: Masticator space: CT and MRI of secondary tumor spread. Am J Roentgenol 189:488-497, 2007 49. Chong V: The skull base in oncologic imaging. Cancer Imaging 4:5-6, 2004 50. Clark MP, Pretorius PM, Byren I, et al: Central or atypical skull base osteomyelitis: Diagnosis and treatment. Skull Base 19:247-254, 2009 51. Seabold JE, Simonson TM, Weber PC, et al: Cranial osteomyelitis: Diagnosis and follow-up with in-111 white blood cell and Tc-99m methylene diphosphonate bone spect, CT, and MR imaging. Radiology 196:779-788, 1995