Neuroimaging: Intrinsic Lesions of the Central Skull Base Region Asim K. Bag, MD, and Philip R. Chapman, MD The sphenoid bone is the osseous foundation of the central skull base. The body of the sphenoid is cuboid in shape and its posterior margin is joined to the basilar occipital bone (basiocciput) via a synchondrosis to form the complete clivus. Traditionally, radiologic discussions of intrinsic disease of the central skull base emphasize marrow spaceoccupying lesions including metastatic disease, myeloma, and chordoma. Based on our practical experience and the anatomical boundaries of the central skull–based region put forth, we include lesions of the sphenoid sinus and petrous apex in our discussion. We describe lesions that might originate within, be confined to, or principally involve the skeletal foundation of the central skull base, including the pneumatized regions contained within. Intrinsic lesions affecting the central skull base are emphasized and the most important computed tomography and magnetic resonance imaging findings that allow for effective diagnosis, planning, and treatment are highlighted. Semin Ultrasound CT MRI 34:412-435 C 2013 Elsevier Inc. All rights reserved.

T

he sphenoid bone provides the central osseous foundation of the skull base. The body of the sphenoid is cuboid in shape, with complex lateral and inferior projections. The sphenoid bone is variably pneumatized by the sphenoid sinus air cells. 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. Its posterior margin is joined to the basilar occipital bone via a synchondrosis to form the complete clivus. The clivus contains significant trabecular bone and thus possesses one of the most conspicuous areas of vascularized marrow space in the skull. Traditionally, radiologic discussions of central skull base intrinsic disease have emphasized marrow space–occupying lesions including metastatic disease and myeloma. Chordomas usually arise from notochordal remnants within the clivus. Chondrosarcomas are generally included as they arise from the contiguous petrooccipital fissure and bear close anatomical-radiologic relationship to chordomas. Based on our practical experience and the anatomical boundaries of the central skull– based region put forth, we have chosen to include lesions of the sphenoid sinus and petrous apex in our discussion.

Project Editor: Suzanne Byan-Parker. Tel.: +1-205-934-4274; +1-205-4823229 (mobile). E-mail: [email protected] Department of Radiology, Section of Neuroradiology, 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: pchapman@ uabmc.edu

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0887-2171/$-see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.sult.2013.08.004

Ultimately, this article attempts to describe lesions that might originate within, be confined to, or principally involve the skeletal foundation of the central skull base, including the pneumatized regions contained within. Moreover, we highlight the most important computed tomography (CT) and magnetic resonance imaging (MRI) findings that allow for effective diagnosis, planning, and treatment.

Sphenoid Sinus Lesions Normal Variant of Sphenoid Sinus Pneumatization The body of the sphenoid bone develops from 2 anterior and 2 posterior ossification centers. Laterally located secondary ossification centers form the greater and lesser wings of sphenoid as well as pterygoid plates. Normal pneumatization of sphenoid sinus progresses from anterior to posterior through a series of changes in a highly coordinated fashion, forming the main sinus within the body of sphenoid, with or without extension to the clivus. Pneumatization of the more laterally located ossification centers results in aeration of the greater and lesser wings, anterior clinoid as well as pterygoid processes.1 As a precursor of pneumatization, the normal red marrow of the body of the sphenoid begins conversion to fatty marrow in infants approximately 4 months old.2 Most children show

Neuroimaging significant marrow conversion by 2 years of age.1 Subsequently, respiratory mucosa enters into the fatty marrow of the sphenoid as fatty marrow conversion and aeration of the sphenoid sinus continue as a congruent process. The most rapid aeration occurs in children aged 1-5 years, although sphenoid sinus continues to expand throughout childhood until it reaches final size at approximately 12-14 years of age.1,3,4 Extent of final pneumatization of sphenoid sinus varies widely. It is important to determine the extent of pneumatization before patients undergo functional endoscopic sinus surgery using the following approaches: transsphenoidal approach to the median (planum sphenoidale and sella), paramedian (cavernous sinus, Meckel cave, middle cranial fossa, and petrous apex), central skull base, as well as an extended transclival endoscopic approach to the posterior fossa.5 Different classification systems have been used to categorize the degree of pneumatization. Pneumatization of the sphenoid sinus is typically classified into 3 types, depending on the relationship of pneumatization to the sella: conchal, presellar, and sellar.6 A broader classification system using the same principle has been suggested by Guldner et al that includes type I or nonpneumatization type (seen in o1%); type II or presellar type (posterior sinus wall anterior to the anterior wall of sella; seen in 7%); type III or sellar type (pneumatization extending between anterior and posterior wall of the sella; seen in 57%); and type IV or postsellar type (pneumatization extends posterior to the posterior wall of the sella; seen in 38%), which is also used in the surgical literature.7 Knowledge of pneumatization of the secondary ossification centers (pterygoid and clinoid processes) is also very important in skull base surgery to avoid catastrophic complications. It has been shown that there is a statistically significant correlation between pneumatization of the pterygoid plates (in approximately 37%-39% of patients) and protrusions of the vidian canal and foramen rotundum into the sinus cavity.8,9 Similarly, pneumatization of the anterior clinoid process (in approximately 17%-23% of patients) is correlated with protrusion of the optic canal into the sphenoid sinus (12%-19%) or having a complete intrasinus course (8%-9%).8-10 DeLano et al11 classified the relationship of optic canal to sphenoid and ethmoid

413 sinuses into 4 types according to the degree of protrusion. There is no statistical correlation between clinoid pneumatization and protrusion of the carotid canal into the sphenoid sinus,8 but recognition of this variant can avoid torrential hemorrhage during endoscopic surgery.

Arrested Pneumatization Arrested pneumatization is the deviation from normal aeration of the sphenoid sinus or other skull base regions (petrous apex and mastoid air cells) with incongruence of fatty marrow conversion and final extent of pneumatization. This occurs because aeration fails to replace fully the areas of already converted fatty marrow. As a result, there are abnormal fatty foci of skull base bone marrow (Fig. 1). Most cases of arrested pneumatization occur in the sphenoid bone and are related to the sphenoid sinus air cells. However, similar phenomena can be seen in other areas of the skull base, including the mastoid segment of the temporal bone and occipital bone. It is important to differentiate arrested pneumatization, which is relatively common, from clinically important pathologies involving the central skull base. Key imaging features of arrested pneumatization include (1) location at a site of normal or accessory pneumatization, (2) CT image findings of a nonexpansile fat-density region with sclerotic margin, with or without, curvilinear matrix calcification,12 and (3) nonenhancing, nonexpansile fat signal abnormality on MRI. The presence of focal fat in these cases can be misleading for the radiologists unfamiliar with this entity. The low-density and heterogeneous appearance on CT may prompt concern for a lytic process. The high signal from fat on T2-weighted MRI may be suggestive of an infiltrative lesion such as a chordoma. Strict evaluation of density measurements on CT and fat-suppressed sequences on MRI should prevent confusion. However, in some rare cases there may be no evidence of fat content.

Hyperpneumatization of the Skull Base Rarely the pneumatization of paranasal sinuses and temporal bones can be extreme. Various terms have been used to

Figure 1 A 30-year-old male with incidental finding of arrested pneumatization of the central skull base. (A) Axial CT scan through the body of the sphenoid demonstrated heterogeneous density of the sphenoid body on the left (arrow). Notice the prominent right-sided sphenoid sinus and essentially absent left-sided air cell. (B) Coronal CT scan through the sphenoid sinus demonstrates bubbly lucency in the left side of the sphenoid bone (arrow). (C) Axial MRI scan reveals conspicuous T1 shortening or hyperintensity (arrow) consistent with fat signal, a hallmark of benign arrested pneumatization.

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414 describe an enlarged sinus. A hypersinus refers to an enlarged paranasal sinus without associated bony expansion or cortical thinning. Pneumosinus dilatans, described in the context of planum sphenoidale meningiomas earlier, refers to an enlarged sinus with associated expansion of the bone itself, without thinning of the cortex. Hyperpneumatization or pneumocele refers to an extreme form of sinus enlargement, with bony expansion and associated marked cortical thinning. Rare cases of extreme hyperpneumatization of the skull base have been reported. These cases demonstrate pneumatization of the skull base and cervicocranial junction well beyond expected boundaries of the sphenoid sinus or temporal bone air cells. Pneumatization can be seen in the occipital bone diffusely including the occipital condyles, the atlas, and even C2.13-15 The mechanism or etiology for these cases is not understood. Although some authors propose a ball valve–type mechanism, others have not confirmed this. Brown et al16 found an association of hyperpneumatization of the skull base with intraosseous lymphangiomatosis. Some cases have been discovered incidentally. Several cases have been associated with superimposed trauma, with skull base fractures leading to deep soft tissue emphysema or epidural air15 (Fig. 2).

Isolated Sphenoid Sinus Disease Isolated sphenoid sinus disease (ISSD) is essentially a radiologic description and refers to complete opacification on imaging studies of 1 or both sphenoid sinus air cells with relative sparing of the remaining paranasal sinuses and nasal vault.17,18 Since the original series by Wyllie et al in 1973, several case series have been published reviewing the various etiologies and pathologies for ISSD, as well as clinical symptoms. ISSD is relatively uncommon, affecting 1%-3% of patients with sinus disease.19 However, given the prevalence of neuroimaging, isolated opacification of the sphenoid sinus is a relatively frequent finding for the radiologist. ISSD is difficult to diagnose and treat given its insidious and nonspecific clinical findings. The most common clinical finding is headache (72%), followed by visual disturbance in approximately 21%.20 It should be remembered that the number of ISSD etiologies is extensive. In the largest ISSD case series to date, Cakmak et al20 found inflammatory lesions in most of the 182 cases: sphenoid sinusitis (29%), mucoceles (24%), and fungal disease (8%). However, the remaining 34 cases (18%) constituted various malignant tumors including squamous cell carcinoma,

Figure 2 A 35-year-old male with motor vehicle crash, headache, and basilar skull fracture. (A) Sagittal reformatted CT scan through the clivus demonstrates marked aeration of the entire clivus (arrow). (B) Sagittal CT reformatted image through the cervicocranial junction on the right shows extension of pneumatization into the skull base including the occipital bone (long arrow) on the right lateral mass of C1 (short arrow). There is trace extraosseous air because of occipital fracture (open arrow). (C) Axial CT scan through the occipital bone reveals pneumatization of the clivus and occipital bone laterally on the right. Small extraosseous gas is seen anteriorly (arrow). (D) Axial CT image through the C1 ring shows marked atypical pneumatization of the C1 ring (arrow). There is extraosseous air in the right vertebral artery foramen, epidural space, and the posterior suboccipital region.

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Figure 3 Thirty-eight-year-old male with long standing history of asthma, aspirin sensitivity, and sinonasal polyposis (Samter's Triad). At surgery, eosinophilic-rich mucin was identified without fungal hyphae or culture. (A) Axial CT through the sphenoid sinus demonstrates generalized hyperostosis and focal dehiscence along the posterior wall (arrow). (B) Axial postcontrast MRI demonstrates marked mucosal thickening and enhancement of the ethmoid and sphenoid sinus air cells with enhancement extending to the epidural margin (arrow). (C) Coronal CT through sphenoid demonstrates additional area of dehiscence (arrow) along roof of the sphenoid sinus. (D) Coronal postcontrast MRI reveals marked mucosal thickening and enhancement, with enhancing mucosa extending through the planum sphenoidale (arrow).

plasmacytoma, lymphoma, metastatic disease, and salivary gland tumors. Sixteen patients had benign tumors.20 When reporting uncomplicated, solitary opacification of the sphenoid sinus on routine unenhanced head or sinus CT, it is reasonable to suggest that benign inflammatory disease is the most likely etiology. However, further evaluation with short-term followup CT or enhanced MRI may be warranted, particularly if there is vision loss, cranial nerve deficit, or persistent or worsening symptoms. Usually, endoscopic evaluation, drainage with or without biopsy is recommended for treatment of ISSD.

Inflammation and Infection of the Sphenoid Sinus Mucosal thickening and opacification of the sphenoid sinus generally occurs as part of a broader pattern of pansinus infection or inflammation. Isolated sphenoid disease is less common.21 Typical sphenoid sinus infection appears similar to sinusitis in other paranasal sinuses, with mucosal thickening, mucosal enhancement, air-fluid levels, and hyperostosis. On CT, sinusitis of the sphenoid sinus can lead to variable appearances depending on chronicity and underlying etiology.

The bony walls of sphenoid sinus may be normal or demonstrate hyperostosis, erosion, or a combination of these findings (Fig. 3). Opacification of the sphenoid sinus may range from relative hypodensity in acute or subacute sinus inflammation to relative hyperdensity in chronic obstructive sinusitis.22 The hyperdensity itself is nonspecific and can be related to inspissated mucous contents or to superimposed fungal colonization.23 Mycetoma, a noninvasive form of fungal sinusitis, should be suspected on CT if the sphenoid sinus shows central hyperdense opacification with fine, round, or linear calcifications; a surrounding zone of thin mucoid density separating the hyperdensity from the bone; sclerotic or hyperostotic margins of the bony walls; and a lack of invasive features. MRI generally shows relative hypodensity on both T1- and T2-weighted images and lack of central enhancement following contrast administration.22,24 Aggressive or invasive sphenoid sinusitis can be caused by bacteria but are more commonly caused by fungal infections and can lead to bony erosion or destruction. Additionally, organisms can traverse vascular channels through the trabecular and cortical bone without overt bone resorption or lytic

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Figure 4 A 34-year-old female with history of heart transplant and diabetes developed sinusitis and vision loss. Emergency sinus surgery was undertaken owing to concerns for invasive fungal sinusitis. Culture yielded Streptococcus viridians. Vision improved immediately after surgery. (A) Preoperative coronal CT demonstrates complete opacification of the sphenoid sinus and subtle dehiscence at the medial base of the right optic strut (arrow), just below the optic nerve. (B) Axial STIR images from preoperative MRI show heterogeneous fluid signal in the sphenoid and posterior sinuses with fluid extending into the base of the optic strut on the right. (C) Coronal postcontrast T1-weighted image through the sphenoid sinus demonstrates loss of enhancement of the mucosa on the right (short solid arrows), subtle dural enhancement along the planum sphenoidale (long open arrow), focal mucosal enhancement in the anterior clinoid process, and subtle perineural enhancement in the optic nerve canal (short open arrow). (D) Axial postcontrast fat-saturated T1-weighted images demonstrate enhancement along the right optic nerve entering the optic nerve canal (arrow) consistent with inflammation. STIR, short TI inversion recovery.

changes. Because of the proximity to critical neurovascular structures, these invasive infections of the sphenoid sinus can lead to devastating intracranial complications, including vision loss, cavernous sinus thrombosis, cranial nerve dysfunction, internal carotid artery thrombosis, and meningitis or cerebritis. Mucor is the major invasive pathogen associated with rhinocerebritis in diabetic patients. Aspergillus is the most common opportunistic fungal agent of the sinuses in oncologically immunocompromised patients. The pneumatized optic strut and anterior clinoid process generally (82%) communicate with the sphenoid sinus but can also arise from posterior ethmoid air cells.25 Infection involving the optic strut and anterior clinoid process can result in visual loss owing to inflammation and compression or vascular compromise of the optic nerve with or without frank bony dehiscence of the cortical margins (Fig. 4). Dehiscence of the superolateral sphenoid sinus wall leads to invasion of the cavernous sinus and results in thrombophlebitis or thrombosis of the cavernous sinus. High-resolution CT with multiplanar reconstruction is necessary in most cases to detect subtle or early erosive changes in the walls of the

sphenoid sinus. Postcontrast CT of the skull base or CT angiography or both are useful to evaluate the cavernous sinuses and cavernous segments of the internal carotid arteries. MRI with and without contrast provides more detailed evaluation of the marrow space and soft tissues of the central skull base region and can detect central skull base osteomyelitis (SBO), cavernous sinus thrombosis, meningeal inflammation, optic nerve inflammation or infarction, empyema, and associated brain parenchymal disease.

Mucocele Unlike other paranasal sinuses, sphenoid sinus mucocele is relatively rare and, depending on the published literature, represents approximately 1%-3% of all paranasal sinus mucoceles.26,27 Mucocele of the sphenoid sinus is usually secondary to sphenoid sinusitis. Typically, mucocele is asymptomatic, but large mucocele can present with treatment-resistant headache and visual field defect secondary to involvement of the optic nerve. Sphenoid sinus mucocele commonly expands anterolaterally to involve the anterior clinoid process and

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Figure 5 A 50-year-old male with Wegener granulomatosis presents with progressive proptosis. (A) Axial CT scan through the sinuses shows marked sclerosis and hyperostosis of the sinus walls with destruction of the turbinates and nasal septum. Retroorbital fat is obliterated on the left (arrow). (B) Axial postcontrast T1-weighted MRI reveals a solidly enhancing inflammatory mass (arrow) in the left orbit and diffuse mucosal thickening and enhancement of the remaining sinuses. (C) Sagittal postcontrast MRI demonstrates confluent, solid-appearing enhancement within the sphenoid sinus (arrow) that could mimic a neoplasm.

orbital apex and can present with vision loss and multiple cranial nerve symptoms. Sphenoid sinus mucoceles have the highest complication rate of the paranasal sinus mucoceles. The radiologist must be able to describe fully the extent of the mucocele and its relationship to the optic nerve, superior orbital fissure, cavernous sinus, and internal carotid artery. Skull base CT scan with bone algorithm best evaluates the degree of bone expansion and any regions of bony dehiscence. The usual CT appearance is an airless, expanded sinus cavity with marked scalloping, or erosion of the bony margins. Mucoceles are filled with hypodense content. However, mucoceles can have variable density depending on the protein content of secretion. Similarly, the T1 and T2 characteristics of the secretions are dependent on the protein content of the secretion and can vary from low T1-high T2 to high T1-high T2.28 A highly proteinaceous secretion can have signal void both on T1- and T2-weighted sequences.28 Treatment is usually endoscopic drainage, but optimum timing of therapy is still controversial in asymptomatic patients.

Wegener Granulomatosis Wegener Granulomatosis (WG) is a systemic, pauci-immune, granulomatous vasculitis. It predominantly involves the smalland medium-sized blood vessels of the upper respiratory tracts and kidneys with histopathologic triad of necrotizing granulomatous lesions of the respiratory tract, segmental necrotizing angiitis of arteries and veins, and necrotizing glomerulitis. Therapy-resistant chronic sinusitis is the most common initial presentation. With time, symptoms related to lower respiratory tract and kidney involvement appear. Typically, patients with WG show positive results for elevated levels of serum cytoplasmic variant of antineutrophil cytoplasmic antibodies against the enzyme proteinase-3. On imaging, there is usually pansinus mucosal thickening with remodeling of the sinus walls with or without destruction of the nasal septum and turbinates. Bone destruction primarily affects the septum, turbinates, cribriform plate, and lamina papyracea with relative sparing of the central skull base. Neo-osteogenesis as well as

complete bony obliteration of the sinus cavities (maxillary, frontal, and sphenoid) has also been described.29 Additionally, there may be calcification of the sinus content, which may mimic fungal sinusitis.30 The sphenoid sinus may demonstrate nonspecific sinusitis or may be partially or completed obliterated owing to hyperostosis (Fig. 5). Rarely, the cavernous sinus, sella, central skull base dura, pituitary gland, and hypophysitis may be secondarily involved with or without definite bony erosion of the sphenoid walls.31-33 There may be heterogeneous enhancement of the sinus contents on postcontrast MRI, with enhancement and infiltration of the pituitary gland and stalk.32,34,35

Sphenochoanal Polyposis Choanal polyps arise from inflamed, edematous paranasal sinus mucosa, most commonly from maxillary sinuses. Rarely, the polyp can arise from the sphenoid sinus and extend into the nasopharynx through the posterior choana. Sphenochoanal polyp can be misdiagnosed as anterochoanal polyp on anterior rhinoscopy. On imaging, a polypoid lesion is seen to arise from the sphenoid sinus and extend into the nasopharynx, through the posterior choana. Even though polypoid mucosal thickening of the sphenoid sinus is the most common etiology of sphenochoanal polyp, inverted papilloma, pituitary macroadenoma, and heterotopic glial tissue can present as isolated sphenochoanal polyp.36,37 CT as well as endoscopic evaluation is critical for accurate identification and selection of proper surgical technique.38 Some authors even suggest preoperative biopsy for surgical planning.36

Neoplasms of the Sphenoid Sinus Sphenoid Sinus Osteoma Osteoma is a benign, expansile proliferation of mature bone arising from either sinus wall or the septum. Osteomas are seen in approximately 1% of imaging studies obtained for sinus symptoms. They most commonly involve frontal and ethmoid

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418 sinuses. Sphenoid sinus is rarely involved. Almost all sinus osteomas are confined to the sinuses, often conforming to the contour of the sinus cavity. Rarely, they may obstruct the outflow tract and can cause obstruction and mucocele formation. Multiple osteomas should raise the concern of Gardner syndrome. Density of osteoma is variable and depends on the degree of ossification of the matrix.

Primary Sphenoid Sinus Squamous Cell Carcinoma Primary malignant tumors of the sphenoid sinus are rare, seen in only 2% of patients presenting with sinonasal tumors. Pathologic entities are variable, but most primary sphenoid cancers are of epithelial origin and include squamous cell carcinoma, adenocarcinoma, and sinonasal undifferentiated carcinoma. Squamous cell carcinoma is reported to be the most common of the epithelial primary lesions of the sphenoid sinus. Squamous cell carcinoma more typically involves the maxillary sinus (50%), the nasal cavity (20%), and the ethmoid sinus (10%). Sphenoid sinus involvement is often seen with

large lesions involving multiple subsites whose exact origin is difficult to establish (Fig. 6). Primary sphenoid cancers present particular challenges to diagnosis and treatment. Unlike cancers located more anteriorly that present with sinonasal symptoms of obstruction, epistaxis, and nasal discharge, primary sphenoid sinus cancers frequently present with headache, visual disturbance, and cranial neuropathies following invasion of the central skull base and the cavernous sinuses.39 Aggressive bone destruction and poorly defined enhancing soft tissue mass are the best clues to identifying sinonasal squamous cell carcinoma with CT. On MRI, lesions are seen as intermediate signal on T1-weighted images and intermediate to high signal (compared to muscle) on T2-weighted images, with moderate heterogeneous or diffuse enhancement. Positron emission tomography (PET) scanning demonstrates avid uptake of fluorine-18 fluorodeoxyglucose (F-18 FDG). Basaloid squamous cell cancer, a distinctive variant of squamous cell cancer, has behavior that is more aggressive, frequently invades intracranial structures, and has a poorer prognosis. In fact, sphenoid sinus basaloid squamous cell cancer can present as a sellar mass.40

Figure 6 A 54-year-old male presents with sinonasal obstruction, sixth nerve palsy, and presumed fungal sinusitis. Biopsy showed invasive sinonasal squamous cell carcinoma. (A) Axial CT scan demonstrates soft tissue density in posterior nasal cavity (open arrow) and sphenoid sinuses with lytic destruction of the clivus (solid arrow). (B) Sagittal postcontrast T1-weighted midsagittal image demonstrates a large posterior sinonasal mass (arrow) obliterating the sphenoid sinus, invading the sphenoid clivus, the pituitary gland, and retroclival dura. (C) Axial T2-weighted image through the central skull base demonstrates isointense mass infiltrating the floor of the sella (open arrow) and adjacent cavernous sinuses. (D) Coronal postcontrast fat-saturated image through the sella shows enhancing mass in the sphenoid sinus or clivus (arrow) contiguous with the sella and bilateral cavernous sinuses.

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Sinonasal Undifferentiated Cancer Sinonasal undifferentiated cancer (SNUC) is a distinct histopathologic entity initially described in 1986. It is assumed that the tumor is derived from schneiderian epithelium or nasal ectoderm of the paranasal sinuses. Clinically, the tumor is aggressive and frequently reaches advanced stage at the time of diagnosis, often with large tumor volume involving the posterior paranasal sinuses and central skull base. Prognosis is poor, with survival generally being less than 1 year after diagnosis. At the microscopic level, SNUC consists of small- to medium-sized polygonal cells, which tend to form nests, sheets, and trabeculae. SNUC cells have high nucleus-tocytoplasm ratios and numerous mitoses. The nuclei are moderately pleomorphic, hyperchromatic, and round to oval in shape. Extensive distension of the vascular lumina and tumor-filling invasion of blood vessels are often the distinguishing features of SNUC. There is also associative evidence of extensive necrosis. SNUC has no characteristic imaging features permitting differentiation from other pathologic entities of this region. They are usually large, expansile, and highly heterogeneous

419 lesions with areas of bone destruction and invasion of adjacent structures including the anterior and central skull base, paranasal sinuses, and orbits. On CT, SNUC appears as soft tissue density tumor with variable contrast enhancement. On MRI, tumors are hypointense on T1-weighted sequence and isointense to hyperintense on T2-weighted sequence with heterogeneous enhancement (Fig. 7). Extensive marrow replacement of the central skull base is common. Owing to high nucleus-tocytoplasm ratios, this tumor frequently demonstrates diffusion restrictions on the diffusion-weighted sequence.

Minor Salivary Gland Tumors Arising From Sphenoid Sinus Similar to other paranasal sinuses, salivary gland tumors can also arise from sphenoid sinus. The most common varieties of salivary tumors are adenoid cystic carcinoma, pleomorphic adenoma, and mucoepidermoid carcinoma. Adenoid cystic carcinomas account for approximately 35% of minor salivary gland tumors. However, the sphenoid sinus is rarely involved. Of all the primary sinonasal salivary gland adenoid cystic

Figure 7 A 56-year-old male presents with headache and diplopia. Sinonasal undifferentiated carcinoma. (A) Sagittal T2-weighted MRI brain demonstrates isointense mass within the sphenoid sinus with invasion of the clivus. (B) Sagittal postcontrast T1-weighted image through the skull base shows heterogeneously enhancing mass infiltrating the clivus, extending to the sella and retroclival epidural space. Note necrosis in the inferior clivus (arrow). (C) Axial noncontrast T1-weighted image demonstrates isointense mass (long arrow) of the clivus with direct infiltration of the left cavernous sinus (short arrow). (D) Axial postcontrast MRI of the skull base shows the lesion to be centrally necrotic (long arrow) with nodular, solidly enhancing advancing front extending into the left cavernous sinus (short arrow).

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420 carcinomas, the sphenoid sinus is involved only in approximately 3%, typically affecting Caucasians between 30 and 60 years of age. Perineural tumor spread is characteristic of this tumor. A dull aching pain deep in the face or retroorbital region is suggestive of perineural tumor spread. Evaluation of the adjacent nerves on MRI is very important step for staging of this tumor. Other salivary gland tumors are rare in the paranasal sinuses, particularly in the sphenoid sinus.

Sphenoid Sinus and Central Skull Base: Sinonasal Lymphoma Sinonasal lymphoma is a relatively rare manifestation of extranodal, extralymphatic lymphoma, which occurs more commonly in Asian populations compared with Western populations.41 Pathologically, sinonasal lymphomas are nonHodgkin’s lymphomas. In Western populations, B-cell lesions are generally considered more common and present most often in elderly males, whereas in Asian populations, T-cell lymphomas predominate and affect middle-aged males. Sinonasal lymphomas, in general, arise in the maxillary sinus or nasal cavity and occasionally are localized to the ethmoid sinus air cells. Primary lymphoma of the sphenoid sinus is rare but is reported occasionally as a case report or in small series. Lymphoma of the sphenoid sinus can be seen in general radiologic practice as part of a disseminated lymphoma that affects the skull base. CT may demonstrate a lobulated, noncalcified mass in the sinonasal region with moderate, diffuse enhancement. Lymphomas are generally more likely to cause bone scalloping than frank destruction but both patterns can be seen. If the sphenoid sinus is involved, there may be direct invasion of the clivus and sella. MRI demonstrates a lesion that has intermediate signal on T1- and T2-weighted images and shows moderate enhancement and relative diffusion restriction.42,43 Primary and secondary lymphomas can be multicompartmental, involving the sinus, the skull base, and intracranial or intraorbital tissues (Fig. 8).

Intrinsic Lesions of Bone Marrow Signal Variability on MRI CT is the best modality for evaluating the cortical bone and for demonstrating detailed anatomy of the cortical margins of the central skull base. However, CT lacks the sensitivity in evaluating marrow space involvement until significant marrow infiltration and subsequent trabecular bone loss has occurred. MRI is more sensitive in evaluating marrow signal abnormalities in the calvarium and skull base because of its superior contrast resolution.44 The clivus contains significant trabecular bone and thus possesses one of the most concentrated areas of localized marrow space in the skull. Evaluation of the marrow space of the central skull base requires knowledge of normal variations and temporal physiological changes that occur in normal marrow.44 The composition of marrow in the clivus, like the axial skeleton and long bones, changes over time. At birth and in early childhood, red marrow predominates, contains hematopoietic cells, and is highly vascularized. The marrow signal is easily evaluated on sagittal MRI sequences through the midline. On T1-weighted images, the marrow signal is hypointense, similar to or slightly hyperintense to muscle. On T2-weighted images, the marrow is slightly hyperintense. On short T1 inversion recovery (STIR) images, the marrow signal retains its hyperintensity. Additionally, the marrow can demonstrate mild, diffuse enhancement normally. Over time, the red marrow is gradually converted to yellow marrow, containing significant fat cells. In general, most of the normal marrow space of the clivus is converted to fatty marrow in adults by the age of 25 years.44 In adults, therefore, the marrow is generally expected to be homogeneously hyperintense on T1-weighted images relative to muscle (and similar to subcutaneous fat), hyperintense on T2-weighted images, and have low signal on short TI inversion recovery. The normal adult clivus does not enhance. The problem, of course, is that variations along this normal spectrum can occur and the

Figure 8 Patient with disseminated NH lymphoma presents with bilateral ophthalmoplegia. (A) Coronal postcontrast MRI through the sella demonstrates hypoenhancing mass infiltrating the body of the sphenoid bone (long arrow), the sella, and bilateral cavernous sinuses (small arrows). (B) Sagittal postcontrast MRI skull base. There is large infiltrating mass completely obliterating the sphenoid sinus and invading the pituitary gland and basilar dura. There is a thumb of tissue protruding posteriorly (arrow), a feature often associated with clival chordoma. NH, non-Hodgkin’s.

Neuroimaging marrow signal can appear heterogeneous on MRI in some patients normally. Red marrow can persist beyond 25 years of age in some normal individuals. With marrow reconversion, fatty marrow is replaced by red marrow. This occurs when there is profound physiological stress, as can be seen in patients with chronic illnesses including chronic anemias, heart failure, or hematologic malignancies.44 The MRI signal pattern then reverts, as the fat signal is lost. In adult patients, diffuse T1 hypointensity in the bone marrow of the clivus, particularly when combined with T1 hypointensity in the calvarium and cervical spine, should be reported. At our institution, the authors generally would indicate that the finding is not specific but raises the possibility of underlying chronic illness and requires clinical correlation and at least a basic blood work analysis.

Chordoma Chordoma is a rare neoplasm arising from transformed remnants of embryonic notochordal tissue and accounts for 1%-4% of all bone cancers. Chordoma has a predilection for the axial skeleton, with the most common sites being the sacrum, the clivus or skull base, and vertebrae. Although the sacrococcygeal region has been generally reported as the most common location, evidence suggests an almost equal distribution in the skull base (32%), spine (32.8%), and sacrum (29.2%).45,46 Virchow first characterized microscopic appearances of chordoma. He described unique intracellular bubblelike vacuoles that he referred to as physaliferous cells, which remain a distinguishing, if not pathognomonic, feature for histopathologic diagnosis of chordomas. Recent molecular phenotyping has confirmed the link between these tumors and notochordal tissue. Chordoma cells express brachyury, a transcription factor expressed in normal undifferentiated embryonic notochord in the axial skeleton.47 Chordoma exhibits various degrees of histologic atypia and manifests as one of the 3 main histological types: classical (conventional), chondroid, and dedifferentiated. Classical chordomas appear as soft, gray-white, lobulated tumors composed of groups of physaliferous cells separated by fibrous septae. Neoplastic cells are typically positive for S-100 and epithelial markers such as MUC1 and cytokeratins.48 Brachyury staining combined with cytokeratin staining reliably differentiate chondroid chordomas from chondrosarcoma with 98% sensitivity and 100% specificity.49 The dedifferentiated variety of chordoma is extremely rare but represents a highly aggressive tumor that is usually found in the sacrococcygeal region rather than skull base. Chordoma can occur at any age, but most skull base chordomas present in adults aged 30-50 years. Men are more commonly affected than women. Rarely, chordoma can be familial.50 Chordoma is uncommon in children, but when present, it is more aggressive than the adult counterpart is.51 Common presenting symptoms of skull base chordomas include orbitofrontal headache, visual disturbances, and upper cranial neuropathies. Large tumors can also jeopardize function of lower cranial nerves.

421 On CT, the skull base chordoma is typically a midline hyperdense mass with lytic changes of the clivus. Common CT features of chordoma include bone destruction, intratumoral soft tissue component, and a sharp margin separating the tumor from adjacent normal soft tissue or bone.52 Intratumoral densities are frequently seen on CT and are thought to represent dispersed fragments of the native bone rather than true new bone formation. Unlike chondrosarcomas, chordomas do not form calcified rings or arcs. Extension into posterior fossa, sella, sphenoid sinus, as well as cavernous sinus is common.52 There is variable enhancement with contrast. However, the soft tissue component of the tumor usually demonstrates nodular or masslike enhancement. MRI better characterizes the soft tissue component of the tumor and the relationship of the tumor with the surrounding structures. On MRI, chordoma can have variable signal on T1-weighted images. The bulk of the tumor is isointense or hypointense on T1 with localized areas of hyperintensity owing to hemorrhage or mucoid materials. On T2-weighted sequence, chordoma is typically (classically) hyperintense secondary to high fluid content or abundance of physaliferous cells with areas of T2 hypointensity secondary to chronic hemorrhage, bone fragments, or highly proteinaceous mucoid material.52,53 Focal scattered areas of hypointensity in the background of hyperintensity on T2-weighted sequence give the appearance of a dirty cauliflower. Usually chordomas enhance intensely with contrast, unless the tumor is completely necrotic (Fig. 9). In the sagittal plane, the clival mass may have a dorsal extension that projects toward the pons, a so-called thumb of tissue. Unfortunately, this appearance is not specific. As the lesion expands superiorly, it usually displaces the pituitary rather than invading it. Careful examination of several sequences may be necessary to distinguish enhancing tumor tissue from enhancing pituitary gland and differentiate from the more common invasive macroadenoma. There is some suggestion that the chondroid chordoma has shorter T1 and T2 relaxation time compared with the classic chordoma; however, for an individual case, this differentiation is not a useful clue.54 There is much controversy about the intradural location of some chordomas, which remains a diagnostic challenge. Several different locations have been described in the literature, including posterior fossa, the pineal gland, Meckel cave, and suprasellar region.55-57 However, the retroclival posterior fossa is the most common location for the intradural chordomas.58,59 Tumor in this location is frequently confused with ecchordosis physaliphora (EP). However, unlike EP, intradural chordomas show enhancement on postcontrast scan. If the tumor is nonenhancing, image-based diagnosis becomes difficult.60 Skull base chordomas are indolent, slow-growing neoplasms. Their location and proximity to critical neurovascular structures makes complete resection impossible in many cases. The optimum therapies of skull base chordomas include maximal safe cytoreductive surgery with radiation. Unlike the classic photon-based radiation therapy, advanced radiation therapy techniques using hadrons (high-dose proton, carbon ion, helium, etc.) may prove useful in precisely delivering high-dose radiation to the central skull base tumor with

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Figure 9 A 47-year-old female with headache and ptosis. (A) Axial T2-weighted image through the central skull base reveals an expansile chordoma in the posterior ethmoid air cells, sphenoid sinus, and clivus, which is predominantly hyperintense on T2-weighted sequence (arrow). (B) Axial postcontrast T1-weighted image shows heterogeneous enhancement of the mass (arrow). (C) Coronal T2-weighted image through the central skull base shows marked hyperintensity of the mass (arrow) within the sphenoid sinus, which might be mistaken for fluid or inflammatory change. (D) Coronal postcontrast MRI reveals heterogeneous enhancement of the central skull base chordoma (arrow).

minimal damage to the surrounding structures. However, the exact role of these techniques is yet to be established.45

Benign Notochordal Cell Tumor Benign notochordal cell tumor (BNCT) is a relatively newly described intraosseous benign lesion of notochord origin in the axial skeleton. Several different terminologies were coined to describe this particular pathology, including giant notochordal hamartoma, benign notochordal cell lesions, notochordal remnants, notochordal vestige, and giant vertebral notochordal rest.61 However, BNCT is now the accepted terminology. Clinically and histopathologically, BNCT lack characteristic features of chordomas. Usually, these lesions are limited within bone lacking soft tissue components. On CT, there is a variable degree of sclerosis due to trabecular thickening with or without bubbly areas of hypodensity within the tumor secondary to fat content.62 On MRI, BNCT is hypointense on T1-weighted sequences and hyperintense on T2-weighted sequences without any enhancement on postcontrast images. There is no cortical destruction or soft tissue component. These lesions do not demonstrate increased uptake on bone scintigraphy. The fat content can be confirmed on MRI as a high signal on both T1- and T2-weighted sequences that completely suppresses on fat-suppressed sequences.62,63

Appropriate treatment of this particular pathology is controversial. As this is a benign nonprogressive lesion, most authorities support nonsurgical approach.61 However, if symptoms can be attributed to this lesion, surgical resection can be advised.61 For incidental lesions, a biopsy may be required to establish the appropriate diagnosis. Follow-up imaging is usually recommended to ensure nonprogression.61

Ecchordosis Physaliphora Ecchordosis physaliphora (EP) is a midline, usually small, gelatinous nodule at the retroclival area and is considered an ectopic notochordal remnant. Although this could be considered an endocranial entity, we include it here, given its notochordal origin and its intimate relationship to the clivus. EP is often diagnosed as an incidental finding. Rarely EP can be symptomatic, presenting with cerebrospinal fluid (CSF) rhinorrhea, intralesional hemorrhage, CSF fistula as well fatal pontine hemorrhage.64-67 CT is not very useful for the evaluation of EP secondary to smaller size of the lesion as well as increased frequency of artifacts in the posterior fossa. However, a bony stalk connecting the lesion with the posterior margin of the clivus (either on bone window or on CT cisternography) is the morphologic

Neuroimaging hallmark of the lesion.68 MRI is superior to CT for evaluation of EP. On MRI, the typical appearance of EP is a small, midline intradural T1 hypointense, T2 hyperintense lesion with or without a connecting stalk (to the clivus) that shows no enhancement on postcontrast images. Inclusion cysts (particularly neurenteric cyst) at this location may have similar signal characteristics.

Metastatic Disease of the Central Skull Base Skull base metastasis from distant tumors occurs in 4% of patients with cancer. Breast, lung, and prostate cancers are the 3 most common cancers metastasizing to skull base, accounting for 40%, 14%, and 12% of cases, respectively.69 Skull base metastasis can be secondary to hematogenous spread (eg, breast and lung) or can be secondary to retrograde seeding through the Batson venous plexuses that connects pelvic structures with the skull through epidural and dural veins.70 Central skull base metastasis can present with headache, cavernous sinus syndrome, or middle fossa syndrome.

423 Cavernous sinus syndrome is characterized by frontal headache, paralysis of oculomotor nerves, and sensory symptoms along the divisions of the trigeminal nerves. Lymphoma has a peculiar tropism for cavernous sinuses and adjacent areas.71 Middle fossa syndrome is characterized by facial paresthesias, numbness, and pain in the frontal region. Rarely, there may be paralysis of the ipsilateral masticator muscles, if the motor root of the trigeminal nerve is involved. Headache is uncommon in middle fossa syndrome.71 CT findings in skull base metastasis are extremely variable. CT is insensitive to marrow replacement by metastatic disease. However, CT imaging of the skull base may demonstrate lytic or permeative destruction of trabecular or cortical bone (Fig. 10). Metastatic disease can manifest as multifocal areas of dense sclerosis in the skull base and calvarium (Fig. 11). On MRI, there is loss of normal marrow signal on precontrast T1weighted sequence associated with heterogeneous T2 signal abnormality and enhancement on the postcontrast T1weighted scan. Associated extraosseous soft tissue tumor generally demonstrates contiguous enhancement and can extend intracranially or extracranially. Sclerotic metastases,

Figure 10 A 54-year-old female with a 3-month history of severe left-sided facial pain. Patient subsequently determined to have widespread metastatic lung cancer. (A) Axial CT scan through the skull base shows lytic destructive lesion involving the base of left pterygoid process (long arrow), adjacent soft tissue density in the left pterygopalatine fossa (short arrow), and erosion of the posterior wall left maxillary sinus. (B) Coronal CT shows destructive lesion of the left sphenoid bone (long arrow) with invasion of the sphenoid sinus (short arrow). (C) Axial T1-weighted MRI shows soft tissue mass destroying the left pterygoid base (arrow) and replacement of normal fat in the left pterygopalatine fossa. (D) Axial postcontrast T1-weighted image shows heterogeneous enhancement of the metastatic lesion in the lateral sphenoid bone (arrow) and pterygopalatine fossa.

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Figure 11 A 66-year-old male with widespread osseous metastatic disease of the prostate. (A) Axial CT image demonstrates sphenoid sinus opacification consistent with chronic sinusitis. The sclerosis in the walls of the sphenoid sinus (arrows) might be mistaken for chronic inflammatory changes. (B) Axial CT image more inferiorly reveals more extensive multifocal sclerosis (arrows) of the skull base and mandibular condyles, consistent with metastatic prostate cancer.

such as prostate cancer, may show marked hypointensity on all sequences with little or no enhancement. Metastases are usually hypermetabolic on 18FDG PET study.

Plasmacytoma or Multiple Myeloma Plasma cell tumors include solitary plasmacytoma, multiple solitary plasmacytoma, monoclonal gammopathy of undetermined significance, and monoclonal gammopathy as well as multiple myeloma.72 These tumors are characterized by monoclonal proliferation of immunoglobulin-secreting plasma cells. Isolated plasmacytomas are rare (only 2% of patients with plasma cell dyscrasias), benign lesions that are classified as either intramedullary or extramedullary based on their association with bone marrow.73 Intramedullary plasmacytomas progress to multiple myeloma more frequently (approximately 50%) than the extramedullary plasmacytomas (approximately 30%).74 The diagnosis of solitary bone plasmacytoma is established by a bone biopsy demonstrating infiltration by plasma cells with absence of clonal plasma cells in a random sample of bone marrow examination and without any evidence of anemia, hypercalcemia, or renal involvement, suggesting systemic myeloma. It is important to note that 90% of all solitary extramedullary plasmacytomas are located in the head and neck, including the sinonasal region. Isolated intracranial plasmacytomas are even more unusual and can affect the calvarium, the cranial base, or the dura. Of these locations, the cranial base has been reported as the strongest predictor for the development of multiple myeloma.74 On the contrary, multiple myeloma is a clonal B-cell neoplasm of plasma cells. It accounts for approximately 10% of hematologic malignancies. The annual incidence of newly diagnosed cases in the United States is 3-4 per 100,000 population per year with approximately 14,000 cases diagnosed every year.75 Plasma cell tumors of the skull base may represent solitary plasmacytomas or myelomatous lesions in the setting of multiple myeloma. The role of imaging in the workup of patients consists of studies that allow recognition of the effects of the tumor on the skeletal system. Both multiple myeloma

and solitary plasmacytomas are lytic lesions that typically affect the clivus and adjacent petrous apex in the skull base. On CT, the typical imaging appearance is expansile lytic lesion with soft tissue component.52 On MRI, these are usually isointense to hypointense on T1-weighted sequence and isointense to hyperintense on T2-weighted sequence with moderate, usually homogenous, enhancement (Fig. 12). Bone scintigraphy is inadequate for detection of myeloma-associated bone lesions owing to minimal osteoblastic activity. However, FDG PET scan can detect bone marrow involvement and is useful in assessing extent of disease at the time of diagnosis, contributing to staging as well as evaluating therapeutic response.76

Giant Cell Tumor Giant cell tumor (GCT) of the cranium represents 1% of all GCTs and preferentially involves the sphenoid bone (the body as well as in the greater wing) and the temporal bone.77 As in other areas of the body, GCT of the sphenoid bone tends to occur in young adults with slight female predominance. GCT of the body of the sphenoid frequently causes local destruction of the adjacent bone and frequently invades the sella. Typical presentation is headache and dysfunction of multiple cranial nerves. Malfunction of the nerves of the cavernous sinus and optic nerve is common.78 The larger mass can extend posteriorly and cause dysfunction of the seventh and eighth cranial nerves.77 Radiographic diagnosis of GCT at the body of the sphenoid is challenging, as the tumor lacks specific radiologic features. However, the most frequent radiologic finding is an expansile and sometimes lytic bone lesion that extends to the adolescent soft tissue and dura as well as sinuses. On MRI, the tumor is hypointense on both T1- as well as T2-weighted sequences. Similar to the other areas of the body, sphenoid bone GCT can transform to aneurysmal bone cyst (ABC). Giant cell reparative granuloma is another uncommon lesion in this location and should be considered in the differential diagnosis of GCT, both radiologically as well as histopathologically.77

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Figure 12 A 68-year-old patient with longstanding multiple myeloma. (A) Axial CT scan reveals lytic changes in the upper clivus centrally (arrow) as well as the petrous apices bilaterally. (B) Axial T1-weighted image reveals homogeneous lowsignal infiltration of the clivus (solid arrow) and petrous apices (open arrows) as tumor replaces normal fat. (C) Axial postcontrast T1-weighted images with fat saturation shows relatively homogeneous enhancement of the tumor involving the clivus (solid arrow) and petrous apices (open arrows).

Aneurysma Bone Cyst Aneurysma bone cyst (ABC) is an extremely rare tumor of the skull base. Similar to other areas of the body, typical radiologic appearances are multiple cystic lesions filled with hemorrhagic fluid-fluid levels. Several types of bone lesions have been considered precursors to ABCs such as GCT, chondroblastoma, osteoblastoma, giant cell granuloma, chondromyxoid fibroma, bone cysts, eosinophilic granuloma, and hemangioma.

Fibrous Dysplasia Fibrous dysplasia (FD) is a benign proliferative developmental abnormality in the bone that results in replacement of normal cancellous bone with varying degrees of fibrous tissue and immature woven bone. FD is predominantly monostotic (70%), but can be polyostotic, and can be associated with McCune-Albright syndrome. FD involving the facial bones, skull base, or calvarium occurs in 10%-25% of patients with monostotic FD and 50% of those with polyostotic disease. Although FD is relatively uncommon, it can be identified frequently in a busy neuroradiology practice, occasionally as an unsuspected or incidental finding. Craniofacial FD most often involves the ethmoid bone, with the sphenoid bone being the next most common site.79 Although the diagnosis is generally straightforward on CT, the MRI appearance can be misleading and mimic an aggressive or malignant process. On CT, FD of the central skull base can be seen as an expansile lesion of bone that involves the medullary cavity and generally preserves the cortex. The transition between the lesion and normal bone is typically abrupt. The classic CT imaging appearance is that of homogeneous ground glass opacity within the lesion. However, the density can be variable, depending on the concentrations of fibrous tissue and osteoid matrix. FD lesions on CT may have a ground glass appearance (56%), homogenously dense pattern (23%), cystic change (21%), or mixed pattern.80 Lesions can be small and focal or multifocal or can be large and involve multiple contiguous bones of the skull base. High-resolution CT imaging can demonstrate bony optic canal encroachment or other foraminal narrowing (Fig. 13).81 The FD MRI appearance is often less specific than CT. Lesions may demonstrate homogeneous or heterogeneously

intermediate to low signal on T1-weighted images depending upon the ratio of fibrous tissue to mineralized matrix.80 Lesions containing more mineralized matrix show hypointensity on T1, whereas lesions that are more fibrous demonstrate intermediate signal. T2-weighted images are more variable and may demonstrate relatively low-signal (owing to highly mineralized matrix), hyperintensity (intralesional cystic component), or mixed signal patterns (mixed contents). Postcontrast images demonstrate variable degrees of contrast enhancement. Fibrous tissue in FD is well vascularized and often demonstrate numerous small vessels at the center and sinusoids at the periphery of the lesion.80 This histopathologic feature explains intense enhancement in some of the lesions, frequently the most disconcerting, and misleading feature on MRI. Lesions affecting the sinuses can lead to obstructive sinusitis, hyperostosis, and mucocele formation, all of which can complicate the radiologic appearance.

Paget Disease Paget disease of bone is common, affecting 3%-4% of Caucasian adults older than 40 years. The precise etiology is not known, but underlying genetic factors play a significant role. Pathologically, bone resorption occurs secondary to abnormal osteoclast activity, followed by disorganized bone repair, fibrosis, and increased vascularization.82 Although the reparative response leads to osseous expansion, the bone strength is diminished. The process can affect one or more sites throughout the body, but preferentially affects the axial skeleton. Skull involvement occurs in up to 65% of affected patients. Paget disease can be divided into lytic, mixed, and blastic phases. In general, the radiologic appearance partially depends on the predominant phase of the disease. The lytic phase corresponds to early resorption as abnormal osteoclastic activity occurs. CT scout image of the head may demonstrate areas of circumscribed lucency in calvarium (osteoporosis circumscripta) during the lytic phase. Patchy bone resorption can affect the skull base as well. In the mixed phase, there is cortical and trabecular thickening. In the blastic phase, sclerosis can predominate and lead to marked thickening of the diploic

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Figure 13 A 29-year-old female with headache and polyostotic fibrous dysplasia involving the skull base. (A) Axial CT scan shows a mixed-density expansile process involving the sphenoid bone, petrous portion of the temporal bone, and occipital bone, including frankly lytic regions (long arrow) and regions of ground glass density (short arrow). (B) Axial T1 image demonstrates expansile process with intermediate signal (arrows) of the central and posterior skull base. (C) Axial T2-weighted image shows the variable signal within the polyostotic process, including areas of both marked hypointensity (solid arrow) and hyperintensity (open arrow). (D) Postcontrast T1-weighted image demonstrates heterogeneous enhancement within the lesion (arrow).

or marrow space and subsequent enlargement of the bone.83 Weakening of the bone may result in basilar invagination. MRI patterns can be variable as well. The bone appears expanded but there is generally some preservation of, or even increased deposition of, yellow marrow (fat). T1 hyperintensity secondary to preserved or increased fat content can be seen in comparison to normal bone. T2 images are characterized by markedly heterogeneous signal. Postcontrast images reveal enhancement in the marrow space.83 Extraosseous soft tissue lesions are not seen in uncomplicated cases (Fig. 14).

Other Diffuse Skull Base Dysplasias, Dysostoses, and Metabolic Diseases There are a variety of bone diseases that can affect the skull base, lead to abnormal growth and morphology and

abnormalities in CT density or MRI signal. Entities include achondroplasia, osteogenesis imperfecta, lysosomal storage disease, osteopetrosis, and Gorham disease. In-depth discussion of these diseases is beyond the scope of this article.

Central Skull Base Osteomyelitis Central skull base osteomyelitis (SBO), first described in 1959 by Meltzer and Kellerman, is relatively uncommon and can be one of the more difficult diagnoses to make in clinical or neuroradiologic practice. SBO often occurs in the elderly, diabetic patients, or patients with acquired immunodeficiencies and is associated with high morbidity and mortality, despite aggressive treatment. SBO is most commonly associated with spread from a necrotizing external otitis, but can be atypical and occur secondary to aggressive sinus disease, trauma, or surgery. Pseudomonas aeruginosa is the commonest pathogenic

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Figure 14 A 79-year-old female with Paget disease of the skull. (A) Lateral plain film demonstrates marked thickening of the skull base with cotton wool appearance. (B) Sagittal T1-weighted MRI reveals extensive fat signal within areas of expanded calvarium and clivus. (C) Axial CT scan shows heterogeneous bone density throughout the skull base with low-density fat within the expanded clivus (arrow).

microorganism and typical presentation is severe otalgia and otorrhea.84 Radiologic diagnosis often requires a combination of highresolution CT scan with bone algorithm and pre- and postMRI. On CT, temporal bone or sinus opacification may be the only early clue to the possibility of SBO. Soft tissue thickening, obscuration of fat planes, and enhancement involving the external auditory canal, preclival soft tissues, or infratemporal fossa may reveal the multispatial nature of the infection. Permeative or erosive changes in the cortical or cancellous bone can occur but are late findings. The term SBO implies overt bacterial or fungal infection of the marrow space of the central cranial base, beyond the pneumatized margins of the sinonasal cavities or temporal bones. This is usually accompanied by extraosseous inflammatory changes, including cellulitis, phlegmon, or abscess that can affect the endocranial and exocranial skull base. With its superior soft tissue discrimination, MRI is superior in evaluating the bone marrow as well as extraosseous soft tissues. Osteomyelitis results in T1 hypointensity, T2 hyperintensity, and postcontrast enhancement in the affected marrow space.

Determination of marrow space enhancement of the sphenoid, occipital, or temporal bones is best accomplished with fat saturation techniques. Infection of the clivus invariably leads to abnormal signal and enhancement of the preclival musculature, namely the longus capitis muscles and adjacent soft tissues. Given the extraosseous involvement and the aggressive appearance on both MRI and CT scan, SBO can be extremely difficult to distinguish from invasive neoplasm of central skull base (Fig. 15).85,86

Lesions of the Petrous Apex and Petrooccipital Fissure Myriad lesions involve the petrous apex; however, both CT and MRI are complimentary in their evaluation of petrous apex lesions. It is often possible to reach a diagnosis based on a combination of CT and MRI findings.

Asymmetric Fatty Marrow Asymmetric fatty marrow in the petrous apex is usually noted as an incidental finding on conventional MR images and may be

Figure 15 A 69-year-old male with neutropenic fever and invasive fungal sinusitis. (A) Axial T1-weighted image demonstrates mottled signal in the central skull base (arrow). (B) Axial postcontrast image shows heterogeneous enhancement of the central skull base consistent with skull base osteomyelitis (arrow).

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428 concerning for a radiologist. Nonpneumatized or partially pneumatized petrous apex on 1 side can be confused as an abnormal mass, particularly if the other side is completely pneumatized. Correct identification of this condition is essential to prevent misdiagnosis as well as unnecessary further workup. Asymmetric fatty marrow is bright on T1-weighted sequence and intermediate to high signal on fast spin-echo T2-weighted sequences.87 Clues to correct diagnosis are as follows: (1) signal intensity follows orbital fat on all imaging sequences; (2) the signal is suppressed on fat-suppressed sequences. If there is any doubt, particularly if only postcontrast T1 sequence is available for review (without precontrast T1-weighted sequence) or if it is confused with T1-bright cholesterol granuloma, a temporal bone CT correctly identifies nonpneumatized marrow.

Petrous Apex Effusion Petrous apex effusion, also known as trapped or retained fluid, is a leave-me-alone lesion of the central skull base most commonly discovered as an incidental finding on routine neuroimaging. This is the most common lesion at the parasagittal central skull base.88 Origin of isolated petrous apex fluid is presumed to represent previous otitis media with subsequent obstruction of outflow of the petrous apex air cells.87 Both on CT and MRI, there is isolated fluid in the petrous apex, usually on 1 side. There is no expansion of the bone or erosion of trabeculae. On MRI, effusion always demonstrates high T2 signal. T1 appearance is variable and depends on protein content of effusion. Low T1 signal intensity is characteristic of uncomplicated petrous apex effusion and if present, no further evaluation is recommended.87 If there is hyperintensity on T1-weighted sequence, further evaluation on CT scan may be warranted to confirm absence of expansion and trabecular erosions and to exclude cholesterol granuloma. Appropriate follow-up is not well defined.

Petrous Apex Cephaloceles Petrous apex cephalocele is a rare leave-me-alone lesion of the petrous apex secondary to inferior herniation of the posterolateral part of the Meckel cave into the petrous apex.89 The exact pathophysiological mechanism for the development of petrous apex cephaloceles is not known. It has been postulated that bony dehiscence is developmental and develops secondary to continuous CSF pulsation over a congenitally thin bone.52 Most cases are unilateral and asymptomatic. In rare instances, cephaloceles can present with fifth nerve dysfunction, otorrhea, recurrent meningitis, and even pulsatile tinnitus.52 On CT scan, petrous apex cephalocele is well-circumscribed, smooth expansion of the petrous apex without any cortical destruction, which demonstrate fluid signal on all MRI sequences. No treatment is necessary in the asymptomatic cases and decision of surgery in symptomatic patients is offered on a case-by-case basis.

Cholesteatoma Cholesteatoma involving the central skull base is usually primary or congenital in origin. However, the central skull

base can also be secondarily involved from the acquired cholesteatomas of the middle ear or mastoid cavity. Primary cholesteatoma arises from slow, expansile growth of epithelial cell rests (ectodermal elements) of the first branchial grooves, instead of normal regression.90 Pathologically, cholesteatoma is epithelial-lined cavity that is filled with concentric layers of keratin and stratified squamous epithelial cells within a highly structured scaffold of pseudoconnective tissue stroma.90 Enlargement of the cholesteatoma occurs as the advancing epithelial margin gradually advances in combination with surrounding bone resorption, secondary to host inflammatory response.52 Cholesteatomas remain asymptomatic for years before producing symptoms secondary to progressive mass effects. Common presenting symptoms are hearing loss, cranial nerve palsies, and headache. Typical appearance of cholesteatoma on temporal bone CT is an expansile low-attenuation lesion at the central skull base. Usually the margin of the lesions is variable and can be ill-defined with wide zone of transition (mimicking aggressive lesions) or can be narrow zone of transition with well-defined smooth margin. There is no matrix calcification. On MRI, typical appearance of cholesteatoma includes hypointensity on T1, hyperintensity on T2, and intermediate signal on fluid-attenuated inversionrecovery (FLAIR). High signal on diffusion-weighted sequence with low apparent diffusion coefficient value (diffusion restriction) is characteristic finding of cholesteatoma. On postcontrast images, there is no enhancement of the matrix. Subtle enhancement of the margin can be seen secondary to host inflammatory changes. If the central skull base is secondarily involved by cholesteatomas arising from other parts of the temporal bone, the imaging appearance of the primary tumor is similar on imaging.

Cholesterol Granuloma Cholesterol granuloma, the most common primary petrous apex lesion, always arises from a well-pneumatized petrous apex. Cholesterol granuloma arises secondary to obstruction of the outflow tracts of the petrous apex air cells. As the mucosa absorbs air, negative pressure and hypoxia develops, leading to intraluminal and submucosal extravasation of blood and serum. Catabolism of hemoglobin leads to formation of cholesterol crystals, which mount an intense inflammatory response leading to the formation of giant cell foreign body reaction.91 Characteristic microscopic features of cholesterol granulomas are chronic inflammatory granulation tissue containing large numbers of rhomboid birefringent cholesterol crystals. Abundant iron deposition is also seen around the cholesterol crystals as well as elsewhere within the matrix of the lesions. The source of iron is mainly from the degradation of hemoglobin. Alterative hypothesis of abundant intralesional iron is overexpression of lactoferrin, an internal anti-infective molecule.90 Macroscopically, cholesterol granuloma is an intraosseous cyst filled with dark, viscous, chocolate brown fluid.88 With continued slow hemorrhage, the lesion gradually enlarges in size and produces symptoms secondary to mass

Neuroimaging effect. The most common symptoms are hearing loss, vertigo, and headache. Other less common symptoms are tinnitus, otalgia, diplopia, and cranial neuropathies. Headache is the most common cause patients seek for intervention.88 On CT scan, the typical appearance of a cholesterol granuloma is an expansile, sharply defined, rounded or ovoid lesion involving aerated petrous apex with cortical thinning and trabecular break down (Fig. 16). There is no matrix calcification. In a large lesion, bony margin can be dehisced. The characteristic appearance on MRI is high signal intensity mass on both T1 and T2-weighted sequences without any suppression of signal on fat-suppressed sequences. Abundance of intralesional iron content is thought to incur the high signal on T1- and T2-weighted sequences. Small lesions may have homogenous signal characteristics. However, larger lesions are typically heterogeneous. The margin of the lesion may demonstrate a T2 hypointense rim, secondary to hemosiderin within retained macrophages. Minimal marginal enhancement may be seen on postcontrast scan secondary to inflammatory changes at the margin.

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Petrous Apex Mucocele Mucoceles are more commonly seen in the paranasal sinuses but occasionally may be seen in the parasagittal central skull base (petrous apex) if mucus secreting cells of petrous apex becomes obstructed.92 Continued production and accumulation of mucoid material in an obstructed air cell with associated inflammation lead to expansion, remodeling, and dehiscence of the margin in some cases. CT demonstrate expansile, usually isodense mass with loss of septae. MRI appearance can be complex but usually demonstrate high signal on T2-weighted sequence. Appearance on T1-weighted sequence depends on protein content. Enhancement of the central component is not a feature of mucocele and suggests an alternative diagnosis. Marginal enhancement can be seen secondary to inflammatory response.

Apical Petrositis (AP) AP typically refers to an acute infection of the pneumatized petrous apex, usually resulting from anteromedial extension of

Figure 16 A 26-year-old patient with headache and left petrous apex cholesterol granuloma. (A) Axial postcontrast CTA source image with bone window demonstrates well-circumscribed lytic lesion on left petrous apex (open arrow) without evidence of vascular enhancement. (B) Axial unenhanced T1 weighted image demonstrates typical T1 shortening within the lesion (arrow). (C) Axial T2 demonstrates heterogeneous hyperintense signal within the lesion (arrow). (D) Following gadolinium injection, the lesion demonstrates no additional T1 hyperintensity to indicate true enhancement, but might be mistaken for an enhancing mass if seen in isolation. CTA, CT angiography.

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Figure 17 A 73-year-old patient with recent mastoidectomy and tympanoplasty on the right for chronic inflammatory disease of middle ear developed fever, severe right ear pain, neck pain, sixth nerve palsy, and headache. (A) Axial T1-weighted images demonstrate postmastoidectomy changes on the right with opacification of the mastoid bowl, middle ear, and petrous apex. Abnormal signal replaces fat signal in the petrous apex (arrow). (B) Axial postcontrast T1-weighted image demonstrates marked enhancement of the temporal bone including the petrous apex (arrow). (C) Axial postcontrast fat-saturated image through the skull base demonstrates abnormal enhancement of the occipital bone (white arrow) and preclival musculature (black arrow). Note periarterial enhancement in the right carotid canal with luminal narrowing of right ICA (open arrow). (D) Coronal postcontrast image demonstrates marked enhancement in the infratemporal fossa (black arrow) as well as the petrous apex on the right (open arrow). ICA, internal carotid artery.

severe otomastoiditis.93 Gram-positive cocci are the most common causative organisms. Isolated infection of the pneumatized petrous apex without otomastoiditis is rare. If infection spreads to adjacent marrow space from the petrous apex air cells, or involves marrow space of the nonpneumatized petrous apex, the authors prefer to use the term skull base osteomyelitis to indicate a more severe infection. Overt malignant otitis externa can also secondarily infect the petrous apex directly or through the venous plexuses around the carotid artery in the carotid canal. The petrous apex can also become infected owing to invasive sinus infection. Initially intact petrous apex air cells are opacified with purulent exudates. With uncontrolled infection, the infection spreads into the adjacent bone from the air cells resulting in localized infection of the bone. Uncontrolled infection can spread to the adjacent structures resulting in meningitis, empyema, intracranial abscess, thrombophlebitis, or dural sinus thrombosis. However, complicated AP is extremely rare in this postantibiotic era. Symptoms are widely variable and

depend on the primary site as well as extent of the infection. Presentation with Gradenigo triad (deep facial pain from involvement of trigeminal nerve and its branches, diplopia secondary to sixth nerve palsy, and otomastoiditis) is rather rare. Patients present more commonly with components of Gradenigo triad. On CT scan, there is opacification of the petrous apex air cells with or without diffuse demineralization of the adjacent bones. Margin between infected bone and normal adjacent tissue is usually blurred. On MRI, there is diffuse loss of normal marrow fat in the involved bones on the precontrast T1-weighted sequence. On T2-weighted sequence, there is heterogeneous T2 hyperintensity in the involved bones. Heterogeneous enhancement is usually seen. Ring enhancement with central diffusion restriction is seen if there is abscess formation. AP used to be treated surgically in the past. Currently, however, this is treated conservatively with antibiotic. Sequential 67Ga is used to monitor response to treatment (Fig. 17).

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Figure 18 A 50-year-old female developed headache and diplopia with left sixth nerve palsy. (A) Axial CT through the skull base demonstrates a lytic lesion at the left POF with destruction of both the left petrous apex and the left side of the basisphenoid. (B) Coronal enhanced T1-weighted image with fat saturation demonstrates infiltrating lesion in the body of the sphenoid (long solid arrow), with extensive enhancing soft tissue expanding the cavernous sinus (short solid arrow) and associated narrowing of the left cavernous ICA (open arrow). POF, petrooccipital fissure; ICA, internal carotid artery.

Inflammatory Pseudotumor

Chondrosarcoma

Inflammatory pseudotumor of the central skull base is an extremely rare, locally aggressive lesion frequently misdiagnosed as aggressive tumor of the central skull base on imaging. There is no specific imaging appearance. The diagnosis is usually made after biopsy is undertaken to exclude malignancy or invasive infection. Fibroblastic proliferation and presence of inflammatory cells are usually seen on microscopy. Pseudotumors of the skull base may be predominantly extraosseous and involve the meninges or extracranial or orbital soft tissues but can occasionally create localized skull base destruction as well (Fig. 18). The lesions may involve facial nerves and the bony labyrinth. Surgical resection is usually offered as the primary therapy with adjuvant steroids and radiation therapy in patients with residual or recurrent disease.94,95 Some lesions show dramatic improvement, at least initially with steroid or methotrexate therapy.

Chondrosarcoma is an uncommon, slow-growing, malignant tumor arising from cartilage-producing cells and comprising approximately 0.1% of all intracranial tumors and 6% of all skull base tumors.96 Most chondrosarcomas arise from the parasagittal central skull base including the sphenopetroclival fissure, petrous apex, and the posteromedial aspect of the petrous part of the temporal bone, between the internal auditory canal and jugular fossa. Residual islands of cartilaginous cell rests are present in these areas either from the cartilage cell remnants of the central skull base synchondroses (petroclival and sphenooccipital) or from embryonic remnants (from endochondral ossification) within skull base bones.97 Chondrosarcomas can also arise from the midline in up to 13% of cases, particularly if it is associated with Ollier disease or Maffucci syndrome.98,99 Most of the chondrosarcomas at the skull base arise de novo from the normal bone. Rarely, secondary chondrosarcoma can arise from other cartilaginous

Figure 19 A 38-year-old male with right sixth nerve palsy and low-grade chondrosarcoma. (A) Axial CT through temporal bone shows well-circumscribed lytic lesion (arrow) involving the right petrous apex and petrooccipital fissure. (B) Axial T2-weighted image demonstrates marked hyperintensity within the expansile mass (arrow). (C) Axial postcontrast T1-weighted image demonstrates diffuse enhancement of the POF chondrosarcoma (arrow). POF, petrooccipital fissure.

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Figure 20 A 23-year-old male presents with headache, visual loss, and hypopituitarism. Biopsy proved intermediate-grade chondrosarcoma. (A) Axial CT with soft tissue windows demonstrates a large expansile mass within the sella that contains multifocal and confluent calcifications (arrow). (B) Axial CT at the same level with bone window setting demonstrate arcs and whorls suggestive of chondrosarcoma (arrow). (C) Sagittal precontrast T1-weighted image shows a large predominantly low-signal lesion (arrow) expanding the sella and extending into the suprasellar cistern. (D) Coronal postcontrast T1-weighted image shows homogeneous enhancement of this unusual centrally located chondrosarcoma (arrow).

tumors, or it may be associated with Paget disease, Maffucci syndrome, and Ollier disease.97 Chondrosarcoma has different histopathologic variants. Conventional chondrosarcomas are either hyaline and myxoid types or combination of the 2. Histopathologically, hyaline types are neoplastic chondrocytes residing within lacunar spaces surrounded by hyaline matrix. On the contrary, myxoid types have areas of chondrocytes that are surrounded by frothy mucous.97 Mesenchymal and dedifferentiated variants are anaplastic in nature and have more aggressive course and poor prognosis. Fortunately, these variants are rare, constituting only 10% of the skull base chondrosarcomas.98 Histopathologically, chondrosarcomas are separated from chordomas by demonstration of morphologic features of chondroid differentiation, positive immunohistochemical staining for S-100 protein, and negative staining for epithelial cell markers (cytokeratin and epithelial membrane antigens) that are typically present in chordomas.98 Rosenberg et al99 have graded skull base chondrosarcomas into 3 types based on cellular differentiation pattern: grade I (well differentiated),

grade II (moderately differentiated), and grade III (poorly differentiated). The most frequent type is grading I (50.5%). Higher-grade tumor has a poorer prognosis. Chondrosarcomas are slow-growing but locally invasive tumors. They can grow medially to involve the clivus, superiorly to involve cavernous sinus, and laterally to involve the posteromedial petrous bone. Clinical feature depends on location of the tumor and mass effect over the adjacent structures. Depending on the published literature, either diplopia (secondary to involvement of the sixth cranial nerve) or headache is the most common presentation.52,100 If the tumor is large enough, it can invade any of the lower cranial nerves. Involvements of cranial nerves III to XII have been described in skull base chondrosarcomas.97 On CT, chondrosarcomas can have variable appearance, depending on the amount of chondroid matrix present. CT images usually demonstrate expansile lytic lesion at the central skull base with a significant soft tissue component that may appear slightly hyperdense on noncontrast CT with variable degree of enhancement. Chondroid-type calcifications (arcs

Neuroimaging and whorls) in the tumor matrix are characteristics of chondrosarcomas but not always present. Usually, the zone of transition is very sharp but is not sclerotic (Fig. 19). MRI can best evaluate skull base chondrosarcomas. Usual appearance on MRI is multilobulated, isointense to hypointense on T1 and fairly hyperintense on T2-weighted sequence. Enhancement with contrast is highly variable but not profound. Small calcifications are not seen on MRI but larger calcifications can be easily seen. From a practical standpoint, it may be difficult to distinguish chondrosarcomas from chordomas in terms of CT and MRI features. Location may be helpful. Chordomas are traditionally considered midline lesions, whereas chondrosarcomas more typically arise off midline. However, this distinction is not always reliable (Fig. 20). Optimal treatment of chondrosarcomas is variable. However, gross total resection is always performed if feasible. There is no effective chemotherapy for chondrosarcomas. Radiation to the surgical margin may be given if there is any concern for positive surgical margin/gross residual tumor. Some institutes routinely radiate the tumor site after resection. There is recent interest in application of newer technologies (proton beam therapy or carbon ion therapy) to skull base chondrosarcomas with improved overall survival rate and better local disease control. However, these modalities are not widely available and support for their use has yet to be established.101,102

Conclusion There are a number of pathologic conditions that arise from or principally affect the osseous and cartilaginous structures of the central skull base, affecting the imaging of central skull base region. We have reviewed the pertinent imaging features of the traditional lesions arising from the central skull base, including metastatic disease, chordoma, and chondrosarcoma. In addition, we have attempted to broaden this central skull base spectrum by including pathologies of the sphenoid sinus and petrous apex in our discussion. This comprehensive approach, though daunting, more realistically reflects the neuroimaging challenges put forth by lesions in the region.

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Neuroimaging: intrinsic lesions of the central skull base region.

The sphenoid bone is the osseous foundation of the central skull base. The body of the sphenoid is cuboid in shape and its posterior margin is joined ...
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