The skull base is a complex anatomical region that harbors many important neurovascular structures in a relatively confined space. The pathology that can develop at this site is varied, and many disease processes may present with similar clinical and neuroimaging findings. While computed tomography maintains a role in the evaluation of many entities and can, for instance, delineate osseous erosion with great detail and characterize calcified tumor matrices, magnetic resonance imaging (MRI) is the mainstay in the neuroimaging assessment of most pathology occurring at the skull base. Various MRI sequences have proven to be robust tools for tissue characterization and can provide information on the presence of lipids, paramagnetic and diamagnetic elements, and tumor cellularity, among others. In addition, currently available MRI techniques are able to generate high spatial resolution images that allow visualization of cranial nerves and their involvement by adjacent pathology. The information obtained from such examinations may aid in the distinction of these disease processes and in the accurate delineation of their extent prior to biopsy or treatment planning.
Characterization of sellar and parasellar lesions is challenging due to the anatomical complexity of the skull base, the extensive breadth of pathology that one may encounter, and the similar imaging appearance and clinical presentations of some entities. The presence of various critical neurovascular structures in a confined space complicates surgical access for tissue diagnosis or resection and underscores the importance of appropriate imaging. We review relevant neuroimaging aspects of sellar and parasellar lesions with particular attention to the anterior skull base.
RELEVANT ANATOMY AND EMBRYOLOGY
The adenohypophysis derives from a diverticulum that arises from the primitive oral cavity (Rathke's pouch) and projects toward the central skull base at around weeks 4 and 5.1,2 A diencephalic infundibulum then projects inferiorly and contacts the diverticulum, which loses its connection to the oral cavity.2 The anterior wall of the pouch fills with cells and forms the pars distalis, leaving a small cleft between it and the posterior wall, which becomes the pars intermedia.3 A small infundibular process grows superiorly and becomes the pars tuberalis.2 The posterior diencephalic tissue evolves into the neurohypophysis.4
Anatomy of the Sphenoid Bone
The sphenoid bone is the largest component of the anterior skull base. Understanding its complex anatomy is key in evaluating sellar and parasellar pathology due to the popularity of endoscopic approaches to access many of these lesions.5 The sphenoid bone holds the pituitary gland within the sella and adjacent structures, including cavernous sinuses and traversing cranial nerves. It features numerous foramina and fissures through which neurovascular structures pass. The sphenoid body is situated in the midline and has a cuboidal shape. Anterolaterally and superiorly, it extends as the greater wings forming the anteromedial aspects of the middle cranial fossae. The carotid sulci are located above their attachment to the sphenoid body lodging the internal carotid arteries and lateral cavernous sinuses. The lesser wings project posterolaterally, forming the superior orbital fissures between themselves and the greater wings. The flat surface connecting the lesser wings is the planum sphenoidale.
The sella is a saddle-shaped concavity in the sphenoid body that is devoid of a bony covering laterally and superiorly. The pituitary gland is lodged in the sella and bounded laterally by cavernous sinuses, which are large venous plexuses between inner and outer layers of dura mater. The cavernous sinuses are interconnected via channels crossing the midline along the anterior, inferior, and posterior pituitary surfaces. A reflection of the inner dural layer above the pituitary gland forms the diaphragma sellae, which has a variable-sized opening for the infundibulum.
Magnetic resonance imaging (MRI) is a mainstay in the evaluation of sellar and parasellar pathology, due to intrinsic high-contrast resolution and availability of advanced sequences that offer high spatial resolution. Isotropic CISS (constructive interference in steady state) or its analog FIESTA-C (fast imaging employing steady-state acquisition cycled phases) are derived from steady-state free precession (SSFP) sequences that are acquired in a way that eliminates phase artifacts and generates fluid-sensitive images with high spatial resolution in a short period of time.6 The isovolumetric data provide excellent contrast between fluid and surrounding tissues and can be reconstructed in any plane.7 CISS and FIESTA-C are particularly useful for delineating the cisternal segments of the cranial nerves due to the bright signal from surrounding cerebrospinal fluid (CSF). Administration of intravenous contrast material in SSFP-based sequences allows visualization of the interdural cranial nerve segments and may be able to depict their relationship with skull base lesions.8,9 High-resolution cranial nerve MRI should be performed at a minimum of 1.5 Tesla and ideally at 3.0 Tesla, as the latter provides higher signal-to-noise ratio and increases the conspicuity of cranial nerves.10 Isovolumetric gradient echo T1-weighted MR sequences (volumetric interpolated brain examination, VIBE, or magnetization-prepared rapid acquisition with gradient echo, MP-RAGE) allow for multiplanar reconstructions. However, contrast enhancement may be less conspicuous compared to spin-echo images.11
For the pituitary gland, thinner slices are acquired with smaller field of views centered at the sella. Dynamic contrast-enhanced sequences are obtained in the coronal plane and acquired every 30 s for 3 min following intravenous contrast injection (Table). These take advantage of the different contrast dynamics of adenomas in relation to pituitary tissue and may be useful for evaluation of small tumors.12,13 Most adenomas enhance more slowly (peak between 60 and 200 s) than the briskly and avidly enhancing pituitary tissue.14
|Sequence||Slice thickness (mm)|
|Sequence||Slice thickness (mm)|
aDynamic scan with 5 slices per acquisition every 30 s for 3 min.
Adenomas account for 10% to 15% of intracranial tumors.15 They have a prevalence of 17% based on a meta-analysis of autopsy and imaging data.16 Adenomas have been arbitrarily classified into microadenomas (<10 mm) and macroadenomas (≥10 mm). Microadenomas may be difficult to detect due to their size but nonetheless be highly symptomatic, while macroadenomas have the potential to exert mass effect. Adenomas can also be classified according to their cellular origin into lactotrophs, somatotrophs, gonadotrophs, corticotrophs, and thyrotrophs. In an epidemiologic study, 74% of macroadenomas and 22% of microadenomas were nonfunctioning.17 Functioning adenomas secrete prolactin in 25% to 41% and adrenocorticotropic and growth hormones in 5% and 2.8% of cases, respectively.16
Identification of small lesions may benefit from dynamic contrast-enhanced MRI or, rarely, computed tomography (CT) techniques (usually when MRI cannot be performed) based on the differential rate of enhancement of adenomas compared to pituitary tissue. Normal pituitary parenchyma shows homogeneous enhancement at 60 to 80 s following administration of contrast material.18 Maximum image contrast between adenomas and pituitary tissue occurs at about 1 min and gradually decreases thereafter (Figure 1).19 However, an adenoma may enhance earlier than pituitary tissue due to direct arterial supply.18 While signal intensity on T2-weighted sequences is variable (particularly in large tumors) due to hemorrhage, cysts, or necrosis,20 T2 hypointensity has been commonly reported in growth-hormone-producing adenomas.21,22
Large adenomas usually infiltrate the gland; therefore, a displaced but otherwise normal-appearing pituitary gland may be helpful in ruling out an adenoma. Up to 10% of adenomas invade the cavernous sinuses (more frequently larger lesions) and tend to be biologically aggressive, with increased surgical morbidity and mortality.22-24 Imaging features that may be indicative or highly suggestive of cavernous sinus invasion (or absence of such) are usually assessed on coronal pre- and postcontrast T1-weighted sequences.24,25 In one study, greater than 67% encasement of the internal carotid artery by tumor (about 241°) yielded a 100% positive predictive value (PPV) for invasion as confirmed by surgery.24 Other imaging findings included obliteration of the carotid sulcus venous compartment (95% PPV) and tumor extension beyond the lateral intercarotid line (85% PPV).24 Features predicting absence of invasion include less than 25% of carotid encasement, presence of pituitary tissue between the tumor and carotid artery, and tumor not extending beyond the medial intercarotid line (Figure 2).24
Five percent to 14% of surgically treated adenomas are classified as “giant” (greater than 4 cm).15 Despite their benign histology, such tumors can infiltrate the skull base and, rarely, extend to the nasopharynx (Figure 3).26 Large adenomas undergo spontaneous infarction at a greater rate than any other central nervous system (CNS) tumor, probably due to outgrowth of their blood supply, vascular compression from expansion, or other intrinsic features.27,28 This occurs with or without hemorrhage and may lead to pituitary apoplexy, which can rarely be complicated by retroclival hematomas.27,29 Pituitary carcinomas are exceedingly rare, and distant spread is the only imaging feature that may distinguish them from adenomas.30
Meningiomas constitute the most common primary CNS neoplasms, with an incidence of 7 in 100 000 individuals according to one population-based study.31 They are 3 times more common in females and are largely tumors of adulthood, with greater than 70% occurring after age 55.31 Meningiomas are rarely seen in children unless syndromic or associated with radiation (less than 2% occur in patients 20 yr of age or younger).32 With 15 subtypes in the 2007 WHO classification, they are histologically heterogeneous, although most (90%) are benign and classified as grade I.33 Six percent of meningiomas are anaplastic (grade II) and 5% frankly malignant (grade III).33 Meningiomas arise from arachnoid cap cells in the leptomeninges, which derive from the mesenchyme and neural crest.34,35 They are almost always dural-based (but may be intraventricular or very rarely extracranial) and commonly occur along dural reflections. Approximately 40% arise in the skull base.36 Roughly one-half of anterior skull base meningiomas arise at the sphenoid wings and the remainder from the tuberculum sellae, limbus sphenoidale, and chiasmatic and olfactory grooves (Figure 4).37 Therefore, extension frequently occurs into the optic canals, cavernous sinuses, or sella (Figure 5). Arterial encasement may be present, which can lead to stenosis.38 Anterior skull base meningiomas can result in abnormal dilatation of an adjacent paranasal sinus (pneumosinus dilatans).39
On noncontrast CT, meningiomas tend to be iso- to hyperdense to cerebral cortex and are difficult to visualize unless they are large or calcified or there is hyperostosis, which appears to be more common in the skull base (occurring in 50% of patients) compared to convexity meningiomas.40 On MRI they are usually well circumscribed, iso- to hypointense on T2, and show avid contrast enhancement, although this will vary according to the degree of calcification or, rarely, cystic degeneration. Although not entirely specific, identification of a dural tail may suggest a meningioma if the original definition is applied: the tail must (a) be thicker closer to the tumor and taper peripherally, (b) enhance to a greater degree than the tumor, and (c) be seen in 2 consecutive tumor sections and more than one plane.41,42 Studies on their apparent diffusion coefficient (ADC) characteristics have yielded variable results.43
Craniopharyngiomas are nonglial epithelial tumors arising from remnants of Rathke's pouch or rests of buccal mucosa at any point along the trajectory of the craniopharyngeal duct.44 While their most common site of origin is the infundibulum, where squamous epithelial rests are known to occur, they may rarely arise primarily within the third ventricle, sphenoid bone, or even nasopharynx.45 They are rare, with an incidence of 0.5 to 2 cases per million persons per year, up to half occurring during childhood and adolescence.46 They constitute 4% of intracranial neoplasms in the pediatric population and have a bimodal distribution, with peaks at 5 to 14 and 50 to 75 yr of age.47,48 Histologically, craniopharyngiomas are almost always benign and have a high survival rate, but they can be locally aggressive and may be associated with significant morbidity.46 Those occurring in childhood are more commonly of the adamantinomatous type and present as heterogeneous cystic and solid suprasellar masses (Figure 6). Approximately 90% of these have calcifications that can be readily identified on CT.44,46 The solid portions show contrast enhancement, and on MRI the cystic components may present with variable signal intensities depending on their contents of protein, cholesterol, or hemorrhage.44 Tumors in adulthood are more commonly of the papillary type and are solid, less commonly calcified, and devoid of cysts.44,46 Due to their preferred infundibular location, age at presentation, and solid appearance with lack of calcification or cystic changes, papillary craniopharyngiomas may be difficult to distinguish from germinomas. While they both usually show strong contrast enhancement, germinomas have lower ADC values by virtue of their higher grade histology and cellularity compared to papillary craniopharyngiomas.49
Most chordomas are histologically low-grade but locally aggressive tumors derived from embryonic remnants of the notochord.50 Three subtypes have been recognized, with the conventional form accounting for the majority, followed by chondroid chordoma with cartilaginous elements and the rare dedifferentiated chordomas, which are malignant and have the worst prognosis.51,52 Half of all chordomas are sacrococcygeal, one-third occur in the skull base, and a minority arise in the spine.51 They are twice as common in males, have a peak incidence between 50 and 60 yr of age, and are rare in children and adolescents.52 These tumors most frequently occur midline at the spheno-occipital synchondrosis, although the chondroid subtype has a tendency to arise laterally at the petroclival junction.50,53 Chordomas are extradural and almost always originate in bone, which may lead to bony sequestra (the chondroid variant may have a true calcific matrix) that can be demonstrated on CT.53 On MRI, chordomas are well circumscribed with a pseudoencapsulated appearance and markedly bright on T2-weighted sequences, probably secondary to mucin and/or necrosis.53 T2 signal in the chondroid variant may be relatively low due to cartilaginous tissue.54 Interlobular septa are formed by epithelioid cells and appear hypointense on T2 with variable degrees of contrast enhancement (Figure 7).53 Chordomas are soft tumors that tend to displace or encase blood vessels, but stenoses are rare.55 Dural transgression can be present, and tumor can extend into the subarachnoid space, increasing the risk of CSF leaks and infection.56 Dural disruption may be identified on T2-weighted imaging but is best visualized on high-resolution SSFP-based sequences, where administration of contrast may aid in delineation of tumor against nonenhancing dura and depiction of its relationship to cranial nerves (Figure 7).8
Chondrosarcomas are often described together with chordomas, as they share similar imaging features, clinical presentations, and locations. Both are rare and usually present in adults between the fourth and fifth decades of life and together constitute less than 1% of intracranial tumors.50,57 Chondrosarcomas represent 6% of all skull base tumors, and only 1% of them occur at this site.58 Compared to chordomas, conventional chondrosarcomas are relatively indolent and have a more favorable prognosis.50 Metastases and recurrence are rare, except for the unusual dedifferentiated subtype, which has an aggressive course.59 While pathogenesis is uncertain, chondrosarcomas are believed to arise from chondrocytes within endochondral cartilage rests and are commonly located at the petroclival (two-third of cases), spheno-occipital, and sphenopetrosal synchondroses, therefore more commonly off-midline compared to chordomas.60,61 CT demonstrates osseous erosions and destructive changes usually with a calcified chondroid matrix that may have typical “rings and arcs” or nonspecific foci of amorphous calcifications.62 Similarly, on MRI, chondrosarcomas are usually well circumscribed and hyperintense on T2-weighted sequences, with variable degrees of heterogeneous contrast enhancement (Figure 8).50,57,61 As opposed to chordomas, chondrosarcomas more often present with ophthalmoplegia, presumably due to their preferred lateral location.61 Finally, while some of their imaging features overlap, a recent study using diffusion-weighted imaging suggests that chondrosarcomas have significantly higher ADC values than chordomas.63
Schwannomas are benign, slow-growing neoplasms that arise from Schwann cells. They make up 8.5% of intracranial tumors and are usually seen in adulthood, the overwhelming majority originating from cranial nerve (CN) VIII (90%), followed by CN V (1%-8%) and CN VII.64 In the parasellar region, CN V schwannomas predominate, while CN III, IV, and VI schwannomas are rare, except in the setting of neurofibromatosis type 2.64 The most common location of CN V schwannomas is the gasserian ganglion, but they can also affect the cisternal or postganglionic segments, and the cavernous sinus is often involved.65,66 Most patients present with CN V dysfunction, although symptoms related to compression of CN VI within Dorello's canal may be present.65 On MRI, schwannomas are iso- to hypointense on T1 and hyperintense on T2-weighted sequences and show avid contrast enhancement (Figure 9).65,67 Their heterogeneity varies according to the presence of cystic changes or, rarely, hemorrhage or calcifications.65-67 Melanotic schwannomas are rare and show intrinsic T1 hyperintensity, in which case they may be confused with lipomas, although the latter suppress on fat-saturated sequences.59,68,69
Optic Pathway Glioma
These lesions represent 3% to 5% of brain tumors in children, and the great majority are diagnosed before age 20.70 They account for 65% of all optic nerve tumors, and roughly one-third are associated with neurofibromatosis type 1.71 They are, in fact, the most common CNS neoplasms in this syndrome, in which they have been reported in 7% to 20% of children.70 Syndromic optic nerve gliomas almost always develop before age 4 and are low grade and generally indolent.72 While it appears that sporadic gliomas are associated with worse outcomes, their presentation differs from syndromic tumors in that the latter are usually found in asymptomatic patients as part of routine screening and their natural history has not been conclusively demonstrated.73 Syndromic status also has implications for tumor location. Optic pathway gliomas in neurofibromatosis type 1 more frequently involve the optic nerves, while sporadic ones favor a chiasmatic or postchiasmatic location (Figures 10 and 11).72,73 On MRI, they infiltrate and cause enlargement of optic nerves, chiasm, and/or optic tracts. They are iso- to hypointense on T1 and hyperintense on T2-weighted sequences and usually well demarcated.73 Enhancement is variable and does not correlate with tumor grade.74 Calcifications are uncommon and hemorrhage is exceedingly rare.70,72,75 Cysts may be present and are more common in sporadic tumors (Figure 12).72 Increased T2 signal is generally not useful to evaluate involvement of the optic tracts, as it may be also related to edema.72 Postcontrast images may be helpful in this regard, since enhancement indicates tumor.74
Germ Cell Tumor
Germinomas represent 50% to 70% of intracranial germ cell tumors, with the rest belonging to the nongerminomatous subtype.76 Germ cell tumors are predominantly seen in children and account for nearly 4% of pediatric intracranial tumors in the United States, most of them diagnosed between 5 and 14 yr of age.48 Pure germinomas are usually diagnosed in patients between 10 and 21 yr of age, while nongerminomatous tumors are more often seen in younger children.76 The incidence of intracranial germ cell tumors is significantly higher in Asian countries and also in patients of Asian descent living in the United States.77 Alfa-fetoprotein and β-human chorionic gonadotropin are commonly found in CSF and serum and may be helpful markers for diagnosis.76,78
Intracranial germ cell tumors favor a midline location around the third ventricle.79 The majority of lesions arising in the pineal region are nongerminomatous, while suprasellar ones are more likely to be germinomas, with these 2 sites constituting the most common locations.80 Five percent to 10% of germinomas are ectopic and occur in basal ganglia or thalami.81 These have an increased incidence in Asia and may be associated with ipsilateral cerebral or brainstem atrophy.82 Rarely, germinomas may occur within the sella turcica.30 While pineal germ cell tumors are most common in males, suprasellar ones are slightly more frequent in females.30,78 Pure germinomas are exquisitely radiosensitive, with survival rates greater than 90% at 5 yr with radiotherapy alone.76
The earliest finding of a suprasellar germinoma may be absence of the pituitary bright spot due to disruption of the infundibulo-neurohypophyseal system.30 Enhancement is typically avid and more homogeneous in germinomas compared to nongerminomatous tumors, which are more commonly hemorrhagic.83 Germinomas are highly cellular and, as a result, show restricted diffusion on MRI and are usually hyperdense on CT.83,84 Compared to nongerminomatous tumors, one study suggests that germinomas may show lower ADC values on MRI (Figure 13).85 However, conventional imaging characteristics of germinomas and nongerminomatous tumors are similar, and reliable discrimination between histologic subtypes is not possible.76 These lesions are iso- to slightly hyperintense on T2-weighted images, and T1 signal intensity is variable.83,84 Cystic changes may be seen, and calcifications are unusual (as opposed to pineal region tumors, in which calcifications may “engulf” the pineal gland).84 Germ cell tumors have a high propensity to seed CSF, and therefore imaging of the entire neuraxis is warranted.30
Plasma cell neoplasms can present systemically as multiple myeloma or as discrete masses in the form of plasmacytomas.86 Plasmacytomas are usually intramedullary and solitary but may occasionally be multiple or extramedullary.86 By definition, solitary plasmacytomas occur without other stigmata of multiple myeloma, such as anemia, renal insufficiency, or scattered skeletal lesions.87 Skull base plasmacytomas usually arise from the nasopharynx, clivus, or petrous apex.88 The great majority of patients with a solitary plasmacytoma will develop multiple myeloma, although this appears to be less common with extramedullary tumors.88,89 While the imaging appearance is nonspecific to allow a preoperative diagnosis, one of their most characteristic features on MRI is perhaps the intrinsic T1 hyperintensity of some tumors, presumably related to densely packed cells and a low water content (Figure 14).88 As opposed to chondrosarcomas or chordomas, which are markedly hyperintense on T2, plasmacytomas are relatively isointense to gray matter due to high cellularity and lack a calcific matrix or areas of sequestered bone.88,90 Plasmacytomas enhance avidly and homogeneously, which are also features of lymphoma, but the latter tends to have intermediate to low signal intensity on both T1 and T2-weighted sequences.30
Metastases to the skull base occur in approximately 4% of cancer patients.91 While various tumors metastasize to this region, in one literature review, the most common primaries in order of frequency were prostate and breast cancer, lymphoma, and lung cancer.92 Most skull base metastases probably spread hematogenously, although seeding may also occur through the valveless Batson venous plexus, which connects the pelvic and thoracic venous systems with intracranial veins and sinuses.92,93 Additionally, the skull base can be involved by direct extension of nasopharyngeal or sphenoid tumors or via perineural spread, commonly along branches of CN V.94,95 Infiltration can lead to a distinct parasellar syndrome, with neuropathies frequently involving CNs III and V.96 On MRI, osseous metastases are typically identified as hypointense geographic areas of marrow replacement on noncontrast T1-weighted sequences. Because marrow fat is normally hyperintense on both T1- and T2-weighted sequences, fat saturation is useful to visualize metastases after administration of intravenous contrast material (Figure 15). Diffusion-weighted imaging may improve detection of calvarial metastases by showing areas of increased marrow signal, with the exception of prostate cancer, which is primarily sclerotic.97,98
In contradistinction, pituitary metastases are relatively rare. They are most frequently secondary to lung and breast cancer (more than one-half of cases), followed by renal and colorectal cancer.99 Pituitary metastasis present as a mass involving the pituitary stalk or gland, may have a waist at the diaphragma sellae, can extend cranially to involve the hypothalamus, and the majority show loss of the pituitary bright spot and avid contrast enhancement.99 Their distinction from adenomas is difficult. While smooth expansion of the sella favors an adenoma, erosion of the sella or skull base does not necessarily indicate a metastasis, as macroadenomas can be large and infiltrative.100 Edema along the optic tracts may be present but is a nonspecific finding that can be seen in a variety of tumors.101
Rathke Cleft Cyst
These are benign, true epithelial cysts believed to arise from remnants of Rathke's pouch due to incomplete obliteration of the embryonic cleft.30 They are more common in females, with symptomatic patients usually presenting between ages 30 and 60, although they may be seen at any age.30,102 While the majority of them are small, asymptomatic, and found incidentally (in up to 33% of pituitary glands at autopsy), they may occasionally grow and cause mass effect.102,103 Their contents are almost always homogeneous, but their signal characteristics on MRI vary according to the presence of protein, mucopolysaccharides, and, rarely, hemorrhage.104 They are located centrally or near centrally within the pituitary gland (typically without deviation of the stalk) between the pars distalis and the pars intermedia but may also have suprasellar extension (Figure 16).18,102 While they are frequently hyperintense on T2-weighted images, lesions that are homogeneously T2 hypointense are highly suggestive of the diagnosis, as is the presence of a T1-hyperintense and T2-hypointense intracystic nodule (representing a mucinous mass with cholesterol and protein) (Figure 17).103,105,106 Aside from intracystic nodules and epithelial rests, they do not have solid components, do not enhance with contrast material, and rarely calcify.104 Rathke cleft cysts may rarely be complicated by infection, hemorrhage, or chemical meningitis.107,108
Dermoid and Epidermoid Tumor
Both of these nonneoplastic lesions develop from inclusion of ectodermal remnants during neural tube closure and differ histopathologically in that dermoids contain dermal appendages and epidermoids do not.109,110 They both accumulate desquamated debris from their capsules and grow slowly.109 Despite being congenital, they are infrequently found in children and usually manifest later in life.109 Both are rare, but dermoids are less common.108 Their location may vary, but dermoids are more commonly found midline while epidermoids tend to occur off-midline or asymmetrically to one side.108,110 Between 40% and 50% of intracranial epidermoids occur in the cerebellopontine angles, 30% are parasellar, and 25% are primarily intraosseous with potential involvement of the subdural space.111
On CT, most epidermoids have low density that approaches CSF and may be difficult to distinguish from arachnoid cysts. Increased density in a minority of them may result from hemorrhage, increased protein, or saponification.112,113 On MRI, their T1 and T2 signal intensities are usually close to those of CSF, but the lesion does not suppress on fluid-attenuated inversion recovery (FLAIR) sequences, on which it shows a “dirty” appearance (Figure 18).114 Classically, they demonstrate very high (“light bulb bright”) signal intensity on diffusion-weighted sequences, with ADC values that may not be as low as expected, probably from a combination of restricted diffusion and T2 shine-through effects.114,115 Calcification is present in 10% of epidermoid tumors.116 Dermoids tend to follow fat characteristics on CT and MRI but may be hypointense on T2 (Figure 19). Except for a thin peripheral rim in some, dermoids and epidermoids do not enhance.116,117 Malignant transformation to squamous cell carcinoma is rare and can be seen in both lesions but is more common in epidermoids.118
Arachnoid cysts are benign nonneoplastic structures filled with CSF and lined by arachnoid cells and collagen.119 They are more common in males and have an imaging prevalence of 1% to 3%, with 75% found in children.120,121 Between one-third and one-half are located in the middle cranial fossa, followed by the retrocerebellar region.121,122 Growth is rare for incidentally found cysts and the majority are asymptomatic, although they occasionally may result in obstructive hydrocephalus, cranial neuropathies, seizures, or headaches.121 Lack of or incomplete communication between the cyst and subarachnoid space is thought to be a factor in the development of mass effect and progressive symptoms.123 While this has traditionally been assessed with CT cisternography, more recent studies have focused on the use of phase-contrast MRI to document jet-like flow within communicating arachnoid cysts or on the demonstration of cyst walls using SSFP sequences (Figure 20).123-125 Arachnoid cysts are well-demarcated, expansile, and unilocular lesions without internal architecture, which often result in osseous remodeling.83,126 Most arachnoid cysts follow CSF signal intensity on all MRI sequences and suppress on FLAIR, except for some that may contain increased proteins or hemorrhage.126
These are benign lesions consisting of neuronal and glial tissues that arise from the inferior hypothalamus or tuber cinereum.127,128 They are classically associated with gelastic seizures, precocious puberty, and developmental delay, although there is considerable variability in their presentation.129 Hamartomas can develop along the floor of the third ventricle or within the hypothalamus proper or be pedunculated, often with attachments to the mammillary bodies or tuber cinereum.129,130 On MRI, they do not strictly follow gray matter signal intensity, and most are mildly hyperintense on T2 (probably due to glial tissue) and iso- to hypointense on T1-weighted sequences (Figure 21).128-130 Like normal brain parenchyma, hamartomas have a functional blood–brain barrier and should not show contrast enhancement.130
Most aneurysms are thought to develop from a combination of hemodynamic stress and acquired or inherited factors (including connective tissue disorders) that result in progressive weakening of the parent vessel.131 Giant aneurysms are greater than 25 mm in diameter and are rare but important lesions that may occur in the suprasellar or parasellar regions.132 Their recognition is critical due to treatment implications and, most importantly, to avoid a catastrophic biopsy. Giant aneurysms can cause osseous erosion of the skull base and may occasionally extend to the parapharyngeal space, paranasal sinuses, or infratemporal fossa.133 Their imaging characteristics may be misleading, with heterogeneous signal intensities and incomplete enhancement owing to thrombosis and blood at various stages. Areas of profound T2 hypointensity within a skull base mass may represent a vascular flow void, and the lesion should be carefully scrutinized to rule out an aneurysm. Pulsation artifact in the phase-encoding direction may be a useful clue when present (Figure 22).
Autopsy studies have found subclinical CNS involvement in 25% of patients with systemic sarcoidosis.134,135 This can involve any part of the brain, meninges, cranial nerves, or calvarium, and its appearance is nonspecific, with leptomeningeal or pachymeningeal thickening and enhancement and/or cerebral or soft-tissue granulomata.136 Infiltration of the pituitary gland and infundibulum/hypothalamus is rare and generally proportional to disease severity but can rarely occur in isolation (which may mimic the appearance of lymphocytic hypophysitis or Langerhans cell histiocytosis).135 More commonly, hypothalamic/pituitary involvement is associated with nodular leptomeningeal basilar enhancement.135-137 As with other granulomatous infections, sarcoid lesions may be hypointense on T2-weighted images, but this finding is nonspecific (Figure 23).137
Lymphocytic hypophysitis is an infiltrative autoimmune/inflammatory disorder that may affect the anterior or posterior pituitary gland or infundibulum and is characterized by variable degrees of pituitary dysfunction.138 It is most commonly seen in pregnant or postpartum patients but has also been linked to a number of autoimmune conditions, including drug related (eg, ipilimumab).139 Its imaging appearance is nonspecific and may be indistinguishable from other infiltrative processes such as Langerhans cell histiocytosis and may also be confused with primary or metastatic neoplasms. It usually leads to thickening and avid enhancement of the affected portions of the gland, sometimes with absence of the pituitary bright spot (Figure 24).140 In some patients, a T2-hypointense area is seen in the parasellar region, which may suggest the diagnosis.138
The sellar and parasellar regions are relatively small areas of the skull base that harbor important neurovascular structures and feature complex anatomy. The pathology that can occur at these sites is varied, and some of the diseases may present with similar clinical and imaging characteristics. Knowledge of relevant neuroimaging features may assist in characterization of these lesions, delineation of their extent, and awareness of potential mimics prior to diagnostic biopsy or resection.
The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.