INTRODUCTION
Meningiomas are the most common intradural, extra-axial lesions of the CNS, accounting for 15% to 18% of intracranial neoplasms in adults (1). They originate from the meningothelial cells of the arachnoid villi (6), the so–called arachnoid "cap" cells, which are located in the arachnoid granulations, roof of the third ventricle, and choroid plexus (3). The locations of these arachnoid cells give rise to the typical sites of meningiomas, which will be detailed below.

DEMOGRAPHICS
Meningiomas are common in adults; however, they are rare in childhood and account for approximately 5% of pediatric CNS neoplasms (3). There is a slight male predilection for meningiomas seen in childhood (5). The peak incidence is at approximately 45 years of age, although a wide range exists, extending from the third decade to the seventh decade (1). Meningiomas are two to four times more common in females. A more striking female predominance is seen in the spine, such that females are ten times more likely to be diagnosed with meningiomas in this region (6). Some authorities believe this female predilection to be related to hormonal influences. Studies by Brentani et al., have demonstrated the presence of gonadal hormone receptors within meningiomas. Receptors have been documented which are estrogen and progesterone–sensitive. These receptors could account for the growth of meningiomas during pregnancy, as well as their association with pituitary adenoma and breast carcinoma (3, 5). A summary of the demographics of meningiomas is listed in Table 1.

PATHOLOGY
Upon gross inspection, meningiomas typically have a rounded or globular, well circumscribed appearance. They exert mass effect upon the underlying brain parenchyma but are readily separated from it. Although it is less common, meningiomas may have a bi–lobed, dumbbell shape (characteristic of falcine or tentorial meningiomas) or they may have a flat, plate–like appearance (seen in the en plaque meningiomas) (3).

There are a variety of histological subtypes that have been described. They are listed in Table 2. It is sufficient to say that, while histological variation in these subtypes is recognized, there are no known distinguishing radiological features of these subtypes and there does not appear to be a prognostic implication for these subtypes.

CLASSIFICATION
Two classification systems exist: the WHO and the Helsinki group classification (5).

The WHO classification is more favored and is as follows:

1. Typical meningioma: these lesions are benign and account for 90% to 95% of lesions.
2. Atypical: these demonstrate intermediate hypercellularity and mitotic activity. They account for approximately 5% of lesions.
3. Anaplastic (malignant): they are characterized by necrosis, abundant mitoses, and parenchymal invasion. They account for 1% to 2% of meningiomas.

The Helsinki group classifies meningiomas into four grades as follows:

1. Benign
2. Atypical
3. Anaplastic
4. Sarcomatous

LESION MORPHOLOGY
Three morphological types are recognized and are as follows:

1. Globular (globose): This is the most common form of meningiomas.
2. En plaque: These are flat lesions and are much less common than the globular form.
3. Multicentric meningioma: This is an uncommon morphology accounting for 1% to 10% of meningiomas. They present at a younger age and tend to be confined to a single hemicranium (Figure 1.)

CLINICAL PRESENTATION
Meningiomas may be completely asymptomatic. Symptomatology is dependant upon lesion location and size. Optic nerve sheath or sphenoid wing meningiomas (Figure 2) may cause proptosis or visual disturbances.

LOCATION
The majority of meningiomas are located supratentorially. A large number are located at the convexities; 20% to 34% occur over the lateral convexity. Another 20% to 25% occur in a parasagittal location, that is, along the falx inferior to the superior sagittal sinus. Approximately 17% to 25% occur in a frontobasal location.

A relatively small fraction (10%) occur in the posterior fossa. Of these, approximately 5% occur at the cerebellar convexity, 2% to 4% occur at the tentorium cerebelli, 2% to 4% occur at the cerebellopontine angle, and less than 1% occur at the clivus.

Meningiomas also occur in atypical locations such as the optic nerve sheath (<2%). Intraventricular meningiomas account for 2% to 5% of lesions. Of these, 80% are in the lateral ventricle (left > right), 15% are located in the third ventricle, and 5% are located in the fourth ventricle. Meningiomas are the most common trigonal intraventricular mass in adults.

Spinal meningiomas account for approximately 10% of meningiomas. In very rare cases (<1%), meningiomas may occur in ectopic locations such as intraosseous, scalp, paranasal sinus, parotid, parapharyngeal space, mediastinum, lung or adrenal gland. Pediatric meningiomas are more frequently atypical in location (5). Table 3 summarizes the locations of meningiomas.

IMAGING CHARACTERISTICS
Conventional Radiographs
While conventional radiographs have a limited role in the current evaluation of meningiomas, there are several important radiographic findings which may aid in the diagnosis of the lesion. Indeed, these findings may occasionally be identified on the topograph from a CT examination.

Osseous changes are frequently seen with meningiomas. The most common osseous change is hyperostosis. This is often focal at the site of dural attachment, however, it may be more extensive in cases of en–plaque meningiomas (7). A characteristic, but non–specific, radiographic finding is the pneumosinus dilitans, which is produced by hyperaeration of the sphenoid sinus in cases of tuberculum sellae or planum sphenoidale meningiomas. Calcifications within the lesion may also be identified radiographically as a calcified mass. Widening of osseous sutures is an uncommon radiographic finding (1).

Computed Tomography
Meningiomas are often identified on CT images. They are extra–axial in location, that is, they are within the dura, and outside of the actual brain parenchyma. It may occasionally be difficult to confirm the extra–axial location of a lesion. One helpful feature to aid in localization of the lesion is the "inward buckling" of cortical white matter and the widening of the ipsilateral cistern seen with extra–axial lesions (3).

Meningiomas are slowly growing, well–circumscribed lesions. Due to their indolent course, they are often large at the time of presentation and exert significant mass effect upon the underlying brain parenchyma (Figure 3). More laterally located meningiomas (for example, lateral suprasellar or sphenoid wing meningiomas) may become large enough to obliterate the suprasellar cistern as a consequence of their mass effect (7).

Meningiomas are associated with underlying parenchymal edema, although the degree of edema is unrelated to the size of the mass. A recent study by Kalapurakal et al., (4) has defined a number of predisposing factors for peri–lesion edema including location (parasagittal, frontoparietal), sagittal sinus occlusion, large size, hormonal receptor positivity, and certain histological types (meningothelial, transitional, and malignant).

A characteristic imaging feature of meningiomas is the "dural tail." While better seen with MR, this sign may occasionally be identified on contrast enhanced CT images. The dural tail represents the dural extension of the lesion which is seen to enhance homogeneously upon the administration of intravenous contrast. While this may be seen in up to 60% of meningiomas (1), it is not specific for meningiomas and may be seen with other lesions.

Unenhanced CT images will demonstrate an extra–axial mass which is most frequently hyperintense to underlying parenchyma, however, it may be iso– or hypointense (9). In rare cases, acute intralesion hemorrhage may be present, producing a focal area of high attenuation (3). Calcification may be seen within the mass in either a circular or radial pattern and, when calcification is present, the lesion must be differentiated from an osteoma (9). Contrast enhanced computed tomography classically demonstrates a homogeneously enhancing lesion. Heterogenous enhancement is possible, but very unusual.

Hyperostosis is a valuable imaging feature found in the majority of meningiomas. It may be focal or diffuse (as seen with en–plaque meningiomas). The presence of hyperostosis in lesions involving the cerebellopontine angle and internal auditory canal can help to differentiate meningiomas from acoustic schwannomas.

A summary chart of the CT and MR characteristics of meningiomas is located in Table 4.

Magnetic Resonance Characteristics
Unenhanced magnetic resonance imaging has a somewhat limited role in the evaluation of meningiomas due to the signal characteristics of the lesion (Figure 4). While MR is superior to CT for localizing a lesion to an extra–axial location (due to its multiplanar capabilities), unenhanced CT is better suited to finding the lesion due to the hyperintensity of the lesion relative to parenchyma on CT.

Meningiomas are iso–intense to grey matter on T1 (T1WI) weighted images and are of variable intensity on T2 (T2WI) weighted images, ranging from iso– to hyperintense (1). Edema sensitive sequences such as inversion recovery may be used to identify peri–lesion edema. Cortical buckling is perhaps the most valuable imaging feature for identifying lesions on unenhanced images. Flow voids may be seen on unenhanced images in lesions with prominent feeding arteries and draining veins (5).

With the advent of gadolinium enhanced images, MR became a valuable tool in the localization and surgical planning of meningiomas. It has surpassed CT for screening for meningiomas and for evaluating the relationship of the lesion to surrounding structures. Upon the administration of intravenous gadolinium, meningiomas demonstrate marked homogenous enhancement. As with CT, the "dural tail" represents enhancement of the dural attachment of the lesion. While this is a sensitive sign of dural involvement, it is not specific and may be seen in various other lesions including neurosarcoidosis, lymphoma, chloroma, aspergillosis, metastatic disease, schwannomas, or as the sequelae of post–operative or radiation induced changes (3).

Angiographic Findings
Although the advent of gadolinium enhanced MR has largely circumvented the requirements for angiography, angiography continues to have a role in the pre–operative work–up of meningiomas. It is valuable in delineating the feeding arteries and draining veins and thus has a role in planning the operative approach.

The classical angiographic appearance is the "mother–in–law" pattern whereby there is an early, intense contrast enhancement which persists into the venous phase (8). A "spokewheel" or "sunburst" pattern of enhancement may be seen with a cloud–like contrast blush (1) (Figures 3c & d).

Meningiomas are typically fed by branches of the external carotid artery, but as they enlarge they often outgrow this blood supply and the circumference of the lesion soon becomes fed by internal carotid branches. The internal carotid artery angiogram therefore gives a "doughnut" appearance with the circumference of the lesion enhancing and the center (external carotid supply) not enhancing until the external carotid is injected. Unlike typical meningiomas, en plaque meningiomas are hypovascular.

SPECIFIC ANATOMICAL SITES
The findings described above are general features of meningiomas. Meningiomas in certain locations deserve special mention and will be discussed below.

Suprasella Cistern
Meningiomas are commonly found in this location. When laterally situated within the suprasellar cistern, they exert mass effect, leading to narrowing of the cistern. If the lesion is large enough, it may completely obliterate the cistern. One unusual feature seen with parasellar meningiomas is their propensity to produce osteolysis and not hyperostosis. This may cast doubt upon the diagnosis, despite other characteristic imaging findings.

The entity which should be considered in the differential diagnosis of a lesion in this region is a vascular abnormality related to the Circle of Willis (located within the suprasellar cistern). Aneurysmal dilatation of a branch of the Circle of Willis may mimic the features of a meningioma. One useful discerning feature is the more rapid enhancement of non–thrombosed aneurysms on contrast enhanced CT as well as the relative hyperintensity of enhancement compared with meningiomas (7). MR angiography may also be of value for differentiating a meningioma from a vascular lesion.

Pituitary macroadenomas may also be considered in the differential diagnosis of a lesion in this location (due to suprasellar extension of a pituitary lesion). Useful, albeit subtle, discerning features of meningiomas include osseous changes in the region of the sphenoid ridge, anterior choroid, and tuberculum sellae. Occasionally, the diaphragma sellae may be identified; if intact, it may separate the lesion from the contents of the pituitary fossa, thereby lending support to the diagnosis of a meningioma. Enhancement characteristics differ for pituitary adenomas and meningiomas, with meningiomas demonstrating more marked and more homogeneous enhancement (7).

Optic Sheath Meningioma
Optic sheath meningiomas have a slightly different imaging appearance than globular meningiomas. They often produce diffuse thickening of the nerve sheath without producing a focal mass. Unenhanced CT demonstrates relatively intense tissue surrounding a low–density optic nerve. Although rare, calcification may be present. There is marked enhancement of the lesion with contrast enhanced CT techniques.

Conventional MR demonstrates a lesion which is isointense to the extra–ocular muscles on T1WI and is generally isointense on T2WI. If calcified, this lesion may be of markedly low signal on T1WI and T2WI. In contradistinction to the CT findings, on unenhanced conventional MR imaging, the optic nerve is usually relatively higher in signal intensity than the surrounding meningioma. Again, marked lesion enhancement is seen (7).

The differential diagnosis of an optic nerve sheath meningioma is an optic nerve glioma. While optic sheath meningiomas are often diffuse in appearance, optic nerve gliomas tend to cause fusiform enlargement of the nerve with tapering at the proximal and distal aspects. Gliomas may also demonstrate cylindrical enlargement, tortuosity or kinking of the nerve, or exophytic growth. The lesion may be heterogeneous on unenhanced CT with areas of cystic change (which are hypodense) and areas of relative hyperdensity.

Conventional MR of optic gliomas demonstrates a lesion which is isointense to the extra–ocular muscles and brain parenchyma on T1WI and minimally hyperintense on T2WI. The lesion may be heterogeneous in appearance secondary to cystic change.

OTHER TYPES OF MENINGIOMAS
Cystic Meningioma
These account for approximately 2% to 4% of meningiomas (1) and are most frequently found in childhood. These lesions are characterized by a cystic component (with variable signal intensity on MR due to proteinaceous material) and a nodular, irregular solid component which is isointense to grey matter. There is marked enhancement of the solid components (7) (Figure 5).

Differential considerations would include pilocytic astrocytoma, hemangioblastoma, and metastatic lesions. Cyst enhancement is a useful discriminator for more aggressive lesions such as metastases, which demonstrate cyst wall enhancement.

Intraventricular Meningiomas
Although this location accounts for only 2% to 5% of meningiomas, meningioma is the most common trigonal intraventricular mass in adults (1). The left lateral ventricle is the most frequent site (80%), followed by the third ventricle (15%) and the fourth ventricle (5%) (Figures 6, 7). Imaging characteristics are similar to those of meningiomas found elsewhere in the cranium with the additional feature of hydrocephalus from outlet obstruction.

Differential considerations would include subependymal giant cell astrocytoma, choroid plexus papilloma and interventricular metastases. The margin of the meningioma tends to be smooth, unlike the choroid plexus papilloma which has a "cauliflower" appearance.

En Plaque Meningiomas
This type of meningioma may be very difficult to recognize due to its morphology. It is often radiographically silent on unenhanced and enhanced CT images and may be difficult to identify on non–contrast MR due to its signal characteristics (iso–intense to grey matter on T1WI). As with most meningiomas, marked contrast enhancement is seen, despite the fact that en–plaque meningiomas tend to be hypovascular (Figure 8). Hyperostosis is frequently seen along the dural attachments, which are often at the skull base, thus aiding in the diagnosis.

MALIGNANT DEGENERATION
This is an uncommon occurrence. Degeneration is sarcomatous. Metastases are uncommon and may be seen with benign or malignant meningiomas. Features suggesting malignancy include tumor necrosis, infrequent calcifications (as compared to benign meningiomas, which have a higher rate of calcification), irregular borders, lobulation, and bosselation. Additional features suggesting malignancy include marked underlying parenchymal edema, heterogeneous contrast enhancement and non–enhancing central low density components (Figure 9). These features are summarized in Table 5.

"Mushrooming" is another feature suggestive of malignancy. This term refers to a pattern where the lesion has an intracranial diameter of incursion which is significantly larger than its dural attachment (7).

PROGNOSIS AND TREATMENT
Treatment is usually by surgical resection, although complete resection may be difficult secondary to size and site of the lesion as well as its relationship to surrounding structures such as vessels. Tumor recurrence is as high as 10% to 20% with incompletely resected lesions (7).

CONCLUSION
Meningiomas are the most common extra–axial neoplasm found in adults. They are typically lesions found in middle aged adults with a predilection for females, likely due to hormone receptors found on the lesions. While the majority are benign, sarcomatous degeneration is a recognized phenomenon.

Associations with neurofibromatosis Type II, hormone receptor activity, breast carcinoma and pituitary adenomas are well known (7). There is also a well documented association with prior radiation therapy with meningiomas arising in regions of prior radiation therapy after a period of latency (2).

Characteristic locations of meningiomas are detailed above, with the majority lying over the convexities. The lesions are isointense to hyperintense on unenhanced CT with homogenous enhancement demonstrated following the administration of intravenous contrast. They are isointense to grey matter on T1WI, making screening and localization difficult if no significant mass effect is present. The advent of contrast enhanced MR has significantly impacted the identification of meningiomas with marked enhancement following the administration of intravenous gadolinium, making contrast enhanced MR a valuable screening tool for suspected meningiomas.