Meningiomas are the most commonly reported intracranial tumor. They represent approximately 38% of all intracranial neoplasms in females and 20% in males.  Meningiomas are also the most common extra-axial tumors in the brain and the most frequently occurring tumors of mesodermal or meningeal origin. They are more common in women than men and are usually diagnosed after age 30. [2, 3, 4, 5, 6]
Advances in radiologic imaging techniques, such as computed tomography (CT) scanning and magnetic resonance imaging (MRI), have improved the ability to predict the success for complete removal of the mass. Imaging information about the dural attachment site, the location and severity of edema,  and the displacement of critical neurovascular structures is useful for planning the operative approach and has an effect on the outcome. [8, 9]
Neuroradiologists and neurosurgeons must be aware of both the typical and atypical imaging appearances of meningiomas, as there is some correlation with different histologic types of tumor.
The World Health Organization (WHO) classifies meningiomas into 15 subtypes under 3 major categories:
Grade 1 (typical or benign), representing 88-94% of cases
Grade II (atypical), representing 5-7% of cases
Grade III (anaplastic or malignant), representing 1-2% of cases
Significant factors contributing to recurrence include atypical and malignant histologic types and heterogeneous tumor contrast enhancement on CT scans.
Meningiomas arise from the arachnoid membranes, specifically from meningothelial cells. Most meningiomas grow inward toward the brain as discrete well-defined, dural-based masses and are spherical or lobulated in contour. Flat tumors, termed en plaque, infiltrate the dura and grow as a thin carpet or sheet of tumor along the convexity dura, falx, or tentorium. Dural attachment of meningiomas can be pedunculated or broad-based (sessile). Because the pia and arachnoid form a membranous barrier between brain and tumor, some meningiomas grow into the subarachnoid space, but invasion of the brain is infrequent.
MRI is preferred for the diagnosis and evaluation of brain meningiomas. [7, 10, 11, 12] It more accurately evaluates en plaque and posterior fossa meningiomas, which may be missed on CT scanning. CT scanning, however, clearly depicts bony hyperostosis, which may be difficult to appreciate on MRI.
CT scanning historically had limitations in performing direct imaging in any plane other than axial. However, with modern spiral CT scanning and multisection or multidetector-row CT (MDCT) scanning, the quality of sagittal and coronal images that can be reconstructed from axial data has increased significantly. CT scanning is less helpful than MRI in differentiating different types of soft tissue.
The differential diagnosis for brain meningioma includes dural metastasis (with breast and prostate cancer being the most common primary malignancies), hemangiopericytoma, granulomatous disease (including sarcoidosis and tuberculosis), idiopathic hypertrophic pachymeningitis, extramedullary hematopoiesis, hemangioma, and dura/venous sinuses. With certain anatomic locations, other differential diagnoses should be considered, including vestibular schwannoma for cerebellopontine angle tumors, pituitary macroadenoma and craniopharyngioma for parasellar tumors, and chordoma/chondrosarcoma for masses around the clivus.
The development of catheters and the continued refinement of embolic materials and radiographically controlled interventional procedures have contributed to improved treatment of patients with brain meningiomas. The clinician must be aware of the active participation of the neurosurgeon and neuroradiologist in the therapy for neurosurgical patients. [13, 14]
The best available treatment for benign meningiomas is complete surgical resection of the tumor. Nevertheless, interventional neuroradiologists commonly contribute in performing preoperative embolization to reduce the blood supply to the tumor. Treatment of meningiomas is benefited by embolization, but especially those with a complex presentation, giant meningiomas, meningiomas exhibiting malignant or angioblastic characteristics, or meningiomas involving the skull base, scalp, or critical vascular structures. The preoperative embolization of meningiomas is commonly used to facilitate surgery.
In a study of the effects of preoperative embolization on overall surgical outcomes after meningioma resection, preembolization and postembolization tumor enhancement patterns on MRI defined as embolization fraction correlated with decreased intraoperative blood loss and better postoperative functional outcomes. Tumor location significantly correlated with the decision to embolize preoperatively. 
Embolization can be carried out at the same time as the diagnostic angiography session or may occur later if detailed procedural planning is required. Distal, homogeneous, and permanent occlusion of the vascular bed by injecting small particles (150-300 microns of polyvinyl alcohol [PVA]) through microcatheters is the goal. Bilateral dural devascularization shortens the surgical resection time and permits total removal of the tumor. The procedure causes tumor necrosis, expanding the spectrum of meningiomas that can be safely resectioned during surgery.
PVA particles ranging in size from 100 to 2000 microns have been used, but the newer class of deformable particles and Bead Block have been shown to be more effective in distal embolotherapy to reach the capillary bed of the meningioma. Embospheres can be tagged with chemotherapeutic agents. Several meningiomas of the convexity have been embolized with Embospheres in our experience.
A cautious approach should be taken regarding pathologic evaluation of preoperatively embolized meningiomas; one study has suggested that preoperatively embolizing meningiomas may risk overgrading the pathologic specimen if the interpreting pathologist is not aware of the recent procedure. Features such as neoangiogenesis (microvessel density), necrosis, and prominent nucleoli should be interpreted with caution in such specimens. 
Approximately 2% of patients have complications associated with embolization that result in neurologic deficits. At the theoretical level, embolization may reduce the likelihood of recurrence. Embolization also may be the only treatment required in older or high-risk patients. See the images below.
In most patients, plain skull radiographs are nondiagnostic, with no features to suggest the presence of a meningioma. Some cases may demonstrate calcification or reactive hyperostosis. Rarely, osteolysis may be observed.
Most plain skull radiographs do not depict signs of meningiomas. Meningiomas en plaque have diffuse hyperostosis, more frequently observed over the sphenoid wing and pterion. This finding results in a high degree of confidence.
Calcification within the tumor is a considerably less frequent plain radiographic manifestation; therefore, false-negative results occur. Most patients with brain meningiomas do not undergo radiographic imaging because the diagnosis has been made directly with CT scanning or MRI.
Computed tomography (CT) scanning is frequently utilized in the assessment of meningiomas. [16, 17, 18, 19, 20, 21, 22, 23] Typical features on unenhanced images include a well-circumscribed, smoothly marginated extra-axial mass abutting the dura.
Approximately 70-75% of meningiomas are hyperattenuating to surrounding brain parenchyma, while roughly 25% are isodense. A rare group of meningiomas (the lipoblastic subtype) contain fat and are thus hypoattenuating.
Calcification is another common finding, seen in approximately 20-25% of cases. The CT nature of the calcification may be nodular, fine and punctate, or dense. Surrounding parenchymal vasogenic edema is common, identified as hypodense brain tissue. Occasionally, the edema is extensive and, as it predominantly affects white matter, can resemble fingers of low attenuation. Edema, however, is absent in approximately 50% of cases because of the neoplasm's slow growth.
An advantage of CT over MRI is the evaluation of bone.  Underlying bone demonstrates hyperostosis in 15-20% of patients. Other bony findings include an increase in vascular markings and cortical irregularity. Less common meningioma findings include hemorrhage, cyst formation, and necrosis. Cystic components of meningiomas may be present inside the tumor or between the tumor and the adjacent brain (so-called trapped CSF).
The administration of intravenous contrast in evaluating meningiomas is helpful, as more than 90% of cases will demonstrate intense homogeneous enhancement. Inhomogeneous enhancement can result because of necrosis or rare hemorrhage. Steinhoff et al observed a nodular blush in 97% of cases, a mixed inhomogeneous blush in 0.5%, and a ring blush in 1.5%.  In a study by Naidich et al, tumor blush was nodular and nearly homogeneous in 70% of cases, inhomogeneous in 24%, and ringlike in 2%. 
Approximately 90% of meningiomas are demonstrated on CT scans. The main role of CT scanning, as opposed to MRI, is the demonstration of adjacent bone changes and calcification within the lesion.
Atypical CT scan features are the primary reason for preoperative misdiagnosis. Posterior fossa meningiomas and some en plaque lesions may be missed by CT. CT scanning can fail to demonstrate cystic changes in intracranial meningiomas. CT scan features, such as irregular areas of nonenhancing mass and well-defined regions of persistent low attenuation, are common reasons for preoperative misdiagnosis.
False-negative findings can occur with cystic changes in brain meningiomas. False-positive findings can occur with large dural calcification, which can mimic the disease.
Magnetic Resonance Imaging
MRI with gadolinium is the best imaging modality for evaluating meningiomas. Important advantages of MRI in the imaging of meningiomas are the superior resolution of different types of soft tissue, multiplanar capability, and 3-dimensional (3-D) reconstruction ability. [18, 19, 27, 28]
MRI can demonstrate tumor vascularity, arterial encasement, venous sinus invasion, and the relationship between the tumor and surrounding structures. This modality is particularly advantageous in depicting the juxtasellar area and the posterior fossa and in demonstrating the rare presence of disseminated disease via the CSF. The multiplanar capability is often the best means to visualize the broad contact of tumors to the meninges, tumor capsules, and meningeal contrast enhancement adjacent to the tumor. [29, 30, 31]
On nonenhanced T1-weighted images, most meningiomas have no signal intensity difference compared with cortical gray matter. Fibromatous meningiomas may be more hypointense than the cerebral cortex. T1-weighted images may be used to asses for necrosis, hemorrhagic products, and cysts. On T2-weighted images, signal is variable. T2-weighted images are useful in assessing for hemorrhagic products and cysts, as well. Additionally, this sequence is used to assess for a CSF cleft between the neoplasm and brain parenchyma, confirming an extra-axial location.
Hyperintensity on T2-weighted images indicates soft-tumor consistency and microhypervascularity. This is seen more often in aggressive, angioblastic, or meningothelial tumors. T2-weighted signal intensity is best correlated with both the histology and the consistency of the meningioma. Generally, low-intensity portions of the tumor indicate a more fibrous and harder character (eg, fibroblasticmeningiomas), whereas higher-intensity portions indicate a softer character (eg, angioblastic tumor). [32, 33, 34]
Fluid-attenuated inversion recovery (FLAIR) sequences are useful to assess for associated edema, as well as for the characteristic feature of a dural tail. The dural tail represents a collar of thickened, enhancing dura that surrounds the tumor's dural attachment. A dural tail occurs in approximately 65% of meningiomas, as well as 15% of other tumors. Although this finding is not specific for meningiomas, it is highly suggestive of the diagnosis.
On MRI and CT, meningiomas exhibit the same enhancement appearance after the injection of contrast medium. Intense enhancement following gadolinium is seen in more than 85% of tumors. A ring appearance may represent a capsule. Gadolinium also enables better visualization of en plaque meningiomas that may be more subtle on unenhanced sequences.
Histologic subtypes may have different MRI appearances, but this does not suffice for a histologic diagnosis by using MRI.
MR spectroscopy (MRS) has been studied to aid in differentiating meningiomas from other mimics. Studies have consistently demonstrated increased levels of alanine, choline, and glutamate-glutamine complex and decreased levels of N-acetylaspartate and creatine.  Specifically, elevated signal intensities for glutamine at 3.8 ppm and alanine at 1.48 ppm have been reported.  Lactate and lipid levels, although correlated well with malignancy of gliomas and metastases, remain controversial in their value in evaluating meningiomas. Low myoinositol and creatine have been shown to be characteristic in meningiomas. 
Phosphorus-31 MRS has shown a characteristic alkaline environment and low levels of phosphocreatine and phosphodiesterases. 
Diffusion-weighted MRI has been evaluated with varying conclusions. Apparent diffusion coefficients (ADC) have been shown to typically be lower than surrounding brain for high-grade neoplasms. Some studies have shown a similar trend for meningiomas, while other studies have found ADC values for grade I and grade II meningiomas to not be statistically different. 
Perfusion-weighted MRI has been studied, with several conclusions. Hypervascular meningiomas will typically demonstrate increased perfusion. Perfusion-weighted MRI has also been evaluated to assess meningioma tumor subtype, as well as to follow postintervention success. 
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
An apparent diffusion coefficient (ADC) of 0.85 using diffusion-weighted MRI was found to differentiate grade I meningioma from grade II and III tumors. In the study of 389 patients, World Health Organization grade I was diagnosed in 271 cases (69.7%), grade II in 103 (26.5%), and grade III in 15 patients (3.9%). 
In general, the sensitivity and specificity of MRI are high in the diagnosis of meningiomas. MRI has proved to be superior in delineation of the tumor and its relation with surrounding structures. However, MRI is unreliable for recognition of tumor calcification, and acute hemorrhage is often difficult to image with this modality.
False-negative findings of tumor calcium must be considered. Delineation of acute hemorrhage into tumor with conventional sequences is a disadvantage of MRI and may generate false findings.
The location of intratumoral hemorrhage, cystic changes inside or outside of the tumor mass, calcifications, invasion of the parenchyma by malignant meningiomas, and lobulated or multilobulated masses is demonstrable only with intraoperative ultrasonography.
Meningiomas can be identified incidentally on bone scans. This is thought to be secondary to factors that affect calcium deposition, as well as the presence/absence of a vascular pool.  Numerous single photon emission computed tomography (SPECT) and positron emission tomography (PET) radiolabeled tracers have been evaluated with respect to meningiomas and may assist in further differentiating between biological features and specific subtypes of meningiomas. Nuclear medicine, specifically SPECT and PET, may have a role in several areas of meningioma management, including establishing a diagnosis and differential diagnosis, planning radiation treatment, predicting tumor grade, predicting likelihood of recurrence, assessing treatment response, and differentiating residual tumor from posttreatment fibrosis. 
SPECT evaluation of meningiomas has demonstrated several characteristic features. Thallium-201 has shown accumulation in several brain tumors, including meningiomas. Studies have suggested that long-lasting uptake in meningiomas could be utilized to predict malignant likelihood and tumor aggressiveness, and it has recently been associated with vascular endothelial growth factor positivity. 
Numerous studies have identified characteristic features of meningiomas using technetium-labeled compounds, such as99m TC-methoxyisobutylisonitrile (MIBI) and99m Tc-tetrofosmin (TF), as well as somatostatin receptor scintigraphy (111 In-octreotide and99m Tc-depreotide). 
PET has been studied using18 F-FDG (18 F-fluorodeoxyglucose), with mixed results: some papers have suggested an FDG avidity to differentiate benign from malignant meningiomas, while other papers contradict this finding. One of the major complicating factors of FDG evaluation is the normal high uptake in cerebral cortex. Multiple other PET tracers have been studied, with mixed results. 
Although magnetic resonance angiography (MRA and magnetic resonance venography [MRV]) have decreased the role of classical angiography, the latter remains a powerful tool for embolization and planning surgery. Angiography is still indispensable if embolization of the tumor is deemed necessary. [42, 51, 52]
Meningiomas are supplied by meningeal branches of the internal and external carotid artery (see the following images). Basal meningiomas of the anterior and middle cranial fossa and meningiomas of the wings of the sphenoid bone are commonly supplied by the internal carotid artery. Other supratentorial meningiomas are supplied by the internal and external carotid arteries.
Tumors that arise along the falx, the sphenoidal ridge, and the convexity are supplied by the middle meningeal artery. Falcine meningiomas can be supplied additionally by the anterior meningeal artery. Parasellar and tentorium tumors are supplied by the hypophyseal meningeal artery. Direct meningeal arteries from the cavernous sinus can supply meningiomas of the middle cranial fossa. Intraventricular tumors are supplied by anterior and posterior choroidal arteries.
External carotid and vertebral branches supply tumors of the posterior fossa. Large meningiomas can be supplied by pia vessels around the tumor.
Meningeal arteries penetrate to a meningioma through its dural attachment with inside branches radially distributed like sunrays, creating the typical "sunburst" appearance. Homogeneous sharp tumor staining is seen early and remains late and, as such, has been called "the mother-in-law sign." Usually, meningiomas do not exhibit drainage veins, although angioblastic types may.
In summary, angiography is useful in delineating the blood supply of the external versus internal carotid arteries and can show encasement of intracranial vessels. Angiography demonstrates an arterial map for preoperative embolization.
As an alternative to traditional catheter angiography, 3-D CT angiography may depict the relationship between skull base meningiomas and neighboring bony and vascular structures clearly, quickly, and with minimal risk to the patient.
Angiography has a high degree of confidence in recognizing the arterial source of the meningioma. Tumor feeding can be identified with a low rate of false-positive and/or false-negative findings.
Arterial findings have a high sensitivity and specificity in the diagnosis of meningiomas. Angiography shows an arterial map for preoperative embolization with a low false-finding rate.