Introduction

Meningioma, a central nervous system tumor characterized by indolent growth, is the most common extraaxial lesion encountered in neuroimaging. The high frequency of incidental diagnosis during neuroimaging for other indications reflects the fact that up to 2% of autopsies reveal a meningioma.1

Historically, meningiomas were characterized by plain roentgenograms and conventional diagnostic angiography, but the introduction of computed tomography (CT) and magnetic resonance imaging (MRI) dramatically improved detection and accuracy of diagnosis. Continuous refinements in neuroimaging technology have led to the exquisitely detailed contemporary images with which we are familiar today.

Recent years have witnessed the development of advanced imaging techniques, such as MR spectroscopy, MR perfusion, indium-111-octreotide scintigraphy, and positron emission tomography (PET), which at times aid in the diagnosis of meningiomas.

Although these studies are becoming more standardized and their usefulness in certain clinical scenarios better defined, these newer techniques remain peripheral at present and serve only as adjuncts to CT and routine MRI sequences.

Computed Tomography

Although MRI is the imaging study of choice for evaluation of suspected meningioma or in the context of known or highly suspected pathology, CT is highly accessible and indicated for rapid screening in urgent settings. As such, many meningiomas are first encountered on CT scans obtained for different reasons. CT has a place in the diagnosis of meningioma because it is superior in demonstrating the effects of this neoplasm on adjacent bone, specifically osseous destruction or hyperostosis, and is more sensitive in detecting psammomatous calcifications in the tumor (seen grossly in ~25% of meningiomas).

Benign meningiomas typically appear as rounded or elongated extraaxial masses that demonstrate a broad attachment to the dura. On CT, they are usually isodense but can occasionally be hyperdense or slightly hypodense compared with cerebrum.

Their extraaxial nature is suggested by a sharp interface with displaced brain parenchyma, the presence of a cerebrospinal fluid attenuation cleft ( Fig. 13.1A ), and intense enhancement. Meningiomas exhibit homogeneous attenuation before and after administration of contrast material but can show some heterogeneity depending on the consistency of tumor (i.e., the presence of calcium, fat, tumor necrosis).

Hyperostosis of adjacent skull is highly suggestive of benign meningioma and is best demonstrated by CT, windowed on bone algorithm, as cortical thickening and hyperdensity ( Fig. 13.1B ). Hyperostosis typically indicates infiltration of bone by meningioma. In one study, meningioma infiltration of radiographically shown hyperostotic bone was identified histologically in 25 of 26 patients.2 Hyperostosis is usually more difficult, but not impossible, to ascertain by MRI.

The World Health Organization (WHO) classifies meningiomas into grade I (benign), atypical (grade II), and malignant (grade III and IV) stages. The latter often, but not always, have an aggressive behavior, invade bone and brain, and tend not to cause hyperostosis but, rather, frank bone destruction. Malignant meningiomas can metastasize as well as spread along the leptomeninges.

Dural metastases or other malignancies (i.e., plasmacytomas, leiomyomas) may have an appearance similar to meningioma, but associated lytic calvarial or skull base lesions may help differentiate these lesions from meningioma.

Calcifications are also typical of meningioma but not pathognomonic because other extraaxial neoplasms, such as chondromas, may also contain calcifications.3 The morphology and volume of calcifications are varied. Parasellar thrombosed aneurysms and schwannomas arising from the cavernous sinus can masquerade as meningioma ( Fig. 13.2 ), so extraaxial skull base masses should be carefully evaluated before assigning a diagnosis of meningioma.

Magnetic Resonance Imaging

Common Imaging Features of Meningiomas on Magnetic Resonance Imaging

Approximately 85 to 90% of meningiomas have typical features, including an extraaxial mass with signal intensity isointense to cortex on T1 and T2 MRI sequences, avid homogeneous enhancement following administration of gadolinium contrast, and an enhancing “dural tail” ( Fig. 13.3A ), which reflects neoplastic dural infiltration or reactive vascularity (or both) draining into the adjacent dura. An eccentric core of lower signal intensity is often seen and indicates the vascular pedicle entering at the original nidus of the meningioma. Low signal intensity within the tumor may often be due to calcification or to vascular flow voids, a distinction sometimes difficult to make, unless the morphology is clearly that of branching vessels.

Meningiomas can be spherical or elongated (en plaque), multiple, and often take origin from a dural sinus, a feature important for surgical planning. These tumors also tend not to respect the dural boundary and may extend on both sides of the falx and the tentorium, which is a distinctive feature not typical of other neoplasms ( Fig. 13.3B ).

Hyperostosis of bone can also be appreciated on MRI, particularly when florid, as seen with en plaque meningiomas ( Fig. 13.4A ). Hyperostosis appears as an area of thicker cortical bone with low signal intensity on T1- and T2-weighted images. The hyperostosis (also Fig. 13.4B ) usually reflects osseous neoplastic infiltration and is best removed at the time of surgery, if possible.

Narrowing of engulfed arteries, so-called encasement, is also a common feature, especially of parasellar meningiomas, which often encase and narrow the supraclinoid internal carotid artery ( Fig. 13.5 ). The encasement, on occasion, can lead to cerebral ischemia.

Although most benign meningiomas are innocuous from the standpoint of metastatic potential, they may result in serious complications secondary to dural sinus invasion ( Fig. 13.6 ) (with or without thrombosis), narrowing and thrombosis of significant arterial structures, and compression of cranial nerves and other important neural structures, such as the brain stem. The use of MR vascular imaging or conventional angiography for pre-operative detailing of the vascular structures adjacent to these tumors before surgical removal is often helpful.4

Edema associated with meningioma is thought to be vasogenic in origin and probably related to tumor secretion of vascular endothelial growth factor (VEGF), rather than a result of direct mass effect on adjacent brain or venous invasion causing vascular congestion.5 The presence of intraaxial edema is said to predict an increased potential for recurrence.6,7 Figure 13.7 demonstrates a meningioma associated with extensive vasogenic edema and low T2 signal serpentine structures within/high vascularity.

“Cystic” meningiomas, named for focal areas of cerebrospinal fluid signal intensity on MRI (or attenuation on CT), are not truly cystic in their entirety ( Fig. 13.8 ). These collections may result from adjacent trapped CSF collections or intratumoral cystic collections. Lipomatous changes within meningiomas are also uncommon and result from accumulation of lipid inside meningothelial cells rather than metaplasia.8

Intraventricular meningiomas arise from arachnoid cell nests within the choroid plexus ( Fig. 13.9 ). These rare tumors represent only ~1 to 2% of all meningiomas9 but are the most common atrial intraventricular neoplasms in adults.10 These lesions can become quite large; yet, despite their size, they produce less mass effect and little edema because they reside within the ventricle.

Atypical and Malignant Meningiomas

Atypical (World Health Organization [WHO] grade II) and malignant (WHO grade III) meningiomas represent ~6% and 2%, respectively, of all meningiomas.1 They are typically characterized by their aggressive and invasive nature and higher rates of recurrence. These tumors tend to penetrate the brain parenchyma through the perivascular subarachnoid spaces ( Fig. 13.10 ) and may erode through the dura into skull and scalp ( Fig. 13.11 ). Irregularity of the margin with brain, so-called mushrooming, has been suggested as a feature of malignant meningiomas, but in the authors’ experience this is an unreliable marker.

Malignant meningiomas’ aggressiveness and tendency to recur are linked and directly proportional to the degree of adjacent skull osteolysis, extent of brain parenchymal invasion, and volume of pial-cortical supply (as opposed to dural-meningeal supply).11

The recurrence rate of grossly resected atypical/malignant meningiomas is reported as high as 48% at 10 years.12 Malignant meningiomas also have a higher tendency than their benign counterparts to disseminate through the cerebrospinal fluid or hematogenously (to lungs and other organs).13