Neuroimaging in the Evaluation of Epilepsy

Naymee J. Velez-Ruiz, MD; Joshua P. Klein, MD, PhD2

Disclosures

Semin Neurol. 2012;32(4):361-373. 

In This Article

Meningiomas

Meningiomas constitute the most common extra-axial neoplasm of the brain. Epilepsy is one of the most common symptoms of intracranial meningiomas with an incidence of 20–50% as the initial symptom.[87] Location of the meningioma at the supratentorium or convexity and evidence of severe peritumoral edema have been associated with the development of epilepsy.[87] Surgical excision of the meningioma stops the seizures in ~60% of the patients.[87] Hence, the importance of its accurate identification.

On unenhanced CT, ~60% of meningiomas are slightly hyperdense compared with normal brain tissue.[88] Also, calcifications within meningiomas can be seen in ~20% of cases.[88] Their extra-axial nature is suggested by a sharp interface with displaced brain parenchyma, the presence of a cerebrospinal fluid attenuation cleft and intense tumor postcontrast enhancement.[89] Enhancement is usually homogeneous, but can show some heterogeneity depending on the consistency of the tumor, i.e., the presence of calcium, fat, and tumor necrosis.[89] Hyperostosis of adjacent skull is highly suggestive of benign meningioma and is best demonstrated by CT, windowed on bone algorithm, as cortical bone thickening and hyperdensity.[89]

MRI is superior to CT in detecting the full extent of meningiomas, vascularity, and intracranial edema.[88] The typical MRI signal intensity characteristic of meningiomas consist of isointensity to slight hypointensity relative to gray matter on the T1WI and isointensity to hyperintensity relative to gray matter on the T2WI. In addition, there is avid homogeneous enhancement following the administration of gadolinium, and an enhancing "dural tail," which reflects neoplastic dural infiltration or reactive vascularity, or both, draining into the adjacent dura.[89] Occasionally, meningiomas may have necrotic centers or calcified portions, which may not enhance. Meningiomas can be nearly spherical or elongated (en plaque), multiple, and often originate from a dural sinus. Although most meningiomas have no metastatic potential, they may result in serious complications secondary to dural sinus invasion, narrowing and thrombosis of important vascular channels, and compression of important neural structures.[89] The degree of parenchymal edema is variable in meningiomas, and it seems to correlate with the size and location of the lesion as well as its rate of growth. Although there are exceptions, larger meningiomas located adjacent to the cortex tend to incite greater edema than smaller meningiomas or than those along the basal cisterns and planum sphenoidale. Edema associated with meningiomas may be caused by compressive ischemia, venous stasis, aggressive growth, or parasitization of pial vessels.[88] Malignant meningiomas occur uncommonly, and these are usually diagnosed when the meningioma exhibits intraparenchymal invasion or markedly rapid growth. Reduced water diffusivity has been correlated with more aggressive tumor behavior and is sometimes seen with atypical/malignant meningiomas.[90] A decrease in ADC values on follow-up of a benign meningioma should raise suspicion for dedifferentiation to a higher tumor grade.[91] However, the utility of this finding is limited by the inability of DWI to identify histologic subtypes. On MRS, meningiomas are characterized by prominent Cho, absence of NAA and Cr, and presence of Ala.[92] This spectral pattern has been reported in both typical and atypical meningiomas and, MRS cannot reliably differentiate one type from the other.[92]

The role of PET imaging in the evaluation of meningiomas is complicated by the variable metabolic presentation in different meningioma types. Given the presence of hypermetabolism in malignant meningiomas, some authors have proposed the use of 18F-FDG PET for the differentiation of benign and malignant meningiomas as well as for the noninvasive prediction of tumor growth.[93] However, the presence of hypermetabolism cofounds their differentiation from other intracranial tumors.

Conventional angiography is most often performed for preoperative endovascular embolization and is intended to minimize blood loss intraoperatively. Meningiomas diagnostically appear as lesions with an angiographic stain (tumor blush) and can have both dural and pial blood supply.

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