Neuroimaging in the Evaluation of Epilepsy

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


Semin Neurol. 2012;32(4):361-373. 

In This Article

Low-grade Intra-axial Tumors

The most common tumor types in patients with intractable epilepsy are ganglioglioma and low-grade astrocytoma.[79] Less common are low-grade oligodendroglioma, mixed glioma, and dysembryoplastic neuroepithelial tumor (DNET).[79] Although gangliogliomas occur with an overall frequency of less than 1% of all patients with brain tumors, almost all patients with this tumor type have epilepsy. Like ganglioglioma, DNET is almost exclusively found in patients with epilepsy (Fig. 3).

Figure 3.

Low-grade tumor. A 44-year-old right-handed woman who had a first convulsive seizure and was found to have a left temporal brain mass. (A) Axial T2- fluid attenuated inversion recovery magnetic resonance imaging (FLAIR MRI) showed a well-circumscribed round T2 hyperintense lesion in the left temporal subcortical white matter, surrounded by a less distinct region of abnormal T2 hyperintensity extending posteriorly and laterally. (B) Axial T1-weighted MRI following intravenous administration of gadolinium contrast showed a focal nodular area of abnormal enhancement at the anteromedial aspect of the round lesion. (C) Axial gradient echo MRI showed multiple foci of susceptibility artifact, consistent with calcification. The radiographic differential for a cystic and nodular mass with internal calcification in a young person with new-onset seizures includes tumors such as ganglioglioma, dysembryoplastic neuroepithelial tumor (DNET), pleomorphic xanthoastrocytoma, and oligodendroglioma, though the presence of enhancement could be consistent with a higher-grade lesion. This lesion has not been biopsied and will be followed closely over time.

Gangliogliomas are encountered in up to 38% of the patients with brain tumors undergoing surgery for refractory seizures.[79] These tumors occur more commonly in children and young adults.[80] The tumor is usually single, well circumscribed, and located in one of the temporal lobes.[80] Almost 60% of gangliogliomas have a cystic component; some even consist of a large cyst with a mural nodule. The most common MRI findings associated with the solid component include increased signal on T2WI, isointense signal on T1WI, and significantly increased signal on proton density images. There is contrast enhancement in ~40% of cases.[81] The cystic component usually exhibits increased signal on T2WI, decreased signal on T1WI, and variable (depending on protein concentration) signal on proton density images.

DNET is typically diagnosed before the age of 30. The lesion is supratentorial, often well demarcated, and usually found in temporal or frontal lobes.[82] The characteristic MRI finding is increased signal on T2WI associated with decreased signal on T1WI. Contrast enhancement has been described in up to one-third of patients.[83] Also approximately one-third of the patients exhibit cystic changes, and one-fourth demonstrates calcification.[84] On proton density images, the signal from cystic-appearing DNET is different from that of cerebrospinal fluid, implying an increased protein concentration, and thereby distinguishing these tumors from subarachnoid cysts.[83]

Gangliogliomas and DNET cannot be reliably differentiated from each other and from other types of low grade tumors using current MRI techniques (including MRS) or 18F-FDG PET. However, prognosis in patients with low-grade tumors appears to be related to location rather than histologic appearance.[85] For the most part, these tumors are benign. If a complete resection of the tumor can be accomplished, the prognosis for seizure control is excellent. One factor that can affect seizure outcome is the coexistence of tumors and FCD. FCD are often seen near but separate from the tumor.[86] Some authors believe that this finding suggests a dysplastic nature of the tumors.[86]