Several neuroimaging techniques can be used to evaluate seizure localization and plan surgical treatment of structural lesions.

CT scans

Computed tomography (CT) scans of the head are usually reserved for the acute evaluation of new-onset seizures, owing to their ready availability in settings such as the emergency room. They can detect etiologies such as acute hemorrhage or trauma50 but they are less sensitive than magnetic resonance imaging (MRI) in detecting structural lesions. Middle and posterior fossa lesions can be masked by bone artifact on CT. 49

MRI

MRI is the gold standard in evaluating epilepsy. It can identify lesions, such as a neoplasm, that may be missed on CT. New-onset nontraumatic seizures should be evaluated with gadolinium-enhanced T1-weighted, T2-weighted, and fluid-attenuated inversion recovery (FLAIR) images. To evaluate a chronic disorder, however, the use of contrast may be unnecessary unless there has been a recent change in seizure type, frequency, or intensity.51

MRI is free of radiation and is therefore very safe.52 It can be used to verify the placement of invasive electrodes in evaluations for epilepsy surgery, and to document the extent of cortical resection after surgery.

Some brain tumors have characteristic findings that can distinguish them from non-neoplastic lesions or even suggest a specific type of tumor. It may be difficult, however, to differentiate low-grade gliomas from other tumor types presenting with epilepsy:

• Astrocytomas characteristically show a T2-signal increase and a nonspecific T1 signal of solid components (see Figure1). Cystic components are isointense to cerebrospinal fluid.

Figure 1. Low-grade astrocytoma. (A) MRI fluid-attenuated inversion recovery (FLAIR) axial image without contrast enhancement. (B) T1-weighted axial image after gadolinium infusion. Images show an abnormal increased signal in the right anterior temporal lobe (arrow).

• Oligodendrogliomas look like astrocytomas but may have a honeycomb and stippled amorphous appearance with areas of superficially distributed calcifications.53
• Gangliogliomas have no specific MRI features distinguishing them from other gliomas (see Figure 2).

Figure 2. Ganglioglioma. (A) T1-weighted axial image of a left frontoparietal space-occupying lesion (arrow). (B) Note no appreciable contrast enhancement (arrow) after gadolinium infusion.

• DNETs (dysembryoplastic neuroepithelial tumors) usually affect the frontal and temporal lobes in children, also appearing similar to low-grade gliomas, except that edema is rarely appreciated (see Figure 3). Kuroiwa et al. described DNETs as hypointense on T1-weighted sequences and hyperintense and well demarcated on T2-weighted sequences with no contrast enhancement noted.54

Figure 3. Dysembryoplastic neuroepithelial tumor (DNET). T2-weighted axial MRI of a right medial temporal infiltrative lesion demonstrating abnormal signal intensity.

• Glioblastoma multiforme (GBM) displays a heterogenous signal on T1- and T2-weighted imaging and inhomogeneous postgadolinium ring enhancement with nonenhancing areas representing necrosis (see Figure 4).55

Figure 4. Glioblastoma multiforme. Gadolinium-enhanced axial T1-weighted MRI showing a left-sided multicystic necrotic lesion with a solid enhancing component in the temporoparietal region.

• Meningiomas are usually isodense on T1- and T2- weighted images and demonstrate intense postgadolinium enhancement in the T1 sequences (see Figure 5).56

Figure 5. Meningioma. Gadolinium-enhanced axial T1- weighted MRI showing a homogenously enhancing extra-axial lesion extending into the left-middle cranial fossa.

Focal calcifications are seen in 25% of tumors, but CT scans show hyperintensity in more than 70%.55 To identify focal lesions, T2-weighted sequences are more sensitive to low-grade tumors, arteriovenous malformations, focal gliosis, and hamartomas.57–59

Other techniques

Other neuroimaging techniques, while not currently primary neurodiagnostic modalities in the evaluation of tumors and epilepsy, are helpful adjuncts. They can increase the level of certainty that the region of seizure activity and eloquent cortex have been correctly identified, provide prognostic tumor data, and guide intracranial electrode placement.

Positron emission tomography (PET) scanning is a functional glucose metabolic imaging technique that assesses cerebral metabolism. Patients with low-grade epileptogenic tumors may demonstrate areas of hypometabolism interictally, and hypermetabolism may be seen during ictal events. These areas tend to be larger than the anatomic abnormality itself.60 PET with fluoro-2-deoxy-D-glucose (FDG) uptake can have prognostic value in low-grade gliomas.61 Those tumors that display areas of increased FDG uptake are likely to recur or progress to higher grades.

Derlon et al. compared metabolic patterns between astrocytomas and oligodendrogliomas using PET FDG (glucose) and 11C-L-methylmethionine (amino acid) uptake.62 Although both tumors showed glucose hypometabolism, amino acid uptake was increased only in oligodendrogliomas. This suggests that specialized PET protocols may aid in distinguishing tumor types. Continued research using this technology may help improve diagnosis, therapy, and assessment of possible tumor progression.

Single photon emission computerized tomography (SPECT) is a functional cerebral perfusion imaging technique used in the noninvasive physiologic evaluation of intractable seizures to help define focal areas of abnormality. SPECT is less expensive than PET and is available in more hospitals. Electrographically, widespread blood flow changes may be seen during a seizure, but well-timed SPECT injections close to ictal onset may identify the origin of seizure onset. SPECT aids in localization by demonstrating cerebral areas that have decreased regional cerebral blood flow (rCBF) interictally and increased areas of rCBF ictally. There are several difficulties in accomplishing ictal SPECT, however. Injecting the radioisotope on time can be a challenge (particularly with nocturnal seizures), given the unpredictability and short duration of some seizures, the need to obtain the isotope, the limit of its half-life, and the limited sensitivity and specificity of postictal data.63

More recently, co-registered subtraction SPECT imaging from MRI has been shown to improve reliability in localizing seizure foci postictally.64

Magnetic resonance spectroscopy (MRS) is a noninvasive technique that measures metabolic activity, allowing for the comparison of neural and tumoral elements containing protons. Relevant metabolites are creatine, N-acetylaspartate, lactate, and carbohydrate-containing phospholipids. Assessment of carbohydrate (CHO) peaks and lac tate levels helps to assess tumor aggressiveness and distinguish radiation damage from tumor recurrence.

In a recent study of 11 pediatric patients with low-grade gliomas, Lazareff et al. showed that MRS CHO values were a viable noninvasive prognostic tool to follow tumor progression, with higher ratios correlating with more rapid tumor growth.65

MRS has also been shown to be of prognostic value when treating recurrent malignant gliomas with radiosurgery.66

Functional MRI (fMRI) is another noninvasive imaging technique that maps changes in rCBF and concentrations of oxyhemoglobin or deoxyhemoglobin on task performance. It is a useful tool to identify regions of eloquent cortex that need to be spared in lesion resection.67,68

Adapted from: Mangano FT, McBride AE, and Schneider SJ. Brain tumors and epilepsy. In: Ettinger AB and Devinsky O, eds. Managing epilepsy and co-existing disorders. Boston: Butterworth-Heinemann; 2002;175–194.
With permission from Elsevier (www.elsevier.com).
Reviewed and revised March 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.

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