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

Imaging Modalities in Epilepsy

Although the routine use of CT for the evaluation of epileptic patients has decreased with availability of MRI, given its accessibility and lower cost, it is still used in emergency scenarios and when high-resolution imaging cannot be obtained. CT can accurately detect hemorrhages, gross structural malformations, large tumors, and calcified lesions. In 1976, shortly after the development of this technique, Gastaut reported a proportion of 34–51% cerebral lesions in epileptic patients.[2] A comparison of early versions of T2-weighted spin-echo MRI and contrast-enhanced CT in patients undergoing surgical evaluation for refractory partial epilepsy showed that these modalities detected 83% and 58% of the causative lesions, respectively.[3]

Brain MRI not only has higher sensitivity than CT, but also better spatial resolution and soft tissue contrast. In addition, it allows multiple plane imaging as well as functional cerebral assessment through different techniques. All these characteristics make MRI the primary imaging modality for the evaluation of patients with epilepsy. MRI with a dedicated epilepsy protocol increases even more the frequency with which epileptogenic lesions are identified.[4] Although protocols vary from institution to institution, most include axial and coronal oblique T2-weighted fast spin echo (FSE) and fluid attenuated inversion recovery (FLAIR) sequences, as well as coronal magnetization-prepared rapid acquisition gradient echo (MPRAGE) and/or spoiled gradient recalled echo (SPGR) volumetric dataset.

Contrast enhanced and gradient echo (GRE) images and single voxel spectroscopy are also included in some protocols because these are useful in the identification of specific pathologies such as tumors and sequelae of traumatic brain injury (TBI). There are newer advanced techniques that can be used to further delineate the epileptogenic substrate and its relationship with surrounding tissue, such as diffusion tensor imaging (DTI), which reveals white matter tracts with great precision. Moreover, functional MRI (fMRI) has been used for the identification of eloquent cortex, especially when related to language, in patients undergoing presurgical evaluation.

Although MRI provides invaluable information for the identification of the anatomic substrate of epilepsy, other complementary neuroimaging modalities are required in complex cases and when planning for surgical resection of the epileptogenic focus. Positron emission tomography (PET), for example, makes use of glucose metabolism (18F-fluorodeoxyglucose [18F-FDG]) as an indirect measure of neuronal function. An epileptogenic focus typically appears as an area of hypometabolism on interictal scans and an area of hypermetabolism on ictal scans. However, its ability to image cerebral metabolism is limited to the interictal state due to the long radiotracer uptake time (30–45 minutes) that is significantly longer than the average seizure duration.[5]

Single photon emission computed tomography (SPECT) entails the use of 99mTc-HMPAO (technetium-99 hexamethylpropylene amine oxime) or 99mTc-ECD (technetium-99m-ethylcysteinate diethylester) as substrate to assess regional cerebral blood flow changes during both the ictal and interictal periods. The epileptogenic focus typically appears as an area of hypoperfusion in the interictal state and hyperperfusion in the ictal state. Interictal SPECT has low sensitivity, but SPECT perfusion difference (ictal-interictal) is believed to have higher sensitivity and specificity than any other noninvasive localizing criterion.[6] However, ictal imaging is technically challenging as it requires dedicated personnel waiting at the bedside to accomplish the tracer injection within the initial few seconds after seizure onset.

Magnetoencephalography (MEG) is another imaging modality utilized for the identification of the source(s) of epileptiform activity during presurgical evaluation. It is especially useful in cases where primary modalities, including ictal EEG and MRI, are not clearly localizing. It is also used for the mapping of presurgical eloquent cortex, such as in the lateralization and regional localization of language.[7,8] Although EEG measures electrical signals that originate from volume-conducted extracellular activity, MEG detects electromagnetic neural activity that arises from intracellular postsynaptic currents.[9,10] MEG is most sensitive to tangentially oriented cortical sources, and it has excellent temporal and good spatial resolution.[11,12] The magnetic dipoles generated by MEG are then superimposed on structural MR images, creating magnetic source imaging (MSI). This technique provides nonredundant clinical information in about one-third of the epilepsy cases undergoing presurgical evaluation.[13] Unfortunately, its use in the localization of epileptogenic activity is limited to the interictal state due to physical constraints such as the size of the machine and the need for a magnetically shielded room.

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