The era of image guidance in epilepsy surgery was inaugurated with the advent of MRI and with the description of findings of mesial temporal sclerosis. A marked increase in seizure-free outcomes is achieved by identification and resection of a small, T2-bright hippocampus with loss of internal laminar architecture, sometimes accompanied by ipsilateral forniceal and/or mammillary body atrophy. Hopes were then high for characterizing focal structural abnormalities in nearly every patient with refractory partial seizures. However, in everyday clinical practice this promise has yet to materialize. Although MRI has been shown to be a reliable tool to identify an abnormal hippocampus in patients with refractory temporal lobe epilepsy, its efficacy for detection of extratemporal neocortical seizure foci is significantly lower. In pediatric patients, for example, the search for focal cortical dysplasia is often time-consuming in the absence of a suggestive electroencephalographic focus. This often results in the performance of additional tests for lateralization and localization of the epileptogenic foci, including PET, SPECT, magnetoencephalography (MEG), fMRI, and sometimes Wada testing.

Neuroradiologists and allied neuroscientists have made significant progress over the last two decades, utilizing a myriad of techniques to identify epileptogenic foci and to characterize secondary abnormalities in the epileptogenic zone and related circuits, eg, limbic system in temporal lobe epilepsy. By applying advanced MR techniques we can appreciate that MR exams of patients with temporal lobe epilepsy are rarely normal, even when negative by visual inspection. The challenge neuroradiologists now face is to bring these tools into our reading rooms. At Emory University, we routinely report automated volumetric measurements of the hippocampi in our clinical exams performed for epilepsy, which also include comparison with a normative database. We are also currently exploring double-inversion recovery (DIR), an MRI sequence that increases the conspicuity of gliotic and dysplastic tissue by suppressing signal of normal white matter and of CSF.¹

In this issue of AJNR Digest, we highlight the application of advanced MR techniques to identify epileptogenic foci below the threshold of

conventional MR imaging. Diffusion tensor imaging has the ability to identify the neocortical epileptogenic zone² or the abnormal hippocampus.³ The abnormal temporal lobe can be identified in the interictal phase by a decreased NAA peak on spectroscopy,⁴ by decreased perfusion using arterial spin-labeling,⁵ or by high ADC values using diffusion-weighted imaging.⁶ Finally, Coan et al demonstrate that T2 relaxometry and automated volumetric analysis improve detection of hippocampal sclerosis in patients with mesial temporal lobe epilepsy.⁷

The techniques reviewed in this issue have the potential to be implemented in the routine clinical readout of epilepsy brain MRI exams. We have invited the authors to detail how these discoveries have impacted their practice. As you will notice, most of these articles address mesial temporal lobe epilepsy; however, the techniques are also suitable to investigate neocortical epilepsy, more frequently encountered in pediatric patients.

References

1. Rugg-Gunn FJ, Boulby PA, Symms MR, et al. Imaging the neocortex in epilepsy with double inversion recovery imaging. Neuroimage 2006;31:39–50
2. Widjaja E, Geibprasert S, Otsubo H, et al. Diffusion tensor imaging assessment of the epileptogenic zone in children with localization-related epilepsy. AJNR Am J Neuroradiol 2011;32:1789–94
3. Assaf BA, Mohamed FB, Abou-Khaled KJ, et al. Diffusion tensor imaging of the hippocampal formation in temporal lobe epilepsy. AJNR Am J Neuroradiol 2003;24:1857–62
4. Capizzano AA, Vermathen P, Laxer KD, et al. Multisection proton MR spectroscopy for mesial temporal lobe epilepsy. AJNR Am J Neuroradiol 2002;23:1359–68
5. Wolf RL, Alsop DC, Levy-Reis I, et al. Detection of mesial temporal lobe hypoperfusion in patients with temporal lobe epilepsy by use of arterial spin labeled perfusion MR imaging. AJNR Am J Neuroradiol 2001;22:1334–41
6. Goncalves Pereira PM, Oliveira E, Rosado P. Apparent diffusion coefficient mapping of the hippocampus and the amygdala in pharmaco-resistant temporal lobe epilepsy. AJNR Am J Neuroradiol 2006;27:671–83
7. Coan AC, Kubota B, Bergo FP, et al. 3T MRI quantification of hippocampal volume and signal in mesial temporal lobe epilepsy improves detection of hippocampal sclerosis. AJNR Am J Neuroradiol 2014;35:77–83

 

Image modified from: Widjaja E, Geibprasert S, Otsubo H, et al. Diffusion tensor imaging assessment of the epileptogenic zone in children with localization-related epilepsy.

Figure legend: Child with MR imaging–negative epilepsy localized to the left frontal lobe. Regions of abnormal fractional anisotropy (orange areas in A) and mean diffusivity (orange areas in B) demonstrate lobar concordance with the magnetoencephalography dipole cluster (white areas) corresponding to the epileptogenic zone. See article for more details.