Introduction:

Magnetoencephalography (MEG) has been used to aid in the localization of epileptogenic foci in epilepsy patients by identifying highly coherent activity in epileptic neural networks [1]. Diffusion tensor imaging (DTI) can identify virtual pathways of major fiber tracts and quantify abnormalities that are thought to reflect disruption of the microstructural environment[2]. We aimed to correlate MEG and DTI findings in patients with mesial temporal lobe epilepsy (mTLE) and examine the ability of each modality to predict laterality in mTLE [3].

Methods:

Preoperative resting state MEG data was analyzed to determine coherence in 54 anatomical sites in 17 adult mTLE patients who subsequently underwent surgical resection and Engel class I outcome, and compared with 12 age- and gender- matched controls. DTI tractography identified the fiber tracts passing through these same anatomical sites. Then, DTI nodal degree and laterality index were calculated and compared with the corresponding MEG coherence and laterality index. Statistical analysis was performed with one way ANOVAs with Bonferroni adjustment.

Results:

MEG coherence laterality showed significant differences for right versus left mTLE in insular cortex and both lateral orbitofrontal and superior temporal gyri (p<0.01, Figure 1). Likewise, DTI nodal degree laterality showed significant differences for right versus left mTLE in gyrus rectus, insular cortex, precuneus and superior temporal gyrus (p<0.01 , Figure 1). In insular cortex, MEG coherence laterality correlated with DTI nodal degree laterality (R^2=0.46; p=0.003) in the cases of mTLE. None of these anatomical sites showed statistically significant differences in coherence laterality between right and left sides of the controls. MEG coherence laterality and DTI nodal degree laterality were in agreement with the declared side of epileptogenicity in 94% and 100% of the patients, respectively.

Conclusions:

MEG coherence laterality was related to the nodal degree laterality extracted by analysis of DTI connectivity. Both indices may be helpful in noninvasive lateralization of mTLE patients.

Patient Care:

The proposed methodology for using MEG and DTI to investigate diffusion abnormalities related to focal epileptogenicity and propagation may provide a further means of noninvasive lateralization. With increasingly sophisticated signal processing methods and the use of multimodal neuroimaging and neurophysiological biomarkers, noninvasive investigational techniques may ultimately supplant invasive monitoring as a means of localizing focal epileptogenicity and establishing surgical candidacy.

Learning Objectives:

By the conclusion of this session, participants should be able to 1)Describe the correlation between diffusion properties of fiber tracts quantified by DTI and coherence measures quantified by MEG 2) Understand how MEG and DTI indices can contribute to noninvasive lateralization of mTLE patients.

References:

Elisevich, K., et al., An assessment of MEG coherence imaging in the study of temporal lobe epilepsy. Epilepsia, 2011: p. no-no. 2. Rugg-Gunn, F.J., Diffusion imaging in epilepsy. Expert Rev Neurother, 2007. 7(8): p. 1043-54. 3. Stefan, H., et al., Periventricular nodular heterotopia: A challenge for epilepsy surgery. Seizure: the journal of the British Epilepsy Association, 2007. 16(1): p. 81-86.