MEG time–frequency analyses for pre- and post-surgical evaluation of patients with epileptic rhythmic fast activity

Magnetoencephalography as a Prognostic Tool in Patients with Medically Intractable Temporal Lobe Epilepsy

BACKGROUND

Most surgical treatments for medically intractable temporal lobe epilepsy are helpful. When a patient has persistent symptoms after surgery, there are no tests that accurately predict whether a patient will have remnant epileptic foci. The aim of this study was to evaluate the usefulness of magnetoencephalography (MEG) as a prognostic tool in patients with temporal lobe epilepsy.

METHODS

From July 2012 to July 2016, 21 patients underwent preoperative and postoperative MEG at our center. Postoperative MEG was performed within 2 weeks after surgery. We analyzed MEG by estimating the time-frequency component of the signal to define gamma oscillations (GOs), which are an indicator of epileptogenic foci. We analyzed the relationship between GOs on MEG and surgical outcomes.

RESULTS

Mean follow-up period was 28.3 months (range, 13-44 months). At the last follow-up visit, patients were divided into 2 groups according to surgical outcome. All patients showed spike waves and GOs on preoperative electroencephalography and MEG. In the seizure control group (16 patients), spike waves (2 patients) and GOs (2 patients) were seen postoperatively despite absence of symptoms. In the recurrent seizure group (5 patients), whereas 3 patients showed spike waves, all 5 patients showed GOs on MEG postoperatively. There was a significant association between presence of GOs on postoperative MEG and surgical outcome (P = 0.01).

CONCLUSIONS

MEG can provide valuable postsurgical information on epileptic foci in patients with recurrent symptoms; GOs on postoperative MEG were especially correlated with epileptic recurrence. Our data show that GOs on postoperative MEG may have prognostic value.

Coregistrating magnetic source and magnetic resonance imaging for epilepsy surgery in focal cortical dysplasia

Background

Epilepsy surgery for focal cortical dysplasia type II (FCD II) offers good chances for seizure freedom, but remains a challenge with respect to lesion detection, defining the epileptogenic zone and the optimal resection strategy. Integrating results from magnetic source imaging from magnetoencephalography (MEG) with magnetic resonance imaging (MRI) including MRI postprocessing may be useful for optimizing these goals.

Methods

We here present data from 21 adult FCD II patients, investigated during a 10 year period and evaluated including magnetic source imaging. 16 patients had epilepsy surgery, i.e. histopathologically verified FCD II, and a long follow up. We present our analysis of epileptogenic zones including MEG in relation to structural data according to MRI data and relate these results to surgical outcomes.

Results

FCD II in our cohort was characterized by high MEG yield and localization accuracy and MEG showed impact on surgical success-rates. MEG source localizations were detected in 95.2% of patients and were as close as 12.3 ± 8,1 mm to the MRI-lesion. After a mean follow up of >3 years, we saw >80% Engel I outcomes, with more favourable outcomes when the MEG source was completely resected (Fishers exact test 0,033).

Conclusion

We argue for a high value of conducting a combined MEG-MRI approach in the presurgical workup and the resection strategy in patients with FCD II related epilepsy.

Intraoperative coregistration of magnetic resonance imaging, positron emission tomography, and electrocorticographic data for neocortical lesional epilepsies may improve the localization of the epileptogenic focus: a pilot study

OBJECTIVE

To objectively mark out abnormal areas of magnetic resonance imaging (MRI), positron emission tomography (PET), and electrocorticography (ECoG) using neuronavigation so as to 1) enhance the accuracy of margins of the epileptogenic zone and 2) understand the relationships of all the three modalities with each other.

METHODS

A prospective study was conducted of 37 patients with intractable epilepsy due to lesional, neocortical pathologies from noneloquent areas. Prior to surgery, fusion and transfer of MRI and PET images onto a neuronavigation system was performed. At surgery, this was correlated to intraoperative ECoG using the electrode as referential points. An objective score was created for every electrode point that was correlated with MRI and PET abnormality at the point. The extent of surgical resection was mapped out using these data.

RESULTS

From a total of the data recorded from 1280 electrode points, 23.5% were located over the lesion. In addition, over the lesions, 93% of PET and 66% of ECoG points were abnormal. Over the perilesional areas, 43% of PET and 45% of ECoG points were abnormal. Using these data for surgery, both lesional and perileisonal areas were resected; 33/37 patients had good outcome (25 Engel I, 8 Engel II) (mean follow-up: 23.6 ± 3.2 months; range 18-31 months).

CONCLUSION

Multimodal imaging and ECoG using this method seems to provide a better objective localization of the epileptogenic foci.

Source localization in magnetoencephalography to identify epileptogenic foci

RATIONALE

Magnetoencephalography (MEG) is useful to localize epileptic foci in epilepsy as MEG has higher spatio-temporal resolution than conventional diagnostic imaging studies; positron emission computed tomography, single photon emission computed tomography and magnetic resonance imaging (MRI).

METHODS

We use 204-channel helmet-shaped MEG with a sampling rate of 600 Hz. A single dipole method calculates equivalent current dipoles to localize epileptic sources. The equivalent current dipoles are superimposed onto MRI as magnetic source imaging (MSI). Ictal MEG data are analyzed using time-frequency analysis. The power spectrum density is calculated using short-time Fourier transform and superimposed onto MRI results.

RESULTS

Clustered equivalent current dipoles represent epileptogenic zones in patients with localization-related epilepsy. The surgical plan is reliably developed from source localizations of dipoles and power spectrum of interictal spike discharges, and ictal frequency.

CONCLUSION

MEG is indispensable in diagnosis and surgical resection for epilepsy to accurately localize the epileptogenic zone.