Poster: 2-1 Clinical MEG: Presurgical evaluation of functional mapping 2-1-1: Comparison of Functional Localization with MEG Somatosensory Evoked Field and Intraoperative or Extraoperative SEP

For surgical treatment of medically intractable epilepsy it is important to localize motor functional area in the brain to avoid involving this area during the resection of seizure onset or epileptogenic zones in order to avoid any impairment of motor function after the surgery. Intraoperative visual identification of the central sulcus may be difficult. For this reason, the median nerve somatosensory evoked potential (SEP) recorded from a cortical grid array has been used to demonstrate the characteristic phase reversal on the central sulcus. However, intraoperative median nerve SEP can be often time consuming. Functional localization with magnetoencephalography (MEG) and whole head magnetometers has shown increasing usefulness and accuracy with new source analysis algorithms. We recorded 8 children, who underwent surgery, with either intraoperative or extraoperative median nerve SEP and median nerve somatosensory evoked field (SEF) with CTF 275-channel MEG system. For the SEF electrical stimuli were given to left and right median nerve alternately with 200μs duration, 3.9 Hz rate, and 500 repetitions with intensity to produce a clearly visible muscle twitch causing abduction of thumb. The source localizations were done with either single equivalent current dipole fit or SAM, or both. Those two modalities were compared with 3D brain images created by co-registration of each patient's MRI and CT with subdural grid with CURRY6 and compared with intraoperative images of grid, dipole fit or SAM peak and brain surface topography. We found good accuracy for MEG central sulcus localization. This methodology may reduce time needed to verify precentral motor gyrus intraoperatively.