| Research Abstract |
We developed a MRI-linked whole head magnetoencephalography (MEG) system as a tool of non-invasive functional brain mapping. In 20 healthy subjects, 80 patients with intracranial structural lesions and 4 patients with intractable seizures, neuromagnetic fields were measured over the entire head, using a helmet shaped 66 channel MEG system.
First, we measured somatosensory evoked field (SEF), auditory evoked field (AEF), and visually evoked field (VEF). We localized generators of above brain activities, using a multiple dipole model with a best-fitting sphere for each subject's head. Finally, the dipoles were superimposed on 3-D MRI.SEF for median and tibial nerve stimuli localized the contralateral central sulcus. AEF for tones localized bilateral superior temporal planes. VEF localized calcarine fissure contralateral to the stimulated half visual field. Positions of the sources were precisely on the responsible cortical surface, with standard deviation of 2-4 mm, either with or without structural lesions.
Second, in patients with epilepsy, interictal epileptic discharges were measured by means of both MEG and EEG.We observed two dipole patterns bilaterally in two cases. These dipolar patterns overlap each other across the midline. A two dipole model estimated "spike" or "wave" dipoles in bilateral neocortices, unilateral neocortex and contralateral mesial temporal structure, or bilateral mesial temporal structures. In the other 2 cases, single dipole pattern appeared on unilateral hemisphere only ; these spike or wave hipoles were estimated within one lobe. The whole head MEG system is useful to localize multifocal epileptic discharges, as well as to evaluate signal propagation between bilateral hemispheres.
In summary, MEG localized cortical function non-invasively, with exceptionally high spatial resolution confirmed by our MRI-linked system. This technique was useful for conventional and intravascular neurosurgery, as well as for stereotaxic radiosurgery.
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