Robotized image-guided transcranial magnetic stimulation, a novel technique for functional brain-mapping

Introduction: One of the major difficulties for neurosurgical interventions in or close of the central region of the brain is the individual distribution of functional areas. This is particularly important when dealing with tumours that cause displacement or migration of such areas. The functional MRI (fMRI) is the most common procedure for non-invasive functional brain-mapping. However, this technique does not completely meet the needs of neurosurgeons, because it does not allow the localization of single functional areas, like motor cortical areas of specific muscles. We here present a novel non-invasive and painless technique for motor cortex mapping: The robotized image-guided transcranial magnetic stimulation (Ri-TMS). Methods: Three patients with brain tumours of the central region and one healthy volunteer were examined by Ri-TMS. A figure-of-eight coil was placed over the central region of the examined persons according to a grid of up to 90 points with a fixed distance of 1cm between the grid points. The coil was positioned by an image-guided robot. Image-guidance was based on an anatomical MRI and a neuronavigaional device, tracking all movements of the patient. All grid points were stimulated by 10 single pulses (interstimulus interval = 5 seconds) successively. Motor evoked potentials (MEPs) of three representative muscles (m. abductor digiti minimi, m. abductor pollicis brevis and m. brachioradialis) were simultaneously recorded by Ag/AgCl surface electrodes connected to an Endeavor CR electromyographic device. The peak-to-peak amplitudes of the averaged MEP curves were measured online. The results obtained for each stimulated spot were used for calculating brain surface maps showing the regions of maximal MEP responses. To proof the reproducibility of Ri-TMS the examinations were repeated after 3-5 days. The results were compared to fMRI images acquired under finger-tapping protocol.