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Motor mapping with navigated transcranial magnetic stimulation (TMS)

Motor mapping with navigated transcranial magnetic stimulation (TMS)

Mapping the motor cortex in patients with a tumor in the proximity of the primary motor cortex is of crucial importance for the outcome of the surgery. For optimal results, the anatomical relation between the functional and pathological tissues needs to be investigated. The surgeons cannot rely on the position of known anatomical landmarks in relation to the pathologic tissue since the tumor can distort the normal anatomy of the cortex. The technique mostly adopted so far for the purpose is the fMRI, which, however, present several disadvantages. First of all, it requires the collaboration of the patient, the signal is very sensible to motion and large vessels might be shown as active sites. Navigated TMS can outcome most of the problem related to fMRI. Mapping with TMS is based on the relationship between the amplitude of the peripheral motor response evoked by applying a magnetic pulse and the density of motoneurons in the stimulated area. With the aid of navigation, the positions of the TMS coil during stimulation can be related to the underlying brain anatomy. The technique has been proved to be reliable, it doesn't require patient collaboration, and it is completely painless for the patient.

ANT has developed a system which integrates navigated TMS and EMG recording, and online processing. visor2 allows to record the evoked motor responses at multiple sites simultaneously, allowing therefore to generate functional maps highlighting the presumed cortical representations of the analyzed muscles.

Mapping of the not affected side.





Based on the estimation of the electrical field induced by the TMS coil, visor2 relates the stimulated sites on the scalp with the underlying brain anatomy. The evoked motor responses are processed online, and the calculated amplitude is related to the corresponding stimulate site in the cortex. Maps can be created by highlighting only the sites presumed to be the cortical representations of all investigated muscles. The generated maps can be then exported to 3D image formats directly readable by operating room navigation systems.


Each mapping session should be carefully analyzed offline: an incorrect placement of the coil or noise in the EMG signal might cause false positives. visor2 allows to review offline, for each of the stimulated sites the position of the coil in the moment of stimulation and the corresponding evoked response, and to discard the outliers.
