Registration of transcranial magnetic stimulation, a visualization tool for brain functions

We describe a method for the registration of functional brain data acquired with transcranial magnetic stimulation (TMS) on MRI brain images. TMS is a non-invasive method largely used in the study of brain functions. For the registration process we acquire 150 points on the patient´s scalp with a magnetic-field digitizer. Then, we minimize the mean square distance between those points and the segmented scalp surface drawn from the MR image. The distance to the scalp surface is computed with the help of a 3D Euclidean distance transformation. For each stimulation, the position of the TMS device is acquired with the digitizer. The registration transformation is applied to the TMS coordinate in order to map TMS data and anatomical information. The results show that the method is precise (4 mm) and reproducible (1 mm).