Localizing Broca's area for transcranial magnetic stimulation: Comparison of surface distance measurements and stereotaxic positioning

Background Precise placement of transcranial magnetic stimulation (TMS) coils over target regions is crucial for correct interpretation of TMS effects. Modern frameless stereotaxic systems yield high accuracy, but require extensive equipment and cannot be used in every setting, for example, during functional imaging sessions. Objective The aim of this study was the development of a method for TMS-coil placement based on individual imaging data without the need for external tracking devices. Methods We compared coil positioning over Broca's area using an advanced stereotaxic navigation system with placement according to the surface distance measurements (SDM) method. By using the SDM-method, 3-dimensional renderings adapted from individual T1-weighted magnetic resonance imaging (MRI) data were created to identify Broca's area and Broca's homologue, respectively, and to define anatomic landmarks on the skin's surface. Distances between these landmarks were used to localize the real target on the individual's head. Results The mean Euclidean distance between surface positions as determined with the two methods was 8.31 mm and the mean difference of estimated virtual electric field intensity at the target point was 7.37 V/m corresponding to 4.01% of maximum field strength. Conclusions Our findings suggest that, compared with a state-of-the-art frameless stereotaxy system, the SDM-method yields a reasonable accuracy for positioning of a TMS-coil over Broca's area in terms of spatial coordinates.