P 12 Integrating nTMS motor mapping of the M1 tongue area into diffusion tractography to reconstruct motor-speech pathways

• Published in: Clinical neurophysiology 2017-10, Vol.128 (10), p.e332-e332
• Main Authors: Pieczewski, J., Nettekoven, C., Neuschmelting, V., Thiele, K., Hoevels, M., Reck, N., Tittgemeyer, M., Grefkes, C., Goldbrunner, R., Lucas, C. Weiss
• Format: Article
• Language: English
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2017.06.091

The preservation of important white matter tracts is extremely important to optimize the functional long-term outcome after brain tumour surgery. The integration of functional localizer data such as navigated transcranial magnetic stimulation (nTMS) as starting region of interest (ROI) within the primary motor cortex (M1) into diffusion-tensor-imaging- (DTI-) tractography algorithms represents a recent advancement of the methodology with regard to the reconstruction of the corticospinal tract (CST). We here studied the use of a single nTMS-derived seeding ROI of M1 of the tongue to reconstruct connected fibres of the motor-speech network. 13 healthy volunteers were investigated by nTMS mapping of M1. The hotspot, i.e. the cortical site of the highest motor evoked potential (MEP) amplitude, of the tongue representation was localized. The 3D-coordinate of the respective voxel (site of maximum calculated electrical-field strength) was obtained and was enlarged by a radius of 5mm. This spheric ROI, representing M1 of the tongue, served as origin for probabilistic fibre tracking (FSL). The reconstructed fibres were segmented according to anatomical knowledge. Moreover, the tractography result was compared to the ”traditional” approach using and anatomical seeding-ROI of equivalent size and depth. Tracts were visualized using MRIcron and Slicer (3D). Segments of the arcuate fasciculus (AF) could be reconstructed in all subjects using the single M1-tongue-ROI. Most prominently, the anterior segment of the AF was observed, linking the triangular and opercular part of the inferior frontal gyrus (IFG), i.e., Broca’s territory, with the inferior parietal lobule (85% of cases). Moreover, also the long segment of the AF, connecting the IFG to the angular and dorsal superior temporal gyrus, could be displayed in most cases (92% of cases). Besides several motor pathways like the CST, other tracts involved in language/speech processing were reconstructed using this approach, e.g., the left frontal aslant tract (85% of cases) for which a crucial involvement in speech production (verbal fluency) has been described. Preliminary results regarding diffusion metrics and numbers of aberrant fibres point towards the use of nTMS for seeding-ROI determination being more specific as compared anatomical ROI-seeding. Implementing the M1 hotspot of the tongue as a unique seeding-ROI for probabilistic diffusion tractography allows for depiction of motor-speech associated fibres, particu