gms | German Medical Science

Objective: Fiber tracking is a popular tool for visualizing the corticospinal tract. However, results may be influenced by the selection of regions of interests, the chosen tracking algorithm and the presence of peritumoral edema. Moreover, intraoperative brain shift may lead to inaccurate results. To overcome these limitations, fMRI and nTMS data were combined with deterministic and probabilistic fiber tracking, whereas brain shift was limited by the use of intraoperative MRI (iMRI).

Method: Eight patients (age 55.2 ± 17.9 years, four women) suffering from perirolandic lesions underwent iMRI-guided surgery. Pre- and postoperatively, fMRI and nTMS were performed for somatotopic motor cortex definition, serving as regions of interest for fiber tracking. Data from intraoperative cortical and subcortical mapping were related to their distances of these tracks.

Results: Mapping sites were closer to fiber tracks based on nTMS data than to fiber tracks using fMRI, independent of the tracking algorithms (p=0.00003). Moreover, deterministic and probabilistic fiber tracking differed significantly in depicting lower limb fibers (p=0.0029), whereas the difference between tracking algorithms did not reach significance for upper limb fiber tracking. In a preliminary subset of data, distances between stimulation sites and fiber tracks showed good correlation with MEP thresholds.

Conclusions: Our results suggest that combining nTMS with probabilistic fiber tracking allows for a more accurate depiction of the corticospinal tract. Furthermore, the correlation between MEP thresholds and distances to the fiber tracks may help to better preserve motor function.