gms | German Medical Science

Objective: To establish a novel approach for fiber tracking based on nTMS mapping of the primary motor cortex and define an algorithm for determination of an individualized fractional anisotropy value.

Methods: 40 patients (17 female, 23 male, median age 58 years (20–80)) with brain tumors compromising the corticospinal tract underwent preoperative MR imaging and transcranial magnetic stimulation. For each patient, stimulation spots outlining the primary motor cortex next to the lesion were imported into the fiber tracking software and set as seed points for tractography. Next the individual FA threshold, i.e. the highest FA value leading to visualization of tracts at a predefined minimum fiber length of 110 mm, was determined. Fiber tracking was then performed at a fractional anisotropy value of 75% of the individual FA threshold. Results of tractography were presented to the surgeon for preoperative planning and integrated into the navigation system for intraoperative orientation and stimulation and its impact rated using a questionnaire.

Results: Mapping of the motor cortex was successful in all patients. A fractional anisotropy threshold could be obtained in every case making an individualised, yet objective TMS-based algorithm for tractography feasible. The obtained data changed or modified surgical strategy in 18 of 40 patients (45%) and facilitated intraoperative orientation and, if applicable, electrical stimulation in 23 of 40 (58%) patients.

Conclusions: Fiber tracking based on nTMS by the proposed standardised algorithm represents an objective visualisation method based on functional data and provides a valuable instrument for preoperative planning and intraoperative orientation and monitoring.