gms | German Medical Science

Integration of DTI based fiber information has evolved into a routine tool in navigated brain tumor surgery to spare functional fiber anatomy during resection. However, fiber anatomy may alter, when brain shift occurs. The aim of this study was to evaluate the feasibility of intraoperative DTI in order to update fiber navigation after intraoperative MRI (iMRI) and to thus estimate the effects of brain shift.

20 patients with astrocytoma of the fronto-temporal region, who were operated in a 3T-iMRI hybrid OR suite, received pre- and intraoperative acquisition of DTI imaging. Pre- and intraoperative tracking of the corticospinal tract (CST) was performed using the same tracking algorithms (FA 2.0, minimum fiber lengths 11cm), the two generated fiber objects (preOP/intraOP) were fused into the same cMPRAGE. Maximum fiber distances on axial sections were measured. In the vicinity of the CST monopolar subcortical stimulation (SCS) (maximum current: 15 mAmps, impulse width: 0.5 msec, interstimulation interval: 4 msec, square pulse) was performed. Minimum stimulation intensity to elicit motor evoked potentials (MEPs) was registered on various acquired points in the neuronavigation. Distance from the acquired points to DTI fiber tracts was measured and matched to stimulation intensity.

Intraoperative fiber tracking was feasible in all cases. Distance between pre- and intraoperative fiber tracts varied between 0 and 9mm (mean: 4.9mm ± 2.2mm). Fiber shift was dependent on size and location of the tumor, as well as on intraoperative corticospinal fluid (CSF) loss. Resected tumor volumes at the time of iMRI ranged from 22,7cm3 to 42,5cm3 (mean: 35,5cm3 ± 24,2cm3). Large temporo-insular tumors resulted in considerable fiber shifts, while with precentral tumors hardly any shift was observed. In absence of relevant fiber shifts, the minimum stimulation intensity to elicit MEPs correlated to the distance to CST in a linear fashion (1mA = 1mm). Altered fiber location was confirmed through modified minimum current amplitudes to elicit MEPs during SCS.

While SCS with electrophysiological mapping remains the gold standard in locating eloquent fibers during surgery, updating fiber visualization after ioMRI may aid the operating surgeon in regaining orientation of the fiber anatomy during the resection of large tumors and may result in better preservation of neurological function as well as better extent of resection.