Object Navigated transcranial magnetic stimulation (nTMS) combined with
diffusion tensor imaging (DTI) is used preoperatively in patients with
eloquent-located brain lesions and allows analyzing non-invasively the spatial
relationship between the tumor and functional areas (e.g. the motor cortex and
the corticospinal tract [CST]). In this study, we examined the diffusion
parameters FA (fractional anisotropy) and ADC (apparent diffusion coefficient)
within the CST in different locations and analyzed their interrater
reliability and usefulness for predicting the patients' motor outcome with a
precise approach of specific region of interest (ROI) seeding based on the
color-coded FA-map. Methods Prospectively collected data of 30 patients
undergoing bihemispheric nTMS mapping followed by nTMS-based DTI fiber
tracking prior to surgery of motor eloquent high-grade gliomas were analyzed
by 2 experienced and 1 unexperienced examiner. The following data were
scrutinized for both hemispheres after tractography based on nTMS-motor
positive cortical seeds and a 2nd region of interest in one layer of the
caudal pons defined by the color-coded FA-map: the pre- and postoperative
motor status (day of discharge und 3 months), the closest distance between the
tracts and the tumor (TTD), the fractional anisotropy (FA) and the apparent
diffusion coefficient (ADC). The latter as an average within the CST as well
as specific values in different locations (peritumoral, mesencephal, pontine).
Results Lower average FA-values within the affected CST as well as higher
average ADC-values are significantly associated with deteriorated
postoperative motor function (p = 0.006 and p = 0.026 respectively). Segmental
analysis within the CST revealed that the diffusion parameters are especially
disturbed on a peritumoral level and that the degree of their impairment
correlates with motor deficits (FA p = 0.065, ADC p = 0.007). No significant
segmental variation was seen in the healthy hemisphere. The interrater
reliability showed perfect agreement for almost all analyzed parameters.
Conclusions Adding diffusion weighted imaging derived information on the
structural integrity of the nTMS-based tractography results improves the
predictive power for postoperative motor outcome. Utilizing a second
subcortical ROI which is specifically seeded based on the color-coded FA map
increases the tracking quality of the CST independently of the examiner's
experience. Further prospective studies are needed to validate the nTMS-based
prediction of the patient's outcome