Improved nTMS- and DTI-derived CST tractography through anatomical ROI seeding on anterior pontine level compared to internal capsule

Abstract

Imaging of the course of the corticospinal tract (CST) by diffusion tensor imaging (DTI) is useful for function-preserving tumour surgery. The integration of functional localizer data into tracking algorithms offers to establish a direct structure–function relationship in DTI data. However, alterations of MRI signals in and adjacent to brain tumours often lead to spurious tracking results. We here compared the impact of subcortical seed regions placed at different positions and the influences of the somatotopic location of the cortical seed and clinical co-factors on fibre tracking plausibility in brain tumour patients.The CST of 32 patients with intracranial tumours was investigated by means of deterministic DTI and neuronavigated transcranial magnetic stimulation (nTMS). The cortical seeds were defined by the nTMS hot spots of the primary motor area (M1) of the hand, the foot and the tongue representation. The CST originating from the contralesional M1 hand area was mapped as intra-individual reference. As subcortical region of interests (ROI), we used the posterior limb of the internal capsule (PLIC) and/or the anterior inferior pontine region (aiP). The plausibility of the fibre trajectories was assessed by a-priori defined anatomical criteria. The following potential co-factors were analysed: Karnofsky Performance Scale (KPS), resting motor threshold (RMT), T1-CE tumour volume, T2 oedema volume, presence of oedema within the PLIC, the fractional anisotropy threshold (FAT) to elicit a minimum amount of fibres and the minimal fibre length.The results showed a higher proportion of plausible fibre tracts for the aiP-ROI compared to the PLIC-ROI. Low FAT values and the presence of peritumoural oedema within the PLIC led to less plausible fibre tracking results. Most plausible results were obtained when the FAT ranged above a cut-off of 0.105. In addition, there was a strong effect of somatotopic location of the seed ROI; best plausibility was obtained for the contralateral hand CST (100%), followed by the ipsilesional hand CST (>95%), the ipsilesional foot (>85%) and tongue (>75%) CST. In summary, we found that the aiP-ROI yielded better tracking results compared to the IC-ROI when using deterministic CST tractography in brain tumour patients, especially when the M1 hand area was tracked. In case of FAT values lower than 0.10, the result of the respective CST tractography should be interpreted with caution with respect to spurious tracking results. Moreover, the presence of oedema within the internal capsule should be considered a negative predictor for plausible CST tracking.

Visualization of subcortical language pathways by diffusion tensor imaging fiber tracking based on rTMS language mapping

“…Additionally, fibers adjacent to tumor margins and/or tumor edemata are prone to false negative results because of low anisotropy, which can lead to inaccurate DTI FT results (Berman et al 2007;Krieg et al 2012a;Weiss et al 2015), although in our cohort peritumoral edema did not interfere the DTI FT results.…”

Multimodal Imaging in Malignant Brain Tumors: Enhancing the Preoperative Risk Evaluation for Motor Deficits with a Combined Hybrid MRI-PET and Navigated Transcranial Magnetic Stimulation Approach

“…As in earlier studies, 13,17 motorevoked potentials were recorded of the abductor pollicis brevis muscle as the M1 hand representation, the plantar toe flexors representing the foot M1 region, and the anterior lateral tongue muscles as the M1 face representation. The "hotspot" was defined as the cortical stimulation site at which coil positioning, orientation, and tilt yielded the highest amplitude of the motor-evoked potentials.…”

“…17,26,27 For each M1 muscle representation, a 5-mm sphere centered around the nTMS hotspot was concentrically enlarged by 5 mm to form a standardized seed volume for the tractography in an anterograde direction for each voxel in the respective ROI (vector step length, 1.6 mm; angular threshold, 30°). 15,26 The course of the reconstructed fibers was verified as part of the CST by an experienced neuroradiologist.…”

“…9,15,16 For example, DTI-based tractography has been developed to visualize the subcortical fiber tracts in individual subjects; this is especially useful in conditions in which the functional anatomy can be heavily distorted such as in patients with brain tumors. 17,18 In addition, DTI can also be used to characterize the altered diffusion metrics within the tumor mass and the surrounding tissue, thereby allowing quantification of the integrity of fiber tracts. 19,20 In summary, there are currently rather limited data on how to preoperatively evaluate patients with brain tumors for operationassociated risks related to motor impairment and recovery thereof.…”

The Pre-Surgical Planning of Brain Neoplasms: From Diffusion Tensor Imaging to More Advanced Approaches

“…Indeed, a recent study has adopted nTMS for subject-specific localization of the motor area and then DTI-based tractography for CST tracking; this approach allowed a good detection of CST, although tractographic reconstructions were found affected by above described DTI limitations, such as the peritumoral edema. 34 The use of CSD-based tractography in a similar study protocol might provide a better subject-specific detection of CST. Finally it will be important to assess potential improvements and advantages for neurosurgeons provided by the integration of these techniques into intraoperative navigation systems.…”