Probabilistic vs. deterministic fiber tracking and the influence of different seed regions to delineate cerebellar-thalamic fibers in deep brain stimulation.

This study compared tractography approaches for identifying cerebellar-thalamic fiber bundles relevant to planning target sites for deep brain stimulation (DBS). In particular, probabilistic and deterministic tracking of the dentate-rubro-thalamic tract (DRTT) and differences between the spatial courses of the DRTT and the cerebello-thalamo-cortical (CTC) tract were compared. Six patients with movement disorders were examined by magnetic resonance imaging (MRI), including two sets of diffusion-weighted images (12 and 64 directions). Probabilistic and deterministic tractography was applied on each diffusion-weighted dataset to delineate the DRTT. Results were compared with regard to their sensitivity in revealing the DRTT and additional fiber tracts and processing time. Two sets of regions-of-interests (ROIs) guided deterministic tractography of the DRTT or the CTC, respectively. Tract distances to an atlas-based reference target were compared. Probabilistic fiber tracking with 64 orientations detected the DRTT in all twelve hemispheres. Deterministic tracking detected the DRTT in nine (12 directions) and in only two (64 directions) hemispheres. Probabilistic tracking was more sensitive in detecting additional fibers (e.g. ansa lenticularis and medial forebrain bundle) than deterministic tracking. Probabilistic tracking lasted substantially longer than deterministic. Deterministic tracking was more sensitive in detecting the CTC than the DRTT. CTC tracts were located adjacent but consistently more posterior to DRTT tracts. These results suggest that probabilistic tracking is more sensitive and robust in detecting the DRTT but harder to implement than deterministic approaches. Although sensitivity of deterministic tracking is higher for the CTC than the DRTT, targets for DBS based on these tracts likely differ.

DTI-based deterministic fibre tracking of the medial forebrain bundle.

BACKGROUND: Deep brain stimulation (DBS) of the medial forebrain bundle (MFB) was reported to reduce symptoms in psychiatric disorders. The aim of our study was to find standardised parameters for diffusion tensor imaging (DTI) based fibre tracking to reliably visualise the MFB. METHODS: Twenty-two cerebral hemispheres in 11 patients were investigated. Three different regions of interest (ROIs) were defined as seed regions for fibre tracking: the ipsilateral and contralateral superior cerebellar peduncle (SCP) and the nucleus raphe dorsalis (NRD). From each seed region the fibres were followed separately through the ventral tegmental area (VTA = second ROI) and their further courses and volumina were documented and compared. Minimal fibre length was set at 30 mm and the FA threshold at 0.12. RESULTS: The fibre tracts starting in seed regions in the ipsilateral SCP and the NRD follow a similar course along the lateral wall of the third ventricle (hypothalamus) and the anterior limb of the internal capsule (ALIC) to inferior fronto-medial brain areas. These fibres are in accordance with the course of the MFB as described in various anatomical atlases. Consistently, a branch leaves the main fibre tract laterally to take a course through the capsula externa to the temporo-parietal cortex. Fibre tracts starting from the contralateral SCP follow a more superior and lateral course, including the dentato-rubro-thalamic and the pyramidal tract. CONCLUSIONS: Deterministic fibre tracking with standardised ROIs provides constant and reproducible delineations of the medial forebrain bundle. Its visualisation might help to adjust targeting in DBS for psychiatric disorders.

Deep brain stimulation for essential tremor: targeting the dentato-rubro-thalamic tract?

OBJECTIVE: The aim of our study was to evaluate the influence of the stimulation site relative to the dentato-rubro-thalamic tract (DRTT) on the alleviation of tremor in deep brain stimulation. METHODS: Ten DRTTs in five patients were investigated using preoperative diffusion tensor imaging (DTI). Regions of interest for fiber tracking were located in the cerebellar dentate nucleus, the superior cerebellar peduncle and the contralateral red nucleus. The position and distance of all intraoperative stimulation sites to the DRTT were measured and correlated to the amount of tremor reduction. RESULTS: Nine of 10 DRTTs could be identified using DTI-based fiber tracking. Better tremor reduction was achieved in locations in or posterior and lateral to the DRTT than in medial and anterior positions (p = 0.001). Stimulation sites inferior to and in the DRTT achieved better results than locations superior to the DRTT (p < 0.05). The vicinity of the stimulation site to the DRTT did not correlate with tremor alleviation. DISCUSSION: In deep brain stimulation targeting for thalamic stimulation sites is limited to statistical, atlas-based coordinates. Diffusion tensor imaging and fiber tracking was used to visualize the dentato-rubro-thalamic tract as a potential, individualized target structure. However, we could not demonstrate that contacts closer to the DRTT provided better clinical effects than distant contacts, in any given direction. DTI sequences with a higher number of read-out directions, probabilistic fiber tracking and three Tesla MRI scanners may lead to different results in the depiction of the chosen fiber tract and may provide a better correlation with stimulation effects. CONCLUSIONS: The results do not provide sufficient evidence to define the DRTT as a new DBS-target for tremor. Further investigations on different fiber tracts, DTI sequences, and fiber tracking algorithms are mandatory.

The variability of atlas-based targets in relation to surrounding major fibre tracts in thalamic deep brain stimulation.

BACKGROUND: In essential tremor (ET), the main target for deep brain stimulation (DBS) is the thalamic ventralis intermedius nucleus (Vim). This target cannot be identified on conventional magnetic resonance imaging (MRI). Therefore, targeting depends on probabilistic coordinates derived from stereotactic atlases. The goal of our study was to investigate the variability of atlas-based Vim targets in relation to surrounding major fibre tracts. METHODS: With the MRI and computed tomography (CT) scan data of ten patients who underwent DBS, we planned atlas based Vim targets in both hemispheres. We also performed deterministic fibre-tracking with diffusion tensor imaging (DTI) of the dentato-rubro-thalamic tract (DRTT), pyramidal tract (PT) and lemniscus medialis (LM) in all 20 hemispheres. Subsequently, we measured the distance from the atlas-based Vim target to each tract along the medial/lateral (x-coordinate), anterior/posterior (y-coordinate) and superior/inferior axis (z-coordinate). RESULTS: Seventeen out of 20 DRTTs could be depicted with our standardised DTI/fibre-tracking parameters. The PT and the LM could be displayed in all 20 hemispheres. The atlas-based Vim target was found inside the DRTT in 11 (concerning the x-coordinate) and 10 hemispheres (concerning the z-coordinate). Regarding the anterior/posterior direction, the target was posterior to the DRTT in 11 cases. In 19 hemispheres the Vim target was located medial and superior to the PT and in 17 hemispheres posterior to it. Concerning the LM, the Vim target was found inside the LM in 16 (regarding the x-coordinate) and in 14 cases (regarding the z-coordinate). In eight cases it was located inside and in 12 cases anterior to the LM concerning the y-coordinate. CONCLUSIONS: We found a considerable variability of the location of atlas-based target points of the ventralis intermedius nucleus in relation to neighbouring major fibre tracts in individual patients. These results suggest that individualised targeting to structures not directly visible on conventional MRI is necessary.