gms | German Medical Science

Objective: Evidence accumulates that targets outside the traditional targets within basal nuclei provide superior results for treatment of tremor in deep brain stimulation. Stereotactic targeting of fiber tracts in the subthalamic space however so far relies on ACPC-coordinates or indirect targeting with delineation of adjacent nuclei in T2-weight MRI. Our experience with integration of tractography for direct visualisation of white matter connections for targeting in DBS is described.

Method: Seven patients with tremor were treated with deep brain stimulation in the posterior subthalamic area. Tractography images based on diffusion tensor imaging (DTI) were coregistered with T2-weight images and high resolution T1-images on a commercially available planning system. Targets were defined integrating tractography, T2-weight images and ACPC-related coordinates derived from the literature.

Additional experiments were conducted using a constrained spherical convolution (CSD) algorithm and compared with DTI.

Results: Tremor control was achieved in all patients with low currents. The cerebellothalamic connections serving as a target in our patients could easily be delineated by tractography. Limitations were motion artefacts and distortions of DWI-images. The long acquisition times of DWI-sequences were problematic with movement disorder patients and furthered motion artefacts. Mirroring of fibers proved difficult to overcome with tractography based on diffusion tensor imaging. Additional calculations with CSD with deterministic and probabilistic methods delineated more crossing fibers. A deterministic and a probabilistic approach were compared. While the probabilistic method yielded the greatest amount of fibers, the amount of anatomically not plausible and probably false connections increased also. Spacial resolution was low.

Conclusions: Subthalamic stimulation appears to be a viable alternative to v.i.m stimulation. Tractography was helpful in defining target points and refining the angle of trajectories. However with the limitations of present techniques fiber tracking must be used with caution. With further refinements (CSD) problems like mirroring can be partially overcome but the appearance of false fibers must be taken into account. A major step forward will only be reached with improved scanner techniques and new sequences allowing for enhanced resolution with lower acquisition times.