Contrasting Connectivity of the Vim and Vop Nuclei of the Motor Thalamus Demonstrated by Probabilistic Tractography.

BACKGROUND:: Targeting of the motor thalamus for the treatment of tremor has traditionally been achieved by a combination of anatomical atlases and neuro-imaging, intra-operative clinical assessment, and physiological recordings. OBJECTIVE:: To evaluate whether thalamic nuclei targeted in tremor surgery could be identified by virtue of their differing connections using non-invasive neuro-imaging, thereby providing an extra factor to aid successful targeting. METHODS:: Diffusion tensor tractography was performed in seventeen healthy control subjects using diffusion data acquired at 1.5T magnetic resonance imaging (60 directions, b-value=1000 s/mm, 2x2x2 mm voxels). The ventralis intermedius (Vim) and ventralis oralis posterior (Vop) nuclei were identified by a stereotactic neurosurgeon and these sites were used as seeds for probabilistic tractography. The expected cortical connections of these nuclei were determined a priori from the literature, namely the primary motor cortex (M1) and contralateral cerebellum for the Vim and M1, the supplementary motor area (SMA) and dorsolateral prefrontal cortex (DLPFC) for the Vop. RESULTS:: Tractogram signal intensity was highest in the DLPFC and SMA after Vop seeding (p<0.0005 using Wilcoxon signed-rank tests). High intensity was seen in M1 after seeding of both nuclei, but was greater with Vim seeding (p<0.0005). Contralateral cerebellar signal was highest with Vim seeding (p<0.0005). CONCLUSION:: Probabilistic tractography can depict differences in connectivity between intimate nuclei within the motor thalamus. These connections are consistent with published anatomical studies and therefore tractography may provide an important adjunct in future targeting in tremor surgery.