Targeting the Thalamic Ventrointermediate Nucleus for Neurosurgical Treatment of Tremor Using Q-Ball Calculation

Objective: Localization of the thalamic nuclei for targeting in deep brain stimulation (DBS) is still challenging, as it is not clearly identifiable on standard magnetic resonance (MR) sequences. The dentate-rubro-thalamic tract connects the thalamic ventrointermediate (Vim) nucleus with the cerebellum. We evaluate the reliability of Q-ball imaging in predicting the position of the functional target compared with coordinate-based localization of the nucleus.

Methods: In this study six patients who received implantation of electrodes in the Vim for treatment of essential tremor in 2011 and 2012 are included. The calculation was performed retrospectively in three and prospectively in three patients. We analyzed the connectivity by Q-ball calculation of the dentate nucleus with the thalamus and thus identified the Vim. Coregistration of the connective imaging target, the atlas-based coordinate preoperative targeting and the postoperative computed tomography coordinate of the active contact of the implanted electrode (which was correlated with the best clinical effect on tremor) was performed. Euclidian distances were calculated between (1) the atlas-based target and (2) the connective image target with respect of the coordinates of the active contact of the implanted electrode.

Results: We were able to localize the Vim according to connectivity with the cerebellum via the dentate-rubro-thalamic tract in most of the six patients. In at least one patient the rubro-thalamic tract did not lead to relevant connectivity values. We observed similar distances from the active contact and (1) the atlas-based target and (2) connective image target.

Conclusion: Connectivity analysis by Q-ball calculation of the Vim position provided direct visualization of the Vim in almost all cases. Our preliminary results suggest that the reliability of Q-ball imaging in determining the preoperative targeting of the Vim is comparable to atlas-based targeting. Larger prospective calculations are needed to determine the robustness of this method in providing a refined information useful for neurosurgical treatment of tremor, including not only DBS, but also radiosurgery.