13.2.1 Targeting

While it is commonplace to utilize both direct and indirect techniques for DBS targeting, commonly available 1.5 T or 3.0 T MRI sequences do not allow for visualization of individual thalamic nuclei parcellations. Therefore, thalamic targeting is dependent primarily on indirect targeting based on the posterior commissure, distance to the wall of the third ventricle and distance to the internal capsule. While it is possible to target the Vim nucleus of the thalamus in the first stereotactic cannulation for placement of the DBS lead, we often prefer to refine stereotactic localization by first targeting the ventrocaudal (Vc) nucleus, the primary sensory relay located posterior to Vim. The Vc is organized with the sensory representation of the face medially and the representation of the lower extremity laterally. The transition between Vc and Vim is typically located 2–4 mm anterior to the posterior commissure (PC) and the upper extremity representation in the Vc located approximately 10–11 mm lateral to the wall of the third ventricle. The bottom of the thalamus is typically at the level of the intercommissural plane. The first microelectrode recording pass can indicate the laterality in relation to the plan. If more tactile units corresponding to the face are identified, this is an indication that the pass was more medial than intended and if more units corresponding to the leg are identified, it suggests a more lateral pass. A second microelectrode recording (MER) pass can be made anteriorly to help define the Vc-Vim transition. The final electrode is typically placed 2–4 mm anterior to the estimated Vc-Vim transition and aimed at the physiological topography of the upper extremity. ⁵ Although we in general avoid transventricular trajectories ⁶ some patients require a transventricular approach due to large ventricles or other limitations such as vascular anatomy that narrows the options for trajectory planning. ^{5 , 7}