2.5. STN MER-MRI trajectory length comparison

One further pertinent way to assess the accuracy of the reconstructed DBS lead relative to STN anatomy was to compare the position of the DBS lead with intraoperative MER results. Therefore, we determined the length of electrophysiological STN activity along the two to three intraoperatively used parallel trajectories and compared it to image-based reconstructed trajectories across the STN according to the four atlases (template space) as well as to the T2-hypointense signal of each patient's STN visualised on MRI (native space).

Native-space based analysis was performed with Brainlab software. Reconstructed trajectories of MER adjacent to the DBS lead were manually placed in inline view with a fixed 2 mm distance to the lead according to known features of the five-channel Ben-Gun. The entry and exit points of the STN were then measured manually in the software to calculate the resulting STN length. Lead-DBS provided a microelectrode tool to determine STN lengths along the different microelectrode trajectories such as central, lateral or medial (Fig. 1).

We then calculated the difference of the image- or atlas-based STN trajectory length (S_MRI/atlas) and the MER-based trajectory length (S_MER) in each patient (ΔS = S_MRI/atlas - S_MER). If no electrophysiological activity was recorded for a given trajectory, S_MER was set to zero. Similarly, if the image- or atlas-based trajectory was projected outside the STN, S_MRI/atlas was set to zero for that trajectory.