Multi-institutional evaluation of deep brain stimulation targeting using probabilistic connectivity-based thalamic segmentation

Clinical article

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Object

Due to the lack of internal anatomical detail with traditional MR imaging, preoperative stereotactic planning for the treatment of tremor usually relies on indirect targeting based on atlas-derived coordinates. The object of this study was to preliminarily investigate the role of probabilistic tractography–based thalamic segmentation for deep brain stimulation (DBS) targeting for the treatment of tremor.

Methods

Six patients undergoing bilateral implantation of DBS electrodes in the thalamus for the treatment of upper-extremity tremor were studied. All patients underwent stereotactic surgical implantation using traditional methods (based on indirect targeting methodologies and intraoperative macrostimulation findings) that were programmed for optimal efficacy, independent of tractography-based segmentations described in this report. Connectivity-based thalamic segmentations were derived by identifying with which of 7 cortical target regions each thalamic voxel had the highest probability of connectivity. The authors retrospectively analyzed the location of the optimal contact for treatment of tremor with connectivity-based thalamic segmentations. Findings from one institution (David Geffen School of Medicine at UCLA) were validated with results from 4 patients at another institution (University of Virginia Health System).

Results

Of 12 electrodes implanted using traditional methodologies, all but one resulted in efficacious tremor control. Connectivity-based thalamic segmentation consistently revealed discrete thalamic regions having unique connectivity patterns with distinct cortical regions. Although the authors initially hypothesized that the most efficacious DBS contact for controlling tremor would colocalize with the thalamic region most highly connected with the primary motor cortex, they instead found it to highly colocalize with those thalamic voxels demonstrating a high probability of connectivity with premotor cortex (center-to-center distance: 0.36 ± 0.55 mm). In contrast to the high degree of colocalization with optimal stimulation site, the precise localization of the premotor cortex–defined thalamic region relative to the anterior and posterior commissures was highly variable. Having defined a connectivity-based target for thalamic stimulation in a cohort of patients at David Geffen School of Medicine at UCLA, the authors validated findings in 4 patients (5 electrodes) who underwent surgery at a different institution (University of Virginia Health System) by a different surgeon.

Conclusions

This report identifies and provides preliminary external validation of a novel means of targeting a patient-specific therapeutic thalamic target for the treatment of tremor based on individualized analysis of thalamic connectivity patterns. This novel thalamic targeting approach is based on identifying the thalamic region with the highest probability of connectivity with premotor and supplementary motor cortices. This approach may prove to be advantageous over traditional preoperative methods of indirect targeting, providing patient-specific targets that could improve the precision, efficacy, and efficiency of deep brain stimulation surgery. Prospective evaluation and development of methodologies to make these analyses more widely available to neurosurgeons are likely warranted.

Abbreviations used in this paper: AC = anterior commissure; DBS = deep brain stimulation; DGSOM = David Geffen School of Medicine at UCLA; DT = diffusion tensor; FTM = Fahn-Tolosa-Marin; PC = posterior commissure; PMC = premotor and supplementary motor cortices; UVAHS = University of Virginia Health System.

Article Information

Address correspondence to: Nader Pouratian, M.D., Ph.D., Department of Neurosurgery, David Geffen School of Medicine at UCLA, 10945 Le Conte Avenue, Suite 2120, Los Angeles, California 90095. email: npouratian@mednet.ucla.edu.

Please include this information when citing this paper: published online August 19, 2011; DOI: 10.3171/2011.7.JNS11250.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Methods for connectivity-based thalamic segmentation. Using previously described methodology, we segmented the thalamus based on differential patterns of connectivity with 7 predefined cortical targets. A: The thalamus was manually masked in each patient. B: Cortical target masks were likewise delineated in each patient. In this figure the prefrontal (green), premotor (red), and primary motor cortex (blue) targets masks are depicted. C–E: Using probabilistic tractography, the probability of each thalamic voxel connecting with the cortical target masks is defined. Specifically, thalamic connectivity with primary motor (C, blue), premotor (D, red-yellow), and prefrontal (E, green) cortices is illustrated. F: Once probabilistic patterns of connectivity with each cortical target have been defined, thalamic voxels are assigned to a group based on the region with which they have the highest probability of connection, resulting in thalamic segmentations reminiscent of previously published reports and known thalamic nuclear organization.

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    Image registration. Final electrode position after DBS implantation was determined by merging postoperative images (a CT scan in this case) with the preoperative high-resolution T1-weighted MR images using a linear transformation. Connectivity-based maps, illustrated in Fig. 1, were similarly merged into the high-resolution T1-weighted MR imaging space for intermodality comparisons.

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    Efficacious contact position relative to thalamic M1 connectivity. Using probabilistic tractography, the probability of connectivity of each thalamic voxel with the anatomically defined primary motor cortex was determined. These probabilistic thalamic maps (blue overlays) and post-DBS implantation imaging (yellow overlays) were merged into a common space to compare relative position. Efficacious contact position (yellow) is consistently anterior to those voxels with the highest probability of connectivity with the primary motor cortex. Light blue indicates the highest probability of M1 connectivity; dark blue, the lower probability of M1 connectivity. Panels A–F correspond to each patient.

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    Efficacious contact for thalamic stimulation colocalizes with thalamic voxels with the highest probability of connectivity with premotor and supplementary motor cortices. Using probabilistic tractography, the probability of connectivity of each thalamic voxel with the premotor cortex target mask (which includes lateral premotor and medial supplementary motor areas) was determined. These probabilistic thalamic maps (orange-yellow overlays) are illustrated for each patient (A–F) in the left column. Post-DBS implantation imaging (green overlays) were merged into a common space to compare relative position and plotted atop the thalamic probabilistic connectivity maps. Efficacious contact position (green) is consistently colocalized with those voxels with the highest probability of connectivity with the premotor cortex target mask. Yellow denotes the highest probability of PMC connectivity; red, the lower probability of PMC connectivity.

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    Variability of PMC connectivity across patients in standard space. Using affine transformation to account for interpatient anatomical variability, thalamic PMC-connectivity maps were transformed into a common space, normalized, and averaged to assess the degree of interpatient spatial variability in PMC connectivity. Percentages denote an average normalized score of connectivity across patients, where 100% would indicate consistent maximal spatial concordance with respect to thalamic connectivity with premotor/supplementary motor areas. The highest score is 60%, suggesting significant interpatient variability in connectivity patterns even after anatomical differences are accounted for.

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    Fiber tract projections from the thalamic region with maximal connectivity with PMC. Tractography from the thalamic region of interest (optimal target for efficacious stimulation for tremor control) demonstrates rostral projections to premotor and supplementary motor cortices and caudal projection to the cerebellum.

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    External validation of colocalization of efficacious contacts and PMC connectivity maps. Four patients (A–D; 5 electrodes) were evaluated from a similar data set at UVAHS, demonstrating similar localization of efficacious contacts within thalamic areas with the highest probability of connectivity with the PMC cortical target mask. Refer to Fig. 4 for explanation of colors.

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