The variability of atlas-based targets in relation to surrounding major fibre tracts in thalamic deep brain stimulation

Connectivity-based segmentation of the thalamic motor region for deep brain stimulation in essential tremor: A comparison of deterministic and probabilistic tractography

OBJECTIVE

Deep brain stimulation (DBS) studies have shown that stimulation of the motor segment of the thalamus based on probabilistic tractography is predictive of improvement in essential tremor (ET). However, probabilistic methods are computationally demanding, requiring the need for alternative tractography methods for use in the clinical setting. The purpose of this study was to compare probabilistic vs deterministic tractography methods for connectivity-based targeting in patients with ET.

METHODS

Probabilistic and deterministic tractography methods were retrospectively applied to diffusion-weighted data sets in 36 patients with refractory ET. The thalamus and precentral gyrus were selected as regions of interest and fiber tracking was performed between these regions to produce connectivity-based thalamic segmentations, per prior methods. The resultant deterministic target maps were compared with those of thresholded probabilistic maps. The center of gravity (CG) of each connectivity map was determined and the differences in spatial distribution between the tractography methods were characterized. Furthermore, the intersection between the connectivity maps and CGs with the therapeutic volume of tissue activated (VTA) was calculated. A mixed linear model was then used to assess clinical improvement in tremor with volume of overlap.

RESULTS

Both tractography methods delineated the region of the thalamus with connectivity to the precentral gyrus to be within the posterolateral aspect of the thalamus. The average CG of deterministic maps was more medial-posterior in both the left (3.7 ± 1.3 mm3) and the right (3.5 ± 2.2 mm3) hemispheres when compared to 30 %-thresholded probabilistic maps. Mixed linear model showed that the volume of overlap between CGs of deterministic and probabilistic targeting maps and therapeutic VTAs were significant predictors of clinical improvement.

CONCLUSIONS

Deterministic tractography can reconstruct DBS thalamic target maps in approximately 5 min comparable to those produced by probabilistic methods that require > 12 h to generate. Despite differences in CG between the methods, both deterministic-based and probabilistic targeting were predictive of clinical improvement in ET.

One-pass deep brain stimulation of subthalamic nucleus and ventral intermediate nucleus for levodopa-resistant tremor-dominant Parkinson's disease

Objective

Tremor-dominant Parkinson's disease (TD-PD) can be further separated into levodopa-responsive and levodopa-resistant types, the latter being considered to have a different pathogenesis. Previous studies indicated that deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the globus pallidus internus (GPi) individually was not sufficient for tremor control, especially for the levodopa-resistant TD-PD (LRTD-PD). The thalamic ventral intermediate nucleus (VIM) has been regarded as a potent DBS target for different kinds of tremors. Therefore, we focused on the LRTD-PD subgroup and performed one-pass combined DBSs of STN and VIM to treat refractory tremors, aiming to investigate the safety and effectiveness of this one-trajectory dual-target DBS scheme.

Methods

We retrospectively collected five LRTD-PD patients who underwent a one-pass combined DBS of STN and VIM via a trans-frontal approach. The targeting of VIM was achieved by probabilistic tractography. Changes in severity of symptoms (measured by the Unified Parkinson Disease Rating Scale part III, UPDRS-III), levodopa equivalent daily doses (LEDD), and disease-specific quality of life (measured by the 39-item Parkinson's Disease Questionnaire, PDQ-39) were evaluated.

Results

Three-dimensional reconstruction of electrodes illustrated that all leads were successfully implanted into predefined positions. The mean improvement rates (%) were 53 ± 6.2 (UPDRS-III), 82.6 ± 11.4 (tremor-related items of UPDRS), and 52.1 ± 11.4 (PDQ-39), respectively, with a mean follow-up of 11.4 months.

Conclusion

One-pass combined DBS of STN and VIM via the trans-frontal approach is an effective and safe strategy to alleviate symptoms for LRTD-PD patients.

Mediation of Tremor Control by the Decussating and Nondecussating Part of the Dentato-Rubro-Thalamic Tract in Deep Brain Stimulation in Essential Tremor: Which Part Should Be Stimulated?

OBJECTIVES

The dentato-rubro-thalamic tract (DRTT) has been found to play a major role in the mechanisms of tremor alleviation by deep brain stimulation (DBS) in essential tremor (ET). Still, the influence of the two different parts of the DRTT, consisting of crossing and nondecussating fibers, is not yet clear with respect to tremor reduction. The aim of this study was to assess the influence of the crossing and the nondecussating part of the DRTT on tremor control in ET.

MATERIALS AND METHODS

We investigated 80 electrode contacts in ten patients with ET who received bilateral DBS of the Nucleus ventralis intermedius of the thalamus (VIM). Preoperatively and with patients under general anesthesia, 3T magnetic resonance imaging scans were performed, including Diffusion Tensor Imaging scans with 64 gradient directions. We calculated the course of the two parts of the DRTT based on a workflow for probabilistic fiber tracking including protocols for correction of susceptibility- and eddy current-induced distortions. Distances of electrode contacts were correlated with clinical data from neurologic single pole testing.

RESULTS

Voltage- and current-steered systems were analyzed separately. Regarding postural tremor, effective contacts showed significantly lower distances to both parts of the DRTT (crossing p < 0.001, nondecussating p < 0.05) in voltage-steered systems. Regarding intentional tremor, significant results were only found for the crossing part (p < 0.01). Regarding both tremor types, effective contacts were closer to the crossing part, unlike less effective contacts. Nonlinear regression analyses using a logistic model showed higher coefficients for the crossing part of the DRTT. Multivariate regression models including distances to both parts of the DRTT showed a significant influence of only the crossing part. Analysis of current-steered systems showed unstable data, probably because of the small number of analyzed patients.

CONCLUSIONS

Our data suggest an involvement of both parts of the DRTT in tremor reduction, indicating mediation of DBS effects by both fiber bundles, although the crossing part showed stronger correlations with good clinical responses. Nevertheless, special attention should be paid to methodologic aspects when using probabilistic tractography for patient-specific targeting to avoid uncertain and inaccurate results.

Tremor suppression following treatment with MRgFUS: skull density ratio consistency and degree of posterior dentatorubrothalamic tract lesioning predicts long-term clinical outcomes in essential tremor

Objectives

Magnetic resonance-guided focussed ultrasound (MRgFUS) is an incisionless ablative procedure, widely used for treatment of Parkinsonian and Essential Tremor (ET). Enhanced understanding of the patient- and treatment-specific factors that influence sustained long-term tremor suppression could help clinicians achieve superior outcomes via improved patient screening and treatment strategy.

Methods

We retrospectively analysed data from 31 subjects with ET, treated with MRgFUS at a single centre. Tremor severity was assessed with parts A, B and C of the Clinical Rating Scale for Tremor (CRST) as well as the combined CRST. Tremor in the dominant and non-dominant hand was assessed with Hand Tremor Scores (HTS), derived from the CRST. Pre- and post-treatment imaging data were analysed to determine ablation volume overlap with automated thalamic segmentations, and the dentatorubrothalamic tract (DRTT) and compared with percentage change in CRST and HTS following treatment.

Results

Tremor symptoms were significantly reduced following treatment. Combined pre-treatment CRST (mean: 60.7 ± 17.3) and HTS (mean: 19.2 ± 5.7) improved by an average of 45.5 and 62.6%, respectively. Percentage change in CRST was found to be significantly negatively associated with age (β = -0.375, p = 0.015), and SDR standard deviation (SDRSD; β = -0.324, p = 0.006), and positively associated with ablation overlap with the posterior DRTT (β = 0.535, p < 0.001). Percentage HTS improvement in the dominant hand decreased significantly with older age (β = -0.576, p < 0.01).

Conclusion

Our results suggest that increased lesioning of the posterior region of the DRTT could result in greater improvements in combined CRST and non-dominant hand HTS, and that subjects with lower SDR standard deviation tended to experience greater improvement in combined CRST.

The role of tractography in the localization of the Vim nucleus of the thalamus and the dentato-rubro-thalamic tract for the treatment of tremor

INTRODUCTION

The ventralis intermedius (Vim) nucleus of the thalamus is the usual surgical target for tremor. However, locating the structure may be difficult as it is not visible with conventional imaging methods; therefore, surgical procedures typically use indirect calculations correlated with clinical and intraoperative neurophysiological findings. Current ablative surgical procedures such as Gamma-Knife thalamotomy and magnetic resonance-guided focused ultrasound require new alternatives for locating the Vim nucleus. In this review, we compare Vim nucleus location for the treatment of tremor using stereotactic procedures versus direct location by means of tractography.

DISCUSSION

The most widely used cytoarchitectonic definition of the Vim nucleus is that established by Schaltenbrand and Wahren. There is a well-defined limit between the motor and the sensory thalamus; Vim neurons respond to passive joint movements and are synchronous with peripheral tremor. The most frequently used stereotactic coordinates for the Vim nucleus are based on indirect calculations referencing the mid-commissural line and third ventricle, which vary between patients. Recent studies suggest that the dentato-rubro-thalamic tract is an optimal target for controlling tremor, citing a clinical improvement; however, this has not yet been corroborated.

CONCLUSIONS

Visualisation of the cerebello-rubro-thalamic pathway by tractography may help in locating the Vim nucleus. The technique has several limitations, and the method requires standardisation to obtain more precise results. The utility of direct targeting by tractography over indirect targeting for patients with tremor remains to be demonstrated in the long-term.

Clinical Characteristics of Patients with Gait Instability after MR-Guided Focused Ultrasound Thalamotomy

Background

MRgFUS thalamotomy is an incisionless procedure which effectively treats patients with tremor, although the procedure can result in adverse side effects including gait instability. By determining whether certain pre-existing conditions predispose patients to developing gait instability, we will be able to better counsel patients regarding risk of MRgFUS thalamotomy.

Methods

All patients diagnosed with essential tremor, mixed tremor syndrome, or tremor predominant Parkinson disease who underwent MRgFUS thalamotomy at Mayo Clinic, Rochester between 2017 and 2020 were retrospectively reviewed. Baseline demographic and clinical data was extracted, and gait symptoms were compared pre- versus post-operatively.

Results

Of 45 patients who underwent MRgFUS thalamotomy, 42 had at least one follow-up visit within twelve months and were included in the study. 39 patients had essential tremor, 1 had tremor predominant Parkinson disease, and 2 had mixed tremor syndrome. 19 out of 42 patients (45%) had gait decline. There were 10 (24%) females, and median age was 77.6 years (IQR 71.5-83.2). Older age was not correlated with gait decline (p = 0.82). Patients with a history of neuropathy and joint replacements were more likely to have gait decline after MRgFUS thalamotomy (p = 0.0099 and p = 0.0376). Patients with pre-existing gait aids were not more likely to have gait instability (p = 0.20).

Conclusion

Patients who undergo MRgFUS thalamotomy for each of the tremor conditions, have an increased risk of experiencing gait decline, when there is a pre-procedure history of peripheral neuropathy, or joint replacement surgery. Older age or pre-existing gait aid use is not associated with worsened gait outcomes.

Highlights

Patients who undergo MRgFUS thalamotomy for tremor syndromes have a significantly increased risk of experiencing gait decline when there is comorbid peripheral neuropathy or joint replacementOlder age or pre-existing gait aid use is not associated with worsened gait outcomes.

Ventral intermediate nucleus structural connectivity-derived segmentation: anatomical reliability and variability

The Ventral intermediate nucleus (Vim) of thalamus is the most targeted structure for the treatment of drug-refractory tremors. Since methodological differences across existing studies are remarkable and no gold-standard pipeline is available, in this study, we tested different parcellation pipelines for tractography-derived putative Vim identification. Thalamic parcellation was performed on a high quality, multi-shell dataset and a downsampled, clinical-like dataset using two different diffusion signal modeling techniques and two different voxel classification criteria, thus implementing a total of four parcellation pipelines. The most reliable pipeline in terms of inter-subject variability has been picked and parcels putatively corresponding to motor thalamic nuclei have been selected by calculating similarity with a histology-based mask of Vim. Then, spatial relations with optimal stimulation points for the treatment of essential tremor have been quantified. Finally, effect of data quality and parcellation pipelines on a volumetric index of connectivity clusters has been assessed. We found that the pipeline characterized by higher-order signal modeling and threshold-based voxel classification criteria was the most reliable in terms of inter-subject variability regardless data quality. The maps putatively corresponding to Vim were those derived by precentral and dentate nucleus-thalamic connectivity. However, tractography-derived functional targets showed remarkable differences in shape and sizes when compared to a ground truth model based on histochemical staining on seriate sections of human brain. Thalamic voxels connected to contralateral dentate nucleus resulted to be the closest to literature-derived stimulation points for essential tremor but at the same time showing the most remarkable inter-subject variability. Finally, the volume of connectivity parcels resulted to be significantly influenced by data quality and parcellation pipelines. Hence, caution is warranted when performing thalamic connectivity-based segmentation for stereotactic targeting.

MRI and tractography techniques to localize the ventral intermediate nucleus and dentatorubrothalamic tract for deep brain stimulation and MR-guided focused ultrasound: a narrative review and update

The thalamic ventral intermediate nucleus (VIM) can be targeted for treatment of tremor by several procedures, including deep brain stimulation (DBS) and, more recently, MR-guided focused ultrasound (MRgFUS). To date, such targeting has relied predominantly on coordinate-based or atlas-based techniques rather than directly targeting the VIM based on imaging features. While general regional differences of features within the thalamus and some related white matter tracts can be distinguished with conventional imaging techniques, internal nuclei such as the VIM are not discretely visualized. Advanced imaging methods such as quantitative susceptibility mapping (QSM) and fast gray matter acquisition T1 inversion recovery (FGATIR) MRI and high-field MRI pulse sequences that improve the ability to image the VIM region are emerging but have not yet been shown to have reliability and accuracy to serve as the primary method of VIM targeting. Currently, the most promising imaging approach to directly identify the VIM region for clinical purposes is MR diffusion tractography.In this review and update, the capabilities and limitations of conventional and emerging advanced methods for evaluation of internal thalamic anatomy are briefly reviewed. The basic principles of tractography most relevant to VIM targeting are provided for familiarization. Next, the key literature to date addressing applications of DTI and tractography for DBS and MRgFUS is summarized, emphasizing use of direct targeting. This literature includes 1-tract (dentatorubrothalamic tract [DRT]), 2-tract (pyramidal and somatosensory), and 3-tract (DRT, pyramidal, and somatosensory) approaches to VIM region localization through tractography.The authors introduce a 3-tract technique used at their institution, illustrating the oblique curved course of the DRT within the inferior thalamus as well as the orientation and relationship of the white matter tracts in the axial plane. The utility of this 3-tract tractography approach to facilitate VIM localization is illustrated with case examples of variable VIM location, targeting superior to the anterior commissure-posterior commissure plane, and treatment in the setting of pathologic derangement of thalamic anatomy. Finally, concepts demonstrated with these case examples and from the prior literature are synthesized to highlight several potential advantages of tractography for VIM region targeting.

Comparative evaluation of tractography-based direct targeting and atlas-based indirect targeting of the ventral intermediate (Vim) nucleus in MRgFUS thalamotomy

To analyze and compare direct and indirect targeting of the Vim for MRgFUS thalamotomy. We retrospectively evaluated 21 patients who underwent unilateral MRgFUS Vim ablation and required targeting repositioning during the procedures. For each patient, in the three spatial coordinates, we recorded: (i) indirect coordinates; (ii) the coordinates where we clinically observed tremor reduction during the verification stage sonications; (iii) direct coordinates, measured on the dentatorubrothalamic tract (DRTT) at the after postprocessing of DTI data. The agreement between direct and indirect coordinates compared to clinically effective coordinates was evaluated through the Bland–Altman test and intraclass correlation coefficient. The median absolute percentage error was also calculated. Compared to indirect targeting, direct targeting showed inferior error values on the RL and AP coordinates (0.019 vs. 0.079 and 0.207 vs. 0.221, respectively) and higher error values on the SI coordinates (0.263 vs. 0.021). The agreement between measurements was higher for tractography along the AP and SI planes and lower along the RL planes. Indirect atlas-based targeting represents a valid approach for MRgFUS thalamotomy. The direct tractography approach is a valuable aid in assessing the possible deviation of the error in cases where no immediate clinical response is achieved.

Increased structural connectivity of thalamic stimulation sites to motor cortex relates to tremor suppression

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Clinically weighted tractography reveals key patterns of therapeutic brain stimulation.

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Thalamic stimulation for tremor preferentially connects to precentral gyrus and cerebellum.

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Thalamic DBS of areas most connected to motor cortex results in superior outcomes.

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Acute and chronic therapeutic outcomes demonstrate converging connectivity patterns.

Deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM-DBS) is a highly successful treatment for medication-refractory essential tremor (ET). Clinical outcomes are dependent on accurate targeting. Here, we aim to develop a framework for connectivity-guided DBS targeting by evaluating probabilistic tractography and clinical response at both initial programming (IP) and clinical follow-up (CF).

Magnetic resonance imaging and clinical outcomes were evaluated in 23 ET patients who were treated by VIM-DBS at the University of California Los Angeles (20 at IP, 18 at CF, 14 at both). Lead-DBS was used to model the volume of tissue activated tissue (VTA) based on programming configurations at both IP and CF. Probabilistic tractography, calculated in FSL, was used to evaluate 1) clinically weighted whole brain connectivity of VTA; 2) connectivity between VTA and freesurfer-derived target regions of interest (ROI) including primary motor, premotor, and prefrontal cortices, and cerebellum; and 3) volume of intersection between VTA and probabilistic tractography-based segmentation of the thalamus. At IP, individual contacts were scored as high or low efficacy based on acute tremor improvement. At CF, clinical response was measured by percent of change of the Clinical Rating Scale for Tremor (CRST) compared to preoperative scores. Contributions from each target ROI to clinical response was measured using logistic regression for IP and linear regression for CF.

The clinically weighted map of whole brain connectivity of VTA shows preferential connectivity to precentral gyrus and brainstem/cerebellum. The volume of intersection between VTA and thalamic segmentation map based on probabilistic connectivity to primary motor cortex was a significant predictor of contact efficacy at IP (OR = 2.26 per 100 mm³ of overlap, p = .04) and percent change in CRST at CF (β = 14.67 per 100 mm³ of overlap, p = .003).

Targeting DBS to the area of thalamus most connected to primary motor cortex based on probabilistic tractography is associated with superior outcomes, providing a potential guide not only for lead targeting but also therapeutic programming.

Tractography-based targeting of the ventral intermediate nucleus: accuracy and clinical utility in MRgFUS thalamotomy

OBJECTIVE

Tractography-based targeting of the thalamic ventral intermediate nucleus (T-VIM) is a novel method conferring patient-specific selection of VIM coordinates for tremor surgery; however, its accuracy and clinical utility in magnetic resonance imaging-guided focused ultrasound (MRgFUS) thalamotomy compared to conventional indirect targeting has not been specifically addressed. This retrospective study sought to compare the treatment locations and potential adverse effect profiles of T-VIM with indirect targeting in a large cohort of MRgFUS thalamotomy patients.

METHODS

T-VIM was performed using diffusion tractography outlining the pyramidal and medial lemniscus tracts in 43 MRgFUS thalamotomy patients. T-VIM coordinates were compared with the indirect treatment coordinates used in the procedure. Thalamotomy lesions were delineated on postoperative T1-weighted images and displaced ("translated") by the anteroposterior and mediolateral difference between T-VIM and treatment coordinates. Both translated and actual lesions were normalized to standard space and subsequently overlaid with areas previously reported to be associated with an increased risk of motor and sensory adverse effects when lesioned during MRgFUS thalamotomy.

RESULTS

T-VIM coordinates were 2.18 mm anterior and 1.82 mm medial to the "final" indirect treatment coordinates. Translated lesions lay more squarely within the boundaries of the VIM compared to nontranslated lesions and showed significantly less overlap with areas associated with sensory adverse effects. Translated lesions overlapped less with areas associated with motor adverse effects; however, this difference was not significant.

CONCLUSIONS

T-VIM leads to the selection of more anterior and medial coordinates than the conventional indirect methods. Lesions moved toward these anteromedial coordinates avoid areas associated with an increased risk of motor and sensory adverse effects, suggesting that T-VIM may improve clinical outcomes.

DeepDTI: High-fidelity six-direction diffusion tensor imaging using deep learning

Diffusion tensor magnetic resonance imaging (DTI) is unsurpassed in its ability to map tissue microstructure and structural connectivity in the living human brain. Nonetheless, the angular sampling requirement for DTI leads to long scan times and poses a critical barrier to performing high-quality DTI in routine clinical practice and large-scale research studies. In this work we present a new processing framework for DTI entitled DeepDTI that minimizes the data requirement of DTI to six diffusion-weighted images (DWIs) required by conventional voxel-wise fitting methods for deriving the six unique unknowns in a diffusion tensor using data-driven supervised deep learning. DeepDTI maps the input non-diffusion-weighted (b ​= ​0) image and six DWI volumes sampled along optimized diffusion-encoding directions, along with T1-weighted and T2-weighted image volumes, to the residuals between the input and high-quality output b = 0 image and DWI volumes using a 10-layer three-dimensional convolutional neural network (CNN). The inputs and outputs of DeepDTI are uniquely formulated, which not only enables residual learning to boost CNN performance but also enables tensor fitting of resultant high-quality DWIs to generate orientational DTI metrics for tractography. The very deep CNN used by DeepDTI leverages the redundancy in local and non-local spatial information and across diffusion-encoding directions and image contrasts in the data. The performance of DeepDTI was systematically quantified in terms of the quality of the output images, DTI metrics, DTI-based tractography and tract-specific analysis results. We demonstrate rotationally-invariant and robust estimation of DTI metrics from DeepDTI that are comparable to those obtained with two b ​= ​0 images and 21 DWIs for the primary eigenvector derived from DTI and two b ​= ​0 images and 26-30 DWIs for various scalar metrics derived from DTI, achieving 3.3-4.6 × ​acceleration, and twice as good as those of a state-of-the-art denoising algorithm at the group level. The twenty major white-matter tracts can be accurately identified from the tractography of DeepDTI results. The mean distance between the core of the major white-matter tracts identified from DeepDTI results and those from the ground-truth results using 18 ​b ​= ​0 images and 90 DWIs measures around 1-1.5 ​mm. DeepDTI leverages domain knowledge of diffusion MRI physics and power of deep learning to render DTI, DTI-based tractography, major white-matter tracts identification and tract-specific analysis more feasible for a wider range of neuroscientific and clinical studies.

Improved Vim targeting for focused ultrasound ablation treatment of essential tremor: A probabilistic and patient-specific approach

Magnetic resonance-guided focused ultrasound (MRgFUS) ablation of the ventral intermediate (Vim) thalamic nucleus is an incisionless treatment for essential tremor (ET). The standard initial targeting method uses an approximate, atlas-based stereotactic approach. We developed a new patient-specific targeting method to identify an individual's Vim and the optimal MRgFUS target region therein for suppression of tremor. In this retrospective study of 14 ET patients treated with MRgFUS, we investigated the ability of WMnMPRAGE, a highly sensitive and robust sequence for imaging gray matter-white matter contrast, to identify the Vim, FUS ablation, and a clinically efficacious region within the Vim in individual patients. We found that WMnMPRAGE can directly visualize the Vim in ET patients, segmenting this nucleus using manual or automated segmentation capabilities developed by our group. WMnMPRAGE also delineated the ablation's core and penumbra, and showed that all patients' ablation cores lay primarily within their Vim segmentations. We found no significant correlations between standard ablation features (e.g., ablation volume, Vim-ablation overlap) and 1-month post-treatment clinical outcome. We then defined a group-based probabilistic target, which was nonlinearly warped to individual brains; this target was located within the Vim for all patients. The overlaps between this target and patient ablation cores correlated significantly with 1-month clinical outcome (r = -.57, p = .03), in contrast to the standard target (r = -.23, p = .44). We conclude that WMnMPRAGE is a highly sensitive sequence for segmenting Vim and ablation boundaries in individual patients, allowing us to find a novel tremor-associated center within Vim and potentially improving MRgFUS treatment for ET.

Thalamic Deep Brain Stimulation for Essential Tremor: Relation of the Dentatorubrothalamic Tract with Stimulation Parameters

BACKGROUND

In deep brain stimulation (DBS) for essential tremor, the primary target ventrointermedius (VIM) nucleus cannot be clearly visualized with structural imaging. As such, there has been much interest in the dentatorubrothalamic tract (DRTT) for target localization, but evidence for the DRTT as a putative stimulation target in tremor suppression is lacking. We evaluated proximity of the DRTT in relation to DBS stimulation parameters.

METHODS

This is a retrospective analysis of 26 consecutive patients who underwent DBS with microelectrode recordings (46 leads). Fiber tracking was performed with a published deterministic technique. Clinically optimized stimulation parameters were obtained in all patients at the time of most recent follow-up (6.2 months). Volume of tissue activated (VTA) around contacts was calculated from a published model.

RESULTS

Tremor severity was reduced in all treated hemispheres, with 70% improvement in the treated hand score of the Clinical Rating Scale for Tremor. At the level of the active contact (2.9 ± 2.0 mm superior to the commissural plane), the center of the DRTT was lateral to the contacts (5.1 ± 2.1 mm). The nearest fibers of the DRTT were 2.4 ± 1.7 mm from the contacts, whereas the radius of the VTA was 2.9 ± 0.7 mm. The VTA overlapped with the DRTT in 77% of active contacts. The distance from active contact to the DRTT was positively correlated with stimulation voltage requirements (Kendall τ = 0.33, P = 0.006), whereas distance to the atlas-based VIM coordinates was not.

CONCLUSIONS

Active contacts in proximity to the DRTT had lower voltage requirements. Data from a large cohort provide support for the DRTT as an effective stimulation target for tremor control.

Prospective Tractography-Based Targeting for Improved Safety of Focused Ultrasound Thalamotomy

BACKGROUND

Focused ultrasound thalamotomy (FUS-T) was recently approved for the treatment of refractory essential tremor (ET). Despite its noninvasive approach, FUS-T reinitiated concerns about the adverse effects and long-term efficacy after lesioning.

OBJECTIVE

To prospectively assess the outcomes of FUS-T in 10 ET patients using tractography-based targeting of the ventral intermediate nucleus (VIM).

METHODS

VIM was identified at the intercommissural plane based on its neighboring tracts: the pyramidal tract and medial lemniscus. FUS-T was performed at the center of tractography-defined VIM. Tremor outcomes, at baseline and 3 mo, were assessed independently by the Tremor Research Group. We analyzed targeting coordinates, clinical outcomes, and adverse events. The FUS-T lesion location was analyzed in relation to unbiased thalamic parcellation using probabilisitic tractography. Quantitative diffusion-weighted imaging changes were also studied in fiber tracts of interest.

RESULTS

The tractography coordinates were more anterior than the standard. Intraoperatively, therapeutic sonications at the tractography target improved tremor (>50% improvement) without motor or sensory side effects. Sustained improvement in tremor was observed at 3 mo (tremor score: 18.3 ± 6.9 vs 8.1 ± 4.4, P = .001). No motor weakness and sensory deficits after FUS-T were observed during 6-mo follow-up. Ataxia was observed in 3 patients. FUS-T lesions overlapped with the VIM parcellated with probablisitic tractography. Significant microstructural changes were observed in the white matter connecting VIM with cerebellum and motor cortex.

CONCLUSION

This is the first report of prospective VIM targeting with tractography for FUS-T. These results suggest that tractography-guided targeting is safe and has satisfactory short-term clinical outcomes.

An ancestor of the stereotactic atlases: the Tabulae Anatomicae of Bartolomeo Eustachio

The authors report the history of the Tabulae Anatomicae of Bartolomeo Eustachio (ca. 1510-1574). In the tables, the anatomical illustrations were drawn inside a numerical frame, with pairs of numbers on the y- and x-axes to identify single anatomical details in the reference table. The measures and the references could be calculated using the graduated margins divided by 5 units for each the x-axis and y-axis. The Tabulae Anatomicae can be considered a precursor to modern anatomical reference systems that are the basis of studies on cerebral localization mainly used for stereotactic procedures.

Targeting of the dentato-rubro-thalamic tract for MR-guided focused ultrasound treatment of essential tremor

BACKGROUND

Magnetic resonance-guided focused ultrasound ablation of the thalamic ventral intermediate nucleus is a safe and effective treatment for medically refractory essential tremor. However, indirect targeting of the ventral intermediate nucleus using stereotactic coordinates from normal neuroanatomy can be inefficient. We therefore evaluated the feasibility of supplementing this method with direct targeting of the dentato-rubro-thalamic tract.

METHODS

We retrospectively identified four patients undergoing magnetic resonance-guided focused ultrasound ablation for essential tremor in which preoperative diffusion tractography imaging of the dentato-rubro-thalamic tract was fused with T2 weighted-imaging and utilized for intra-procedural targeting. The size and location of the dentato-rubro-thalamic tract and 24-hour lesion, as well as the center of the stereotactic coordinates, was evaluated. Finally, the amount of overlap between the dentato-rubro-thalamic tract and the lesion was calculated.

RESULTS

The 24-hour lesion size was homogeneous in the cohort (mean 31.3 mm², range 30-32 mm²), while there was substantial variation in the dentato-rubro-thalamic tract area (mean 14.3 mm², range 3-24 mm²). The center of the stereotactic coordinates and dentato-rubro-thalamic tract diverged by more than 1 mm in mediolateral and anterposterior directions in all patients, while the dentato-rubro-thalamic tract and lesion centers were in close proximity (mean mediolateral separation 1 mm, range 0.1-2.2 mm; mean anteroposterior separation 0.75 mm, range 0.4-1.2 mm). There was greater than 50% coverage of the dentato-rubro-thalamic tract by the lesion in all patients (mean 82.9%, range 66.7-100%). All patients experienced durable tremor relief.

CONCLUSION

Direct targeting of the dentato-rubro-thalamic tract using diffusion tractography imaging fused to T2 weighted-imaging may be a useful strategy for focused ultrasound treatment of essential tremor. Further investigation of the technique is warranted.

Microstimulation-induced inhibition of thalamic reticular nucleus in non-human primates

The thalamic reticular nucleus (TRN) modulates activity in the thalamus and controls excitatory input from corticothalamic and thalamocortical glutamatergic projections. It is made up of GABAergic neurons which project topographically to the thalamus. The TRN also receives inhibitory projections from the globus pallidus and the substantia nigra pars reticulata. Photostimulation of the TRN results in local inhibition in rat slice preparations but the effects of local stimulation in vivo are not known. This study aimed to characterize stimulation-evoked responses in the TRN of non-human primates (NHPs). Microelectrodes were inserted into the TRN and neurons were stimulated at 5, 10, 15, and 20 µA using 0.5 s trains at 100 Hz and the subsequent response was recorded from the same electrode. Stimulation in surrounding nuclei and the internal capsule was used for mapping the anatomical borders of the TRN. Stimulation as low as 10 µA resulted in predominantly inhibition, recorded in both single units and background unit activity (BUA). The duration of inhibition (~ 1-3 s) increased with increasing stimulation amplitude and was significantly increased in BUA when single units were present. At 20 µA of current, 93% of the single units (41/44) and 92% of BUA sites (67/73) were inhibited. Therefore, microstimulation of the NHP TRN with low currents results in current-dependent inhibition of single units and BUA. This finding may be useful to further aid in localization of deep thalamic and subthalamic nuclei during brain surgery.

Validation of diffusion tensor imaging tractography to visualize the dentatorubrothalamic tract for surgical planning

OBJECTIVE The dentatorubrothalamic tract (DRTT) has been suggested as the anatomical substrate for deep brain stimulation (DBS)-induced tremor alleviation. So far, little is known about how accurately and reliably tracking results correspond to the anatomical DRTT. The objective of this study was to systematically investigate and validate the results of different tractography approaches for surgical planning. METHODS The authors retrospectively analyzed 4 methodological approaches for diffusion tensor imaging (DTI)-based fiber tracking using different regions of interest in 6 patients with essential tremor. Tracking results were analyzed and validated with reference to MRI-based anatomical landmarks, were projected onto the stereotactic atlas of Morel at 3 predetermined levels (vertical levels -3.6, -1.8, and 0 mm below the anterior commissure-posterior commissure line), and were correlated to clinical outcome. RESULTS The 4 different methodologies for tracking the DRTT led to divergent results with respect to the MRI-based anatomical landmarks and when projected onto the stereotactic atlas of Morel. There was a statistically significant difference in the lateral and anteroposterior coordinates at the 3 vertical levels (p < 0.001, 2-way ANOVA). Different fractional anisotropy values ranging from 0.1 to 0.46 were required for anatomically plausible tracking results and led to varying degrees of success. Tracking results were not correlated to postoperative tremor reduction. CONCLUSIONS Different tracking methods can yield results with good anatomical approximation. The authors recommend using 3 regions of interest including the dentate nucleus of the cerebellum, the posterior subthalamic area, and the precentral gyrus to visualize the DRTT. Tracking results must be cautiously evaluated for anatomical plausibility and accuracy in each patient.

Structural and functional connectivity of the nondecussating dentato-rubro-thalamic tract

The dentato-rubro-thalamic tract (DRTT) regulates motor control, connecting the cerebellum to the thalamus. This tract is modulated by deep-brain stimulation in the surgical treatment of medically refractory tremor, especially in essential tremor, where high-frequency stimulation of the thalamus can improve symptoms. The DRTT is classically described as a decussating pathway, ascending to the contralateral thalamus. However, the existence of a nondecussating (i.e. ipsilateral) DRTT in humans was recently demonstrated, and these tracts are arranged in distinct regions of the superior cerebellar peduncle. We hypothesized that the ipsilateral DRTT is connected to specific thalamic nuclei and therefore may have unique functional relevance. The goals of this study were to confirm the presence of the decussating and nondecussating DRTT pathways, identify thalamic termination zones of each tract, and compare whether structural connectivity findings agree with functional connectivity. Diffusion-weighted imaging was used to perform probabilistic tractography of the decussating and nondecussating DRTT in young healthy subjects from the Human Connectome Project (n = 91) scanned using multi-shell diffusion-weighted imaging (270 directions; TR/TE = 5500/89 ms; spatial resolution = 1.25 mm isotropic). To define thalamic anatomical landmarks, a segmentation procedure based on the Morel Atlas was employed, and DRTT targeting was quantified based on the proportion of streamlines arriving at each nucleus. In parallel, functional connectivity analysis was performed using resting-state functional MRI (TR/TE = 720/33 ms; spatial resolution = 2 mm isotropic). It was found that the decussating and nondecussating DRTTs have significantly different thalamic endpoints, with the former preferentially targeting relatively anterior and lateral thalamic nuclei, and the latter connected to more posterior and medial nuclei (p < 0.001). Functional and structural connectivity measures were found to be significantly correlated (r = 0.45, p = 0.031). These findings provide new insight into pathways through which unilateral cerebellum can exert bilateral influence on movement and raise questions about the functional implications of ipsilateral cerebellar efferents.

Diffusion MRI tractography for improved transcranial MRI-guided focused ultrasound thalamotomy targeting for essential tremor

Purpose

To evaluate the use of diffusion magnetic resonance imaging (MRI) tractography for neurosurgical guidance of transcranial MRI-guided focused ultrasound (tcMRgFUS) thalamotomy for essential tremor (ET).

Materials and methods

Eight patients with medication-refractory ET were treated with tcMRgFUS targeting the ventral intermediate nucleus (Vim) of the thalamus contralateral to their dominant hand. Diffusion and structural MRI data and clinical evaluations were acquired pre-treatment and post-treatment. To identify the optimal target location, tractography was performed on pre-treatment diffusion MRI data between the treated thalamus and the hand-knob region of the ipsilateral motor cortex, the entire ipsilateral motor cortex and the contralateral dentate nucleus. The tractography-identified locations were compared to the lesion location delineated on 1 year post-treatment T₂-weighted MR image. Their overlap was correlated with the clinical outcomes measured by the percentage change of the Clinical Rating Scale for Tremor scores acquired pre-treatment, as well as 1 month, 3 months, 6 months and 1 year post-treatment.

Results

The probabilistic tractography was consistent from subject-to-subject and followed the expected anatomy of the thalamocortical radiation and the dentatothalamic tract. Higher overlap between the tractography-identified location and the tcMRgFUS treatment-induced lesion highly correlated with better treatment outcome (r = −0.929, −0.75, −0.643, p = 0.00675, 0.0663, 0.139 for the tractography between the treated thalamus and the hand-knob region of the ipsilateral motor cortex, the entire ipsilateral motor cortex and the contralateral dentate nucleus, respectively, at 1 year post-treatment). The correlation for the tractography between the treated thalamus and the hand-knob region of the ipsilateral motor cortex is the highest for all time points (r = −0.719, −0.976, −0.707, −0.929, p = 0.0519, 0.000397, 0.0595, 0.00675 at 1 month, 3 months, 6 months and 1 year post-treatment, respectively).

Conclusion

Our data support the use of diffusion tractography as a complementary approach to current targeting methods for tcMRgFUS thalamotomy.

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Retrospectively used tractography to define a target for MRgFUS thalamotomy for ET.

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Larger overlap between tractography and lesion correlates with better outcomes.

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Strongest correlations for tract between the thalamus and motor hand-knob region

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Diffusion tractography is a complementary approach to current targeting methods.

Connectivity derived thalamic segmentation in deep brain stimulation for tremor

The ventral intermediate nucleus (VIM) of the thalamus is an established surgical target for stereotactic ablation and deep brain stimulation (DBS) in the treatment of tremor in Parkinson's disease (PD) and essential tremor (ET). It is centrally placed on a cerebello-thalamo-cortical network connecting the primary motor cortex, to the dentate nucleus of the contralateral cerebellum through the dentato-rubro-thalamic tract (DRT). The VIM is not readily visible on conventional MR imaging, so identifying the surgical target traditionally involved indirect targeting that relies on atlas-defined coordinates. Unfortunately, this approach does not fully account for individual variability and requires surgery to be performed with the patient awake to allow for intraoperative targeting confirmation. The aim of this study is to identify the VIM and the DRT using probabilistic tractography in patients that will undergo thalamic DBS for tremor. Four male patients with tremor dominant PD and five patients (three female) with ET underwent high angular resolution diffusion imaging (HARDI) (128 diffusion directions, 1.5 mm isotropic voxels and b value = 1500) preoperatively. Patients received VIM-DBS using an MR image guided and MR image verified approach with indirect targeting. Postoperatively, using parallel Graphical Processing Unit (GPU) processing, thalamic areas with the highest diffusion connectivity to the primary motor area (M1), supplementary motor area (SMA), primary sensory area (S1) and contralateral dentate nucleus were identified. Additionally, volume of tissue activation (VTA) corresponding to active DBS contacts were modelled. Response to treatment was defined as 40% reduction in the total Fahn-Tolosa-Martin Tremor Rating Score (FTMTRS) with DBS-ON, one year from surgery. Three out of nine patients had a suboptimal, long-term response to treatment. The segmented thalamic areas corresponded well to anatomically known counterparts in the ventrolateral (VL) and ventroposterior (VP) thalamus. The dentate-thalamic area, lay within the M1-thalamic area in a ventral and lateral location. Streamlines corresponding to the DRT connected M1 to the contralateral dentate nucleus via the dentate-thalamic area, clearly crossing the midline in the mesencephalon. Good response was seen when the active contact VTA was in the thalamic area with highest connectivity to the contralateral dentate nucleus. Non-responders had active contact VTAs outside the dentate-thalamic area. We conclude that probabilistic tractography techniques can be used to segment the VL and VP thalamus based on cortical and cerebellar connectivity. The thalamic area, best representing the VIM, is connected to the contralateral dentate cerebellar nucleus. Connectivity based segmentation of the VIM can be achieved in individual patients in a clinically feasible timescale, using HARDI and high performance computing with parallel GPU processing. This same technique can map out the DRT tract with clear mesencephalic crossing.

Intraoperative test stimulation versus stereotactic accuracy as a surgical end point: a comparison of essential tremor outcomes after ventral intermediate nucleus deep brain stimulation

OBJECTIVE Ventral intermediate nucleus deep brain stimulation (DBS) for essential tremor is traditionally performed with intraoperative test stimulation and conscious sedation, without general anesthesia (GA). Recently, the authors reported retrospective data on 17 patients undergoing DBS after induction of GA with standardized anatomical coordinates on T1-weighted MRI sequences used for indirect targeting. Here, they compare prospectively collected data from essential tremor patients undergoing DBS both with GA and without GA (non-GA). METHODS Clinical outcomes were prospectively collected at baseline and 3-month follow-up for patients undergoing DBS surgery performed by a single surgeon. Stereotactic, euclidean, and radial errors of lead placement were calculated. Functional (activities of daily living), quality of life (Quality of Life in Essential Tremor [QUEST] questionnaire), and tremor severity outcomes were compared between groups. RESULTS Fifty-six patients underwent surgery: 16 without GA (24 electrodes) and 40 with GA (66 electrodes). The mean baseline functional scores and QUEST summary indices were not different between groups (p = 0.91 and p = 0.59, respectively). Non-GA and GA groups did not differ significantly regarding mean postoperative percentages of functional improvement (non-GA, 47.9% vs GA, 48.1%; p = 0.96) or QUEST summary indices (non-GA, 79.9% vs GA, 74.8%; p = 0.50). Accuracy was comparable between groups (mean radial error 0.9 ± 0.3 mm for non-GA and 0.9 ± 0.4 mm for GA patients) (p = 0.75). The mean euclidean error was also similar between groups (non-GA, 1.1 ± 0.6 mm vs GA, 1.2 ± 0.5 mm; p = 0.92). No patient had an intraoperative complication, and the number of postoperative complications was not different between groups (non-GA, n = 1 vs GA, n = 10; p = 0.16). CONCLUSIONS DBS performed with the patient under GA to treat essential tremor is as safe and effective as traditional DBS surgery with intraoperative test stimulation while the patient is under conscious sedation without GA.

Does the Use of Intraoperative Microelectrode Recording Influence the Final Location of Lead Implants in the Ventral Intermediate Nucleus for Deep Brain Stimulation?

To determine if the use of intraoperative microelectrode recording (MER) influences the final location of lead implant in deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM), and to evaluate the incidence of associated complications. The usefulness of intraoperative MER in DBS is debated, some centers suggesting it increases complications without additional benefit. We conducted a retrospective chart review of all patients who underwent VIM DBS with MER at the University of Texas Health Science Center in Houston from June 1, 2009 to October 1, 2013. Initial (MRI determined) and final (intraoperative MER determined) coordinates of implant were compared. To assess incidences of hemorrhagic and infectious complications, we reviewed postoperative CT scans and follow-up notes. Forty-five lead implants on 24 patients were reviewed. The mean age at implantation was 62.42 years (range 18-83). The average duration from diagnosis to surgery was 21.5 years (range 1-52). A statistically significant mean difference was observed in the superior-inferior plane (0.52 ± 0.80 mm inferiorly, p < 0.05) and the anterior-posterior plane (0.45 ± 0.86 mm posteriorly, p < 0.05). A non-statistically significant difference was also observed in the medial-lateral plane (0.02± 0.15 mm, p > 0.05). One patient developed an infectious complication (4.2 %) that required removal of leads; two patients had minimal asymptomatic intra-ventricular bleeding (8.3 %). In our DBS center, intraoperative MER in VIM DBS implant does not seem to have a higher rate of surgical complications compared to historical series not using MER, and might also be useful in determining the final lead location.

Probabilistic vs. deterministic fiber tracking and the influence of different seed regions to delineate cerebellar-thalamic fibers in deep brain stimulation

This study compared tractography approaches for identifying cerebellar-thalamic fiber bundles relevant to planning target sites for deep brain stimulation (DBS). In particular, probabilistic and deterministic tracking of the dentate-rubro-thalamic tract (DRTT) and differences between the spatial courses of the DRTT and the cerebello-thalamo-cortical (CTC) tract were compared. Six patients with movement disorders were examined by magnetic resonance imaging (MRI), including two sets of diffusion-weighted images (12 and 64 directions). Probabilistic and deterministic tractography was applied on each diffusion-weighted dataset to delineate the DRTT. Results were compared with regard to their sensitivity in revealing the DRTT and additional fiber tracts and processing time. Two sets of regions-of-interests (ROIs) guided deterministic tractography of the DRTT or the CTC, respectively. Tract distances to an atlas-based reference target were compared. Probabilistic fiber tracking with 64 orientations detected the DRTT in all twelve hemispheres. Deterministic tracking detected the DRTT in nine (12 directions) and in only two (64 directions) hemispheres. Probabilistic tracking was more sensitive in detecting additional fibers (e.g. ansa lenticularis and medial forebrain bundle) than deterministic tracking. Probabilistic tracking lasted substantially longer than deterministic. Deterministic tracking was more sensitive in detecting the CTC than the DRTT. CTC tracts were located adjacent but consistently more posterior to DRTT tracts. These results suggest that probabilistic tracking is more sensitive and robust in detecting the DRTT but harder to implement than deterministic approaches. Although sensitivity of deterministic tracking is higher for the CTC than the DRTT, targets for DBS based on these tracts likely differ.

Microelectrode recording findings within the tractography-defined ventral intermediate nucleus

OBJECTIVE

The ventral intermediate nucleus (VIM) of the thalamus is not visible on structural MRI. Therefore, direct VIM targeting methods for stereotactic tremor surgery are desirable. The authors previously described a direct targeting method for visualizing the VIM and its structural connectivity using deterministic tractography. In this combined electrophysiology and imaging study, the authors investigated the electrophysiology within this tractography-defined VIM (T-VIM).

METHODS

Thalamic neurons were classified based on their relative location to the T-VIM: dorsal, within, and ventral to the T-VIM. The authors identified the movement-responsive cells (kinesthetic and tremor cells), performed spike analysis (firing rate and burst index), and local field potential analysis (area under the curve for 13–30 Hz). Tremor efficacy in response to microstimulation along the electrode trajectory was also assessed in relation to the T-VIM.

RESULTS

Seventy-three cells from a total of 9 microelectrode tracks were included for this analysis. Movement-responsive cells (20 kinesthetic cells and 26 tremor cells) were identified throughout the electrode trajectories. The mean firing rate and burst index of cells (n = 27) within the T-VIM are 18.8 ± 9.8 Hz and 4.5 ± 5.4, respectively. Significant local field potential beta power was identified within the T-VIM (area under the curve for 13–30 Hz = 6.6 ± 7.7) with a trend toward higher beta power in the dorsal T-VIM. The most significant reduction in tremor was also observed in the dorsal T-VIM.

CONCLUSIONS

The electrophysiological findings within the VIM thalamus defined by tractography, or T-VIM, correspond with the known microelectrode recording characteristics of the VIM in patients with tremor.

Efficacy of Fiber Tractography in the Stereotactic Surgery of the Thalamus for Patients with Essential Tremor

Several targets and targeting methods are utilized in stereotactic surgery to achieve tremor suppression for patients with intractable tremor. Recent developments in magnetic resonance imaging, including diffusion tensor imaging, have enabled the setting of appropriate targets in stereotactic surgery. In this retrospective study, the optimal target to suppress tremors in stereotactic surgery was explored using diffusion tensor image-based fiber tractography. Four tracts were focused on in this study, namely: the cerebello-thalamo-premotor cortical fiber tract, cerebello-thalamo-primary motor cortical fiber tract, spino-thalamo-somatosensory cortical fiber tract, and pyramidal tract. In 10 patients with essential tremor, we evaluated the thalamotomy lesions and active contacts of the lead in thalamic stimulation by diffusion tensor image-based fiber tractography to reveal which part of the cerebral cortex is most affected by stereotactic surgery. Tremor suppression and adverse events were also evaluated in the patients involved in this study. Consequently, the good tremor suppression was achieved in all patients. There had been no permanent adverse events 3 months after surgery. Twelve lesions in thalamotomy patients or active contacts of the lead in thalamic stimulation patients were on the cerebello-thalamo-premotor cortical fiber tract (12/14 lesions or active contacts: 86%). In conclusion, the cerebello-thalamo-premotor cortical fiber tract may be an optimal target for tremor suppression. Diffusion tensor image-based fiber tractography may enable us to both determine the optimal target to achieve strong tremor suppression and to reduce the number of adverse events by keeping lesions or electrodes away from important fiber tracts, such as the pyramidal tract and spinothalamic fibers.

Patient-Specific Electric Field Simulations and Acceleration Measurements for Objective Analysis of Intraoperative Stimulation Tests in the Thalamus

Despite an increasing use of deep brain stimulation (DBS) the fundamental mechanisms of action remain largely unknown. Simulation of electric entities has previously been proposed for chronic DBS combined with subjective symptom evaluations, but not for intraoperative stimulation tests. The present paper introduces a method for an objective exploitation of intraoperative stimulation test data to identify the optimal implant position of the chronic DBS lead by relating the electric field (EF) simulations to the patient-specific anatomy and the clinical effects quantified by accelerometry. To illustrate the feasibility of this approach, it was applied to five patients with essential tremor bilaterally implanted in the ventral intermediate nucleus (VIM). The VIM and its neighborhood structures were preoperatively outlined in 3D on white matter attenuated inversion recovery MR images. Quantitative intraoperative clinical assessments were performed using accelerometry. EF simulations (n = 272) for intraoperative stimulation test data performed along two trajectories per side were set-up using the finite element method for 143 stimulation test positions. The resulting EF isosurface of 0.2 V/mm was superimposed to the outlined anatomical structures. The percentage of volume of each structure’s overlap was calculated and related to the corresponding clinical improvement. The proposed concept has been successfully applied to the five patients. For higher clinical improvements, not only the VIM but as well other neighboring structures were covered by the EF isosurfaces. The percentage of the volumes of the VIM, of the nucleus intermediate lateral of the thalamus and the prelemniscal radiations within the prerubral field of Forel increased for clinical improvements higher than 50% compared to improvements lower than 50%. The presented new concept allows a detailed and objective analysis of a high amount of intraoperative data to identify the optimal stimulation target. First results indicate agreement with published data hypothesizing that the stimulation of other structures than the VIM might be responsible for good clinical effects in essential tremor. (Clinical trial reference number: Ref: 2011-A00774-37/AU905)

Tractography-Based Ventral Intermediate Nucleus Targeting: Novel Methodology and Intraoperative Validation

BACKGROUND

The ventral intermediate nucleus of the thalamus is not readily visible on structural magnetic resonance imaging. Therefore, a method for its visualization for stereotactic targeting is desirable.

OBJECTIVE

The objective of this study was to define a tractography-based methodology for the stereotactic targeting of the ventral intermediate nucleus.

METHODS

The lateral and posterior borders of the ventral intermediate nucleus were defined by tracking the pyramidal tract and medial lemniscus, respectively. A thalamic seed was then created 3 mm medial and anterior to these borders, and its structural connections were analyzed. The application of this method was assessed in an imaging cohort of 14 tremor patients and 15 healthy controls, in which we compared the tractography-based targeting to conventional targeting. In a separate surgical cohort (3 tremor and 3 tremor-dominant Parkinson's disease patients), we analyzed the accuracy of this method by correlating it with intraoperative neurophysiology.

RESULTS

Tractography of the thalamic seed revealed the tracts corresponding to cerebellar input and motor cortical output fibers. The tractography-based target was more lateral (12.5 [1.2] mm vs 11.5 mm for conventional targeting) and anterior (8.5 [1.1] mm vs 6.7 [0.3] mm, anterior to the posterior commissure). In the surgical cohort, the Euclidian distance between the ventral intermediate nucleus identified by tractography and the surgical target was 1.6 [1.1] mm. The locations of the sensory thalamus, lemniscus, and pyramidal tracts were concordant within <1 mm between tractography and neurophysiology.

INTERPRETATION

The tractography-based methodology for identification of the ventral intermediate nucleus is accurate and useful. This method may be used to improve stereotactic targeting in functional neurosurgery procedures. © 2016 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Feasibility of Diffusion Tractography for the Reconstruction of Intra-Thalamic and Cerebello-Thalamic Targets for Functional Neurosurgery: A Multi-Vendor Pilot Study in Four Subjects

Functional stereotactic neurosurgery by means of deep brain stimulation or ablation provides an effective treatment for movement disorders, but the outcome of surgical interventions depends on the accuracy by which the target structures are reached. The purpose of this pilot study was to evaluate the feasibility of diffusion tensor imaging (DTI) based probabilistic tractography of deep brain structures that are commonly used for pre- and perioperative targeting for functional neurosurgery. Three targets were reconstructed based on their significance as intervention sites or as a no-go area to avoid adverse side effects: the connections propagating from the thalamus to (1) primary and supplementary motor areas, (2) to somatosensory areas and the cerebello-thalamic tract (CTT). We evaluated the overlap of the reconstructed connectivity based targets with corresponding atlas based data, and tested the inter-subject and inter-scanner variability by acquiring repeated DTI from four volunteers, and on three MRI scanners with similar sequence parameters. Compared to a 3D histological atlas of the human thalamus, moderate overlaps of 35-50% were measured between connectivity- and atlas based volumes, while the minimal distance between the centerpoints of atlas and connectivity targets was 2.5 mm. The variability caused by the MRI scanner was similar to the inter-subject variability, except for connections with the postcentral gyrus where it was higher. While CTT resolved the anatomically correct trajectory of the tract individually, high volumetric variability was found across subjects and between scanners. DTI can be applied in the clinical, preoperative setting to reconstruct the CTT and to localize subdivisions within the lateral thalamus. In our pilot study, such subdivisions moderately matched the borders of the ventrolateral-posteroventral (VL_pv) nucleus and the ventral-posterolateral (VPL) nucleus. Limitations of the currently used standard DTI protocols were exacerbated by large scanner-to-scanner variability of the connectivity-based targets.

Multimodal 7T Imaging of Thalamic Nuclei for Preclinical Deep Brain Stimulation Applications

Precise neurosurgical targeting of electrode arrays within the brain is essential to the successful treatment of a range of brain disorders with deep brain stimulation (DBS) therapy. Here, we describe a set of computational tools to generate in vivo, subject-specific atlases of individual thalamic nuclei thus improving the ability to visualize thalamic targets for preclinical DBS applications on a subject-specific basis. A sequential nonlinear atlas warping technique and a Bayesian estimation technique for probabilistic crossing fiber tractography were applied to high field (7T) susceptibility-weighted and diffusion-weighted imaging, respectively, in seven rhesus macaques. Image contrast, including contrast within thalamus from the susceptibility-weighted images, informed the atlas warping process and guided the seed point placement for fiber tractography. The susceptibility-weighted imaging resulted in relative hyperintensity of the intralaminar nuclei and relative hypointensity in the medial dorsal nucleus, pulvinar, and the medial/ventral border of the ventral posterior nuclei, providing context to demarcate borders of the ventral nuclei of thalamus, which are often targeted for DBS applications. Additionally, ascending fiber tractography of the medial lemniscus, superior cerebellar peduncle, and pallidofugal pathways into thalamus provided structural demarcation of the ventral nuclei of thalamus. The thalamic substructure boundaries were validated through in vivo electrophysiological recordings and post-mortem blockface tissue sectioning. Together, these imaging tools for visualizing and segmenting thalamus have the potential to improve the neurosurgical targeting of DBS implants and enhance the selection of stimulation settings through more accurate computational models of DBS.

“Asleep” deep brain stimulation for essential tremor

OBJECT

Deep brain stimulation (DBS) performed under general anesthesia (“asleep” DBS) has not been previously reported for essential tremor. This is in part due to the inability to visualize the target (the ventral intermediate nucleus [VIM]) on MRI. The authors evaluate the efficacy of this asleep technique in treating essential tremor by indirect VIM targeting.

METHODS

The authors retrospectively reviewed consecutive cases of initial DBS for essential tremor performed by a single surgeon. DBS was performed with patients awake (n = 40, intraoperative test stimulation without microelectrode recording) or asleep (n = 17, under general anesthesia). Targeting proceeded with standardized anatomical coordinates on preoperative MRI. Intraoperative CT was used for stereotactic registration and lead position confirmation. Functional outcomes were evaluated with pre- and postoperative Bain and Findley Tremor Activities of Daily Living scores.

RESULTS

A total of 29 leads were placed in asleep patients, and 60 were placed in awake patients. Bain and Findley Tremor Activities of Daily Living Questionnaire scores were not significantly different preoperatively for awake versus asleep cohorts (p = 0.2). The percentage of postoperative improvement was not significantly different between asleep (48.6%) and awake (45.5%) cohorts (p = 0.35). Euclidean error (mm) was higher for awake versus asleep patients (1.7 ± 0.8 vs 1.2 ± 0.4, p = 0.01), and radial error (mm) trended higherfor awake versus asleep patients (1.3 ± 0.8 vs 0.9 ± 0.5, p = 0.06). There were no perioperative complications.

CONCLUSIONS

In the authors’ initial experience, asleep VIM DBS for essential tremor without intraoperative test stimulation can be performed safely and effectively.

Personalized mapping of the deep brain with a white matter attenuated inversion recovery (WAIR) sequence at 1.5-tesla: Experience based on a series of 156 patients

OBJECTIVE

Deep brain mapping has been proposed for direct targeting in stereotactic functional surgery, aiming to personalize electrode implantation according to individual MRI anatomy without atlas or statistical template. We report our clinical experience of direct targeting in a series of 156 patients operated on using a dedicated Inversion Recovery Turbo Spin Echo sequence at 1.5-tesla, called White Matter Attenuated Inversion Recovery (WAIR).

METHODS

After manual contouring of all pertinent structures and 3D planning of trajectories, 312 DBS electrodes were implanted. Detailed anatomy of close neighbouring structures, whether gray nuclei or white matter regions, was identified during each planning procedure. We gathered the experience of these 312 deep brain mappings and elaborated consistent procedures of anatomical MRI mapping for pallidal, subthalamic and ventral thalamic regions. We studied the number of times the central track anatomically optimized was selected for implantation of definitive electrodes.

RESULTS

WAIR sequence provided high-quality images of most common functional targets, successfully used for pure direct stereotactic targeting: the central track corresponding to the optimized primary anatomical trajectory was chosen for implantation of definitive electrodes in 90.38%.

CONCLUSION

WAIR sequence is anatomically reliable, enabling precise deep brain mapping and direct stereotactic targeting under routine clinical conditions.

Diffusion Tractography in Deep Brain Stimulation Surgery: A Review

Deep brain stimulation (DBS) is believed to exert its therapeutic effects through modulation of brain circuitry, yet conventional preoperative planning does not allow direct targeting or visualization of white matter pathways. Diffusion MRI tractography (DT) is virtually the only non-invasive method of visualizing structural connectivity in the brain, leading many to suggest its use to guide DBS targeting. DT-guided DBS not only has the potential to allow direct white matter targeting for established applications [e.g., Parkinson’s disease (PD), essential tremor (ET), dystonia], but may also aid in the discovery of new therapeutic targets for a variety of other neurologic and psychiatric diseases. Despite these exciting opportunities, DT lacks standardization and rigorous anatomic validation, raising significant concern for the use of such data in stereotactic brain surgery. This review covers the technical details, proposed methods, and initial clinical data for the use of DT in DBS surgery. Rather than focusing on specific disease applications, this review focuses on methods that can be applied to virtually any DBS target.

DTI-based deterministic fibre tracking of the medial forebrain bundle

BACKGROUND

Deep brain stimulation (DBS) of the medial forebrain bundle (MFB) was reported to reduce symptoms in psychiatric disorders. The aim of our study was to find standardised parameters for diffusion tensor imaging (DTI) based fibre tracking to reliably visualise the MFB.

METHODS

Twenty-two cerebral hemispheres in 11 patients were investigated. Three different regions of interest (ROIs) were defined as seed regions for fibre tracking: the ipsilateral and contralateral superior cerebellar peduncle (SCP) and the nucleus raphe dorsalis (NRD). From each seed region the fibres were followed separately through the ventral tegmental area (VTA = second ROI) and their further courses and volumina were documented and compared. Minimal fibre length was set at 30 mm and the FA threshold at 0.12.

RESULTS

The fibre tracts starting in seed regions in the ipsilateral SCP and the NRD follow a similar course along the lateral wall of the third ventricle (hypothalamus) and the anterior limb of the internal capsule (ALIC) to inferior fronto-medial brain areas. These fibres are in accordance with the course of the MFB as described in various anatomical atlases. Consistently, a branch leaves the main fibre tract laterally to take a course through the capsula externa to the temporo-parietal cortex. Fibre tracts starting from the contralateral SCP follow a more superior and lateral course, including the dentato-rubro-thalamic and the pyramidal tract.

CONCLUSIONS

Deterministic fibre tracking with standardised ROIs provides constant and reproducible delineations of the medial forebrain bundle. Its visualisation might help to adjust targeting in DBS for psychiatric disorders.