Quantification and Visualization of the Three-Dimensional Inconsistency of the Subthalamic Nucleus in the Schaltenbrand-Wahren Brain Atlas

A Brief History of Stereotactic Atlases: Their Evolution and Importance in Stereotactic Neurosurgery

Following the recent acquisition of unprecedented anatomical details through state-of-the-art neuroimaging, stereotactic procedures such as microelectrode recording (MER) or deep brain stimulation (DBS) can now rely on direct and accurately individualized topographic targeting. Nevertheless, both modern brain atlases derived from appropriate histological techniques involving post-mortem studies of human brain tissue and the methods based on neuroimaging and functional information represent a valuable tool to avoid targeting errors due to imaging artifacts or insufficient anatomical details. Hence, they have thus far been considered a reference guide for functional neurosurgical procedures by neuroscientists and neurosurgeons. In fact, brain atlases, ranging from the ones based on histology and histochemistry to the probabilistic ones grounded on data derived from large clinical databases, are the result of a long and inspiring journey made possible thanks to genial intuitions of great minds in the field of neurosurgery and to the technical advancement of neuroimaging and computational science. The aim of this text is to review the principal characteristics highlighting the milestones of their evolution.

Defining the optimal target for anterior thalamic deep brain stimulation in patients with drug-refractory epilepsy

OBJECTIVE

The anterior thalamic nucleus (ATN) is a common target for deep brain stimulation (DBS) for the treatment of drug-refractory epilepsy. However, no atlas-based optimal DBS (active contacts) target within the ATN has been definitively identified. The object of this retrospective study was to analyze the relationship between the active contact location and seizure reduction to establish an atlas-based optimal target for ATN DBS.

METHODS

From among 25 patients who had undergone ATN DBS surgery for drug-resistant epilepsy between 2016 and 2018, those who had follow-up evaluations for more than 1 year were eligible for study inclusion. After an initial stimulation period of 6 months, patients were classified as responsive (≥ 50% median decrease in seizure frequency) or nonresponsive (< 50% median decrease in seizure frequency) to treatment. Stimulation parameters and/or active contact positions were adjusted in nonresponsive patients, and their responsiveness was monitored for at least 1 year. Postoperative CT scans were coregistered nonlinearly with preoperative MR images to determine the center coordinate and atlas-based anatomical localizations of all active contacts in the Montreal Neurological Institute (MNI) 152 space.

RESULTS

Nineteen patients with drug-resistant epilepsy were followed up for at least a year following bilateral DBS electrode implantation targeting the ATN. Active contacts located more adjacent to the center of gravity of the anterior half of the ATN volume, defined as the anterior center (AC), were associated with greater seizure reduction than those not in this location. Intriguingly, the initially nonresponsive patients could end up with much improved seizure reduction by adjusting the active contacts closer to the AC at the final postoperative follow-up.

CONCLUSIONS

Patients with stimulation targeting the AC may have a favorable seizure reduction. Moreover, the authors were able to obtain additional good outcomes after electrode repositioning in the initially nonresponsive patients. Purposeful and strategic trajectory planning to target this optimal region may predict favorable outcomes of ATN DBS.

High thickness histological sections as alternative to study the three-dimensional microscopic human sub-cortical neuroanatomy

Stereotaxy is based on the precise image-guided spatial localization of targets within the human brain. Even with the recent advances in MRI technology, histological examination renders different (and complementary) information of the nervous tissue. Although several maps have been selected as a basis for correlating imaging results with the anatomical locations of sub-cortical structures, technical limitations interfere in a point-to-point correlation between imaging and anatomy due to the lack of precise correction for post-mortem tissue deformations caused by tissue fixation and processing. We present an alternative method to parcellate human brain cytoarchitectural regions, minimizing deformations caused by post-mortem and tissue-processing artifacts and enhancing segmentation by means of modified high thickness histological techniques and registration with MRI of the same specimen and into MNI space (ICBM152). A three-dimensional (3D) histological atlas of the human thalamus, basal ganglia, and basal forebrain cholinergic system is displayed. Structure's segmentations were performed in high-resolution dark-field and light-field microscopy. Bidimensional non-linear registration of the histological slices was followed by 3D registration with in situ MRI of the same subject. Manual and automated registration procedures were adopted and compared. To evaluate the quality of the registration procedures, Dice similarity coefficient and normalized weighted spectral distance were calculated and the results indicate good overlap between registered volumes and a small shape difference between them in both manual and automated registration methods. High thickness high-resolution histological slices in combination with registration to in situ MRI of the same subject provide an effective alternative method to study nuclear boundaries in the human brain, enhancing segmentation and demanding less resources and time for tissue processing than traditional methods.

7T MRI subthalamic nucleus atlas for use with 3T MRI

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) reduces motor symptoms in most patients with Parkinson disease (PD), yet may produce untoward effects. Investigation of DBS effects requires accurate localization of the STN, which can be difficult to identify on magnetic resonance images collected with clinically available 3T scanners. The goal of this study is to develop a high-quality STN atlas that can be applied to standard 3T images. We created a high-definition STN atlas derived from seven older participants imaged at 7T. This atlas was nonlinearly registered to a standard template representing 56 patients with PD imaged at 3T. This process required development of methodology for nonlinear multimodal image registration. We demonstrate mm-scale STN localization accuracy by comparison of our 3T atlas with a publicly available 7T atlas. We also demonstrate less agreement with an earlier histological atlas. STN localization error in the 56 patients imaged at 3T was less than 1 mm on average. Our methodology enables accurate STN localization in individuals imaged at 3T. The STN atlas and underlying 3T average template in MNI space are freely available to the research community. The image registration methodology developed in the course of this work may be generally applicable to other datasets.

Human brain atlasing: past, present and future

We have recently witnessed an explosion of large-scale initiatives and projects addressing mapping, modeling, simulation and atlasing of the human brain, including the BRAIN Initiative, the Human Brain Project, the Human Connectome Project (HCP), the Big Brain, the Blue Brain Project, the Allen Brain Atlas, the Brainnetome, among others. Besides these large and international initiatives, there are numerous mid-size and small brain atlas-related projects. My contribution to these global efforts has been to create adult human brain atlases in health and disease, and to develop atlas-based applications. For over two decades with my R&D lab I developed 35 brain atlases, licensed to 67 companies and made available in about 100 countries. This paper has two objectives. First, it provides an overview of the state of the art in brain atlasing. Second, as it is already 20 years from the release of our first brain atlas, I summarise my past and present efforts, share my experience in atlas creation, validation and commercialisation, compare with the state of the art, and propose future directions.

Subthalamic nucleus volumes are highly consistent but decrease age-dependently-a combined magnetic resonance imaging and stereology approach in humans

The subthalamic nucleus (STN) is a main target structure of deep brain stimulation (DBS) in idiopathic Parkinson's disease. Nevertheless, there is an ongoing discussion regarding human STN volumes and neuron count, which could potentially have an impact on STN-DBS. Moreover, a suspected functional subdivision forms the basis of the tripartite hypothesis, which has not yet been morphologically substantiated. In this study, it was aimed to investigate the human STN by means of combined magnetic resonance imaging (MRI) and stereology. STN volumes were obtained from 14 individuals (ranging from 65 to 96 years, 25 hemispheres) in 3 T MRI and in luxol-stained histology slices. Neuron number and cell densities were investigated stereologically over the entire STN and in pre-defined subregions in anti-human neuronal protein HuC/D-stained slices. STN volumes measured with MRI were smaller than in stereology but appeared to be highly consistent, measuring on average 99 ± 6 mm³ (MRI) and 132 ± 20 mm³ (stereology). The neuron count was 431,088 ± 72,172. Both STN volumes and cell count decreased age-dependently. Neuron density was different for the dorsal, medial and ventral subregion with significantly higher values ventrally than dorsally. Small variations in STN volumes in both MRI and stereology contradict previous findings of large variations in STN size. Age-dependent decreases in STN volumes and neuron numbers might influence the efficacy of STN-DBS in a geriatric population. Though the study is limited in sample size, site-dependent differences for the STN subregions form a morphological basis for the tripartite theory. Hum Brain Mapp 38:909-922, 2017. © 2016 Wiley Periodicals, Inc.

What You See Is What You Get: Lead Location Within Deep Brain Structures Is Accurately Depicted by Stereotactic Magnetic Resonance Imaging

BACKGROUND

Magnetic resonance imaging (MRI)-verified deep brain stimulation relies on the correct interpretation of stereotactic imaging documenting lead location in relation to visible anatomic target. However, it has been suggested that local signal distortion from the lead itself renders its depiction on MRI unreliable.

OBJECTIVE

To compare lead location on stereotactic MRI with subsequent location of its brain track after removal.

METHODS

Patients underwent deep brain stimulation with the use of MRI-guided and MRI-verified Leksell frame approach. Infection or suboptimal efficacy required lead removal and subsequent reimplantation by using the same technique. Postimplantation stereotactic MR images were analyzed. Lateral (x) and anteroposterior (y) distances from midcommissural point to center of the lead hypointensity were recorded at the anterior commissure-posterior commissure plane (pallidal electrode) or z = -4 (subthalamic electrode). Stereotactic MRI before the second procedure, x and y distances from the center of the visible lead track hypointensity to midcommissural point were independently recorded. Vectorial distance from center of the lead hypointensity to the center of its track was calculated.

RESULTS

Sixteen electrode tracks were studied in 10 patients. Mean differences between lead artifact location and lead track location were: x coordinate 0.4 mm ± 0.2; y coordinate 0.6 mm ± 0.3. Mean vectorial distance was 0.7 mm ± 0.2.

CONCLUSION

Stereotactic distance between lead location and subsequent brain track location on MRI was small. The mean discrepancy was approximately half the deep brain stimulation lead width. This suggests that lead hypointensity seen on postimplantation MRI is indeed an accurate representation of its real location within deep brain structures.

A Three-dimensional Deformable Brain Atlas for DBS Targeting. I. Methodology for Atlas Creation and Artifact Reduction

Background:

Targeting in deep brain stimulation (DBS) relies heavily on the ability to accurately localize particular anatomic brain structures. Direct targeting of subcortical structures has been limited by the ability to visualize relevant DBS targets.

Methods and Results:

In this work, we describe the development and implementation, of a methodology utilized to create a three dimensional deformable atlas for DBS surgery. This atlas was designed to correspond to the print version of the Schaltenbrand-Bailey atlas structural contours. We employed a smoothing technique to reduce artifacts inherent in the print version.

Conclusions:

We present the methodology used to create a three dimensional patient specific DBS atlas which may in the future be tested for clinical utility.

Improving targeting in image-guided frame-based deep brain stimulation

BACKGROUND

Deep brain stimulation (DBS) is commonly used in the treatment of movement disorders such as Parkinson disease (PD), dystonia, and other tremors.

OBJECTIVE

To examine systematic errors in image-guided DBS electrode placement and to explore a calibration strategy for stereotactic targeting.

METHODS

Pre- and postoperative stereotactic MR images were analyzed in 165 patients. The perpendicular error between planned target coordinates and electrode trajectory was calculated geometrically for all 312 DBS electrodes implanted. Improvement in motor unified PD rating scale III subscore was calculated for those patients with PD with at least 6 months of follow-up after bilateral subthalamic DBS.

RESULTS

Mean (standard deviation) scalar error of all electrodes was 1.4(0.9) mm with a significant difference between left and right hemispheres. Targeting error was significantly higher for electrodes with coronal approach angle (ARC) ≥10° (P < .001). Mean vector error was X: -0.6, Y: -0.7, and Z: -0.4 mm (medial, posterior, and superior directions, respectively). Targeting error was significantly improved by using a systematic calibration strategy based on ARC and target hemisphere (mean: 0.6 mm, P < .001) for 47 electrodes implanted in 24 patients. Retrospective theoretical calibration for all 312 electrodes would have reduced the mean (standard deviation) scalar error from 1.4(0.9) mm to 0.9(0.5) mm (36% improvement). With calibration, 97% of all electrodes would be within 2 mm of the intended target as opposed to 81% before calibration. There was no significant correlation between the degree of error and clinical outcome from bilateral subthalamic nucleus DBS (R = 0.07).

CONCLUSION

After calibration of a systematic targeting error an MR image-guided stereotactic approach would be expected to deliver 97% of all electrodes to within 2 mm of the intended target point with a single brain pass.

Magnetic resonance imaging techniques for visualization of the subthalamic nucleus

The authors reviewed 70 publications on MR imaging-based targeting techniques for identifying the subthalamic nucleus (STN) for deep brain stimulation in patients with Parkinson disease. Of these 70 publications, 33 presented quantitatively validated results. There is still no consensus on which targeting technique to use for surgery planning; methods vary greatly between centers. Some groups apply indirect methods involving anatomical landmarks, or atlases incorporating anatomical or functional data. Others perform direct visualization on MR imaging, using T2-weighted spin echo or inversion recovery protocols. The combined studies do not offer a straightforward conclusion on the best targeting protocol. Indirect methods are not patient specific, leading to varying results between cases. On the other hand, direct targeting on MR imaging suffers from lack of contrast within the subthalamic region, resulting in a poor delineation of the STN. These deficiencies result in a need for intraoperative adaptation of the original target based on test stimulation with or without microelectrode recording. It is expected that future advances in MR imaging technology will lead to improvements in direct targeting. The use of new MR imaging modalities such as diffusion MR imaging might even lead to the specific identification of the different functional parts of the STN, such as the dorsolateral sensorimotor part, the target for deep brain stimulation.

Patient-specific analysis of the relationship between the volume of tissue activated during DBS and verbal fluency

Deep brain stimulation (DBS) for the treatment of advanced Parkinson's disease involves implantation of a lead with four small contacts usually within the subthalamic nucleus (STN) or globus pallidus internus (GPi). While generally safe from a cognitive standpoint, STN DBS has been commonly associated with a decrease in the speeded production of words, a skill referred to as verbal fluency. Virtually all studies comparing presurgical to postsurgical verbal fluency performance have detected a decrease with DBS. The decline may be attributable in part to the surgical procedures, yet the relative contributions of stimulation effects are not known. In the present study, we used patient-specific DBS computer models to investigate the effects of stimulation on verbal fluency performance. Specifically, we investigated relationships of the volume and locus of activated STN tissue to verbal fluency outcome. Stimulation of different electrode contacts within the STN did not affect total verbal fluency scores. However, models of activation revealed subtle relationships between the locus and volume of activated tissue and verbal fluency performance. At ventral contacts, more tissue activation inside the STN was associated with decreased letter fluency performance. At optimal contacts, more tissue activation within the STN was associated with improved letter fluency performance. These findings suggest subtle effects of stimulation on verbal fluency performance, consistent with the functional nonmotor subregions/somatotopy of the STN.

Quantification of spatial consistency in the Talairach and Tournoux stereotactic atlas

BACKGROUND

The Talairach-Tournoux (TT) atlas is one of the most prevalent brain atlases. Although its spatial inconsistencies were reported earlier, there has been no systematic quantification of them across the entire atlas, which is addressed here.

METHOD

The consistency of the TT atlas, defined as uniformity of labeling across all three orthogonal atlas orientations, is calculated and presented as maps. It is analyzed in function of discrepancy measuring spatial offset in labeling.

FINDINGS

The TT atlas has 27.4% consistency and 37.7% inconsistency. The most consistent structure is the thalamus (85.7% consistency, 5.4% inconsistency). The consistency of the basal ganglia is good. For 3-mm discrepancy, the inconsistency of major subcortical gray matter structures is very low: 0% (globus pallidus medial and putamen), 0.7% (thalamus), 2.2% (globus pallidus lateral), 4.8% (hippocampus) and 4.9% (caudate nucleus). The inconsistency of all subcortical structures is relatively high (16.8%), caused by a very high inconsistency of white matter tracts. The consistency of stereotactic targets is 69.2% (GPi), 50.0% (STN) and 42.9% (VPL). The overall TT consistency increases by 20% for 1-mm discrepancy, constantly grows by 10% for 2-4-mm discrepancy and slows down to 3% for 5-6-mm discrepancy.

CONCLUSION

This work enhances our understanding of the TT atlas and its variable spatial consistency. It is helpful in using multiple atlas orientations simultaneously. It also may be useful in atlas interpolation and construction of a fully consistent 3D atlas. As the consistency of the main stereotactic targets is medium, the use of the TT atlas in stereotactic procedures requires a great deal of care and understanding of its limitations.

Automated 3-dimensional brain atlas fitting to microelectrode recordings from deep brain stimulation surgeries

OBJECTIVE

Deep brain stimulation (DBS) surgeries commonly rely on brain atlases and microelectrode recordings (MER) to help identify the target location for electrode implantation. We present an automated method for optimally fitting a 3-dimensional brain atlas to intraoperative MER and predicting a target DBS electrode location in stereotactic coordinates for the patient.

METHODS

We retrospectively fit a 3-dimensional brain atlas to MER points from 10 DBS surgeries targeting the subthalamic nucleus (STN). We used a constrained optimization algorithm to maximize the MER points correctly fitted (i.e., contained) within the appropriate atlas nuclei. We compared our optimization approach to conventional anterior commissure-posterior commissure (AC/PC) scaling, and to manual fits performed by four experts. A theoretical DBS electrode target location in the dorsal STN was customized to each patient as part of the fitting process and compared to the location of the clinically defined therapeutic stimulation contact.

RESULTS

The human expert and computer optimization fits achieved significantly better fits than the AC/PC scaling (80, 81, and 41% of correctly fitted MER, respectively). However, the optimization fits were performed in less time than the expert fits and converged to a single solution for each patient, eliminating interexpert variance.

CONCLUSIONS AND SIGNIFICANCE

DBS therapeutic outcomes are directly related to electrode implantation accuracy. Our automated fitting techniques may aid in the surgical decision-making process by optimally integrating brain atlas and intraoperative neurophysiological data to provide a visual guide for target identification.

Deep brain stimulation activation volumes and their association with neurophysiological mapping and therapeutic outcomes

OBJECTIVE

Despite the clinical success of deep brain stimulation (DBS) for the treatment of Parkinson's disease (PD), little is known about the electrical spread of the stimulation. The primary goal of this study was to integrate neuroimaging, neurophysiology and neurostimulation data sets from 10 patients with PD, unilaterally implanted with subthalamic nucleus (STN) DBS electrodes, to identify the theoretical volume of tissue activated (VTA) by clinically defined therapeutic stimulation parameters.

METHODS

Each patient specific model was created with a series of five steps: (1) definition of the neurosurgical stereotactic coordinate system within the context of preoperative imaging data; (2) entry of intraoperative microelectrode recording locations from neurophysiologically defined thalamic, subthalamic and substantia nigra neurons into the context of the imaging data; (3) fitting a three dimensional brain atlas to the neuroanatomy and neurophysiology of the patient; (4) positioning the DBS electrode in the documented stereotactic location, verified by postoperative imaging data; and (5) calculation of the VTA using a diffusion tensor based finite element neurostimulation model.

RESULTS

The patient specific models show that therapeutic benefit was achieved with direct stimulation of a wide range of anatomical structures in the subthalamic region. Interestingly, of the five patients exhibiting a greater than 40% improvement in their Unified PD Rating Scale (UPDRS), all but one had the majority of their VTA outside the atlas defined borders of the STN. Furthermore, of the five patients with less than 40% UPDRS improvement, all but one had the majority of their VTA inside the STN.

CONCLUSIONS

Our results are consistent with previous studies suggesting that therapeutic benefit is associated with electrode contacts near the dorsal border of the STN, and provide quantitative estimates of the electrical spread of the stimulation in a clinically relevant context.

Quantification and visualization of three-dimensional inconsistency of the ventrointermediate nucleus of the thalamus in the Schaltenbrand-Wahren brain atlas

BACKGROUND

This work quantifies and visualises 3D inconsistencies of the ventrointermediate nucleus (VIM) of the thalamus, including the VIM externum (VIMe) and VIM internum (VIMi), in the Schaltenbrand-Wahren (SW) brain atlas.

METHOD

For each VIM, VIMe, VIMi the 3D models, 3D-A, 3D-C and 3D-S were reconstructed from the SW axial, coronal and sagittal microseries, respectively, by applying a shape-based method. All 3D models, placed in the SW coordinate system, were compared quantitatively in terms of location (centroids), size (volumes), shape (normalised eigen values), orientation (eigen vectors), and mutual spatial relationships (overlaps and inclusions).

FINDINGS

The reconstructed 3D models differ significantly in location, size, shape, and inclusion rate. The centroid of 3D-A/VIM differs considerably from those of 3D-C/VIM and 3D-S/VIM. The difference between the centroids of 3D-C/VIM and 3D-S/VIM is in laterality only: that of 3D-C/VIM is located more medially (11.85 mm) than that of 3D-S/VIM (14.62 mm). 3D-A/VIM has the smallest volume (69.00 mm(3)); 3D-C/VIM is 3.71 and 3D-S/VIM 3.89 times larger. The overlap is also highly variable: 104.88 mm(3) for 3D-C/VIM with 3D-S/VIM, and very low (3.22 and 7.45 mm(3)) when 3D-A/VIM is involved. The highest inclusion rate is for 3D-C/VIM with 3D-S/VIM (39.10 and 40.97%) and the lowest for 3D-A/VIM with 3D-C/VIM (1.26 and 4.66%). The centroid of 3D-A/VIMe differs noticeably from those of 3D-C/VIMe and 3D-S/VIMe. The difference between the centroids of 3D-C/VIMe and 3D-S/VIMe is mainly in laterality: that of 3D-C/VIMe is located more medially (12.91 mm) than that of 3D-S/VIMe (16.65 mm). 3D-A/VIMe has the smallest volume (49.87 mm(3)); 3D-S/VIMe is 3.24 and 3D-C/VIMe 3.36 times larger. The overlap sizes are low: 32.72 mm(3) for 3D-C/VIMe with 3D-S/VIMe, and very low (1.32 and 2.01 mm(3)) when 3D-A/VIMe is involved. The inclusion rates are also low: the highest is for 3D-C/VIMe with 3D-S/VIMe (19.53 and 20.29%) and the lowest for 3D-A/VIMe with 3D-C/VIMe (1.19 and 4.01%). Lateral scaling of the coronal microseries by 1.2897 to match the 3D-C/VIMe and 3D-S/VIMe centroids increases the inclusion rates for the sagittal microseries by more than twice. The volume of scaled 3D-C enlarges to 216.24 mm(3) which is 1.34 bigger than that of 3D-S. There are substantial differences among the centroids of 3D-A/VIMi, 3D-C/VIMi and 3D-S/VIMi. The centroid of 3D-A/VIMi is located more anteriorly (-1.92 mm) than that of 3D-C/VIMi (-5.02 mm). The centroid of 3D-A/VIMi is located more ventrally (2.88 mm) than those of 3D-C/VIMi and 3D-S/VIMi (each at 5.34 mm). 3D-A/VIMi has the smallest volume (19.75 mm(3)); 3D-S/VIMi is 3.23 and 3D-C/VIMi 4.30 times larger. 3D-A/VIMi practically does not overlap with 3D-C/VIMi and 3D-S/VIMi. The inclusion rates for 3D-C/VIMi with 3D-S/VIMi are medium (32.63 and 43.43%).

CONCLUSION

Each VIM, VIMe, VIMi as reconstructed from the SW atlas has a significant 3D inaccuracy within each orientation and across them. Therefore, absolute and direct reliance on the original SW atlas is unreliable and unsafe, and this atlas has to be used with great care and understanding of its strengths and limitations.

3D segmentation and quantification of a masticatory muscle from MR data using patient-specific models and matching distributions

A method is proposed for 3D segmentation and quantification of the masseter muscle from magnetic resonance (MR) images, which is often performed in pre-surgical planning and diagnosis. Because of a lack of suitable automatic techniques, a common practice is for clinicians to manually trace out all relevant regions from the image slices which is extremely time-consuming. The proposed method allows significant time savings. In the proposed method, a patient-specific masseter model is built from a test dataset after determining the dominant slices that represent the salient features of the 3D muscle shape from training datasets. Segmentation is carried out only on these slices in the test dataset, with shape-based interpolation then applied to build the patient-specific model, which serves as a coarse segmentation of the masseter. This is first refined by matching the intensity distribution within the masseter volume against the distribution estimated from the segmentations in the dominant slices, and further refined through boundary analysis where the homogeneity of the intensities of the boundary pixels is analyzed and outliers removed. It was observed that the left and right masseter muscles' volumes in young adults (28.54 and 27.72 cm(3)) are higher than those of older (ethnic group removed) adults (23.16 and 22.13 cm(3)). Evaluation indicates good agreement between the segmentations and manual tracings, with average overlap indexes for the left and right masseters at 86.6% and 87.5% respectively.

Towards construction of an ideal stereotactic brain atlas

BACKGROUND

The role of the brain atlas is changing in many aspects with the advancements in stereotactic and functional neurosurgery. Therefore, there is a critical need to construct a new atlas. This paper addresses the definition and construction of an atlas, ideal (in our opinion) for stereotactic and functional neurosurgery. The essence of the new atlas is not only its population-based structural and functional content, but also its continuous "self-updatability" with the new clinical results obtained.

METHOD

The ideal atlas defined here contains four major components: brain models, knowledge database, tools, and clinical results. Towards its creation, a multi-atlas is proposed. The construction of the initial version of the multi-atlas is detailed with the probabilistic functional atlas (PFA), interpolated Talairach-Tournoux atlas, and enhanced Schaltenbrand-Wahren atlas. These atlases are put in a spatial register by matching their AC-PC distances and heights of the thalamus; the Schaltenbrand coronal and sagittal microseries are scaled laterally to match the target structure centroids with the locations of the best targets of the PFA.

FINDINGS

Construction of an initial version of the ideal stereotactic atlas is feasible at present from the available resources. To achieve that, our three atlases (PFA, Talairach and Schaltenbrand) are enhanced and combined together. A single lateral scaling factor per target structure is feasible to co-register the Schaltenbrand atlas with PFA in four situations (compensated against the third ventricle, non-compensated, bilateral, and non-bilateral). The STN has to be stretched by 18% more than the VIM on the Schaltenbrand coronal microseries, and the VIM has to be compressed by 13% less than the STN on the Schaltenbrand sagittal microseries.

CONCLUSION

The new multi-atlas can potentially be more useful than the currently employed atlases and will facilitate further development of the ideal atlas for stereotactic and functional neurosurgery.

Patient-specific analysis of the volume of tissue activated during deep brain stimulation

Despite the clinical success of deep brain stimulation (DBS) for the treatment of movement disorders, many questions remain about its effects on the nervous system. This study presents a methodology to predict the volume of tissue activated (VTA) by DBS on a patient-specific basis. Our goals were to identify the intersection between the VTA and surrounding anatomical structures and to compare activation of these structures with clinical outcomes. The model system consisted of three fundamental components: (1) a 3D anatomical model of the subcortical nuclei and DBS electrode position in the brain, each derived from magnetic resonance imaging (MRI); (2) a finite element model of the DBS electrode and electric field transmitted to the brain, with tissue conductivity properties derived from diffusion tensor MRI; (3) VTA prediction derived from the response of myelinated axons to the applied electric field, which is a function of the stimulation parameters (contact, impedance, voltage, pulse width, frequency). We used this model system to analyze the effects of subthalamic nucleus (STN) DBS in a patient with Parkinson's disease. Quantitative measurements of bradykinesia, rigidity, and corticospinal tract (CST) motor thresholds were evaluated over a range of stimulation parameter settings. Our model predictions showed good agreement with CST thresholds. Additionally, stimulation through electrode contacts that improved bradykinesia and rigidity generated VTAs that overlapped the zona incerta/fields of Forel (ZI/H2). Application of DBS technology to various neurological disorders has preceded scientific characterization of the volume of tissue directly affected by the stimulation. Synergistic integration of clinical analysis, neuroimaging, neuroanatomy, and neurostimulation modeling provides an opportunity to address wide ranging questions on the factors linked with the therapeutic benefits and side effects of DBS.

Quantification and Visualization of Three-Dimensional Inconsistency of the Globus Pallidus Internus in the Schaltenbrand-Wahren Brain Atlas

The major shortcomings of the Schaltenbrand-Wahren (SW) brain atlas include 3-dimensional (3D) inconsistency and spatial sparseness. This work quantifies and visualizes 3D inconsistency of the globus pallidus internus (GPi), a stereotactic target for the treatment of Parkinson's disease, dystonia and Huntington disease. The GPi 3D models 3D-A, 3D-C and 3D-S are reconstructed from the SW axial, coronal and sagittal microseries, respectively, by applying a shape-based (Nonuniform Rational B Splines) method. All three 3D models, placed in the SW coordinate system, are compared quantitatively in terms of location (centroids), size (volumes), shape (normalized eigen values), orientation (eigen vectors) and mutual spatial relationships (overlaps and inclusions). The analysis is done in 3D within each orientation and across them. The reconstructed 3D GPi models substantially differ in location, size and inclusion rate. The centroid of 3D-C is located more medially (15.6 mm) than those of 3D-A (17.5 mm) and 3D-S (18.2 mm), and that of 3D-A more ventrally (-2.3 mm) than those of 3D-C (-0.1 mm) and 3D-S (-0.4 mm). 3D-S has the smallest volume (347.3 mm3); 3D-A is 1.18 and 3D-C 1.85 times larger. The highest inclusion rate is for 3D-S (54.3 and 56.3%) and the lowest for 3D-C (28.8 and 30.6%). A smaller variability is observed in shape, orientation and overlap size (196.8, 196.1 and 185.5 mm3). To get a better correspondence between 3D-C and 3D-S, the coronal microseries were scaled laterally by 1.1667. This results in a substantial improvement of the inclusion rate of 3D-S (87.9%), though raising the volume mismatch to 2.16. The GPi in the SW atlas has a substantial 3D inaccuracy within each orientation and across them. Therefore, absolute and direct reliance on the original atlas is unsafe, and this atlas has to be used with great care and understanding of its limitations. As matching various SW microseries by global scaling is not feasible, we propose the target-dependent scaling based on structure centroid matching.