Accuracy evaluation of microTargeting Platforms for deep-brain stimulation using virtual targets

Towards MR-Guided Robotic Intracerebral Hemorrhage Evacuation: Aiming Device Design and ex vivo Ovine Head Trial

Stereotactic neurosurgery is a well-established surgical technique for navigation and guidance during treatment of intracranial pathologies. Intracerebral hemorrhage (ICH) is an example of various neurosurgical conditions that can benefit from stereotactic neurosurgery. As a part of our ongoing work toward real-time MR-guided ICH evacuation, we aim to address an unmet clinical need for a skull-mounted frameless stereotactic aiming device that can be used with minimally invasive robotic systems for MR-guided interventions. In this paper, we present NICE-Aiming, a Neurosurgical, Interventional, Configurable device for Effective-Aiming in MR-guided robotic neurosurgical interventions. A kinematic model was developed and the system was used with a concentric tube robot (CTR) for ICH evacuation in (i) a skull phantom and (ii) in the first ever reported ex vivo CTR ICH evacuation using an ex vivo ovine head. The NICE-Aiming prototype provided a tip accuracy of 1.41±0.35 mm in free-space. In the MR-guided gel phantom experiment, the targeting accuracy was 2.07±0.42 mm and the residual hematoma volume was 12.87 mL (24.32% of the original volume). In the MR-guided ex vivo ovine head experiment, the targeting accuracy was 2.48±0.48 mm and the residual hematoma volume was 1.42 mL (25.08% of the original volume).

Analysis of the Technical Accuracy of a Patient-Specific Stereotaxy Platform for Brain Biopsy

The use of stereotactic frames is a common practice in neurosurgical interventions such as brain biopsy and deep brain stimulation. However, conventional stereotactic frames have been shown to require modification and adaptation regarding patient and surgeon comfort as well as the increasing demand for individualized medical treatment. To meet these requirements for carrying out state-of-the-art neurosurgery, a 3D print-based, patient-specific stereotactic system was developed and examined for technical accuracy. Sixteen patient-specific frames, each with two target points, were additively manufactured from PA12 using the Multi Jet Fusion process. The 32 target points aim to maximize the variability of biopsy targets and depths for tissue sample retrieval in the brain. Following manufacturing, the frames were measured three-dimensionally using an optical scanner. The frames underwent an autoclave sterilization process prior to rescanning. The scan-generated models were compared with the planned CAD models and the deviation of the planned target points in the XY-plane, Z-direction and in the resulting direction were determined. Significantly lower (p < 0.01) deviations were observed when comparing CAD vs. print and print vs. sterile in the Z-direction (0.17 mm and 0.06 mm, respectively) than in the XY-plane (0.46 mm and 0.16 mm, respectively). The resulting target point deviation (0.51 mm) and the XY-plane (0.46 mm) are significantly higher (p < 0.01) in the CAD vs. print comparison than in the print vs. sterile comparison (0.18 mm and 0.16 mm, respectively). On average, the results from the 32 target positions examined exceeded the clinically required accuracy for a brain biopsy (2 mm) by more than four times. The patient-specific stereotaxic frames meet the requirements of modern neurosurgical navigation and make no compromises when it comes to accuracy. In addition, the material is suitable for autoclave sterilization due to resistance to distortion.

Awake Deep Brain Stimulation Surgery Without Intraoperative Imaging Is Accurate and Effective: A Case Series

BACKGROUND

The success of deep brain stimulation (DBS) surgery depends on the accuracy of electrode placement. Several factors can affect this such as brain shift, the quality of preoperative planning, and technical factors. It is crucial to determine whether techniques yield accurate lead placement and effective symptom relief. Many of the studies establishing the accuracy of frameless techniques used intraoperative imaging to further refine lead placement.

OBJECTIVE

To determine whether awake lead placement without intraoperative imaging can achieve similar minimal targeting error while preserving clinical results.

METHODS

Eighty-two trajectories in 47 patients who underwent awake, frameless DBS lead placement with the Fred Haer Corporation STarFix system for essential tremor or Parkinson's disease were analyzed. Neurological testing during lead placement was used to determine appropriate lead locations, and no intraoperative imaging was performed. Accuracy data were compared with previously performed studies.

RESULTS

The Euclidean error for the patient cohort was 1.79 ± 1.02 mm, and the Pythagorean error was 1.40 ± 0.95 mm. The percentage symptom improvement evaluated by the Unified Parkinson's Disease Rating Scale for Parkinson's disease or the Fahn-Tolosa-Marin scale for essential tremor was similar to reported values at 58% ± 17.2% and 67.4% ± 24.7%, respectively. The operative time was 95.0 ± 30.3 minutes for all study patients.

CONCLUSION

Awake, frameless DBS surgery with the Fred Haer Corporation STarFix system does not require intraoperative imaging for stereotactic accuracy or clinical effectiveness.

Impact of brain shift on neural pathways in deep brain stimulation: a preliminary analysis via multi-physics finite element models

OBJECTIVE

The effectiveness of deep brain stimulation (DBS) depends on electrode placement accuracy, which can be compromised by brain shift during surgery. While there have been efforts in assessing the impact of electrode misplacement due to brain shift using preop- and postop- imaging data, such analysis using preop- and intraop- imaging data via biophysical modeling has not been conducted. This work presents a preliminary study that applies a multi-physics analysis framework using finite element biomechanical and bioelectric models to examine the impact of realistic intraoperative shift on neural pathways determined by tractography.

APPROACH

The study examined six patients who had undergone interventional magnetic resonance (iMR)-guided DBS surgery. The modeling framework utilized a biomechanical approach to update preoperative MR to reflect shift-induced anatomical changes. Using this anatomically deformed image and its undeformed counterpart, bioelectric effects from shifting electrode leads could be simulated and neural activation differences were approximated. Specifically, for each configuration, volume of tissue activation (VTA) was computed and subsequently used for tractography estimation. Total tract volume and overlapping volume with motor regions as well as connectivity profile were compared. In addition, volumetric overlap between different fiber bundles among configurations was computed and correlated to estimated shift.

MAIN RESULT

The study found deformation-induced differences in tract volume, motor region overlap, and connectivity behavior, suggesting the impact of shift. There is a strong correlation (R=-0.83) between shift from intended target and intended neural pathway recruitment, where at threshold of ~2.94 mm, intended recruitment completely degrades. The determined threshold is consistent with and provides quantitative support to prior observations and literature that deviations of 2-3 mm are detrimental.

SIGNIFICANCE

The findings support and advance prior studies and understanding to illustrate the need to account for shift in DBS and the potentiality of computational modeling for estimating influence of shift on neural activation.

Feasibility and performance of a frameless stereotactic system for targeting subcortical nuclei in nonhuman primates

OBJECTIVE

Deep brain stimulation (DBS) is an effective therapy for different neurological diseases, despite the lack of comprehension of its mechanism of action. The use of nonhuman primates (NHPs) has been historically important in advancing this field and presents a unique opportunity to uncover the therapeutic mechanisms of DBS, opening the way for optimization of current applications and the development of new ones. To be informative, research using NHPs should make use of appropriate electrode implantation tools. In the present work, the authors report on the feasibility and accuracy of targeting different deep brain regions in NHPs using a commercially available frameless stereotactic system (microTargeting platform).

METHODS

Seven NHPs were implanted with DBS electrodes, either in the subthalamic nucleus or in the cerebellar dentate nucleus. A microTargeting platform was designed for each animal and used to guide implantation of the electrode. Imaging studies were acquired preoperatively for each animal, and were subsequently analyzed by two independent evaluators to estimate the electrode placement error (EPE). The interobserver variability was assessed as well.

RESULTS

The radial and vector components of the EPE were estimated separately. The magnitude of the vector of EPE was 1.29 ± 0.41 mm and the mean radial EPE was 0.96 ± 0.63 mm. The interobserver variability was considered negligible.

CONCLUSIONS

These results reveal the suitability of this commercial system to enhance the surgical insertion of DBS leads in the primate brain, in comparison to rigid traditional frames. Furthermore, our results open up the possibility of performing frameless stereotaxy in primates without the necessity of relying on expensive methods based on intraoperative imaging.

Concept description and accuracy evaluation of a moldable surgical targeting system

Purpose: We explain our concept for customization of a guidance instrument, present a prototype, and describe a set of experiments to evaluate its positioning and drilling accuracy.

Methods: Our concept is characterized by the use of bone cement, which enables fixation of a specific configuration for each individual surgical template. This well-established medical product was selected to ensure future intraoperative fabrication of the template under sterile conditions. For customization, a manually operated alignment device is proposed that temporary defines the planned trajectory until the bone cement is hardened. Experiments (n=10) with half-skull phantoms were performed. Analysis of accuracy comprises targeting validations and experiments including drilling in bone substitutes.

Results: The resulting mean positioning error was found to be 0.41±0.30  mm at the level of the target point whereas drilling was possible with a mean accuracy of 0.35±0.30  mm.

Conclusion: We proposed a cost-effective, easy-to-use approach for accurate instrument guidance that enables template fabrication under sterile conditions. The utilization of bone cement was proven to fulfill the demands of an easy, quick, and prospectively intraoperatively doable customization. We could demonstrate sufficient accuracy for many surgical applications, e.g., in neurosurgery. The system in this early development stage already outperforms conventional stereotactic frames and image-guided surgery systems in terms of targeting accuracy.

Stereotactic Neuro-Navigation Phantom Designs: A Systematic Review

Diverse stereotactic neuro-navigation systems are used daily in neurosurgery and novel systems are continuously being developed. Prior to clinical implementation of new surgical tools, methods or instruments, in vitro experiments on phantoms should be conducted. A stereotactic neuro-navigation phantom denotes a rigid or deformable structure resembling the cranium with the intracranial area. The use of phantoms is essential for the testing of complete procedures and their workflows, as well as for the final validation of the application accuracy. The aim of this study is to provide a systematic review of stereotactic neuro-navigation phantom designs, to identify their most relevant features, and to identify methodologies for measuring the target point error, the entry point error, and the angular error (α). The literature on phantom designs used for evaluating the accuracy of stereotactic neuro-navigation systems, i.e., robotic navigation systems, stereotactic frames, frameless navigation systems, and aiming devices, was searched. Eligible articles among the articles written in English in the period 2000-2020 were identified through the electronic databases PubMed, IEEE, Web of Science, and Scopus. The majority of phantom designs presented in those articles provide a suitable methodology for measuring the target point error, while there is a lack of objective measurements of the entry point error and angular error. We identified the need for a universal phantom design, which would be compatible with most common imaging techniques (e.g., computed tomography and magnetic resonance imaging) and suitable for simultaneous measurement of the target point, entry point, and angular errors.

Deep Brain Stimulation Lead Implantation Using a Customized Rapidly Manufactured Stereotactic Fixture with Submillimetric Euclidean Accuracy

BACKGROUND

The microTargetingTM MicrotableTM Platform is a novel stereotactic system that can be more rapidly fabricated than currently available 3D-printed alternatives. We present the first case series of patients who underwent deep brain stimulation (DBS) surgery guided by this platform and demonstrate its in vivo accuracy.

METHODS

Ten patients underwent DBS at a single institution by the senior author and 15 leads were placed. The mean age was 69.1 years; four were female. The ventralis intermedius nucleus was targeted for patients with essential tremor and the subthalamic nucleus was targeted for patients with Parkinson's disease.

RESULTS

Nine DBS leads in 6 patients were appropriately imaged to enable measurement of accuracy. The mean Euclidean electrode placement error (EPE) was 0.97 ± 0.37 mm, and the mean radial error was 0.80 ± 0.41 mm (n = 9). In the subset of CT scans performed greater than 1 month postoperatively (n = 3), the mean Euclidean EPE was 0.75 ± 0.17 mm and the mean radial error was 0.69 ± 0.17 mm. There were no surgical complications.

CONCLUSION

The MicrotableTM platform is capable of submillimetric accuracy in patients undergoing stereotactic surgery. It has achieved clinical efficacy in our patients without surgical complications and has demonstrated the potential for superior accuracy compared to both traditional stereotactic frames and other common frameless systems.

Custom mastoid-fitting templates to improve cochlear implant electrode insertion trajectory

PURPOSE

Insertion trajectory affects final intracochlear cochlear implant (CI) positioning, but limited information is available intraoperatively regarding ideal trajectory. We sought to improve intracochlear positioning CI electrodes using custom templates to specify insertion trajectory.

METHODS

3D reconstructions were created from computed tomography of three cadaveric temporal bones. Trajectories co-planar with the straight segment of the cochlea's basal turn were considered ideal. Templates were designed to fit against the drilled mastoid's surface and convey this guided trajectory via a hollow cylinder. Templates were 3D-printed using stereolithography. Mastoidectomy was performed. Template accuracy was tested by measuring target registration error (TRE) for four templates. A novel, roller-based insertion tool (designed to fit within the template cylinder) constrained insertions to intended trajectories. Insertions were performed with MED-EL Standard electrodes in three bones with three conditions: guided trajectory with insertion tool, non-guided trajectory with insertion tool and guided trajectory with surgical forceps. For the final condition, the template was used to mark the mastoid to convey trajectory. Insertion was stopped when electrode buckling occurred.

RESULTS

TRE ranged from 0.23 to 0.73 mm. Mean TRE ± standard deviation was 0.55 ± 0.19 mm. Insertions along guided versus non-guided trajectories averaged more intracochlear electrodes (9, 8, 8 vs. 7, 7, 8) and greater angular insertion depths (AID) (377°, 341°, 320° vs. 278°, 302°, 290°). Insertions performed with forceps using templates as a guide also achieved excellent results (intracochlear electrodes: 10, 7, 8; AID: 478°, 318°, 333°). No translocations occurred.

CONCLUSION

Custom mastoid-fitting templates reliably specify intended insertion trajectory and provide sufficient information for recreation of that trajectory with manual insertion after template removal. The templates can accurately target structures within the temporal bone with a TRE of 0.55 ± 0.19 mm. Our roller-based insertion tool achieves results comparable to manual insertion using surgical forceps.

Accounting for Deformation in Deep Brain Stimulation Surgery With Models: Comparison to Interventional Magnetic Resonance Imaging

The efficacy of deep brain stimulation (DBS) depends on electrode placement accuracy, which can be jeopardized by brain shift due to burr hole and dura opening during surgery. Brain shift violates assumed rigid alignment between preoperative image and intraoperative anatomy, negatively impacting therapy.

OBJECTIVE

This study presents a deformation-atlas biomechanical model-based approach to address shift.

METHODS

Six patients, who underwent interventional magnetic resonance (iMR) image-guided DBS burr hole surgery, were studied. A patient-specific model was employed under varying surgical conditions, generating a collection of possible intraoperative shift estimations or a 'deformation atlas.' An inverse problem was driven by sparse measurements derived from iMR to determine an optimal fit of solutions of the atlas. This fit was then used to obtain a volumetric deformation field, which was utilized to update preoperative MR and estimate shift at surgical target region localized on iMR. Model performance was examined by quantitatively comparing intraoperative subsurface measurements to their model-predicted counterparts, and qualitatively comparing iMR, preoperative MR, and model updated MR. A nonrigid image registration was introduced as a comparator.

RESULTS

Model-based approach reduced general parenchyma shift from 8.2 ± 2.2 to 2.7 ± 1.1 mm (∼66.8% correction), and produced updated MR with better agreement to iMR than that of preoperative MR. The average model estimated shift at target region was 1.2 mm.

CONCLUSIONS

This study demonstrates the feasibility of a model-based shift correction strategy in DBS surgery with only sparse data.

SIGNIFICANCE

The developed strategy has the potential to complement and/or enhance current clinical approaches in addressing shift.

Computational Modeling and Neuroimaging Techniques for Targeting during Deep Brain Stimulation

Accurate surgical localization of the varied targets for deep brain stimulation (DBS) is a process undergoing constant evolution, with increasingly sophisticated techniques to allow for highly precise targeting. However, despite the fastidious placement of electrodes into specific structures within the brain, there is increasing evidence to suggest that the clinical effects of DBS are likely due to the activation of widespread neuronal networks directly and indirectly influenced by the stimulation of a given target. Selective activation of these complex and inter-connected pathways may further improve the outcomes of currently treated diseases by targeting specific fiber tracts responsible for a particular symptom in a patient-specific manner. Moreover, the delivery of such focused stimulation may aid in the discovery of new targets for electrical stimulation to treat additional neurological, psychiatric, and even cognitive disorders. As such, advancements in surgical targeting, computational modeling, engineering designs, and neuroimaging techniques play a critical role in this process. This article reviews the progress of these applications, discussing the importance of target localization for DBS, and the role of computational modeling and novel neuroimaging in improving our understanding of the pathophysiology of diseases, and thus paving the way for improved selective target localization using DBS.

Fully automated targeting using nonrigid image registration matches accuracy and exceeds precision of best manual approaches to subthalamic deep brain stimulation targeting in Parkinson disease

BACKGROUND

Finding the optimal location for the implantation of the electrode in deep brain stimulation (DBS) surgery is crucial for maximizing the therapeutic benefit to the patient. Such targeting is challenging for several reasons, including anatomic variability between patients as well as the lack of consensus about the location of the optimal target.

OBJECTIVE

To compare the performance of popular manual targeting methods against a fully automatic nonrigid image registration-based approach.

METHODS

In 71 Parkinson disease subthalamic nucleus (STN)-DBS implantations, an experienced functional neurosurgeon selected the target manually using 3 different approaches: indirect targeting using standard stereotactic coordinates, direct targeting based on the patient magnetic resonance imaging, and indirect targeting relative to the red nucleus. Targets were also automatically predicted by using a leave-one-out approach to populate the CranialVault atlas with the use of nonrigid image registration. The different targeting methods were compared against the location of the final active contact, determined through iterative clinical programming in each individual patient.

RESULTS

Targeting by using standard stereotactic coordinates corresponding to the center of the motor territory of the STN had the largest targeting error (3.69 mm), followed by direct targeting (3.44 mm), average stereotactic coordinates of active contacts from this study (3.02 mm), red nucleus-based targeting (2.75 mm), and nonrigid image registration-based automatic predictions using the CranialVault atlas (2.70 mm). The CranialVault atlas method had statistically smaller variance than all manual approaches.

CONCLUSION

Fully automatic targeting based on nonrigid image registration with the use of the CranialVault atlas is as accurate and more precise than popular manual methods for STN-DBS.

3D Rapid Prototyping for Otolaryngology-Head and Neck Surgery: Applications in Image-Guidance, Surgical Simulation and Patient-Specific Modeling

The aim of this study was to demonstrate the role of advanced fabrication technology across a broad spectrum of head and neck surgical procedures, including applications in endoscopic sinus surgery, skull base surgery, and maxillofacial reconstruction. The initial case studies demonstrated three applications of rapid prototyping technology are in head and neck surgery: i) a mono-material paranasal sinus phantom for endoscopy training ii) a multi-material skull base simulator and iii) 3D patient-specific mandible templates. Digital processing of these phantoms is based on real patient or cadaveric 3D images such as CT or MRI data. Three endoscopic sinus surgeons examined the realism of the endoscopist training phantom. One experienced endoscopic skull base surgeon conducted advanced sinus procedures on the high-fidelity multi-material skull base simulator. Ten patients participated in a prospective clinical study examining patient-specific modeling for mandibular reconstructive surgery. Qualitative feedback to assess the realism of the endoscopy training phantom and high-fidelity multi-material phantom was acquired. Conformance comparisons using assessments from the blinded reconstructive surgeons measured the geometric performance between intra-operative and pre-operative reconstruction mandible plates. Both the endoscopy training phantom and the high-fidelity multi-material phantom received positive feedback on the realistic structure of the phantom models. Results suggested further improvement on the soft tissue structure of the phantom models is necessary. In the patient-specific mandible template study, the pre-operative plates were judged by two blinded surgeons as providing optimal conformance in 7 out of 10 cases. No statistical differences were found in plate fabrication time and conformance, with pre-operative plating providing the advantage of reducing time spent in the operation room. The applicability of common model design and fabrication techniques across a variety of otolaryngological sub-specialties suggests an emerging role for rapid prototyping technology in surgical education, procedure simulation, and clinical practice.

A Compact, Bone-Attached Robot for Mastoidectomy

Otologic surgery often involves a mastoidectomy, which is the removal of a portion of the mastoid region of the temporal bone, to safely access the middle and inner ear. The surgery is challenging because many critical structures are embedded within the bone, making them difficult to see and requiring a high level of accuracy with the surgical dissection instrument, a high-speed drill. We propose to automate the mastoidectomy portion of the surgery using a compact, bone-attached robot. The system described in this paper is a milling robot with four degrees-of-freedom (DOF) that is fixed to the patient during surgery using a rigid positioning frame screwed into the surface of the bone. The target volume to be removed is manually identified by the surgeon pre-operatively in a computed tomography (CT) scan and converted to a milling path for the robot. The surgeon attaches the robot to the patient in the operating room and monitors the procedure. Several design considerations are discussed in the paper as well as the proposed surgical workflow. The mean targeting error of the system in free space was measured to be 0.5 mm or less at vital structures. Four mastoidectomies were then performed in cadaveric temporal bones, and the error at the edges of the target volume was measured by registering a postoperative computed tomography (CT) to the pre-operative CT. The mean error along the border of the milled cavity was 0.38 mm, and all critical anatomical structures were preserved.

Minimally invasive multiport surgery of the lateral skull base

OBJECTIVE

Minimally invasive procedures minimize iatrogenic tissue damage and lead to a lower complication rate and high patient satisfaction. To date only experimental minimally invasive single-port approaches to the lateral skull base have been attempted. The aim of this study was to verify the feasibility of a minimally invasive multiport approach for advanced manipulation capability and visual control and develop a software tool for preoperative planning.

METHODS

Anatomical 3D models were extracted from twenty regular temporal bone CT scans. Collision-free trajectories, targeting the internal auditory canal, round window, and petrous apex, were simulated with a specially designed planning software tool. A set of three collision-free trajectories was selected by skull base surgeons concerning the maximization of the distance to critical structures and the angles between the trajectories.

RESULTS

A set of three collision-free trajectories could be successfully simulated to the three targets in each temporal bone model without violating critical anatomical structures.

CONCLUSION

A minimally invasive multiport approach to the lateral skull base is feasible. The developed software is the first step for preoperative planning. Further studies will focus on cadaveric and clinical translation.

A personalized stereotactic fixture for implantation of depth electrodes in stereoelectroencephalography

BACKGROUND

The stereoelectroencephalographic (SEEG) implantation procedures still represent a challenge due to the intrinsic complexity of the method and the number of depth electrodes required.

OBJECTIVES

We aim at designing and evaluating the accuracy of a custom stereotactic fixture based on the StarFix™ technology (FHC Inc., Bowdoin, ME) that significantly simplifies and optimizes the implantation of depth electrodes used in presurgical evaluation of patients with drug-resistant epilepsy.

METHODS

Fiducial markers that also serve as anchors for the fixture are implanted into the patient's skull prior to surgery. A 3D fixture model is designed within the surgical planning software, with the planned trajectories incorporated in its design, aligned with the patient's anatomy. The stereotactic fixture is built using 3D laser sintering technology based on the computer-generated model. Bilateral rectangular grids of guide holes orthogonal to the midsagittal plane and centered on the midcommissural point are incorporated in the fixture design, allowing a wide selection of orthogonal trajectories. Up to two additional grids can be accommodated for targeting structures where oblique trajectories are required. The frame has no adjustable parts, this feature reducing the risk of inaccurate coordinate settings while simultaneously reducing procedure time significantly.

RESULTS

We have used the fixture for the implantation of depth electrodes for presurgical evaluation of 4 patients with drug-resistant focal epilepsy, with nearly 2-fold reduction in the duration of the implantation procedure. We have obtained a high accuracy with a submillimetric mean positioning error of 0.68 mm for the anchor bolts placed at the trajectory entry point and 1.64 mm at target.

CONCLUSIONS

The custom stereotactic fixture design greatly simplifies the planning procedure and significantly reduces the time in the operating room, while maintaining a high accuracy.

Preliminary Testing of a Compact, Bone-Attached Robot for Otologic Surgery

Otologic surgery often involves a mastoidectomy procedure, in which part of the temporal bone is milled away in order to visualize critical structures embedded in the bone and safely access the middle and inner ear. We propose to automate this portion of the surgery using a compact, bone-attached milling robot. A high level of accuracy is required to avoid damage to vital anatomy along the surgical path, most notably the facial nerve, making this procedure well-suited for robotic intervention. In this study, several of the design considerations are discussed and a robot design and prototype are presented. The prototype is a 4 degrees-of-freedom robot similar to a four-axis milling machine that mounts to the patient's skull. A positioning frame, containing fiducial markers and attachment points for the robot, is rigidly attached to the skull of the patient, and a CT scan is acquired. The target bone volume is manually segmented in the CT by the surgeon and automatically converted to a milling path and robot trajectory. The robot is then attached to the positioning frame and is used to drill the desired volume. The accuracy of the entire system (image processing, planning, robot) was evaluated at several critical locations within or near the target bone volume with a mean free space accuracy result of 0.50 mm or less at all points. A milling test in a phantom material was then performed to evaluate the surgical workflow. The resulting milled volume did not violate any critical structures.

Clinical testing of an alternate method of inserting bone-implanted fiducial markers

BACKGROUND

Deep brain stimulation (DBS) surgery utilizes image guidance via bone-implanted fiducial markers to achieve the desired submillimetric accuracy and to provide means for attaching microstereotactic frames. For maximal benefit, the markers must be inserted to the correct depth since over-insertion leads to stripping and under-insertion leads to instability.

PURPOSE

The purpose of the study was to test clinically a depth-release drive system, the PosiSeat™, versus manual insertion (pilot hole followed by manual screwing until tactile determined correct seating) for implanting fiducial markers into the bone.

METHODS

With institutional review board approval, the PosiSeat™ was used to implant markers in 15 DBS patients (57 fiducials). On post-insertion CT scans, the depth of the gap between the shoulder of the fiducial markers and the closest bone surface was measured. Similar depth measurements were performed on the CT scans of 64 DBS patients (250 fiducials), who underwent manual fiducial insertion.

RESULTS

Median of shoulder-to-bone distance for PosiSeat™ and manual insertion group were 0.03 and 1.06 mm, respectively. Fifty percent of the fiducials had the shoulder-to-bone distances within 0.01-0.09 mm range for the PosiSeat group and 0.04-1.45 mm range for the manual insertion group. These differences were statistically significant.

CONCLUSIONS

A depth-release drive system achieves more consistent placement of bone-implanted fiducial markers than manual insertion.

Effect of data normalization on the creation of neuro-probabilistic atlases

In the past 15 years, rapid improvements in imaging technology and methodology have had a tremendous impact on how we study the human brain. During deep brain stimulation surgeries, detailed anatomical images can be combined with physiological data obtained by microelectrode recordings and microstimulations to address questions relating to the location of specific motor or sensorial functions. The main advantage of techniques such as microelectrode recordings and microstimulations over brain imaging is their ability to localize patient physiological activity with a high degree of spatial resolution. Aggregating data acquired from large populations permits to build what are commonly referred to as statistical atlases. Data points from statistical atlases can be combined to produce probabilistic maps. A crucial step in this process is the intersubject spatial normalization that is required to relate a position in one subject's brain to a position in another subject's brain. In this paper, we study the impact of spatial normalization techniques on building statistical atlases. We find that the Talairach or anterior-posterior commissure coordinate system commonly used in the medical literature produces atlases that are more dispersed than those obtained with normalization methods that rely on nonlinear volumetric image registration. We also find that the maps produced using nonlinear techniques correlate with their expected anatomic positions.

Design of a bone-attached parallel robot for percutaneous cochlear implantation

Access to the cochlea requires drilling in close proximity to bone-embedded nerves, blood vessels, and other structures, the violation of which can result in complications for the patient. It has recently been shown that microstereotactic frames can enable an image-guided percutaneous approach, removing reliance on human experience and hand-eye coordination, and reducing trauma. However, constructing current microstereotactic frames disrupts the clinical workflow, requiring multiday intrasurgical manufacturing delays, or an on-call machine shop in or near the hospital. In this paper, we describe a new kind of microsterotactic frame that obviates these delay and infrastructure issues by being repositionable. Inspired by the prior success of bone-attached parallel robots in knee and spinal procedures, we present an automated image-guided microstereotactic frame. Experiments demonstrate a mean accuracy at the cochlea of 0.20 ± 0.07 mm in phantom testing with trajectories taken from a human clinical dataset. We also describe a cadaver experiment evaluating the entire image-guided surgery pipeline, where we achieved an accuracy of 0.38 mm at the cochlea.

Customized, miniature rapid-prototype stereotactic frames for use in deep brain stimulator surgery: initial clinical methodology and experience from 263 patients from 2002 to 2008

BACKGROUND

The microTargeting™ platform (MTP) stereotaxy system (FHC Inc., Bowdoin, Me., USA) was FDA approved in 2001 utilizing rapid-prototyping technology to create custom platforms for human stereotaxy procedures. It has also been called the STarFix (surgical targeting fixture) system since it is based on the concept of a patient- and procedure-specific surgical fixture. This is an alternative stereotactic method by which planned trajectories are incorporated into custom-built, miniature stereotactic platforms mounted onto bone fiducial markers. Our goal is to report the clinical experience with this system over a 6-year period.

METHODS

We present the largest reported series of patients who underwent deep brain stimulation (DBS) implantations using customized rapidly prototyped stereotactic frames (MTP). Clinical experience and technical features for the use of this stereotactic system are described. Final lead location analysis using postoperative CT was performed to measure the clinical accuracy of the stereotactic system.

RESULTS

Our series included 263 patients who underwent 284 DBS implantation surgeries at one institution over a 6-year period. The clinical targeting error without accounting for brain shift in this series was found to be 1.99 mm (SD 0.9). Operating room time was reduced through earlier incision time by 2 h per case.

CONCLUSION

Customized, miniature stereotactic frames, namely STarFix platforms, are an acceptable and efficient alternative method for DBS implantation. Its clinical accuracy and outcome are comparable to those associated with traditional stereotactic frame systems.

The role of registration in accurate surgical guidance

Registration is presented as the central issue of surgical guidance. The focus is on the accuracy of approaches employed today, all of which use pre-operative images to guide surgery on rigid anatomy. The three most well-established approaches to guidance, namely the stereotactic frame, point fiducials, and surface matching, are examined in detail, together with two new approaches based on microstereotactic frames. It is shown that each method relies on the registration of points in the image to corresponding points in the operating room, and therefore that the error patterns associated with point registration are similar for all of them. Three types of registration error, namely fiducial localization error (FLE), fiducial registration error (FRE), and target registration error (TRE), are highlighted, as well as two additional guidance errors, namely target localization error and total targeting error, the latter of which is the overall error of the guidance system. Statistical relationships between TRE and FLE, between FRE and FLE, and between TRE, TLE, and TTE are given. Finally some myths concerning fiducial registration are highlighted.

Design of a Novel Device to Provide Assured Seating of Bone Implanted Fiducial Markers

Microstereotactic frames are a subset of stereotactic frames. They are smaller than traditional frames, and they employ small discrete anchors that are screwed into a patient’s bony anatomy, typically the skull. These anchors provide points of attachment for the frame and also serve as markers that provide a bridge between image space and physical space. Both these markers and the anatomy of interest are imaged in a CT or MRI scanner, and their positions are established relative to one another. The microstereotactic frame is then custom made so that, when it is attached to the anchors, it will guide a surgical instrument to an anatomical target. These frames offer advantages over stereotactic frames, including increased targeting accuracy and patient comfort. Firm placement of these anchors is critical as any movement will cause errors in the imaging localization, frame design, or frame attachment. Anchor placement is complicated by the variation in bone density and by the obscuring tissue and bleeding that make visual confirmation of seating very difficult. A novel device called the PosiSeat™ is presented that provides assured seating for bone-implanted anchors.

Effect of MR distortion on targeting for deep-brain stimulation

Deep-brain stimulation (DBS) surgery involves placing electrodes within specific deep-brain target nuclei. Surgeons employ MR imaging for preoperative selection of targets and computed tomography (CT) imaging for designing stereotactic frames used for intraoperative placement of electrodes at the targets. MR distortion may contribute to target-selection error in the MR scan and also to MR-CT registration error, each of which contributes to error in electrode placement. In this paper, we analyze the error contributed by the MR distortion to the total DBS targeting error. Distortion in conventional MR scans, both T1 and T2 weighted, were analyzed for six bilateral DBS patients in the typical areas of brain using typical scans on a 3-T clinical scanner. Mean targeting error due to MR distortion in T2 was found to be 0.07 +/- 0.025 mm with a maximum of 0.13 mm over 12 targets; error in the T1 images was smaller by 4%.

Percutaneous cochlear implant drilling via customized frames: an in vitro study

OBJECTIVE

Percutaneous cochlear implantation (PCI) surgery uses patient-specific customized microstereotactic frames to achieve a single drill-pass from the lateral skull to the cochlea, avoiding vital anatomy. We demonstrate the use of a specific microstereotactic frame, called a "microtable," to perform PCI surgery on cadaveric temporal bone specimens.

STUDY DESIGN

Feasibility study using cadaveric temporal bones.

SUBJECTS AND METHODS

PCI drilling was performed on six cadaveric temporal bone specimens. The main steps involved were 1) placing three bone-implanted markers surrounding the ear, 2) obtaining a CT scan, 3) planning a safe surgical path to the cochlea avoiding vital anatomy, 4) constructing a microstereotactic frame to constrain the drill to the planned path, and 5) affixing the frame to the markers and using it to drill to the cochlea. The specimens were CT scanned after drilling to show the achieved path. Deviation of the drilled path from the desired path was computed, and the closest distance of the mid-axis of the drilled path from critical structures was measured.

RESULTS

In all six specimens, we drilled successfully to the cochlea, preserving the facial nerve and ossicles. In four of six specimens, the chorda tympani was preserved, and in two of six specimens, it was sacrificed. The mean +/- standard deviation error at the target was found to be 0.31 +/- 0.10 mm. The closest distances of the mid-axis of the drilled path to structures were 1.28 +/- 0.17 mm to the facial nerve, 1.31 +/- 0.36 mm to the chorda tympani, and 1.59 +/- 0.43 mm to the ossicles.

CONCLUSION

In a cadaveric model, PCI drilling is safe and effective.

Clinical validation study of percutaneous cochlear access using patient-customized microstereotactic frames

OBJECTIVE

Percutaneous cochlear implant (PCI) surgery consists of drilling a single trough from the lateral cranium to the cochlea avoiding vital anatomy. To accomplish PCI, we use a patient-customized microstereotactic frame, which we call a "microtable" because it consists of a small tabletop sitting on legs. The orientation of the legs controls the alignment of the tabletop such that it is perpendicular to a specified trajectory.

STUDY DESIGN

Prospective.

SETTING

Tertiary referral center.

PATIENTS

Thirteen patients (18 ears) undergoing traditional cochlear implant surgery.

INTERVENTIONS

With institutional review board approval, each patient had 3 fiducial markers implanted in bone surrounding the ear. Temporal bone computed tomographic scans were obtained, and the markers were localized, as was vital anatomy. A linear trajectory from the lateral cranium through the facial recess to the cochlea was planned. A microtable was fabricated to follow the specified trajectory.

MAIN OUTCOME MEASURES

After mastoidectomy and posterior tympanotomy, accuracy of trajectories was validated by mounting the microtables on the bone-implanted markers and then passing sham drill bits across the facial recess to the cochlea. The distance from the drill to vital anatomy was measured.

RESULTS

Microtables were constructed on a computer-numeric-control milling machine in less than 5 minutes each. Successful access across the facial recess to the cochlea was achieved in all 18 cases. The mean +/- SD distance was 1.20 +/- 0.36 mm from midportion of the drill to the facial nerve and 1.25 +/- 0.33 mm from the chorda tympani.

CONCLUSION

These results demonstrate the feasibility of PCI access using customized microstereotactic frames.

Clinical accuracy of a customized stereotactic platform for deep brain stimulation after accounting for brain shift

Previous studies have evaluated the accuracy of several approaches for the placement of electrodes for deep brain stimulation. In this paper, we present a strategy to minimize the effect of brain shift on the estimation of the electrode placement error (EPE) for a stereotactic platform in the absence of intraoperative imaging data, and we apply it to the StarFix microTargeting Platform (FHC Inc., Bowdoin, Me., USA). This method involves comparing the intraoperative stereotactic coordinates of the implant with its position in the postoperative CT images in a population for which the effect of brain shift is minimal. The study we have conducted on 75 patients demonstrates that the EPE is overestimated at least by about 60% if brain shift is not taken into account, and shows a clinical accuracy of 1.24 +/- 0.37 mm for the StarFix frame, which is similar to the reported G frame accuracy and better than the reported Nexframe accuracy (2.5 +/- 1.4 mm) [Stereotact Funct Neurosurg 2007;85:235-242].

Effect of brain shift on the creation of functional atlases for deep brain stimulation surgery

PURPOSE

In the recent past many groups have tried to build functional atlases of the deep brain using intra-operatively acquired information such as stimulation responses or micro-electrode recordings. An underlying assumption in building such atlases is that anatomical structures do not move between pre-operative imaging and intra-operative recording. In this study, we present evidences that this assumption is not valid. We quantify the effect of brain shift between pre-operative imaging and intra-operative recording on the creation of functional atlases using intra-operative somatotopy recordings and stimulation response data.

METHODS

A total of 73 somatotopy points from 24 bilateral subthalamic nucleus (STN) implantations and 52 eye deviation stimulation response points from 17 bilateral STN implantations were used. These points were spatially normalized on a magnetic resonance imaging (MRI) atlas using a fully automatic non-rigid registration algorithm. Each implantation was categorized as having low, medium or large brain shift based on the amount of pneumocephalus visible on post-operative CT. The locations of somatotopy clusters and stimulation maps were analyzed for each category.

RESULTS

The centroid of the large brain shift cluster of the somatotopy data (posterior, lateral, inferior: 3.06, 11.27, 5.36 mm) was found posterior, medial and inferior to that of the medium cluster (2.90, 13.57, 4.53 mm) which was posterior, medial and inferior to that of the low shift cluster (1.94, 13.92, 3.20 mm). The coordinates are referenced with respect to the mid-commissural point. Euclidean distances between the centroids were 1.68, 2.44 and 3.59 mm, respectively for low-medium, medium-large and low-large shift clusters. We found similar trends for the positions of the stimulation maps. The Euclidian distance between the highest probability locations on the low and medium-large shift maps was 4.06 mm.

CONCLUSION

The effect of brain shift in deep brain stimulation (DBS) surgery has been demonstrated using intra-operative somatotopy recordings as well as stimulation response data. The results not only indicate that considerable brain shift happens before micro-electrode recordings in DBS but also that brain shift affects the creation of accurate functional atlases. Therefore, care must be taken when building and using such atlases of intra-operative data and also when using intra-operative data to validate anatomical atlases.

Customized, rapid-production microstereotactic table for surgical targeting: description of concept and in vitro validation

PURPOSE

To introduce a novel microstereotactic frame, called the Microtable, consisting of a tabletop that mounts on bone-implanted spherical markers. The microtable is customized for individual patient anatomy to guide a surgical instrument to a specified target.

METHODS

Fiducial markers are bone-implanted, and CT scanning is performed. A microtable is custom-designed for the location of the markers and the desired surgical trajectory and is constructed using a computer-numerical-control machine. Validation studies were performed on phantoms with geometry similar to that for cochlear implant surgery. Two designs were tested with two different types of fiducial markers.

RESULTS

Mean targeting error of the microtables for the two designs were 0.37 +/- 0.18 and 0.60 +/- 0.21 mm (n = 5). Construction of each microtable required approximately 6 min.

CONCLUSIONS

The new frame achieves both high accuracy and rapid fabrication. We are currently using the microtable for clinical testing of the concept of percutaneous cochlear implant surgery.