Article

Accuracy of robotic coil positioning during transcranial magnetic stimulation

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Objective: Robotic positioning systems for transcranial magnetic stimulation (TMS) promise improved accuracy and stability of coil placement, but there is limited data on their performance. Investigate the usability, accuracy, and limitations of robotic coil placement with a commercial system, ANT Neuro, in a TMS study. Approach: 21 subjects underwent a total of 79 TMS sessions corresponding to 160 hours under robotic coil control. Coil position and orientation were monitored concurrently through an additional neuronavigation system. Main Results: Robot setup took on average 14.5 min. The robot achieved low position and orientation error with median 1.34 mm and 3.48°. The error increased over time at a rate of 0.4%/minute for both position and orientation. Significance: Robotic TMS systems can provide accurate and stable coil position and orientation in long TMS sessions. Lack of pressure feedback and of manual adjustment of all coil degrees of freedom were limitations of this robotic system.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... A further limitation of TMS procedures is that they have a limited, and often unquantified, precision and accuracy of determining and maintaining the coil placement. Even with gold-standard neuronavigation and robotic coil placement, coil position error can exceed 5 mm [24][25][26]. As such, there is an uncertainty in the TMS induced E-field resulting from uncertainty in the coil placement. ...
... The geodesic distance on the scalp is computed using the heat method described in [53]. The values of Δ and Δ are specific to the TMS methods used and should be based on available data [24][25][26]. Other distributions (e.g., Gaussians) could also be used to discretize the coil position and orientation uncertainty whenever appropriate. ...
Preprint
Background During transcranial magnetic stimulation (TMS) a coil placed on the scalp is used to non-invasively modulate activity of targeted brain networks via a magnetically induced electric field (E-field). Ideally, the E-field induced during TMS is concentrated on a targeted cortical region of interest (ROI). Objective To improve the accuracy of TMS we have developed a fast computational auxiliary dipole method (ADM) for determining the optimum coil position and orientation. The optimum coil placement maximizes the E-field along a predetermined direction or, alternatively, the overall E-field magnitude in the targeted ROI. Furthermore, ADM can assess E-field uncertainty resulting from precision limitations of TMS coil placement protocols. Method ADM leverages the electromagnetic reciprocity principle to compute rapidly the TMS induced E-field in the ROI by using the E-field generated by a virtual constant current source residing in the ROI. The framework starts by solving for the conduction currents resulting from this ROI current source. Then, it rapidly determines the average E-field induced in the ROI for each coil position by using the conduction currents and a fast-multipole method. To further speed-up the computations, the coil is approximated using auxiliary dipoles enabling it to represent all coil orientations for a given coil position with less than 600 dipoles. Results Using ADM, the E-fields generated in an MRI-derived head model when the coil is placed at 5,900 different scalp positions and 360 coil orientations per position (over 2.1 million unique configurations) can be determined in under 15 minutes on a standard laptop computer. This enables rapid extraction of the optimum coil position and orientation as well as the E-field variation resulting from coil positioning uncertainty. Conclusion ADM enables the rapid determination of coil placement that maximizes E-field delivery to a specific brain target. This method can find the optimum coil placement in under 15 minutes enabling its routine use for TMS. Furthermore, it enables the fast quantification of uncertainty in the induced E-field due to limited precision of TMS coil placement protocols, enabling minimization and statistical analysis of the E-field dose variability. Highlights Auxiliary dipole method (ADM) optimizes TMS coil placement in under 8 minutes Optimum coil position is up to 14 mm away from conventional targeting Optimum coil orientation is typically near normal to the sulcal wall TMS induced E-field is less sensitive to orientation than position errors
... A further limitation of TMS procedures is that they have a limited, and often unquantified, precision and accuracy of determining and maintaining the coil placement. Even with goldstandard neuronavigation and robotic coil placement, coil position error can exceed 5 mm (Goetz et al., 2019;Ruohonen and Karhu, 2010;Sparing et al., 2008). As such, there is an uncertainty in the TMS induced E-field resulting from uncertainty in the coil placement. ...
... The geodesic distance on the scalp is computed using the heat method described in (Crane et al., 2013). The values of and are specific to the TMS methods used and should be based on available data (Goetz et al., 2019;Ruohonen and Karhu, 2010;Sparing et al., 2008). Other distributions (e.g., Gaussians) could also be used to discretize the coil position and orientation uncertainty whenever appropriate. ...
Article
Full-text available
Background During transcranial magnetic stimulation (TMS) a coil placed on the scalp is used to non-invasively modulate activity of targeted brain networks via a magnetically induced electric field (E-field). Ideally, the E-field induced during TMS is concentrated on a targeted cortical region of interest (ROI). Determination of the coil position and orientation that best achieve this objective presently requires a large computational effort. Objective To improve the accuracy of TMS we have developed a fast computational auxiliary dipole method (ADM) for determining the optimum coil position and orientation. The optimum coil placement maximizes the E-field along a predetermined direction or, alternatively, the overall E-field magnitude in the targeted ROI. Furthermore, ADM can assess E-field uncertainty resulting from precision limitations of TMS coil placement protocols. Method ADM leverages the electromagnetic reciprocity principle to compute rapidly the TMS induced E-field in the ROI by using the E-field generated by a virtual constant current source residing in the ROI. The framework starts by solving for the conduction currents resulting from this ROI current source. Then, it rapidly determines the average E-field induced in the ROI for each coil position by using the conduction currents and a fast-multipole method. To further speed-up the computations, the coil is approximated using auxiliary dipoles enabling it to represent all coil orientations for a given coil position with less than 600 dipoles. Results Using ADM, the E-fields generated in an MRI-derived head model when the coil is placed at 5,900 different scalp positions and 360 coil orientations per position (over 2.1 million unique configurations) can be determined in under 15 minutes on a standard laptop computer. This enables rapid extraction of the optimum coil position and orientation as well as the E-field variation resulting from coil positioning uncertainty. ADM is implemented in SimNIBS 3.2. Conclusion ADM enables the rapid determination of coil placement that maximizes E-field delivery to a specific brain target. This method can find the optimum coil placement in under 15 minutes enabling its routine use for TMS. Furthermore, it enables the fast quantification of uncertainty in the induced E-field due to limited precision of TMS coil placement protocols, enabling minimization and statistical analysis of the E-field dose variability.
... Several groups propose the use of robotic systems combined with IR cameras to provide real-time localization of both the head and the coil in the space. The implementation of a robotic arm to hold and move the TMS coil coupled with IR passive markers and a commercial stereo camera is advised in different approaches [51,53,126,127]. This type of robotic arm provides a wide range of movement around the head and is suitable for different types of coils. ...
... The topic of robotic systems for positioning is still open for improvement (RQ4). Current robotic guiding systems [51] combine off-line head segmentation with real-time positioning using IR cameras [48], active markers [45], and image tags (QR codes) [53]. However, coil positioning is still limited by time-consuming calibration protocols to be carried out before each therapy [45], which prolongs the time required for each TMS session. ...
Article
Full-text available
The technology for transcranial magnetic stimulation (TMS) has significantly changed over the years, with important improvements in the signal generators, the coils, the positioning systems, and the software for modeling, optimization, and therapy planning. In this systematic literature review (SLR), the evolution of each component of TMS technology is presented and analyzed to assess the limitations to overcome. This SLR was carried out following the PRISMA 2020 statement. Published articles of TMS were searched for in four databases (Web of Science, PubMed, Scopus, IEEE). Conference papers and other reviews were excluded. Records were filtered using terms about TMS technology with a semi-automatic software; articles that did not present new technology developments were excluded manually. After this screening, 101 records were included, with 19 articles proposing new stimulator designs (18.8%), 46 presenting or adapting coils (45.5%), 18 proposing systems for coil placement (17.8%), and 43 implementing algorithms for coil optimization (42.6%). The articles were blindly classified by the authors to reduce the risk of bias. However, our results could have been influenced by our research interests, which would affect conclusions for applications in psychiatric and neurological diseases. Our analysis indicates that more emphasis should be placed on optimizing the current technology with a special focus on the experimental validation of models. With this review, we expect to establish the base for future TMS technological developments.
... TMS-related outcome measures are variable, and thus their reliability has attracted considerable interest, that is, TMS can provide accurate and consistent measurements for individuals without physiological changes. Although some new techniques such as neuronavigation, robot TMS coil holding arm, and EEG-TMS closed-loop triggering strategy which could improve the accuracy of hotspot localization and reduce the variability of TMS-induced MEP (Goetz et al., 2019), these advanced techniques are more widely applied in some high-level lab for research studies. For clinical evaluation, the conventional methods of TMS using hand-hold assessment are still used by most medical institutions. ...
Article
Full-text available
Objective: The objective of this study was to determine the reliability of corticomotor excitability measurements using the conventional hand-hold transcranial magnetic stimulation (TMS) method for the tibialis anterior (TA) muscle in healthy adults and the number of stimuli required for reliable assessment. Methods: Forty healthy adults participated in three repeated sessions of corticomotor excitability assessment in terms of resting motor threshold (rMT), slope of recruitment curve (RC), peak motor evoked potential amplitude (pMEP), and MEP latency using conventional TMS method. The first two sessions were conducted with a rest interval of 1 h, and the last session was conducted 7–10 days afterward. With the exception of rMT, the other three outcomes measure elicited with the block of first 3–10 stimuli were analyzed respectively. The within-day (session 1 vs. 2) and between-day (session 1 vs. 3) reliability for all four outcome measures were assessed using intraclass correlation coefficient (ICC), standard error of measurement, and minimum detectable difference at 95% confidence interval. Results: Good to excellent within-day and between-day reliability was found for TMS-induced outcome measures examined using 10 stimuli (ICC ≥ 0.823), except in pMEP, which showed between-day reliability at moderate level (ICC = 0.730). The number of three stimuli was adequate to achieve minimum acceptable within-day reliability for all TMS-induced parameters and between-day reliability for MEP latency. With regard to between-day reliability of RC slope and pMEP, at least seven and nine stimuli were recommended respectively. Conclusion: Our findings indicated the high reliability of corticomotor excitability measurement by TMS with adequate number of stimuli for the TA muscle in healthy adults. This result should be interpreted with caveats for the specific methodological choices, equipment setting, and the characteristics of the sample in the current study. Clinical Trial Registration: http://www.chictr.org.cn , identifier ChiCTR2100045141.
... To adjust the stimulated cortical location, a TMS coil is typically moved manually. Robotic TMS systems offer an alternative approach [4][5][6]; however, the mechanical coil movement is relatively slow due to inertia and safety limitations. Thus, with a single-coil TMS system, it is practically impossible to adjust the stimulated spot fast, in the neuronally meaningful, millisecond timescale. ...
Preprint
Full-text available
Background Transcranial magnetic stimulation (TMS) allows non-invasive stimulation of the cortex. In multi-locus TMS (mTMS), the stimulating electric field (E-field) is controlled electronically without coil movement by adjusting currents in the coils of a transducer. Objective To develop an mTMS system that allows adjusting the location and orientation of the E-field maximum within a cortical region. Methods We designed and manufactured a planar 5-coil mTMS transducer to allow controlling the maximum of the induced E-field within a cortical region approximately 30 mm in diameter. We developed electronics with a design consisting of independently controlled H-bridge circuits to drive up to six TMS coils. To control the hardware, we programmed software that runs on a field-programmable gate array and a computer. To induce the desired E-field in the cortex, we developed an optimization method to calculate the currents needed in the coils. We characterized the mTMS system and conducted a proof-of-concept motor-mapping experiment on a healthy volunteer. In the motor mapping, we kept the transducer placement fixed while electronically shifting the E-field maximum on the precentral gyrus and measuring electromyography from the contralateral hand. Results The transducer consists of an oval coil, two figure-of-eight coils, and two four-leaf-clover coils stacked on top of each other. The technical characterization indicated that the mTMS system performs as designed. The measured motor evoked potential amplitudes varied consistently as a function of the location of the E-field maximum. Conclusion The developed mTMS system enables electronically targeted brain stimulation within a cortical region.
... The unique feature of such a large-scale TMS array system is that the location of the 'hot spot' can be moved rapidly under electronic control and therefore different targets can be activated with millisecond-level temporal accuracy. While robotic TMS coil positioning ( Lancaster et al., 2004 ) provides arguably the most accurate targeting ( Goetz et al., 2019 ), moving the coil from one target to another will be inherently sluggish due to the obvious safety concerns associated with automated rapid physical coil movement. While the TMS array concept is simple, construction of the first set of large-scale systems has necessitated tackling several technological challenges. ...
Article
Full-text available
Purpose: Multichannel Transcranial Magnetic Stimulation (mTMS) arrays enable multiple sites to be stimulated simultaneously or sequentially under electronic control without moving the system's stimulation coils. Here, we build and characterize the performance of a novel modular 3-axis TMS coil that can be utilized as a unit element in large-scale multichannel TMS arrays. Methods: We determined the basic physical characteristics of the 3-axis TMS coil x-, y- and z-elements using a custom 2-channel programmable stimulator prototype. We mapped the temporal rate-of-change of the induced magnetic field (dB/dt) on a 2D plane parallel to the coil surface (including an extended line for full spatial coverage) and compared those values with predictions from magnetic field simulations. Temperature measurements were carried out to assess the incorporated air-cooling method. We measured the mutual and self-inductances of the x/y/z-elements to assess coupling between them. Additionally, we measured and calculated the coupling between z-elements in the array configuration. Finally, we performed electric field simulations to evaluate the stimulation intensity and focality of the coil and compared the results to conventional TMS coils as well as demonstrated suitability of the 3-axis coil for a multichannel array configuration. Results: The experimentally obtained dB/dt values validated the computational model of the 3-axis coil and therefore confirmed that both the coil and stimulator system are operating as intended. The air-cooling system was effective for brief high-frequency pulse trains and extended single- and paired-pulse TMS protocols. The electromagnetic simulations suggested that an array of the 3-axis coils would have comparable stimulation intensity to conventional TMS coils, therefore enabling clearly suprathreshold stimulation of the human brain. The recorded coil coupling between the x/y/z-elements was < 1% and the maximal coupling between z-elements in the array configuration was 1.8% and 3.4% for the measured and calculated values, respectively. Conclusion: We presented a 3-axis coil intended for multichannel TMS arrays. The electromagnetic measurements and simulations verified that the coil fabrication met the desired specifications and that the inductive coupling between the elements was negligible. The air-cooled 3-axis TMS coil appears suitable to be used as an element in multichannel TMS arrays.
... Using an optical detection camera system (Polaris, NDI Medical Solutions), participants' anatomical images were co-registered with sensors. A figure-ofeight 70 mm Air-Film coil (Magstim) accurately maintained its alignment and position in near real-time (1 cm/second) (Ginhoux et al., 2013;Goetz et al., 2019;Grab et al., 2018). Using the neuronavigation software, a 12 × 12 rectangular grid with 7 mm spacing was superimposed on the reconstructed curvilinear brain and centered over the anatomical hand-knob ( Figure 1a) (Yousry et al., 1997). ...
Article
Full-text available
Introduction: Transcranial magnetic stimulation (TMS) motor mapping can characterize the neurophysiology of the motor system. Limitations including human error and the challenges of pediatric populations may be overcome by emerging robotic systems. We aimed to show that neuronavigated robotic motor mapping in adolescents could efficiently produce discrete maps of individual upper extremity muscles, the characteristics of which would correlate with motor behavior. Methods: Typically developing adolescents (TDA) underwent neuronavigated robotic TMS mapping of bilateral motor cortex. Representative maps of first dorsal interosseous (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles in each hand were created. Map features including area (primary), volume, and center of gravity were analyzed across different excitability regions (R100%, R75%, R50%, R25%). Correlations between map metrics and validated tests of hand motor function (Purdue Pegboard Test as primary) were explored. Results: Twenty-four right-handed participants (range 12-18 years, median 15.5 years, 52% female) completed bilateral mapping and motor assessments with no serious adverse events or dropouts. Gender and age were associated with hand function and motor map characteristics. Full motor maps (R100%) for FDI did not correlate with motor function in either hand. Smaller excitability subset regions demonstrated reduced variance and dose-dependent correlations between primary map variables and motor function in the dominant hemisphere. Conclusions: Hand function in TDA correlates with smaller subset excitability regions of robotic TMS motor map outcomes. Refined motor maps may have less variance and greater potential to quantify interventional neuroplasticity. Robotic TMS mapping is safe and feasible in adolescents.
... Using the Ziegler-Nichols tuning table (illustrated in Table 3 The first test addressed to the robot positioning assessment. The studies found that the changes in head position are approximately in the ranges of 0-40 mm [15]. Then, the ranges of the dummy head movements in this test were defined as the range of 0 to the maximum of 40 mm with 10 mm resolution in both x-y axes. ...
Article
Full-text available
This paper presents the development of a robotized Transcranial Magnetic Stimulation (TMS) system, as robotic assistance brings crucial benefits of the TMS to provide a more adequate, accuracy and reliable manner. A 6-DOF KUKA KR-16 robot, an ATI-multi-axis force/torque sensor and a Kinect 3D camera were used to develop the robotized TMS. All electrical signals were strategically processed by a host ROS-computer. Real-time hybrid position/force control based on Proportional Integral and Derivative (PID) control and Fuzzy Logic Control (FLC) was successfully implemented to ensure effective human-robot collaboration. The results claimed that the performance of the control schemes based on the PID and FLC control approaches were evidently acceptable for the robotized TMS application. However, in-depth observation exposed that the FLC method resulted to be slightly superior to the PID control by actively compensating for the dynamics in the non-linear system.
... A 12 × 12 rectangular grid with 7 mm spacing was superimposed on the reconstructed curvilinear brain and centered over the anatomical hand-knob of left and right M1 (Yousry et al., 1997) to generate targets for motor mapping. Each grid-point trajectory was aligned tangentially to the cortical surface and maintained at 45 • in relation to the interhemispheric fissure using a figure-of-eight 70 mm Air-Film coil (Magstim, Dyfed, United Kingdom), accurately maintaining position and motion correction in near real-time (1 cm/s) (Ginhoux et al., 2013;Goetz et al., 2019). ...
Article
Full-text available
Introduction: Conventional transcranial direct current stimulation (tDCS) and high-definition tDCS (HD-tDCS) may improve motor learning in children. Mechanisms are not understood. Neuronavigated robotic transcranial magnetic stimulation (TMS) can produce individualised maps of primary motor cortex (M1) topography. We aimed to determine the effects of tDCS- and HD-tDCS-enhanced motor learning on motor maps. Methods: Typically developing children aged 12–18 years were randomised to right M1 anodal tDCS, HD-tDCS, or Sham during training of their left-hand on the Purdue Pegboard Task (PPT) over 5 days. Bilateral motor mapping was performed at baseline (pre), day 5 (post), and 6-weeks retention time (RT). Primary muscle was the first dorsal interosseous (FDI) with secondary muscles of abductor pollicis brevis (APB) and adductor digiti minimi (ADM). Primary mapping outcomes were volume (mm ² /mV) and area (mm ² ). Secondary outcomes were centre of gravity (COG, mm) and hotspot magnitude (mV). Linear mixed-effects modelling was employed to investigate effects of time and stimulation type (tDCS, HD-tDCS, Sham) on motor map characteristics. Results: Twenty-four right-handed participants (median age 15.5 years, 52% female) completed the study with no serious adverse events or dropouts. Quality maps could not be obtained in two participants. No effect of time or group were observed on map area or volume. LFDI COG (mm) differed in the medial-lateral plane (x-axis) between tDCS and Sham (p = 0.038) from pre-to-post mapping sessions. Shifts in map COG were also observed for secondary left-hand muscles. Map metrics did not correlate with behavioural changes. Conclusion: Robotic TMS mapping can safely assess motor cortex neurophysiology in children undergoing motor learning and neuromodulation interventions. Large effects on map area and volume were not observed while changes in COG may occur. Larger controlled studies are required to understand the role of motor maps in interventional neuroplasticity in children.
... To adjust the stimulated cortical location, a TMS coil is typically moved manually. Robotic TMS systems offer an alternative approach [4][5][6]; however, the mechanical coil movement is relatively slow due to inertia and safety limitations. Thus, with a single-coil TMS system, it is practically impossible to adjust the stimulated spot fast, in the neuronally meaningful, millisecond timescale. ...
Article
Full-text available
Background Transcranial magnetic stimulation (TMS) allows non-invasive stimulation of the cortex. In multi-locus TMS (mTMS), the stimulating electric field (E-field) is controlled electronically without coil movement by adjusting currents in the coils of a transducer. Objective To develop an mTMS system that allows adjusting the location and orientation of the E-field maximum within a cortical region. Methods We designed and manufactured a planar 5-coil mTMS transducer to allow controlling the maximum of the induced E-field within a cortical region approximately 30 mm in diameter. We developed electronics with a design consisting of independently controlled H-bridge circuits to drive up to six TMS coils. To control the hardware, we programmed software that runs on a field-programmable gate array and a computer. To induce the desired E-field in the cortex, we developed an optimization method to calculate the currents needed in the coils. We characterized the mTMS system and conducted a proof-of-concept motor-mapping experiment on a healthy volunteer. In the motor mapping, we kept the transducer placement fixed while electronically shifting the E-field maximum on the precentral gyrus and measuring electromyography from the contralateral hand. Results The transducer consists of an oval coil, two figure-of-eight coils, and two four-leaf-clover coils stacked on top of each other. The technical characterization indicated that the mTMS system performs as designed. The measured motor evoked potential amplitudes varied consistently as a function of the location of the E-field maximum. Conclusion The developed mTMS system enables electronically targeted brain stimulation within a cortical region.
... The coil position with maximum MEP output amplitude in the FDI muscle was identified manually before the procedure. A robotic coil holder maintained the position of the coil throughout the session and compensated subject movements, while subjects were instructed to sit as still as possible [41,42]. MEPs were recorded and sampled synchronously to the TMS pulse trigger through surface Ag/AgCl electrodes and an MEP amplifier (K800 with SX230FW pre-amplifier, Biometrics Ltd., Gwent, UK) at 5 kHz and 16 bit. ...
Preprint
Background: Motor-evoked potentials (MEP) are one of the most prominent responses to brain stimulation, such as supra-threshold transcranial magnetic stimulation (TMS) and electrical stimulation. Understanding of the neurophysiology and the determination of the lowest stimulation strength that evokes responses requires the detection of even smaller responses, e.g., from single motor units. However, available detection and quantization methods suffer from a large noise floor. Objective: This paper develops a detection method that extracts MEPs hidden below the noise floor. With this method, we aim to estimate excitatory activations of the corticospinal pathways well below the conventional detection level. Methods: The presented MEP detection method presents a self-learning matched-filter approach for improved robustness against noise. The filter is adaptively generated per subject through iterative learning. For responses that are reliably detected by conventional detection, the new approach is fully compatible with established peak-to-peak readings and provides the same results but extends the dynamic range below the conventional noise floor. Results: In contrast to the conventional peak-to-peak measure, the proposed method increases the signal-to-noise ratio by more than a factor of 5. The first detectable responses appear to be substantially lower than the conventional threshold definition of 50 μV median peak-to-peak amplitude. Conclusion: The proposed method shows that stimuli well below the conventional 50 μV threshold definition can consistently and repeatably evoke muscular responses and thus activate excitable neuron populations in the brain. As a consequence, the IO curve is extended at the lower end, and the noise cut-off is shifted. Importantly, the IO curve extends so far that the 50 μV point turns out to be closer to the center of the logarithmic sigmoid curve rather than close to the first detectable responses. The underlying method is applicable to a wide range of evoked potentials and other biosignals, such as in electroencephalography.
... TMS neuronavigation systems are known to have a coil placement accuracy of 5-6 mm on average [41,42]. Excluding errors from the registration and shifts of the head tracker relative to the head, the median tangential deviation of the coil position and orientation of 1.34 mm and 3.48 • , respectively, was reported for a neuronavigated robotic coil holder [43]. Moreover, an inter-session position error of approximately 2.5 mm was reported for neuronavigated manual coil placement [44]. ...
Article
Objective: Transcranial magnetic stimulation (TMS) can modulate brain function via an electric field (E-field) induced in a brain region of interest (ROI). The ROI E-field can be computationally maximized and set to match a specific reference using individualized head models to find the optimal coil placement and stimulus intensity. However, the available software lacks many practical features for prospective planning of TMS interventions and retrospective evaluation of the experimental targeting accuracy. Approach: The TMS targeting and analysis pipeline (TAP) software uses an MRI/fMRI-derived brain target to optimize coil placement considering experimental parameters such as the subject's hair thickness and coil placement restrictions. The coil placement optimization is implemented in SimNIBS 3.2, for which an additional graphical user interface (TargetingNavigator) is provided to visualize/adjust procedural parameters. The coil optimization process also computes the E-field at the target, allowing the selection of the TMS device intensity setting to achieve specific E-field strengths. The optimized coil placement information is prepared for neuronavigation software, which supports targeting during the TMS procedure. The neuronavigation system can record the coil placement during the experiment, and these data can be processed in TAP to quantify the accuracy of the experimental TMS coil placement and induced E-field. Main results: TAP was demonstrated in a study consisting of three repetitive TMS sessions in five subjects. TMS was delivered by an experienced operator under neuronavigation with the computationally optimized coil placement. Analysis of the experimental accuracy from the recorded neuronavigation data indicated coil location and orientation deviations up to about 2 mm and 2°, respectively, resulting in an 8% median decrease in the target E-field magnitude compared to the optimal placement. Significance: TAP supports navigated TMS with a variety of features for rigorous and reproducible stimulation delivery, including planning and evaluation of coil placement and intensity selection for E-field-based dosing.
... Robotic TMS (Axilum Robotics, France) was utilized to perform all motor mapping procedures, the details of which are described elsewhere (Giuffre et al., 2019;Grab et al., 2018) and below. The Axilum TMS robot accommodates a 70-mm Air-Film coil (Magstim, UK), allows precise neuronavigation, near real-time motion correction, and compensates for human operation errors associated with manual mapping (Goetz et al., 2019). ...
Article
Full-text available
Brain stimulation combined with intensive therapy may improve hand function in children with perinatal stroke-induced unilateral cerebral palsy (UCP). However, response to therapy varies and underlying neuroplasticity mechanisms remain unclear. Here, we aimed to characterize robotic motor mapping outcomes in children with UCP. Twenty-nine children with perinatal stroke and UCP (median age 11 ± 2 years) were compared to 24 typically developing controls (TDC). Robotic, neuronavigated transcranial magnetic stimulation was employed to define bilateral motor maps including area, volume, and peak motor evoked potential (MEP). Map outcomes were compared to the primary clinical outcome of the Jebsen-Taylor Test of Hand Function (JTT). Maps were reliably obtained in the contralesional motor cortex (24/29) but challenging in the lesioned hemisphere (5/29). Within the contralesional M1 of participants with UCP, area and peak MEP amplitude of the unaffected map were larger than the affected map. When comparing bilateral maps within the contralesional M1 in children with UCP to that of TDC, only peak MEP amplitudes were different, being smaller for the affected hand as compared to TDC. We observed correlations between the unaffected map when stimulating the contralesional M1 and function of the unaffected hand. Robotic motor mapping can characterize motor cortex neurophysiology in children with perinatal stroke. Map area and peak MEP amplitude may represent discrete biomarkers of developmental plasticity in the contralesional M1. Correlations between map metrics and hand function suggest clinical relevance and utility in studies of interventional plasticity.
... 6b,c are remarkably close to each other and are both relatively close to one. This indicates an acceptable stability of the solution with regard to the position uncertainties observed for the navigated TMS (nTMS) [49], [50] and for robotic TMS where the corresponding values are approximately 3.5 mm (overall, 1.3 mm without coil-head spacing) and 3.5° [51] or 2 mm [52]. Fig. 6. a) -A cube with 3 mm sides created for stability evaluation; a similar cube with sides of 6 degrees is created in the angular search space. ...
Preprint
A particular yet computationally successful solution of an inverse transcranial magnetic stimulation (TMS) problem is reported. The goal has been focusing the normal unsigned electric field at the inner cortical surface and its vicinity (the D wave activation site) given a unique high-resolution gyral pattern of a subject and a precise coil model. For 16 subjects and 32 arbitrary target points, the solution decreases the mean deviation of the maximum-field domain from the target by a factor of 2 on average. The reduction in the maximum-field area is expected to quadruple. The average final deviation is 6 mm. Rotation about the coil axis is the most significantly altered parameter, and the coil moves 10 mm on average during optimization. The maximum electric field magnitude decreases by 16% on average. Stability of the solution is enforced. The relative average de-focalization is below 1.2 when position/orientation accuracies are within 3 mm/6 degrees, respectively. The solution for the maximum normal field may also maximize the total field and its gradient for neighboring cortical layers III-V (I wave activation).
Chapter
The pathophysiological mechanisms that underlie the generation and maintenance of tinnitus are being unraveled progressively. Based on this knowledge, a large variety of different neuromodulatory interventions have been developed and are still being designed, adapting to the progressive mechanistic insights in the pathophysiology of tinnitus. rTMS targeting the temporal, temporoparietal, and the frontal cortex has been the mainstay of non-invasive neuromodulation. Yet, the evidence is still unclear, and therefore systematic meta-analyses are needed for drawing conclusions on the effectiveness of rTMS in chronic tinnitus. Different forms of transcranial electrical stimulation (tDCS, tACS, tRNS), applied over the frontal and temporal cortex, have been investigated in tinnitus patients, also without robust evidence for universal efficacy. Cortex and deep brain stimulation with implanted electrodes have shown benefit, yet there is insufficient data to support their routine clinical use. Recently, bimodal stimulation approaches have revealed promising results and it appears that targeting different sensory modalities in temporally combined manners may be more promising than single target approaches.
Article
Objective Investigate the variability previously found with cortical stimulation and handheld transcranial magnetic stimulation (TMS) coils, criticized for its high potential of coil position fluctuations, bypassing the cortex using deep brain electrical stimulation (DBS) of the corticospinal tract with fixed electrodes where both latent variations of the coil position of TMS are eliminated and cortical excitation fluctuations should be absent. Methods Ten input–output curves were recorded from five anesthetized cats with implanted DBS electrodes targeting the corticospinal tract. Goodness of fit of regressions with a conventional single variability source as well as a dual variability source model was quantified using a Schwarz Bayesian Information approach to avoid overfitting. Results Motor evoked potentials (MEPs) through DBS of the corticospinal tract revealed short-term fluctuations in excitability of the targeted neuron pathway reflecting endogenous input-side variability at similar magnitude as TMS despite bypassing cortical networks. Conclusion Input-side variability, i.e., variability resulting in changing MEP amplitudes as if the stimulation strength was modulated, also emerges in electrical stimulation at a similar degree and is not primarily a result of varying stimulation, such as minor coil movements in TMS. More importantly, this variability component is present, although the cortex is bypassed. Thus, it may be of spinal origin, which can include cortical input from spinal projections. Further, the nonlinearity of the compound variability entails complex heteroscedastic non-Gaussian distributions and typically does not allow simple linear averages in statistical analysis of MEPs. As the average is dominated by outliers, it risks bias. With appropriate regression, the net effects of excitatory and inhibitory inputs to the targeted neuron pathways become noninvasively observable and quantifiable. Significance The neural responses evoked by artificial stimulation in the cerebral cortex are variable. For example, MEPs in response to repeated presentations of the same stimulus can vary from no response to saturation across trials. Several sources of such variability have been suggested, and most of them may be technical in nature, but localization is missing.
Article
Transcranial magnetic stimulation is a powerful and non-invasive technique to stimulate nerve cells in the brain to improve symptoms of depression. It needs accurate contact force and positioning of a stimulating coil to offer effective treatment. Conventionally, a neurologist has to manually place the coil on the patient’s head at a predefined position and orientation throughout the 30-to-45-minute treatment. This paper consequently highlights the design of a robot control framework used for the transcranial magnetic stimulation. The hybrid Proportional-Derivative position and Proportional–Integral force control scheme was successfully implemented on the effective robotic system. The system can track the head movement by using the Kinect camera and simultaneously maintain the contact force between the magnetic coil and the head in real-time. The optimal gain tuning technique was adopted. It delivered PD gains of K p = 75.0, K d =2.0 and PI gains of K p = 0.09 and K i = 0.011, respectively. This provides a faster response of the robot end-effector with a small interactive force error. The experimental results show that when the head has translational and rotational motions, the robot-assisted transcranial magnetic stimulation control system performs with a safe and reliable manner validated by the quantitative measurement.
Article
Objective: To present and disseminate our transcranial magnetic stimulation (TMS) modeling software toolkit, including several new algorithmic developments, and to apply this software to realistic TMS modeling scenarios given a high-resolution model of the human head including cortical geometry and an accurate coil model. Approach: The recently developed charge-based boundary element fast multipole method (BEM-FMM) is employed as an alternative to the 1st order finite element method (FEM) most commonly used today. The BEM-FMM approach provides high accuracy and unconstrained numerical field resolution close to and across cortical interfaces. Here, the previously proposed BEM-FMM algorithm has been improved in several novel ways. Main results: The improvements resulted in a threefold increase in computational speed while maintaining the same solution accuracy. The computational code based on the MATLAB® platform is made available to all interested researchers, along with a coil model repository and examples to create custom coils, head model repository, and supporting documentation. The presented software toolkit may be useful for post-hoc analyses of navigated TMS data using high-resolution subject-specific head models as well as accurate and fast modeling for the purposes of TMS coil/hardware development. Significance: TMS is currently the only non-invasive neurostimulation modality that enables painless and safe supra-threshold stimulation by employing electromagnetic induction to efficiently penetrate the skull. Accurate, fast, and high resolution modeling of the electric fields may significantly improve individualized targeting and dosing of TMS and therefore enhance the efficiency of existing clinical protocols as well as help establish new application domains.
Preprint
Background Transcranial magnetic stimulation (TMS) is currently the only non-invasive neurostimulation modality that enables painless and safe supra-threshold stimulation by employing electromagnetic induction to efficiently penetrate the skull. Accurate, fast, and high resolution modeling of the electric fields (E-fields) may significantly improve individualized targeting and dosing of TMS and therefore enhance the efficiency of existing clinical protocols as well as help establish new application domains. Objective To present and disseminate our TMS modeling software toolkit, including several new algorithmic developments, and to apply this software to realistic TMS modeling scenarios given a high-resolution model of the human head including cortical geometry and an accurate coil model. Method The recently developed charge-based boundary element fast multipole method (BEM-FMM) is employed as an alternative to the 1st order finite element method (FEM) most commonly used today. The BEM-FMM approach provides high accuracy and unconstrained field resolution close to and across cortical interfaces. Here, the previously proposed BEM-FMM algorithm has been improved in several novel ways. Results and Conclusions The improvements resulted in a threefold increase in computational speed while maintaining the same solution accuracy. The computational code based on the MATLAB ® platform is made available to all interested researchers, along with a coil model repository and examples to create custom coils, head model repository, and supporting documentation. The presented software toolkit may be useful for post-hoc analyses of navigated TMS data using high-resolution subject-specific head models as well as accurate and fast modeling for the purposes of TMS coil/hardware development.
Article
Full-text available
Correctly determining individual's resting motor threshold (rMT) is crucial for accurate and reliable mapping by navigated transcranial magnetic stimulation (nTMS), which is especially true for preoperative motor mapping in brain tumor patients. However, systematic data analysis on clinical factors underlying inter-individual rMT variability in neurosurgical motor mapping is sparse. The present study examined 14 preselected clinical factors that may underlie inter-individual rMT variability by performing multiple regression analysis (backward, followed by forward model comparisons) on the nTMS motor mapping data of 100 brain tumor patients. Data were collected from preoperative motor mapping of abductor pollicis brevis (APB), abductor digiti minimi (ADM), and flexor carpi radialis (FCR) muscle representations among these patients. While edema and age at exam in the ADM model only jointly reduced the unexplained variance significantly, the other factors kept in the ADM model (gender, antiepileptic drug intake, and motor deficit) and each of the factors kept in the APB and FCR models independently significantly reduced the unexplained variance. Hence, several clinical parameters contribute to inter-individual rMT variability and should be taken into account during initial and follow-up motor mappings. Thus, the present study adds basic evidence on inter-individual rMT variability, whereby some of the parameters are specific to brain tumor patients.
Article
Full-text available
In this study the head circumference (HC), height and weight of 408 adults were measured. An adult’s HC was been shown to be affected by his/her height and weight. Our data suggest that absolute measurements of HC without regard to stature and weight are inadequate for demonstrating clinically significant abnormalities of head size. On average a male’s head is 1.33 cm larger than that of a female. We produced centile charts for adult HCs to accurately detect relative abnormalities. The pediatric charts are also appropriate for use in adults of either sex. [Journal of Turgut Özal Medical Center 1997;4(3):261-264]
Article
Full-text available
We study the impact of coil orientation on the motor threshold (MT) and present an optimal coil orientation for stimulation of the foot. The result can be compared to results of models that predict this orientation from electrodynamic properties of the media in the skull and from orientations of cells, respectively. We used a robotized TMS system for precise coil placement and recorded motor-evoked potentials with surface electrodes on the abductor hallucis muscle of the right foot in 8 healthy control subjects. First, we performed a hot-spot search in standard (lateral) orientation and then rotated the coil in steps of 10° or 20°. At each step we estimated the MT. For navigated stimulation and for correlation with the underlying anatomy a structural MRI scan was obtained. Optimal coil orientation was 33.1±18.3° anteriorly in relation to the standard lateral orientation. In this orientation the threshold was 54±18% in units of maximum stimulator output. There was a significant difference of 8.0±5.9% between the MTs at optimal and at standard orientation. The optimal coil orientations were significantly correlated with the direction perpendicular to the postcentral gyrus ([Formula: see text]). Robotized TMS facilitates sufficiently precise coil positioning and orientation to study even small variations of the MT with coil orientation. The deviations from standard orientation are more closely matched by models based on field propagation in media than by models based on orientations of pyramidal cells.
Article
Full-text available
Low-cost piezoresistive sensors can be of great interest in robotic applications due not only to their advantageous cost but to their dimension as well, which enables an advanced mechanical integration. In this paper, a comparison of two commercial piezoresistive sensors based on different technologies is performed in the case of a medical robotics application. The existence of significant nonlinearities in their dynamic behavior is demonstrated, and a nonlinear modeling is proposed. A compensation scheme is developed for the sensor with the largest nonlinearities before discussing the selection of a sensor for dynamic applications. It is shown that force control is achievable with these kinds of sensors, in spite of their drawbacks. Experiments with both types of sensors are presented, including force control with a medical robot.
Article
Full-text available
Transcranial magnetic stimulation is a noninvasive brain stimulation technique. It is based on current induction in the brain with a stimulation coil emitting a strong varying magnetic field. Its development is currently limited by the lack of accuracy and repeatability of manual coil positioning. A dedicated robotic system is proposed in this paper. Contrary to previous approaches in the field, a custom design is introduced to maximize the safety of the subject. Furthermore, the control of the force applied by the coil on the subject's head is implemented. The architecture is original and its experimental evaluation demonstrates its interest: the compensation of the head motion is combined with the force control to ensure accuracy and safety during the stimulation.
Article
Full-text available
Shape and exact location of motor cortical areas varies among individuals. The exact knowledge of these locations is crucial for planning of neurosurgical procedures. In this study, we have used robot-assisted image-guided transcranial magnetic stimulation (Ri-TMS) to elicit MEP response recorded for individual muscles and reconstruct functional motor maps of the primary motor cortex. One healthy volunteer and five patients with intracranial tumors neighboring the precentral gyrus were selected for this pilot study. Conventional MRI and fMRI were obtained. Transcranial magnetic stimulation was performed using a MagPro X100 stimulator and a standard figure-of-eight coil positioned by an Adept Viper s850 robot. The fMRI activation/Ri-TMS response pattern were compared. In two cases, Ri-TMS was additionally compared to intraoperative direct electrical cortical stimulation. Maximal MEP response of the m. abductor digiti minimi was located in an area corresponding to the "hand knob" of the precentral gyrus for both hemispheres. Repeated Ri-TMS measurements showed a high reproducibility. Simultaneous registration of the MEP response for m. brachioradialis, m. abductor pollicis brevis, and m. abductor digiti minimi demonstrated individual peak areas of maximal MEP response for the individual muscle groups. Ri-TMS mapping was compared to the corresponding fMRI studies. The areas of maximal MEP response localized within the "finger tapping" activated areas by fMRI in all six individuals. Ri-TMS is suitable for high resolution non-invasive preoperative somatotopic mapping of the motor cortex. Ri-TMS may help in the planning of neurosurgical procedures and may be directly used in navigation systems.
Conference Paper
Full-text available
This paper presents a new robotic system for automated image-guided transcranial magnetic stimulation, a noninvasive technique for the treatment of neurologic pathologies such as depression. This stimulation technique requires the accurate positioning of a magnetic coil in order to induce a specific cortical excitation. The neurologist currently positions the coil manually by means of a navigation system, which does not allow the precise clinical evaluation of the procedure. In this paper, a novel robotic system is proposed to assist the neurologist during a TMS session. The proposed system and its control are designed to satisfy simultaneously safety and accuracy requirements.
Article
In order to ensure successful application of transcranial magnetic stimulation (TMS), practitioners must be certain that they are targeting the correct cortical location. To aid in this, a number of clinicians and practitioners have begun utilizing various neuronavigation systems to track coil and participant-head position in space for the duration of a stimulation session. In this chapter, I explore the history of neuronavigation and the developments that made combining this technology with TMS possible. Following this, I discuss the practical aspects of properly utilizing a neuronavigation system: including MRI acquisition, 3D-reconstruction, head and coil co-registration, cortical targeting, motion tracking, and electric field modeling. I conclude with a brief discussion of incorporating robotic assistance in coil positioning and tracking.
Article
Electroconvulsive therapy (ECT) and magnetic seizure therapy (MST) are conventionally applied with a fixed stimulus current amplitude, which may result in differences in the neural stimulation strength and focality across patients due to interindividual anatomical variability. The objective of this study is to quantify the effect of head anatomical variability associated with age, sex, and individual differences on the induced electric field characteristics in ECT and MST. Six stimulation modalities were modeled including bilateral and right unilateral ECT, focal electrically administered seizure therapy (FEAST), and MST with circular, cap, and double-cone coils. The electric field was computed using the finite element method in a parameterized spherical head model representing the variability in the general population. Head tissue layer thicknesses and conductivities were varied to examine the impact of interindividual anatomical differences on the stimulation strength, depth, and focality. Skull conductivity most strongly affects the ECT electric field, whereas the MST electric field is independent of tissue conductivity variation in this model but is markedly affected by differences in head diameter. Focal ECT electrode configurations such as FEAST is more sensitive to anatomical variability than that of less focal paradigms such as BL ECT. In MST, anatomical variability has stronger influence on the electric field of the cap and circular coils compared to the double-cone coil, possibly due to the more superficial field of the former. The variability of the ECT and MST electric field due to anatomical differences should be considered in the interpretation of existing studies and in efforts to improve dosing approaches for better control of stimulation strength and focality across patients, such as individualization of the current amplitude. The conventional approach to individualizing dosage by titrating the number of pulses cannot compensate for differences in the spatial extent of stimulation that result from anatomical variability.
Article
In this article, we discuss industrial robot characteristics of accuracy and repeatability. The factors that affect these characteristics are identified, and an error tree is developed. Subsequently, the accuracy and repeatability are investigated utilizing the Denavit-Hartenberg kinematics parameters, the homogeneous transformation matrix, and the differential transformation matrix theory, and corresponding measures are developed. The formulation indicates that the influence matrices associated with joint variables are constant. A new measure called degree of influence is established that qualitatively assesses the relative contribution of each kinematic parameter variation to the accuracy and repeatability of rigid manipulators. The developed formulation provides for easy evaluation of the degree of influence measures for rigid manipulators in either numerical or symbolic form. A numerical example is included in which the degree of influence of the kinematics parameters for an articulated manipulator, PUMA 560, are evaluated and analysed.
Article
Background: Transcranial Magnetic Stimulation (TMS) is based on a changing magnetic field inducing an electric field in the brain. Conventionally, the TMS coil is mounted to a static holder and the subject is asked to avoid head motion. Additionally, head resting frames have been used. In contrast, our robotized TMS system employs active motion compensation (MC) to maintain the correct coil position. Objective/hypothesis: We study the effect of patient motion on TMS. In particular, we compare different coil positioning techniques with respect to the induced electric field. Methods: We recorded head motion for six subjects in three scenarios: (a) avoiding head motion, (b) using a head rest, and (c) moving the head freely. Subsequently, the motion traces were replayed using a second robot to move a sensor to measure the electric field in the target region. These head movements were combined with 2 types of coil positioning: (1) using a coil holder and (2) using robotized TMS with MC. Results: After 30 min the induced electric field was reduced by 32.0% and 19.7% for scenarios (1a) and (1b), respectively. For scenarios (2a)-(2c) it was reduced by only 4.9%, 1.4% and 2.0%, respectively, which is a significant improvement (P < 0.05). Furthermore, the orientation of the induced field changed by 5.5°, 7.6°, 0.4°, 0.2°, 0.2° for scenarios (1a)-(2c). Conclusion: While none of the scenarios required rigid head fixation, using a simple holder to position a coil during TMS can lead to substantial deviations in the induced electric field. In contrast, robotic motion compensation results in clinically acceptable positioning throughout treatment.
Movement-related magnetic fields (MRMFs) accompanying left and right unilateral and bilateral finger flexions were studied in 6 right-handed subjects. Six different MRMF components occurring prior to, and during both unilateral and bilateral movements are described: a slow pre-movement readiness field (RF, 1–0.5 sec prior to movement onset); a motor field (MF) starting shortly before EMG onset; 3 separate “movement-evoked” fields following EMG onset (MEFI at 100 msec; MEFII at 225 msec; and MEFIII at 320 msec); and a “post-movement” field (PMF) following the movement itself. The bilateral topography of the RF and MF for both unilateral and bilateral movements suggested bilateral generators for both conditions. Least-squares fitting of equivalent current dipole sources also indicated bilateral sources for MF prior to both unilateral and bilateral movements with significantly greater strength of contralateral sources in the case of unilateral movements. Differences in pre-movement field patterns for left versus right unilateral movements indicated possible cerebral dominance effects as well. A single current dipole in the contralateral sensorimotor cortex could account for the MEFI for unilateral movements and bilateral sensorimotor sources for bilateral movements. Other MRMF components following EMG onset indicated similar sources in sensorimotor cortex related to sensory feedback or internal monitoring of the movement. The results are discussed with respect to the possible generators active in sensorimotor cortex during unilateral and bilateral movement preparation and execution and their significance for the study of cortical organization of voluntary movement.
Article
Direct tracking is more robust than tracking that is based on additional markers. 3D laser scans can be used for direct tracking because they result in a 3D data set of surface points of the scanned object. For head-navigated robotized systems, it is crucial to know where the patient's head is positioned relatively to the robot. We present a novel method to use a 3D laserscanner for direct head navigation in the robotized TMS system that places a coil on the patient's head using an industrial robot. First experimental results showed a translational error < 2mm in the robot hand-eye-calibration with the laserscanner. The rotational error was 0.75° and the scaling error < 0.001. Furthermore, we found that the error of a scanned head to a reference head image was < 0.2mm using ICP. These results have shown that a direct head navigation is feasible for the robotized TMS system. Additional effort has to be made in future systems to speed up the compution time for real time capability.
Article
Transcranial magnetic stimulation (TMS) is a unique method for non-invasive brain imaging. The fundamental difference between TMS and other available non-invasive brain imaging techniques is that when a physiological response is evoked by stimulation of a cortical area, that specific cortical area is causally related to the response. With other imaging methods, it is only possible to detect and map a brain area that participates in a given task or reaction. TMS has been shown to be clinically accurate and effective in mapping cortical motor areas and applicable to the functional assessment of motor tracts following stroke, for example. Many hundreds of studies have been published indicating that repetitive TMS (rTMS) may also have multiple therapeutic applications. Techniques and protocols for individually targeting and dosing rTMS urgently need to be developed in order to ascertain the accuracy, repeatability and reproducibility required of TMS in clinical applications. We review the basic concepts behind navigated TMS and evaluate the currently accepted physical and physiological factors contributing to the accuracy and reproducibility of navigated TMS. The advantages of navigated TMS over functional MRI in motor cortex mapping are briefly discussed. Illustrative cases utilizing navigated TMS are shown in presurgical mapping of the motor cortex, in therapy for depression, and in the follow-up of recovery from stroke.
Article
Forensic anthropology typically uses osteological and/or dental data either to estimate characteristics of unidentified individuals or to serve as evidence in cases where there is a putative identification. In the estimation context, the problem is to describe aspects of an individual that may lead to their eventual identification, whereas in the evidentiary context, the problem is to provide the relative support for the identification. In either context, individual characteristics such as sex and race may be useful. Using a previously published forensic case (Steadman et al. (2006) Am J Phys Anthropol 131:15-26) and a large (N = 3,167) reference sample, we show that the sex of the individual can be reliably estimated using a small set of 11 craniometric variables. The likelihood ratio from sex (assuming a 1:1 sex ratio for the "population at large") is, however, relatively uninformative in "making" the identification. Similarly, the known "race" of the individual is relatively uninformative in "making" the identification, because the individual was recovered from an area where the 2000 US census provides a very homogenous picture of (self-identified) race. Of interest in this analysis is the fact that the individual, who was recovered from Eastern Iowa, classifies very clearly with [Howells 1973. Cranial Variation in Man: A Study by Multivariate Analysis of Patterns of Difference Among Recent Human Populations. Cambridge, MA: Peabody Museum of Archaeology and Ethnology; 1989. Skull Shape and the Map: Craniometric Analyses in the Dispersion of Modern Homo. Cambridge, MA: Harvard University Press]. Easter Islander sample in an analysis with uninformative priors. When the Iowa 2000 Census data on self-reported race are used for informative priors, the individual is clearly identified as "American White." This analysis shows the extreme importance of an informative prior in any forensic application.
Article
The emergence of transcranial magnetic stimulation (TMS) as a tool for investigating the brain has been remarkable over the past decade. While many centers are now using TMS, little has been done to automate the delivery of planned TMS stimulation for research and/or clinical use. We report on an image-guided robotically positioned TMS system (irTMS) developed for this purpose. Stimulation sites are selected from functional images overlaid onto anatomical MR images, and the system calculates a treatment plan and robotically positions the TMS coil following that plan. A new theory, stating that cortical response to TMS is highest when the induced E-field is oriented parallel to cortical columns, is used by the irTMS system for planning the position and orientation of the TMS coil. This automated approach to TMS planning and delivery provides a consistent and optimized method for TMS stimulation of cortical regions of the brain. We evaluated the positional accuracy and utility of the irTMS system with a B-shaped TMS coil. Treatment plans were evaluated for sites widely distributed about a head phantom with well-defined landmarks. The overall accuracy in positioning the planned site of the TMS coil was approximately 2 mm, similar to that reported for the robot alone. The estimated maximum range of error in planned vs. delivered E-field strength was +4%, suggesting a high degree of accuracy and reproducibility in the planned use of the irTMS system.
Article
In cognitive neuroscience, optically tracked frameless stereotaxic navigation has been successfully used to precisely guide transcranial magnetic stimulation (TMS) to desired cortical areas for brain-mapping purposes. Thereby, potential sources of imprecision are the fixation of a reference frame to the head of the subject and the referencing procedure according to certain landmarks (LM). The aim of our study was to evaluate the accuracy of frameless stereotaxic coil positioning in a standard experimental setting. A parameter for accuracy is the reproducibility of LM coordinates. In order to test the stability of the referencing for stereotaxic positioning within a single TMS session (within-session stability), the coordinates of six predefined facial LM in nine subjects were recorded first after the initial registration and second after a 20 minutes TMS session. The two sets of coordinates were then compared. The reliability of the positioning coordinates between different TMS sessions (inter-session repeatability) was addressed by registering the subjects LM coordinates in two independent TMS sessions. The variance of the recorded coordinates was analyzed. Altogether, LM were registered 1728 times (192 measures per subject). Within-session stability: The mean Euclidean distance (MED) between the LM position coordinates before and after a TMS session was 1.6 mm, when pooling over all LM. Inter-session repeatability: The MED between the LM positions recorded after the reference procedures of two different sessions showed an average deviation of 2.5 mm. In conclusion, optically tracked frameless stereotaxic coil positioning is from the technical viewpoint of high stability and repeatability. It is therefore a precise method for TMS brain mapping studies or for repeated TMS treatments, with the need of topographically exact stimulation.
Article
Transcranial magnetic stimulation (TMS) can be interleaved with fMRI to visualize regional brain activity in response to direct, non-invasive, cortical stimulation, making it a promising tool for studying brain function. A major practical difficulty is accurately positioning the TMS coil within the MRI scanner for stimulating a particular area of brain cortex. The objective of this work was to design and build a self-contained hardware/software system for MR-guided TMS coil positioning in interleaved TMS/fMRI studies. A compact, manually operated, articulated TMS coil positioner/holder with 6 calibrated degrees of freedom was developed for use inside a cylindrical RF head coil, along with a software package for transforming between MR image coordinates, MR scanner space coordinates, and positioner/holder settings. Phantom calibration studies gave an accuracy for positioning within setups of dx=+/-1.9 mm, dy=+/-1.4 mm, dz=+/-0.8 mm and a precision for multiple setups of dx=+/-0.8 mm, dy=+/-0.1 mm, dz=+/-0.1 mm. This self-contained, integrated MR-guided TMS system for interleaved TMS/fMRI studies provides fast, accurate location of motor cortex stimulation sites traditionally located functionally, and a means of consistent, anatomy-based TMS coil positioning for stimulation of brain areas without overt response.
Towards direct head navigation for robot-guided transcranial magnetic stimulation using 3D laserscans: idea, setup and feasibility
  • L Richter
Richter L et al 2010 Towards direct head navigation for robot-guided transcranial magnetic stimulation using 3D laserscans: idea, setup and feasibility 2010 Ann. Int. Conf. IEEE Eng. Med. Biol. Soc. pp 2283-6