Solenoidal Micromagnetic Stimulation Enables Activation of Axons With Specific Orientation

Electrical stimulation of the central and peripheral nervous systems - such as deep brain stimulation, spinal cord stimulation, and epidural cortical stimulation are common therapeutic options increasingly used to treat a large variety of neurological and psychiatric conditions. Despite their remarkable success, there are limitations which if overcome, could enhance outcomes and potentially reduce common side-effects. Micromagnetic stimulation (μMS) was introduced to address some of these limitations. One of the most remarkable properties is that μMS is theoretically capable of activating neurons with specific axonal orientations. Here, we used computational electromagnetic models of the μMS coils adjacent to neuronal tissue combined with axon cable models to investigate μMS orientation-specific properties. We found a 20-fold reduction in the stimulation threshold of the preferred axonal orientation compared to the orthogonal direction. We also studied the directional specificity of μMS coils by recording the responses evoked in the inferior colliculus of rodents when a pulsed magnetic stimulus was applied to the surface of the dorsal cochlear nucleus. The results confirmed that the neuronal responses were highly sensitive to changes in the μMS coil orientation. Accordingly, our results suggest that μMS has the potential of stimulating target nuclei in the brain without affecting the surrounding white matter tracts

Electromagnetic Modelling of Planar Circuits Embedded in Laterally Shielded Multilayered Media

Multilayered media with printed circuits embedded between their dielectric layers are one of the most successful technologies for manufacturing planar structures with a very promising performance-to-price ratio. These planar structures include many different geometries ranging from cavity-backed microstrip antennas through Frequency Selective Surfaces (FSS) and photonic band-gap structures, to shielded printed circuits and ultra-wide-band slot-line microwave filters. The electromagnetic characteristics of such structures can be modeled with a wide range of numerical techniques. In recent years we have witnessed a remarkable progress in developing numerical solvers specialized in analyzing specific electromagnetic problems including the Integral Equation technique (IE) solved by the Method of Moments (MoM) which has proven to be very robust and efficient in the framework of partially open or shielded planar structures. As a part of this study an efficient CAD tool was developed based on the IE formulation and implemented in the context of the MoM technique. It includes a rigorous mathematical formulation of the electromagnetic problem which automatically incorporates the effects of the metallic shield (cavity) and the multilayered substrate which supports different layers of metallization (interfaces). A comprehensive study has been carried out to point out the convergence of the proposed algorithm along with a detailed validation, verification and benchmarking plan. The tool is developed in such a way that is capable of working with a hybrid mesh (combination of both rectangular and triangular cells) which gives it a great flexibility in analyzing any arbitrarily-shaped embedded circuitry. To enhance the overall performance of the algorithm, symmetry options have been formulated and implemented. These additional features have shown to dramatically reduce the computational and temporal requirements and thus expand the range of microwave devices which can be efficiently analyzed by the software, such as filters based on waveguide structures and FSSs. Other enhancement techniques have been envisaged based on selective MoM matrix member calculation which successfully reduce the number of unknowns in large microstrip layouts while maintaining an acceptable accuracy. Mathematical formulations are developed to provide the capability of handling lumped ports (representing coaxial-line feeds) and waveguide excitations. The implementation of combined ports in the framework of planar multilayered structures is shown to be practically useful in analyzing systems that have both waveguide connections and planar circuitries in their structure as it enables the application of highly specialized algorithms in the analysis of each part of the system and subsequent communication of the network characterizations at the ports' interfaces. This approach has considerably enhanced the overall performance of the analysis tool by reducing the computation time and memory requirements to a great extent

Combined Field Integral Equation Technique to Model Fine Slots Embedded in Laterally Shielded Multilayered Media

This paper presents the details of development and computer implementation of Combined Field Integral Equation (CFIE) technique used in the framework of Method of Moments to address the problem of simulating fine slots appearing in grounded screens embedded in laterally shielded multilayered media. Narrow slots are encountered extensively in the new generation of ultra wideband microwave filters which use slotlines as a part of their coupling structure. The mathematical formulation is described comprehensively to allow the reader proceeding straightforwardly with the computer implementation. The considerable advantage of using magnetic currents to represent fine slots instead of using the conventional electric currents representing metallic parts is emphasized and the effects of using both current types (electric and magnetic) on the convergence of modal series representing Green’s functions is studied

Analysis of fractal electrodes for efficient neural stimulation

Planar electrodes are increasingly used in therapeutic neural stimulation techniques such as functional electrical stimulation, epidural spinal cord stimulation and cortical stimulation. Recently, optimized electrode geometries have been shown to increase the efficiency of neural stimulation by increasing the variation of current density on the electrode surface. In the present work, a new family of modified fractal electrode geometries is developed to enhance the efficiency of neural stimulation. It is shown that a promising approach in increasing the neural activation function is to increase the "edginess of the electrode surface, a concept that is explained and quantified by fractal mathematics. Rigorous finite element simulations were performed to compute electric potential produced by proposed modified fractal geometries. The activation of 256 model axons positioned around the electrodes was then quantified, showing that modified fractal geometries required a 22% less input power while maintaining the same level of neural activation. Preliminary in-vivo experiments investigating muscle evoked potentials due to median nerve stimulation showed encouraging results, supporting the feasibility of increasing neural stimulation efficiency using modified fractal geometries. © 2013 Golestanirad, Elahi, Molina_arribere, Mosig, Pollo and Graham

Applying combined field integral equation technique to analyze quasi-waveguide structures

In this paper a detailed mathematical procedure is described to evaluate modal scattering parameters conventionally defined in planar multilayered structures excited by wave ports. Combined Field Integral Equation technique is implemented and solved using method of moments. The developed formulation then is successfully applied to analyze waveguide structures and proved to be very efficient in the term of accuracy and performance

Complete network characterisation of stratified planar circuits using the method of moments technique: an integrated approach to lumped and wave ports

This study gives a full treatment of the network characterisation of multi-port planar circuits embedded in shielded stratified media with an integrated approach to lumped ports and wave ports. The details of the mathematical formulation for the retrieval of scattering parameters are given in three cases: networks possessing only lumped ports, networks possessing only wave ports and networks with a combination of both types. A detailed description of the computer algorithm is presented to help the reader easily reproduce our results. The implementation of combined ports in the framework of planar multi-layered structures is particularly useful in analysing systems that have waveguide connections and planar circuitries in their structure, such as cavity-backed antennas and their associated feeding systems, dielectric resonator antennas or compound microwave filters. In such structures one can divide the system into the waveguide structure and the planar circuitries and analyse each part separately with an appropriate and efficient numerical technique. Network characterisations will be consequently communicated at the port interfaces. This strategy enhances the total performance of the computational scheme to a great extent, while not affecting the accuracy of the full-wave analysis

A preliminary fMRI study of a novel self-paced written fluency task: observation of left-hemispheric activation, and increased frontal activation in late vs. early task phases

Neuropsychological tests of verbal fluency are very widely used to characterize impaired cognitive function. For clinical neuroscience studies and potential medical applications, measuring the brain activity that underlies such tests with functional magnetic resonance imaging (fMRI) is of significant interest—but a challenging proposition because overt speech can cause signal artifacts, which tend to worsen as the duration of speech tasks becomes longer. In a novel approach, we present the group brain activity of 12 subjects who performed a self-paced written version of phonemic fluency using fMRI-compatible tablet technology that recorded responses and provided task-related feedback on a projection screen display, over long-duration task blocks (60 s). As predicted, we observed robust activation in the left anterior inferior and medial frontal gyri, consistent with previously reported results of verbal fluency tasks which established the role of these areas in strategic word retrieval. In addition, the number of words produced in the late phase (last 30 s) of written phonemic fluency was significantly less (p < 0.05) than the number produced in the early phase (first 30 s). Activation during the late phase vs. the early phase was also assessed from the first 20 s and last 20 s of task performance, which eliminated the possibility that the sluggish hemodynamic response from the early phase would affect the activation estimates of the late phase. The last 20 s produced greater activation maps covering extended areas in bilateral precuneus, cuneus, middle temporal gyrus, insula, middle frontal gyrus and cingulate gyrus. Among these areas, greater activation was observed in the bilateral middle frontal gyrus (Brodmann area BA 9) and cingulate gyrus (BA 24, 32) likely as part of the initiation, maintenance, and shifting of attentional resources. Consistent with previous pertinent fMRI literature involving overt and covert verbal responses, these findings highlight the promise and practicality of fMRI of written phonemic fluency