Multiple Dipole Model Research Articles

Method for control by orbital spacecraft magnetic cleanliness based on multiple magnetic dipole models with consideration of their uncertainty

Aim. Development of method for control by orbital spacecraft magnetic cleanliness based on multiple magnetic dipole models using compensation of the initial magnetic field with consideration of magnetic characteristics uncertainty. Methodology. Orbital spacecraft multiple magnetic dipole models calculated as solution of nonlinear minimax optimization problem based on near field measurements for prediction orbital spacecraft far magnetic field magnitude. Nonlinear objective function calculated as the weighted sum of squared residuals between the measured and predicted magnetic field. Weight matrix calculated as inverse covariance matrix of random errors vector. Values of magnetic moments and coordinates of placement of compensating magnetic dipoles for compensation of the orbital spacecraft initial magnetic field also calculated as solution of nonlinear minimax optimization problem. Both solutions of this nonlinear minimax optimization problems calculated based on particle swarm nonlinear optimization algorithms. Results. Results of prediction spacecraft far magnetic field magnitude based on orbital spacecraft multiple magnetic dipole models using near field measurements and compensation of the initial magnetic field with consideration of orbital spacecraft magnetic characteristics uncertainty for ensuring the orbital spacecraft magnetic cleanliness. Originality. The method for control by orbital spacecraft magnetic cleanliness based on multiple magnetic dipole models using compensation of the initial magnetic field with consideration of magnetic characteristics uncertainty is developed. Practical value. An important practical problem of ensuring orbital spacecraft magnetic cleanliness based on orbital spacecraft multiple magnetic dipole models using near field measurements and compensation of the initial magnetic field with consideration of orbital spacecraft magnetic characteristics uncertainty solved.

A unique cardiac electrocardiographic 3D model. Toward interpretable AI diagnosis.

Mathematical models of cardiac electrical activity are one of the most important tools for elucidating information about heart diagnostics. In this paper, we present an efficient mathematical formulation for this modeling simple enough to be easily parameterized and rich enough to provide realistic signals. It relies on a five dipole representation of the cardiac electric source, each one associated with the well-known waves of the electrocardiogram signal. Beyond the physical basis of the model, the parameters are physiologically interpretable as they characterize the wave shape, similar to what a physician would look for in signals, thus making them very useful in diagnosis. The model accurately reproduces the electrocardiogram signals of any diseased or healthy heart. This new discovery represents a significant advance in electrocardiography research. It is especially useful for diagnosis, patient follow-up or decision-making on new therapies; is also a promising tool for well-performing, transparent and interpretable AI approaches.

Magnetic Dipole Moment Determination Using Near-Field Magnetic Gradient Tensor Data

Determination of the magnetic dipole moment of a spacecraft is a key issue for magnetic cleanliness and attitude control of various space scientific spacecraft. Two main methods, namely the multiple magnetic dipole modeling method and near-field analysis, have been used with good results inside a coil facility for prelaunch magnetic testing of a spacecraft. When field-canceling coil sets are unavailable, or coil feedback controls are inadequate to compensate for time-varying geomagnetic fields, determination of the magnetic signature of a spacecraft from measurements of field components is compromised. Therefore, we propose a magnetic dipole moment determination method using magnetic gradient tensor data without any coil facility, which is based on the fact that the time variation of the gradient tensor of the geomagnetic field is very small compared with that of field. We improve the near-field analysis method by expanding the magnetic gradient tensor in a series of spherical harmonics, obtain sets of near-field equations using Fourier analysis, and solve these equations to determine the dipole-moment components. The proposed tensor method can effectively suppress the interference from time variations of ambient magnetic fields. Simulations and experiments have been conducted to prove the validity, robustness, and accuracy of the tensor method.

A Software-Based Calibration Technique for Characterizing the Magnetic Signature of EUTs in Measuring Facilities

In this paper, a software-based stochastic approach for calibration purposes in measuring facilities is introduced. The algorithm of the calibration procedure is tested with simulation analysis and evaluated with real measurements in a facility with two magnetometers. The results validate that the algorithm can accurately predict the actual positions of the magnetometers, making the multiple dipole model prediction more accurate and enabling the precise extrapolation of the magnetic field. This study illustrates the importance of the calibration process verification tests for equipment having magnetic cleanliness requirements in space missions.

Genetic algorithms for dipole location of fetal magnetocardiography.

In this paper, we explore the use of Maximum Likelihood (ML) method with Genetic Algorithms (GA) as global optimization procedure for source reconstruction in fetal magnetocardiography (fMCG) data. A multiple equivalent current dipole (ECD) model was used for sources active in different time samples. Inverse solutions across time were obtained for a single-dipole approximation to estimate the trajectory of the dipole position. We compared the GA and SIMPLEX methods in a simulation environment under noise conditions. Methods are applied on a real fMCG data. Results show robust estimators of the cardiac sources when GA is used as optimization technique.

DC magnetic compatibility of satellites

Magnetic effects of satellites should be carefully studied starting from the design phase by creating a stringent magnetic cleanliness program. This program is required to maintain DC magnetic compatibility between units and reduce the interaction of satellite magnetic field with external magnetic field resulting parasitic torques. DC magnetic compatibility is especially important for satellites aiming scientific missions requiring sensitive magnetic equipment. Magnetic behavior of satellites and its units can be understood by test and analysis. Although testing is the most reliable method to evaluate magnetic assessment quantitatively, analysis becomes the first step because of cost and time restrictions. In this study, common analysis methods such as centered dipole approximation, multiple dipole modeling, spherical harmonics methods are discussed for their powerful and weak points. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:330–334, 2016.

A Semianalytical Heuristic Approach for Prediction of Eut's Multiple Dipole Model by Reducing the Number of Heuristics

A novel multiple dipole modeling (MDM) calculation scheme is proposed for prediction of the magnetic behavior of an equipment under test (EUT). The MDM calculation scheme is based on a semianalytical approach leading to less heuristics needed to be determined; thus, leading to a more efficient MDM determination. A genetic algorithm approach is adopted to minimize the root mean square error between the measured magnetic fields of the EUT and the MDM. To validate the proposed scheme and investigate its performance, numerous simulations have been employed. Simulation results regarding a mix of different EUT and the corresponding MDMs magnetic fields demonstrate the capability of the proposed scheme to optimally determine the MDMs and explore some topics about the measuring procedure of an EUT, depending on the distance and the number of the observation points. Furthermore, it is shown that the number of dipoles chosen to compose the MDM is of crucial importance.

Particle Swarm Optimization for Multiple Dipole Modeling of Space Equipment

An advanced modeling algorithm based on particle swarm optimization (PSO) has been developed to solve multiple dipole modeling (MDM) problems in space applications. MDM is a method to represent spacecraft units as a set of equivalent magnetic dipoles able to reconstruct, in the far-field distance, the same magnetostatic field. This procedure allows preparing a magnetic model of the spacecraft during design and development phases. Moreover, it allows refined prediction of magnetic cleanliness for space missions with equipment susceptible to magnetic fields. Indeed, owing to the increase of missions requiring magnetostatic cleanliness, such characterization becomes increasingly important. To validate the PSO procedure, synthetic data have been initially used, generated using a software simulator. Algorithm performance has been tested through measured data acquired using the Mobile Coil Facility located at the European Space Research and Technology Centre in The Netherlands. Starting from measured data, the algorithm iteratively identifies the values of the unknowns, positions, and magnetic moments of the equivalent dipoles that best match the measured field. Since the problem is ill posed, several solutions are possible. To develop a reliable algorithm, some test cases have been analyzed where the expected solution is known. This allowed improving the algorithm leading to satisfying results.

Noninvasive finding of local repolarization changes in the heart using dipole models and simplified torso geometry

Two inverse methods using dipole models for noninvasive assessment of local repolarization changes were investigated and compared in the simulation study. Lesions with changed repolarization were modeled by shortening of the action potential durations in ventricular regions typically influenced by occlusion of coronary arteries. Corresponding body surface potentials were computed using a multiple dipole model of the cardiac generator and an inhomogeneous torso model. Position of each lesion was then estimated by an inverse solution to a single dipole and to a group of five neighbouring dipoles. For both methods the lesion localization error was evaluated and its dependence on the lesion size and the noise in input data was studied. When no noise was present in the input data, the use of the inverse method to a group of dipoles instead of a single dipole resulted in an unsubstantial reduction of the mean localization error of small lesions from 0.6 to 0.5cm. For medium and especially for large lesions the mean localization errors decreased significantly from 1.1 to 0.6cm and from 2.3 to 1.0cm, respectively. The inverse solution to a group of five dipoles was more sensitive to noise. However, for large lesions it still gave better results than the solution to a single dipole if the signal to noise ratio was higher than 30dB.

Subject position affects EEG magnitudes

EEG (electroencephalography) has been used for decades in thousands of research studies and is today a routine clinical tool despite the small magnitude of measured scalp potentials. It is widely accepted that the currents originating in the brain are strongly influenced by the high resistivity of skull bone, but it is less well known that the thin layer of CSF (cerebrospinal fluid) has perhaps an even more important effect on EEG scalp magnitude by spatially blurring the signals. Here it is shown that brain shift and the resulting small changes in CSF layer thickness, induced by changing the subject's position, have a significant effect on EEG signal magnitudes in several standard visual paradigms. For spatially incoherent high-frequency activity the effect produced by switching from prone to supine can be dramatic, increasing occipital signal power by several times for some subjects (on average 80%). MRI measurements showed that the occipital CSF layer between the brain and skull decreases by approximately 30% in thickness when a subject moves from prone to supine position. A multiple dipole model demonstrated that this can indeed lead to occipital EEG signal power increases in the same direction and order of magnitude as those observed here. These results suggest that future EEG studies should control for subjects' posture, and that some studies may consider placing their subjects into the most favorable position for the experiment. These findings also imply that special consideration should be given to EEG measurements from subjects with brain atrophy due to normal aging or neurodegenerative diseases, since the resulting increase in CSF layer thickness could profoundly decrease scalp potential measurements.

PREDICTION OF MULTIPLE MAGNETIC DIPOLE MODEL PARAMETERS FROM NEAR FIELD MEASUREMENTS EMPLOYING STOCHASTIC ALGORITHMS

In this paper, the problem of predicting far fleld magnitude from near fleld measurements of an equipment under test (EUT) is studied. Firstly, a multiple magnetic dipole model is developed to simulate the magnetic behavior of the EUT. The parameters of the model (dipoles positions and magnetic moments) are calculated using the values of the near fleld applying the Particle Swarm Optimization (PSO) algorithm. For the evaluation of the method, extended simulations were conducted, producing theoretical values and distorting them with noise, and then the developed algorithm was used to create the proper model. Finally, the theoretical results are compared to the fleld assessments the proper models produced.

Comparison of cardiac activation models using 3D MCG data

Event Abstract Back to Event Comparison of cardiac activation models using 3D MCG data Massimo D. Melis1* and Yoshinori Uchikawa1 1 Tokyo Denki University, Japan In the study of the inverse problem in biomagnetism, the measured signals are used to localize the source that generates a given magnetic field distribution. The problem has no unique solution, since the same magnetic field map (MFM) can be generated by an infinite number of possible source configurations. To restrict the set of possible solutions, physiological information can be used. In magnetocardiography (MCG), the knowledge of the heart functionality may help in making a suitable choice to model the current source in the human heart. A possible choice for the current model is a single dipole, with varying position, orientation and amplitude inside the heart volume, the so-called source space. An alternative solution can be a multiple dipole model, characterized by a limited number of dipoles, with varying positions and orientations, or a limited range of variability for the position inside the source space. A more general description of the wavefront propagation can be provided by a distributed sources model, which considers an elevated number of dipolar sources, located in fixed positions inside the heart volume region, having varying amplitudes and directions, to model the current flow during the heart cycle. A single dipole model is a suitable one only for short segments of the MCG signals, whereas a distributed model allows estimating the current density in the source space. In this study the source localizations of MCG signals were performed using different source models on the same data sets, to evaluate whether the differences in the source models descriptions can be minimized in the MCG data sets, or in some of them. The analyses are repeated on normal subjects and WPW patients, using normal components data and vector components data of the measured MFMs, to study the effect of the information provided by the additional tangential components. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: MCG: Instrumentation, Modeling, Basic and Clinical Studies Citation: Melis MD and Uchikawa Y (2010). Comparison of cardiac activation models using 3D MCG data. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00341 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 07 Apr 2010; Published Online: 07 Apr 2010. * Correspondence: Massimo D Melis, Tokyo Denki University, Tokyo, Japan, mdemelis@hotmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Massimo D Melis Yoshinori Uchikawa Google Massimo D Melis Yoshinori Uchikawa Google Scholar Massimo D Melis Yoshinori Uchikawa PubMed Massimo D Melis Yoshinori Uchikawa Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Spatio-temporal Modeling of Evoked Brain Activity During Memory Encoding and Target Comparison in Visual Tasks

In this paper, the temporal pattern of activity and approximate locations of brain areas related to selective attention and visual working memory processes were studied with event related potential (ERP) recordings in healthy humans. Three experimental series included pairs of the following conditions: Face comparison (familiar faces), Pattern comparison (abstract dot patterns), and Passive viewing. Participants compared pairs of consecutive targets presented in composite images on a computer screen. Spatio-temporal multiple dipole models were developed for 128-channel ERPs. Keeping dipole locations and orientations constant, we compared the source activities for ERPs recorded (1) in different tasks for task-specificity of activations, (2) after the first and second stimuli in the pair, i.e. on the encoding and comparison stages of the task, and (3) after the first stimulus in different series to compare encoding in different conditions. Sources located in the inferotemporal brain areas, especially in the left hemisphere, showed increased activity after 200 ms from the first stimulus onset that may indicate encoding into visual working memory. The anterior sources, located near midline and showing activity around or after 300 ms, presumably reflect non-specific memory processes and attentional control. Major task-specific differences were observed in the temporo-parieto-occipital region in 250-500 ms.

Medial profrontal cortex and anterior cingulate cortex in the generation of alpha activity induced by transcendental meditation: a magnetoencephalographic study.

Previous EEG studies have shown that transcendental meditation (TM) increases frontal and central alpha activity. The present study was aimed at identifying the source of this alpha activity using magnetoencephalography (MEG) and electroencephalography (EEG) simultaneously on eight TM practitioners before, during, and after TM. The magnetic field potentials corresponding to TM-induced alpha activities on EEG recordings were extracted, and we attempted to localize the dipole sources using the multiple signal classification (MUSIC) algorithm, equivalent current dipole source analysis, and the multiple spatio-temporal dipole model. Since the dipoles were mapped to both the medial prefrontal cortex (mPFC) and anterior cingulate cortex (ACC), it is suggested that the mPFC and ACC play an important role in brain activity induced by TM.

Non-stationary magnetoencephalography by Bayesian filtering of dipolemodels

In this paper, we consider the biomagnetic inverse problem of estimating atime-varying source current from magnetic field measurements. It is assumed thatthe data are severely corrupted by measurement noise. This setting is a model formagnetoencephalography (MEG) when the dynamic nature of the sourceprevents us from effecting noise reduction by averaging over consecutivemeasurements. Thus, the potential applications of this approach include the singletrial estimation of the brain activity, in particular from the spontaneousMEG data. Our approach is based on non-stationary Bayesian estimation,and we propose the use of particle filters. The source model in this workis either a single dipole or multiple dipole model. Part of the problemconsists of the model determination. Numerical simulations are presented.

Frontal and Temporal Dysfunction of Auditory Stimulus Processing in Schizophrenia

Attentiondeficits have been consistently described in schizophrenia. Functional neuroimaging and electrophysiological studies have focused on anterior cingulate cortex (ACC) dysfunction as a possible mediator. However, recent basic research has suggested that the effect of attention is also observed as a relative amplification of activity in modality-associated cortical areas. In the present study, the question was addressed whether an amplification deficit is seen in the auditory cortex of schizophrenic patients during an attention-requiring choice reaction task. Twenty-one drug-free schizophrenic patients and 21 age- and sex-matched healthy controls were studied (32-channel EEG). The underlying generators of the event-related N1 component were separated in neuroanatomic space using a minimum-norm (LORETA) and a multiple dipole (BESA) approach. Both methods revealed activation in the primary auditory cortex (peak latency ≈ 100 ms) and in the area of the ACC (peak latency ≈ 130 ms). In addition, the adapted multiple dipole model also showed a temporal-radial source activation in nonprimary auditory areas (peak latency ≈ 140 ms). In schizophrenic patients, significant activation deficits were found in the ACC as well as in the left nonprimary auditory areas that differentially correlated with negative and positive symptoms. The results suggest that (1) the source in the nonprimary auditory cortex is detected only with a multiple dipole approach and (2) that the N1 generators in the ACC and in the nonprimary auditory cortex are dysfunctional in schizophrenia. This would be in line with the notion that attention deficits in schizophrenia involve an extended cortical network.

Sources of movement-related cortical potentials derived from foot, finger, and mouth movements.

Movement-related cortical potentials (MRCPs) register brain electrical activity before and during movement execution. In an attempt to delineate the components of MRCPs that reflect common sources to various movements and that are movement-specific, simple self-paced voluntary foot, finger, and mouth movements were studied. MRCPs were recorded in eight healthy volunteers with 30 electrodes placed on the scalp. Data were analyzed using Brain Electric Source Analysis software, and multiple equivalent dipole models were developed to separate spatial and temporal aspects of brain activity related to the execution of voluntary movements. Independent models were separately developed for the grand average data and for the individual subjects' data for each movement type. MRCPs derived from foot movements were accounted for using a 5-dipole model, finger movements using an 8-dipole model, and mouth movements with a 7-dipole model, yielding the grand average residual variances of 3%, 2%, and 6%, respectively. Based on individual models, intersubject variability of dipole locations was less than 10 mm (+/- SD). Overlaying the mean dipole coordinates onto the stereotaxic atlas provided proof that the sensorimotor cortical areas, supplementary motor area, and also cerebellum and thalamus were active in all three movements. Locations of the dipoles in the contralateral sensorimotor area clearly implied well-known medial to lateral somatotopic organization of foot, finger, and mouth movements. Temporal separation of the activity spread over different brain areas was demonstrated by evolution in the moments of dipole source potentials. The authors' models support the view of simultaneous activation of the primary motor cortex and supplementary motor area at the time of movement execution. Multiple equivalent dipole models developed in this study implied the activity originating in corresponding brain areas as previously detected by positron emission tomography or functional magnetic resonance imaging. However, MRCPs provided additional information regarding the temporal evolution of the brain activity related to the execution of voluntary movements. Thus, the concurrent use of MRCPs and other imaging techniques may provide complementary information not easily obtained by the other imaging techniques themselves.

Noninvasive biomagnetic imaging in coronary artery disease based on individual current density maps of the heart

Objective: In this paper we present an attempt at noninvasive imaging of distributed myocardial electrical activity in patients suffering from myocardial infarction and in healthy subjects. Although advances have been made, noninvasive three-dimensional imaging of cardiac electrophysiological activity is still in its infancy and extending our knowledge of cardiac electrophysiological properties may be a valuable guide in the treatment of patients with coronary artery disease. Methods: Magnetic field mapping data formed the input for an inverse solution that is based on a multiple dipole model. The lead field normalized minimum norm least square criterion was applied to predefined myocardial source geometry. Current density distributions were calculated for the left ventricle during ventricular depolarization. Images from two patients with previous myocardial infarction were compared to images from two healthy subjects. Results: Low regional and global current density was found in the infarction patients. Regions of low current density corresponded to infarcted segments. The images of the healthy subjects displayed less marked areas of low current density. Conclusion: The proposed multiple dipole model may be able to distinguish viable from scarred myocardium. A prospective clinical study should be undertaken to investigate the spatial resolution and the diagnostic performance of this method.

Source localization of P300 from oddball, single stimulus, and omitted-stimulus paradigms.

Three different auditory stimulus paradigms were used to elicit P300 potentials. Normal subjects were tested on the classical rare target stimulus, single-stimulus and omitted-stimulus conditions. Noninvasive identification of the cerebral sources of the event-related potentials (ERPs) was performed using spatio-temporal multiple dipole modeling (BESA software) with individually sized spherical head models. The grand average data of each condition was first independently modeled and these models were used as starting values for modeling each individual subject's data. Models for the rare-stimulus condition and single-stimulus condition both consisted of 6 dipoles. Models for the omitted-stimulus condition consisted of 2 dipoles. The dipole locations of the final individual 6-dipole models for the rare and single-stimulus conditions did not differ significantly from each other or from one previous result obtained from a another group of subjects (Tarkka et al. 1995). Super-imposition of the dipole coordinates on the sterotaxic brain atlas suggests that bilateral deep medial temporal lobe structures are the major contributors to rare and single-stimulus P300s. Because both the wave form morphology and the source model of the P300 elicited by single stimulus were close to those of the rare-stimulus P300 it may be that the underlying neural mechanisms eliciting these P300 potentials are essentially the same.

The functional organization of the interictal spike complex in benign rolandic epilepsy.

We studied the functional organization of the interictal epileptic spike complex in patients with benign rolandic epilepsy of childhood (BREC). We recorded interictal epileptiform spikes and somatosensory evoked potentials after median nerve stimulation, providing a biologic marker for the location of the central sulcus in 12 patients with BREC. We used multiple dipole modeling to assess the number, the three-dimensional intracerebral location, and the time activity of the underlying neuronal sources. Although the interictal spike complex could be modeled by a single tangential dipolar source in seven patients (group 1), in the remaining five patients, two sources-a radial and a tangential dipole-were necessary adequately to explain the interictal spikes (group 2). The tangential source was located deeper than the radial source and was characterized by a frontal positivity and a centroparietal negativity with a phase reversal across the central sulcus, suggesting that the interictal spikes originated in the anterior wall of the central sulcus. The radial source showed a single electronegativity over the ipsilateral central region, which would be compatible with involvement of the top of either the pre- or postcentral gyrus. Both sources showed biphasic time patterns with an average latency difference of 30 ms. Our results indicate that in some patients with typical BREC, the interictal epileptiform spike complex is generated by multiple, simultaneously active neuronal populations within the central region and that epileptiform activity is propagated between these two adjacent cortical areas.