Multi-source localization in MEG using simulated annealing: model order determination and parameter accuracy

Empirical neuromagnetic studies have reported that multiple brain regions are active at single instants in time as well as across time intervals of interest. Determining the number of active regions, however, required a systematic search across increasing model orders using reduced chi-square measure of goodness-of-fit and multiple starting points within each model order assumed. Simulated annealing was recently proposed for noiseless biomagnetic data as an effective global minimizer. A modified cost function was also proposed to effectively deal with an unknown number of dipoles for noiseless, multi-source biomagnetic data. Numerical simulation studies were conducted using simulated annealing to examine effects of a systematic increase in model order using both reduced chi-square as a cost function as well as a modified cost function, and effects of overmodeling on parameter estimation accuracy. Effects of different choices of weighting factors are also discussed. Simulated annealing was also applied to visually evoked neuromagnetic data and the effectiveness of both cost functions in determining the number of active regions was demonstrated

Sequential Monte Carlo samplers for semilinear inverse problems and application to magnetoencephalography

We discuss the use of a recent class of sequential Monte Carlo methods for solving inverse problems characterized by a semi-linear structure, i.e. where the data depend linearly on a subset of variables and nonlinearly on the remaining ones. In this type of problems, under proper Gaussian assumptions one can marginalize the linear variables. This means that the Monte Carlo procedure needs only to be applied to the nonlinear variables, while the linear ones can be treated analytically; as a result, the Monte Carlo variance and/or the computational cost decrease. We use this approach to solve the inverse problem of magnetoencephalography, with a multi-dipole model for the sources. Here, data depend nonlinearly on the number of sources and their locations, and depend linearly on their current vectors. The semi-analytic approach enables us to estimate the number of dipoles and their location from a whole time-series, rather than a single time point, while keeping a low computational cost.Comment: 26 pages, 6 figure

Independent component analysis of magnetoencephalographic signals

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Strategies for optimal design of biomagnetic sensor systems

Magnetic field imaging (MFI) is a technique to record contact free the magnetic field distribution and estimate the underlying source distribution in the heart. Currently, the cardiomagnetic fields are recorded with superconducting quantum interference devices (SQUIDs), which are restricted to the inside of a cryostat filled with liquid helium or nitrogen. New room temperature optical magnetometers allow less restrictive sensor positioning, which raises the question of how to optimally place the sensors for robust field reconstruction. The objective in this study is to develop a generic object-oriented framework for optimizing sensor arrangements (sensor positions and orientations) which supports the necessary constraints of a limited search volume (only outside the body) and the technical minimum distance of sensors (e.g. 1 cm). In order to test the framework, a new quasi-continuous particle swarm optimizer (PSO) component is developed as well as an exemplary goal function component using the condition number (CN) of the leadfield matrix. Generic constraint handling algorithms are designed and implemented, that decompose complex constraints into basic ones. The constraint components interface to an operational exemplary optimization strategy which is validated on the magnetocardiographic sensor arrangement problem. The simulation setup includes a three compartment boundary element model of a torso with a fitted multi-dipole heart model. The results show that the CN, representing the reconstruction robustness of the inverse problem, can be reduced with our optimization by one order of magnitude within a sensor plane (the cryostat bottom) in front of the torso compared to a regular sensor grid. Reduction of another order of magnitude is achieved by optimizing sensor positions on the entire torso surface. Results also indicate that the number of sensors may be reduced to 20-30 without loss of robustness in terms of CN. The original contributions are the generic reusable framework and exemplary components, the quasicontinuous PSO algorithm with constraint support and the composite constraint handling algorithms

Computational methods for Bayesian estimation of neuromagnetic sources

The electromagnetic inverse problem in human brain research consists of determining underlying source currents in the brain based on measurements outside the head. Solution to the inverse problem is ambiguous, necessitating the use of prior information and modeling assumptions for obtaining reasonable inverse estimates. In this study, we create new and improve existing computational methods for estimating neuromagnetic sources in the human brain. One straightforward way of incorporating presumptions to this problem is to formulate it in a probabilistic Bayesian manner. Bayesian statistics is largely based on modeling uncertainties associated with parameters constituting the model by representing them with probability distributions. In this work, existing neuroscientific knowledge and information from anatomical and functional magnetic resonance imaging are used as prior assumptions in model implementation. The neuromagnetic inverse problem is resolved with two different approaches. First, we perform the analysis using distributed source current modeling and infer some arbitrary parameter choices and the source currents from the measurement data by using numerical sampling methods. We apply similar strategies to cortically constrained current dipole localization and suggest using functional magnetic resonance imaging data for guiding the sampling algorithm. The models are tested with simulated and measured data. The presented methods are rather automatic, yielding plausible and robust inverse estimates of cortical current sources. With the spatiotemporal dipole localization model, the inclusion of functional magnetic resonance imaging data improves performance of the numerical sampling method. However, apparent multimodality of the parameter posterior distribution causes complications especially with empirical data. We suggest using loose cortical orientation constraints for smoothing down the complicated posterior distribution instead of marginal improvements to the sampling scheme. This might help to overcome the somewhat limited mixing properties of the sampling algorithm and ease the inconvenient multimodality of the posterior distribution.Ihmisaivojen tutkimukseen liittyvällä sähkömagneettisella käänteisongelmalla tarkoitetaan aivojen virtalähteiden paikantamista pään ulkopuolisten mittausten perusteella. Ongelmaan ei ole yksikäsitteistä ratkaisua, joten mallintamisessa on käytettävä ennakko-oletuksia järkevien ratkaisujen tuottamiseksi. Tässä tutkimuksessa kehitämme uusia ja parannamme olemassaolevia laskennallisia menetelmiä aivoissa syntyvien magneettikenttiä tuottavien lähteiden paikantamiseksi. Kenties yksinkertaisin tapa lisätä ennakko-oletuksia tähän ongelmaan on käyttää bayesilaista mallintamista. Bayesilainen tilastotiede perustuu pitkälti parametrien epävarmuuksien mallintamiseen ja esittämiseen todennäköisyysjakaumin. Työn mallien muodostamisessa käytetään apuna aivojen toiminnallisesta ja rakenteellisesta magneettikuvauksesta saatavaa neurotieteellistä ennakkotietoa. Sähkömagneettisen käänteisongelman ratkaisuun käytämme kahta eri menetelmää. Aluksi analysoimme aivojen pinnalle muodostettuja virtalähdejakaumamalleja ja pyrimme laskennallisia otantamenetelmiä käyttäen arvioimaan virtojen sekä muuten etukäteen mielivaltaisesti valittavien parametrien arvoja mittausaineistosta. Sovellamme samantyyppistä otantamenetelmää malliin, missä dipolaarisia virtalähteitä rajoittaa aivojen kuorikerroksen anatomia ja fysiologia. Ehdotamme lisäksi toiminnallisen magneettikuvauksen tuottaman mittausaineiston käyttöä otantamenetelmän apuna. Malleja testataan sekä simuloidulla että kokeellisella mittausaineistolla. Kehitetyt menetelmät ovat hyvin automaattisia ja tuottavat järkeviä ratkaisuja magneettisten mittausten lähteiksi. Dipolaaristen virtalähteiden paikallis-ajalliseen määrittämiseen käytetyn otantamenetelmän suorituskyky parantuu toiminnallisesta magneettikuvauksesta saatavan tiedon avulla. Mallin parametrien todennäköisyysjakauma on kuitenkin selvästi monihuippuinen aiheuttaen ongelmia erityisesti kokeellisen mittausaineiston kanssa. Otantamenetelmän parannusten sijaan ehdotamme väljempien aivojen kuorikerroksen anatomiaan perustuvien rajoitteiden käyttöä, jolloin itse parametrien todennäköisyysjakauma saattaa muuttua helpommin käsiteltäväksi. Tämä parantanee myös nykyisen otantamenetelmän tehokkuutta tässä ongelmassa ja helpottaa siten monihuippuisten jakaumien jatkokäsittelyä.reviewe

Spatio-temporal evolution of interictal epileptic activity : a study with unaveraged multichannel MEG data in association with MRIs.

This thesis addresses issues relating to MEG modelling, analysis and interpretation of results. A source model employing current density distributions, namely Magnetic Field Tomography (MET), is used to obtain the MEG results. The first issue of concern refers to the registration of MEG data with structural MR images in an attempt to improve the localisation capability of MEG/MET. Simulations testing some spatial and tem poral aspects of the reconstruction capability of MET are also provided. A novel way of conducting MET studies in depth is suggested and implemented: the iterative use of a source space designed to cover deep situated structures on either side of the brain. The main bulk of this thesis is concerned with research into interictal epileptic activity as recorded by means of multichannel MEG system s and analysed using MET. The major aim is to investigate whether or not MET analysis of unaveraged MEG data (single epochs) is feasible in cases of pathophysiological signals and more specifically interictal signals from patients with epilepsy of a complex partial type. The investigation is undertaken against the "traditional" view of the impropriety and absurdity of using single epoch records in the MEG analysis due to noise dominance; we provide evidence that analysis of single, unaveraged epileptic spikes is actually feasible: we demonstrate spatio-temporal coherence in the MET results of the various single interictal events and show that activity extracted from the "averaged event" is made up of activity contributions which occur intermittently and at variable latencies. Our statements are drawn from the study of both superficial and deep activity

Magnetoencephalography

This is a practical book on MEG that covers a wide range of topics. The book begins with a series of reviews on the use of MEG for clinical applications, the study of cognitive functions in various diseases, and one chapter focusing specifically on studies of memory with MEG. There are sections with chapters that describe source localization issues, the use of beamformers and dipole source methods, as well as phase-based analyses, and a step-by-step guide to using dipoles for epilepsy spike analyses. The book ends with a section describing new innovations in MEG systems, namely an on-line real-time MEG data acquisition system, novel applications for MEG research, and a proposal for a helium re-circulation system. With such breadth of topics, there will be a chapter that is of interest to every MEG researcher or clinician