Méthodes entropiques appliqués au problème inverse en magnétoencéphalographie

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal

Numerical methods for improved signal to noise ratios in spatiotemporal biomedical data

Magnetocardiography (MCG) is a technique to measure the magnetic fields produced by electrical activity in the heart. The interpretation of MCG signals is difficult because of different disturbances and noise. Several methods have been suggested for noise reduction in MCG data such as averaging, pass or stop band filters, and statistical based methods, but a unified framework that takes into account different typologies of MCG signals (rest, stress, and patients with an already ICD– Implanted Cardioverter Defibrillator- implanted) using an adequate number of recordings is still missing. Consequently, the main aim of the thesis is to develop methods for noise and artifacts treatment. Due to the non-stationarity (NS) of the noise, the conventional ensemble averaging of the data does not yield the theoretical improvement. In order to overcome this problem an average procedure that ignores the noisiest beats is applied. The results of this averaging procedure confirms that in case of NS, the Signal to Noise Ratio (SNR) does not behave as expected, but reaches a maximum after a certain number of selected beats. Furthermore, a theoretical proof of this result is given. The second part of the thesis deals with techniques based on Blind Source Separation (BSS), as preprocessing step for the averaging procedure, in case of MCG signals with low SNR. Different BSS algorithms are compared in order to find the best one in terms of noise reduction, separation, and computational time for each data typology. A drawback of BSS techniques is the order of the sources that cannot be determined a priori; for this reason 3 methods (based on different statistical principles) have been developed for the retrieval of cardiac signals. The last part of the thesis deals with the application of BSS methods to a category of signals not yet analyzed: patients with ICD implanted. It is shown that it is possible to extract the cardiac signal also in such noisy data, although not automatically. The Temporal Decorrelation source SEParation (TDSEP) algorithm outperforms the other BSS methods. This thesis shows that, applying novel automatic routines for the removal of noise and artifacts, MCG data could be used in clinical environments

BioSig: The Free and Open Source Software Library for Biomedical Signal Processing

BioSig is an open source software library for biomedical signal processing. The aim of the BioSig project is to foster research in biomedical signal processing by providing free and open source software tools for many different application areas. Some of the areas where BioSig can be employed are neuroinformatics, brain-computer interfaces, neurophysiology, psychology, cardiovascular systems, and sleep research. Moreover, the analysis of biosignals such as the electroencephalogram (EEG), electrocorticogram (ECoG), electrocardiogram (ECG), electrooculogram (EOG), electromyogram (EMG), or respiration signals is a very relevant element of the BioSig project. Specifically, BioSig provides solutions for data acquisition, artifact processing, quality control, feature extraction, classification, modeling, and data visualization, to name a few. In this paper, we highlight several methods to help students and researchers to work more efficiently with biomedical signals

The Development Of Biomagnetic Systems : Planar Gradiometers And Software Tools

This thesis is concerned with two aspects of the design and construction of biomagnetic systems. Firstly, it considers the optimum design of planar gradiometers. The modelling of gradiometers is discussed and an algorithm for optimising the sensitivity of a specific type of gradiometer is presented. A test thin-film procedure for the manufacture of a planar gradiometer is outlined. The performance of three different types of gradiometer in recovering test current distributions, using a distributed current analysis technique, is assessed. Secondly, four major software tools that are essential in the analysis of data from large multi-channel biomagnetic systems are presented. These tools are then used to analyze data from a visual evoked response experiment. The system used to collect data was the Helsinki multi-channel system which consists of 24 planar gradiometers. The results confirm the retinotopic mapping of visual field information, and suggest that the time evolution of activity in different parts of the visual cortex is similar for early latencies

Use of Multiscale Entropy to Characterize Fetal Autonomic Development

The idea that uterine environment and adverse events during fetal development could increase the chances of the diseases in adulthood was first published by David Barker in 1998. Since then, investigators have been employing several methods and methodologies for studying and characterizing the ontological development of the fetus, e.g., fetal movement, growth and cardiac metrics. Even with most recent and developed methods such as fetal magnetocardiography (fMCG), investigators are continuously challenged to study fetal development; the fetus is inaccessible. Finding metrics that realize the full capacity of characterizing fetal ontological development remains a technological challenge. In this thesis, the use and value of multiscale entropy to characterize fetal maturation across third trimester of gestation is studied. Using multiscale entropy obtained from participants of a clinical trial, we show that MSE can characterize increasing complexity due to maturation in the fetus, and can distinguish a growing and developing fetal system from a mature system where loss of irregularity is due to compromised complexity from increasing physiologic load. MSE scales add a nonlinear metric that seems to accurately reflect the ontological development of the fetus and hold promise for future use to investigate the effects of maternal stress, intrauterine growth restriction, or predict risk for sudden infant death syndrome

The Development Of Biomagnetic Systems : Planar Gradiometers And Software Tools

This thesis is concerned with two aspects of the design and construction of biomagnetic systems. Firstly, it considers the optimum design of planar gradiometers. The modelling of gradiometers is discussed and an algorithm for optimising the sensitivity of a specific type of gradiometer is presented. A test thin-film procedure for the manufacture of a planar gradiometer is outlined. The performance of three different types of gradiometer in recovering test current distributions, using a distributed current analysis technique, is assessed. Secondly, four major software tools that are essential in the analysis of data from large multi-channel biomagnetic systems are presented. These tools are then used to analyze data from a visual evoked response experiment. The system used to collect data was the Helsinki multi-channel system which consists of 24 planar gradiometers. The results confirm the retinotopic mapping of visual field information, and suggest that the time evolution of activity in different parts of the visual cortex is similar for early latencies

Independent component analysis of magnetoencephalographic signals

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Single Epoch Analysis and Bi-hemisphere Study of Magnetoencephalographic (MEG) Signals using Vector Signal Transformation V3 and Magnetic Field Tomography (MFT)

The biomagnetic inverse problem has no unique solution, nevertheless even a cursory look at the features shown in raw signal can often suffice to highlight strong superficial activity. To do a proper single epoch analysis is normally prohibitively expensive in terms of computing demands. Hence the original aim of this thesis was to use simple efficient signal transformations to characterize superficial generators and contrast the single epoch signature with that extracted from the average signal. The results have intrigued us sufficiently to go beyond the original goal and extract very preliminary estimates of activity across the cerebral hemisphere in single trials. The original tool, and one that we have used for much of the work, is a simple vector signal transformation called V3. This signal transformation highlights nearby sources; it is a crude but quick estimator of generators directly from the raw MEG signals. Together with Magnetic Field Tomography (MFT), which relies on distributed source analysis of the MEG signals, we have tackled the following specific problems relating to aspects of normal brain function: efficient estimation of generators of magnetic fields; relationship between the average signal and single trials; and interhemispheric differences and relationship between the activity in the left and right hemispheres of the brain. During the project, we have used as examples auditory evoked MEG measurements obtained from two multichannel systems and applied the V3 and MFT analysis to both the average and single trial signals. In particular, we chose the 40-Hz (or gamma band) auditory response as the study subject. We found that in single epochs similar patterns of high frequency activity are observed in the area around the auditory cortex well before, close to and well after stimulus onset; the sequence of events observed in the average can only represent the evolution of events in single trials in a statistical way; and deep and central areas of the brain may be the seeds for the main deflections observed in the auditory responses

Detection and Magnetic Source Imaging of Fast Oscillations (40–160 Hz) Recorded with Magnetoencephalography in Focal Epilepsy Patients

We present a framework to detect fast oscillations (FOs) in magnetoencephalography (MEG) and to perform magnetic source imaging (MSI) to determine the location and extent of their generators in the cortex. FOs can be of physiologic origin associated to sensory processing and memory consolidation. In epilepsy, FOs are of pathologic origin and biomarkers of the epileptogenic zone. Seventeen patients with focal epilepsy previously confirmed with identified FOs in scalp electroencephalography (EEG) were evaluated. To handle data deriving from large number of sensors (275 axial gradiometers) we used an automatic detector with high sensitivity. False positives were discarded by two human experts. MSI of the FOs was performed with the wavelet based maximum entropy on the mean method. We found FOs in 11/17 patients, in only one patient the channel with highest FO rate was not concordant with the epileptogenic region and might correspond to physiologic oscillations. MEG FOs rates were very low: 0.02–4.55 per minute. Compared to scalp EEG, detection sensitivity was lower, but the specificity higher in MEG. MSI of FOs showed concordance or partial concordance with proven generators of seizures and epileptiform activity in 10/11 patients. We have validated the proposed framework for the non-invasive study of FOs with MEG. The excellent overall concordance with other clinical gold standard evaluation tools indicates that MEG FOs can provide relevant information to guide implantation for intracranial EEG pre-surgical evaluation and for surgical treatment, and demonstrates the important added value of choosing appropriate FOs detection and source localization methods.Facultad de IngenieríaInstituto de Investigaciones en Electrónica, Control y Procesamiento de Señale