16 research outputs found
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 ICDâ 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
Independent component analysis of magnetoencephalographic signals
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Innovative Processing Algorithms for Fetal Magnetoencephalographic Data
Fetale Magnetenzepahalographie (fMEG) ermöglicht die Untersuchung der Entwicklung
des zentralen und des autonomen Nervensystems bei Feten ab der 20. Schwangerschaftswoche.
Wie normale Magnetenzephalographie bei Erwachsenen und Kindern ist
auch fMEG eine nicht-invasive Methode und in der Anwendung vollkommen harmlos
fĂŒr Mutter und Kind. Die magnetischen Sensoren sind hierbei um das Abdomen der
schwangeren Frau angeordnet. Die gute rÀumliche und zeitliche Auflösung erlaubt es,
mĂŒtterliche und fetale Magnetokardiogramme gleichzeitig mit der fetalen HirnaktivitĂ€t
zu messen. Die Signale der fetalen Magnetoenzephalographie werden vor allem zur
Messung von auditiven und visuellen ereignisbezogenen Hirnreaktionen oder der spontanen
HirnaktivitÀt verwendet. Wichtige Fragen zum Entwicklungsprozess des fetalen
Gehirns und des autonomen Nervensystems sowie der mĂŒtterliche Einfluss auf den
metabolischen und kognitiven Zustand des Neugeborenen können durch die Analyse der
fetalen Magnetoenzephalographie-Signale geklÀrt werden.
Die Auswertung der fetalen HirnaktivitÀt birgt einige Herausforderungen, da die Signale
der fetalen und mĂŒtterlichen HerzaktivitĂ€t etwa 10-1000 mal stĂ€rker sind als das fetale
Hirnsignal. Daher ist es zwingend erforderlich, die HerzaktivitÀt der Mutter und des Fetus
zu erkennen und zu entfernen, bevor die fetale HirnaktivitÀt analysiert wird.
Die derzeit verwendeten Methoden fĂŒr die Erkennung und Entfernung der HerzaktivitĂ€t
funktionieren fĂŒr die meisten DatensĂ€tze zuverlĂ€ssig, die Verarbeitung enthĂ€lt jedoch
einige manuelle Schritte, was das Ganze sehr zeitaufwĂ€ndig macht. DarĂŒber hinaus ist die
Signal Redistribution beim Entfernen der HerzaktivitÀt ein bekanntes Problem, welches
es schwierig macht, die HirnaktivitÀt spÀter zu identifizieren. Das Ziel dieser Arbeit war
es, die Auswertung der fMEG Daten schneller, besser und trotzdem leicht handhabbar zu
machen.
In dieser Arbeit werden zwei neue vollautomatisierte Methoden zur Erkennung und Entfernung
der HerzaktivitÀt vorgestellt. Der vollautomatisierte R-Peak Erkennungsalgorithmus
(FLORA) verbessert die R-Peak Erkennung, indem er die Vorteile der zuvor verwendeten
Methoden kombiniert und erweitert. Der Algorithmus zur vollautomatisierten Subtraktion
der HerzaktivitÀt (FAUNA) verbessert die SignalqualitÀt und vereinfacht die Erkennung
der HirnaktivitÀt, ohne Redistribution. Die ZuverlÀssigkeit der Daten wird dadurch erhöht,
da keine manuelle Auswahl getroffen werden muss. Die Kombination beider Methoden in
einem Programm zur vollautomatisierten Verarbeitung fĂŒr die fetale Magnetoenzephalographie
(FAIRY) macht die Datenauswertung nun einfach und schnell. Damit wird die
fMEG Datenverarbeitung auf die "Big Data"- und "Automated Science"-Ăra vorbereitet.
Des Weiteren wurde eine Studie ĂŒber die autonome und zentralnervöse Reaktion von Feten
und Neugeborenen auf die mĂŒtterliche Stimme (AURORA) mit den neuen Datenverarbeitungsmethoden
durchgefĂŒhrt. Die Ergebnisse zeigten eine reduzierte Bewegung der
Feten zwischen der 26. und 32. Schwangerschaftswoche und eine niedrigere Herzfrequenz
wÀhrend der ersten 20 Sekunden der Stimulation in den letzten Schwangerschaftswochen,
als Reaktion auf die mĂŒtterliche Stimme. ZusĂ€tzlich fanden wir eine höhere Amplitude der
Gehirnreaktion als Reaktion auf eine fremde Frauenstimme bei Neugeborenen.Fetal magnetoencephalography (fMEG) facilitates the investigation of both the nature
and development of the fetal central and autonomic nervous system, starting at 20
weeks of gestational age. Like magnetoencephalography in children and adults, fetal
magnetoencephalography is a noninvasive method and therefore completely harmless for
both the mother and the child. Magnetic sensors in fMEG devices are arranged around
the abdomen of the pregnant woman. The good spatial and temporal resolution allows to
measure maternal and fetal magnetocardiograms simultaneously with fetal brain activity.
The fMEG signals are mainly used to measure the auditory and visual event-related
brain responses or the spontaneous brain activity. Important questions concerning the
developmental process of the fetal brain, as well as the maternal influence on the metabolic
and cognitive state of the newborn, can be clarified by the analysis of fMEG signals.
The evaluation of the fetal brain activity poses some challenges, as the signals of fetal and
maternal heart activity are 10-1000 times stronger than the fetal brain signal. Therefore,
it is mandatory to detect and remove the heart activity of both the mother and the fetus
before analyzing the fetal brain activity. The currently used methods for this detection and
removal work well for most datasets, but the processing includes numerous manual steps
and is therefore very time consuming. Furthermore, signal redistribution is a problem with
the current methods, which makes later detection of the fetal brain activity challenging.
The aim of this work was to make the evaluation of fMEG data faster, better and nevertheless,
easy to use. In this thesis two new fully-automated procedures for the detection
and removal of the heart activity are presented. The fully automated R-peak detection
algorithm (FLORA) improves R-peak detection by combining and extending the advantages
of the previously used methods. The algorithm for the fully automated subtraction
of heart activity (FAUNA) improves the signal quality and facilitates detection of brain
activity without the problem of redistribution. Furthermore these methods lead to a higher
reliability of the data analysis since no manual interventions are necessary. Combining
both methods in a tool for fully automated processing for fetal magnetoencephalography
(FAIRY) makes data evaluation now easy and fast. This prepares the processing of fMEG
data for the era of "Big Data" and "Automated Science".
Additionally a study about the fetal and neonatal autonomous and central nervous response
to maternal voice (AURORA) was performed using the new data processing methods. The
results showed a reduced movement of fetuses between 26 and 32 weeks of pregnancy and
a lower heart rate during the fist 20 seconds of stimulation in the last weeks of pregnancy
as a reaction to maternal voice. We additionally found a higher amplitude of the brain
response to voice onset of a stranger female voice in newborns
High-temperature superconducting magnetometers for on-scalp MEG
In the growing field of on-scalp magnetoencephalography (MEG), brain activity is studied by non-invasively mapping the magnetic fields generated by neuronal currents with sensors that are flexibly placed in close proximity to the subject\u27s head. This thesis focuses on high-temperature superconducting magnetometers made from YBa2Cu3Ox-7 (YBCO), which enables a reduction in the sensor-to-room temperature standoff distance from roughly 2 cm (for conventional MEG systems) down to 1 mm. Because of the higher neuromagnetic signal magnitudes available to on-scalp sensors, simulations predict that even a relatively low-sensitivity (higher noise) full-head on-scalp MEG system can extract more information about brain activity than conventional systems.In the first part of this thesis, the development of high critical temperature (high-Tc) superconducting quantum interference device (SQUID) magnetometers for a 7-channel on-scalp MEG system is described. The sensors are single layer magnetometers with a directly coupled pickup loop made on 10 mm
7 10 mm substrates using bicrystal grain boundary Josephson junctions. We found that the kinetic inductance strongly varies with film quality and temperature. Determination of all SQUID parameters by combining measurements and inductance simulations led to excellent agreement between experimental results and theoretical predictions. This allowed us to perform an in-depth magnetometer optimization. The best magnetometers achieve a magnetic field noise level of 44 fT/âHz at 78 K. Fabricated test SQUIDs provide evidence that noise levels below 30 fT/âHz are possible for high quality junctions with fairly low critical currents and in combination with the optimized pickup loop design. Different feedback methods for operation in a densely-packed on-scalp MEG system were also investigated. Direct injection of current into the SQUID loop was identified as the best on-chip feedback method with feedback flux crosstalk below 0.5%. By reducing the operation temperature, the noise level can be further reduced, however, the effective area also decreases because of the decreasing kinetic inductance contribution. We present a method that allows for one-time sensor calibration independent of temperature.In the second part, the design, operation, and performance of the constructed 7-channel on-scalp MEG system based on the fabricated magnetometers is presented. With a dense (2 mm edge-to-edge) hexagonal head-aligned array, the system achieves a small sensor-to-head standoff distance of 1-3 mm and dense spatial sampling. The magnetic field noise levels are 50-130 fT/âHz and the sensor-to-sensor feedback flux crosstalk is below 0.6%. MEG measurements with the system demonstrate the feasibility of the approach and indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG.In the third part, two alternative magnetometer types are studied for the next generation system. The first alternative is magnetometers based on Dayem bridge junctions instead of bicrystal grain boundary junctions. With a magnetometer based on the novel grooved Dayem bridge junctions, a magnetic field noise level of 63 fT/âHz could be achieved, which shows that Dayem bridge junctions are starting to become a viable option for single layer magnetometers. The second alternative are high-Tc SQUID magnetometers with an inductively coupled flux transformer. The best device with bicrystal grain boundary junctions reaches a magnetic field noise level below 11 fT/âHz and outperforms the best single layer device for frequencies above 20 Hz.In the last part, the potential of kinetic inductance magnetometers (KIMs) is investigated. We demonstrate the first high-Tc KIMs, which can be operated in fields of 9-28 \ub5T and achieve a noise level of 4 pT/âHz at 10 kHz
Extraction et débruitage de signaux ECG du foetus.
Les malformations cardiaques congĂ©nitales sont la premiĂšre cause de dĂ©cĂšs liĂ©s Ă une anomalie congĂ©nitale. L electrocardiogramme du fĆtus (ECGf), qui est censĂ© contenir beaucoup plus d informations par rapport aux mĂ©thodes Ă©chographiques conventionnelles, peut ĂȘtre mesurĂ© e par des Ă©lectrodes sur l abdomen de la mĂšre. Cependant, il est tres faible et mĂ©langĂ© avec plusieurs sources de bruit et interfĂ©rence y compris l ECG de la mĂšre (ECGm) dont le niveau est trĂšs fort. Dans les Ă©tudes prĂ©cĂ©dentes, plusieurs mĂ©thodes ont Ă©tĂ© proposĂ©es pour l extraction de l ECGf Ă partir des signaux enregistrĂ©s par des Ă©lectrodes placĂ©es Ă la surface du corps de la mĂšre. Cependant, ces mĂ©thodes nĂ©cessitent un nombre de capteurs important, et s avĂšrent inefficaces avec un ou deux capteurs. Dans cette Ă©tude trois approches innovantes reposant sur une paramĂ©trisation algĂ©brique, statistique ou par variables d Ă©tat sont proposĂ©es. Ces trois mĂ©thodes mettent en Ćuvre des modĂ©lisations diffĂ©rentes de la quasi-pĂ©riodicitĂ© du signal cardiaque. Dans la premiĂšre approche, le signal cardiaque et sa variabilitĂ© sont modĂ©lisĂ©s par un filtre de Kalman. Dans la seconde approche, le signal est dĂ©coupĂ© en fenĂȘtres selon les battements, et l empilage constitue un tenseur dont on cherchera la dĂ©composition. Dans la troisiĂšme approche, le signal n est pas modĂ©lisĂ© directement, mais il est considĂ©rĂ© comme un processus Gaussien, caractĂ©risĂ© par ses statistiques Ă l ordre deux. Dans les diffĂ©rentes modĂšles, contrairement aux Ă©tudes prĂ©cĂ©dentes, l ECGm et le (ou les) ECGf sont modĂ©lisĂ©s explicitement. Les performances des mĂ©thodes proposĂ©es, qui utilisent un nombre minimum de capteurs, sont Ă©valuĂ©es sur des donnĂ©es synthĂ©tiques et des enregistrements rĂ©els, y compris les signaux cardiaques des fĆtus jumeaux.Congenital heart defects are the leading cause of birth defect-related deaths. The fetal electrocardiogram (fECG), which is believed to contain much more information as compared with conventional sonographic methods, can be measured by placing electrodes on the mother s abdomen. However, it has very low power and is mixed with several sources of noise and interference, including the strong maternal ECG (mECG). In previous studies, several methods have been proposed for the extraction of fECG signals recorded from the maternal body surface. However, these methods require a large number of sensors, and are ineffective with only one or two sensors. In this study, state modeling, statistical and deterministic approaches are proposed for capturing weak traces of fetal cardiac signals. These three methods implement different models of the quasi-periodicity of the cardiac signal. In the first approach, the heart rate and its variability are modeled by a Kalman filter. In the second approach, the signal is divided into windows according to the beats. Stacking the windows constructs a tensor that is then decomposed. In a third approach, the signal is not directly modeled, but it is considered as a Gaussian process characterized by its second order statistics. In all the different proposed methods, unlike previous studies, mECG and fECG(s) are explicitly modeled. The performances of the proposed methods, which utilize a minimal number of electrodes, are assessed on synthetic data and actual recordings including twin fetal cardiac signals.SAVOIE-SCD - Bib.Ă©lectronique (730659901) / SudocGRENOBLE1/INP-Bib.Ă©lectronique (384210012) / SudocGRENOBLE2/3-Bib.Ă©lectronique (384219901) / SudocSudocFranceF
Signal processing algorithms for enhanced image fusion performance and assessment
The dissertation presents several signal processing algorithms for image fusion in noisy multimodal
conditions. It introduces a novel image fusion method which performs well for image
sets heavily corrupted by noise. As opposed to current image fusion schemes, the method has
no requirements for a priori knowledge of the noise component. The image is decomposed with
Chebyshev polynomials (CP) being used as basis functions to perform fusion at feature level. The
properties of CP, namely fast convergence and smooth approximation, renders it ideal for heuristic
and indiscriminate denoising fusion tasks. Quantitative evaluation using objective fusion assessment
methods show favourable performance of the proposed scheme compared to previous efforts
on image fusion, notably in heavily corrupted images.
The approach is further improved by incorporating the advantages of CP with a state-of-the-art
fusion technique named independent component analysis (ICA), for joint-fusion processing
based on region saliency. Whilst CP fusion is robust under severe noise conditions, it is prone to
eliminating high frequency information of the images involved, thereby limiting image sharpness.
Fusion using ICA, on the other hand, performs well in transferring edges and other salient features
of the input images into the composite output. The combination of both methods, coupled with
several mathematical morphological operations in an algorithm fusion framework, is considered a
viable solution. Again, according to the quantitative metrics the results of our proposed approach
are very encouraging as far as joint fusion and denoising are concerned.
Another focus of this dissertation is on a novel metric for image fusion evaluation that is based
on texture. The conservation of background textural details is considered important in many fusion
applications as they help define the image depth and structure, which may prove crucial in
many surveillance and remote sensing applications. Our work aims to evaluate the performance of image fusion algorithms based on their ability to retain textural details from the fusion process.
This is done by utilising the gray-level co-occurrence matrix (GLCM) model to extract second-order
statistical features for the derivation of an image textural measure, which is then used to
replace the edge-based calculations in an objective-based fusion metric. Performance evaluation
on established fusion methods verifies that the proposed metric is viable, especially for multimodal
scenarios
Brain and Human Body Modeling
This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this bookâs coverage of the latest developments in computational modelling and human phantom development to assess a given technologyâs safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields
Brain and Human Body Modeling
This open access book describes modern applications of computational human modeling with specific emphasis in the areas of neurology and neuroelectromagnetics, depression and cancer treatments, radio-frequency studies and wireless communications. Special consideration is also given to the use of human modeling to the computational assessment of relevant regulatory and safety requirements. Readers working on applications that may expose human subjects to electromagnetic radiation will benefit from this bookâs coverage of the latest developments in computational modelling and human phantom development to assess a given technologyâs safety and efficacy in a timely manner. Describes construction and application of computational human models including anatomically detailed and subject specific models; Explains new practices in computational human modeling for neuroelectromagnetics, electromagnetic safety, and exposure evaluations; Includes a survey of modern applications for which computational human models are critical; Describes cellular-level interactions between the human body and electromagnetic fields