La cartographie du temps de relaxation longitudinale T1 est une technique d'IRM quantitative pour caractériser les tissus myocardiques. Plusieurs études ont déjà montré la corrélation entre la mesure de T1 et la présence de fibrose. Celle-ci est souvent observée dans les pathologies cardiaques telles que les cardiomyopathies ou l'infarctus du myocarde. Cependant, l'acquisition d'une carte T1 du coeur reste techniquement difficile. Actuellement, la quantification T1 du myocarde humain est réalisée en apnée à l'aide de séquences 2D qui sont spécifiques aux constructeurs et donc peu disponibles. Afin de pallier aux limitations de ces séquences, nous proposons une méthode basée sur une séquence 3D clinique. Cette technique, utilisant la variation des angles de bascule avec intégration d'une correction B1, a été adaptée pour une utilisation en imagerie cardiaque. Des essais sur fantôme ont permis de sélectionner les paramètres optimaux et de montrer la reproductibilité de la méthode. Puis, une étude sur volontaires sains a permis de valider la méthode en double synchronisation (cardiaque et respiratoire). Enfin, une méthode de reconstruction intégrant des signaux physiologiques de mouvement a également été utilisée afin de faire de la quantification T1 en respiration libre et de diminuer le temps d'acquisition. Les valeurs de T1 myocardique sur volontaires sont comprises entre 1289 ± 66 ms et 1376 ± 43 ms, correspondant aux valeurs de la littérature. Ces travaux ouvrent la voie à l'utilisation de la cartographie T1 chez les patients avec pour objectifs une meilleure caractérisation des pathologies et une meilleure adaptation des stratégies thérapeutiques / T1 mapping is a useful quantitative MR technique for cardiac tissue characterization. Several studies have shown that T1 measurements are correlated with fibrosis, which is observed in cardiac diseases such as cardiomyopathy or myocardial infarction. However, cardiac T1 mapping remains challenging, mainly because of long acquisition times and interference from cardiac and respiratory motions. T1 quantification on the human myocardium is generally performed on breath-hold with 2D specific sequences. Unfortunately these sequences are scanner specific and poorly available for clinical use. To overcome these limitations, we propose a new method based on a 3D clinical sequence. This technique, using a variable flip angle approach that integrates B1 correction, was adapted in cardiac imaging. Phantom tests were used to select the optimal parameters and to show the method reproducibility. Then, the method was validated with a volunteer study using double synchronization (cardiac and respiratory). Moreover, a reconstruction method integrating physiological signals of motion was also used to perform T1 quantification in free breathing and to reduce the total acquisition time. The myocardial T1 values on volunteers ranged between 1289 ± 66 ms and 1376 ± 43 ms, which was in good agreement with previously published works. These studies allow the use of T1 mapping in patients with better characterization of pathologies and a better adaptation to therapeutic strategies

Ziel der Untersuchungen war es, neue Strategien zu entwickeln, die zu einer Verbesserung der MR-Diagnostik von Gliomen führen. Im Vordergrund des Interesses stand dabei die MR-Charakterisierung von experimentellen Gliomen mittels superparamagnetischer Eisenoxide, wobei MION ("Monocrystalline-Iron-Oxide-Nanopartikel") als Modellsubstanz für einen Großteil der Untersuchungen benutzt wurde. In Experimenten zur Blut-Hirn-Schranke (BHS) wurde gezeigt, dass normales Hirngewebe jenseits der BHS mit Eisenoxiden erreicht werden kann, wenn artifiziell die BHS zuvor hyperosmotisch durch Mannitol-Infusion temporär geöffnet wurde. Neben der intrazellulären Aufnahme in Astrozyten werden Eisenoxide nach erfolgter BHS-Öffnung in signifikant höherem Maße von aktivierter Mikroglia phagozytiert. Nach selektiver Öffnung der BHS durch Bradykinin-Injektion in die A. carotis interna lässt sich selektiv der Transport von Eisenoxiden in das Gliomzentrum vergrößern. Am experimentellen Gliommodell ließ sich zeigen, dass intravenöse MION-Gabe zu einem charakteristischen ringförmigen Erscheinungsbild in der MRT der großen Tumoren führt: Die histologischen Untersuchungen bei mehreren Gliomarten (C6 und 9L) zeigen eindeutig die Phagozytose von Eisenpartikeln durch Gliomzellen selbst. Verglichen mit der Eisenoxid-Aufnahme in die Gliomzelle ist die Phagozytose der Eisenpartikel durch ortsständige Mikrogliazellen und Blutmakrophagen jedoch signifikant größer. Die intrazelluläre Aufnahme von MION durch Tumorzellen lies sich in Zellkulturexperimenten an verschiedenen Gliom- (C6, 9L) und Karzinom-Zelllinien (LX-1) bestätigen. In vitro konnte gezeigt werden dass die Konjugation von Transferrin (Tf) an eine Eisenoxidverbindung zu einer verstärkten intrazelluläre Aufnahme verglichen mit unkonjugiertem Verbindungen führt. Die Untersuchungen zur Bildgebung der Tumorvaskularisation von experimentellen Gliomen ergaben, dass durch die kombinatorische Anwendung eines kleinmolekularen und eines großmolekularen Markers mit anschließender einfacher Bildsubtraktion die vaskulären und interstitiellen Volumenfraktionen (VVF, IVF) ermittelt werden können. Auch die alleinige Injektion der noch experimentellen Blut-Pool-Marker MPEG-Pl-GdDTPA und Gadomer-17 ermöglicht im Tiermodell die quantitative Bestimmung sowohl des vaskulären Volumens (CBV) als auch der Permeabilität (P). Bei den Eisenoxiden verfälschen deren starke Suszeptibilitätseffekte die quantitative Bestimmung von Blutflussdaten, auch die Quantifizierung der Gefäßpermeabilität ist mit diesen Verbindungen mittels dynanischer MRT nicht möglich. / The aim of the studies was to develop new strategies for improving magnetic resonance imaging (MRI) of gliomas. In the majority of experiments the focus was on the characterization of experimental gliomas after administration of superparamagnetic iron oxides using MION (Monocrystalline Iron Oxide Nanoparticles) as a model compound. Experiments on the blood-brain barrier (BBB) demonstrated that iron oxides reach normal brain tissue beyond the BBB after their artificial, transient hyperosmotic opening by mannitol infusion. Upon opening of the BBB, iron oxides not only show intracellular uptake by astrocytes but are also phagocytosed in significantly higher amounts by activated microglia. Selective opening of the BBB by bradykinin injection into the internal carotid artery specifically increases the transport of iron oxides into the center of gliomas. Using an experimental glioma model, it was shown that intravenous administration of MION produces a characteristic ring enhancement of large tumors on MR images. Histologic studies of different types of gliomas (C6 and 9L) unequivocally demonstrated that iron oxide particles were phagocytosed by the glioma cells themselves. However, iron oxide uptake by glioma cells is significantly less pronounced compared to the phagocytosis of iron oxide particles by local microglial cells and blood macrophages. The incorporation of MION by tumor cells was confirmed in cell culture experiments using different glioma (C6, 9L) and carcinoma cell lines (LX-1). In vitro studies showed that conjugation of transferrin to an iron oxide compound enhanced intracellular uptake compared to unconjugated compounds. The imaging studies investigating tumor vascularization in experimental gliomas demonstrated that the combined use of a small-molecular and a large-molecular marker and simple image subtraction allow for determining vascular and interstitial volume fractions (VVF, IVF). Furthermore, injection of the blood pool markers MPEG-P1-Gd-DTPA and gadomer-17 alone likewise enables quantitative determination of both vascular volume (CBV) and permeability (P) in the animal model. Iron oxide particles, on the other hand, have pronounced susceptibility effects, which impair the quantitative determination of blood flow data. Nor do the particles allow for quantifying vascular permeability by dynamic MR imaging.

Lokal parametrische Modelle werden häufig im Kontext der nichtparametrischen Schätzung verwendet. Bei einer punktweisen Schätzung der Zielfunktion können die parametrischen Umgebungen mithilfe von Gewichten beschrieben werden, die entweder von den Designpunkten oder (zusätzlich) von den Beobachtungen abhängen. Der Vergleich von verrauschten Beobachtungen in einzelnen Punkten leidet allerdings unter einem Mangel an Robustheit. Der Propagations-Separations-Ansatz von Polzehl und Spokoiny [2006] verwendet daher einen Multiskalen-Ansatz mit iterativ aktualisierten Gewichten. Wir präsentieren hier eine theoretische Studie und numerische Resultate, die ein besseres Verständnis des Verfahrens ermöglichen. Zu diesem Zweck definieren und untersuchen wir eine neue Strategie für die Wahl des entscheidenden Parameters des Verfahrens, der Adaptationsbandweite. Insbesondere untersuchen wir ihre Variabilität in Abhängigkeit von der unbekannten Zielfunktion. Unsere Resultate rechtfertigen eine Wahl, die unabhängig von den jeweils vorliegenden Beobachtungen ist. Die neue Parameterwahl liefert für stückweise konstante und stückweise beschränkte Funktionen theoretische Beweise der Haupteigenschaften des Algorithmus. Für den Fall eines falsch spezifizierten Modells führen wir eine spezielle Stufenfunktion ein und weisen eine punktweise Fehlerschranke im Vergleich zum Schätzer des Algorithmus nach. Des Weiteren entwickeln wir eine neue Methode zur Entrauschung von diffusionsgewichteten Magnetresonanzdaten. Unser neues Verfahren (ms)POAS basiert auf einer speziellen Beschreibung der Daten, die eine zeitgleiche Glättung bezüglich der gemessenen Positionen und der Richtungen der verwendeten Diffusionsgradienten ermöglicht. Für den kombinierten Messraum schlagen wir zwei Distanzfunktionen vor, deren Eignung wir mithilfe eines differentialgeometrischen Ansatzes nachweisen. Schließlich demonstrieren wir das große Potential von (ms)POAS auf simulierten und experimentellen Daten. / In statistics, nonparametric estimation is often based on local parametric modeling. For pointwise estimation of the target function, the parametric neighborhoods can be described by weights that depend on design points or on observations. As it turned out, the comparison of noisy observations at single points suffers from a lack of robustness. The Propagation-Separation Approach by Polzehl and Spokoiny [2006] overcomes this problem by using a multiscale approach with iteratively updated weights. The method has been successfully applied to a large variety of statistical problems. Here, we present a theoretical study and numerical results, which provide a better understanding of this versatile procedure. For this purpose, we introduce and analyse a novel strategy for the choice of the crucial parameter of the algorithm, namely the adaptation bandwidth. In particular, we study its variability with respect to the unknown target function. This justifies a choice independent of the data at hand. For piecewise constant and piecewise bounded functions, this choice enables theoretical proofs of the main heuristic properties of the algorithm. Additionally, we consider the case of a misspecified model. Here, we introduce a specific step function, and we establish a pointwise error bound between this function and the corresponding estimates of the Propagation-Separation Approach. Finally, we develop a method for the denoising of diffusion-weighted magnetic resonance data, which is based on the Propagation-Separation Approach. Our new procedure, called (ms)POAS, relies on a specific description of the data, which enables simultaneous smoothing in the measured positions and with respect to the directions of the applied diffusion-weighting magnetic field gradients. We define and justify two distance functions on the combined measurement space, where we follow a differential geometric approach. We demonstrate the capability of (ms)POAS on simulated and experimental data.

Les matériaux de chaussée dits “à froid”, formés d’un mélange de granulats et d’une émulsion de bitume, constituent une alternative plus économique et respectueuse de l’environnement aux matériaux de chaussée classiques. Cependant, ces mélanges incorporent une quantité significative d’eau, retardant la consolidation du mélange granulaire. L’objectif de cette thèse est de comprendre le déroulement du séchage au sein de l’enrobé, un milieu granulaire poreux saturé en émulsion de bitume, et quel en est l’effet sur sa structure et ses propriétés. Les principaux mécanismes du séchage de milieux poreux simples initialement saturés en eau sont relativement bien connus : celui-ci se déroule selon une longue première période à vitesse constante associée à une désaturation homogène du milieu et durant laquelle environ 90% d’eau est extraite, suivie d’une période à vitesse décroissante. Quelques travaux ont récemment montré que l’on peut largement s’éloigner de ce schéma dès que le fluide interstitiel est plus complexe (solution saline, suspension, gel), du fait du transport et/ou de l’accumulation des composants non-évaporables contenus dans le fluide. Nous avons choisi de décomposer le problème en étudiant d’abord le séchage du fluide complexe seul, autrement dit l’émulsion de bitume (ainsi que des émulsions modèles de silicone afin de mieux comprendre les mécanismes), puis le séchage de cette même émulsion en milieu poreux. Notre travail s’appuie sur le suivi « macroscopique » du séchage (i.e. pesée), et un suivi des caractéristiques internes (distribution de de liquide au sein du matériau) par Imagerie par Résonance Magnétique, qui fournit des informations originales très complètes. Nous montrons ainsi que l’extraction d’eau résultant du séchage d’émulsions modèles de silicone et de bitume seules est essentiellement homogène sur toute l’épaisseur (centimétrique) de l’émulsion, et associée à la compaction et la déformation progressives des gouttes de phase dispersée. Cependant, un gradient de concentration existe près de la surface libre et il s’avère que dans cette zone la coalescence de gouttes de phase dispersée ralentit très fortement le séchage. Nous montrons ensuite que la vitesse de séchage de milieux poreux initialement saturés en émulsion diminue dès le départ et continuellement jusqu’à de très faibles valeurs. La formation et la progression rapide d’un front sec dans le milieu poreux ainsi que l’absence de migration de bitume dans le milieu poreux, observés par IRM, permettent d’expliquer en partie ce phénomène, mais il faut probablement y ajouter les caractéristiques propres du séchage de l’émulsion de bitume, qui contribuent également à ralentir la vitesse d’évaporation / The preparation of cold mix asphalts, composed of aggregates and bitumen emulsion, represent a substantial economic and environmental potential but the presence of water delays the strengthening of the material. Our objective is to understand how the mix dries and how this affects its properties. The drying of pure liquids in simple porous media has been extensively studied : a constant drying rate period as well as a homogeneous distribution of water are observed until 90% of the water has been extracted; afterwards the drying rate decreases. Studies have shown that the drying of complex fluids (ionic solutions, suspensions, gels) is very different due to the transport and/or accumulation of the fluid’s non-vaporizable elements. We chose to study first the drying of the complex fluid i.e. the bitumen emulsion as well as silicone oil emulsions to understand better the drying mechanisms, then the drying of the complex fluid in a porous media. We used Magnetic Resonance Imaging (MRI) to monitor the internal characteristics of our systems. This allowed us to show that water is extracted uniformly over the drying emulsion’s entire thickness ($approx$cm), leading to the progressive droplet compaction and deformation. However, a water concentration gradient forms near the free surface, which ultimately slows the drying as droplets coalesce. We then show that the drying rate of a porous medium initially saturated with emulsion decreases from the beginning of drying. The rapid formation and progression of a dry area in the porous medium and the absence of bitumen transport partly explain this but it is very likely that the bitumen emulsion’s drying characteristics contribute to the decreasing drying rate

Doctor of Philosophy (PhD) / Schizophrenia is a severe mental illness that affects approximately 1% of the population worldwide, and which typically has a devastating effect on the lives of its sufferers. The characteristic symptoms of the disease include hallucinations, delusions, disorganized thought and reduced emotional expression. While many of the early theories of schizophrenia focused on its psychosocial foundations, more recent theories have focused on the neurobiological underpinnings of the disease. This thesis has four primary aims: 1) to use magnetic resonance imaging (MRI) to identify the structural brain abnormalities present in patients suffering from their first episode of schizophrenia (FES), 2) to elucidate whether these abnormalities were static or progressive over the first 2-3 years of patients’ illness, 3) to identify the relationship between these neuroanatomical abnormalities and patients’ clinical profile, and 4) to identify the normative relationship between longitudinal changes in neuroanatomy and electrophysiology in healthy participants, and to compare this to the relationship observed between these two indices in patients with FES. The aim of Chapter 2 was to use MRI to identify the neuroanatomical changes that occur over adolescence in healthy participants, and to identify the normative relationship between the neuroanatomical changes and electrophysiological changes associated with healthy periadolescent brain maturation. MRI and electroencephalographic (EEG) scans were acquired from 138 healthy participants between the ages of 10 and 30 years. The MRI scans were segmented into grey matter (GM) and white matter (WM) images, before being parcellated into the frontal, temporal, parietal and occipital lobes. Absolute EEG power was calculated for the slow-wave, alpha and beta frequency bands, for the corresponding cortical regions. The age-related changes in regional tissue volumes and regional EEG power were inferred with a regression model. The results indicated that the healthy participants experienced accelerated GM loss, EEG power loss and WM gain in the frontal and parietal lobes between the ages of 10 and 20 years, which decelerated between the ages of 20 and 30 years. A linear relationship was also observed between the maturational changes in regional GM volumes and EEG power in the frontal and parietal lobes. These results indicate that the periadolescent period is a time of great structural and electrophysiological change in the healthy human brain. The aim of Chapter 3 was to identify the GM abnormalities present in patients with FES, both at the time of their first presentation to mental health services (baseline), and over the first 2-3 years of their illness (follow-up). MRI scans were acquired from 41 patients with FES at baseline, and 47 matched healthy control subjects. Of these participants, 25 FES patients and 26 controls returned 2-3 years later for a follow-up scan. The analysis technique of voxel-based morphometry (VBM) was used in conjunction with the Statistical Parametric Mapping (SPM) software package in order to identify the regions of GM difference between the groups at baseline. The related analysis technique of tensor-based morphometry (TBM) was used to identify subjects’ longitudinal GM change over the follow-up interval. Relative to the healthy controls, the FES patients were observed to exhibit widespread GM reductions in the frontal, parietal and temporal cortices and cerebellum at baseline, as well as more circumscribed regions of GM increase, particularly in the occipital lobe. Furthermore, the FES patients lost considerably more GM over the follow-up interval than the controls, particularly in the parietal and temporal cortices. These results indicate that patients with FES exhibit significant structural brain abnormalities very early in the course of their illness, and that these abnormalities progress over the first few years of their illness. Chapter 4 employed the same methodology to investigate the white matter abnormalities exhibited by the FES subjects relative to the controls, both at baseline and over the follow-up interval. Compared to controls, the FES patients exhibited volumetric WM deficits in the frontal and temporal lobes at baseline, as well as volumetric increases at the fronto-parietal junction bilaterally. Furthermore, the FES patients lost considerably more WM over the follow-up interval than did the controls in the middle and inferior temporal cortex bilaterally. While there is substantial evidence indicating that abnormalities in the maturational processes of myelination play a significant role in the development of WM abnormalities in FES, the observed longitudinal reductions in WM were consistent with the death of a select population of temporal lobe neurons over the follow-up interval. The aim of Chapter 5 was to investigate the clinical correlates of the GM abnormalities exhibited by the FES patients at baseline. The volumes of four distinct cerebral regions where 31 patients with FES exhibited reduced GM volumes relative to 30 matched controls were calculated and correlated with patients’ scores on three primary symptom dimensions: Disorganization, Reality Distortion and Psychomotor Poverty. The results indicated that the greater the degree of atrophy exhibited by the FES patients in three of these four ‘regions-of-reduction’, the less severe their degree of Reality Distortion. These results suggest that an excessive amount of GM atrophy may in fact preclude the formation of hallucinations or highly systematized delusions in patients with FES. The aim of Chapter 6 was to identify the relationship between the longitudinal changes in brain structure and brain electrophysiology exhibited by 19 FES patients over the first 2-3 years of their illness, and to compare it to the normative relationship between the two indices reported in Chapter 2. The methodology employed for the parcellation of the MRI and EEG data was identical to Chapter 2. The results indicated that, in contrast to the healthy controls, the longitudinal reduction in GM volume exhibited by the FES patients was not associated with a corresponding reduction in EEG power in any brain lobe. In contrast, EEG power was observed to be maintained or even to increase over the follow-up interval in these patients. These results were consistent with the FES patients experiencing an abnormal elevation of neural synchrony. Such an abnormality in neural synchrony could potentially form the basis of the dysfunctional neural connectivity that has been widely proposed to underlie the functional deficits present in patients with schizophrenia. The primary aim of Chapter 7 was to assimilate the findings from the preceding empirical chapters with the theoretical framework provided in the literature, into an integrated and testable model of schizophrenia. The model emphasized dysfunctions in brain maturation, specifically in the normative processes of synaptic ‘pruning’ and axonal myelination, as playing a key role in the development of disintegrated neural activity and the subsequent onset of schizophrenic symptoms. The model concluded with the novel proposal that disintegrated neural activity arises from abnormal elevations in the synchrony of synaptic activity in patients with first-episode schizophrenia.

<p>Mice have become the preferred model system for studying brain function and disease. With the powerful genetic tools available, mouse models can be created to study the underlying molecular basis of neurobiology in vivo. Just as magnetic resonance imaging is the dominant tool for evaluating the human brain, high-resolution MRI--magnetic resonance microscopy (MRM)--is a useful tool for studying the brain of mouse models. However, the need for high spatial resolution limits the signal-to-noise ratio (SNR) of the MRM images. To address this problem, T1-shortening contrast agents can be used, which not only improve the tissue contrast-to-noise ratio (CNR) but also increase SNR by allowing the MR signal to recover faster between pulses. By "actively staining" the tissue with these T1-shortening agents, MRM can be performed with higher resolution, greater contrast, and shorter scan times. In this work, active staining with T1-shortening agents was used to enhance three types of in vivo mouse brain MRM: (1) angiographic imaging of the neurovasculature, (2) anatomical imaging of the brain parenchyma, and (3) functional imaging of neuronal activity.</p> <p></p> <p>For magnetic resonance angiography (MRA) of the mouse, typical contrast agents are not useful because they are quickly cleared by the body and/or extravasate from the blood pool before a high-resolution image can be acquired. To address these limitations, a novel contrast agent--SC-Gd liposomes--has been developed, which is cleared slowly by the body and is too large to extravasate from the blood pool. In this work, MRA protocols were optimized for both the standard technique (time-of-flight contrast) and SC-Gd liposomes. When the blood was stained with SC-Gd liposomes, small vessel CNR improved to 250% that of time-of-flight. The SC-Gd liposomes could also be used to reduce scan time by 75% while still improving CNR by 32%.</p> <p>For MRM of the mouse brain parenchyma, active staining has been used to make dramatic improvements in the imaging of ex vivo specimens. However for in vivo imaging, the blood-brain barrier (BBB) prevents T1-shortening agents from entering the brain parenchyma. In this work, a noninvasive technique was developed for BBB opening with microbubbles and ultrasound (BOMUS). Using BOMUS, the parenchyma of the brain could be actively stained with the T1-shortening contrast agent, Gd-DTPA, and MRM images could be acquired in vivo with unprecedented resolution (52 x 52 x 100 micrometers3) in less than 1 hour.</p> <p>Functional MRI (fMRI), which uses blood oxygen level dependant (BOLD) contrast to detect neuronal activity, has been a revolutionary technique for studying brain function in humans. However, in mice, BOLD contrast has been difficult to detect and thus routine fMRI in mice has not been feasible. An alternative approach for detecting neuronal activity uses manganese (Mn2+). Mn2+ is a T1-shortening agent that can enter depolarized neurons via calcium channels. Thus, Mn2+ is a functional contrast agent with affinity for active neurons. In this work, Mn2+ (administered with the BOMUS technique) was used to map the neuronal response to stimulation of the vibrissae. The resultant activation map showed close agreement to published maps of the posterior-lateral and anterior-medial barrel field of the primary sensory cortex.</p> <p>The use of T1-shortening agents to actively stain tissues of interest--blood, brain parenchyma, or active neurons--will facilitate the use of MRM for studying mouse models of brain development, function, and disease.</p> / Dissertation

Recently, the use of permanent magnets as magnetic field sources in biomedical applications has become widespread. However, usage of permanent magnets in magnetic resonance imaging (MRI) is limited due to their inhomogeneous magnetic field distributions. In this thesis, shape and geometry optimization of a magnet is performed. Moreover, placement of more than one magnet is optimized to obtain desired magnetic field distribution in specific region of space. However, obtained magnetic field distribution can not be used in the conventional MRI image reconstruction techniques. Consequently, an image reconstruction technique for MRI in inhomogeneous magnetic fields is developed. Apart from these, since any reconstruction technique requires signal data, an MRI simulator in inhomogeneous magnetic fields is constructed as a part of this thesis. Obtained results show that the theory developed in this thesis is valid. Consequently, new MRI devices that have permanent magnets as magnetic field sources can be constructed in the future.

Mikrochips zur Tierkennzeichnung bestehen aus verschiedenen metallischen Materialien. Diese treten in der Magnetresonanztomographie in Wechselwirkung mit den elektromagnetischen Feldern. So verursachen die ferromagnetischen Materialen der Mikrochips gravierende fokale Bildstörungen. Diese Suszeptibilitätsartefakte können die Beurteilbarkeit der Halsregion erheblich einschränken. Ziel der Studie war, den Einfluss des Sequenztyps auf die Größe des Artefakts zu untersuchen und herauszufinden, welche Möglichkeiten zur Artefaktreduktion bei Veränderung bestimmter Sequenzparameter bestehen. Zusätzlich sollte geklärt werden, wie groß der Abstand zwischen Spinalkanal und Mikrochip mindestens sein muss, um spinale Strukturen beurteilen zu können. In das Untersuchungsgut der Studie gingen die Kadaver von 26 Katzen und 2 Hunden ein. An einem 0,5-Tesla-MRT wurde für verschiedene Sequenztypen (SE-Sequenzen, TSE-Sequenzen, GRE-Sequenzen) und Kombinationen modifizierter Sequenzparameter (Echozeit (TE), Voxelgröße, Ausleserichtung) das Ausmaß der Artefakte ermittelt. Berechnet wurde der Flächeninhalt des Artefakts (cm2). Dieser wurde dann als prozentualer Anteil zur Fläche des Halsquerschnitts angegeben. Diese Berechnung erfolgte für alle untersuchten Einstellungen an transversalen Aufnahmen. Eine ergänzende computertomografische Untersuchung dienste dazu, die Distanz zwischen Spinalkanalund Mikrochip zu messen. Die Untersuchungen der Studie haben gezeigt, dass TSE-Sequenzen wegen ihrer geringeren Artefaktanfälligkeit den SE- und GRE-Sequenzen vorgezogen werden sollten. Besonders kleine Artefakte konnten bei einer T1-TSE-Sequenz mit kleiner TE (10 ms) und kleiner Voxelgröße (große Akquisitionsmatrix von 256 x 256 Pixel, kleines Field of View (FOV) von 160 mm, geringe Schichtdicke (ST) von 2 mm) erreicht werden. Durch Anpassung der Kodierrichtung war es möglich, die Form und Richtung des Artefaktes zu beeinflussen. Lag das Zentrum des Mikrochips näher als 19 mm von der Mitte des Wirbelkanals entfernt, ließen sich auch mit dieser optimierten Sequenz die spinalen Strukturen auf Höhe des Mikrochips nicht beurteilen. Die Größe und Form der Suszeptibilitätsartefakte konnten durch die Wahl des Sequenztyps und Modifikation von Sequenzparametern verändert werden. Dies ist besonders bei kleinen Tieren von Bedeutung. Bei diesen kann es aufgrund der geringen Distanz zwischen Mikrochip und Wirbelsäule zur Beeinträchtigung der MR-Bildauswertung kommen. Eine T1-gewichtete TSE-Sequenz mit kleiner Echozeit (10 ms) und kleiner Voxelgröße (Akquisitionsmatrix 256 x 256 Pixel, FOV 160 mm, ST 2 mm) bietet bei 0,5 Tesla das größte Potenzial zur Artefaktreduktion.

No description available.

This thesis describes the electrical and physiological characterization of cardiac tissue with myocardial infarction (MI) responsible for abnormal cardiac rhythms such as ventricular tachycardia (VT), using a newly-developed magnetic resonance imaging (MRI) electrophysiology system. In electrophysiology (EP), radiofrequency (RF) catheter ablation combined with cardioverter-defibrillator implantation is a first-line action to manage ventricular VT. Unfortunately, this therapy is known to have sub-optimal success rates in a large number of patients because of difficulties to accurately identifying the arrhythmic target regions. Currently, characterization of post-MI scars is performed by using catheters to measure electrical signals of the endocardial tissue (electroanatomical mapping), under x-ray fluoroscopy guidance. Prolonged radiation exposure to both the cardiologist and the patient have made the use of MRI extremely attractive; further, unlike x-ray imaging, MRI provides post-MI scars with direct visualization, characterization in three dimensions and the ability to visualize ablation lesions. Although recent research has focused on registration between pre-acquired MR images and electroanatomical maps, a potentially more useful approach is to use real-time MRI to directly locate and characterize potential arrhythmogenic regions during the EP procedure. A real-time MR-guided EP system was developed and validated to perform EP diagnostic procedures, such as mapping and pacing. In a series of animal studies, the system demonstrated the ability to use active catheter tracking and intra-procedural MR imaging to navigate to specific regions in the left ventricle and record intracardiac electrical signals. A study correlating myocardial fibrotic scar detected by multicontrast late enhancement (MCLE) MRI and electroanatomical voltage mapping demonstrated that MRI information (transmurality, tissue classification, and relaxation rate) can accurately predict areas of myocardial fibrosis identified with bipolar voltage mapping. Finally, MCLE-derived gray zone was shown to have a high correspondence to regions with a high proportion of abnormal intracardiac signals. The methods described in this thesis help advance the understanding of infarcted tissue responsible for ventricular tachycardia. Further studies are proposed to perform RF ablation lesions and correlate pre- and post-ablation tissue electrophysiological properties with MRI.