Littérature scientifique sur le sujet « Ultra-High-Field (UHF) Magnetic Resonance Imaging (MRI) »

Magnetic resonance imaging (MRI) at ultra-high field (UHF) strengths (7 T and above) offers unique opportunities for studying the human brain with increased spatial resolution, contrast and sensitivity. However, its reliability can be compromised by factors such as head motion, image distortion and non-neural fluctuations of the functional MRI signal. The objective of this review is to provide a critical discussion of the advantages and trade-offs associated with UHF imaging, focusing on the application to studying brain–heart interactions. We describe how UHF MRI may provide contrast and resolution benefits for measuring neural activity of regions involved in the control and mediation of autonomic processes, and in delineating such regions based on anatomical MRI contrast. Limitations arising from confounding signals are discussed, including challenges with distinguishing non-neural physiological effects from the neural signals of interest that reflect cardiorespiratory function. We also consider how recently developed data analysis techniques may be applied to high-field imaging data to uncover novel information about brain–heart interactions.

Background:Fibrocartilaginous enthesis is composed of different histological zones which are commonly referred to the tendon distal extremity (a lamellar tissue with a low cell density, collagen and connective tissue), the fibrocartilaginous zone (with chondrocytes), a progressively mineralized zone and the bone. The MRI visualization of the water content of entheses is challenging given the very short relation time so that entheses has been very poorly assessed using MRI (1).Objectives:The main objective of the study was to assess the structural elements of the knee enthesis based on the quantitative T2* measurements using Ultra High Field (UHF) MRI.Methods:Twelve healthy subjects without any osteoarticular pathology were included in the study after they provided their informed consent. 3D gradient echo sequence with a 4.3 ms echo time and T2* mapping were performed. The lateral internal, external and crossed ligaments, patellar and quadricipital tendons were assessed. T2* measurements were performed specifically on the quadricipital tendon.Results:The quadricipital tendon and the bone trabeculation could be visualized on the UHF MR image. The T2* mapping analysis illustrated a large value (16.4 ± 4 ms) for the subchondral bone and much lower values for the trabecular bone (11 ± 4.5 ms) and the different zones of the keen entheses (7.7 ± 1.9 ms).Conclusion:Based on T2* measurements performed using UHF MRI, the different structural elements of the knee entheses were distinguished. This quantitative stratification could be used to assess changes in pathological conditions such as SpA and trauma.References:[1]Benjamin M, Bydder GM. Magnetic resonance imaging of entheses using ultrashort TE (UTE) pulse sequences. Journal of magnetic resonance imaging: JMRI. 2007;25(2):381-9.Disclosure of Interests:None declared

Improvements in transmission and reception sensitivities of radiofrequency (RF) coils used in ultra-high field (UHF) magnetic resonance imaging (MRI) are needed to reduce specific absorption rates (SAR) and RF power deposition, albeit without applying high-power RF. Here, we propose a method to simultaneously improve transmission efficiency and reception sensitivity of a band-pass birdcage RF coil (BP-BC RF coil) by combining a multi-channel wireless RF element (MCWE) with a high permittivity material (HPM) in a 7.0 T MRI. Electromagnetic field (EM-field) simulations, performed using two types of phantoms, viz., a cylindrical phantom filled with oil and a human head model, were used to compare the effects of MCWE and HPM on BP-BC RF coils. EM-fields were calculated using the finite difference time-domain (FDTD) method and analyzed using Matlab software. Next, to improve RF transmission efficiency, we compared two HPM structures, namely, a hollow cylinder shape HPM (hcHPM) and segmented cylinder shape HPM (scHPM). The scHPM and MCWE model comprised 16 elements (16-rad BP-BC RF coil) and this coil configuration demonstrated superior RF transmission efficiency and reception sensitivity along with an acceptable SAR. We expect wider clinical application of this combination in 7.0 T MRIs, which were recently approved by the United States Food and Drug Administration.

Deep brain stimulation (DBS) of the subthalamic nucleus is a neurosurgical intervention for Parkinson’s disease patients who no longer appropriately respond to drug treatments. A small fraction of patients will fail to respond to DBS, develop psychiatric and cognitive side-effects, or incur surgery-related complications such as infections and hemorrhagic events. In these cases, DBS may require recalibration, reimplantation, or removal. These negative responses to treatment can partly be attributed to suboptimal pre-operative planning procedures via direct targeting through low-field and low-resolution magnetic resonance imaging (MRI). One solution for increasing the success and efficacy of DBS is to optimize preoperative planning procedures via sophisticated neuroimaging techniques such as high-resolution MRI and higher field strengths to improve visualization of DBS targets and vasculature. We discuss targeting approaches, MRI acquisition, parameters, and post-acquisition analyses. Additionally, we highlight a number of approaches including the use of ultra-high field (UHF) MRI to overcome limitations of standard settings. There is a trade-off between spatial resolution, motion artifacts, and acquisition time, which could potentially be dissolved through the use of UHF-MRI. Image registration, correction, and post-processing techniques may require combined expertise of traditional radiologists, clinicians, and fundamental researchers. The optimization of pre-operative planning with MRI can therefore be best achieved through direct collaboration between researchers and clinicians.

The aim of this work was to assess two issues concerning magnetic resonance imaging (MRI) including device functionality and image artifacts for the presence of ultra high frequency (UHF) radio frequency identification (RFID) devices in connection with 0.3 Tesla at 12.7 MHz MRI and computed tomography (CT) scanning. A total of 15 samples of RFID tags with two dissimilar sizes (wristband and ID card types) were tested. The tags were exposed to a several numbers of MR-imaging conditions during MRI examination and X-rays of CT scan. During the test, the tags were oriented in three different directions (axial, coronal and sagittal) pertaining to MRI system in order to encompass all possible situations with respect to the patient undergoing MRI and CT scanning, wearing a RFID tag on wrist. In addition to the device functionality test and imaging artifacts, we also analyzed the reading performance of the RFID reader considering significant factors in MRI scan area. We observed that the tags did not experience physical damage with its functionality remained unchanged even after MRI and CT scanning, and there was no modification in previously stored data as well. In addition, no evidence of artifact was observed in the acquired MR and CT images. Therefore, we can conclude that the use of passive UHF RFID tag is safe for a patient undergoing MRI at 0.3 T/12.7 MHz and CT scanning. However, the reading performance of the RFID reader got affected depending on whether the MRI machine was on or off and also by the angle of the reader antenna.

AbstractDipole radiofrequency (RF) elements have been successfully used to compose multi-channel RF coils for ultrahigh fields (UHF) magnetic resonance imaging (MRI). As magnetic components of RF fields (B1) can be very inhomogeneous at UHF (B0≥7T), dielectric pads with high dielectric constants were proposed to improve the B1 efficiency and homogeneity [1]. Dielectric pads can be used as a passive B1 shimmimg technique thanks to inducing a strong secondary magnetic field in their vicinity. The use of such dielectric pads affect not only the B1 field but also the electric field. This in turn affects the specific absorption rate (SAR) and consequently the temperature distribution inside the patient’s body. To study these effects, a 29 cm-long transmission dipole RF coil element terminated by two meander was used for 7T MRI [2]. Using a cylindrical agarose-gel phantom, numerical and experimental results were analyzed with respect to homogeneity and amplitude of the magnetic and electric fields generated by the RF element in various configurations with and without dielectric pads. Calculated and measured B1 results were cross-checked and found to be in good agreement. When using dielectric pads B1 homogeneity and magnitude increase in regions where it was previously weak or insufficient. Calculations suggest that SAR distribution will change when using the pads.

In ultrahigh-field (UHF) magnetic resonance imaging (MRI) system, the RF power required to excite the nuclei of the target object increases. As the strength of the main magnetic field (B0 field) increases, the improvement of the RF transmit field (B1+ field) efficiency and receive field (B1− field) sensitivity of radio-frequency (RF) coils is essential to reduce their specific absorption rate and power deposition in UHF MRI. To address these problems, we previously proposed a method to simultaneously improve the B1+ field efficiency and B1− field sensitivity of 16-leg bandpass birdcage RF coils (BP-BC RF coils) by combining a multichannel wireless RF element (MCWE) and segmented cylindrical high-permittivity material (scHPM) comprising 16 elements in 7.0 T MRI. In this work, we further improved the performance of transmit/receive RF coils. A new combination of RF coil with wireless element and HPM was proposed by comparing the BP-BC RF coil with the MCWE and the scHPM proposed in the previous study and the multichannel RF coils with a birdcage RF coil-type wireless element (BCWE) and the scHPM proposed in this study. The proposed 16-ch RF coils with the BCWE and scHPM provided excellent B1+ field efficiency and B1− field sensitivity improvement.

For ultra-high field and frequency (UHF) magnetic resonance imaging (MRI), the associated short wavelengths in biological tissues leads to penetration and homogeneity issues at 10.5 tesla (T) and require antenna transmit arrays for efficiently generated 447 MHz B1+ fields (defined as the transmit radiofrequency (RF) magnetic field generated by RF coils). Previously, we evaluated a 16-channel combined loop + dipole antenna (LD) 10.5 T head array. While the LD array configuration did not achieve the desired B1+ efficiency, it showed an improvement of the specific absorption rate (SAR) efficiency compared to the separate 8-channel loop and separate 8-channel dipole antenna arrays at 10.5 T. Here we compare a 16-channel dipole antenna array with a 16-channel LD array of the same dimensions to evaluate B1+ efficiency, 10 g SAR, and SAR efficiency. The 16-channel dipole antenna array achieved a 24% increase in B1+ efficiency in the electromagnetic simulation and MR experiment compared to the LD array, as measured in the central region of a phantom. Based on the simulation results with a human model, we estimate that a 16-channel dipole antenna array for human brain imaging can increase B1+ efficiency by 15% with similar SAR efficiency compared to a 16-channel LD head array.

In this paper, we present a study on the effects of varying the position of a single tuning capacitor in a circular loop coil as a mechanism to control and produce non-symmetric current distribution, such that could be used for magnetic resonance imaging (MRI) operating at ultra-high frequency (UHF). This study aims to demonstrate that the position of the tuning capacitor of a circular loop could improve the coupling between adjacent coils, used to optimize transmission field uniformity or intensity, improve signal-to-noise ratio (SNR) or specific absorption rate (SAR). A typical loop coil used in MRI consists of symmetrically distributed capacitors along the coil; this design is able to produce uniform current distributions inside the coil. However, in UHF conditions, the magnetic flux density (|B1+|) field produced by this setup may exhibit field distortion, requiring a method of controlling the field distribution and improving the field intensity of the circular loop coil. The control mechanism investigated in this study is based on the position of the tuning capacitor in the circular coil, the capacitor position was varied from 15° to 345°, in steps of 15°. We performed electromagnetic (EM) simulations, fabricated the coils, and performed MRI experiments at 7T, with each of the coils with capacitor position from 15° to 345° to determine the effects on field intensity, coupling between adjacent coils, SAR, and applications for field uniformity optimization. For the case of free space, a coil with capacitor position at 15° showed higher field intensity compared to the reference coil; while an improved decoupling was achieved when a coil had the capacitor placed at 180° and the other coil at 90°; in a similar matter, we discuss the results for SAR, field uniformity and an application with an array coil for the spinal cord.

Abstract High magnetic fields play a critical role in the development of modern science and technology, breeding many significant scientific discoveries and boosting the generation of new technologies. In the last few years, China has undertaken a great deal of work on the application of ultra-high-field (UHF) superconducting magnet technology, such as for the Synergetic Extreme Condition User Facility in Beijing, the UHF nuclear magnetic resonance/magnetic resonance imaging, nuclear fusion energy, particle accelerator, and so on. This paper reports the research status of UHF superconducting magnets in China from different perspectives, including design options, technical features, experimental progress, opportunities, and challenges.

The main goal of this Thesis is the design and development of Radio-Frequency (RF) coils for sodium Magnetic Resonance Imaging (MRI) at Ultra High Field (UHF). The advantage of using UHF MR scanners is due to the possibility to achieve improved Signal-to-Noise-Ratio (SNR) and spatial resolution. These characteristics are fundamental in case of imaging with nuclei different from proton, which provide an intrinsically lower signal because of their lower in-vivo concentration and lower gyromagnetic ratio. Moreover, the overlap between sodium and proton images allows the accurate localization of regions with an anomalous sodium concentration, thanks to the anatomically more detailed proton images. For this reason, in case of imaging with non-proton nuclei, Dual-Tuned (DT) coils are preferable, since they allow the signal acquisition in a fixed spatial orientation of the subject, thus removing the need of patient’s repositioning between two consecutive acquisitions with two different RF coil resonating at the two Larmor frequencies of proton and sodium, respectively. Therefore, with a DT coil, automatically co-registered images can be obtained. The cost to pay is an increase in the design and development complexity with respect to a standard RF coil. In this Thesis, RF coils prototypes for sodium imaging (Larmor frequency ≃ 79 MHz) have been designed and developed for two different applications: human knee and human head imaging. Concerning the knee imaging, both surface coils, suitable for the signal reception, and volume coils, suitable for the sample excitation, have been designed and developed. All surface coils for knee imaging are dual-tuned. The first DT-RF coil prototype has been developed to study and characterize the issues related to the coupling between the two resonant structures, which usually compose a DT coil. New decoupling strategies have been proposed and developed as an alternative to the standard decoupling by using trap circuits, including models based on PIN diodes and Micro-Electro-Mechanical System (MEMS) switches. The volume RF coil for the knee imaging, built to be sensitive to the sodium signal only, has been designed according to the birdcage model. It has been developed to face with potential issues related to sodium volume coil interfacing with the MR system and signal acquisition before starting the construction of DT volume coils. Concerning the head imaging, an imbricated DT-RF coil, consisting of two concentrically placed birdcages, and the related electronic interface needed to connect the coil to the MR system, have been developed. Finally, an unconventional DT volume coil model (four-ring model), consisting of two birdcage-like resonant structures arranged on the same cylindrical surface and tuned at the two frequencies of interest, has been taken into account. The four-ring model has been optimized though electromagnetic simulations, with the main purpose of increasing the magnetic field homogeneity at the proton Larmor frequency at 7 T (≃ 300 MHz), and finally compared with the imbricated model.

The push toward higher magnetic fields in MRI has consistently thrown up new challenges in hardware development. The recent development of a new generation of ultra-high field scanners for human imaging is no exception. The earch presented in this thesis aims to provide solutions to new technical challenges in radio-frequency probe design. All probe designs were developed for use at 7T on a Philips Acheiva full body scanner.

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xiv, 222 p. : ill. (some col.). Advisor: Pierre-Marie Luc Robitaille, Dept. of Emergency Medicine. Includes bibliographical references.

Equine fetlock region disease is responsible for significant morbidity and mortality. Diagnosis of sesamoidean ligament, cartilage and subchondral bone injury has been obtained by clinical MRI. Low-field MRI provides images helpful in the investigation of MCPJ/MTPJ region pathology in horses in the clinical setting but the greater resolution of high and ultra-field MR images has the potential to aid interpretation through a better understanding of MRI anatomy. Quantitative MRI could provide a non-invasive technique to determine tissue biochemical properties associated with the early onset of articular cartilage degenerative conditions such as osteoarthritis. So far, ultra-high field MRI has not been used in equine research and practice. However, recently 3T MRI has been introduced in equine hospitals in Europe and the US. The general objectives of this project, which utilised cadaver limbs, was to improve understanding of the MRI anatomy of the equine MCPJ/MTPJ region and to evaluate the use of MRI for the non-invasive, quantitative assessment of articular cartilage from the same region. The first specific objective was to describe the appearance of the normal anatomy of the equine MCPJ/MTPJ region, especially the SDFT & DDFT and DSLs, using high field (1.5T) and ultra high field (7T) MRI and to compare the images obtained with the two systems. The second objective was to determine the accuracy and precision of articular cartilage thickness measurements using 1.5T and 7T MRI and comparing the measurements with those made from histological sections of the MCPJ/MTPJ. The third objective was to measure T1 & T2 MRI sequence relaxation times for normal horse articular cartilage pre and post gadolinium contrast (dGEMRIC) administration and to determine their correlation with GAG concentration, including a description of topographical variation. The fourth objective was to compare sodium concentration in normal equine MCPJ/MTPJ articular cartilage measured using 7T MR imaging with a dual tuned quadrature 23Na/1H coil with the biochemical properties (sodium concentration determined by flame photometry and GAG concentration). The final objective was to evaluate MR sodium imaging for the assessment of enzymatically degraded equine cartilage. The findings demonstrated that 7T MRI produces high resolution images, which enable better evaluation of the hard and soft tissues of the equine MCPJ/MTPJ region than images from lower field MR systems and which permit accurate and precise articular cartilage thickness measurements to be made. Moreover, it was found that the dGEMRIC technique appears to provide a feasible quantitative tool for evaluating the articular cartilage properties. However, the quantitative parameters determined by the dGEMRIC method cannot fully characterise the biochemical properties of the cartilage. Moreover, delayed gadolinium-enhanced (dGEMRIC) techniques are time consuming, requiring relatively long incubation and scanning times. The measurement of T2 time is a very complex method. The work described in the last chapters demonstrated that sodium MRI was significantly correlated with the biochemical properties of the equine articular cartilage. Therefore the sodium MRI technique showed promise in imaging articular cartilage and providing useful information on the biochemical properties of the cartilage.

L’ambition des très hauts champs magnétiques (≥ 7T) à forts gradients (≥ 300mT/m) est de dépasser la résolution millimétrique imposée à plus bas champ pour atteindre l’échelle mésoscopique en neuroimagerie. Etudier le cerveau à cette échelle est essentiel pour comprendre le lien entre fonction et substrat anatomique. Malgré les progrès réalisés sur les aimants cliniques à 7T, il n’en est pas de même des gradients. Cette thèse vise à cartographier le cerveau humain à l’échelle mésoscopique via l’étude de pièces anatomiques post mortem. Une approche alternative a été choisie, reposant sur l'utilisation d'imageurs précliniques à très hauts champs (7T et 11.7T) et forts gradients (780mT/m). Après une première étape de préparation (extraction et fixation) opérée au CHU de Tours, une pièce anatomique complète a été scannée à 3T, avant découpe de l’hémisphère gauche en sept blocs. Un protocole d’acquisition IRM ciblant une résolution mésoscopique a ensuite été mis en place à 11.7T. Ce protocole, incluant des séquences anatomiques, relaxométriques, et de diffusion, a été validé à l’aide de deux structures clé: un hippocampe et un tronc cérébral. Les données anatomiques et de diffusion acquises à une résolution mésoscopique sur l’hippocampe ont permis de segmenter ses sous-champs, d’extraire le circuit polysynaptique et d’observer l’existence d’un gradient de connectivité et de densité neuritique positif dans la direction postéro-antérieure de l’hippocampe. L’utilisation de modèles avancés d’étude de la microstructure a également révélé l’apport de ces techniques pour la segmentation de l’hippocampe, les cartes de densité neuritique révélant les trois couches des champs ammoniens. Un tronc cérébral a ensuite été scanné, avec une résolution atteignant la centaine de micromètres. Une segmentation de 53 de ses 71 noyaux a été réalisée au sein du CHU de Tours, permettant d’établir la cartographie IRM du tronc cérébral humain la plus complète à ce jour. Les principaux faisceaux de la substance blanche ont été reconstruits, ainsi que les projections du locus coeruleus, structure connue pour être atteinte dans le maladie de Parkinson. Forts de ces résultats, la campagne d'acquisition de l'hémisphère gauche, d’une durée de 10 mois, a été initiée. Le protocole d’acquisition à 11.7T intègre des séquences anatomiques (100/150µm) ainsi que des séquences d'imagerie 3D pondérées en diffusion (b=1500/4500/8000 s/mm², 25/60/90 directions) à 200µm. Des acquisitions complémentaires réalisées à 7T comprenant des séquence d’écho de spin rapide avec inversion-récupération ont par ailleurs permis d’étudier la myéloarchitecture du cortex cérébral et d’identifier automatiquement sa structure laminaire. Un nouveau modèle de mélange de Gaussiennes a été développé, intégrant les informations myéloarchitecturales issues de la cartographie T1 et les informations cytoarchitecturales issues de l’imagerie de diffusion. Il a ainsi pu être démontré que l’utilisation conjointe de ces deux informations permettait de mettre en évidence des couches du cortex visuel, l’information myéloarchitecturale favorisant l’extraction des couches externes et la densité neuritique celle des couches plus profondes. Enfin, l’exploitation des données IRM acquises à 11.7T sur les différents blocs a nécessité la mise en place d’une chaîne de prétraitements pour corriger les artéfacts d’imagerie et reconstruire l’hémisphère entier à l’aide de stratégies de recalage difféomorphe avancées. L’objectif de ce projet est l’obtention d’un jeu de données IRM de très haute résolution spatio-angulaire de l’hémisphère gauche. Ce jeu de données anatomique et de diffusion unique permettra à terme de constituer un nouvel atlas IRM mésoscopique de la structure, de la connectivité et de la cytoarchitecture du cerveau humain
The aim of ultra-high field strength (≥7T) and ultra-strong gradient systems (≥300mT/m) is to go beyond the millimeter resolution imposed at lower field and to reach the mesoscopic scale in neuroimaging. This scale is essential to understand the link between brain structure and function. However, despite recent technological improvements of clinical UHF-MRI, gradient systems remain too limited to reach this resolution. This thesis aims at answering the need for mapping the human brain at a mesoscopic scale by the study of post mortem samples. An alternative approach has been developed, based on the use of preclinical systems equipped with ultra-high fields (7T/11.7T) and strong gradients (780mT). After its extraction and fixation at Bretonneau University Hospital (Tours), an entire human brain specimen was scanned on a 3T clinical system, before separating its two hemispheres and cutting each hemisphere into seven blocks that could fit into the small bore of an 11.7T preclinical system. An MRI acquisition protocol targeting a mesoscopic resolution was then set up at 11.7T. This protocol, including anatomical, quantitative, and diffusion-weighted sequences, was validated through the study of two key structures: the hippocampus and the brainstem. From the high resolution anatomical and diffusion dataset of the human hippocampus, it was possible to segment the hippocampal subfields, to extract the polysynaptic pathway, and to observe a positive gradient of connectivity and neuritic density in the posterior-anterior direction of the hippocampal formation. The use of advanced microstructural models (NODDI) also highlighted the potential of these techniques to reveal the laminar structure of the Ammon’s horn. A high resolution anatomical and diffusion MRI dataset was obtained from the human brainstem with an enhanced resolution of a hundred micrometers. The segmentation of 53 of its 71 nuclei was performed at the Bretonneau University Hospital, making it the most complete MR-based segmentation of the human brainstem to date. Major white matter bundles were reconstructed, as well as projections of the locus coeruleus, a structure known to be impaired in Parkinson’s disease. Buoyed by these results, a dedicated acquisition campaign targeting the entire left hemisphere was launched for total scan duration of 10 months. The acquisition protocol was performed at 11.7T and included high resolution anatomical sequences (100/150μm) as well as 3D diffusion-weighted sequences (b=1500/4500/8000 s/mm², 25/60/90 directions, 200μm). In addition, T1-weighted inversion recovery turbo spin echo scans were performed at 7T to further investigate the myeloarchitecture of the cortical ribbon at 300µm, revealing its laminar structure. A new method to automatically segment the cortical layers was developed relying on a Gaussian mixture model integrating both T1-based myeloarchitectural information and diffusion-based cytoarchitectural information. The results gave evidence that the combination of these two contrasts highlighted the layers of the visual cortex, the myeloarchitectural information favoring the extraction of the outer layers and the neuritic density favoring the extraction of the deeper layers. Finally, the analysis of the MRI dataset acquired at 11.7T on the seven blocks required the development of a preprocessing pipeline to correct artifacts and to reconstruct the entire hemisphere using advanced registration methods. The aim was to obtain an ultra-high spatio-angular resolution MRI dataset of the left hemisphere, in order to establish a new mesoscopic post mortem MRI atlas of the human brain, including key information about its structure, connectivity and microstructure

L’imagerie par résonance magnétique (IRM) est un outil d’investigation majeur donnant accès de manière non invasive à des nombreuses informations quantitatives et fonctionnelles. La qualité des images obtenues (rapport-signal-sur-bruit, RSB) est cependant limitée dans certaines applications nécessitant des résolutions spatiales et/ou temporelles poussées. Afin d’améliorer la sensibilité de détection des équipements d’IRM, diverses orientations peuvent être suivies telles qu’augmenter l’intensité du champ magnétique des imageurs, améliorer les performances des systèmes de détection radiofréquence (RF), ou encore développer des séquences d’acquisition et des techniques de reconstruction d’images plus efficaces. La thématique globale dans laquelle s’inscrit cette thèse concerne le développement des systèmes de détection RF à haute sensibilité pour l’IRM à haut champ chez l’homme. En particulier, des antennes auto-résonantes basées sur le principe des lignes de transmission sont utilisées parce qu’elles peuvent être réalisée sur substrat souple. Cette adaptabilité géométrique du résonateur permet d’ajuster précisément sa forme aux spécificités morphologiques de la zone anatomique observée, et ainsi d’augmenter le RSB. La première visée technologique de ce projet concerne le développement, de la conception jusqu’à la mise en œuvre dans un appareil 7 T corps entier, d’un système de détection RF flexible à haute sensibilité, utilisant des antennes miniatures associées en réseau. L’utilisation d’un réseau d’antennes miniatures permet d’obtenir des images sur un champ de vue élargi tout en conservant la haute sensibilité inhérente à chaque antenne miniature. De plus, la technologie de l’imagerie parallèle devient accessible, ce qui permet d’accélérer l’acquisition des images. De surcroît, un nouveau schéma de résonateur de ligne transmission avec un degré de liberté supplémentaire est introduit, ce qui permet de réaliser de grands résonateurs multi-tours pour l’IRM à haut champ. Cette thèse décrit le développement, la mise en œuvre et l’évaluation des nouveaux systèmes de détection RF au moyen de simulations analytiques et numériques, et des études expérimentales
Magnetic resonance imaging (MRI), among other imaging techniques, has become a major backbone of modern medical diagnostics. MRI enables the non-invasive combined, identification of anatomical structures, functional and chemical properties, especially in soft tissues. Nonetheless, applications requiring very high spatial and/or temporal resolution are often limited by the available signal-to-noise ratio (SNR) in MR experiments. Since first clinical applications, image quality in MRI has been constantly improved by applying one or several of the following strategies: increasing the static magnetic field strength, improvement of the radiofrequency (RF) detection system, development of specialized acquisition sequences and optimization of image reconstruction techniques. This work is concerned with the development of highly sensitive RF detection systems for biomedical ultra-high field MRI. In particular, auto-resonant RF coils based on transmission line technology are investigated. These resonators may be fabricated on flexible substrate which enables form-fitting of the RF detector to the target anatomy, leading to a significant SNR gain. The main objective of this work is the development of a flexible RF coil array for high-resolution MRI on a human whole-body 7 T MR scanner. With coil arrays, the intrinsically high SNR of small surface coils may be exploited for an extended field of view. Further, parallel imaging techniques are accessible with RF array technology, allowing acceleration of the image acquisition. Secondly, in this PhD project a novel design for transmission line resonators is developed, that brings an additional degree of freedom in geometric design and enables the fabrication of large multi-turn resonators for high field MR applications. This thesis describes the development, successful implementation and evaluation of novel, mechanically flexible RF devices by analytical and 3D electromagnetic simulations, in bench measurements and in MRI experiments

La sclérose en plaques (SEP) est une maladie inflammatoire démyélinisante du système nerveux central, qui représente la première cause de handicap non-traumatique du jeune adulte. Il a été démontré que le principal mécanisme impliqué dans la progression du handicap était une atteinte neuronale dégénérative. Les mécanismes à l’origine de la neurodégénérescence sont cependant peu connus in vivo. Des études post-mortem ont mis en évidence le rôle d’une inflammation persistante impliquant le système immunitaire innée, en association avec un échec de la remyélination dans la substance blanche ainsi que les lésions corticales. Cette thèse a pour objectif de développer des techniques d’imagerie permettant de quantifier et cartographier l’activation des cellules de l’immunité innée ainsi que les changements en myéline, en combinant la tomographie par émission de positons (TEP) et l’IRM à haut champ. À cette fin, j'ai tout d'abord mis au point une méthodologie de post-traitement permettant de générer des cartes individuelles et régionales de l’atteinte et de la réparation des tissus, utilisée pour la quantification de l'activation des cellules immunitaires innées ainsi que pour l’étude de la démyélinisation/remyélinisation dans les lésions de la substance blanche ainsi que dans le cortex. J’ai combiné la TEP au [18F]-DPA714, ciblant principalement les cellules immunitaires innées activées, et l’IRM multimodale à 3T évaluant les dommages structurels dans une cohorte de patients SEP présentant une forme rémittente ou progressive de la maladie. J'ai montré que les patients SEP étaient caractérisés par un niveau de neuroinflammation très hétérogène et qu'un grand sous-ensemble de lésions de la substance blanche considérées comme inactives en IRM étaient en réalité très actives en TEP, suggérant des lésions actives chroniques. J'ai montré que cette inflammation persistante était en lien avec les trajectoires individuelles du handicap. Ensuite, j’ai étudié si la proximité du liquide céphalo-rachidien (LCR) pouvait influencer les cellules immunitaires innées des substances blanche et grise profondes. J'ai démontré la présence d’un gradient d'activation des cellules immunitaires innées à proximité du LCR ventriculaire, corrélé au gradient périventriculaire d’atteintes microstructurales, pouvant également expliquer en partie du handicap clinique. Dans le but de quantifier la dynamique de la myéline dans le cortex, j'ai appliqué l'imagerie par transfert de magnétisation et généré des cartes individuelles de démyélinisation et de remyélinisation dans les tissus corticaux. J'ai ensuite montré que les profils individuels de remyélinisation de la substance blanche étaient hétérogènes selon les sujets et contribuaient au handicap des patients SEP. Enfin, j'ai appliqué pour la première fois la transmission parallèle utilisant la modulation RF dynamique sur un système IRM 7T afin de visualiser les lésions corticales au niveau du cerveau entier, ouvrant ainsi la voie à une meilleure détection de ces lésions dans les études futures. Les résultats acquis dans ce travail devraient permettre d’appliquer de nouveaux outils d’imagerie permettant de cartographier les mécanismes à l’origine de la neurodégénérescence dans la SEP dans les études futures, d’ouvrir la perspective de la stratification du patient, de concevoir de nouvelles méthodes d’essais de réparation et de neuroprotection et d’optimiser les soins
Multiple Sclerosis (MS), an inflammatory and demyelinating disease of the central nervous system, is the leading cause of non-traumatic neurological disability in young adults in western countries. It is now well accepted that neurodegeneration is the key mechanism underlying disability progression in this disease. The biological mechanisms leading to neurodegeneration remains poorly understood in vivo, but pathological post-mortem studies have pointed the potential contribution of a persisting inflammation involving the innate immune system together with a failure of endogenous repair in white matter (WM) and cortical lesions. This thesis aimed at developing imaging tools able to quantify and map innate immune cell activation and myelin dynamics though a combination of positron emission tomography (PET) and advanced high field magnetic resonance imaging (MRI) in patients with MS. For this purpose I have first developed a post processing methodology that allows the generation of individual and regional maps of tissue damage and repair, which was used for the quantification of innate immune cell activation as well as for the investigation of demyelination/remyelination in WM lesions and in the cortex. We have combined TSPO PET with [18F]-DPA714, targeting mainly activated innate immune cells, and multimodal 3T MRI, assessing structural damage, in a cohort of MS patients with either a relapsing or a progressive form of the disease. I showed that patients with MS were characterized by a very heterogeneous level of neuroinflammation, and that a large subset of WM lesions considered as inactive on MRI were actually very active on PET, a finding suggestive of chronic active lesions. I further showed that this persisting inflammation correlated with individual trajectories of disability. Then, I have questioned whether the proximity to the cerebrospinal fluid (CSF) could influence innate immune cells in the deep white and grey matter. I demonstrated a clear gradient of innate immune cells activation in vicinity to ventricular CSF, which correlated with the periventricular gradient of microstructural damage, and could also explain part of clinical disability. Aiming to quantify myelin dynamics in the cortex I have applied Magnetization Transfer Imaging and generated individual maps of demyelination and remyelination in cortical tissues. I then showed that cortical and WM individual remyelination profiles were heterogeneous among subjects, and were synergistically contributing to disability in MS. Finally, I have applied for the first time the parallel transmission using dynamic RF-shimming on a 7T MRI system to visualize cortical lesions at the whole brain level, paving the way for an improved detection of these lesions in future studies. Results acquired in this work should allow to apply new imaging tools mapping the mechanisms that drive neurodegeneration in MS in future studies, opening the perspective of patient stratification, novel design for repair and neuroprotection trials, and optimization of care

By increasing the field strength of the magnet used for magnetic resonance imaging (MRI), the available signal from the patient is enhanced, and this basic physics principle has driven the clinical MRI market to ever higher field strengths. Seven Tesla (7 T) scanners yield 4-5 times more signal than 1.5 T scanners; this signal-to-noise ratio increase facilitates high-resolution imaging, faster imaging when using accelerated techniques such as SENSE and GRAPPA, and greater sensitivity to low-concentration metabolites. Magnetic resonance spectroscopy acquisitions also benefit, owing to the greater chemical shift dispersion at ultra-high field. A significant difficulty is due to the radiofrequency excitation required that oscillates at 300 MHz, which results in destructive interference of the excitation fields and heating of the patient, and hence requires expensive additional hardware. While 7 T presents a great opportunity to cardiovascular MRI research, it is not yet a routine clinical tool, owing to the compound challenges of high cost, limited availability, and the difficulties of radiofrequency excitation at 300 MHz.