Forced Current Excitation in Selectable Field of View Coils for 7T MRI and MRS

High field magnetic resonance imaging (MRI) provides improved signal-to-noise ratio (SNR) which can be translated to higher image resolution or reduced scan time. 7 Tesla (T) breast imaging and 7 T spine imaging are of clinical value, but they are challenging for several reasons: A bilateral breast coil requires the use of closely-spaced elements that are subject to severe mutual coupling which leads to uncontrollable current distribution and non-uniform field pattern; A spine coil at 7T requires a large field of view (FOV) in the z direction and good RF penetration into the human body. Additionally, the ability to switch FOV without the use of expensive high power RF amplifiers is desired in both applications. This capability would allow reconfigurable power distribution and avoid unnecessary heat deposition into human body. Forced-Current Excitation (FCE) is a transmission line-based method that maintains equal current distribution across an array, alleviating mutual coupling effects and allowing current/field replication across a large FOV. At the same time, the nature of this method enables selectable FOV with the inclusion of PIN diodes and a controller. In this doctoral work, the theory of FCE is explained in detail, along with its benefits and drawbacks. Electromagnetic simulation considerations of FCE-driven coils are also discussed. Two FCE-driven coils were designed and implemented: a switchable bilateral/unilateral 7T breast coil, and a segmented dipole for spine imaging at 7T with reconfigurable length. For the breast coil, shielded loop elements were used to form a volume coil, whereas for the spine coil, a segmented dipole was chosen as the final design due to improved RF penetration. Electromagnetic simulations were performed to assist the design of the two coils as well as to predict the SAR (specific absorption rate) generated in the phantom. The coils were evaluated on bench and through MRI experiments in different configurations to validate the design. The switchable breast coil provides uniform excitation in both unilateral and bilateral mode. In unilateral mode, the signal in the contralateral breast is successfully suppressed and higher power is concentrated into the breast of interest; The segmented dipole was compared to a regular dipole with the same length used for 7T spine imaging. The segmented dipole shows a large FOV in the long mode. In the short mode, the residual signal from other part of the dipole is successfully suppressed. The ability to switch FOV and reconfigure the power distribution improves the B1 generated with unit specific absorption rate towards the edge of the dipole, compared to the regular dipole

Forced Current Excitation in Selectable Field of View Coils for 7T MRI and MRS

High field magnetic resonance imaging (MRI) provides improved signal-to-noise ratio (SNR) which can be translated to higher image resolution or reduced scan time. 7 Tesla (T) breast imaging and 7 T spine imaging are of clinical value, but they are challenging for several reasons: A bilateral breast coil requires the use of closely-spaced elements that are subject to severe mutual coupling which leads to uncontrollable current distribution and non-uniform field pattern; A spine coil at 7T requires a large field of view (FOV) in the z direction and good RF penetration into the human body. Additionally, the ability to switch FOV without the use of expensive high power RF amplifiers is desired in both applications. This capability would allow reconfigurable power distribution and avoid unnecessary heat deposition into human body. Forced-Current Excitation (FCE) is a transmission line-based method that maintains equal current distribution across an array, alleviating mutual coupling effects and allowing current/field replication across a large FOV. At the same time, the nature of this method enables selectable FOV with the inclusion of PIN diodes and a controller. In this doctoral work, the theory of FCE is explained in detail, along with its benefits and drawbacks. Electromagnetic simulation considerations of FCE-driven coils are also discussed. Two FCE-driven coils were designed and implemented: a switchable bilateral/unilateral 7T breast coil, and a segmented dipole for spine imaging at 7T with reconfigurable length. For the breast coil, shielded loop elements were used to form a volume coil, whereas for the spine coil, a segmented dipole was chosen as the final design due to improved RF penetration. Electromagnetic simulations were performed to assist the design of the two coils as well as to predict the SAR (specific absorption rate) generated in the phantom. The coils were evaluated on bench and through MRI experiments in different configurations to validate the design. The switchable breast coil provides uniform excitation in both unilateral and bilateral mode. In unilateral mode, the signal in the contralateral breast is successfully suppressed and higher power is concentrated into the breast of interest; The segmented dipole was compared to a regular dipole with the same length used for 7T spine imaging. The segmented dipole shows a large FOV in the long mode. In the short mode, the residual signal from other part of the dipole is successfully suppressed. The ability to switch FOV and reconfigure the power distribution improves the B1 generated with unit specific absorption rate towards the edge of the dipole, compared to the regular dipole

Design and Construction of a Highly Sensitive Coil for MRI of the Spinal Cord

RÉSUMÉ Un grand nombre de pathologies (sclérose en plaque, lésions, etc.) peuvent toucher la moelle épinière, des techniques non invasives de diagnostic tel que l’imagerie par résonance magnétique (IRM) sont généralement utilisées pour les dépister. Les antennes commerciales pour l’IRM sont conçues pour accommoder une large population, mais elles ne sont pas optimisées pour le rapport signal sur bruit (S/B). L'objectif principal de ce mémoire a été de concevoir et de construire une antenne radiofréquence (RF) en réseau phasé avec six bobines en réception pour l’IRM de la moelle épinière cervicale chez des sujets humains. La configuration optimale de l’antenne avec six canaux a été déterminée à l'aide de simulations électromagnétiques pour modéliser l’antenne en réception. L’antenne a été conçue et construite pour s’ajuster au plus près du sujet humain tout en étant compatible avec l'interface du scanner IRM. Les performances de l’antenne ont été évaluées sur le banc à l'aide d'un fantôme, ainsi que dans l'IRM sur des sujets humains. Les résultats montrent une amélioration moyenne du rapport S/B par un facteur 2 par rapport à l’antenne commerciale. Cette amélioration permet d’avoir une haute résolution qui facilite la représentation des fins détails comme les petites lésions présentes dans la sclérose en plaques. De plus, la géométrie optimisée de l’antenne permet d'utiliser de hauts facteurs d'accélération (par exemple 3), réduisant considérablement le temps d'acquisition. Pour conclure, l’antenne en réseau phasé avec six bobines pourrait servir à l'imagerie anatomique de haute résolution (0,3 mm dans le plan), l'IRM fonctionnelle (IRMf), IRM de diffusion et dans études par spectroscopie pour caractériser le métabolisme des tissus présents dans la moelle épinière et les sections inférieures du cerveau.----------ABSTRACT Spinal cord injuries affect a large number of people, therefore a non-invasive technique such as magnetic resonance imaging (MRI) can be used for diagnosis purposes. While current commercial coils are designed to fit a diverse population, they are not optimized for signal-to-noise ratio (SNR). The major objective of this thesis was to design and construct a six-channel radio-frequency (RF) receive-only phased array coil for MRI of the cervical spinal cord in human subjects. The optimal configuration of the six-channel coil array was determined using electromagnetic simulations framework for modelling the array behavior in the receiving mode. The design and construction of the coil array were focused on offering a tight fit of the human subject, while being compatible with the scanner interface. The RF coil performances were evaluated at the bench using a phantom. Furthermore, it was validated in the MRI on human subjects. The results show an average improvement in SNR by a factor of two compared to the commercial coil. This enhancement enables higher resolution and therefore better depiction of small pathologies such as small lesions in multiple sclerosis. Moreover, the optimized geometry of the RF coil enables the use of aggressive acceleration factors (e.g., 3), which reduces significantly the acquisition time. In conclusion, the six-channel coil array could be used in high resolution anatomical imaging (0.3mm in-plane), functional MRI (fMRI), diffusion tensor imaging (DTI) and spectroscopy studies for characterizing the metabolism of different tissues present in the spinal cord and lower brain sections

Multimodal surface coils for low-field MR imaging

Leveraging the potential of low-field Magnetic Resonance Imaging (MRI), our study introduces the multimodal surface RF coil, a design tailored to overcome the limitations of conventional coils in this context. The inherent challenges of low-field MRI, notably suboptimal signal-to-noise ratio (SNR) and the need for specialized RF coils, are effectively addressed by our novel design. The multimodal surface coil is characterized by a unique assembly of resonators, optimized for both B1 efficiency and low-frequency tuning capabilities, essential for low-field applications. This paper provides a thorough investigation of the conceptual framework, design intricacies, and bench test validation of the multimodal surface coil. Through detailed simulations and comparative analyses, we demonstrate its superior performance in terms of B1 field efficiency, outperforming conventional surface coils

An eight‐channel Tx dipole and 20‐channel Rx loop coil array for MRI of the cervical spinal cord at 7 tesla

RÉSUMÉ: The quality of cervical spinal cord images can be improved by the use of tailored radiofrequency (RF) coil solutions for ultrahigh field imaging; however, very few commercial and research 7-T RF coils currently exist for the spinal cord, and in particular, those with parallel transmission (pTx) capabilities. This work presents the design, testing, and validation of a pTx/Rx coil for the human neck and cervical/upper thoracic spinal cord. The pTx portion is composed of eight dipoles to ensure high homogeneity over this large region of the spinal cord. The Rx portion is made up of twenty semiadaptable overlapping loops to produce high signal-to-noise ratio (SNR) across the patient population. The coil housing is designed to facilitate patient positioning and comfort, while also being tight fitting to ensure high sensitivity. We demonstrate RF shimming capabilities to optimize B1+ uniformity, power efficiency, and/or specific absorption rate efficiency. B1+ homogeneity, SNR, and g-factor were evaluated in adult volunteers and demonstrated excellent performance from the occipital lobe down to the T4-T5 level. We compared the proposed coil with two state-of-the-art head and head/neck coils, confirming its superiority in the cervical and upper thoracic regions of the spinal cord. This coil solution therefore provides a convincing platform for producing the high image quality necessary for clinical and research scanning of the upper spinal cord

Streamlining the Design and Use of Array Coils for In Vivo Magnetic Resonance Imaging of Small Animals

Small-animal models such as rodents and non-human primates play an important pre-clinical role in the study of human disease, with particular application to cancer, cardiovascular, and neuroscience models. To study these animal models, magnetic resonance imaging (MRI) is advantageous as a non-invasive technique due to its versatile contrast mechanisms, large and flexible field of view, and straightforward comparison/translation to human applications. However, signal-to-noise ratio (SNR) limits the practicality of achieving the high-resolution necessary to image the smaller features of animals in an amount of time suitable for in vivo animal MRI. In human MRI, it is standard to achieve an increase in SNR through the use of array coils; however, the design, construction, and use of array coils for animal imaging remains challenging due to copper-loss related issues from small array elements and design complexities of incorporating multiple elements and associated array hardware in a limited space. In this work, a streamlined strategy for animal coil array design, construction, and use is presented and the use for multiple animal models is demonstrated. New matching network circuits, materials, assembly techniques, body-restraining systems and integrated mechanical designs are demonstrated for streamlining high-resolution MRI of both anesthetized and awake animals. The increased SNR achieved with the arrays is shown to enable high-resolution in vivo imaging of mice and common marmosets with a reduced time for experimental setup

Dual-tuned Coaxial-transmission-line RF coils with Independent Tuning Capabilities for X-nuclear Metabolic MRS Imaging at Ultrahigh Magnetic Fields

Information on the metabolism of tissues in both healthy and diseased states has potential for detecting tumors, neurodegeneration diseases, diabetes, and many metabolic disorders in biomedical studies. Hyperpolarized carbon-13 magnetic resonance imaging (13C-HPMRI) and deuterium metabolic imaging (2H-DMI) are two emerging X-nuclei used as practical imaging tools to investigate tissue metabolism. However due to their low gyromagnetic ratios ($\gamma_{13C}$ = 10.7 MHz/T; $\gamma_{2H}$ = 6.5 MHz/T) and natural abundance, such method required the use of a sophisticated dual-tuned radio frequency (RF) coil where the X-nucleus signal is associated with the proton signal used for anatomical reference. Here, we report a dual-tuned coaxial transmission line (CTL) RF coil agile for metabolite information operating at 7T with independent tuning capability. Analysis based on full-wave simulation has demonstrated how both resonant frequencies can be individually controlled by simply varying the constituent of the design parameters. A broadband tuning range capability is obtained, covering most of the X-nucleus signal, especially the 13C and 2H spectra at 7T. Numerical results has demonstrated the effectiveness of the magnetic field produced by the proposed dual-tuned 1H/13C and 1H/2H CTLs RF coils. Furthermore, in order to validate the feasibility of the proposed design, both dual-tuned CTLs prototypes are designed and fabricated using a semi-flexible RG-405 .086" coaxial cable and bench test results (scattering parameters and magnetic field efficiency/distributions) are successfully obtained.Comment: 9 pages, 7 figure

7-Tesla-Ultrahochfeld Magnetresonanztomographie im Kopfund Halsbereich mittels 64-Kanal-Signaldetektion und integrierter paralleler 16-Kanal-Sendespule

Die MRT hat sich als wertvolles Diagnosewerkzeug im klinischen Alltag gezeigt und sich seit seiner Einführung konstant weiterentwickelt. So wurde erst kürzlich der erste MRT durch die United States Food and Drug Administration zur klinischen Nutzung freigegeben. Mit einer höheren Magnetfeldstärke ändern sich die physikalischen Effekte und Parameter, die eine positive (kontrastverstärkende Auswirkung), aber auch eine negative (artefaktverstärkende) Auswirkung auf die Bildgebung haben können. So ist es aus technischen Gründen derzeit noch nicht möglich, eine Ganzkörperaufnahme bei einer Feldstärke von 7 T zu generieren, wie es beispielsweise bei 1,5 T möglich ist. Die derzeitige Bildgebung bei 7 T-MRTs beschränkt sich hauptsächlich auf lokale Bereiche wie z.B. das Gehirn, oder das Fuß-, Arm- oder Kniegelenk. Auf Basis der großen Nachfrage aus dem klinischen Bereich, das Bildfeld zu erweitern, ergibt sich die Fragestellung dieser Dissertation: Ist es möglich, mit dem aktuellen Stand der Technik unter Berücksichtigung der maximal verfügbaren Sende- und Empfangskanäle, die ein derzeitiges kommerzielles 7 T-MRT bietet, ein Bildfeld zu generieren, welches den Kopf- und Halsbereich abdeckt? Diese Fragestellung wurde durch die Entwicklung von morphologisch angepassten Signalgeneratoren als auch Signaldetektoren gelöst. Das Bildfeld wurde von der Gehirnregion auf den Halsbereich bei 7 T erweitert. Die entwickelte Hardware wurde entworfen, simuliert, konstruiert, getestet und mit einer kommerziell verfügbaren 7 T Gehirnspule verglichen. Ein Fortschritt zum aktuellen Stand der Technik wurde quantifiziert. Die neu entwickelten Methoden zur Gestaltung und Konstruktion der Sende- und Empfangsstruktur bei 7 T, als auch die zur Prüfung der Funktionalität verwendete Hardware, wurde direkt in abgewandelter Form bei ähnlichen MRT-Forschungsprojekten bei einer Feldstärke von 3 T verwendet und publiziert. Zusammenfassend wurde mit diesem Projekt sowohl der Grundstein für die klinische Bildgebung als auch für weitere Forschung im kombinierten Kopf- und Halsbereich bei der 7 T gelegt. Der Einfluss dieses Projekts wird sich voraussichtlich in den nächsten Jahren in klinischen Studien zeigen. Limitierende Faktoren wie beispielsweise die SAR können durch Softwaremaßnahmen und der Ansteuerung der Spulen in weiteren Doktorarbeiten optimiert werden, um den Bildgebungsprozess zu optimieren

Electric Field and SAR Reduction in High Impedance RF Arrays by Using High Permittivity Materials for 7T MR Imaging

Higher frequencies and shorter wavelengths present significant design issues at ultra-high fields, making multi-channel array setup a critical component for ultra-high field MR imaging. The requirement for multi-channel arrays, as well as ongoing efforts to increase the number of channels in an array, are always limited by the major issue known as inter-element coupling. This coupling affects the current and field distribution, noise correlation between channels, and frequency of array elements, lowering imaging quality and performance. To realize the full potential of UHF MRI, we must ensure that the coupling between array elements is kept to a minimum. High-impedance coils allow array systems to completely realize their potential by providing optimal isolation while requiring minimal design modifications. These minor design changes, which demand the use of low capacitance on the conventional loop to induce elevated impedance, result in a significant safety hazard that cannot be overlooked. High electric fields are formed across these low capacitance lumped elements, which may result in higher SAR values in the imaging subject, depositing more power and, ultimately, providing a greater risk of tissue heating-related injury to the human sample. We propose an innovative method of utilizing high-dielectric material to effectively reduce electric fields and SAR values in the imaging sample while preserving the B1 efficiency and inter-element decoupling between the array elements to address this important safety concern with minimal changes to the existing array design comprising high-impedance coils.Comment: 12 pages, 18 figures, 2 table

Novel magnetic resonance antennas and applications

This dissertation describes novel magnetic resonance imaging (MRI) surface antennas and arrays, and their applications at both 3 and 7 Tesla. While the first half of this work describes flexible lightweight antenna arrays, the other half focuses on the use of solid ceramic high-permittivity materials as a substantial part of the antenna. NWOLUMC / Geneeskund