Electrodynamics and radiofrequency antenna concepts for human magnetic resonance at 23.5 T (1 GHz) and beyond

Objective: This work investigates electrodynamic constraints, explores RF antenna concepts and examines the transmission fields (B 1 + ) and RF power deposition of dipole antenna arrays for 1H magnetic resonance of the human brain at 1 GHz (23.5 T). Materials and methods: Electromagnetic field (EMF) simulations are performed in phantoms with average tissue simulants for dipole antennae using discrete frequencies [300 MHz (7.0 T) to 3 GHz (70.0 T)]. To advance to a human setup EMF simulations are conducted in anatomical human voxel models of the human head using a 20-element dipole array operating at 1 GHz. Results: Our results demonstrate that transmission fields suitable for 1H MR of the human brain can be achieved at 1 GHz. An increase in transmit channel density around the human head helps to enhance B 1 + in the center of the brain. The calculated relative increase in specific absorption rate at 23.5 versus 7.0 T was below 1.4 (in-phase phase setting) and 2.7 (circular polarized phase setting) for the dipole antennae array. Conclusion: The benefits of multi-channel dipole antennae at higher frequencies render MR at 23.5 T feasible from an electrodynamic standpoint. This very preliminary finding opens the door on further explorations that might be catalyzed into a 20-T class human MR system

Radio Frequency Antenna Designs and Methodologies for Human Brain Computer Interface and Ultrahigh Field Magnetic Resonance Imaging

Brain Computer Interface (BCI) and Magnetic Resonance Imaging (MRI) are two powerful medical diagnostic techniques used for human brain studies. However, wired power connection is a huge impediment for the clinical application of BCI, and most current BCIs have only been designed for immobile users in a carefully controlled environment. For the ultrahigh field (≥7T) MRI, limitations such as inhomogeneous distribution of the transmit field (B1+) and potential high power deposition inside the human tissues have not yet been fully combated by existing methods and are central in making ultrahigh field MRI practical for clinical use. In this dissertation, radio frequency (RF) methods are applied and RF antennas/coils are designed and optimized in order to overcome these barriers. These methods include: 1) designing implanted miniature antennas to transmit power wirelessly for implanted BCIs; 2) optimizing a new 20-channel transmit array design for 7 Tesla MRI neuroimaging applications; and 3) developing and implementing a dual-optimization method to design the RF shielding for fast MRI imaging methods. First, three miniaturized implanted antennas are designed and results obtained using finite difference time domain (FDTD) simulations demonstrate that a maximum RF power of up to 1.8 miliwatts can be received at 2 GHz when the antennas are implanted at the dura, without violating the government safety regulations. Second, Eigenmode arrangement of the 20-channel transmit coil allows control of RF excitation not only at the XY plane but also along the Z direction. The presented results show the optimized eigenmode could generate 3D uniform transmit B1+ excitations. The optimization results have been verified by in-vivo experiments, and they are applied with different protocol sequences on a Siemens 7 Tesla MRI human whole body scanner equipped with 8 parallel transmit channels. Third, echo planar imaging (EPI), B1+ maps and S matrix measurements are used to verify that the proposed RF shielding can suppress the eddy currents while maintaining the RF characteristics of the transmit coil. The contributions presented here will provide a long-term and safer power transmission path compared to the wire-connected implanted BCIs and will bring ultrahigh field MRI technology closer to clinical applications

Entwicklung von Fluor-19 und Protonen-Magnetresonanztomographie und ihre Anwendung bei Neuroentzündung

The experimental autoimmune encephalomyelitis (EAE) is used to study multiple sclerosis (MS) pathology and develop novel technologies to quantify inflammation over time. Magnetic resonance imaging (MRI) with gadolinium-based contrast agents (GBCAs) is the state-of-the-art method to assess inflammation in MS patients and its animal model. Fluorine (19F)-MRI is one novel technology to quantify inflammatory immune cells in vivo using 19F-nanoparticles. T1 mapping of contrast-enhancing images is another method that could be implemented to quantify inflammatory lesions. Transient macroscopic changes in the EAE brain confound quantification and necessitate registration methods to spatially align images in longitudinal studies. For 19F-MRI, an additional challenge is the low signal-to-noise ratio (SNR) due to low number of 19F-labeled immune cells in vivo. Transceive surface radiofrequency (RF) probes and SNR-efficient imaging techniques such as RARE (Rapid Acquisition with Relaxation Enhancement) are combined to increase sensitivity in 19F-MRI. However, the strong spatially-varying RF field (B1 inhomogeneity) of transceive surface RF probes further hampers quantification. Retrospective B1 correction methods typically use signal intensity equations, unavailable for complex acquisition methods like RARE. The main goal of this work is to investigate novel B1 correction and registration methods to enable the study of inflammatory diseases using 1H- and 19F-MRI following GBCA and 19F-nanoparticle administration, respectively. For correcting B1 inhomogeneities in 1H- and 19F-MR transceive surface RF probes, a model-based method was developed using empirical measurements and simulations, and then validated and compared with a sensitivity method and a hybrid of both. For 19F-MRI, a workflow to measure anatomical images in vivo and a method to compute 19F-concentration uncertainty after correction using Monte Carlo simulations were developed. To overcome the challenges of EAE brain macroscopic changes, a pipeline for registering images throughout longitudinal studies was developed. The proposed B1 correction methods demonstrated dramatic improvements in signal quantification and T1 contrast on images of test phantoms and mouse brains, allowing quantitative measurement with transceive surface RF probes. For low-SNR scenarios, the model-based method yielded reliable 19F-quantifications when compared to volume resonators. Uncertainty after correction depended linearly on the SNR (≤10% with SNR≥10.1, ≤25% when SNR≥4.25). The implemented registration approach provided successful image alignment despite substantial morphological changes in the EAE brain over time. Consequently, T1 mapping was shown to objectively quantify gadolinium lesion burden as a measure of inflammatory activity in EAE. The 1H- and 19F-MRI methods proposed here are highly relevant for quantitative MR of neuroinflammatory diseases, enabling future (pre)clinical investigations.Die experimentelle Autoimmun-Enzephalomyelitis (EAE) wird zur Untersuchung Multipler Sklerose (MS) und zur Entwicklung neuer Technologien zur Entzündungsquantifizierung eingesetzt. Magnetresonanztomographie (MRT) mit Gadolinium-haltigen Kontrastmitteln (GBCAs) ist die modernste Methode zur Beurteilung von Entzündungen bei MS-Patienten und im Tiermodell. Fluor (19F)-MRT unter Verwendung von 19F-Nanopartikeln ist eine neue Technologie zur Quantifizierung entzündlicher Immunzellen in vivo. T1-Kartierung ist eine MRT-Methode, die zur Quantifizierung entzündlicher Läsionen eingesetzt werden könnte. Temporäremorphologische Veränderungen im EAE-Gehirn erschweren die Quantifizierung und erfordern Registrierungsmethoden, um MRT-Bilder in Längsschnittstudien räumlichabzugleichen. Das niedrige Signal-Rausch-Verhältnis (SNR) ist aufgrund der geringen Anzahl 19F-markierter Immunzellen in vivo eine zusätzliche Herausforderung der 19F-MRT. Um deren Empfindlichkeit zu erhöhen, werden Sende-/Empfangsoberflächen-Hochfrequenzspulen (TX/RX-HF-Spule) und SNR-effiziente MRT-Techniken wie RARE (Rapid Acquisition with Relaxation Enhancement) kombiniert. Jedoch verhindert die starke räumliche Variation des HF-Feldes (B1-Inhomogenität) dieser Spulen die Signalquantifizierung. Retrospektive B1-Korrekturmethoden verwenden in der Regel Signalintensitätsgleichungen, die für komplexe MRT-Techniken wie RARE nicht existieren. Das Hauptziel dieser Arbeit ist die Untersuchung neuartiger B1-Korrektur- und Bildregistrierungsmethoden, um in vivo 1H- und 19F-MRT Studien von Entzündungsprozessen zu ermöglichen. Zur Korrektur von B1-Inhomogenitäten wurde eine modellbasierte Methode entwickelt. Diese verwendet empirische Messungen und Simulationen, wurde in Phantomexperimenten validiert und mit Referenzmethoden verglichen. Für 19F-MRT wurden ein Protokoll zur Messung anatomischer Bilder in vivo und eine Methode zur Berechnung der 19F-Konzentrationsunsicherheit nach Korrektur mittels Monte-Carlo-Simulationen entwickelt. Um morphologische Veränderungen im EAE-Gehirn in longitudinalen Studien zu kompensieren, wurde zur Bildregistrierung eine Software-Bibliothek entwickelt. Die B1-Korrekturmethoden zeigten in Testobjekten und Mäusehirnen drastische Verbesserungen der Signal- und T1 Quantifizierung und ermöglichten so quantitative Messungen mit TX/RX-HF-Spulen. Die modellbasierte Methode lieferte für geringe SNRs zuverlässige 19F-Quantifizierungen, deren Genauigkeit mit dem SNR korrelierte. Die implementierte Registrierungsmethode ermöglichte einen erfolgreichen Abgleich von Bildserientrotz erheblicher morphologischer Veränderungen im EAE-Hirn. Folglich wurde gezeigt, dass MRT basierte T1-Kartierung die Gadolinium-Läsionslast als Maß entzündlicher Aktivität bei EAE objektiv quantifizieren kann. Die hier unterscuhten Methoden sind für quantitative 1H- und 19F-MRT neuroinflammatorischer Erkrankungen sehr relevant und ermöglichen künftige (prä)klinische Untersuchungen

High spatial resolution and temporally resolved t(2) (*) mapping of normal human myocardium at 7.0 tesla: an ultrahigh field magnetic resonance feasibility study

Myocardial tissue characterization using T(2) (*) relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T(2) (*) mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T(2) (*) imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T(2) (*) mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T(2) (*) maps. In vivo studies showed that the peak-to-peak B(0) difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T(2) (*) weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T(2) (*) values were found for anterior (T(2) (*) = 14.0 ms), anteroseptal (T(2) (*) = 17.2 ms) and inferoseptal (T(2) (*) = 16.5 ms) myocardial segments. Shorter T(2) (*) values were observed for inferior (T(2) (*) = 10.6 ms) and inferolateral (T(2) (*) = 11.4 ms) segments. A significant difference (p = 0.002) in T(2) (*) values was observed between end-diastole and end-systole with T(2) (*) changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T(2) (*) mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes

W(h)ither human cardiac and body magnetic resonance at ultrahigh fields? Technical advances, practical considerations, applications, and clinical opportunities.

The objective of this study was to document and review advances and groundbreaking progress in cardiac and body MR at ultrahigh fields (UHF, B0 ≥ 7.0 T) with the goal to attract talent, clinical adopters, collaborations and resources to the biomedical and diagnostic imaging communities. This review surveys traits, advantages and challenges of cardiac and body MR at 7.0 T. The considerations run the gamut from technical advances to clinical opportunities. Key concepts, emerging technologies, practical considerations, frontier applications and future directions of UHF body and cardiac MR are provided. Examples of UHF cardiac and body imaging strategies are demonstrated. Their added value over the kindred counterparts at lower fields is explored along with an outline of research promises. The achievements of cardiac and body UHF-MR are powerful motivators and enablers, since extra speed, signal and imaging capabilities may be invested to overcome the fundamental constraints that continue to hamper traditional cardiac and body MR applications. If practical obstacles, concomitant physics effects and technical impediments can be overcome in equal measure, sophisticated cardiac and body UHF-MR will help to open the door to new MRI and MRS approaches for basic research and clinical science, with the lessons learned at 7.0 T being transferred into broad clinical use including diagnostics and therapy guiding at lower field

RF Studies for Ultrahigh Field MRI RF Coils and Arrays

Over the past few decades, different research groups have worked on different facets of Ultra-High Field (UHF) Magnetic Resonance Imaging (MRI); these developments culminated with the FDA approval of the first clinical 7 Tesla (T) MR scanner, Siemens MAGNETOM Terra in late-2017. MRI is still the preferred non-invasive multi-modal imaging technique for visualization of structural and functional correlates in-vivo and clinical diagnosis. Key issues with UHF MRI are in homogeneities in electric and magnetic fields as the size of imaged object becomes comparable with or larger than the radiofrequency (RF) wavelength. This inherent electromagnetic field inhomogeneity and elevated RF power deposition associated with UHF human imaging can have detrimental effects on the quality and safety in high field MRI. To address these challenges, the research work presented in this study 1) evaluated different cylindrical loop receive (Rx) array geometry to establish their effect on the transmit (Tx) coil RF fields. 2) performed detailed analysis, phantom and in-vivo, comparing the performance of the Tic Tac Toe (TTT) coil with a 16-element Transverse Electromagnetic (TEM) coil using multiple anatomical head models and in-vivo. The abovementioned areas of research included: Rx geometry model extraction from CAD models, and development of multiple anatomically detailed models and evaluation of MR coils simulations using full wave Maxwell's equations. Furthermore, an important part of the thesis work was bench marking of transmit coil performance for efficient and safe use in-vivo. The transmit arrays were tested for reproducibility, reliability and safe usage across multiple studies. Finite Difference Time Domain simulations of the Tx and composite of five head models were used to optimize parameters, to obtain homogenous whole brain excitation with low RF absorption or specific absorption rate (SAR)

SAFETY AND PERFORMANCE CONSIDERATIONS FOR INTERACTIONS BETWEEN ELECTROMAGNETIC FIELDS AND BIOLOGICAL TISSUE: APPLICATIONS TO HIGH FIELD HUMAN MAGNETIC RESONANCE IMAGING AND TISSUE-IMPLANTED DEVICES

The principal advantage of magnetic resonance imaging (MRI) at high field is the increase in signal to noise ratio (SNR), however, high field imaging also leads to an increased Larmor (operating) frequency, thus the wavelength in tissue can become comparable to the size of the load and/or the coil. The performance of the radiofrequency (RF) coil, as a result, becomes increasingly dependent on its electromagnetic interactions with the load, human body or head.Invasive brain machine interface (BMI) technology uses implanted microelectrodes to capture the action potentials of many individual neurons, especially those that code for movement or its intent. Traditionally, stimulating nerves or brain tissue involves cumbersome wiring to power/communicate with the chip. To avoid the limit of the BMI's mobility and freedom, RF powered wireless implementation of a BMI chip has been proposed to widely extend BMI applications. It is essential to perform an analysis of electromagnetic power deposition throughout the human head to determine the amount of power available to BMI devices.In this dissertation, a complete electromagnetic computational (full wave) analysis, the finite difference time domain (FDTD) method, is applied to calculate the interaction between the radio frequency (RF) magnetic field and the subjects during ultra high field MRI exams and wireless BMI operations. The interactions between the high frequency RF fields with the human head and the body models severely affect the performances of MRI and BMI operations, and they also cause heating safety concerns to the tissues exposed to the RF radiation. Through precisely numerical calculations, we accomplished in this dissertation 1) an improved optimization scheme using variable phase and variable amplitude excitation to improve the performance of RF transverse electromagnetic (TEM) coils in MRI with safety concerns; and 2) evaluations of the performance and safety for a prototype of the wireless invasive BMI. Temperature changes caused by RF power deposition are calculated in both MRI and BMI applications

32‐Channel self‐grounded bow‐tie transceiver array for cardiac MR at 7.0T

Purpose Design, implementation, evaluation, and application of a 32‐channel Self‐Grounded Bow‐Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. Methods The array consists of 32 compact SGBT building blocks. Transmission field (B1+) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo B1+ efficiency mapping was conducted with actual flip angle imaging. The array’s applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n = 7). Results B1+ shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured B1+ efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5) mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R = 4 (mean signal‐to‐noise ratio (whole heart) ≥ 16.7, mean contrast‐to‐noise ratio ≥ 13.5) without impairing image quality significantly. Conclusion The compact SGBT building block facilitates a modular high‐density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques.EC/H2020/743077/EU/Thermal Magnetic Resonance: A New Instrument to Define the Role of Temperature in Biological Systems and Disease for Diagnosis and Therapy/ThermalMRBMBF, 01QE1815, Verbundprojekt: Seeing is Believing: Revolution der bildgebenden Diagnostik und Therapiekontrolle des Körperstammes durch superaufgelöste Hochfeld-Magnetresonanztomographie; Teilprojekt: Industrielle Forschung und Entwicklung lokaler Radiofrequenz-Antennen für hochauflösende Hochfeld-MRT des Körperstamme

Technological innovations in magnetic resonance for early detection of cardiovascular diseases

Most recent technical innovations in cardiovascular MR imaging (CMRI) are presented in this review. They include hardware and software developments, and novelties in parametric mapping. All these recent improvements lead to high spatial and temporal resolution and quantitative information on the heart structure and function. They make it achievable ambitious goals in the field of mapletic resonance, such as the early detection of cardiovascular pathologies. In this review article, we present recent innovations in CMRI, emphasizing the progresses performed and the solutions proposed to some yet opened technical problems

Ultra-high spatial resolution BOLD fMRI in humans using combined segmented-accelerated VFA-FLEET with a recursive RF pulse design

Purpose To alleviate the spatial encoding limitations of single-shot EPI by developing multi-shot segmented EPI for ultra-high-resolution fMRI with reduced ghosting artifacts from subject motion and respiration. Methods Segmented EPI can reduce readout duration and reduce acceleration factors, however, the time elapsed between segment acquisitions (on the order of seconds) can result in intermittent ghosting, limiting its use for fMRI. Here, "FLEET" segment ordering--where segments are looped over before slices--was combined with a variable flip angle progression (VFA-FLEET) to improve inter-segment fidelity and maximize signal for fMRI. Scaling a sinc pulse's flip angle for each segment (VFA-FLEET-Sinc) produced inconsistent slice profiles and ghosting, therefore, a recursive Shinnar-Le Roux (SLR) RF pulse design was developed (VFA-FLEET-SLR) to generate unique pulses for every segment that together produce consistent slice profiles and signals. Results The temporal stability of VFA-FLEET-SLR was compared against conventional-segmented EPI and VFA-FLEET-Sinc at 3 T and 7 T. VFA-FLEET-SLR showed reductions in both intermittent and stable ghosting compared to conventional-segmented and VFA-FLEET-Sinc, resulting in improved image quality with a minor trade-off in temporal SNR. Combining VFA-FLEET-SLR with acceleration, we achieved a 0.6-mm isotropic acquisition at 7 T--without zoomed imaging or partial Fourier--demonstrating reliable detection of BOLD responses to a visual stimulus. To counteract the increased repetition time from segmentation, simultaneous multi-slice VFA-FLEET-SLR was demonstrated using RF-encoded controlled aliasing. Conclusions VFA-FLEET with a recursive RF pulse design supports acquisitions with low levels of artifact and spatial blur, enabling fMRI at previously inaccessible spatial resolutions with a "full-brain" field of view.Comment: 51 pages (including supplement), 8 main figures, 6 supporting figures. For supporting videos (8), please visit https://github.com/aveberman/vfa-fleet. Note: this work has been accepted for publication at Magnetic Resonance in Medicin