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)

Optimization of Radiofrequency Coils for Human Brain Magnetic Resonance Spectroscopy at Ultra-High Field Strength

Magnetic resonance spectroscopy (MRS) is a non-invasive and non-ionizing technique to acquire localized spectra of metabolites in vivo. With increasing static magnetic field strength, the spectral separation of the metabolites and the signal-to-noise ratio (SNR) of the spectrum increase. Consequently, the number of detectable metabolites and the spatial specificity are enhanced at ultra-high fields. At the same time, the wavelength of the radiofrequency (RF) field is decreased. For proton spectroscopy at ultra-high fields, the wavelength of the RF field in tissue is smaller than the typical dimension of a human head. From the perspective of electromagnetic theory, this means that a quasistatic approximation of Maxwell's equations is not valid anymore and the electromagnetic field must be calculated with the full system of coupled partial differential equations. Therefore, RF coil designs based on the quasistatic approximation, such as the birdcage coil or loop-only receive arrays, have suboptimal performance at ultra-high fields. This PhD project explored the optimization of RF coils for ultra-high field MRS. The optimization was based on an equivalent surface current distribution surrounding a human head model. It could be shown, that the equivalent surface current distribution can be separated into curl- and divergence-free components. The full-wave electromagnetic field problem was solved by a newly developed dyadic Green's functions approach. As a first optimization goal, the SNR was maximized in a spherical- and later in a realistic human head model. By optimizing the complete set of curl- and divergence-free surface current components, an upper threshold for the achievable SNR of any receive array could be calculated; this so-called ultimate intrinsic SNR (UISNR) was studied at all practically relevant field strengths regarding human head applications. The UISNR increased superlinearly with main magnetic field in central regions of the human brain. In a next step, the SNR optimization was done separately for curl- and divergence-free current components. This yielded a direct performance measure of how close loop-only and dipole-only receive arrays were able to approach the UISNR in the human head. Based upon this analysis, field strength specific design guidelines for RF receive arrays were deduced. In conclusion, at ultra-high field strength a combination of loop and dipole elements is necessary to achieve the best possible SNR at any position in the human head. As a second optimization goal, the coupling of multi-channel RF arrays was minimized. For that, a fast analytical model describing the complex mutual coupling between two surface loops was introduced. To understand and eliminate both electric and magnetic coupling between the loops, the influence of the loop geometry and loading by the sample was systematically examined. For the first time, it was demonstrated that at 400 MHz it is possible to eliminate both, electric and magnetic coupling simultaneously by proper adjustment of the loop width and overlap. A fully decoupled two channel prototype array was constructed having superior transmit and receive performance over a previously used gapped design.Die Magnetresonanz-Spektroskopie ist ein nichtinvasives und nichtionisierendes In-vivo-Verfahren mit dem Stoffwechselvorgänge im menschlichen Körper ortsaufgelöst erfasst werden können. Mit zunehmender statischer Feldstärke vergrößert sich sowohl die relative Frequenzverschiebung der Metaboliten zueinander, als auch das Signal-zu-Rauschverhältnis (SNR) des gesamten Spektrums. Infolgedessen können im Ultrahochfeld eine größere Anzahl an Metaboliten unterschieden werden. Ein weiterer Vorteil der Ultrahochfeld-Spektroskopie besteht darin, dass sich die Metaboliten mit höherer örtlicher Spezifität erfassen lassen. Gleichzeitig weist das von einer Hochfrequenzspule angeregte elektromagnetische Wechselfeld eine kürzere Wellenlänge auf. In menschlichem Gewebe unterschreitet die Wellenlänge des Hochfrequenzfeldes bei Protonenanregung im Ultrahochfeld die typischen Abmessungen des menschlichen Kopfes. Vom Standpunkt der elektromagnetischen Feldtheorie aus betrachtet bedeutet dies, dass eine quasistatische Näherung der Maxwell-Gleichungen nicht mehr anwendbar ist und das vollständige, gekoppelte, partielle Differentialgleichungssystem gelöst werden muss. Daher ist die Leistungsfähigkeit herkömmlicher Spulenkonzepte (z.B. Birdcage Spule oder Schleifenarrays), die auf einer quasistatischen Näherung beruhen, im Ultrahochfeld suboptimal. Im Rahmen dieser Promotion wurde die Optimierung von Hochfrequenzspulen für die Hochfeldmagnetresonanz-Spektroskopie erforscht. Als Optimierungsvariable wurde eine äquivalente Oberflächenstromverteilung definiert, die den menschlichen Kopf umgab. Es wurde gezeigt, dass diese Oberflächenstromverteilung in divergenz- und rotationsfreie Komponenten zerlegt werden kann. Das elektromagnetische Feldproblem wurde mithilfe eines neuartigen Ansatzes, der auf dyadischen Green'schen Funktionen beruht, gelöst. Das erste Optimierungsziel bestand darin, das SNR in einem kugelförmigen, später in einem realistischen Kopfmodell zu maximieren. Zunächst wurde die vollständige Oberflächenstromverteilung aus divergenz- und rotationsfreien Komponenten optimiert. Auf diese Weise konnte das bestmöglich erzielbare SNR beliebiger Empfangsarrays angegeben werden. Dieses optimale intrinsische SNR (UISNR) wurde für sämtliche, praktisch relevante Magnetfeldstärken im menschlichen Kopf untersucht. Dabei stieg das UISNR in zentralen Hirnregionen überproportional mit der Magnetfeldstärke an. Als nächstes wurde die Fragestellung eruiert, inwieweit sich der zuvor theoretisch ermittelte SNR-Grenzwert mit Schleifen- bzw. Dipolarrays erreichen lässt. Dazu wurde die Optimierung jeweils mit divergenz- und rotationsfreien Oberflächenstromkomponenten separat durchgeführt und entsprechende Kenngrößen für die Empfangseigenschaften von Schleifen- und Dipolarrays abgeleitet. Aus diesen Ergebnissen wurden frequenzabhängige Designrichtlinien für Hochfrequenz-Empfangsspulen formuliert. Es zeigte sich, dass eine Kombination aus Schleifen- und Dipolelementen im Ultrahochfeld unerlässlich ist, um das bestmögliche SNR im menschlichen Kopf zu erzielen. Das zweite Optimierungsziel bestand darin, die Kopplung von Mehrkanal-Antennenarrays zu minimieren. Zunächst wurden der elektrische und magnetische Koppelfaktor zweier benachbarter Leiterschleifen durch ein neu entwickeltes analytisches Modell beschrieben. Mithilfe des neu entwickelten Modells wurden anschließend die Schleifengeometrie und der Abstand der Schleifen zu einem zylinderförmigen Kopfmodell systematisch variiert. So konnte erstmals gezeigt werden, dass durch geschickte Überlappung und eine optimierte Schleifenbreite bei 400 MHz die elektrische und die magnetische Kopplung gleichzeitig ausgelöscht werden können. Als Machbarkeitsstudie wurde ein vollständig entkoppeltes Zweikanalarray aufgebaut und mit einem früheren Design aus nicht-überlappenden Schleifenelementen verglichen. Das neu entwickelte, überlappende Arraydesign zeigte signifikant verbesserte Sende- und Empfangseigenschaften gegenüber der früheren Version

Multiple resonant multiconductor transmission line resonator design using circulant block matrix algebra

The purpose of this dissertation is to provide a theoretical model to design RF coils using multiconductor transmission line (MTL) structures for MRI applications. In this research, an MTL structure is represented as a multiport network using its port admittance matrix. Resonant conditions and closed-form solutions for different port resonant modes are calculated by solving the eigenvalue problem of port admittance matrix using block matrix algebra. A mathematical proof to show that the solution of the characteristic equation of the port admittance matrix is equivalent to solving the source side input impedance is presented. The proof is derived by writing the transmission chain parameter matrix of an MTL structure, and mathematically manipulating the chain parameter matrix to produce a solution to the characteristic equation of the port admittance matrix. A port admittance matrix can be formulated to take one of the forms depending on the type of MTL structure: a circulant matrix, or a circulant block matrix (CB), or a block circulant circulant block matrix (BCCB). A circulant matrix can be diagonalized by a simple Fourier matrix, and a BCCB matrix can be diagonalized by using matrices formed from Kronecker products of Fourier matrices. For a CB matrix, instead of diagonalizing to compute the eigenvalues, a powerful technique called “reduced dimension method� can be used. In the reduced dimension method, the eigenvalues of a circulant block matrix are computed as a set of the eigenvalues of matrices of reduced dimension. The required reduced dimension matrices are created using a combination of the polynomial representor of a circulant matrix and a permutation matrix. A detailed mathematical formulation of the reduced dimension method is presented in this thesis. With the application of the reduced dimension method for a 2n+1 MTL structure, the computation of eigenvalues for a 4n X 4n port admittance matrix is simplified to the computation of eigenvalues of 2n matrices of size 2 X 2. In addition to reduced computations, the model also facilitates analytical formulations for coil resonant conditions. To demonstrate the effectiveness of the proposed methods (2n port model and reduced dimension method), a two-step approach was adopted. First, a standard published RF coil was analyzed using the proposed models. The obtained resonant conditions are then compared with the published values and are verified by full-wave numerical simulations. Second, two new dual tuned coils, a surface coil design using the 2n port model, and a volume coil design using the reduced dimensions method are proposed, constructed, and bench tested. Their validation was carried out by employing 3D EM simulations as well as undertaking MR imaging on clinical scanners. Imaging experiments were conducted on phantoms, and the investigations indicate that the RF coils achieve good performance characteristics and a high signal-to-noise ratio in the regions of interest

Design of Radio-Frequency Arrays for Ultra-High Field MRI

Magnetic Resonance Imaging (MRI) is an indispensable, non-invasive diagnostic tool for the assessment of disease and function. As an investigational device, MRI has found routine use in both basic science research and medicine for both human and non-human subjects. Due to the potential increase in spatial resolution, signal-to-noise ratio (SNR), and the ability to exploit novel tissue contrasts, the main magnetic field strength of human MRI scanners has steadily increased since inception. Beginning in the early 1980’s, 0.15 T human MRI scanners have steadily risen in main magnetic field strength with ultra-high field (UHF) 8 T MRI systems deemed to be insignificant risk by the FDA (as of 2016). However, at UHF the electromagnetic fields describing the collective behaviour of spin dynamics in human tissue assume ‘wave-like’ behaviour due to an increase in the processional frequency of nuclei at UHF. At these frequencies, the electromagnetic interactions transition from purely near-field interactions to a mixture of near- and far-field mechanisms. Due to this, the transmission field at UHF can produce areas of localized power deposition – leading to tissue heating – as well as tissue-independent contrast in the reconstructed images. Correcting for these difficulties is typically achieved via multi-channel radio-frequency (RF) arrays. This technology allows multiple transmitting elements to synthesize a more uniform field that can selectively minimize areas of local power deposition and remove transmission field weighting from the final reconstructed image. This thesis provides several advancements in the design and construction of these arrays. First, in Chapter 2 a general framework for modeling the electromagnetic interactions occurring inside an RF array is adopted from multiply-coupled waveguide filters and applied to a subset of decoupling problems encountered when constructing RF arrays. It is demonstrated that using classic filter synthesis, RF arrays of arbitrary size and geometry can be decoupled via coupling matrix synthesis. Secondly, in Chapters 3 and 4 this framework is extended for designing distributed filters for simple decoupling of RF arrays and removing the iterative tuning portion of utilizing decoupling circuits when constructing RF arrays. Lastly, in Chapter 5 the coupling matrix synthesis framework is applied to the construction of a conformal transmit/receive RF array that is shape optimized to minimize power deposition in the human head during any routine MRI examination

Shielded-coaxial-cable coils as receive and transceive array elements for 7T human MRI

Purpose: To investigate the use of shielded-coaxial-cable (SCC) coils as elements for multi-channel receive-only and transceive arrays for 7T human MRI and to compare their performance with equivalently sized conventional loop coils.Methods: The SCC coil element consists of a coaxial loop with interrupted central conductor at the feed-point side and an interrupted shield at the opposite point. Interelement decoupling, transmit efficiency, and sample heating were compared with results from conventional capacitively segmented loop coils. Three multichannel arrays (a 4-channel receive-only array and 8- and 5-channel transceive arrays) were constructed. Their inter-element decoupling was characterized via measured noise correlation matrices and additionally under different flexing conditions of the coils. Thermal measurements were performed and in vivo images were acquired.Results: The measured and simulated B-1(+) maps of both SCC and conventional loops were very similar. For all the arrays constructed, the inter-element decoupling was much greater for the SCC elements than the conventional ones. Even under high degrees of flexion, the coupling coefficients were lower than -10 dB, with a much smaller frequency shift than for the conventional coils.Conclusion: Arrays constructed from SCC elements are mechanically flexible and much less sensitive to changes of the coil shape from circular to elongated than arrays constructed from conventional loop coils, which makes them suitable for construction of size adjustable arrays.Radiolog

Radio Frequency Coils for Ultra-high Field Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) has become a powerful tool not only to analyze the anatomical structures of the human body non invasively but also to investigate brain activity with functional MRI. The promise of increase in signal to noise ratio and spectral resolution proportional to the main magnetic field strength motivated a few research laboratories to pursue even higher field strengths. The 9.4T whole body human scanner and the 16.4T animal scanner installed at the Max Planck Institute for Biological Cybernetics, Tuebingen were, for many years, the worlds strongest magnets in their respective categories. In addition to the strong magnets, radio frequency (RF) coils are also equally important in realising the benefits offered by the high field MRI scanners. The aim of this thesis work is to develop optimized RF coils and RF hardware for ultra-high high field MRI

Master of Science

thesisThe purpose of this work was to design and construct a radio-frequency coil optimized for imaging the Optic Nerve (ON) on a Siemens 3T magnetic resonance imaging (MRI) scanner. The specific goals were to optimize signal sensitivity from the orbit to the optic chiasm and improve SNR over designs currently in use. The constructed coil features two fiberglass formers that can slide over each other to accommodate any arbitrary head size, while maintaining close coupling near the eyes and around the head in general. This design eliminates the air void regions that occur between the coil elements and the forehead when smaller heads are imaged in one-piece, nonadjustable coil formers. The 28 coil elements were placed using a soccer-ball pattern layout to maximize head coverage. rSNR profiles from phantom imaging studies show that the ON coil provides approximately 55% greater rSNR at the region of the optic chiasm and approximately 400% near the orbits compared to the 12-channel commercial coil. The improved rSNR in the optic nerve region allows performance of high resolution DTI, which provides a qualitative measurement for evaluating optic neuritis. Images from volunteer and patient studies with the ON coil reveal plaques that correspond well with the patient disease history of chronic bilateral optic neuritis. Correspondence of image findings with patient disease histories demonstrates that optic neuritis can be visualized and detected in patients using 3T MRI with advanced imaging coils, providing improved patient care

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

Application of Parallel Imaging to Murine Magnetic Resonance Imaging

The use of parallel imaging techniques for image acceleration is now common in clinical magnetic resonance imaging (MRI). There has been limited work, however, in translating the parallel imaging techniques to routine animal imaging. This dissertation describes foundational level work to enable parallel imaging of mice on a 4.7 Tesla/40 cm bore research scanner. Reducing the size of the hardware setup associated with typical parallel imaging was an integral part of achieving the work, as animal scanners are typically small-bore systems. To that end, an array element design is described that inherently decouples from a homogenous transmit field, potentially allowing for elimination of typically necessary active detuning switches. The unbalanced feed of this "dual-plane pair" element also eliminates the need for baluns in this case. The use of the element design in a 10-channel adjustable array coil for mouse imaging is presented, styled as a human cardiac top-bottom half-rack design. The design and construction of the homogenous transmit birdcage coil used is also described, one of the necessary components to eliminating the active detuning networks on the array elements. In addition, the design of a compact, modular multi-channel isolation preamplifier board is described, removing the preamplifiers from the elements and saving space in the bore. Several additions/improvements to existing laboratory infrastructure needed for parallel imaging of live mice are also described, including readying an animal preparation area and developing the ability to maintain isoflurane anesthesia delivery during scanning. In addition, the ability to trigger the MRI scanner to the ECG and respiratory signals from the mouse in order to achieve images free from physiological motion artifacts is described. The imaging results from the compact 10-channel mouse array coils are presented, and the challenges associated with the work are described, including difficulty achieving sample-loss dominance and signal-to-noise ratio (SNR) limitations. In conclusion, in vivo imaging of mice with cardiac and respiratory gating has been demonstrated. Compact array coils tailored for mice have been studied and potential future work and design improvements for our lab in this area are discussed

Engineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla

Magnetic resonance imaging is widely used in medical diagnosis to obtain anatomical details of the human body in a non-invasive way. Clinical MR scanners typically operate at a static magnetic field strength (B0) of 1.5T or 3T. However, going to higher field is of great interest since the signal-to-noise ratio is proportional to B0. Therefore, higher image resolution and better contrast between the human tissues could be achieved. Nevertheless, new challenges arise when increasing B0. The wavelength associated with the radio-frequency field B1+ has smaller dimensions - approx. 12 cm for human brain tissues - than the human brain itself (20 cm in length), the organ of interest in this thesis. The main consequence is that the transmit field distribution pattern (B1+) is altered and the final MR images present bright and dark signal spots. These effects prevent the ultra-high field MR scanners (>= 7T) to be used for routine clinical diagnosis. Parallel-transmit is one approach to address these new challenges. Instead of using an RF coil connected to a single power input as it is commonly done at lower magnetic fields, multiple RF coils are used with independent power inputs. The subsequent distinct RF signals can be manipulated separately, which provides an additional degree of freedom to generate homogeneous B1+-field distributions over large or specific regions in the human body. A transmit/receive RF coil array optimized for whole-brain MR imaging was developed and is described in this thesis. Dipoles antennas were used since they could provide a large longitudinal (vertical axis-head to neck) coverage and high transmit field efficiency. Results demonstrated a complete coverage of the human brain, and particularly high homogeneity over the cerebellum. However, since the receive sensitivity over large field-of-views is related to the number of channels available to detect the NMR signal, the next work was to add a 32-channel receive loop coil array to the transmit coil array. The complete coverage of the human brain was assessed with a substantial increase in signal-to-noise compared to the transmit/receive dipole coil array alone. Moreover, acquisition time was shortened since higher acceleration factors could be used. To optimize the individual RF fields and generate an homogeneous B1+-field, a method was developed making use of the particle-swarm algorithm. A user-friendly graphical interface was implemented. Good homogeneity could be achieved over the whole-brain after optimization with the coil array built in this study. Moreover, the optimization was shown to be robust across multiple subjects. The last project was focused on the single transmit system. Local volume coils (single transmit) present pronounced transmit field inhomogeneities in specific regions of the human brain such as the temporal lobes. A widely used approach to address locally these challenges is to add dielectric pads inside the volume coils to enhance the local transmit field efficiency. It was shown in this thesis that constructing dedicated surface coils is a valuable alternative to the dielectric pads in terms of transmit field efficiency and MR spectroscopy results. Two RF coil setups were developed for the temporal and frontal lobes of the human brain, respectively. This thesis provides extensive insight on MR engineering of RF coils at ultra-high field and the potential of parallel-transmit to address the future needs in clinical applications