1,014 research outputs found
Comparison of 7T 16-channel Dual-row Transmit Arrays
We evaluated and compared the performance of an inductively decoupled and overlapped dual-row transmit arrays for MRI at 7T. For the evaluated designs, the coupling between adjacent elements in the same row was higher for the overlapped compared to the non-overlapped configuration. However the transmit efficiencies for the circular polarization mode of both coils were similar. For comparisons of array transmit performance, consideration of array-internal losses as well as reflected and radiated power is very important, because their sum can be as high as 55% of the total transmit power
Double Cross Magnetic Wall Decoupling for Quadrature Transceiver RF Array Coils using Common-Mode Differential-mode Resonators
In contrast to linearly polarized RF coil arrays, quadrature transceiver coil
arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution
and parallel imaging performance. Owing to a reduced excitation power, low
specific absorption rate can be also obtained using quadrature RF coils.
However, due to the complex nature of their structure and their electromagnetic
proprieties, it is challenging to achieve sufficient electromagnetic decoupling
while designing multichannel quadrature RF coil arrays, particularly at
ultrahigh fields. In this work, we proposed a double cross magnetic wall
decoupling for quadrature transceiver RF arrays and implemented the decoupling
method on common-mode differential mode quadrature (CMDM) quadrature
transceiver arrays at ultrahigh field of 7T. The proposed magnetic decoupling
wall comprised of two intrinsic decoupled loops is used to reduce the mutual
coupling between all the multi-mode current present in the quadrature CMDM
array. The decoupling network has no physical connection with the CMDMs' coils
giving leverage over size adjustable RF arrays. In order to validate the
feasibility of the proposed cross magnetic decoupling wall, systematic studies
on the decoupling performance based on the impedance of two intrinsic loops are
numerically performed. A pair of quadrature transceiver CMDMs is constructed
along with the proposed decoupling network and their scattering matrix is
characterized using a network analyzer. The measured results show all the
current modes coupling are concurrently suppressed using the proposed cross
magnetic wall. Moreover, field distribution, and SNR intensity are numerically
obtained for a well-decoupled 8-channel quadrature knee-coil array.Comment: 9 pages, 10 Figure
Twisted Pair Transmission Line Coil -- A Flexible, Self-Decoupled and Extremely Robust Element for 7T MRI
This study evaluates the performance of a twisted pair transmission line coil
as a transceive element for 7T MRI in terms of physical flexibility, robustness
to shape deformations, and interelement decoupling. Each coil element was
created by shaping a twisted pair of wires into a circle. One wire was
interrupted at the top, while the other was interrupted at the bottom, and
connected to the matching circuit. Electromagnetic simulations were conducted
to determine the optimal number of twists per length (in terms of B field
efficiency, SAR efficiency, sensitivity to elongation and interelement
decoupling properties) and for investigating the fundamental operational
principle of the coil through fields streamline visualization. A comparison
between the twisted pair coil and a conventional loop coil in terms of B
fields, maxSAR10g, and stability of when the coil was deformed, was
performed. Experimentally measured interelement coupling between individual
elements of multichannel arrays was also investigated. Increasing the number of
twists per length resulted in a more physically robust coil. Poynting vector
streamline visualization showed that the twisted pair coil concentrated most of
the energy in the near field. The twisted pair coil exhibited comparable
B fields and improved maxSAR10g to the conventional coil but demonstrated
exceptional stability with respect to coil deformation and a strong
self-decoupling nature when placed in an array configuration. The findings
highlight the robustness of the twisted pair coil, showcasing its stability
under shape variations. This coil holds great potential as a flexible RF coil
for various imaging applications using multiple-element arrays, benefiting from
its inherent decoupling.Comment: Revised version; 20 pages, 16 figures, preprin
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)
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
Loop radiofrequency coils for clinical magnetic resonance imaging at 7 TESLA
To date, the 7 T magnetic resonance imaging (MRI) scanner remains a pure research system and there is still a long way ahead till full clinical integration. Key challenges are the absence of a body transmit radiofrequency (RF) coil as well as of dedicated RF coils in general, short RF wavelengths of the excitation field in the order of the dimensions of a human body leading to signal inhomogeneities, and severe limitations with respect to the specific absorption rate. They all result in a strong need for RF engineering and sequence optimization to explore the potential of MRI at 7 T, and to pave the way for its future clinical application. In this thesis, high-resolution MRI with a rather small field-of-view (FOV) in the head and neck region (parotid gland/duct and carotid arteries), and of the musculoskeletal system as well as with a very large FOV in the abdomen (spine) were presented. Therefore, a variety of RF coils were used: from a commercially available single-loop coil to novel, specially developed phased array coils each consisting of eight loop elements. Methods to thoroughly characterize and test the developed RF coils were presented, including numerical simulations, bench and MRI measurements. Characterization with respect to performance for parallel acquisition techniques and an extensive compliance testing for patient safety were described in detail. All aspects of the engineering part, from design to optimization, and finally, to the in vivo application in volunteers and patients were covered. Since clinical applicability has always been the purpose, optimized imaging protocols along with a discussion on the clinical relevance was included in each study. The presented RF loop coils widely expand the options for clinical research at 7 T and advance the integration of this technology in a clinical setting
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
The UTE and ZTE Sequences at Ultra-High Magnetic Field Strengths: A Survey
UTE (Ultrashort Echo Time) and ZTE (Zero Echo Time) sequences have been
developed to detect short T2 relaxation signals coming from regions that are
unable to be detected by conventional MRI methods. Due to the high
dipole-dipole interactions in solid and semi-solid tissues, the echo time
generated is simply not enough to produce a signal using conventional imaging
method, often leading to void signal coming from the discussed areas. By the
application of these techniques, solid and semi-solid areas can be imaged which
can have a profound impact in clinical imaging. High and Ultra-high field
strength (UHF) provides a vital advantage in providing better sensitivity and
specificity of MR imaging. When coupled with the UTE and ZTE sequences, the
image can recover void signals as well as a much-improved signal quality. To
further this strategy, secondary data from various research tools was obtained
to further validate the research while addressing the drawbacks to this
approach. It was found that UTE and ZTE sequences coupled with some techniques
such as qualitative imaging and new trajectories are very crucial for accurate
image depiction of the areas of the musculoskeletal system, neural system, lung
imaging and dental imaging
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