12 research outputs found
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
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 ( = 10.7
MHz/T; = 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
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
Hairpin RF Resonators for Transceiver Arrays with High Inter-channel Isolation and B1 Efficiency at Ultrahigh Field 7T MR Imaging
Electromagnetic decoupling among a close-fitting or high-density transceiver
RF array elements is required to maintain the integrity of the magnetic flux
density from individual channel for enhanced performance in detection
sensitivity and parallel imaging. High-impedance RF coils have demonstrated to
be a prominent design method to circumvent these coupling issues. Yet, inherent
characteristics of these coils have ramification on the B1 field efficiency and
SNR. In this work, we propose a hairpin high impedance RF resonator design for
highly decoupled multichannel transceiver arrays at ultrahigh magnetic fields.
Due to the high impedance property of the hairpin resonators, the proposed
transceiver array can provide high decoupling performance without using any
dedicated decoupling circuit among the resonant elements. Because of
elimination of lumped inductors in the resonator circuit, higher B1 field
efficiency in imaging subjects can be expected. In order to validate the
feasibility of the proposed hairpin RF coils, systematical studies on
decoupling performance, field distribution, and SNR are performed, and the
results are compared with those obtained from existing high-impedance RF coil,
e.g., "self-decoupled RF coil". To further investigate its performance, an
8-channel head coil array using the proposed hairpin resonators loaded with a
cylindrical phantom is designed, demonstrating a 19 % increase of the B1+ field
intensity compared to the "self-decoupled" coils at 7T. Furthermore, the
characteristics of the hairpin RF coils are evaluated using a more realistic
human head voxel model numerically. The proposed hairpin RF coil provides
excellent decoupling performance and superior RF magnetic field efficiency
compared to the self-decoupled high impedance coils. Bench test of a pair of
fabricated hairpin coils prove to be in good accordance with numerical results.Comment: 10 pages, 12 figures, 2 tables. Second version: Add bench test
results and One dimensional profile of the simulated B1
Optimal Value of Series Capacitors for Uniform Field Distribution in Transmission Line MRI Coils
Transmission lines are often used as coils in high field magnetic resonance imaging (MRI). Due to the distributed nature of transmission lines, coils based on them produce inhomogeneous field. This work investigates application of series capacitors to improve field homogeneity along the coil. The equations for optimal values of evenly distributed capacitors are derived and expressed in terms of the implemented transmission line parameters. The achieved magnetic field homogeneity is estimated under quasistatic approximation and compared to the regular transmission line resonator. Finally, a more practical case of a microstrip line coil with two series capacitors is considered
Practical design of multi-channel MOSFETRF transmission system for 7 T animal MR imaging
In this work, we developed and tested a multi-channel radio frequency (RF) transmission system with compact metal-oxide semiconductor field effect transistor (MOSFET) amplifiers for parallel excitation in 7 T animal MRI scanner. The system is composed of a multi-channel RF controller and four independent RF power amplifiers. Each power amplifier contains two amplification stages. The design was validated by simulation and bench test. The power gain for the amplifier is 18.7 dB at 300 MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel excitation applications at 7 T. This compact RF power amplifier can be potentially used for on-coil amplification in multichannel RF array system
Analytical Approach for MRI RF Array Coils Decoupling by Using Counter-Coupled Passive Resonators
We introduce an analytical approach to design decoupling filters for MRI radiofrequency array elements, adopting counter-coupled passive resonators as unit-cells. Specifically, our method is based on a magneto-static hypothesis, thus a deep comprehension of the physical interactions between all the elements in the system and design guidelines can be achieved. In particular, the couplings between adjacent and next-nearest neighbors coils pairs are both modeled, hence addressing the requirements for MRI arrays. The analytically-obtained filter solution is subsequently refined resorting to targeted full-wave simulations, reducing the computational effort. To prove the validity of the proposed approach, we conceived a test-case consisting of three planar RF coils, tuned at the 7T proton Larmor frequency. We demonstrated through full-wave simulations that the analytical design method is accurate and effective. Moreover, we fabricated a prototype and we performed benchtop measurements, both in unloaded conditions and in the presence of a biological phantom, resulting in excellent agreement with simulations. The developed analytical framework can be useful to model and control the mutual interactions between the various elements of an RF MRI system. In addition, the possibility to print the decoupling elements and the RF coils on the same dielectric substrate leads to a mechanically robust prototype
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
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