Keywords: RF Arrays & Systems, RF Arrays & Systems

Motivation: To explore the SNR performance of Loop and Sleeve antennas for multi-channel human head imaging at 10.5T.

Goal(s): Validation of achievable receive performance of loop and sleeve antennas with same number of channels. 

Approach: Simulation based analysis of SNR between loop and sleeve antennas with same channels.

Results: We obtained and compared simulated SNR data of loop and sleeve antennas. For the same number of channels (1, 3, 5, and 32-channels), each SNR map was compared with similar imaging coverage. Our experimental data indicates an enhancement of SNR achieved with the 32-channel sleeve antenna array compared to the 32-channel loop array.

Impact: Our simulations indicate that a high-density sleeve antenna array compared to a classical loop array can achieve enhancement of SNR for human head imaging at 10.5T.

Keywords: Non-Array RF Coils, Antennas & Waveguides, High-Field MRI, Split Ring Resonator; Metamaterial surface; Loop Array

Motivation: Metasurfaces are conceptually appealing for enhancing RF coil performance due to added degrees of freedom for shaping electromagnetic fields. 

Goal(s): This work focuses on  development and validation of a novel split ring resonator (SRR) based metasurface for improving the performance of a two-channel surface loop array at 7.0 T.  

Approach: Application of a magnetic field perpendicular to SRR induces electromotive force, forming an LC circuit with resonance frequency as currents circulate between the rings. This property benefits MRI transmission field enhancement. 

Results: Our simulations and experimental results demonstrate that  wireless metasurfaces enhance transmission efficiency of a two-channel surface loop coil array at 7T.

Impact: Our approach provides technical foundation for development, implementation and validation of novel metasurfaces for RF arrays customized for UHF-MRI. Our metasurface offers customizable resonance properties by adjusting unit cells, periodicity, or structure placement to enhance transmit field efficiency and uniformity.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Challenges, Materials

Motivation: In MRI of tissues with short T₂, materials of RF coils may be detected and can cause background artefacts. Current solutions to this problem either compromise sequence performance or impose restrictions on coil design.

Goal(s): Making RF coils MR-invisible.

Approach: The material used for constructing the coil housing is filled with ferrimagnetic material, leading to effective signal spoiling. Coil formers are created by additive manufacturing using custom filaments made from magnetite-filled polymer.

Results: Unwanted signals from the RF coil are eliminated by using coil formers made from magnetically filled polymer. Hence, background-free short-T₂ imaging is enabled.

Impact: RF coils made MR-invisible by using magnetically-filled materials simplify coil design and manufacturing, and improve the performance of MRI of tissues with short T₂, such as bone, tendon, lung, or myelin.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides

Motivation: Wave-impedance-match is key to ensure maximized power transmission in travelling-wave (TW) MRI. Currently, only coaxial waveguide method has been proposed. It requires adding metallic cylinder inside magnet and gradient coils, which introduces risks of mechanical instability and patient safety due to eddy current.

Goal(s): Propose a novel wave-impedance-match method for TW excitation with non-metallic materials.

Approach: Dielectric cubes made up of easy-accessible distilled water were modelled and placed between feed and the load. Power-flow-density and power-loss results were used to evaluate wave-impedance-match.

Results: Wave-impedance-match varies with length and width of dielectric cube. The well-matched conditions with dielectric cube and coaxial waveguide are similar.

Impact: Non-metallic wave-impedance-match method was proposed for TW excitation. This has the potential to accelerate maturity of traveling-wave excitation method in UHF MRI.

Keywords: Non-Array RF Coils, Antennas & Waveguides, RF Arrays & Systems

Motivation: To develop a new generation of head coils for ultrahigh field MRI using loop-dipole combined dielectric resonator antenna arrays.

Goal(s): To construct and evaluate an 8-channel loop-coupled dielectric resonator antenna array for brain MRI at 7T.

Approach: Electromagnetic field simulations in a spherical phantom and Duke for different types of RF feeds were performed. An 8-channel, loop-coupled dielectric resonator antenna array was constructed using 8 ceramic, rectangular dielectric blocks (ε_r=275, σ=0.068 S/m).

Results: An 8-channel loop-coupled dielectric resonator antenna array was successfully constructed and evaluated at the bench as well as in preliminary phantom experiments at 7T using pTX system.

Impact: This study is an important step to improve transmit and receive performance of head coils for neuroimaging at 7T.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides

Motivation: Metamaterial-based designs have the potential to locally increase the RF field and to serve as resonators in MRI. However, many of the structure either include a high dielectric layer substrate or require large amount of lumped elements.

Goal(s): Our goal was to design a metamaterial without the need for either.

Approach: A novel metasurface was constructed from concentric split-rings alternatingly rotated by 90° - generating a maze-like configuration - which allowed to lower the resonant frequency.

Results: The novel maze-like metasurface achieved an RF field increase in the range of x1.5-2 compared to using a surface-coil of the same dimensions.

Impact: We demonstrated a new metasurface geometry that provides an efficient resonator at 298 MHz for 7T MRI. This design does not require high dielectric or lumped elements, which offers a simple implementation, a flexible setup and a high efficiency resonator.

Keywords: Non-Array RF Coils, Antennas & Waveguides, RF Arrays & Systems, stretchable

Motivation: Most commercial MRI coil designs are rigid, prohibiting optimal SNR and patient comfort. Flexible coils conform to anatomies varying in size/shape and significantly improve image quality in applications such as breast imaging.

Goal(s): We optimize manufacturing of our previously developed stretchable coils by direct integration of liquid metal within the polymeric coil substrate to allow for rapid one-step fabrication of multi-element arrays in the future.

Approach: Liquid metal is directly 3D printed on the substrate to expedite/integrate fabrication.

Results: A stretchable single element is fabricated and resonance at the 3T Larmor frequency is demonstrated.

Impact: Our novel 3D printing technique simplifies the production of stretchable liquid metal MRI coils, ensuring consistent quality and efficiency. With reduced fabrication time and elimination of manual injection steps, this technology facilitates seamless construction of multi-channel stretchable coil arrays.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Software Tools, Birdcage Coil Design

Motivation: Currently the well-known BirdcageBuilder software tool for providing a first prediction of capacitor values for Birdcage Coil design is available only as an app for Android devices.  

Goal(s): We introduce an Open-source version of BirdcageBuilder available for the MR community to use and improve on GitHub.

Approach: From Java code for BirdcageBuilder Mobile, we developed a version in JavaScript. The GUI is an HTML web page, so the webapp runs in any modern web browser on any device. Outputs were validated against the original publication and the Mobile version.

Results: The software and code are available to all for use and improvement. 

Impact: A freely-available open-source version of the BirdcageBuilder software facilitates rapid design of birdcage coils and allows improvement by the MR community. Additional functionalities could include prediction of all resonances of a coil, noncylindrical coils, and output of a CAD model.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides, High Temperature Superconductor

Motivation: Multi-Loop Coils (MLCs) allow to reduce the sample-induced noise while achieving a large Field Of View. Their sensitivity would be improved with the use of superconducting materials.

Goal(s): We aim to develop a superconducting MLC achieving both a large Field Of View and high quality factor.

Approach: An initial design is modified to render a MLC self-resonant and optimized using electromagnetic simulations.

Results: The HTS Multi Loop Transmission Line Resonator coil characterizations are presented. Q-factor and resonance frequency decline in presence of the static magnetic field or of a conductive sample but the coil still exhibits higher performances than copper MLCs.

Impact: Multi Loop Transmission Line Resonator represents a promising design strategy to increase the Field Of View of HTS reception coils without compromising with the coil sensitivity.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides

Motivation: The DHD coil in the previous study had a narrow sensitivity region. We developed a DHD coil with improved sensitivity region and B1 uniformity to image large samples while maintaining SNR.

Goal(s): Imaging of all organs, especially the brain of a human embryo, using a coil with a wider sensitivity region.

Approach: We performed electromagnetic field simulations to find the shape that would give the best performance, and fabricated the coil.

Results: The imageable area was widened by 30% and the insensitive area was reduced. Not only the brain of a human embryo, but also organs could be imaged at the same time.

Impact: The DHD coil's geometry was determined by electromagnetic field simulation, which allows a 30% wider range in the z-axis than conventional coils. The human embryonic brain and organs such as the heart could be simultaneously imaged with sufficient SNR.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides, Metamaterial, Metasurface

Motivation: Although lower B₀ field strength generally lead to lower SNR, metasurface resonators can greatly increase the sensitivity of scanner-integrated coils to practically perform wireless imaging.

Goal(s): This work investigates the dependence of the resonance frequency on metasurface enhancements and evaluates the potential towards developments designed for low-field applications.

Approach: The capabilities of three geometrically identical metasurfaces were experimentally compared at field strengths of 0.55T, 1.5T and 3T. Images were acquired using the table-integrated spine-coils.

Results: The achieved SNR enhancements beneath the metasurface increased with lower field strength and were highest at 0.55T with 27-fold gain.

Impact: The significant SNR gain achieved at low-field paves the way for further development and implementation of wireless metasurface technologies in MRI. As flexible and cost-effective alternatives or additions to conventional coils, they can additionally ease patient postitioning.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides

Motivation: The demand for improving RF coil designs to maximize the potential of high-field MRI continues to grow. Exploring innovative RF coil elements holds the potential to improve RF coil performance and diagnostic image quality.

Goal(s): Our goal is to investigate the performance of a cycloid dipole antenna as a possible RF coil element for high-field MRI.

Approach: FDTD simulations were performed to compare the performance of a standard half-wavelength dipole at 7 T with two variations of the cycloid dipole.

Results: Cycloid dipole exhibits a shorter resonance structure, higher B₁⁺ and  B₁⁺/√SAR_{10g_max} efficiencies when compared to the standard dipole antenna.  

Impact: The study introduces cycloid dipoles as potential MRI coil elements. This opens opportunities for future investigations to optimize cycloid antennas within specific MRI coil designs, improving clinical imaging quality.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Simulations, High Permittivity Materials, Fetal, RF shimming, Specific Absorption Rate

Motivation: Performing MRI on fetus at 3T can be clinically valuable but often encounters difficulties in transmit field inhomogeneity and SAR increase. 

Goal(s): Determine dominant designing factors for optimal dielectric pad configurations to improve B₁ homogeneity and reduce SAR for fetus imaging at 3T.

Approach: B₁ field distributions of transmit body coil with dielectric pads of various designs in different positions around a pregnant woman model were evaluated using computer simulations.

Results: The placement of dielectric pad could significantly improve B₁ field homogeneity (up to 84%) in the fetus brain. A generalizable routine can be developed for optimization of B₁ shimming using dielectric pads.

Impact: Passive RF shimming with HPM is a viable ancillary approach for reduction of transmit field inhomogeneity artifacts for body imaging in high field MRI (≥3T), which affect the quality and safety of fetal MRI at 3T.

Keywords: Analysis/Processing, Safety, Quality Control

Motivation: RF coil failures are often not visually recognizable. Quality control is only done weekly or monthly, leading to days to weeks where diagnostic images may be negatively impacted.

Goal(s): Identify RF coil failures on patient images using deep transfer learning.

Approach: >10,000 passed and failed images from 50 patients were used to train 4 pre-trained deep learning models using 2 different pipelines: (1) shuffled all images into train and test, and (2) shuffled by each patients’ images.

Results: EfficientNet V2 (L) was the highest performing model, achieving 99% accuracy for pipeline 1, and 55% for pipeline 2. Other models showed similar results.

Impact: Introducing a deep learning model that can identify radiofrequency coil failures on patient images would avoid costly rescans of patients whose images were only determined to be poor after a failure was detected on a later quality control scan.

Keywords: Low-Field MRI, RF Arrays & Systems, Wireless, Low-Field, Portable, Cellular

Motivation: Wireless transmission of MRI data acquired with low-field portable MRI scanners within EMT vehicles over cellular/satellite networks drastically decreases time between stoke onset and imaging for improved patient outcomes. 

Goal(s): Our goal is to enable wireless communication with an iRFW coil design for simultaneously imaging and wireless cellular/satellite data transfer from within the scanner and an EMT vehicle.

Approach: iRFW-Cellular simulations within a portable 70 mT scanner are performed to evaluate its SNR and far-field gain patterns for wireless communication.  

Results: The iRFW-Cellular simulations showed a uniform SNR in the head and gain patterns appropriate for the wireless transmission of MRI data. 

Impact: The iRFW-Cellular spiral coil design potentially enables wireless MRI data transfer from a low-field portable MRI scanner inside, or out, of an EMT vehicle for better stroke onset to imaging times. 

Keywords: RF Arrays & Systems, Parallel Imaging

Motivation: Dual-tuned cable traps, instead of two separate single-tuned ones in series, are preferred in multi-nuclear MRI and MRS.

Goal(s): We introduce a frequency-independent dual-tuned cable trap. For proton nuclear, standard solenoid cable trap was employed. For X-nuclear, an additional solenoid resonator was employed to block the common-mode signal.

Approach: The dual-tuned cable trap was analyzed in electromagnetic simulation, fabricated and its performance is evaluated by bench test.

 

Results: The dual-tuned trap exhibits exceptional common-mode current suppression abilities at both frequencies. And two frequencies of dual-tuned trap can be adjusted independently without mutual interference.

Impact: This novel dual-tuned cable trap fills an important gap in dual-tuned MRI hardware for multi-nuclear magnetic resonance studies, potentially enable next-generation dual-tuned coils.