Keywords: Low-Field MRI, RF Arrays & Systems

Motivation: RF Coil design for low frequencies.

Goal(s): Demonstrate the impact of  losses in different matching network topologies

Approach: we compared three- and four-element matching networks on 12 cm diameter coils at 10, 23.55, and 63.6 MHz. and evaluated coil loss and SNR. 

Results: At 10 and 23.55MHz, the coils with three-element (single stage) matching networks exhibited approximately 50% lower unloaded Q compared to coils with four-element matching networks, whereas unloaded Q values were within 3% for three- and four-element matched coils at 63.6MHz. SNR at 23.55MHz (0.55T) was approximately 30% higher for the four-element matched coil over the three-element matched coil.

Impact: Resistive losses in four-element matching networks can be significantly lower than those of three-element networks at low frequencies or for lightly loaded coils, resulting in higher quality factor and SNR.

Keywords: High-Field MRI, New Devices

Motivation: Eddy currents induced in the RF shield cause vibrations which generate acoustic noise.   

Goal(s): To develop an RF shield that minimizes eddy currents and preserve the B₁⁺ efficiency.

Approach: It was established that eddy-currents induced in the RF shield of the transmit coil are the primary source of acoustic noise in our 7T scanner. Therefore, the conventional slotted double layered RF shield was replaced by a segmented phosphor bronze mesh (PBM) to minimize eddy-currents and acoustic noise.

Results: Measurements performed at both ear locations of an anthropometric phantom demonstrate that the segmented PBM-based RF shield reduces the acoustic noise by up to 10dB.

Impact: Acoustic noise reduction could be achieved by using segmented PBM without compromising the transmit B1 field compared to the conventional double-layered slotted shield. Further improvements might be achieved with different mesh configurations besides the one implemented in this work.

Keywords: High-Field MRI, High-Field MRI, artifacts, MSK

Motivation: Advanced metal-artifact reduction sequences (MARS) are currently only available in parallel transmission (pTx) research mode at 7T, while clinically approved RF coils are single-channel transmit (sTx) coils.

Goal(s): Our goal was to build an adapter to connect a sTx knee coil to a pTx system, and to evaluate MARS techniques in phantom and in vivo scans.

Approach: An evaluation of artifact reduction using advanced MARS techniques was performed in a phantom study. Knees of three volunteers with metallic suture plates were imaged.

Results: Optimized bandwidth and view-angle-tilting (VAT) effectively reduced metal artifacts while keeping acquisition time and SAR load acceptable.

Impact: A hardware adaption for connecting a sTx coil to a pTx system was evaluated for safe in vivo usage. The application of MARS was demonstrated for the first time at 7T in volunteers with metallic suture plates after ACL reconstruction.

Keywords: High-Field MRI, Motion Correction

Motivation: Long scan durations and ultra-high field both facilitate the acquisition of high-resolution quantitative brain MRI, but they increase motion sensitivity.

Goal(s): We aimed to limit the occurrence of deliberate motion by testing a device (‘MinMo’) designed to increase head stability at 7T.

Approach: Using two k-space phase encoding orders with different motion sensitivity profiles we obtained data with and without the MinMo device.

Results: This showed that the MinMo increased image quality visually and as measured quantitatively via reduced gradient entropy in scans of ~10 and ~20 minute duration.     

Impact: Reducing head motion would have a significant impact on image quality in high-resolution long duration research scans and clinical imaging.  A preliminary investigation of a prototype device aiming to stabilize the head showed efficacy in most subjects, warranting further investigation.

Keywords: High-Field MRI, High-Field MRI, human 14T RF front-end

Motivation: The ultra-high field magnetic resonance imaging system can obtain ultra-high sensitivity and signal-to-noise ratio magnetic resonance imaging information,  has become an important tool for basic research in brain science .

Goal(s): However, with the increase of field strength and operating frequency, the RF signal processing module, faces problems such as clutter interference,  mirror frequency interference, which seriously affects the performance of the RF system and the final image quality. 

Approach: We have solved these problems using methods such as secondary mixing.

Results: After key parameter testing and verification, it meets the application requirements of the human 14T ultra-high field magnetic resonance imaging system.
 

Impact: Our work has provided conditions for the construction of multi-channel RF modules in ultra-high fields, improving the quality of RF signal processing. Provided a hardware foundation for improving the quality of MRI images in ultra-high fields.

Keywords: System Imperfections, System Imperfections: Measurement & Correction

Motivation: NMR field probe methods for field monitoring are limited by probe signal loss due to relaxation, dephasing, or both.

Goal(s): To develop a method for characterizing sequences with higher resolution or readout length than allowed by current field monitoring approaches.

Approach: Long-duration 2D spiral readout gradients were characterized by acquiring and stitching multiple segment-specific dynamic field measurements per segmentation of the readout gradient determined by a signal loss model.

Results: For both long-duration and ultrahigh-resolution readouts, our method resulted in plausible 0^th-2^nd order dynamic field measurements throughout the entire spiral readout, correcting erroneous k-space traversal observed with a traditional approach.

Impact: The proposed stitching method provides an effective means to characterize challenging imaging gradients using commercially available hardware and without assuming a linear gradient system, thereby having utility for dynamic field measurements in ultrahigh-resolution MRI using a standard field monitoring system.

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

Motivation: Low-field MRI has garnered significant attention in recent years due to its unique advantages in safety, cost-effectiveness and accessibility. However, lower field strength comes with an inherently lower SNR as its primary limitation.

Goal(s): In this work, we introduce a novel volume RF coil design using coupled stack-up resonators to mitigate this challenge.

Approach: To demonstrate the proposed design, we take 0.5T as an example field strength and designed a prototype coupled stack-up volume coil operating in the 20MHz range. 

Results: Compared to the birdcage coil, the proposed design significantly improves RF field efficiency and homogeneity, ultimately enhancing the performance of low-field MRI.

Impact: The proposed stack-up volume coil outperforms the standard birdcage coil in B1 efficiency and field homogeneity at low fields, ultimately improving the performance of low-field MRI and advancing its applications.

Keywords: Non-Array RF Coils, Antennas & Waveguides, High-Field MRI

Motivation: In this work, we investigated the performance of a coaxial dipole antenna compared with a single wire for dental MRI at 3 T.

Goal(s): A fully flexible coaxial dipole antenna was designed and optimized with an aim to improve the signal-to-noise ratio, image quality, and sensitivity while maintaining safety performances.

Approach: Based on electromagnetic simulations and MRI results it is demonstrated that in single conductor has a current distribution, which is strongly inhomogeneous leading to inhomogeneous B1⁺ field distribution. 

Results: In contrast, coaxial cable with multiple gaps offers homogenous current distribution yielded to optimum B1⁺ field distribution.  

Impact: The development of a fully flexible coaxial intraoral antenna with lightweight, capable of overcoming impedance and homogeneity challenges, has the potential to revolutionize dental MRI. Enhanced patient comfort and adaptability improve image quality, advancing dental MRI's utility in medical imaging.

Keywords: Hybrid & Novel Systems Technology, High-Field MRI, RF Systems, Parallel transmit

Motivation: Ultra-high field imaging has several advantages like high SNR but require expensive RF shimming hardware to mitigate image shading artifacts caused by transmit RF field inhomogeneity.

Goal(s): We developed a low-cost, highly customizable hardware solution to extend conventional RF shimming capabilities.

Approach: We used array compressed parallel transmission system with an optimal network and fixed optimal weights designed to sit inside the bore with the coil array

Results: The optimal network achieved about two-fold improvement in RF homogeneity compared to conventional circularly polarized network.

Impact: This is the first time a design framework is described to optimize a full acpTx system for a desired imaging application. This hardware-centric approach has no additional pre-scan preparation or time considerations, lending itself to future clinical use.

Keywords: High-Field MRI, High-Field MRI, Radiofrequency coil, Neurovascular imaging

Motivation: Current methods for neuroimaging at 7T require separate coils for brain and c-spine imaging or rely on a 16-channel transmission setup for combined imaging.

Goal(s): To develop a neurovascular (NV) coil with eight transmit channels (industry standard) with extended longitudinal coverage from the brain to the neck region. 

Approach: Based on coupled electromagnetic and circuit optimization, a modified coil configuration with six-upper and two-lower transceiver elements was constructed and integrated with a 56-channel receive array.

Results: Measurements in vivo suggest that the proposed NV coil delivers diagnostic-quality images of the brain and spinal cord in a single acquisition.  

Impact: The 7T neurovascular coil, designed for concurrent brain and c-spine imaging, will extend the high-resolution capability of 7T-MRI to the routine diagnosis of diseases affecting soft tissues and vessels in the head and neck regions in clinical practice.

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband, 7 Tesla, Spinal Cord, Universal Shim

Motivation: Addressing the issue of lengthy parallel transmit (pTx) adjustment times caused by the absence of a dedicated spinal cord (SC) coil setting, which limits SC imaging at ultra-high field.

Goal(s): Enhance imaging efficiency by universal shim modes that can be applied without the need for additional adjustment time for different SC regions and coils.

Approach: We have built a library of channel-wise B₁⁺ maps and optimized universal RF shims to optimize transmit homogeneity and efficiency.

Results: The proposed universal shims significantly improve B₁⁺ efficiency, achieving a 50% enhancement compared to the default shim mode, while eliminating the need for subject-specific pTx adjustments.

Impact: The development of universal shims not only enhances SC imaging efficiency at ultra-high field but also streamlines the process by eliminating lengthy subject-specific pTx adjustments, expecting SC imaging to become more usable for non-pTx experts.

Keywords: RF Arrays & Systems, Pediatric, RF Coil, High Field MRI, 7T

Motivation: To develop a safe, size-adaptive RF head coil with an integrated commercial field monitoring system for pediatric imaging at 7T MRI.

Goal(s): Performance of the size-adaptive RF head coil was quantitatively evaluated and compared to that of a commercial receive adult head coil (Nova Medical).

Approach: The coil performance was evaluated at the largest and smallest dimensions of the receive former, with and without integrated field probes.

Results: Simulations of transmit coil demonstrated that the coil is safe for pediatric imaging of subjects below 30 kg at 7T. The SNR performance of the coil was comparable to the commercial coil. 

Impact: The eight channel dipole transmit and 32 channel size-adaptive receive array with integrated 16 NMR commercial field monitoring probes for imaging pediatric (4-9 years old) brain at 7T enables safe and high-quality imaging of subjects below 30 kg at 7T.

Keywords: RF Arrays & Systems, RF Arrays & Systems

Motivation: RF challenges increase with field strength, and novel RF coil solutions are essential to capture the potential benefits at 11.7T

Goal(s): To develop an efficient 8-channel transmit array in combination with a 31-channel receive array for whole brain MRI at 11.7T

Approach: Combined electromagnetic and RF pulse design simulations were performed. A folded-end RF shield was developed to minimise signal loss due to wave propagation. The receive array was optimally integrated to preserve the spatial distribution of the B₁⁺ field.

Results: Whole-brain coverage was achieved with 8-channel transmit array and excellent agreement between simulation and measurements was observed.

Impact: Identifying the coil losses and engineering solutions to mitigate them provides substantial gains in transmit performance, especially at 11.7T where the losses due to radiation is high. This approach can be extended to transmit array designs at other field strengths.  

Keywords: Low-Field MRI, Low-Field MRI

Motivation: Ultra-low MRI systems provide a reduced SNR.

Goal(s): Our goal is to improve the SNR by incorporating SQUID-based RF detection into the ULF MRI system.

Approach: We build a custom-made SQUID MRI system and acquire the first 2D image of a phantom as proof of concept.

Results: we successfully acquired a 2D image of a phantom in 4 hours with a resolution of 15×12.5 mm²

Impact: Striving for MRI accessibility, we develop a portable ultra-low field scanner using highly sensitive SQUID detection. Our first 2D image marks the initial stride toward achieving clinically competitive image quality.

Keywords: RF Arrays & Systems, RF Arrays & Systems

Motivation: Accurate RF excitation in MRI requires a linear response from the radio-frequency power amplifier (RFPA). However, the conventional RF power amplifier (RFPA) used in MRI is typically located in the equipment room, which introduces complexity and significant signal losses. Additionally, it poses challenges to direct control of RF signals at the RF coil side.

Goal(s): Design high power and high linearity RFPA which can operate in a strong magnetic field environment.

Approach: The design incorporates non-magnetic high-power circuits and a negative feedback control module.

Results: The amplifier exhibits high power and linearity characteristics during the bench testing.

Impact: The proposed design method for non-magnetic RFPA can enhance the efficiency and linearity of RF excitation in MRI, multiple non-magnetic RFPAs will be fabricated to facilitate parallel transmission in 5T MRI.

Keywords: Data Acquisition, Software Tools, Radiofrequency, MaRCoS, open-source

Motivation: MaRCoS is an open-source tool integrating hardware, firmware, and software for low-field MRI system control. But releases lacked user-friendly features, requiring technicians to handle automatable operations manually.

Goal(s): To simplify clinical workflows, we developed a device for RF coil impedance tuning and matching in systems controlled by MaRCoS.

Approach: Our approach involved designing a TM device using switchable capacitors and testing it in a real clinical environment on a portable 72 mT system.

Results: The auto-TM system, tested on 20 volunteers (40 protocols), significantly accelerated and improved workflow compared to manual methods, marking a substantial advancement in MRI system efficiency and usability.

Impact: The introduction of our automatic tuning and matching device eases low-field MRI systems controlled by MaRCoS. By simplifying clinical workflows and improving efficiency, this innovation enhances the user experience, simplifing the workflow under clinical environments.