Keywords: High-Field MRI, High-Field MRI, multi-tuned body coils, triple-tuned body coils, efficiency improvement

Motivation: Low B₁⁺ field 1)increases the RF pulse duration which jeopardizes SNR in the short T2 of 23Na spins and 2)limits of the excitation bandwidth which might be insufficient to excite the full chemical shift dispersion of 31P spins.

Goal(s): To improve the efficiency of a triple-tuned (2H/23Na/31P) body coil.

Approach: Reducing the total loss on a birdcage design by decreasing the number of multi-tuned circuits. 

Results: The 8-rungs (i.e., 16 triple-tuned circuits) design wins 3dB in efficiency against a conventional 24-rungs (i.e., 48 double-tuned circuits) design, while covering one more excitation frequency, and the homogeneity is practically unaffected.

Impact: The high loss of the multi-tuned circuits is a determinant in multi-tuned borecoil designs. Reducing the number of multi-tuned circuits used in the design substantially improves the B1 efficiency of a body coil facilitating even triple tuning.

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

Motivation: Radiative excitation (e.g., travelling-wave, TW) has strength in large coverage and low SAR. It’s well compatibility with single-channel (clinical-mode) makes it promising as whole-body excitation solution at UHF. But it has apparent weakness in efficiency.

Goal(s): Improve excitation efficiency as well as homogeneity of TW MRI under clinical-mode.

Approach: Subwavelength dielectric waveguide was designed to enhance excitation efficiency as well as homogeneity through mode conversion, power-focusing, wave-impedance-matching and phase-velocity-matching.

Results: The excitation efficiency was improved by 114% over brain compared to classic TW . SAR efficiency was 10.8% higher than birdcage. The B₁⁺ RMSE in brain was reduced by 21.9% compared to birdcage.

Impact: Our results offered insights into the design of new generation TW MRI excitation systems at UHF. The improved TW MRI systems operated under single channel may hold promises to whole-body imaging at UHF in clinical scenarios.

Keywords: RF Arrays & Systems, RF Arrays & Systems, X Nuclei, Multinuclear coil, Flexible coil

Motivation: X-nuclear MRI is used to monitor metabolic processes but requires RF coils that are individually tuned to each resonance frequency.

Goal(s): To introduce a modular, flexible transmission line resonator array element for 5 resonance frequencies to enable multiple X-nuclear applications with a single coil.

Approach: A quintuple-tuned shielded loop resonator (SLR⁵) was designed consisting of three stacked shielded loop resonators with one element for ¹³C and ²³Na, another for ³¹P, and a third for ¹⁹F and ¹H. Switching between nuclei was realized by PIN diodes.

Results: SLR⁵ can be used to acquire X-nuclear signals of 5 nuclei.

Impact: The SLR⁵ concept might facilitate the implementation of X-nuclear MRI methods for metabolic imaging in early diagnosis and monitoring of disease. SLR⁵ coils can be arranged favorably in coil arrays due to the inherently low coupling of SLR coils.

Keywords: Bioeffects & Magnetic Fields, Gradients, PNS thresholds, EM exposure, neurodynamic modeling, sequence development

Motivation: Peripheral nerve stimulation (PNS) limits the current generation of MRI gradient coils. We seek a sequence-based approach to improve scanner performance without hardware changes.

Goal(s): Experimentally demonstrate that pre-excitation targeting of the potassium system (PRE-TAPS) using a kHz-frequency preconditioner waveform can be used to increase PNS thresholds.

Approach: We measure changes in the PNS threshold of a 1.1kHz frequency probe waveform when a 10kHz preconditioner waveform is played immediately before the probe waveform and compare measured results to our model predictions. 

Results: We found up to 10% greater PNS thresholds using PRE-TAPS in one subject and qualitative agreement with our PNS model.

Impact: Waveform-based modulation of PNS thresholds, such as pre-excitation targeting of the potassium system (PRE-TAPS) with a kHz-frequency preconditioner waveform, may enable increased performance in PNS-limited sequences such as EPI.

Keywords: Safety, Safety

Motivation: Metallic implant compatibility at ultra-high field strengths (≥7T) continues to be often contraindicated as RF induced heating can result in surrounding tissue damage. Metasurface technology has been shown in the past to locally null the B₁⁺ field, thereby providing a potential solution for shielding implants.

Goal(s): To demonstrate a metasurface based method for shielding metallic implants to reduce RF heating.

Approach: Eight implants were tested in a polyacrylic acid (PAA) phantom using a high-SAR sequence with and without a prototype metasurface.

Results: On average the metasurface design reduced RF induced implant heating in the phantom by 41.6%.

Impact: Patient undergoing invasive brain or trauma surgeries typically have passive metallic devices placed, making them unable to receive MRI scans at ultra-high field strengths due to RF induced heating. This work benefits this patient population by reducing RF heating.

Keywords: Safety, Modelling, Medical Implants, Transfer Function, ISO/TS 10974

Motivation: Radiofrequency (RF) tissue heating is a known safety risk for patients with cardiac implantable electronic devices (CIEDs), especially children with epicardial systems. 

Goal(s): We present the first cumulative transfer function based on the guidelines in ISO/TS 10974 to evaluate the RF heating of a CIED with a bipolar epicardial lead.

Approach: We measured, calibrated, and validated cumulative transfer functions for predicting RF heating through in vitro experiments and electromagnetic simulations.

Results: Our cumulative transfer functions accurately predicted RF heating around the electrode ends for 48 unique lead trajectories (r=0.90-0.97, p<0.05). 

Impact: We introduce a high-fidelity cumulative transfer function model of a CIED with a bipolar epicardial lead to enable the evaluation of RF heating of bifurcating leads during MRI.

Keywords: Safety, Safety, epicardial leads;implants

Motivation: Once epicardial CIEDs are implanted in pediatric patients, they pose a relative contraindication for MRI scans due to the increased risk of RF heating.

Goal(s): Enhance MRI safety by reducing RF heating through surgical modification of the lead configuration.

Approach: Mathematical models of the CIED with different lead configurations were developed and validated using the transfer function approach. Low heating trajectories were identified and implemented in a patient.

Results: Placing the excess lead length on the heart’s inferior surface resulted in an average 4-fold reduction in RF heating compared to anterior placement.

Impact: Implementing physics-based surgical modifications to the trajectory of epicardial leads can consistently and significantly reduce RF heating in children undergoing MRI at 1.5T.

Keywords: Safety, Safety, Ultra-High-Field, Electroencephalography (EEG)

Motivation: The electroencephalography (EEG) in combination with MRI allows performing multi-modal imaging. The presence of EEG-caps can increase SAR of tight-fit transceiver RF-arrays at Ultra-High-Field.

Goal(s): To numerically evaluate SAR generated by a tight-fit array at 9.4T in the presence of EEG-caps.

Approach: Numerical models of 8-channel 9.4T transceiver arrays with EEG-electrodes were constructed. B₁⁺ and SAR were simulated for the human head voxel models using CST Studio.

Results: In this work, we numerically showed that EEG-caps don’t significantly change B₁⁺ and SAR of the arrays at 9.4T. Furthermore, the created models of the caps can be used in future simulations.

Impact: We numerically showed that EEG caps don’t significantly change B₁⁺ and SAR of the arrays at 9.4T. The developed cap models can be used in future simulations

Keywords: Safety, Safety

Motivation: RF-induced temperature rise is considered a safety risk for MRI.

Goal(s): To study temperature changes in the brain due to (generally considered safe) heat or cool pads placed on a subject's forehead for comparison to RF-induced temperature elevations. 

Approach: Using the Projection onto Dipole Fields method, susceptibility and drift induced field changes are separated from temperature effects to enable assessment of temperature rise in the brain.

Results: Up to 2.5 °C temperature rise was measured at the brain edge in 2 volunteers. Similarly, a 2.0 °C decrease was observed at the brain edge when a coolpad was placed on the forehead.

Impact: RF-induced temperature changes in the brain can be considered modest compared to temperatures induced by (generally considered safe) external heating/cooling sources. The temperature increase due to a heatpad on the forehead is much larger than previously measured RF-induced temperature increases.

Keywords: Safety, Bioeffects & Magnetic Fields

Motivation: For silent whole-body MRI using ultrasonic encoding at 20kHz, very high slew rates will be experienced by body parts such as the torso with PNS risk. However, PNS thresholds for these body parts at ultrasonic frequencies are not well known.

Goal(s): To estimate PNS thresholds for the torso at 20kHz.

Approach: We test volunteers in a whole-body gradient coil driven at 20kHz to determine when PNS occurs and compare to simulations and current IEC guidelines.

Results: PNS occurs at much higher dB/dt values than predicted by IEC guidelines at 20kHz, where we found a mean threshold value of 1316T/s for the torso.

Impact: PNS is a concern for body MRI using ultrasonic encoding. Here, we show much higher measured PNS thresholds than IEC guideline predictions for 20kHz gradient switching, suggesting that whole-body silent MRI at 20kHz is possible with reduced risk of PNS.

Keywords: New Devices, New Devices

Motivation: Ultra-high field imaging (≥7T) lacks transmit (B₁⁺) field inhomogeneity due to the shortened RF wavelengths, often resulting in poor image quality. Novel metasurface designs have previously been effective at improving image quality at lower field strengths but have not yet been implemented at 7T.

Goal(s): To optimize and demonstrate a novel metasurface design for in vivo usage at 7T.

Approach: Empirical optimization and phantom testing produced the final design, while image enhancement was assessed via in vivo calf skeletal muscle imaging.

Results: The metasurface produced a 126.5% increase in image relative SNR and a 27.6% increase in transmit efficiency.

Impact: The work impacts 7T imaging by presenting a novel piece of hardware that can effectively improve image quality. Future work on this project will include further optimization via variable distributed capacitance across the metasurface.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides, travelling wave MRI

Motivation: The utilization of traveling-wave MRI in conjunction with a parallel-plate waveguide provides an alternative for acquiring images at ultrahigh magnetic fields, circumventing the resonant frequency limitations.

Goal(s): To improve the image quality and to investigate the application of metamaterials within the waveguide to improve performance.

Approach: A bio-inspired surface coil in the transceiver mode and a C-shaped unit metamaterial made from copper sheets laminated onto a nonconductive board within the parallel-plate waveguide filled with a saline solution were used. 

Results: Images acquired with the metamaterial-loaded waveguide demonstrated improved signal-to-noise ratio and closely matched the image quality achieved with in-house birdcage coils

Impact: To provide an effective alternative for remote detection in MRI, enabling high-quality image acquisition with efficiency.

Keywords: RF Arrays & Systems, RF Arrays & Systems, Implantable Coils, Flexible Coils

Motivation: Component-less MRI coils have the potential to increase the strength and flexibility of array elements making them suitable for implantables, multi-modal imaging, and body conformal applications.

Goal(s): As a result, this work strives to demonstrate a resonant, matched MRI coil without the use of discrete components.  

Approach: A Twstr coil is composed of a single Twisted-Pair wire manipulated by twisting and cutting such that the coil presents a match at its resonant frequency.

Results: Twstr coils have a high quality factor, similar SNR performance in a Rx-Only configuration, and outstanding TRx capabilities when compared to a standard loop coil.

Impact: Twstr coils are component-less, flexible MRI coils made from a single piece of Twisted-Pair wire. Such coils eliminate the need for discrete components on a coil providing a viable solution for implantable and multi-modal imaging coils without compromising performance. 

Keywords: MR-Guided Interventions, MR-Guided Interventions

Motivation: Simultaneous ultrasound (U/S) and MR imaging has been reported on whole-body MRI systems with conventional gradients (CVG) but remains unexplored on high-performance gradient (HPG) platforms.

Goal(s): We assessed the MR and U/S image quality, alongside thermal impact on an MR-compatible U/S probe at CVG and HPG configurations.

Approach: Fast spoiled gradient echo imaging, echo planar imaging (EPI), B₀-field map and U/S probe temperature were measured.

Results: Moderate-to-severe susceptibility-induced signal loss (due to U/S probe) extended ~8 mm into phantom at gradient isocenter with no visible U/S image artifacts. A substantial temperature rise was observed on U/S probe during EPI at HPG configuration.

Impact: This study evaluates and compares image quality and thermal risks of simultaneous MR and U/S imaging on a high-performance gradient system versus a conventional clinical system. Our findings can help optimize the protocols for image-guided U/S intervention using high-performance gradients.

Keywords: Non-Array RF Coils, Antennas & Waveguides, RF Arrays & Systems, Wireless Coil, Detune, Litzcage, Birdcage, RF Coil.

Motivation: Detuning wireless volume coils is challenging due to their complex structure, multiple resonant modes and multiple detuning circuits.

Goal(s): Developing an efficient method to geometrically decouple from the body coil

Approach: Designing an inductive birdcage coil featuring a figure-of-eight conductor pattern within the rungs, conducting volunteer and phantom image for compare its performance with the body coil and a receive array.

Results: The wireless Litzcage coil offers ~3.9 times higher SNR than the body coil. A 10% boost in the central area, a 21% reduction at the surface, and similar head image quality compared to a commercial 12-channel Head coil.

Impact: Applies to 0.55T, 3.0T, and 7T MRI systems, and expands to extremity, breast and body imaging. Simplifies coil design, improves detuning, and lowers costs. Lightweight and user-friendly, enabling MRI-guided therapy and streamlined clinical processes.

Keywords: Non-Array RF Coils, Antennas & Waveguides, Low-Field MRI, Cyrogenic RF receive coil, Portable MRI

Motivation: Cooled coils could provide an boost SNR for portable low field MRI as an alternative to increasing the field strength using larger, heavier magnets.  

Goal(s): Test whether cooling RF receive coils with liquid nitrogen could provide an appreciable boost to signal-to-noise ratio (SNR) on low-field portable MRI scanners, where noise is dominated by copper losses in the coil.

Approach: A 3.04 MHz multi-turn surface coil was cooled to ~77K using liquid nitrogen and performance was assessed using bench Q measurements and imaging data.

Results: The unloaded Q increases from 220 at 293K to 541 at 77K. SNR for the cooled coil increased 4.15x.

Impact: Liquid nitrogen cooled radiofrequency receive coils could improve image quality for low field portable MRI scanners, broadening the impact of these systems.

Keywords: RF Arrays & Systems, RF Arrays & Systems, Novel soil design, Brain MRI, 7T, Hyperthermia, Thermal MR, Theranostics, Simulations, Experiments

Motivation: Thermal MR adds an interventional dimension to an MRI device. Ultrahigh-field MRI is an excellent fit for ThermalMR since it benefits from multi-channel transmission in the short wavelength regime

Goal(s): This work investigates the MRI and RF heating performance gain of a  multi-channel Tx in a helmet array versus an annular array. 

Approach: We aim to enhance B₁⁺ uniformity for brain MRI at 7.0T and enhance SAR_10g for brain thermal therapy using perpendicular RF elements.

Results: Our simulations and phantom experiments demonstrate superior mean B₁⁺ coverage and better B₁⁺ uniformity for MRI and enhanced SAR_10g for thermal therapy using the helmet configuration. 

Impact: Dome-shaped helmet transmit RF applicators provide a viable alternative for theranostics involving ultrahigh-field MRI and targeted RF-heating for thermal therapy The performance gain of the helmet transmit RF applicator  is demonstrated in numerical simulations and in experiments at 7.0 T. 

Keywords: RF Arrays & Systems, RF Arrays & Systems, Birdcage coil

Motivation: The main goal is to enhance the performance of an RF coil used in preclinical MRI imaging, specifically at a field strength of 7 T.

Goal(s): The researchers proposed a modification to a standard birdcage (BC) coil. This modification involves adding a "chain mail layout" to the rungs of the coil.

Approach: The idea for the chain mail layout was inspired by previous low pass wiring schemes used in chain mail coils, as reported by Mansfield. Chain mail refers to a type of flexible armor composed of interlinked metal rings.

Results: Phantom images were acquired and used to compute uniformity profiles.

Impact: These results obtained with the chain mail BC coil contributes to our overarching goal of achieving improved MRI coil performance for preclinical applications at 7 T.

Keywords: Analysis/Processing, Shims

Motivation: Inhomogeneities of the MRI transmit field cause image shading and hinder diagnosis. In dielectric shimming, pads of high permittivity are used to recover signal in low intensity areas, but full-wave calculation of the resulting fields is too slow for real-time use at the scanner.

Goal(s): We study feasibility of using AI to rapidly predict the transmit field with dielectric pads.

Approach: An AI pipeline is trained using a small simulated data set for proof of concept.

Results: We obtain a structural similarity of 97% with a mean squared error of 0.02%, demonstrating feasibility and the potential for a real-time implementation in the future.

Impact: This work improves image shading and diagnostics. Dielectric shimming in particular and rapid calculation of electromagnetic fields in general especially apply to ultra-high field strengths such as 7T, 9.4T and 10.5T, where significant inhomogeneity is hindering proper evaluation.