Keywords: RF Arrays & Systems, Parallel Transmit & Multiband, 31P/1H imaging, Dual-tuned array design, ultra-high field, parallel transmit system

Motivation: Integrating multinuclear applications into parallel transmit system (PTx) at ultra-high field MRI systems will provide uniform transmit field and good SNR for both proton and the x-nuclei.

Goal(s): Design, build and integrate a 7T ³¹P/¹H head array into a 7T PTx system that supports multinuclear applications.

Approach: Use 8 loops and 8 folded dipoles as ¹H transceivers, and a birdcage as ³¹P transceiver, for ³¹P/¹H MRS and MRI in a 7T PTx system

Results: Phantom and in vivo experimental results show that the coil can provide homogeneous and good SNR signals for both ³¹P and ¹H in ROI

Impact: Elevating multinuclear studies with 7T parallel transmit system, providing uniform transmit fields and good receive sensitivity profile for both proton and x-nuclei in ROI. A potential game-changer for metabolic studies in body applications at ultra-high fields, advancing medical research.

Keywords: RF Arrays & Systems, Brain, Array, UHF, dipole, coverage

Motivation: At ultra-high fields, homogeneity of brain MRI is deteriorated by the subject-specific non-uniform distribution of RF magnetic field B₁⁺.

Goal(s): To design a 9.4T eight-channel transceiver dipole array with improved homogeneity of B₁⁺ in the axial direction with better whole-brain coverage.

Approach: We used an array consisting of paired passively coupled folded-end dipoles. We numerically optimized the B₁⁺ homogeneity by adjusting the overlap between the folded ends of the active and passive dipoles and the load impedance of the passive one.

Results: The proposed array demonstrated improved B₁⁺ whole-brain homogeneity including the upper C-spine compared to several state-of-the-art dipole and loop arrays. 

Impact: The presented antenna element of coupled folded dipoles can be used in designing UHF array coils with improved longitudinal whole-brain coverage. Such coils can be beneficial for studies where imaging of the entire brain including the upper C-spine is required.

Keywords: Parallel Transmit & Multiband, Hybrid & Novel Systems Technology, Dual-tuned, Hybrid & Novel Systems Technology, RF Array, Parallel Imaging

Motivation: The capability of coaxial-transmission-line (CTL) RF coils to exhibit multimode operating frequencies make them versatile for applications involving dual-nuclear coil resonators for Magnetic Resonance Imaging  and Magnetic Resonance Spectroscopy studies.  

Goal(s): Design eight-channel dual-tuned CTL coils array for human head imaging at 10.5 Tesla and address the electromagnetic coupling issue between elements of the array. 

Approach: The high impedance feature of the high-frequency mode helps to achieve sufficient decoupling for ¹H channels while the magnetic wall technique is used to decouple the low-frequency mode tuned to heteronuclear frequency.

Results: A well-decoupled array of dual-tuned CTL coils is feasible for head imaging at 10.5T. 

Impact: The feasibility of advanced multichannel dual-tuned RF coil array can contribute to more informative imaging and spectroscopy, which is valuable in various medical and research applications.   

Keywords: Safety, Safety, specific absorption rate; local SAR; deep learning; Bayesian deep learning; uncertainty estimation

Motivation: Local SAR cannot be measured during an MRI examination. Deep learning approaches are proving to be a solution for on-line subject-specific SAR assessment.

Goal(s): The brain is the region of greatest clinical interest for ultra-high field MRI. Therefore, we apply for brain imaging a deep learning approach presented for local SAR assessment for body imaging.

Approach: The Bayesian deep-learning approach maps the relation between subject-specific complex B₁⁺-maps and the corresponding local SAR distribution, and predicts the spatial distribution of uncertainty at the same time

Results: The Bayesian deep-learning approach for local SAR assessment in brain outperforms the previous application for prostate imaging.

Impact: The application of Bayesian deep-learning can allow the reduction of overly conservative RF safety constraints that limit the performance of UHF-MRI. Furthermore, the joint estimation of uncertainty can help the acceptance of such methods in clinical contexts.

Keywords: Safety, Safety

Motivation: The safety factor, which scales SAR matrices used in real-time SAR monitoring, is commonly dominated by inter-subject variability. However, the contributions from various sources to that variability have not been fully evaluated. 

Goal(s): Assess the impact of different human model variations and different patient positions inside the coil, on the predicted peak local SAR for 10.5T head applications.

Approach: The SAR inter-subject variability was investigated via EM simulations of two human models along with a wide variety of head positions inside an 8-channel coil at 10.5T.

Results: The variability between head models was significantly more consequential than variations in a head model’s position.

Impact: Simulating realistic scenarios with wide appropriate variables, to calculate SAR with a more accurate inter-subject variability on peak local SAR, has the potential to improve patient safety without compromising the scanning quality at ultrahigh field MRI. 

Keywords: Safety, Safety, UHF-MRI, SAR, pTx design

Motivation: Specific absorption rate (SAR), a proxy measure for tissue heating, is affected by patient motion. SAR safety factors during MRI scanning are intentionally overconservative. 

Goal(s): While designed to ensure patient safety, overconservativeness impedes the utility of scanning with parallel-transmit (pTx) 7T MRI. We aim to relieve pTx MRI from overconservativeness while maintaining patient safety. 

Approach: We used deep learning to predict the location of hot-spots during head motion and applied them to a pTx design method which considers patient motion. 

Results: We report that hot-spots are overcalculated almost 1.5-fold when the degree of motion is not included compared to when it is.

Impact: Deep learning-estimated local specific absorption rate (SAR) variations caused by patient motion may be combined with within-scan motion tracking and subject-specific SAR models to create personalized SAR supervision for patients who cannot remain still for high-performance scanning while ensuring patient-safety.  

Keywords: RF Arrays & Systems, RF Arrays & Systems

Motivation: Traditional dense transmit arrays for 7T imaging face decoupling challenges. A novel approach using self-decoupled coils can potentially overcome these limitations and enhance transmit efficiency.

Goal(s): Evaluating an in-house developed 20-element array combining dipoles with self-decoupled overlapped loop coils.

Approach: Bench measurements, electromagnetic simulations on phantom and Duke, and MRI acquisitions were conducted to investigate the coil array's performance, assessing individual channel B₁⁺-map efficiencies and decoupling capacities

Results: Decoupled individual B₁⁺ maps for dipoles, self-decoupled and overlapped loops were obtained for 20-channel head array using MRI measurement and simulations on phantom at 7T. S-matrix results on bench and simulations showed decoupled coil elements.

Impact: Searching for alternative and complex coil array design for transmit and receive arrays might enable for more efficient and fast imaging at MRI with high resolution to enable better diagnostics and improved treatment planning.

Keywords: Safety, Fetus, Electromagnetic simulations

Motivation: Due to the large variation of the body shape and size in the pregnant population, several pregnant body models are needed in fetal MRI safety studies. However, creating detailed pregnant body models is challenging.

Goal(s): Our goal is to investigate the effect of reducing maternal tissue detail on maternal and fetal SAR.

Approach: We simplified >18 maternal tissues into 3 (muscle, lung and fat) in 7 pregnant body models and quantified the change in maternal and fetal SAR.

Results: The value and location of maternal and fetal SAR changes by up to 13% and 34 mm with maternal tissue detail reduction.

Impact: Simplification of maternal tissues into muscle, lung and fat changes maternal and fetal SAR (<14%) significantly less than the variation of SAR between pregnant subjects (>x2) and might provide a good trade-off between accuracy and practicality in RF safety studies.

Keywords: Safety, Safety

Motivation: Advancements in MRI technology towards high fields demand rapid and accurate SAR estimation tools for enhancing MRI safety, currently hindered by the computational cost of conventional physics-based simulators.

Goal(s): The goal is to develop an efficient machine learning framework capable of estimating subject-specific SAR values rapidly.

Approach: The study employs 3D U-net deep learning models with their variants to achieve rapid and accurate SAR estimations.

Results: The proposed neural network model provides SAR estimations within 183ms, achieving approximately 10,000x acceleration over traditional physics-based simulators, with a mean relative error of 7.6%.

Impact: The near real-time accurate SAR estimation achieved by proposed machine learning framework will allow (i) checking patient-specific SAR while patient is lying in the MRI machine and (ii) performing ultra-fast optimization and uncertainty quantification studies while designing new high-field coils.

Keywords: Shims, Shims, high-permittivity, sfaty, hydrogel pad

Motivation: Utilize new material to fabricate high-permittivity pads for addressing the B1 inhomogeneity and safety issues in ultra-high-field MRI scanning.

Goal(s): We aim to develop a new high permittivity pad with flexibility which can fit various body shapes. Different sizes and mass proportions of barium titanate powder were explored to achieve optimized mechanical properties and desired permittivity value.

Approach: The gel pad is fabricated using the PVA freeze-thaw method, resulting in a stable gel structure that incorporates barium titanate powder, thereby possessing excellent mechanical properties and high permittivity.

Results: Experimental results demonstrate of the gel pad's potential for clinical applications in ultra-high-field MRI scanning.

Impact: This abstract introduces a novel gel pad with high permittivity that can be conformed to different shapes of scanned objects. It highlights the potential of the gel pad in addressing safety issues in ultra-high-field MRI scanning.

Keywords: System Imperfections, System Imperfections: Measurement & Correction

Motivation: Magnetic Resonance Fingerprinting (MRF) and preparation-based reference B₁⁺ mapping approaches yielded an unexpected B₁⁺ offset.

Goal(s): Identify the origin of the B₁⁺ offset.

Approach: Pick-up coil measurements to evaluate the actual RF pulse shape using a commercial 1Tx/32Rx head-coil at 7T.

Results: For the preparation-based B₁⁺ mapping method with rectangular pulses, longer RF rise times (~100 µs) and phase variations (~10°) were observed. Furthermore, during the 21s-long MRF acquisition using sinc-pulses, an increased RF amplitude (~2.5%) was observed. By incorporating the measured RF alterations, the B₁⁺ offset could be substantially reduced.

Impact: Unidentified effects of the RF transmission such as deviation in envelope and temporal instability affect measurements. Characterizing the RF transmission enables quantification and correction of these deviations.

Keywords: High-Field MRI, Arterial spin labelling

Motivation: As the most promising method for measuring tissue perfusion, ASL suffers from low signal-to-noise ratio (SNR), with tagged blood typically causing only few percent of signal difference. 

Goal(s): Explore if the use of 5T MRI could lead to higher SNR and CNR for renal ASL than 3T.

Approach: We performed FAIR-EPI on the kidneys of three healthy subjects at 6 inversion times on both 3T and 5T systems. SNR and CNR of difference images were calculated and compared.

Results: SNR at 5T was 10.72±2.81, higher than 7.75±2.65 (P = 0.008) at 3T, and CNR were 9.08±4.08 and 6.53±2.14 (P=0.029), respectively.

Impact: 5T ASL provides difference images with higher SNR and CNR to allow faster acquisition and better corticomedullary differentiation while maintaining comparable image quality.

Keywords: Shims, High-Field MRI, non-human primate, B0 shimming, fMRI

Motivation: Functional MRI faces challenges due to B0 field inhomogeneities, which are amplified in non-human primate studies, especially at ultra-high fields.

Goal(s): Our aim was to develop a Multi-Coil Array (MCA) specifically designed for whole brain shimming of non-human primates.

Approach: A semi-heuristic approach was used to design an MCA, based on the Principal Components Analysis of subject-optimal stream functions. The MCA was then constructed and used to image an in-vivo NHP brain.

Results: The designed MCA effectively reduced B0 inhomogeneities in the primate brain, resulting in significant improvements in B0 homogenization and enhanced image quality in functional MRI images at 7T.

Impact: The development of a Multi-Coil Shim Array designed specifically for non-human primates significantly improves functional MRI image quality by effectively reducing B0 inhomogeneities. This advancement could pave the way for more accurate and detailed primate brain studies at ultra-high fields. 

Keywords: Gradients, Gradients, Peripheral nerve stimulation, magnet heating

Motivation: Two interactions between gradient coils and their environment severely limit imaging performance: stimulation of peripheral nerves in human subjects and deposition of heat energy into the superconducting main magnet leading to helium loss or quench. This heating increases significantly for higher B₀ and gradient switching frequencies.

Goal(s): To address the above limitations by recasting the gradient design problem in a unique way to simultaneously minimize both of these interactions.

Approach: Using these new methods, we designed and modeled a head gradient coil: H12.

Results: We achieved >1.4 fold PNS improvement and >6 fold lower magnet heating compared to existing state-of-the-art head gradient coils.

Impact: Our method allows the design and real-world use of stronger and faster-switching gradients than any in existence, while minimizing peripheral nerve stimulation and excessive magnet heating. The potential impact is especially high for head gradients operating at ultra high field.

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband, SAR;RF Shimming;CMR;

Motivation: Enhance RF shim efficiency, reduce power, and provide uniformity for ultra-high field cardiac imaging.

Goal(s): To significantly enhance excitation efficiency, reduce excitation power, and mitigate the risk of SAR exceeding safe levels, while minimizing loss of uniformity.

Approach: Optimizing excitation intensity and uniformity through joint loss function and validating the approach with simulation models and in vivo imaging.

Results: The research results indicate that, when compared to MLS, the proposed Eff-MLS leads to an average 3.3% reduction in CV, while improving safety excitation efficiency by 442.26% in the simulation model. Similar effects were also observed in in-vivo experiments.

Impact: By enhancing RF shim efficiency and reducing the required excitation power while maintaining uniformity, this approach paves the way for more precise, high-resolution cardiac imaging at ultra-high field strengths. The potential impact includes broader applications of ultra-high field CMR.

Keywords: Safety, Safety

Motivation: Sodium(²³Na)/proton(¹H) MRI of the extremities have great potential in diagnosis and treatment of musculoskeletal conditions and oncology applications (e.g.soft-tissue sarcomas).

Goal(s): Assessing the performance of a novel 16-channel ¹H/²³Na MRI coil across varied leg positions and establish safe RF power limits for clinical applications.

Approach: Electromagnetic simulations to analyse coil performance in four leg positions. SAR_10g levels were compared across configurations while maintaining B₁⁺ field efficacy.

Results: The coil showed consistent B₁⁺ performance across all tested positions. The highest SAR levels occurred with calf-to-coil contact, aligning with predictions. Safe RF power limits were established, supporting the coil's clinical potential for extremity imaging

Impact: This study validates an innovative 16-channel ¹H/²³Na coil, setting benchmarks for safe RF power limits, and enhancing disease characterisation, enabling clinical research in musculoskeletal pathologies and soft-tissue sarcoma in the extremities.

Keywords: Safety, Safety

Motivation: The existing safety validation process for multi-channel RF coils requires MR thermometry tests, which are difficult to carry out. 

Goal(s): To propose an alternative approach to MRT in the validation process of multi-channel RF coils. 

Approach: The simulated B₁⁺ error was measured and propagated to the SAR error using Monte-Carlo simulations. To validate this approach MRT experiments were conducted with a 16-channel body coil at 10.5T, and the measured SAR errors were compared against the predicted ones.

Results: For two different excitation patterns, measured SAR errors were 36% and 40%, while predicted SAR errors were 49% and 46%, respectively.

Impact: The new strategy presented replaces MR Thermometry in the safety validation process of multi-channel RF coils. This approach can simplify and accelerate the development and testing of innovative multi-channel RF coils which are essential for UHF MRI applications.

Keywords: Safety, High-Field MRI, SAR, EM simulation

Motivation: Ensuring subject safety, in particular to limit tissue heating, is a critical aspect of self-developed pTx RF array coils for UHF applications.

Goal(s): Establishing a dependable workflow for accurate simulation, data processing and realization of online supervision of power deposition.

Approach: The workflow relies on cross-comparisons of EM simulation results and intermediate processing steps based on representative excitation modes, and on comparison of measured and simulated field maps.

Results: Consistency was achieved in all cross-comparison steps. Residual discrepancies between measured and simulated B₁⁺ maps require further investigation, but their safety implications can be addressed by an appropriate safety factor.

Impact: A reliable workflow for EM simulation, subsequent data processing, and realization of online SAR monitoring for home-built RF human head array coils at 9.4T is presented. This is an essential building block to ensure subject safety in experimental UHF studies.

Keywords: Safety, Safety, Prostheses, active auditory implants

Motivation: While scanning individuals with active hearing implants (e.g., cochlear implants) at 1.5 and 3 T has become commonplace, scanning these patients at ultra-high field strengths (e.g., 7 T) is likely to be associated with an increased risks of damaging the device or causing soft-tissue damage, pain, and/or discomfort.

Goal(s): To evaluate the safety of three different active hearing implants in the 7 T MR environment.

Approach: Five potential interactions between the devices and the MR environment were assessed using industry-recommended practices for 1.5/3 T.

Results: Preliminary findings show no adverse effects within the predefined test conditions.

Impact: While scanning individuals with active hearing implants at 1.5/3 T has become commonplace, 7 T is associated with significantly greater safety risks. This study is the first to provide some confidence in the safety of such implants at 7 T.

Keywords: Spectroscopy, Spectroscopy, 31P MRSI, NAD+, NADH

Motivation: Nicotinamide adenine dinucleotide (NAD) is vital in cellular metabolism, existing in an oxidized (NAD+) and reduced (NADH) form. Its submillimolar concentration in the human brain makes its whole brain mapping challenging.

Goal(s): To explore the feasibility of whole-brain NAD mapping in human.

Approach: 31P MRSI data were acquired from two volunteers using a 31P 32-channel array coil at 7T. Metabolites were quantified with LCModel.

Results: 3D whole-brain NAD maps were acquired with decent SNR within 45min. The measured NAD level and NAD+/NADH ratio were stable across two subjects and aligned with previous single-voxel studies.

Impact: Our preliminary data demonstrated the feasibility of whole-brain NAD mapping in humans at 7T, which offers the potential to study regional-specific bioenergetics under different pathological conditions.