Keywords: Gradients, Gradients, Eddy current, Electromagnetic Simulations

Motivation: Existing eddy current calculation methods aren't suitable for fast eddy power loss calculations.

Goal(s): We propose a quick calculation to estimate the time-average eddy power loss within the cryostat, generated by gradient assembly (array or conventional) and arbitrary pulse sequences.

Approach: The frequency response of the fields generated by the gradient assembly elements (array or conventional) on the cryostat's surface is combined with the harmonic components of the driving pulse sequences.

Results: A typical pulse sequence feeds a whole-body conventional x-gradient coil, and the proposed method estimates the time-average dissipated power on the fly, comparable with simulation results reported by commercial software.

Impact: Our method optimizes gradient assembly (array/conventional) tuning, empowering MRI engineers with fast and precise dissipated power estimates. This approach sparks novel research paths, enhancing MRI system efficiency and enabling tailored pulse sequences for gradient coils, advancing the field significantly.

Keywords: Hybrid & Novel Systems Technology, Motion Correction, Cardiovascular

Motivation: Beat Pilot Tone (BPT) has been proposed as a non-contact and accurate cardiac sensing method seamlessly integrated with MR systems. However, BPT is sensitive to interference, thereby limiting its ability to robustly obtain fine-grained cardiac waveform.

Goal(s): Our goal was to demonstrate main factors that could lead to corrupted BPT signal.

Approach: We conducted a series of comparative experiments on the hypothesized influencing factors.

Results: The experiments suggested that BPT transmit power, imaging orientation and other physiological motion like respiration caused varying degradation to the BPT signal.

Impact: The demonstration of interfering factors of BPT cardiac sensing could guide the correct BPT setup and interference suppression method design. This helps BPT to robustly obtain fine-grained cardiac waveform, which could be used for MR motion correction and clinical diagnosis.

Keywords: Safety, Gradients

Motivation: Conductive objects such as RF shield can pose heating risk due to eddy current in fast-switching gradient fields.

Goal(s): To develop a systematic method to calculate eddy current heating when multiple gradient coils pulse simultaneously with independent waveforms. We explicitly consider interaction of different coils.

Approach: Our method was tested against experimental measurement of temperature rise in a high-performance head-only gradient coil (MAGNUS) at 3T.

Results: Predicted and measured local eddy current heating showed good qualitative agreement. Importance of coil coupling was demonstrated by the experimental data.

Impact: We present a systematic method to calculate eddy-current heating induced by multiple independent field coils. The work permits accurate prediction of RF shield heating in high-performance gradient systems to ensure patient safety.  

Keywords: Bioeffects & Magnetic Fields, Safety, Peripheral Nerve Stimulation (PNS), gradient coils, fast imaging

Motivation: The on-scanner PNS monitor must estimate each sequence’s stimulation potential. The current SAFE model is overly conservative, unnecessarily restricting gradient performance by up to 1.8X.

Goal(s): Develop a PNS monitor model that rapidly and accurately characterizes a sequence’s PNS thresholds.

Approach: We propose a model (SAFE2) closely inspired by the mechanisms of PNS to capture critical aspects such as E-field cancelations from different gradient axes and extend the training data using detailed PNS modeling reflecting a more diverse set of waveforms.

Results: SAFE2 improves PNS-prediction accuracy by 2X compared to SAFE, boosting usable image encoding performance by up to 30% without hardware changes.

Impact: PNS restricts the usable protocol parameters of EPI, bSSFP, Radial-GRE, etc., yielding suboptimal imaging performance. The current PNS monitoring approach (SAFE) is very conservative, therefore, we propose an improved model yielding up to 30% gradient performance boost without hardware modification.

Keywords: System Imperfections, Relaxometry

Motivation: Non-Cartesian sampling strategies enable free-breathing imaging due to their robustness against motion but are susceptible to artifacts related to gradient imperfections. Such artifacts may manifest as bias or structured noise in quantitative imaging applications.

Goal(s): In this study, we explore the effects of gradient imperfections and evaluate the performance of gradient correction methods in non-Cartesian, confounder-corrected T₁ mapping using both phantom and in vivo imaging experiments.

Approach: For this purpose, we compare confounder-corrected T₁ maps reconstructed with data-driven and calibration-based gradient correction approaches.

Results: Our initial results indicate that gradient correction methods are essential for mitigating the bias due to gradient imperfections.

Impact: This study confirms that gradient imperfections result in bias in non-Cartesian quantitative imaging applications. Our findings indicate that T₁ relaxometry is less susceptible to gradient imperfections than PDFF and R₂* quantification. The application of gradient correction methods mitigates this bias.

Keywords: Shims, Artifacts, Acquisition Methods, Hybrid & Novel Systems Technology, New Devices, Software Tools, Spinal Cord

Motivation: The static magnetic field around the spinal cord has complex and small-scale non-uniformities, making it difficult to shim with low-order spherical harmonics.

Goal(s): Assess the impact of slice-wise shimming in the cervico-thoracic spinal cord using a 15-channel AC/DC coil.

Approach: Measure the magnetic field and acquire echo-planar images using three different shimming scenarios involving volume-wise 0-2^nd order spherical harmonics, volume-wise multi-coil shimming, and slice-wise multi-coil shimming.

Results: Multi-coil shimming decreased magnetic field inhomogeneities by 22% compared to 0-2^nd order spherical harmonics when used across the whole imaging volume, and by 36% when used on a slice-wise basis.

Impact: Advanced shimming (using multi-coils) significantly improves the quality of spinal cord MRI images by mitigating susceptibility artifacts. This leap forward facilitates the clinical adoption of challenging imaging techniques like functional MRI in the spinal cord.

Keywords: Gradients, Diffusion/other diffusion imaging techniques

Motivation: Second-order motion-compensated spin-echo (MCSE) DT-CMR is limited by long echo-times (TE), resulting in T₂-related signal-loss.

Goal(s): Use the ultrahigh gradient-strength of the Connectom scanner to reduce TE of second-order MCSE DT-CMR.

Approach: A second-order MCSE DT-CMR pulse-sequence was developed. Ultra-high (180mT/m) and a high gradient-strength (80mT/m) were compared in acquiring in-vivo DT-CMR data at two cardiac phases.

Results: The substantial reduction in TE enabled by ultra-high gradient-strength resulted in improvements in signal-to-noise ratio (SNR) in both cardiac phases. To our knowledge this is the first report comparing the performance of second-order MCSE DT-CMR at ultra-high gradient-strength (Connectom) to widely available high gradient-strengths.

Impact: Second-order MCSE DT-CMR acquired using ultra-high diffusion gradient strengths increases SNR in both cardiac phases, paving the way for future clinical translation of efficient multiphase DT-CMR.

Keywords: Diffusion Acquisition, Shims, Diffusion

Motivation: High-resolution diffusion MRI in high-gradient head-only systems, which is promising for advancing our understanding of human brain microstructure, is still prone to distortion from B0 field inhomogeneity.

Goal(s): To reduce B0 field inhomogeneity for improved image quality of high-resolution diffusion MRI. 

Approach: Dynamic slice-by-slice B0 shimming of 0^th and 1^st order was implemented in diffusion MRI. 

Results: Preliminary results showed reduced image shift and distortion of slices at different locations in axial b=0 s/mm² echo planar image with 1-mm isotropic resolution with dynamic slice-dependent B0 shimming, demonstrating the potential of high image quality of high-resolution diffusion MRI for brain microstructure imaging.
 

Impact: Neuroimaging scientists and MR physicists, who use high-resolution diffusion MRI for studying brain circuits, connectivity, and microstructure, can benefit from improved image quality with reduced distortion from dynamic slice-by-slice B0 shimming technique in high-performance gradient MRI systems.

Keywords: Signal Modeling, System Imperfections: Measurement & Correction, Nonlinear Encoding

Motivation: Nonlinear gradient imaging is impeded by time-consuming field mapping and the encoding ability is yet to be unleashed.

Goal(s): To accelerate nonlinear field mapping and make use of nonlinear encoding.

Approach: We report a fast and robust estimation method for nonlinear field mapping and studied the utility of simultaneously applying nonlinear and linear gradients, referred to as FRONSAC-wave, where the high-frequency nonlinear field waveform is complementary to a lower frequency linear field waveform. 

Results: The fast field mapping decreases the field mapping time from 10 hours to <0.5 hour. FRONSAC-wave demonstrates better imaging ability compared to wave at various acquisition settings.

Impact: This study demonstrates the advantage of combined sinusoidal waveforms on linear and nonlinear gradients and proves the feasibility of fast mapping for nonlinear fields. It opens prospects for utilization of nonlinear field encoding in clinical scenarios.

Keywords: Myocardium, Cardiovascular

Motivation: Balanced SSFP sequences in cardiovascular MRI suffer from dark band artifacts due to B₀ inhomogeneity. Limited detail of in vivo B₀ conditions apparent in various population groups impede the development of optimal cardiac B₀ shim methods.  

Goal(s): To validate our recently-published high-resolution B₀ simulation approach of B₀ conditions in the human heart based on structural CT images.

Approach: Validation was achieved through direct comparison of B₀ conditions computationally derived from structural CT images, vs. in vivo B₀ maps obtained experimentally on the same five subjects.

Results: Excellent agreement occurred between simulated and in vivo B₀ maps, with an average spatial correlation of 0.91.

Impact: The validated cardiac B₀ simulation from readily-available structural CT images enables characterization of B₀ conditions in populations with wide-spread demographics of age, sex, height, weight and the development of advanced B₀ shimming methods to improve diagnostic accuracy in cardiovascular MRI.

Keywords: Bioeffects & Magnetic Fields, Bioeffects & Magnetic Fields, Peripheral Nerve Stimulation, Magnetic Particle Imaging

Motivation: Two previous peripheral nerve magnetostimulation experiments reported increasing thresholds above 25 kHz, which deviates from the hyperbolic strength-duration curve describing thresholds versus frequency. However, high-frequency PNS measurements are sparse and established neurodynamic models have not been validated above 1 kHz.

Goal(s): Characterize PNS thresholds in a solenoidal head coil between 200 Hz and 88.1 kHz.

Approach: We measure PNS thresholds in four healthy volunteers and compare to predictions of our electromagnetic-neurodynamic PNS model.

Results: The measured thresholds increase 36% on average from 16.9 kHz to 66.7 kHz, which is at odds both with the hyperbolic scaling as well as our detailed PNS modeling.

Impact: Our strength-duration measurements show that the greatest stimulation propensity is ~17 kHz and PNS thresholds remain relatively low at frequencies greater than 20 kHz, which is important for informing the design of MRI and MPI coils.

Keywords: Hybrid & Novel Systems Technology, Diffusion/other diffusion imaging techniques, TMS-MRI

Motivation: Mapping of gray matter microstructure by diffusion MRI is limited by gradient strength, which sets the resolution limit toward small cells and axons.

Goal(s): To use a multi-channel 3-axis TMS coil array as a miniature gradient system and generate strong local gradients.

Approach: We ran simulations and ex-vivo experiments using the TMS coil as a strong local gradient for diffusion encoding.

Results: The measured gradient strength matched simulated values. The TMS-gradient system was used to estimate mean diffusivity in ex-vivo tissue. Simulations show it has the potential to generate even stronger gradient fields with higher current and more coil elements.

Impact: Innovations in TMS-MRI gradient design could significantly boost the achievable gradient strength for diffusion MRI in the living human brain, advancing neuroscientific research within a cost-effective design.

Keywords: Machine Learning/Artificial Intelligence, System Imperfections: Measurement & Correction, Reinforcement learning

Motivation: Gradient hardware chains can exhibit dynamic nonlinearities that cannot be easily corrected with linear models and require more sophisticated approaches.  

Goal(s): Our goal was to develop a flexible and dynamic approach to correct nonlinear MRI system imperfections.

Approach:  We developed a reinforcement learning method for predicting gradient preemphasis and evaluated it in a realistic simulated environment with obscured state information.  

Results: Reinforcement learning is able to accurately predict gradient preemphasis even when system state information is unknown. 

Impact: The ability to dynamically correct system imperfections through reinforcement learning may allow the development of more robust imaging systems that can adapt to complex, nonlinear distortions, reducing the need for expensive hardware corrections or inflexible, system-specific system models.

Keywords: RF Pulse Design & Fields, High-Field MRI

Motivation: Following the commissioning of the Iseult CEA 11.7 T whole-body MRI system, first in vivo human brain images have been acquired at 11.7T.

Goal(s): Our aim is to demonstrate the feasibility of whole brain imaging using an 8TX/32RX home-built RF coil and to test subject-tailored pulses as well as calibration-pTX pulses.

Approach: Using 9 in-vivo B1 maps of the brain, we prepared and tested tailored kT-point pulses and universal GRAPE pulses to be used in non-selective 3D sequences.

Results: Our retrospective pulse performance analysis confirms the feasibility of whole brain imaging at 11.7T both using subject tailored and calibration free pTX.

Impact: . At 11.7T, the heterogeneity of the pseudo-CP mode, the transmit efficiency and the SAR level are such that dynamic pTX and extensive RF pulse optimizations are essential for whole brain imaging applications.

Keywords: PET/MR, PET/MR, ASL, MATT, PET arrival time

Motivation: Arterial Transit Time (ATT) is an important hemodynamic biomarker for cerebrovascular diseases such as stroke and although ¹⁵O-water PET is considered the gold standard CBF imaging modality, its capabilities in measuring transit time has not been demonstrated.

Goal(s): To map tracer arrival time in ¹⁵O-water PET exams.

Approach: Reconstructed short frames during the early PET acquisition with consistent image quality and used a 5D noise filtering method to measure time-activity-curve and then PET-arrival-time for each voxel.

Results: PET-arrival-time can map perfusion related abnormalities apparent in simultaneously acquired ASL-ATT

Impact: Even though 15O-water PET is considered the gold standard CBF imaging modality, it lacks measuring an important hemodynamic biomarker, arterial-transit-time. By using MR-priors, we measured the PET-arrival-time on PET/MR and showed it correlates well with MATT measured by ASL simultaneously.

Keywords: PET/MR, Arterial spin labelling, Cerebral Oxygen Metabolism

Motivation: To develop a non-invasive method of imaging CMRO₂ by hybrid PET/MRI.

Goal(s): Translate a previously validated PET/MR technique capable of imaging CBF, OEF, and CMRO₂ to human studies.

Approach: Technique combines [¹⁵O]O₂-PET with MRI measurements of whole-brain CMRO₂ and arterial spin labeling images of CBF.

Results: Good agreement to literature regional CMRO₂ values.

Impact: Hybrid PET/MRI can simplify the PET-only technique of measuring regional CMRO₂ by avoiding arterial sampling and only requiring one radiotracer, which reduces scan time and radiation dose, while enabling simultaneous acquisition of perfusion and tissue oxygen extraction.