Keywords: Gradients, Gradients

Motivation: To enable silent whole-body MRI using ultrasonic encoding, an additional low inductance gradient coil can be integrated between the RF-rods and RF-shield of a bore coil. However, the resulting effect on RF efficiency is unclear.

Goal(s): To demonstrate the feasibility of placing a gradient coil under the RF-shield without affecting RF efficiency.  

Approach: A Maxwell gradient coil was integrated into a bore coil. We measured the gradient field and compared RF efficiency with and without the extra gradient.

Results: A gradient field was successfully induced without greatly affecting RF efficiency. For efficiency validations, volunteer images were obtained with and without the integrated coil.

Impact: Placing a low inductance gradient coil within the empty space in a bore coil allows an independent gradient axis to be driven at 20kHz. This may address high-voltage and PNS concerns when extending to whole-body silent MRI. 

Keywords: Gradients, High-Field MRI

Motivation: High-performance gradient coils can induce substantial eddy current in the magnet causing heating and artifacts. Motional eddy current at high B0 exacerbates the problem.

Goal(s): To investigate the feasibility of patterned, additional passive shielding to reduce both electromagnetically-indued and motional eddy currents.

Approach: A proof-of-concept experiment was conducted in flat geometry, where shielding efficiency at 3T was compared between a solid copper and a patterned copper plate that mimicked the eddy current image of a driving coil.

Results: The leakage field of patterned copper agreed with theory and was more localized in frequency and space. The shielding efficiency was improved at high frequencies.

Impact: We demonstrated that vibration-induced motional eddy current of a passive conductive shield can be dramatically altered by eddy current cut-outs. The results can be used to build a compact, high-field scanners with reduced magnet-gradient interaction.

Keywords: System Imperfections, System Imperfections: Measurement & Correction, Linear Time-invariant Model

Motivation: The gradient system transfer function (GSTF) can correct k-space distortions arising from gradient imperfections, but is limited by assuming a linear, time-invariant gradient system.

Goal(s): We wanted to investigate the linearity of the gradient signal path before and after the gradient power amplifier.

Approach: We compared transfer functions for different segments of the gradient chain, measured with positive and negative test pulses. We then predicted the waveform of a trapezoidal gradient with the transfer functions, and quantified the error of the predicted k-space.

Results: Using a transfer function based on the gradient amplifier currents halved the errors on the predicted k-space coordinates.

Impact: The current-based transfer function could enable a fully automated k-space trajectory determination with doubled accuracy compared to the gradient system transfer function, without special hardware or lengthening scan times. This could help with the clinical translation of non-Cartesian MRI acquisitions.

Keywords: System Imperfections, Gradients, Eddy-Currents, Multi-Coil

Motivation: The eddy current (EC) behavior of a multi-coil array for image encoding in a novel head-only MR scanner has been estimated theoretically, however, to date, the physical measurements have been outstanding.

Goal(s): Our goal was the spatiotemporal characterization of the ECs of this system.

Approach: We measured the ECs via local field probes by acquiring FIDs at a series of delays after the switching of individual MC channels.

Results: Measurements at over 40 positions in the FOV enabled a comprehensive analysis of the ECs. The longest EC components last for more than a second, however, we show that EC compensation is possible.

Impact: The presented eddy-current characterization is an important stepping stone towards eddy-current compensation for DYNAMITE image encoding in non-traditional MR scanner designs.

Keywords: System Imperfections, Diffusion/other diffusion imaging techniques

Motivation: To correct undesirable eddy-current and concomitant-fields effect in MRI acquisitions.

Goal(s): To achieve successful eddy-current and concomitant-fields mitigations, paving the way for high-quality 3D-invivo diffusion MRI with high-SNR.

Approach: We developed a dynamic shim approach with a multi-coil AC/DC shim-array to correct eddy-currents induced-phase in diffusion-prep encoding, and concomitant-field in long spiral in double-oblique positions.

Results: Invivo experiments were performed on the AC/DC shim-array to demonstrate cardiac-gated, multi-shot 3D-diffusion-prepared DTI acquisition without need for SNR-zapping stabilizer. High-quality, high-b-value(2000 s/mm²) acquisition was achieved where a synergistic combination of shim-array and pre-pulse gradient mitigation strategies were employed to minimize eddy-current phase in this difficult case.

Impact: We proposed a dynamic shim approach for robust invivo eddy-current correction in 3D-DTI with short preparation-time and high-SNR. Our results demonstrates the AC/DC coil's effectiveness in compensating challenging concomitant-fields during double-oblique slice acquisitions, especially with long-spiral sampling and high-performance gradients.

Keywords: System Imperfections, Gradients

Motivation: Gradient imperfections from eddy currents and trajectory error cause image artifacts. NMR field probes can accurately measure these imperfections to achieve high-quality imaging, but require additional hardware and cost.

Goal(s): Develop an alternative imaging-based approach to estimate gradient imperfections by leverage the encoding capability of modern multi-channel receivers and neural networks for implicit Fourier phase representation.

Approach: Simulated spiral imaging acquisition with eddy current and low-gradient sequences without eddy effects. Trained MLPs to convolutionally represent gradient phase imperfections in k-space as a function of time.

Results: MLP estimates spatio-temporal phase to high accuracy, showing promise for high-order phase estimation without NMR field probes.

Impact: We propose an algorithmic imaging-based alternative approach to field probe for gradient characterization. This enables enhanced image reconstruction for high-slew MRI without external hardware, potentially revolutionizing fast acquisition MRI techniques and broadening their application.

Keywords: Safety, Safety, Peripheral Nerve Stimulation, Gradient Array

Motivation: High-performance gradient coils are markedly restricted by peripheral nerve stimulation rather than hardware limitations.

Goal(s): We aimed to reduce the gradient-induced electric fields on the body and consequently increase peripheral nerve stimulation thresholds.

Approach: We introduce a head-only Z-gradient array coil and optimize current weightings of the array configuration to minimize induced electric fields while generating the target gradient within a customizable region with flexible linearity levels.

Results: Utilizing the array configuration reduces the maximum electric field up to 50%, 61%, and 78% compared to a conventional mode for a 22cm spherical region, the disk-shape region at the center, and off-center disks, respectively.

Impact: Peripheral nerve stimulation thresholds of head gradient coils can be increased using array technology; thus, higher gradient strengths and slew rates can be achieved without causing PNS.

Keywords: High-Field MRI, Brain, spiral imaging, field monitoring

Motivation: There has been an increasing interest to pursue spiral imaging at ultrahigh field owing to its improved sampling efficiency. 

Goal(s): Our goal was to demonstrate the feasibility of spiral imaging in humans at 10.5 Tesla.

Approach: Highly-accelerated single-shot 2D spiral GRE images were collected using 128 receive channels and a sequence developed in an open source environment. Dynamic field changes associated with the spiral readout gradients were measured in a separate session using 16 NMR probes. 

Results: Quality T₂*-weighted single-shot spiral imaging of the human brain was achieved by simultaneous corrections of static off-resonances and dynamic field changes through image reconstruction.

Impact: The demonstrated feasibility of spiral imaging in humans at 10.5 Tesla may shed light on how best to implement spiral imaging at ultrahigh field, paving the way for many applications that would benefit from a spiral readout.

Keywords: Hybrid & Novel Systems Technology, Spectroscopy, ECLIPSE

Motivation: ECLIPSE¹ is a novel method to achieve robust outer volume suppression (OVS) in ¹H-MRSI of the human brain. To-date ECLIPSE has been performed with an unshielded-Z2 coil that led to significant B0-eddy currents, necessitating the use of pre/post gradient pulses^2,3 for MEGA-edited MRSI acquisitions.

Goal(s): An inner volume selection (IVS) based ECLIPSE approach would provide B₁ and T₁ independent lipid suppression, however editing efficiency is compromised due to B0-eddy currents.

Approach: A home-built 64-channel gradient controller⁴ was extended with B0-compensation capabilities to drive a Z0-shim coil.

Results: B₀-eddy currents were attenuated by 200-fold following B0-compensation, and allows robust MEGA-edited ECLIPSE-IVS based MRSI acquisitions.

Impact: Constructing a shielded-Z2 coil for ECLIPSE is complex compared to an unshielded coil. A simpler approach is to construct an unshielded-Z2 with B0 compensation and gradient pre-emphasis, provided that eddy currents induced by switching an unshielded-Z2 gradient can be well-characterized.

Keywords: Magnets (B0), Magnets (B0)

Motivation: Experimental characterization and understanding of the power deposition in the He bath of an ultra-high field magnet is important for magnet safety.

Goal(s): We report a quasi-analytical model for the Z gradient axis power deposition and compare it with measurements. 

Approach: The model was based on three cylinders consisting of the He vessel, the cryoshield and the outer vacuum chamber. Coupled partial differential equations involving both electromagnetism and elasticity were derived and solved. 

Results: Our model predicts accurately the positions of the cryogenic peaks but underestimates by about a factor of 2-3 the largest ones.  

Impact: When the symmetry and underlying assumptions are met, the model allows determining the location of the resonance peaks and can alert the user of potential dangerous frequency zones where caution during first experiments should be exercised.

Keywords: High-Field MRI, System Imperfections: Measurement & Correction, dielectric pads

Motivation: At ultra-high fields, B₁⁺ inhomogeneities induce unwanted signal loss or hyper-intensity. Current shimming approaches lack an easy-to-apply solution like dielectric pads, that would also allow subject-specific tailoring.

Goal(s): To evaluate a novel remote shimming device based on actively-coupled dielectric pads.

Approach: A switchboard was constructed using MR-compatible PIN-diodes. Previously proposed dielectric pocket manual coupling configuration was compared to the equivalent remotely enabled connection, in a 3x3 dielectric pad array.

Results: Remote shimming device achieved up to 8% modulation across the ROI, compared to X% in the equivalent manual coupling. Shimming location was movable with the choice of coupled dielectric pockets within the array.

Impact: At ultra-high field imaging, B₁⁺ inhomogeneities become increasingly prominent. Current shimming approaches lack wide-availability with an option for subject-specific tailoring. In this work, we evaluate a novel remote shimming device based on a dielectric pad array with PIN-diode switchboards.

Keywords: Shims, Shims

Motivation: Multicoil B₀ shimming improves signal quality in many MRI applications but either requires pre-calibrated ridged shim coils or lengthy coil re-calibration scans.
This has precluded the use of flexible arrays and limited body applications.

Goal(s): Accelerate B₀ shim coil re-calibration to enable body-conforming flexible shim coil arrays.

Approach: Field calibration is accelerated using physics-based reconstruction, interleaving few measured slices, and de-tangling contributions of simultaneously active shim coils.

Results: Measured data is used to demonstrate the effectiveness of shim field recovery from few-slice measurements, and simulation is used to demonstrate simultaneous multi shim coil calibration, resulting in a 6.7x total acceleration.

Impact: Physics-based reconstruction of coil fields from few interleaved slices, and detangling fields from simultaneous B0 shim coil activation, achieves rapid calibration. Fast subject-specific calibration will enable the use of body-conforming, flexible B₀ shim arrays.

Keywords: Data Acquisition, Data Acquisition

Motivation: Nonlinear magnetic fields have been used as an effective alternative to linear counterparts to offer improved spatial encoding efficiency.

Goal(s): Nonlinear encoding methods often encounter center imaging blurring and require additional information for non-bijective encoding fields to differentiate mis-localized intensities.

Approach: We propose to use pseudo-linear spatial encoding magnetic fields to address these challenges and enhance encoding efficiency. Based on that, two encoding strategies were proposed.

Results: Center resolution discrepancy could be mitigated according to numerical simulations and preliminary experiments.

Impact: The mitigation of the center blurring presents a reference for future research employing nonlinear gradient encoding.

Keywords: System Imperfections, System Imperfections: Measurement & Correction, Data Acquisition, Data Processing, Gradients, Image Reconstruction

Motivation: For EPI, beat phenomena can occur and result in a slowly varying, erroneous phase component caused by mechanical resonances of the gradient system. Current phase correction approaches do not account for this error.

Goal(s): Improving EPI correction by characterization of gradient-induced, time-varying errors of zeroth and first spatial order.

Approach: EPI image projections of a spherical phantom were acquired by omitting the phase encoding blips. The resulting  x-ky-data enable a characterization of zeroth- and first-order effects separately.

Results: The mechanical properties of the gradient system result in a transient state during which an erroneous, temporally and spatially modulated magnetic field gradient occurs.

Impact: EPI is a widely applied fast imaging technique but sensitive to gradient-induced magnetic field deviations. Therefore, phase correction is of utmost importance and may be improved by the proposed characterization of slowly varying, erroneous phases.

Keywords: High-Field MRI, Shims

Motivation: Adequate B0 shimming of deep brain regions at 7T using 3^rd order shims.

Goal(s): Reproduce B0 shim matrix calibration after hardware replacement and estimate possible compensation of residual fields in vivo.

Approach: Calibration matrix of the 3^rd order B0 shimming system was measured after replacement of the gradient coil. Results were compared with previous calibration. Whole-brain and ROI-specific spherical harmonic decompositions of B0 maps were performed after optimized whole-brain shimming.

Results: Shimming calibration matrices could be reproduced with high accuracy on the new hardware. Residual B0 inhomogeneity could potentially be improved with existing shimming hardware.

Impact: B0 shimmming calibration matrix  is well reproducible  on a 7T system equipped with 3^rd order shim coils using single and multi-channel receiver coil configurations. Residual B0 inhomogeneity after optimized whole-brain shimming could be potentially improved using available shimming hardware.

Keywords: Hybrid & Novel Systems Technology, New Devices, Safety, Prevention, Safety, Prevention , Audio Communication, Patient Comfort, Acoustic Noise Reduction, Hearing Protection

Motivation: : Improve audio communication, patient comfort and safety during MRI exams.

Goal(s): Enable verbal communication without interrupting MRI acquisition and real-time monitoring of acoustic noise exposure and protection.

Approach: Use microphones inside and outside of acoustic noise insulated earpiece and advanced signal processing algorithms to remove MR gradient noise while preserving audio signal and to measure the sound pressure level (SPL) in the ear canal.

Results: Good intelligibility in two-way audio communication is demonstrated during MR acquisitions in a trial of healthy volunteers. No significant artifact from earpieces in clinical brain imaging.

Impact: A novel MRI audio system can provide clear and seamless two-way communication, real-time monitoring of acoustic noise exposure and protection, high-quality audio for patient comfort during the entire MRI exam.