Field Imperfections Research Articles

Phase contrast MRI with minimized background phase errors.

Phase contrast MRI (PC-MRI) is used clinically to measure velocities in the body, but systematic background phase errors caused by magnetic field imperfections corrupt the velocity measurements with offsets that limit clinical utility. This work aims to minimize systematic background phase errors in PC-MRI, thereby maximizing the accuracy of velocity measurements. The MRI scanner's background phase errors from eddy currents and mechanical oscillations were modeled using the gradient impulse response function (GIRF). Gradient waveforms were then numerically optimized using the GIRF to create pulse sequences that minimize the background phase errors. The pulse sequences were tested in a static phantom where the predicted response could be compared directly to the measured background velocity. The optimized acquisitions were then tested in human subjects, where flow rates and background errors were compared to conventional PC-MRI. When using the GIRF-optimized gradient waveforms, the predicted background phase was within 0.6 [95% CI = -3.4, 5.4] mm/s of the measured background phase in a static phantom. Excellent agreement was seen for in vivo blood flow values (flow rate agreement = 0.96), and the background phase was reduced by 78.8 18.7%. This work shows that using a GIRF to model the effects of magnetic field imperfections combined with numerically optimized gradient waveforms enables PC-MRI waveforms to be designed to produce a minimal background phase in the most time-efficient manner. The methodology could be adapted for other MRI sequences where similar magnetic field errors affect measurements.

Spherical echo-planar time-resolved imaging (sEPTI) for rapid 3D quantitative and susceptibility imaging.

To develop a 3D spherical EPTI (sEPTI) acquisition and a comprehensive reconstruction pipeline for rapid high-quality whole-brain submillimeter and QSM quantification. For the sEPTI acquisition, spherical k-space coverage is utilized with variable echo-spacing and maximum kx ramp-sampling to improve efficiency and signal incoherency compared to existing EPTI approaches. For reconstruction, an iterative rank-shrinking B0 estimation and odd-even high-order phase correction algorithms were incorporated into the reconstruction to better mitigate artifacts from field imperfections. A physics-informed unrolled network was utilized to boost the SNR, where 1-mm and 0.75-mm isotropic whole-brain imaging were performed in 45 and 90 s at 3 T, respectively. These protocols were validated through simulations, phantom, and in vivo experiments. Ten healthy subjects were recruited to provide sufficient data for the unrolled network. The entire pipeline was validated on additional five healthy subjects where different EPTI sampling approaches were compared. Two additional pediatric patients with epilepsy were recruited to demonstrate the generalizability of the unrolled reconstruction. sEPTI achieved 1.4 faster imaging with improved image quality and quantitative map precision compared to existing EPTI approaches. The B0 update and the phase correction provide improved reconstruction performance with lower artifacts. The unrolled network boosted the SNR, achieving high-quality and QSM quantification with single average data. High-quality reconstruction was also obtained in the pediatric patients using this network. sEPTI achieved whole-brain distortion-free multi-echo imaging and and QSM quantification at 0.75 mm in 90 s which has the potential to be useful for wide clinical applications.

Impact of field imperfections correction on BOLD sensitivity in 3D-SPARKLING fMRI data.

Static and dynamic field imperfections are detrimental to functional MRI (fMRI) applications, especially at ultra-high magnetic fields (UHF). In this work, a field camera is used to assess the benefits of retrospectively correcting field perturbations on Blood Oxygen Level Dependent (BOLD) sensitivity in non-Cartesian three-dimensional (3D)-SPARKLING fMRI acquisitions. fMRI data were acquired at 1 mm and for a 2.4s-TR while concurrently monitoring in real-time field perturbations using a Skope Clip-on field camera in a novel experimental setting involving a shorter TR than the required minimal TR of the field probes. Measurements of the dynamic field deviations were used along with a static map to retrospectively correct static and dynamic field imperfections, respectively. In order to evaluate the impact of such a correction on fMRI volumes, a comparative study was conducted on healthy volunteers. Correction of deviations improved image quality and yielded between 20% and 30% increase in median temporal signal-to-noise ratio (tSNR).Using fMRI data collected during a retinotopic mapping experiment, we demonstrated a significant increase in sensitivity to the BOLD contrast and improved accuracy of the BOLD phase maps: 44% (resp., 159%) more activated voxels were retrieved when using a significance control level based on a p-value of 0.001 without correcting for multiple comparisons (resp., 0.05 with a false discovery rate correction). 3D-SPARKLING fMRI hugely benefits from static and dynamic imperfections correction. However, the proposed experimental protocol is flexible enough to be deployed on a large spectrum of encoding schemes, including arbitrary non-Cartesian readouts.

Resonance Curves and Jump Frequencies in a Dual-Frequency Paul Trap on Account of Octopole Field Imperfection

Resonance Curves and Jump Frequencies in a Dual-Frequency Paul Trap on Account of Octopole Field Imperfection

Probing the percolation in the quantum anomalous Hall insulator

The percolation plays an essential role in the physics of plateau transition, localization, and breakdown in quantum Hall (QH) systems. In practice, it always exists probably due to sample imperfections and has to be addressed before realizing the full potentials of topological electronics and qubits. Here, we investigate the cause, distribution, and number of the percolation in a quantum anomalous Hall (QAH) insulator of an anti-Hall bar geometry with two perimeters, which allows for probing both the inter- and intra-perimeter percolations by injecting currents into either or both perimeters. We discover the dual-QAH effect with opposite chiralities from these two perimeters, which exhibits linear modulations by the currents applied to both perimeters. By solving the formulation of such modulations with the Landauer–Büttiker formalism, the distribution and number of the inter-perimeter percolative channels could be identified. Strikingly, a dissipative constituent is detected in the transport of the QAH state, as revealed by the linear scalings in longitudinal conductivities versus the sum of currents injected to both perimeters, similar to that in the trivial-insulating state. Such a behavior unveils the quasi-2D nature of the intra-perimeter percolation, which superimposes onto and perturbs the dissipationless chiral edge transport. The formation of percolations is ascribed to the joint effect of the electric field, finite conductivity, and sample imperfections.

Adiabatically prepared spin-lock could reduce the R1ρ dispersion.

R1ρ (or spin-lock) imaging is prone to artifacts arising from field inhomogeneities that may impact the R1ρ quantification. Previous research has proposed two types of method to manage the artifacts in continuous-wave constant amplitude spin-lock, one is based on the composite block pulses to compensate for the field imperfections, another category uses adiabatic pulses in the R1ρ pre-pulse to excite and reverse the magnetization (named adiabatic prepared approach). Although both methods have proved their efficiency in alleviating artifacts, we observed that the adiabatic pulse approach could produce much lower R1ρ dispersion in human knee cartilage than the block pulse method (characterized by the R1ρ difference ∆R1ρ =11.4 Hz (from spin-lock field 50 to 500 Hz) for the block pulse method vs. ∆R1ρ =4.5 Hz for the adiabatic pulse approach). Prompted by this observation, the purpose of this study was to investigate the underlying factors that may affect the R1ρ dispersion through numerical simulations based on the two-pool exchanging Bloch-McConnell equations. The effects of free water pool size Pa (from 0.80 to 0.95), chemical exchange rate kb (from the bound to free water pool, ranged from 500 to 3,000 Hz), adiabatic pulse duration Tp (from 5.0 to 25 ms), and the chemical shift of the bound pool ppmb (from 1.0 to 5.0 ppm) were examined on the degree of the R1ρ dispersion for the two R1ρ imaging methods. In general, the greater the ppmb, kb, Tp, and the smaller Pa, the more significant difference in R1ρ dispersion between the block and adiabatic approaches, with the dispersion curve of the adiabatic method becoming flatter. The adiabatic prepared approach may compromise the R1ρ dispersion, the effect is determined by the combination of the tissue and radiofrequency (RF) pulse properties. It is suggested that care should be taken when using the adiabatically prepared approach to study R1ρ dispersion.

A fast MR fingerprinting simulator for direct error estimation and sequence optimization

Magnetic resonance fingerprinting (MRF) is a novel quantitative MR technique that simultaneously provides multiple tissue property maps. When optimizing MRF scans, modeling undersampling errors and field imperfections in cost functions for direct measurement of quantitative errors will make the optimization results more practical and robust. However, optimizing such cost function is computationally expensive and impractical for MRF optimization with tens of thousands of iterations. Here, we introduce a fast MRF simulator to simulate aliased images from actual scan scenarios including undersampling and system imperfections, which substantially reduces computational time and allows for direct error estimation of the quantitative maps and efficient sequence optimization. We evaluate the performance and computational speed of the proposed approach by simulations and in vivo experiments. The simulations from the proposed method closely approximate the signals and MRF maps from in vivo scans, with 158 times shorter processing time than the conventional simulation method using Non-uniform Fourier transform. We also demonstrate the power of applying the fast MRF simulator in MRF sequence optimization. The optimized sequences are validated with in vivo scans to assess the image quality and accuracy. The optimized sequences produce artifact-free T1 and T2 maps in 2D and 3D scans with equivalent mapping accuracy as the human-designed sequence but at shorter scan times. Incorporating the proposed simulator in the MRF optimization framework makes direct estimation of undersampling errors during the optimization process feasible, and provide optimized MRF sequences that are robust against undersampling artifacts and field inhomogeneity.

High-fidelity control of spin ensemble dynamics via artificial intelligence: from quantum computing to NMR spectroscopy and imaging.

High-fidelity control of spin ensemble dynamics is essential for many research areas, spanning from quantum computing and radio-frequency (RF) engineering to NMR spectroscopy and imaging. However, attaining robust and high-fidelity spin operations remains an unmet challenge. Using an evolutionary algorithm and artificial intelligence (AI), we designed new RF pulses with customizable spatial or temporal field inhomogeneity compensation. Compared with the standard RF shapes, the new AI-generated pulses show superior performance for bandwidth, robustness, and tolerance to field imperfections. As a benchmark, we constructed a spin entanglement operator for the weakly coupled two-spin-1/2 system of 13CHCl3, achieving high-fidelity transformations under multiple inhomogeneity sources. We then generated band-selective and ultra-broadband RF pulses typical of biomolecular NMR spectroscopy. When implemented in multipulse NMR experiments, the AI-generated pulses significantly increased the sensitivity of medium-size and large protein spectra relative to standard pulse sequences. Finally, we applied the new pulses to typical imaging experiments, showing a remarkable tolerance to changes in the RF field. These AI-generated RF pulses can be directly implemented in quantum information, NMR spectroscopy of biomolecules, magnetic resonance imaging techniques for in vivo and materials sciences.

Field Imperfection Induced Effects in a Planar Dual Frequency Paul Trap

The article investigates the influence of electric field imperfections that manifest themselves as hexapole and octopole aberrations in a dual frequency planar Paul trap. The characteristics of the charged particle motion are analyzed by formulating equation of motion in a pseudo-potential well wherein the hexapole and octopole field effects get superimposed to yield a Duffing type of an equation. Perturbation and harmonic balance methods are applied to estimate the axial frequency shifts and the displacement of the charged particle from the axis of the trap. The axial frequency is dependent upon whereas the displacement of the charged particle from the center of the axis is independent of the variation in voltage and frequency ratio applied to the Paul trap.

Advances in spiral fMRI: A high-resolution study with single-shot acquisition

Spiral fMRI has been put forward as a viable alternative to rectilinear echo-planar imaging, in particular due to its enhanced average k-space speed and thus high acquisition efficiency. This renders spirals attractive for contemporary fMRI applications that require high spatiotemporal resolution, such as laminar or columnar fMRI. However, in practice, spiral fMRI is typically hampered by its reduced robustness and ensuing blurring artifacts, which arise from imperfections in both static and dynamic magnetic fields.Recently, these limitations have been overcome by the concerted application of an expanded signal model that accounts for such field imperfections, and its inversion by iterative image reconstruction. In the challenging ultra-high field environment of 7 Tesla, where field inhomogeneity effects are aggravated, both multi-shot and single-shot 2D spiral imaging at sub-millimeter resolution was demonstrated with high depiction quality and anatomical congruency.In this work, we further these advances towards a time series application of spiral readouts, namely, single-shot spiral BOLD fMRI at 0.8 mm in-plane resolution. We demonstrate that high-resolution spiral fMRI at 7 T is not only feasible, but delivers both excellent image quality, BOLD sensitivity, and spatial specificity of the activation maps, with little artifactual blurring. Furthermore, we show the versatility of the approach with a combined in/out spiral readout at a more typical resolution (1.5 mm), where the high acquisition efficiency allows to acquire two images per shot for improved sensitivity by echo combination.

High-temperature superconducting screens for magnetic field-error cancellation in accelerator magnets

Accelerators magnets must have minimal magnetic field imperfections to reduce particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the magnetization due to persistent currents adds an undesired field contribution, potentially degrading the magnetic field quality. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field component, such that only the undesired field components are compensated. The screens are self-regulating, and do not require any externally applied source of energy. Measurements in liquid nitrogen at show for dipole-field configurations a significant reduction of the magnetic field error up to a factor of four. The residual error is explained via numerical simulations accounting for the geometric defects in the HTS screens, achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at , field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets.

Alpha-buckets in High Energy Electron Storage Rings (Review of Existing Experiments and Feasibility Studies for Future Developments)

Electron storage rings operating at high energies have proven to be invaluable source of synchrotron radiation. Two and even three simultaneous beams of particles have been observed at different light source facilities worldwide. So called alpha-buckets were studied at Metrological Light Source (MLS) in Berlin (Germany), SOLEIL facility in France, DIAMOND light source in UK, NSLS ring in Brookhaven (USA). It is widely recognized that alpha buckets are general phenomena which is not restricted to a certain storage ring. However, earlier measurements showed essential limitations on parameters of alpha-buckets, strong dependence on high order (sextupole and octupoles) magnetic field imperfections and associated fast decay of electron current as well as reduced life time. Also a rising relevance of high order non-linear longitudinal beam dynamics is associated with new generation of diffraction limited light sources (DLSR) approaches, which all suffer nonlinear momentum compaction factor. A large variety of future generation electron synchrotrons require a comprehensive investigation of the physical processes involved into the operation of such rings. In this paper, we present review of high order non-linear longitudinal beam dynamics based on the longitudinal equations of motion and Hamiltonian expanded to a high order of the momentum compaction factor. Roots of the third order equation with a free term are derived in a form suitable for analytical estimations. The momentum independent term of orbit lengthening due to particle transverse excursions is estimated and taken into account. The results from simulations were benchmarked against existing experiments at the Karlsruhe Research Accelerator (KARA), SOLEIL and MLS rings. Parameters of three simultaneous beams and alpha buckets at MLS and SOLEIL have been reproduced with high accuracy. General conditions for stable operation of alpha buckets are presented. Based on analytical formulas and computer simulations, studies of longitudinal motion at KARA have been performed with an objective to estimate feasibility of filling and storing of beam in α-buckets. A Computer model was used to describe the behavior and dynamics of simultaneous beams in the KARA storage ring

Imaging simulation of a dark field imperfection evaluation arrangement for spherical optical surface based on reverse ray tracing

Imaging simulation of a dark field imperfection evaluation arrangement for spherical optical surface is proposed on the basis of reverse ray tracing. A typical evaluation arrangement for optical element surface imperfections seen in reflection under dark field condition is introduced. As for thin lens, it is a common case that light spots naturally emerge in the field of view (FOV) and dark field condition is no longer satisfied because multiple optical surfaces reflect rays into the camera lenses. A virtual arrangement model composed of a camera model, sample and light geometries, optical properties is established to further discuss the imaging of various lenses and imperfection geometries. The finite aperture camera model is adopted to generate abundant reverse rays for dark field scattering imaging of imperfections such as scratches and digs. But only a small portion of emitted rays whose paths can be traced to lights during propagation will contribute to the final image. Image simulation is performed by solving illuminance integral equation with Monte Carlo techniques. Simulated images of various lens geometries show the bright spots with respect to radius of curvature and centre thickness, which are numerically evaluated by our own defined spot coefficient. To eliminate the blind spot area caused by multiple reflections, lights are switched in turn and an image sequence is captured. A result image with reduced blind area is obtained by image fusion based on the accurate prediction of spot size and location for each frame in sequence. Imaging experiments of 4 thin lenses show that the distributions of spots correspond well with our simulation and proposed fusion method can reduce the blind area for spherical surface inspection while remains high sensitive for imperfections.

Median plane and beam diagnosis in the central region for compact superconducting cyclotron

The magnetic median plane in the cyclotron does not generally coincide with the geometrical mid-plane of magnetic system. This results in radial field imperfection in the acceleration region around the mid-plane causing severe beam loss as observed in the central region of compact K500 superconducting cyclotron at VECC, Kolkata. This is primarily due to small focusing force (axial) and vertical acceptance in the central region of the cyclotron. The radial field being difficult to measure under the backdrop of large main (axial) field, a search coil based system is developed to locate the median plane. Measurement is carried out by introducing median plane perturbations in the central region by sending varying currents in the upper and lower part of the circular trim coil in an effort to diagnose the error. This causes change in median plane position which allows to find the vertical mis-alignment of iron-parts and current element in the central region. The radial field distribution is calculated analytically and then the mis-alignments of magnetic and current elements thus obtained. The present paper describes the methodology used to estimate radial field and its application for different beam analysis in the central region. The results are compared with different beam diagnostic measurements to identify the causes of beam loss. The scheme is used successfully for median plane measurement and correction in the central region of the cyclotron.

Balanced spin-lock preparation for B1 -insensitive and B0 -insensitive quantification of the rotating frame relaxation time T1ρ.

Accurate and artifact-free T1ρ quantification is still a major challenge due to a susceptibility of the spin-locking module to B0 and/or B1 field inhomogeneities. In this study, we present a novel spin-lock preparation module (B-SL) that enables an almost full compensation of both types of inhomogeneities. The new B-SL module contains a second 180° refocusing pulse to compensate each pulse in the preparation block by a corresponding pulse with opposite phase. For evaluation and validation of B-SL, extensive simulations as well as phantom measurements were performed. Furthermore, the new module was compared to three common established compensation methods. Both simulations and measurements demonstrate a much lower susceptibility to artifacts for the B-SL module, therefore providing an improved accuracy in T1ρ quantification. In the presence of field inhomogeneities, measurements revealed an increased banding compensation by 79% compared with the frequently used composite module. The goodness of the mono-exponential T1ρ fitting procedure was improved by 58%. The B-SL preparation enables the generation of accurate relaxation maps with significantly reduced artifacts, even in the case of large field imperfections. Therefore, the B-SL module is suggested to be highly beneficial for in vivo T1ρ quantification.

Comparison of velocity-selective arterial spin labeling schemes.

In velocity-selective (VS) arterial spin labeling, strategies using multiple saturation modules or using VS inversion (VSI) pulse can provide improved SNR efficiency compared to the original labeling scheme using one VS saturation (VSS) module. Their performance improvement, however, has not been directly compared. Different VS labeling schemes were evaluated by Bloch simulation for their SNR efficiency, eddy current sensitivity, and robustness against B1 and B0 variation. These schemes included dual-module double-refocused hyperbolic secant and symmetric 8-segment B1 -insensitive rotation (sBIR8-) VSS pulses, the original and modified Fourier transform-based VSI pulses. A subset of the labeling schemes was examined further in phantom and in vivo experiments for their eddy current sensitivity and SNR performance. An additional sBIR8-VSS with a built-in inversion (sBIR8-VSS-inversion) was evaluated for the effects of partial background suppression to allow a fairer comparison to VSI. According to the simulations, the sBIR8-VSS was the most robust against field imperfections and had similarly high SNR efficiency (dual-module, dual-sBIR8-VSS) compared with the best VSI pulse (sinc-modulated, sinc-VSI). These were confirmed by the phantom and in vivo data. Without additional background suppression, the sinc-VSI pulses had the highest temporal SNR, closely followed by the sBIR8-VSS-inversion pulse, both benefited from partial background suppression effects. Dual-sBIR8-VSS and sinc-VSI measured the highest SNR efficiency among the VS labeling schemes. Dual-sBIR8-VSS was the most robust against field imperfections, whereas sinc-VSI may provide a higher SNR efficiency if its immunity to field imperfections can be improved.

Optimization of compensation of magnetic field imperfection in K500 superconducting cyclotron

Large first Fourier harmonic of the main magnetic field near the injection and extraction regions of the K500 superconducting cyclotron at VECC, Kolkata adversely affects the beam, guiding it off-center and inducing large amplitude coherent oscillations, thus, making efficient extraction very difficult. Copper-liners over the iron-poles make the poles inaccessible for local iron-shimming. In this paper we discuss an optimal correction of the field imperfection. We consider the magnetic field measurements data, analyze the contributions from different sources of error harmonics and find that it is mostly due to the misalignment of major components of the machine. An algorithm to provide effective correction of the first harmonic of the main magnetic field is proposed and successfully adopted in the K500 beam operation.

Real-time sextupole tuning for a long in-plane polarization at storage rings

A long in-plane beam polarization can be a desired feature for spin measurement experiments in storage rings. The spin precession of the particles within a beam can be controlled by means of the frozen spin method and beam bunching via RF cavities, eventually yielding a polarization lifetime of 10–100 s. Previous studies have shown that it can be further improved by sextupoles, which correct the second order effects related to the chromaticity of the beam. However, sextupoles can require readjustment after slight changes in ring parameters. This work presents a real-time sextupole tuning method that relies on a feedback algorithm. It adjusts the sextupole strength during storage, targeting a zero average radial spin component. Satisfying this condition results in a longer polarization lifetime. Simulation studies show that roughly determined feedback coefficients in this method work effectively for a wide range of ring parameters, with practical field imperfections and measurement errors taken into account. Alternatively, this technique can be used to optimize sextupole strengths in a test run without intervening the measurement.

Correctors’ Magnets for the NICA Booster and Collider

The NICA accelerator complex will consist of two injector chains, a new 600 MeV/u superconducting booster synchrotron, the existing superconducting synchrotron—Nuclotron, and the new superconducting collider that has two storage rings each of about 503 m in circumference. The influence of the fringe fields and misalignments of the structure magnets, the field imperfections and natural chromaticity should be corrected by the correction system. The main parameters, arrangements and the field calculations of the corrector magnets are presented.

Dynamic multicoil technique (DYNAMITE) MRI on human brain.

Spatial encoding for MRI is generally based on linear x, y, and z magnetic field gradients generated by a set of dedicated gradient coils. We recently introduced the dynamic multicoil technique (DYNAMITE) for B0 field control and demonstrated DYNAMITE MRI in a preclinical MR environment. In this study, we report the first realization of DYNAMITE MRI of the in vivo human head. Gradient fields for DYNAMITE MRI were generated with a 28-channel multicoil hardware arranged in 4 rows of 7 coils on a cylindrical surface (length 359mm, diameter 344 mm, maximum 5A per coil). DYNAMITE MRIs of a resolution phantom and in vivo human heads were acquired with multislice gradient-echo, multislice spin-echo, and 3D gradient-echo sequences. The resultant image fidelity was compared to that obtained with conventional gradient coil technology. DYNAMITE field control enabled the realization of all imaging sequences with average gradient errors ≤ 1%. DYNAMITE MRI provided image quality and sensitivity comparable to conventional gradient technology without any obvious artifacts. Some minor geometric deformations were noticed primarily in the image periphery as the result of regional field imperfections. The imperfections can be readily approximated theoretically through numerical integration of the Biot-Savart law and removed through image distortion correction. The first realization of DYNAMITE MRI of the in vivo human head has been presented. The obtained image fidelity is comparable to MRI with conventional gradient coils, paving the way for full-fledged DYNAMITE MRI and B0 shim systems for human applications.