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There has been a long-standing demand for noninvasive neuroimaging methods that can detect neuronal activity at both high temporal and high spatial resolution. We present a two-dimensional fast line-scan approach that enables direct imaging of neuronal activity with millisecond precision while retaining the high spatial resolution of magnetic resonance imaging (MRI). This approach was demonstrated through in vivo mouse brain imaging at 9.4 tesla during electrical whisker-pad stimulation. In vivo spike recording and optogenetics confirmed the high correlation of the observed MRI signal with neural activity. It also captured the sequential and laminar-specific propagation of neuronal activity along the thalamocortical pathway. This high-resolution, direct imaging of neuronal activity will open up new avenues in brain science by providing a deeper understanding of the brain’s functional organization, including the temporospatial dynamics of neural networks.

Asymmetric gradient coils introduce zeroth- and first-order concomitant field terms, in addition to higher-order terms common to both asymmetric and symmetric gradients. Salient to compensation strategies is the accurate calibration of the concomitant field spatial offset parameters for asymmetric coils. A method that allows for one-time calibration of the offset parameters is described.A modified phase contrast pulse sequence with single-sided bipolar flow encoding is proposed to calibrate the offsets for asymmetric, transverse gradient coils. By fitting the measured phase offsets to different gradient amplitudes, the spatial offsets were calculated by fitting the phase variation. This was used for calibrating real-time pre-emphasis compensation of the zeroth- and first-order concomitant fields.Image quality improvement with the proposed corrections was demonstrated in phantom and healthy volunteers with non-Cartesian and Cartesian trajectory acquisitions. Concomitant field compensation using the calibrated offsets resulted in a residual phase errorlt;3% at the highest gradient amplitude and demonstrated substantial reduction of image blur and slice position/selection artifacts.The proposed implementation provides an accurate method for calibrating spatial offsets that can be used for real-time concomitant field compensation of zeroth and first-order terms, substantially reducing artifacts without retrospective correction or sequence specific waveform modifications.

A theoretical method is described to analytically calculate a pair of surface current densities, which produce a desired static magnetic field in one region of the space and zero magnetic field in another. The analysis is based on the known relationship between a surface current density and a stream function, the equivalence of stream functions and surface magnetic dipole density, and the scalar potential representation of the associated magnetic field in free space. From these relations, we formulate the magnetostatic problem, which is often treated as a vector field problem, as a scalar field problem in which a two-dimensional scalar field (stream function) is related to a three-dimensional one (magnetic scalar potential) via the differentiation of the electrostatic Green's function 1/|r–rs|. It is shown that, in a coordinate system in which a separated form of the Green's function exists (separable coordinate system), there exists a simple relationship between a harmonic component of a stream function and a harmonic component of the magnetic scalar potential. The method is applied to calculate idealized surface current patterns for actively shielded, linear gradient field coils in the Cartesian, cylindrical, and spherical coordinates.

Purpose Most previous work on the calculation of susceptibility-induced static magnetic field (B0 ) inhomogeneity has considered strictly unidirectional magnetic fields. Here, we present the theory and implementation of a computational method to rapidly calculate static magnetic field vectors produced by an arbitrary distribution of voxelated magnetization vectors. Theory and methods Two existing B0 calculation methods were systematically extended to include arbitrary orientations of the magnetization and the magnetic field; they are (1) Fourier-domain convolution with k-space-discretized (KD) dipolar field, and (2) generalized susceptibility voxel convolution (gSVC). The methods were tested on an analytical ellipsoid model and a tilted human head model, as well as against experimentally measured B0 fields induced by a stainless-steel implant located in an inhomogeneous region of a clinical 3T MRI magnet. Results Both methods were capable of correctly calculating B0 fields inside a magnetized ellipsoid in all tested orientations. The KD method generally required a larger grid and longer computation time to achieve accuracy comparable to gSVC. Measured B0 fields due to the implant showed a good match with the gSVC-calculated fields that accounted for the spatial variation of the applied magnetic field including the radial components. Conclusion Our method can provide a reliable and efficient computational tool to calculate B0 perturbation by magnetized objects under a variety of circumstances, including those with inhomogeneous magnetizing fields, anisotropic susceptibility, and a rotated coordinate system.

Collaborative edge computing (CEC) is a promising technology for supporting latency-sensitive Internet of Things (IoT) applications by distributing computation tasks among edge nodes. However, the edge resources are limited compared to the traditional cloud. Therefore, it is important to decide when and where to execute tasks from IoT applications to meet the latency requirements of the applications. In this study, we propose a task scheduling algorithm in the CEC environment, which considers the priority (i.e., deadline) of tasks and network flows with the aim of reducing the deadline miss ratio (DMR) of IoT applications. The simulation results demonstrate that the proposed algorithm can significantly reduce the DMR of other benchmark methods.

Two-dimensional (2D) line scan-based dynamic magnetic resonance imaging (MRI) is examined as a means to capture the interior of objects under repetitive motion with high spatiotemporal resolutions. The method was demonstrated in a 9.4-T animal MRI scanner where line-by-line segmented k-space acquisition enabled recording movements of an agarose phantom and quail eggs in different conditions—raw and cooked. A custom MR-compatible actuator which utilized the Lorentz force on its wire loops in the scanner’s main magnetic field effectively induced the required periodic movements of the objects inside the magnet. The line-by-line k-space segmentation was achieved by acquiring a single k-space line for every frame in a motion period before acquisition of another line with a different phase-encode gradient in the succeeding motion period. The reconstructed time-course images accurately represented the objects’ displacements with temporal resolutions up to 5.5 ms. The proposed method can drastically increase the temporal resolution of MRI for imaging rapid periodic motion of objects while preserving adequate spatial resolution for internal details when their movements are driven by a reliable motion-inducing mechanism.

A theoretical method is described to analytically calculate a pair of surface current densities which produce a desired static magnetic field in one region of the space, and zero magnetic field in another. The analysis is based on the known relationship between a surface current density and a stream function, the equivalence of stream functions and surface magnetic dipole density, and the scalar potential representation of the associated magnetic field in free space. From these relations, we formulate the magnetostatics problem, which is often treated as a vector field problem, as a scalar field problem in which a two-dimensional scalar field (stream function) is related to a three-dimensional one (magnetic scalar potential) via the differentiation of the electrostatic Green’s function 1/|r-rs|. It is shown that, in a coordinate system in which a separated form of the Green’s function exists (separable coordinate system), there exists a simple relationship between a harmonic component of a stream function and a harmonic component of the magnetic scalar potential. The method is applied to calculate idealized surface current patterns for actively shielded, magnetic resonance imaging (MRI) gradient coils in the Cartesian, cylindrical, and spherical coordinates.

To demonstrate systematic, linear algebra-based, dimensional analysis to derive a scaling relationship among the design parameters of MRI gradient and harmonic shim coils.The dimensions of five physical quantities relevant for gradient coil design (inductance, gradient amplitude, inner diameter [Only when the unit of turns was included did the linear algebra-based analysis uniquely produce the known scaling relationship that gradient inductance is proportional to gradient efficiency squared timesSystematic application of linear algebra-based dimensional analysis can provide new insight in gradient and shim coil design by revealing fundamental scaling relations and helping to guide the design and comparison of coils with different diameters.

The habenula plays an important role in brain reward circuitry and psychiatric conditions. While much work has been done on the function and structure of the habenula in animal models, in vivo imaging studies of the human habenula have been relatively scarce due to its small size, deep brain location, and lack of clear biomarkers for its heterogeneous substructure. In this paper, we report high-resolution (0.5 × 0.5 × 0.8 mm3) MRI of the human habenula with quantitative susceptibility mapping (QSM) at 3 T. By analyzing 48 scan datasets collected from 21 healthy subjects, we found that magnetic susceptibility contrast is highly non-uniform within the habenula and across the subjects. In particular, we observed high prevalence of elevated susceptibility in the posterior subregion of the habenula. Correlation analysis between the susceptibility and the effective transverse relaxation rate (R2*) indicated that localized susceptibility enhancement in the habenula is more associated with increased paramagnetic (such as iron) rather than decreased diamagnetic (such as myelin) sources. Our results suggest that high-resolution QSM could make a potentially useful tool for substructure-resolved in vivo habenula imaging, and provide a groundwork for the future development of magnetic susceptibility as a quantitative biomarker for human habenula studies.

A histology coil is a U-shaped radio-frequency (RF) coil that is designed to provide a high signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI) of microscopically thin samples. In this work, we demonstrate an inductively coupled histology coil that can be easily integrated with a human MRI scanner, providing a simple method to conduct novel, microscopic imaging experiments in a clinical scanner. All experiments were conducted in a 3T whole-body MRI scanner. The coil consisted of a U-shaped copper foil with tuning capacitors shunting the ends of the “U”. This design maximizes the B1-field efficiency and the homogeneity in the coil’s cavity. For inductive coupling, the histology coil was connected to an untuned single-turn pick-up loop via a coaxial cable. A standard 3T receive-only surface coil, connected to the scanner’s standard receive circuitry, was then inductively coupled to the pick-up loop. Separately, the RF pulse transmitted from the MRI body coil inductively drove the histology coil. In gel phantom imaging, we confirmed a higher (threefold) SNR for the histology coil than for the scanner’s standard surface coil, as well as high B1 homogeneity for the histology coil. Gradient echo images of a 40-µm-thick rat brain slice, at an in-plane resolution of 219 × 219 µm2, could be obtained with SNR > 10 (in the cortex) in about 2 hours. Our work demonstrates the feasibility of imaging microscopically thin tissue slices in a clinical MRI scanner using an inductively coupled histology coil. The method can be applied to multi-modality and multi-orientation imaging of ex-vivo and engineered tissue samples.

PURPOSE Many conventional ex vivo magnetic resonance imaging (MRI) setups utilize cylindrical or other nonspherical tissue containers which can cause static field (B0 ) inhomogeneity affecting the accuracy of the measurements in an orientation-dependent manner. In this work we demonstrate an experimental method to obtain MRI of ex vivo tissue samples held in a spherical container in order to minimize bulk susceptibility-induced B0 inhomogeneity in arbitrary orientations. METHODS B0 inhomogeneity caused by tissue-air susceptibility mismatch can be theoretically eliminated if the surface of susceptibility discontinuity is spherical. This situation can be approximated by putting a tissue sample in a spherical shell filled with materials with tissue-like magnetic susceptibility. We achieved this on an intact monkey brain by (a) holding the brain with a three-dimensional (3D)-printed holder with tissue-like (within 0.5 ppm) susceptibility, and (b) enclosing the brain and the holder in an acrylic spherical shell filled with diamagnetic liquid. Furthermore, the sphere and the radio-frequency coil for MRI were mounted on a 3D-printed frame designed to reduce B0 inhomogeneity contributions. The sphere could be rotated freely without disturbing the RF coil to facilitate multi-orientation imaging. We verified our setup by mapping B0 in the monkey brain at 13 different orientations in a human 7T scanner, and measuring orientation-dependent R2∗ relaxation rates in the white and gray matters of the brain. The results were then compared with a setup where the brain was held inside a cylindrical container. RESULTS In all orientations, the B0 standard deviation in the brain in the spherical setup (converted to Larmor frequency offset) was less than about 10 Hz. This corresponds to two-sigma deviation of B0 of

Objective Focused ultrasound has been applied in brain therapeutics. Although focusing ultrasonic beams on multiple arbitrary regions under the guidance of magnetic resonance imaging(MRI) is needed for precise treatments, current therapeutic transducers with large pitch sizes have been optimized to focus on deep brain regions. While annular arrays can adjust the beam foci from cortical to deep regions, their circular shape may generate eddy current-induced magnetic flux during MRI. In this study, a quadrisected annular array is proposed to address these limitations. Methods Conventional and quadrisected annular arrays with three elements were implemented by loading the electrode patterns onto an 850kHz 1-3 composite PZT disc, with a diameter of 31mm, including three rings. MR compatibilities were demonstrated by imaging an MRI phantom with pulse sequences for B0 and B1 mapping and spin-echo imaging. Acoustic beam profiles, with and without a macaque monkey skull, were measured. A quadrisected transducer was also used to open the blood-brain barrier(BBB). Results The flip angle distortion improved by 20% in spin-echo MR imaging. The acoustic beam distortions shifting the focal point from 36 to 41mm and elongating the focal zone from 10 to 15 mm could be recovered to nearly the original values. BBB openings in the hippocampus and basal region were also demonstrated. Conclusion The MR compatibility was improved by the increased resistance of the electrodes in the quadrisected array maintaining dynamic focusing capabilities. Significance The quadrisected annular design can be a fundamental structure for a larger MR-compatible segmented array transducer generating multiple acoustic foci.

There has been a longstanding demand for noninvasive neuroimaging methods capable of detecting neuronal activity at both high temporal and spatial resolution. Here, we propose a novel method that enables Direct Imaging of Neuronal Activity for functional MRI (termed DIANA-fMRI) that can dynamically image spiking activity in milliseconds precision, while retaining the original benefit of high spatial resolution of MRI. DIANA-fMRI was demonstrated through in vivo mice brain imaging at 9.4 T applying electrical whisker-pad stimulation, directly imaging the spiking activity as well as capturing its sequential propagation along the thalamocortical pathway, as further confirmed through in vivo spike recording and optogenetics. DIANA-fMRI will open up new avenues in brain science by providing a deeper understanding of the brain’s functional organization including neural networks.

To develop multifunctional fibers for high-strength strain-hardening cementitious composites (HS-SHCC), a silver-plating technique that incorporated fibers with various pre-treatment methods using dopamine and plasma was adopted for polyethylene (PE) fibers. The effect of this plating technique on the mechanical and electrical properties of HS-SHCC was evaluated. For this, the HS-SHCC mixture with a high compressive strength of 84.1 MPa and tensile strength and strain capacity of 12.7 MPa and 8.27% was first developed. Dopamine-activated, silvered PE fiber was most effective on electroless silver plating and decreased bulk resistance of HS-SHCC by approximately 15%, potentially applicable to pavement deicing, self-sensing, electromagnetic interference shielding, etc. The PE fiber was not directly coated by the silver plating without pre-treatment, and the plasma-treated fiber provided an intermediate silver coating efficiency. The silver nanoparticles deposited on the surface of the PE fibers minimally affected the compressive strength of HS-SHCC in the range of −9% to +3% but deteriorated the tensile performance in general. The mixtures with plasma or hybrid treated silvered PE fibers provided similar tensile strengths of approximately 12 MPa to the control mixture. The silvered PE fiber with dopamine pre-treatment resulted in approximately 12% and 24% lower tensile strength and strain capacity (approximately 11.2 MPa and 6.28%, respectively) compared with the plain PE fiber; however, these tensile properties are still sufficiently outstanding, relative to other HS-SHCC types available.

A method is presented to measure the radio-frequency (RF) vector magnetic field inside an object using magnetic resonance imaging (MRI). Conventional “ $\text{B}_{{1}}$ mapping” in MRI can measure the proton co-rotating component ( $\text{B}_{{1}}^{+}{)}$ of the RF field produced by a transmit coil. Here we show that by repeating $\text{B}_{{1}}^{+}$ mapping on the same object and coil at multiple (8) specific orientations with respect to the main magnet, the magnitudes and relative phases of all (x, y, z) Cartesian components of the RF field can be determined unambiguously. We demonstrate the method on a circularly polarized volume coil and a loop coil tuned at 123.25 MHz in a 3 Tesla MRI scanner, with liquid phantoms. The volume coil measurement showed the axial component of the RF field, which is normally unmeasurable in MRI, away from the center of the coil. The measured RF vector field maps of both coils compared favorably with numerical simulation, with volumetric normalized root-mean-square difference in the range of 7~20%. While the proposed method cannot be applied to human imaging at present, applications to phantoms and small animals could provide a useful experimental tool to validate RF simulation and verify certain assumptions in $\text{B}_{{1}}^{+}$ map-based electrical properties tomography (EPT).

Background Although iron dyshomeostasis is associated with Parkinson’s disease (PD) pathogenesis, the relationship between iron deposition and non-motor involvement in PD is not fully understood. In this study, we investigated basal ganglia and extra-basal ganglia system iron contents and their correlation with non-motor symptoms in drug-naïve, early-stage PD patients. Methods We enrolled 14 drug-naïve, early-stage PD patients and 12 age/sex-matched normal controls. All participants underwent brain magnetic resonance imaging to obtain the effective transverse relaxation rate (R2*) and quantitative susceptibility mapping (QSM). Deep brain structures, including the nucleus accumbens, caudate nucleus, putamen, globus pallidus, thalamus, hippocampus, and amygdala, were delineated using the FSL-FIRST; the substantia nigra, red nucleus, and dentate nucleus were segmented manually. Inter-group differences in R2* and QSM values, as well as their association with clinical parameters of PD, were investigated. Results Substantia nigra and putamen R2* values were significantly higher in PD patients than in normal controls, despite no significant difference in QSM values. Regarding the non-motor symptom scales, PD sleep scale score negatively correlated with R2* values in the red nucleus and right amygdala, Scales for Outcomes in Parkinson’s disease-Autonomic scores were positively correlated with R2* values in the right amygdala and left hippocampus, and cardiovascular sub-score of Non-Motor Symptoms Scale for PD was positively associated with the QSM value in the left hippocampus. Conclusion In this study, iron content in the extra-basal ganglia system was significantly correlated with non-motor symptoms, especially sleep problems and dysautonomia, even in early-stage PD.

Purpose To develop an ultrafast 3D gradient echo-based MRI method with constant TE and high tolerance to B0 inhomogeneity, dubbed ERASE (equal-TE rapid acquisition with sequential excitation), and to introduce its use in BOLD functional MRI (fMRI). Theory and methods Essential features of ERASE, including spin behavior, were characterized, and a comparison study was conducted with conventional EPI. To demonstrate high tolerance to B0 inhomogeneity, in vivo imaging of the mouse brain with a fiber-optic implant was performed at 9.4 T, and human brain imaging (including the orbitofrontal cortex) was performed at 3 T and 7 T. To evaluate the performance of ERASE in BOLD-fMRI, the characteristics of SNR and temporal SNR were analyzed for in vivo rat brains at 9.4 T in comparison with multislice gradient-echo EPI. Percent signal changes and t-scores are also presented. Results For both mouse brain and human brain imaging, ERASE exhibited a high tolerance to magnetic susceptibility artifacts, showing much lower distortion and signal dropout, especially in the regions involving large magnetic susceptibility effects. For BOLD-fMRI, ERASE provided higher temporal SNR and t-scores than EPI, but exhibited similar percent signal changes in in vivo rat brains at 9.4 T. Conclusion When compared with conventional EPI, ERASE is much less sensitive, not only to EPI-related artifacts such as Nyquist ghosting, but also to B0 inhomogeneity including magnetic susceptibility effects. It is promising for use in BOLD-fMRI, providing higher temporal SNR and t-scores with constant TE when compared with EPI, although further optimization is needed for human fMRI.

Purpose To build and evaluate a small-footprint, lightweight, high-performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole-body clinical 3T MRI scanners, while achieving substantial reductions in installation costs. Methods A conduction-cooled magnet was developed that uses less than 12 liters of liquid helium in a gas-charged sealed system, and standard NbTi wire, and weighs approximately 2000 kg. A 42-cm inner-diameter gradient coil with asymmetric transverse axes was developed to provide patient access for head and extremity exams, while minimizing magnet-gradient interactions that adversely affect image quality. The gradient coil was designed to achieve simultaneous operation of 80-mT/m peak gradient amplitude at a slew rate of 700 T/m/s on each gradient axis using readily available 1-MVA gradient drivers. Results In a comparison of anatomical imaging in 16 patients using T2 -weighted 3D fluid-attenuated inversion recovery (FLAIR) between the compact 3T and whole-body 3T, image quality was assessed as equivalent to or better across several metrics. The ability to fully use a high slew rate of 700 T/m/s simultaneously with 80-mT/m maximum gradient amplitude resulted in improvements in image quality across EPI, DWI, and anatomical imaging of the brain. Conclusions The compact 3T MRI system has been in continuous operation at the Mayo Clinic since March 2016. To date, over 200 patient studies have been completed, including 96 comparison studies with a clinical 3T whole-body MRI. The increased gradient performance has reliably resulted in consistently improved image quality.

This research aimed to evaluate the potential therapeutic effects of Rg3 on endometriosis and identify target miRNAs. We designed an in vitro study using human endometrial stromal cells (HESCs) obtained from patients with endometriosis and an in vivo study using mouse models. HESCs were treated with Rg3-enhanced red ginseng extract (Rg3E); real-time PCR and microarray profiling, transfection, and western blot were performed. Mouse endometriosis models were developed and supplemented with Rg3E for 8 weeks. Gross lesion size and fibrotic character were analyzed in the mouse models. RNA levels of Ki-67, col-1, CTGF, fibronectin, TGF-β1, MMP2 and MMP9 significantly decreased in HESCs after Rg3E treatment. Microarray analysis revealed downregulation of miR-27b-3p, which is related to fibrosis modulation. Expression of miR-27b-3p was significantly higher in HESCs from patients with endometriosis than that of controls, and Rg3E treatment significantly decreased its expression; the contraction and migration assay revealed significant reductions in both fibrosis and migration potential in Rg3E-treated HESCs from endometriosis patients. A decrease in size and fibrotic character of endometrial lesions from the Rg3E groups was observed in vivo. In conclusion, Rg3 effectively altered fibrotic properties of HESCs from patients with endometriosis, which is likely associated with miR-27b-3p modulation.

Purpose Imaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time-varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second-order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth-order or B0) and linear (first-order) spatial dependence. Methods This work demonstrates a general solution to eliminate the zeroth-order concomitant field by applying the correct B0 frequency shift in real time to counteract the concomitant fields. Results are demonstrated for phase contrast, spiral, echo-planar imaging (EPI), and fast spin-echo imaging. Results A global phase offset is reduced in the phase-contrast exam, and blurring is virtually eliminated in spiral images. The bulk image shift in the phase-encode direction is compensated for in EPI, whereas signal loss, ghosting, and blurring are corrected in the fast-spin echo images. Conclusion A user-transparent method to compensate the zeroth-order concomitant field term by center frequency shifting is proposed and implemented. This solution allows all the existing pulse sequences—both product and research—to be retained without any modifications. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Fabrication of a spherical multi-compartment MRI phantom is demonstrated that can be used to validate magnetic resonance (MR)-based susceptibility imaging reconstruction. The phantom consists of a 10 cm diameter gelatin sphere that encloses multiple smaller gelatin spheres doped with different concentrations of paramagnetic contrast agents. Compared to previous multi-compartment phantoms with cylindrical geometry, the phantom provides the following benefits: (1) no compartmental barrier materials are used that can introduce signal voids and spurious phase; (2) compartmental geometry is reproducible; (3) spherical susceptibility boundaries possess a ground-truth analytical phase solution for easy experimental validation; (4) spherical geometry of the overall phantom eliminates background phase due to air-phantom boundary in any scan orientation. The susceptibility of individual compartments can be controlled independently by doping. During fabrication, formalin cross-linking and water-proof surface coating effectively blocked water diffusion between the compartments to preserve the phantom's integrity. The spherical shapes were realized by molding the inner gel compartments in acrylic spherical shells, 3 cm in diameter, and constructing the whole phantom inside a larger acrylic shell. From gradient echo images obtained at 3T, we verified that the phantom produced phase images in agreement with the theoretical prediction. Factors that limit the agreement include: air bubbles trapped at the gel interfaces, imperfect magnet shimming, and the susceptibility of external materials such as the phantom support hardware. The phantom images were used to validate publicly available codes for quantitative susceptibility mapping. We believe that the proposed phantom can provide a useful testbed for validation of MR phase imaging and MR-based magnetic susceptibility reconstruction.

Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimer’s Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26-cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.

Purpose To investigate the 1H spin contribution (0.004 parts per million (ppm)) to the water magnetic susceptibility and discuss its implications for high-precision phase mapping and tissue susceptibility measurement. Methods Free induction decay (FID) signals were acquired at 3 Tesla (T) and 9.4T from thin square phantoms at a range of tip angles. The FID frequency shift was examined at a high resolution (

PURPOSE To compare the effectiveness of prospective, retrospective, and combined (prospective + retrospective) EPI distortion correction methods in bilateral breast diffusion-weighted imaging (DWI) scans. METHODS Five healthy female subjects underwent an axial bilateral breast DWI exam with and without prospective B0 inhomogeneity correction using slice-by-slice linear shimming. In each case, an additional b=0 DWI scan was performed with the polarity of the phase-encoding gradient reversed, to generate an estimated B0 map; this map or a separately acquired B0 map was used for retrospective correction, either alone or in combination with the prospective correction. The alignment between an undistorted, anatomical reference scan with similar contrast and the corrected b=0 DWI images with different correction schemes was assessed. RESULTS The average cross-correlation coefficient between the DWI images and the anatomical reference scan was increased from 0.82 to 0.92 over the five volunteers when combined prospective and retrospective distortion correction was applied. Furthermore, such correction substantially reduced patient-to-patient variation of the image alignment and the variability of the average apparent diffusion coefficient in normal glandular tissue. CONCLUSION Combined prospective and retrospective distortion correction can provide an efficient way to reduce susceptibility-induced image distortions and enhance the reliability of breast DWI exams. Magn Reson Med 78:247-253, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Purpose To develop a gradient pre-emphasis scheme that prospectively counteracts the effects of the first-order concomitant fields for any arbitrary gradient waveform played on asymmetric gradient systems, and to demonstrate the effectiveness of this approach using a real-time implementation on a compact gradient system. Methods After reviewing the first-order concomitant fields that are present on asymmetric gradients, we developed a generalized gradient pre-emphasis model assuming arbitrary gradient waveforms to counteract their effects. A numerically straightforward, easily implemented approximate solution to this pre-emphasis problem was derived that was compatible with the current hardware infrastructure of conventional MRI scanners for eddy current compensation. The proposed method was implemented on the gradient driver subsystem, and its real-time use was tested using a series of phantom and in vivo data acquired from two-dimensional Cartesian phase-difference, echo-planar imaging, and spiral acquisitions. Results The phantom and in vivo results demonstrated that unless accounted for, first-order concomitant fields introduce considerable phase estimation error into the measured data and result in images with spatially dependent blurring/distortion. The resulting artifacts were effectively prevented using the proposed gradient pre-emphasis. Conclusion We have developed an efficient and effective gradient pre-emphasis framework to counteract the effects of first-order concomitant fields of asymmetric gradient systems. Magn Reson Med 77:2250–2262, 2017. © 2016 International Society for Magnetic Resonance in Medicine

Purpose To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head-only gradient coil. Materials and Methods Simulation studies were first performed to determine the expected echo spacing and distortion reduction in EPI. A head gradient of 42-cm inner diameter and with asymmetric transverse coils was then installed in a whole-body, conventional 3T magnetic resonance imaging (MRI) system. Human subject imaging was performed on five subjects to determine the effects of EPI on echo spacing and signal dropout at various gradient slew rates. The feasibility of whole-brain imaging at 1.5 mm-isotropic spatial resolution was demonstrated with gradient-echo and spin-echo diffusion-weighted EPI. Results As compared to a whole-body gradient coil, the EPI echo spacing in the head-only gradient coil was reduced by 48%. Simulation and in vivo results, respectively, showed up to 25–26% and 19% improvement in signal dropout. Whole-brain imaging with EPI at 1.5 mm spatial resolution provided good whole-brain coverage, spatial linearity, and low spatial distortion effects. Conclusion Our results of human brain imaging with EPI using the compact head gradient coil at slew rates higher than in conventional whole-body MR systems demonstrate substantially improved image distortion, and point to a potential for benefits to non-EPI pulse sequences. J. Magn. Reson. Imaging 2016;44:653–664.

Purpose: A compact, three-tesla magnetic resonance imaging(MRI) system has been developed. It features a 37 cm patient aperture, allowing the use of commercial receiver coils. Its design allows simultaneously for gradient amplitudes of 85 millitesla per meter (mT/m) sustained and 700 tesla per meter per second (T/m/s) slew rates. The size of the gradient system allows for these simultaneous performance targets to be achieved with little or no peripheral nerve stimulation, but also raises a concern about the geometric distortion as much of the imaging will be done near the system’s maximum 26 cm field-of-view. Additionally, the fast switching capability raises acoustic noise concerns. This work evaluates the system for both the American College of Radiology’s (ACR) MRIimage quality protocol and the Food and Drug Administration’s (FDA) nonsignificant risk (NSR) acoustic noise limits for MR. Passing these two tests is critical for clinical acceptance. Methods: In this work, the gradient system was operated at the maximum amplitude and slew rate of 80 mT/m and 500 T/m/s, respectively. The geometric distortion correction was accomplished by iteratively determining up to the tenth order spherical harmonic coefficients using a fiducial phantom and position-tracking software, with seventh order correction utilized in the ACR test. Acoustic noise was measured with several standard clinical pulse sequences. Results: The system passes all the ACR image quality tests. The acoustic noise as measured when the gradient coil was inserted into a whole-body MRI system conforms to the FDA NSR limits. Conclusions: The compact system simultaneously allows for high gradient amplitude and high slew rate. Geometric distortion concerns have been mitigated by extending the spherical harmonic correction to higher orders. Acoustic noise is within the FDA limits.

Susceptibility-induced static field (B0) inhomogeneity near the nasal cavity degrades high-field MRI image quality. Many studies have addressed this problem by hardware- or sequence-based methods to improve local B0 shimming or minimize the impact of inhomogeneity. Here, we investigate the feasibility of the head-tilted brain scan as an easily accessible way to reduce B0 inhomogeneity and associated gradient echo signal loss in the prefrontal cortex (PFC). We exploit the fact that the region of intense local B0 gradient can be steered away from the PFC by head reorientation with respect to the main magnetic field. We found that the required chin-up head tilting by a substantial angle (> 30°) can be readily achieved for a group of healthy subjects when their back was raised by about 10 cm. Eleven subjects were scanned at 3T, using a standard 20 channel head-neck coil, for whole-head B0 mapping and gradient-echo EPI-based functional MRI (fMRI) performing a reward-punishment task in normal and tilted head orientations. Additionally, multi-echo gradient echo and resting-state fMRI scans were performed on six subjects in both orientations. Head-tilted sessions, which lasted for at least 20 min, were well-tolerated by all subjects and demonstrated a marked reduction of localized signal loss in the gradient echo-based images and EPI images in the PFC compared to normal orientation scans. Imaging in tilted orientation reduced the group-averaged B0 standard deviation and peak B0 gradient in the orbital gyrus beyond what was possible with simulated 3rd order shimming. The behavioral performance in the head-tilted fMRI scans indicated that the subjects were able to perform a cognitive task with little difficulty, and the tilted fMRI scans successfully produced a robust whole-brain functional activation map consistent with the literature. Our study proposes that the back-raised, head-tilted imaging can benefit the shimming of the prefrontal brain regions while being compatible with moderate-length neuroimaging scans on healthy, cooperating subjects.

MR-based electrical properties tomography converts the MRI transmit/receive RF field measurements to tissue electrical property maps through dedicated reconstruction algorithms. Recent reports showed that despite limitations, electrical properties tomography holds promise for generating additional contrast for tumor detection and patient-specific modeling of tissue-RF field interactions. This review summarizes the available tissue electrical property contrasts and compares them with the capabilities of the most commonly used electrical properties tomography reconstruction method. Future directions and prospects of clinical translation are discussed.

PURPOSE To characterize peripheral nerve stimulation (PNS) of an asymmetric head-only gradient coil that is compatible with a commercial high-channel-count receive-only array. METHODS Two prototypes of an asymmetric head-only gradient coil set with a 42-cm inner diameter were constructed for brain imaging at 3T with maximum performance specifications of up to 85 mT/m and 708 T/m/s. Tests were performed in 24 volunteers to measure PNS thresholds with the transverse (x = left-right; y = anterior-posterior [A/P]) gradient coils of both prototypes. Fourteen of these 24 volunteers were also tested for the z-gradient PNS in the second prototype and were scanned with high-slew-rate echo planar imaging (EPI) immediately after the PNS tests. RESULTS For both prototypes, the y-gradient PNS threshold was markedly higher than the x-gradient threshold. The z-gradient threshold was intermediate between those for the x- and y-coils. Of the 24 volunteers, only two experienced y-gradient PNS at 80 mT/m and 500 T/m/s. All volunteers underwent the EPI scan without PNS when the readout direction was set to A/P. CONCLUSION Measured PNS characteristics of asymmetric head-only gradient coil prototypes indicate that such coils, especially in the A/P direction, can be used for fast EPI readout in high-performance neuroimaging scans with substantially reduced PNS concerns compared with conventional whole body gradient coils. Magn Reson Med 76:1939-1950, 2016. © 2015 International Society for Magnetic Resonance in Medicine.