main magnetic field
Recently Published Documents

There is not only space weather; there is also space climate. Space climate includes the ionospheric climate, which is affected by long-term trends in the ionosphere. One of the most important ionospheric parameters is the critical frequency of the ionospheric F2 layer, foF2, which corresponds to the maximum ionospheric electron density, NmF2. Observational data series of foF2 have been collected at some stations for as long as over 60 years and continents are relatively well covered by a network of ionosondes, instruments that measure, among others, foF2. Trends in foF2 are relatively weak. The main global driver of long-term trends in foF2 is the increasing concentration of greenhouse gases, namely CO2, in the atmosphere. The impact of the other important trend driver, the secular change in the Earth’s main magnetic field, is very regional, being positive in some regions, negative in others, and neither in the rest. There are various sources of uncertainty in foF2 trends. One is the inhomogeneity of long foF2 data series. The main driver of year-to-year changes in foF2 is the quasi-eleven-year solar cycle. The removal of its effect is another source of uncertainty. Different methods might provide somewhat different strengths among trends in foF2. All this is briefly reviewed in the paper.

Purpose: Quantitative susceptibility mapping (QSM) is increasingly used for clinical research where oblique image acquisition is commonplace but its effects on QSM accuracy are not well understood. Theory and Methods: The QSM processing pipeline involves defining the unit magnetic dipole kernel, which requires knowledge of the direction of the main magnetic field B0 with respect to the acquired image volume axes. The direction of B0 is dependent upon the axis and angle of rotation in oblique acquisition. Using both a numerical brain phantom and in-vivo acquisitions, we analysed the effects of oblique acquisition on magnetic susceptibility maps. We compared three tilt correction schemes at each step in the QSM pipeline: phase unwrapping, background field removal and susceptibility calculation, using the root-mean-squared error and QSM-tuned structural similarity index (XSIM). Results: Rotation of wrapped phase images gave severe artefacts. Background field removal with projection onto dipole fields gave the most accurate susceptibilities when the field map was first rotated into alignment with B0. LBV and VSHARP background field removal methods gave accurate results without tilt correction. For susceptibility calculation, thresholded k-space division, iterative Tikhonov regularisation and weighted linear total variation regularisation all performed most accurately when local field maps were rotated into alignment with B0 before susceptibility calculation. Conclusion: For accurate QSM, oblique acquisition must be taken into account. Rotation of images into alignment with B0 should be carried out after phase unwrapping and before background field removal. We provide open-source tilt-correction code to incorporate easily into existing pipelines: https://github.com/o-snow/QSM_TiltCorrection.git.

AbstractEach International Geomagnetic Reference Field (IGRF) model released under the auspices of the International Association of Geomagnetism and Aeronomy comprises a secular variation component that describes the evolution of the main magnetic field anticipated for the 5 years to come. Every Gauss coefficient, up to spherical harmonic degree and order 8, is assumed to undergo its own independent linear evolution. With a mathematical model of the core magnetic field and its time rate of change constructed from geomagnetic observations at hand, a standard prediction of the secular variation (SV) consists of taking the time rate of change of each Gauss coefficient at the final time of analysis as the predicted rate of change. The last three generations of the IGRF have additionally witnessed a growing number of candidate SV models relying upon physics-based forecasts. This surge is motivated by satellite data that now span more than two decades and by the concurrent progress in the numerical modelling of Earth’s core dynamics. Satellite data reveal rapid (interannual) geomagnetic features whose imprint can be detrimental to the quality of the IGRF prediction. This calls for forecasting frameworks able to incorporate at least part of the processes responsible for short-term geomagnetic variations. In this letter, we perform a retrospective analysis of the performance of past IGRF SV models and candidates over the past 35 years; we emphasize that over the satellite era, the quality of the 5-year forecasts worsens at times of rapid geomagnetic changes. After the definition of the time scales that are relevant for the IGRF prediction exercise, we cover the strategies followed by past physics-based candidates, which we categorize into a “‘core–surface flow” family and a “dynamo” family, noting that both strategies resort to “input” models of the main field and its secular variation constructed from observations. We next review practical lessons learned from our previous attempts. Finally, we discuss possible improvements on the current state of affairs in two directions: the feasibility of incorporating rapid physical processes into the analysis on the one hand, and the accuracy and quantification of the uncertainty impacting input models on the other hand.

Abstract. Long-lived spin order-based approaches for magnetic resonance rely on the transition between two magnetic environments of different symmetries, one governed by the magnetic field of the spectrometer and the other where this strong magnetic field is inconsequential. Research on the excitation of magnetic-symmetry transitions in nuclear spins is a scientific field that debuted in Southampton in the year 2000. We advanced in this field carrying the baggage of pre-established directions in NMR spectroscopy. We propose to reveal herein the part of discoveries that may have been obscured by our choice to only look at them through the experience of such pre-established directions at the time. The methodological developments that are emphasised herein are the mechanisms of translation between the symmetric and non-symmetric environments with respect to the main magnetic field B0. More specifically, we look again thoroughly at zero-quantum rotations in the starting blocks of long-lived state populations, magnetisation transfers between hyperpolarised heteronuclei, and protons. These pulse sequences seed subsequent magnetic mechanisms that contribute to further applications. For instance, we show how some of the introduced coherence rotations were combined with classical pulse blocks to obtain two-dimensional correlations between protons and heteronuclei. We hope the pulse sequence building blocks discussed herein will open further perspectives for magnetic resonance experiments with long-lived spin order.

The design of gradient coils is sometimes perceived as complex and counterintuitive. However, a current density is connected to a stream function in fact by a simple relation. Here we present an intuitive open source code collection to derive stream functions from current densities on simple surface geometries. Discrete thin wires, oriented orthogonally to the main magnetic field direction are used to describe a surface current density. An inverse problem is solved and stream functions are derived to find coil designs in the current and stream function domains. The flexibility of the design method is demonstrated by deriving gradient coil designs on several different surface topologies. This collection is primarily intended for teaching, as well as for demonstrating all gradient coil design steps with openly available software tools.

Magnetic field stability plays a fundamental role in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) experiments, guaranteeing accuracy and reproducibility of results. While high levels of stabilization can be achieved for standard NMR techniques, this task becomes particularly challenging for Fast Field Cycling (FFC) NMR and MRI, where the main magnetic field is switched to higher or lower levels during the pulse sequence, and field stabilization must be guaranteed within a very short time after switching. Recent works have addressed the problem with rigorous tools from control system theory, proposing a model based approach for the synthesis of magnetic field controllers for FFC-NMR. While an experimental proof of concept has underlined the correctness of the approach for a complete FFC-NMR setup, the application of the novel, model based Field-Frequency Lock (FFL) system to a FFC-MRI scanner requires proper handling of field encoding gradients. Furthermore, the proof of concept work has also stressed how further advances in the hardware and firmware could improve the overall performances of the magnetic field control loop. The main aim of this perspective paper is then discussing the key challenges that arise in the development of the FFL system suitable for a complete MRI scanner, as well as defining possible research directions by means of preliminary, simulated experiments, with the final goal of favoring the development of a novel, model based FFL system for FFC-MRI.

Abstract The topic of this paper are parts of modern MR devices, in which the magnet windings are located. MR scanner magnets are made of four types of electromagnetic windings: 1) The main magnet, made of superconducting material, creates a variable magnetic field; 2) X coil, made of a resistive material, creates a variable magnetic field, horizontally, from left to right, across scanning tube; 3) Y coil creates varaing magnetic field, vertically, from botom to top; 4) Z coil creates varaing magnetic field, longitudinally, from head to toe, within scanning tube.Superconductors, which create the main magnetic field, should be cooled by liquid helium and liquid nitrogen. Main magnets made of superconductors should use cryostat, with cooling vessels with liquid helium and liquid nitrogen, thermal insulation and other protective elements of magnet system. The types of magnets that exist in the basic configurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity create their own, magnetic, fields by passing current through the solenoid, which is held at the temperature of the superconductor. The superconductors used exclusively are: niobium-titanium (NbTi), niobium-tin (Nb3Sn), vanadium-gallium (V3Ga) and magnesium-diboride (MgB2). Only magnesium diboride is a high temperature superconductor, with a critical temperature Tc = 390K. The three remaining superconductors are low temperature. New high-temperature superconductors have been discovered, as well as room-temperature superconductors. Newly discovered superconducting materials are not used in MR scanners. The magnet structure of the MR scanner is complex. The resonant frequency changes at each point of the field in a controlled manner. The windings of the main magnet made of superconducting material in the form of microsial fibers are built into the copper core. The nonlinear gradient field is created by windings of conductive material. It is added to the main magnetic field. Thus, the resulting magnetic field is obtained.

Abstract. Long-lived spin order-based approaches for magnetic resonance rely on the transition between two magnetic environments of different symmetry, one governed by the magnetic field of the spectrometer and the other where this strong magnetic field is inconsequential. Research on the excitation of magnetic-symmetry transitions in nuclear spins is a scientific field that debuted in Southampton in the years 2000. We advanced in this field carrying the baggage of pre-established directions in NMR spectroscopy. We propose to reveal in this text the part of discoveries that may have been obscured by our choice to only look at them through the experience of such pre-established directions, at the time. Focussing on potential applications, we may have insufficiently emphasised in the manuscripts the methodological developments that necessitated most scientific effort. Such methods developments foster most of the progress in NMR. Thus, we present the contributed mechanisms of translation between the symmetric and non-symmetric environments with respect to the main magnetic field B0, free of any utilitarian perspective. The concept of zero-quantum rotations in the starting blocks of long-lived state populations, magnetisation transfers between hyperpolarised heteronuclei and protons, and selective inversion for long-lived coherences are discussed, as well as hybrid 2D methods based on both insensitive nuclei excitation (“INEPT”) and long-lived spin order. We can see at this point that these magnetic wheels will take a longer time than we initially thought to set in motion new applications in studies of slow diffusion, angiography, or large-protein structure. However, these pulse sequences seed subsequent magnetic mechanisms that are sure to contribute to applications. For instance, some of the introduced coherence rotations were combined with classical pulse blocks to obtain 2D correlations between protons and heteronuclei. We hope the pulse sequence building blocks discussed herein open further perspectives for magnetic resonance experiments with long-lived spin order.

AbstractTwo-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.