Measurement Of Static Magnetic Field Research Articles

AAPM Task Group Report 325: MRI static magnetic field homogeneity measurement and evaluation procedures - Guidance and resources.

Measurement of static magnetic field (B0) homogeneity is an essential component of routine MRI system evaluation. This report summarizes the work of AAPM Task Group (TG) 325 on vendor-specific methods of B0 homogeneity measurement and evaluation. TG 325 was charged with producing a set of detailed, step-by-step instructions to implement B0 homogeneity measurement methods discussed in the American College of Radiology (ACR) MRI Quality Control Manual using specific makes and models of MRI scanners. The TG produced such instructions for as many approaches as was relevant and practical on six currently available vendor platforms including details of software/tools, settings, phantoms, and other experimental details needed for a reproducible protocol. Because edits to these instructions may need to be made as vendors enter and exit the market and change available tools, interfaces, and access levels over time, the step-by-step instructions are published as a living document on the AAPM website. This summary document provides an introduction to B0 homogeneity testing in MRI and several of the common methods for its measurement and evaluation. A living document on the AAPM website provides vendor-specific step-by-step instructions for performing these tests to facilitate accurate and reproducible B0 homogeneity evaluation on a routine basis.

Modeling and Characterization of a ReBCO HTS Degaussing Demonstrator

The magnetic modelling and experimental validation of a superconducting degaussing system for maritime vessels is discussed. Degaussing coils compensate for the distortion in the earths’ magnetic field by the magnetized steel hull of a ship, thus rendering it ‘invisible’ for magnetic field sensors. Whereas typical power requirements with copper coils are of the order of 100 kW, a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO HTS degaussing system in principle allows to reduce this by an order of magnitude. In order to validate such efficiency estimates and to demonstrate the required hardware, a table-top test setup was realized with magnetic ship steel. The vessel-imitating cylindrical demonstrator is equipped with six degaussing coils, grouped in three sets that act in two different directions, with each set consisting of one copper and one <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Re</i> BCO coil, the latter one equipped with a sub-cooled forced-flow liquid nitrogen system. Static magnetic field measurements are reported and compared to both analytical and numeric finite element models. The results illustrate how even relatively simple analytical models can be used as a powerful tool to extrapolate design parameters and thus to predict the power requirements of large-scale degaussing systems.

Static magnetic field measurements of smart phones and watches and applicability to triggering magnet modes in implantable pacemakers and implantable cardioverter-defibrillators

Implantable pacemakers and implantable cardioverter-defibrillators (ICDs) are designed to include a "magnet mode" feature that can be activated from magnets stronger than 10 G. This feature is designed to be used when a patient is undergoing a procedure where electromagnetic interference is possible, or anytime suspension of tachycardia detection and therapy is needed. A publication in Heart Rhythm demonstrates an iPhone 12 triggering the magnet mode of a Medtronic ICD. The purpose of this study is to determine the separation distance between consumer electronic devices that may create magnetic interference, including cell phones and smart watches, and implantable pacemakers and ICDs where magnet mode can be triggered. The static magnetic fields of the iPhone 12 models and Apple Watch were measured at several planes in 1 cm resolution using an FW Bell 5180 Gauss Meter with STD18-0404 Transverse probe (unidirectional probe). All iPhone 12 and Apple Watch 6 models tested have static magnetic fields significantly greater than 10 G in close proximity (1-11 mm), which attenuates to below 10 G between 11 and 20 mm. The findings of this study support the US Food and Drug Administration recommendation that patients keep any consumer electronic devices that may create magnetic interference, including cell phones and smart watches, at least 6 inches away from implanted medical devices, in particular pacemakers and cardiac defibrillators.

A mechanical amplifier based high-finesse fiber-optic Fabry–Perot interferometric sensor for the measurement of static magnetic field

A high-finesse fiber-optic extrinsic Fabry–Perot interferometer (EFPI)-based sensor for the measurement of static magnetic fields is proposed and experimentally demonstrated. The high-finesse EFPI is composed of two aligned fiber ends, each of which has a micro-lens mirror. The high-finesse EFPI is mounted on a specially designed mechanical amplification structure which is driven by a TbDyFe rod. Under the applied bias magnetic field, the TbDyFe rod works in the linear region, and its length changes with the change of the magnetic field strength. The mechanical amplification structure amplifies the elongation of the TbDyFe rod in the magnetic field, thus changing the optical cavity length of the high-finesse fiber-optic EFPI. The magnetic field strength can be obtained by measuring the spectral resonance wavelength. Experimental results show that the wavelength sensitivity with the applied magnetic field is 0.218 nm μT−1, the magnetic field resolution is 4.59 nT, and the magnification of the mechanical amplifier is 5.66 times.

High-Resolution Pinning GMR Sensors for Extremely Low Frequencies Powered by a Simple Alternating Current–Biased Scheme

This paper presents a simple scheme for enhancing the resolution of giant magnetoresistance (GMR) sensors by making use of an AC-biased method. In doing so, an alternating voltage was used to power the GMR sensor. As in the case of high-frequency bias, significant suppression of low-frequency (1/f) noise was experimentally observed. With the bare GMR sensor, the 1/f noise value at 1 Hz was recorded to be 15 nT/√Hz, whereas applying the AC bias, the noise was reduced significantly by a factor of 30, with a minimum noise level of 0.5 nT/√Hz@1 Hz. Furthermore, the hysteresis (Hc) of the sensor was also collapsed from 0.1 to 0.015 mT with a negligible offset. The performance of the AC-biased scheme was verified by measuring the Earth’s magnetic field with the measured error being minimized from 2.5 μT down to 0.4 μT. The favorable results indicate that the proposed scheme is a pertinent way to improve the resolution of pinning GMR sensors in ultra-weak magnetic fields and static magnetic field measurements.

Magnetic Field Homogeneity Adjustment for Magnetic Resonance Imaging Equipment

Imaging based on magnetic resonance (MR) methods needs a source of the highest homogeneous magnetic field in the whole volume of the investigated object. The goal of this paper is to demonstrate three methods of correcting coils (shim coils) current calculations and homogeneity adjustment on a whole-body MR imager, based on three methods for static magnetic field measurements. Measurements are carried out using: magnetometer based on the Hall effect, nuclear MR point magnetometer, and gradient-echo imaging method applied to circular test tubes. All three measurements measure the magnetic field on a circle (12 measuring points) and one point in the center. A set of linear equations, determination of a target function, and optimization computations are procedures that provide optimal values of currents for correcting coils. Results of all three methods are compared and discussed. The proposed methods are suitable for periodic testing and correction of magnet inhomogeneities for MRI magnets.

Generation of experimental plasma parameter maps around the baffle aperture of a Kaufman (UK-25) ion thruster

The plasma produced by a Kaufman (UK-25) ion thruster possesses four distinct regions. The interaction of two of these regions, the coupling and discharge plasmas, is important for thruster design and efficiency, since it controls the production of the primary electrons responsible for most of the ionisation in the thruster. This paper details the measurement and analysis techniques used to obtain two-dimensional experimental plasma parameter maps across a plasma double layer in the baffle aperture region of this type of ion thruster. This plasma-interaction region was mapped experimentally using Langmuir probes. The Langmuir probe data were collected and analysed to produce maps of plasma potential, Maxwellian and primary electron energy and number density. Detailed static magnetic field measurements using Hall-effect probes were also obtained. A number of derived plasma parameter maps were then made possible, such as electron pressure. The application of the analysis techniques used here allowed ion thruster plasma property maps to be constructed of a spatial extent and resolution previously unseen. Two-dimensional maps of the spatial location of primary electrons are presented, as effected by applied magnetic field changes, along with the detection of regions showing depletion of primary electron energies.

Analysis and Measurements of Magnetic Field Exposures for Healthcare Workers in SelectedMR Environments

There are concerns about workers repeatedly exposed to magnetic fields exceeding regulatory limits with respect to modern magnetic resonance imaging (MRI). As a result, there is need for an ambulatory magnetic field dosimeter capable of measuring these fields in and around an MRI scanner in order to evaluate the regulatory guidelines and determine any underlying exposure risks. This study presents results of tri-axial measurements using an ambulatory magnetic field dosimeter worn by workers during normal working shifts. We recorded and analyzed magnetic field exposures in and around 1.5 T, 2 T, and 4 T magnets during routine patient procedures. The data was integrated and averaged over time and evaluated against the latest exposure standards. Time-varying magnetic fields occur when individuals move through spatially non-uniform static magnetic fields or during gradient-pulsed magnetic fields or a combination of both. Our previous numerical analysis shows that at certain positions surrounding the MRI scanner ends, such fields may induce current densities and electric fields that may exceed the relevant EU, ICNIRP, and IEEE standards. A high-speed acquisition version of the dosimeter measured gradient- pulsed fields at positions accessible by MRI workers near the scanner ends, and the results were evaluated and compared against the numerical simulations and the standards. Our measurements confirm that workers can be exposed to magnetic fields exceeding the guidelines at positions near the gradient coil ends during clinical imaging and a high degree of correlation exists with the numerical results. While the time-weighted average magnetic field exposures in 1.5 T, 2 T, and 4 T were all within the regulatory limits during static magnetic field measurements, the peak limits for the head can be exceeded in some circumstances. This study presents a small number of routine shifts of data that provide indicative results of magnetic field exposure in real situations.

Magnetization identification problem

This paper deals with the problem of magnetization identification. We consider a ferromagnetic body placed in an inductor field. The goal of this work is, from static magnetic field measurements taken around the device, to obtain an accurate model of its magnetization. This inverse problem is usually ill‐posed and its solution is non‐unique. It is then necessary to use mathematical regularization. However, we prefer to transform it to a better posed one by incorporating our physical knowledge of the problem. Our approach is tested on the magnetization's identification of a real ferromagnetic sheet.

A magnetostrictive sensor interrogated by fiber gratings for DC-current and temperature discrimination

A magnetostrictive sensor head with temperature compensation has been developed for measurement of static magnetic fields. The device consists on two different alloys with similar thermal expansion coefficient one of which has a giant magnetostriction, the expansion of both materials produced by heat and magnetism is detected by two fiber gratings. One of the gratings measures the temperature of the sensor and the difference between the wavelengths reflected by the gratings is a measurement of the magnetically induced strain.

Microwave properties of (Fe0.82B0.18)0.09La0.05R0.05 amorphous metal alloys

Static magnetic field measurements have been reported on amorphous metal alloys of (Fe0.82B0.18)0.90La0.05R0.05, where R=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er. Microwave properties of these alloys are now reported. The microwave measurements consisted of fixed (9 GHz) and swept (0.01–18 GHz) frequency magnetic resonance experiments. The ferromagnetic resonance (FMR) linewidth, ΔH and g factor were measured. ΔH has a minimum for R=Gd and is equal to 100 Oe. ΔH appears to scale with 〈L⋅S〉 where L and S are the spin and orbital angular momentum operators of the R ion. The measured g factor obeys the Wangsness relation, as R is varied.