Split Magnet Research Articles

Combining MRI with PET for partial volume correction improves image-derived input functions in mice.

Accurate kinetic modelling using dynamic PET requires knowledge of the tracer concentration in plasma, known as the arterial input function (AIF). AIFs are usually determined by invasive blood sampling, but this is prohibitive in murine studies due to low total blood volumes. As a result of the low spatial resolution of PET, image-derived input functions (IDIFs) must be extracted from left ventricular blood pool (LVBP) ROIs of the mouse heart. This is challenging because of partial volume and spillover effects between the LVBP and myocardium, contaminating IDIFs with tissue signal. We have applied the geometric transfer matrix (GTM) method of partial volume correction (PVC) to 12 mice injected with 18F-FDG affected by a Myocardial Infarction (MI), of which 6 were treated with a drug which reduced infarction size [1]. We utilised high resolution MRI to assist in segmenting mouse hearts into 5 classes: LVBP, infarcted myocardium, healthy myocardium, lungs/body and background. The signal contribution from these 5 classes was convolved with the point spread function (PSF) of the Cambridge split magnet PET scanner and a non-linear fit was performed on the 5 measured signal components. The corrected IDIF was taken as the fitted LVBP component. It was found that the GTM PVC method could recover an IDIF with less contamination from spillover than an IDIF extracted from PET data alone. More realistic values of Ki were achieved using GTM IDIFs, which were shown to be significantly different (p<0.05) between the treated and untreated groups.

Fabrication and Test of an 8-T Superconducting Split Magnet System With Large Crossing Warm Bore

A conduction-cooled superconducting split magnet system with large crossing warm bore has been successfully constructed in our laboratory for material processing applications. The magnet system design was described in a previous paper. The magnet is composed of six NbTi low temperature superconducting coils, which generate 5.5-T central magnetic field and two Bi2223/Ag high temperature superconducting (HTS) insert coils, which generate 2.5-T central magnetic field and assembled in the form of split coil groups. The magnet has a 136-mm split gap to accommodate the crossing warm bore of 100 mm in diameter. The magnet system is cooled by two GM cryocoolers. The initial cooldown takes 9.2 days and the final temperature of the magnet is about 4.0 K. The HTS coils and NbTi coils are to be operated in the driven mode with two independent power supplies, under the operating currents of 200 A (HTS) and 139 A (NbTi), respectively. The magnet is successfully powered up to 8 T with a the ramp time of 290 min. In this paper, the fabrication and test of the superconducting split magnet system are presented.

풍력터빈 PM형 동기발전기의 와전류손실과 열 해석에 관한 연구

In this paper, eddy current loss, iron loss and heat transfer of PMSG with 2,000kW capacities were analyzed for wind turbine. The PMSG with 3 split magnet was analyzed using ansoft maxwell commercial program and, generator was tested by Back to Back converter with no load condition at laboratory. Rotor surface temperature was measured by Pt100 sensors for investigating heat transfer from rotor to atmosphere. The simulation results shows 27.4㎾ eddy current loss in no load condition and 50.2㎾ eddy current loss in rated load condition with 3 split magnet, and also shows 4.3㎾ iron loss in no load condition and 7.3kW iron loss rated load condition. The heat transfer coefficient of convection between rotor surface and atmosphere was investigated by 9.6 W/㎡·K. Therefore the heat transfer from rotor to atmosphere was about 17㎾(54%) and from rotor to air-gap was about 14.6㎾(46%) in no load condition. It is identified that the cooling system for stator have to include the 46% of iron loss, and heat dissipation structure of rotor surface have to be suggested and designed for efficiency improvement of generator.

풍력터빈 적용을 위한 PMSG용 영구자석의 와전류손실에 관한 연구

The objective of this paper is to suggest a design topology of permanent magnet synchronous generator with 2,000kW capacities for wind turbine. The suggested topology is to provide 3 split magnet PMSG instead of single magnet, and performed an analysis of eddy current loss and iron loss for suggested type using ansoft maxwell commercial program. The simulation results of suggested magnet type show there duction of eddy current loss as 13.87kW with loadless conditions and23.48kW with rated conditions, but iron loss for rotor yoke show the in creasing trend as2.2kW with loadless conditions and 0.2kW with rated conditions. The suggested 3 split maget type is to identified as more useful for 2,000kW PMSG.

Numerical Safety Study of Currents Induced in the Patient During Rotations in the Static Field Produced by a Hybrid MRI-LINAC System

MRI-LINAC is a new image-guided radiotherapy treatment system that combines magnetic resonance imaging (MRI) and a linear particle accelerator (LINAC) into a single unit. Moving (i.e., rotating or translating) the patient inside the strong magnetic field of the split MRI-LINAC magnet can potentially induce high levels of electric fields and corresponding current densities in the conducting tissues. The prediction and assessment of patient safety in terms of electromagnetic field exposure have received very little attention for a split cylindrical MRI magnet configuration, especially in the vicinity of the gap region. In this novel numerical study, based on the quasi-static finite-difference method, rotation-induced electric fields and current densities are calculated considering a split 1-T magnet and a tissue-accurate 2-mm-resolution human body model. The patient was modeled in both axial and radial orientations relative to the magnet gap in a number of treatment/imaging scenarios. It was found that rotating the patient in the radial orientation produced an order of magnitude larger field exposure in the central nervous system than when the patient was rotated in the axial orientation. Also, rotating the patient with periods lower than about Trot = 43.3 s may result in field exposures above the limits set out in the international safety guidelines. The novel results of this investigation can provide useful insights into the safe use of the MRI-LINAC technology and optimal orientations of the patient during the treatment.

HTS Vector Magnet for Magnetic Circular Dichroism Measurement

In this study, we developed a two-dimensional vector magnet with Y-Ba-Cu-O (YBCO) tape conductors for use in an X-ray magnetic circular dichroism measurement system. The characteristics of this high- Tc superconducting vector magnet are as follows: (a) it is a split magnet comprising four unit coils that enable an X-ray magnetic circular dichroism system to access the center sample space along three axes; (b) it is compact with a high magnetic field, and it exhibits a cone-shaped design to enable the split unit coils to come close together and to achieve a maximum magnetic field up to 1 T at the center; and (c) it is conduction-cooled by a cryocooler and a cryogen-free system. We fabricated four unit coils with YBCO tape conductors (manufactured by SuperPower Inc.). Two pairs of the unit coils were configured at right angles to each other in a cryostat and cooled below 30 K by the cryocooler. We confirmed that the magnetic field orientation can be controlled by adjusting the current of the unit coils. However, we observed hysteresis and drift of the magnetic field owing to the screening current induced in the YBCO tape conductors. Here, we design a high- Tc superconducting vector magnet and present its excitation test results.

PET/MRI in the infarcted mouse heart with the Cambridge split magnet

Chronic heart failure, as a result of acute myocardial infarction, is a leading cause of death worldwide. Combining diagnostic imaging modalities may aid the direct assessment of experimental treatments targeting heart failure in vivo. Here we present preliminary data using the Cambridge combined PET/MRI imaging system in a mouse model of acute myocardial infarction. The split-magnet design can deliver uncompromised MRI and PET performance, for better assessment of disease and treatment in a preclinical environment.

Reliability of using a fixed matrix in coregistration of combined PET–MRI in a split magnet design

We consider the effects of using a fixed linear transformation to match positron emission tomography (PET) and magnetic resonance imaging (MRI) data acquired simultaneously using a split-magnet system. Estimates of the frequency offset in MRI scans were used to calculate geometric variability in MRI reconstruction as a consequence of mis-setting this parameter in addition to repeated estimation of the transformation matrix by manual measurements. None of the measured variability approached the resolution of the PET images, so we concluded that a fixed matrix can be reliably used in such a system.

Simulation and analysis of the interactions between split gradient coils and a split magnet cryostat in an MRI–PET system

Splitting a magnetic resonance imaging (MRI) magnet into two halves can provide a central region to accommodate other modalities, such as positron emission tomography (PET). This approach, however, produces challenges in the design of the gradient coils in terms of gradient performance and fabrication. In this paper, the impact of a central gap in a split MRI system was theoretically studied by analysing the performance of split, actively-shielded transverse gradient coils. In addition, the effects of the eddy currents induced in the cryostat on power loss, mechanical vibration and magnetic field harmonics were also investigated. It was found, as expected, that the gradient performance tended to decrease as the central gap increased. Furthermore, the effects of the eddy currents were heightened as a consequence of splitting the gradient assembly into two halves. An optimal central gap size was found, such that the split gradient coils designed with this central gap size could produce an engineering solution with an acceptable trade-off between gradient performance and eddy current effects. These investigations provide useful information on the inherent trade-offs in hybrid MRI imaging systems.

Fabrication and Assembly of the NHMFL 25 T Resistive Split Magnet

The NHMFL has developed a high-field split resistive magnet for use in far-infrared photon scattering experiments. The magnet includes four large scattering ports of elliptical shape at the mid-plane amounting to a total solid angle of 0.5 steradians of available user space. Such a magnet configuration results in unique design challenges being especially severe for the windings in the mid-plane region of the innermost coils. Consequently, the NHMFL developed, tested and employed a new technology called Split Florida-Helix. Next, the coil design of the split user magnet, to be operated at our own facility, has been completed in 2009. This user magnet consist of 5 resistive coils consuming a total of less than 28 MW of dc power and providing a flux-density of 25 T available to the user space. To meet the unique design challenges, Split Florida-Helix technology is used around the mid-plane of the two inner most coils and state-of-the-art Florida-Bitter technology is used for all the regular winding in all 5 coils.

1-G HTS Split Coil Magnet for Research Purposes

A practical split coil magnet using Bi-2223 HTS tapes has been developed and tested. The design parameter is the maximal magnetic field at the aperture of a coil. 1-G Bi-2223 tape was used for magnet's winding produced by Sumitomo Electric Industry Co. The maximum magnetic field in the magnet is determined by the dependence of 1-G tape's critical current on magnetic field normal to the HTS surface of a tape. Reduction of the normal component of the magnetic field to the HTS tape was achieved by using the iron plates. The finite element modeling has been used to design the magnet configurations. The simulation results are in good agreement with the measurement data. The maximum magnetic field at the aperture of the magnet reached 0.75 T at 77.4 K with ~1 T of maximum field on winding. The split coil is used to test anisotropy of critical currents of HTS tapes in our standard HTS wires characterization test program. The details of split coil design and magnet test results are presented.

Design and Fabrication of HTS Coils for a Vector Magnet

We are developing a high-Tc superconducting vector magnet for use in solid-state physics apparatus. The vector magnet features are (a) 1 T peak and two-dimensional magnetic field vector controllable (b) comprising four split magnet units enabling free access in three directions to a sample space (c) cryogen-free conduction-cooled magnet. To obtain a vector controllable magnetic field as high as 1 T using a compact magnet, we adopted a tapered design for split magnet units. We fabricated a test magnet unit with YBCO tape conductors and examined its basic performance. The magnet unit generated the rated magnetic field. This paper describes the vector magnet design, fabrication procedures, and the magnet unit test results.

Explosive-driven Shock Wave Demagnetization of Nd 2 Fe 14 B Hard Ferromagnets

Abstract Magnetic materials subjected to shock wave compression will undergo shock wave demagnetization. To further investigate the demagnetization performance of Nd 2 Fe 14 B ferromagnets, an open magnetic circuit cylinder was used as the energy-carrying body. The discharge process and circuit characteristics during shock wave demagnetization were analyzed. A series of the shock wave demagnetization tests were performed using the shock wave from the detonation of high explosives acting along the axial direction of the cylindrical magnets. The induced electromotive force and current were measured. The results indicate that Nd 2 Fe 14 B ferromagnet begins to demagnetize at approximately 0.28 μs after the shock wave enters its body. The induced electromotive force and current in coils are caused by the destruction of the magnetism in the smallest units of magnetic domains rather than the mechanical damage of the magnet structure. The influence of the shock wave pressure on the effect of demagnetization was investigated by altering the pressure of the shock wave acting the magnets. The results show that Nd 2 Fe 14 B ferromagnets can be absolutely demagnetized and destroyed by a high shock wave pressure, and that demagnetization is insufficient under a low 13 GPa shock wave pressure. In order to reduce eddy current losses during the process of the shock wave demagnetization, a new design of a split magnet with insulation layers was proposed. This process is very effective and beneficial to improve the energy conversion during the shock wave demagnetization.

Explosive-driven Shock Wave Demagnetization of Nd 2 Fe 14 B Hard Ferromagnets

Abstract Magnetic materials subjected to shock wave compression will undergo shock wave demagnetization. To further investigate the demagnetization performance of Nd 2 Fe 14 B ferromagnets, an open magnetic circuit cylinder was used as the energy-carrying body. The discharge process and circuit characteristics during shock wave demagnetization were analyzed. A series of the shock wave demagnetization tests were performed using the shock wave from the detonation of high explosives acting along the axial direction of the cylindrical magnets. The induced electromotive force and current were measured. The results indicate that Nd 2 Fe 14 B ferromagnet begins to demagnetize at approximately 0.28 μs after the shock wave enters its body. The induced electromotive force and current in coils are caused by the destruction of the magnetism in the smallest units of magnetic domains rather than the mechanical damage of the magnet structure. The influence of the shock wave pressure on the effect of demagnetization was investigated by altering the pressure of the shock wave acting the magnets. The results show that Nd 2 Fe 14 B ferromagnets can be absolutely demagnetized and destroyed by a high shock wave pressure, and that demagnetization is insufficient under a low 13 GPa shock wave pressure. In order to reduce eddy current losses during the process of the shock wave demagnetization, a new design of a split magnet with insulation layers was proposed. This process is very effective and beneficial to improve the energy conversion during the shock wave demagnetization.

US builds strongest split magnet

US builds strongest split magnet

Aspects of superconducting magnet design for neutron scattering sample environments

The majority of superconducting magnets for neutron scattering experiments take the form of split pairs with magnetic field vector in the vertical plane. Sample environment access is along the vertical magnetic axis whilst neutron access is in the horizontal plane. Split pair superconducting magnets present significantly more challenges in terms of design than the simpler solenoid type arrangement and the addition of requirements for neutron access further complicates the situation. Many of the requirements of split pair magnets for neutron scattering are conflicting and often compromises have to be made. Presented here are some of the more important design criteria and the ways in which these are met in practical magnet designs. Topics covered range from the choice of superconducting material through to the control of magnetic flux density profiles and mechanical aspects of the magnet former providing the neutron access between the coils. Most of the information presented is based on recent or current production magnets manufactured by Oxford Instruments for a range of neutron related applications.

Qualification Measurements of the Mid-Field and Low-Field CICC for the Series-Connected Hybrid Magnet With Effects of Electromagnetic Load Cycling and Longitudinal Strain

The cable-in-conduit conductors (CICC) designed for the National High Magnetic Field Laboratory (NHMFL) and Helmholtz Zentrum Berlin (HZB) Series-Connected Hybrid magnets have been qualified. The superconducting coils for the hybrid magnets consist of three CICC configurations graded for different designed fields. Representative samples with the same Nb3Sn type, cable pattern, and CICC configuration were fabricated and the current sharing temperature (TCS) and critical current were measured with the intention to quantify the level of degradation from cyclic, transverse electromagnetic loads. In addition the intrinsic strain for the CICC was determined by measuring the TCS and IC as a function of applied longitudinal strain. Two conductor grades, for the middle and low fields regions of the coil (MF and LF), are tested at the NHMFL in a split magnet that is equipped with a system that can apply up to 250 kN force in a direction longitudinal to the samples. The samples are hydraulically connected to a cryogenic system which delivers temperature controlled supercritical helium. The reduction in TCS from electromagnetic load cycling is shown to be insignificant and measurements as a function of strain shown that the MF and LF CICC have intrinsic strains of approximately 0.65% and 0.70% respectively.

Design Study of High Field Resistive Magnets for Diffraction Experiments

The European Synchrotron Radiation Facility (ESRF), and the Institut Laue Langevin neutron facility (ILL) in collaboration with the Laboratoire National des Champs Magne¿tiques Intenses (LNCMI-CNRS) have performed during 2008 a design study for the implementation of continuous high magnetic fields suitable for diffraction studies. Two main designs have been studied: a horizontal field magnet suitable for back scattering and absorption experiments and a split magnet.

FEA-Aided Design of a Powered Scattering Magnet at the NHMFL

The NHMFL has completed the design of all major components of a high-field split resistive magnet for use in far-infrared photon scattering experiments. The magnet includes four large scattering ports of elliptical shape at the mid-plane. Such a magnet configuration results in unique design challenges being especially severe for the windings in the mid-plane region of the innermost coils. Consequently, the NHMFL incorporated its newly developed technology called split Florida-Helix previously tested at the NHMFL with diverse working models. The user magnet, to be operated at our own facility, will consist of 5 resistive coils consuming a total of less than 28 MW of dc power and providing a flux-density of at least 25 T available at the center of the user space. All coils employ axial current grading for field optimization and stress management. Advanced finite element analysis (FEA) served as the essential tool guiding the design optimization of the overall system and the various components. This paper provides a systematic discussion of the critical features and techniques utilized in the complex model-based analysis and the authors present a variety of detailed FEA results and design parameters critical for the integration of the split Florida-helix in conjunction with the traditional Florida Bitter disc design.

Resistive Solenoid Development at the NHMFL

The National High Magnetic Field Laboratory (NHMFL) has several resistive solenoid magnet projects underway presently, including upgrade of four existing magnets, design of the conical bore insert for the Helmholtz Center Berlin (HZB), design of insert coils of the Series-Connected-Hybrid (SCH) for NHMFL and design of resistive split magnet. The upgrades and the design of the conical bore insert are discussed in detail while other programs are presented briefly.