Magnetic Resonance Imaging Magnet Research Articles

A Novel Design Method of Independent Zonal Superconducting Shim Coil

In this paper, a novel active shimming method is proposed to minimize the shim coil–magnet and the shim coil–shim coil magnetic couplings simultaneously. In standard superconducting shim coil designs, each shim coil is usually designed individually to correct specific field harmonic components of the inhomogeneous field without considering the magnetic couplings among the shim coils and the main magnet. In particular, a strong interaction among the zonal shim coils and the main magnet can lead to a series of problems, like the drift of main field, instability of field homogeneity, and even destruction of coils. In this paper, magnetic decoupling techniques for reducing magnetic interactions among the zonal shim coils and the main magnet are developed. To complete a set of independent shim coils design, a nonlinear optimization model considering the field deviation and magnetic coupling is established. To improve the efficiency and solution quality of the involved optimization problem, the nonlinear model is first converted to a linear one by using the McCormick envelopes scheme. Through linear programming (LP), the optimal topology and preliminary positions of the shim coils are determined. To further optimize the position and turn numbers of the shim coil, a nonlinear programming (NLP) is then implemented to achieve desired field uniformity. In order to verify the feasibility and effectiveness of the proposed method, a simulation design of two sets of shim coils, Z2 and Z4, for a practical 9.4T magnet for whole-body magnetic resonance imaging (MRI) is presented. Compared with coils designed by conventional methods, magnetic coupling between the zonal shim coils and the main magnet had been effectively reduced. And also, magnetic coupling between the zonal shim coils had also been minimized as much as possible, which consequently improve the stability and safety of the MRI magnet system.

Proton beam behavior in a parallel configured MRI-proton therapy hybrid: Effects of time-varying gradient magnetic fields.

Real-time magnetic resonance (MR) guidance is of interest to various groups globally because the superior soft tissue contrast MR images offer over other x-ray-based imaging modalities. Because of the precision required in proton therapy, proton therapy treatments rely heavily on image guidance. Integrating a magnetic resonance imaging (MRI) into a proton therapy treatment is a challenge. The charged particles (protons) used in proton therapy experience magnetic forces when travelling through the MRI magnetic fields. Given that it is desired that proton beams can be delivered with submillimeter accuracy, it is important that all potential sources of beam displacement are well modeled and understood. This study investigated the behavior of monoenergetic proton beams in the presence of a simulated set of realistic three-dimensional (3D) vector magnetic gradient fields required for spatial localization during imaging. This deflecting source has not been previously investigated. Three-dimensional magnetic vector fields from a superconducting 0.5 T open bore MRI magnet model (previously developed in-house) and 3D magnetic fields from an in-house gradient coil model were applied to two types of computer simulations. In all simulations, monoenergetic proton pencil beams (from 80 to 250 MeV) were used. The initial directions of proton beams were varied. In all simulations, the orientation of the B0 field coincided with the positive z-axis in the simulation geometry. The first type of simulation is based on an analytic magnetic force equation (analytic simulations) while the second type is a full Monte Carlo (MC) simulation. The analytic simulations were limited to propagating the proton beams in vacuum but could be rapidly calculated in a desktop computer while the MC simulations were calculated in a cluster computer. The proton beam locations and dose profiles at the central plane (z = 0 cm) with or without magnetic fields were extracted and used to quantify the effect of the presence of the different magnetic fields on the proton beam. The analytic simulations agree with MC results within 0.025 mm, thus acting as the verification of MC calculations. The presence of the B0 field caused the beam to follow a helical trajectory which resulted in angular offsets of 4.9o , 3.6o , and 2.8o for the 80, 150, and 250 MeV, respectively. Magnetic field deflections caused by a rapid MRI sequence (bSSFP, with maximum gradient strength of 40 mT/m) show a pattern of distortion which remained spatially invariant in the MR's field of view. For the 80 MeV beam, this pattern shows a maximum ranged in the y direction of 1.5 mm. The presence of the B0 field during the bSSFP simulations adds the same beam rotation to the observed during the B0 only simulations. This investigation reveals that time-varying gradient magnetic fields required for image generation can cause a small spread in the proton beams used in the study which are independent of the effects arising from the B0 field. Further, studies where clinical beam kernels were convolved with this spread show that these magnetic fields are expected to have an insignificant impact on the beam's entrance dose.

Wireless video transmission into the MRI magnet room: implementation and evaluation at 1.5T, 3T and 7T

Purpose To analyze the interference between a wireless high definition multimedia interface (WHDMI) and magnetic resonance imaging (MRI) image quality at 1.5T, 3T and 7T. Materials and methods A wireless video transmission system (WVTS) consisting of a WHDMI and a projector was used to transmit and display a video stream into the magnet room. MR image quality was analyzed at 1.5T, 3T and 7T. Signal-to-noise-ratio (SNR¯) $(\overline {{\rm{SNR}}} )$ and radio frequency (RF)-noise spectrum were measured at three transmitter positions (A: inside the cabin, B: in front of the waveguide and C: in the control room). WVTS system functionality tests included measurements of reliability, delay and image quality. Results With the WVTS mean SNR¯ $\overline {{\rm{SNR}}} $ values significantly decreased in comparison to the reference for all positions and fieldstrenghts, while the spectra's baseline is elevated at 1.5T and 3T. Peaks related to continuous wave interferences are apparent at all field strenghts. For WHDMI alone mean SNR¯ $\overline {{\rm{SNR}}} $ values were stable without significant differences to the reference. No elevation of the spectra's baseline could be observed. Functionality measurements confirmed high connection reliability with stable image quality and no delays for all field strengths. Conclusion We conclude that wireless transmission of video streams into the MRI magnet room is feasible at all field strengths without hampering image quality.

Silent cerebral events/lesions after second-generation cryoballoon ablation: How can we reduce the risk of silent strokes?

Atrial fibrillation (AF) ablation is associated with a substantial risk of silent cerebral events/lesions (SCEs/SCLs) detected on magnetic resonance imaging (MRI). The purpose of this study was to investigate the factors associated with the incidence of SCEs/SCLs during second-generation cryoballoon ablation. Two hundred fifty-six AF patients underwent brain MRI 1 day after pulmonary vein (PV) isolation using second-generation cryoballoons with a single 28-mm balloon and short freeze strategy. Overall, 991 of 1016 PVs (97.5%) were successfully isolated by 4.9 ± 1.3 cryoballoon applications per patient, and 25 PVs required touch-up radiofrequency ablation. The total procedure time was 72.7 ± 26.1 minutes. SCEs and SCLs were detected in 68 (26.5%) and 27 (10.5%) patients, respectively. None of the patients reported any neurologic symptoms. Reinsertion of once withdrawn cryoballoons and subsequent applications significantly increased the incidence of SCEs (odds ratio [OR] 2.057; 95% confidential interval [CI] 1.051-4.028; P = .035), and additional left atrial mapping with a multielectrode catheter significantly increased the incidence of SCLs (OR 3.317; 95% CI 1.365-8.056; P = .008). Transient coronary air embolisms were significantly associated with the incidence of SCLs (OR 3.447; 95% CI 1.015-11.702; P = 0.047). On the contrary, an uninterrupted anticoagulation regimen, use of radiofrequency deliveries for transseptal access, cryoballoon air removal with extracorporeal balloon inflations, strength of the MRI magnet, internal electrical cardioversion, and touch-up ablation were not associated with the incidence of SCEs/SCLs. A significant number of SCE/SCL occurrences was observed after second-generation cryoballoon ablation procedures. These results suggest that air embolisms are the main mechanism of SCEs/SCLs, and the injected air volume might determine the lesion type.

Liquid-State 13C Polarization of 30% through Photoinduced Nonpersistent Radicals

Hyperpolarization via dissolution dynamic nuclear polarization (dDNP) is crucial to significantly increasing the magnetic resonance imaging (MRI) sensitivity, opening up in vivo real-time MRI using 13C-labeled substrates. The range of applications, however, is limited by the relatively fast decay of the nuclear spin polarization together with the constraint of having to polarize the spins near the MRI magnet. As recently demonstrated, the employment of UV-induced nonpersistent radicals represents an elegant solution to tackling these drawbacks. Nevertheless, since its introduction, the spread of the technique has been prevented by the relatively low achievable polarization, slow buildup time, and time-consuming sample preparation. In the present work, thanks to a thorough investigation of the radical generation step, we provide a robust protocol to enhance the efficiency and performance of the UV-radical technique. Under optimal conditions, it was possible to produce up to 60 mM radical in less than 5 min and reach maximum DNP enhancement with a buildup time constant of approximately 25 min at 6.7 T and 1 K, resulting in 30% 13C liquid-state polarization.

Quench analysis of 1.5 T superconducting MRI magnet

A national project on indigenous development of whole body 1.5 T superconducting magnetic resonance imaging (MRI) system has been initiated by Meity Govt. of India. This MRI magnet will have multi-coil solenoidal structure to be operated at a current density of 140 A/mm2. The total stored magnetic energy will be 4.5 MJ. This high stored energy of the magnet makes it crucial to design an efficient quench protection system for the magnet to prevent any damage due to either localized temperature rise or any voltage arcing during the quench. This paper presents the details of the quench characteristics of the MRI magnet simulated using OPERA. The transient thermal mapping of the magnet is also discussed in detail in the paper. A comparison has been made of the quench behavior of the magnet using different types of quench protection systems.

Computational homogenization of the elastic and thermal properties of superconducting composite MgB2 wire

Magnetic Resonance Imaging (MRI) background magnets are made from superconducting composite wires that are a metal matrix composite (MMC). Thermal and mechanical properties of the specific wire matrix must be known to optimize the wire configuration for MRI magnet designs. Computational analysis techniques based on numerical homogenization may provide an accurate characterization of a multifilament MMC wire and can reduce the expenses and time required for experimental tests. Recent developments in magnesium diboride (MgB2) superconducting wire has demonstrated their feasibility in a liquid helium (LHe) free conduction-cooled MRI magnet. Computational analysis of an entire superconducting magnet design requires the elastic and thermal properties of these wires. In this work, the temperature dependent elastic modulus, Poisson’s ratio, thermal expansion coefficients, thermal conductivity, and specific heat are estimated using Finite Element Analysis (FEA) and compared to analytical approaches. The orthotropic elastic and thermal properties of four different superconducting composite wire configurations are evaluated. The experimental thermal conductivity and specific heat at room temperature agree well with its corresponding computational results. FEA based computational homogenization has been demonstrated as an acceptable method to estimate the required material properties of the MMC superconducting wire for magnet analysis of MRI manufacturing and operation.

Mobile refrigeration system for precool and warm up of superconducting magnets

Conservation of helium has become more important in recent years due to global shortages in supply. Magnetic resonance imaging (MRI) superconducting magnets use approximately 20% of the world’s helium reserves in liquid form to cool down and maintain operating temperatures at 4 K. This paper describes a mobile cryogenic refrigeration system, which has been developed by Sumitomo (SHI) Cryogenics of America, Inc. to conserve helium by shipping MRI magnets warm and cooling them down or servicing them on site at a medical facility.The system can cool a typical magnet from room temperature to below 40K in less than a week. The system consists of four single stage Displex®-type Gifford-McMahon (GM) expanders in a cryostat with heat exchangers integrated on the cold ends that cool the helium gas, which is circulated in a closed-loop system through the magnet by a cryogenic fan. The system is configured with heaters on the heat exchangers to effectively warm up a magnet. The system includes a scroll vacuum pump, which is used to evacuate the helium circuit with or without the magnet and turbo pump to evacuate the cryostat. Vacuum-jacketed transfer lines connect the cryostat to the magnet. The system is designed with its own controller for continuous operation of precool, warm up and evacuation processes with automatic and manual controls. The cryostat, pumps and gas controls are mounted on a dewar cart. One compressor and the system controller are mounted on a compressor and control cart, and the other three compressors are mounted on separate carts.

Note: Progress on the use of MgB2 superconducting joint technique for the development of MgB2 magnets for magnetic resonance imaging (MRI).

This note presents a superconducting joint technique for the development of MgB2 magnetic resonance imaging (MRI) magnets. The MgB2 superconducting joint was fabricated by a powder processing method using Mg and B powders to establish a wire-bulk-wire connection. The joint resistance measured using a field-decay method was <10-14 Ω, demonstrating that the proposed joint technique could be employed for developing "next-generation" MgB2 MRI magnets operating in the persistent current mode.

Acoustic field characterization of a clinical magnetic resonance-guided high-intensity focused ultrasound system inside the magnet bore.

With the expanding clinical application of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), acoustic field characterization of MR-HIFU systems is needed for facilitating regulatory approval and ensuring consistent and safe power output of HIFU transducers. However, the established acoustic field measurement techniques typically use equipment that cannot be used in a magnetic resonance imaging (MRI) suite, thus posing a challenge to the development and execution of HIFU acoustic field characterization techniques. In this study, we developed and characterized a technique for HIFU acoustic field calibration within the MRI magnet bore, and validated the technique with standard hydrophone measurements outside of the MRI suite. A clinical Philips MR-HIFU system (Sonalleve V2, Philips, Vantaa, Finland) was used to assess the proposed technique. A fiber-optic hydrophone with a long fiber was inserted through a 24-gauge angiocatheter and fixed inside a water tank that was placed on the HIFU patient table above the acoustic window. The long fiber allowed the hydrophone control unit to be placed outside of the magnet room. The location of the fiber tip was traced on MR images, and the HIFU focal point was positioned at the fiber tip using the MR-HIFU therapy planning software. To perform acoustic field mapping inside the magnet, the HIFU focus was positioned relative to the fiber tip using an MRI-compatible 5-axis robotic transducer positioning system embedded in the HIFU patient table. To perform validation measurements of the acoustic fields, the HIFU table was moved out of the MRI suite, and a standard laboratory hydrophone measurement setup was used to perform acoustic field measurements outside the magnetic field. The pressure field scans along and across the acoustic beam path obtained inside the MRI bore were in good agreement with those obtained outside of the MRI suite. At the HIFU focus with varying nominal acoustic powers of 10-500W, the peak positive pressure and peak negative pressure measured inside the magnet bore were 3.87-68.67MPa and 3.56-12.06MPa, respectively, while outside the MRI suite the corresponding pressures were 3.27-67.32MPa and 3.06-12.39MPa, respectively. There was no statistically significant difference (P>0.05) between measurements inside the magnet bore and outside the MRI suite for the p+ and p- at any acoustic power level. The spatial-peak pulse-average intensities (ISPPA ) for these powers were 312-17816W/cm2 and 220-15698W/cm2 for measurements inside and outside the magnet room, respectively. In addition, when the scanning step size of the HIFU focus was increased from 100μmto 500μm, the execution time for scanning a 4×4mm2 area decreased from 210min to 10min, the peak positive pressure decreased by 14%, the peak negative pressure decreased by 5%, and the lateral full width at half maximum dimension of pressure profiles increased from 1.15mm to 1.55mm. The proposed hydrophone measurement technique offers a convenient and reliable method for characterizing the acoustic fields of clinical MR-HIFU systems inside the magnet bore. The technique was validated for use by measurements outside the MRI suite using a standard hydrophone calibration technique. This technique can be a useful tool in MR-HIFU quality assurance and acoustic field assessment.

Hypothermic Cooling Measured by Thermal Magnetic Resonance Imaging; Feasibility and Implications for Virtual Imaging in the Urogenital Pelvis

To study the combination of thermal magnetic resonance imaging (MRI) and novel hypothermic cooling, via an endorectal cooling balloon (ECB), to assess the effective dispersion and temperature drop in pelvic tissue to potentially reduce inflammatory cascade in surgical applications. Three male subjects, before undergoing robot-assisted radical prostatectomy, were cooled via an ECB, rendered MRI compatible for patient safety before ECB hypothermia. MRI studies were performed using a 3T scanner and included T2-weighted anatomic scan for the pelvic structures, followed by a temperature mapping scan. The sequence was performed repeatedly during the cooling experiment, whereas the phase data were collected using an integrated MR-high-intensity focused ultrasound workstation in real time. Pelvic cooling was instituted with a cooling console located outside the MRI magnet room. The feasibility of pelvic cooling measured a temperature drop of the ECB of 20-25 degrees in real time was achieved after an initial time delay of 10-15 seconds for the ECB to cool. The thermal MRI anatomic images of the prostate and neurovascular bundle demonstrate cooling at this interface to be 10-15 degrees, and also that cooling extends into the prostate itself ~5 degrees, and disperses into the pelvic region as well. An MRI-compatible ECB coupled with thermal MRI is a feasible method to assess effective hypothermic diffusion and saturation to pelvic structures. By inference, hypothermia-induced rectal cooling could potentially reduce inflammation, scarring, and fistula in radical prostatectomy, as well as other urologic tissue procedures of high-intensity focused ultrasound, external beam radiation therapy, radioactive seed implants, transurethral microwave therapy, and transurethral resection of the prostate.

Fluid-Attenuated Inversion Recovery Signal Intensity as a Predictor of Gadolinium Enhancement in Relapsing-Remitting Multiple Sclerosis.

Magnetic resonance imaging (MRI) is used to diagnose and monitor disease activity in relapsing-remitting multiple sclerosis (RRMS). The objective of this study was to explore the association of "ultrabright" axial fluid-attenuated inversion recovery (FLAIR) lesions with gadolinium enhancement in patients with RRMS using qualitative and quantitative approaches. MRIs from patients with RRMS from 2010 to 2015 were reviewed. Two radiologists independently identified ultrabright lesions on axial FLAIR sequences. The contrast-to-noise ratio (CNR) was measured for ultrabright and control lesions. Of 301 lesions included in the study, 77 (26%) were identified by both radiologists as ultrabright. Interrater agreement was moderate (κ = 0.77, P < .001). Lesions identified by both radiologists as ultrabright demonstrated an association with gadolinium enhancement (χ21 = 30.8, P < .001) but were not associated with MRI magnet strength (χ21 = 0.24, P = .65). Higher CNR values were associated with gadolinium enhancement for 1.5-T studies (OR, 1.05; 95% CI, 1.02-1.07; P = .001) and 3-T studies (OR, 1.02; 95% CI, 1.02-1.03; P < .001). Diagnostic accuracy of the quantitative model was good for 1.5-T studies (area under the curve, 0.79; 95% CI, 0.68-0.9; P < .001) and 3-T studies (area under the curve, 0.78; 95% CI, 0.73-0.84; P < .001). Positive predictive value of 100% was obtained for CNR values of 92 for 1.5-T and 184 for 3-T studies. Qualitatively and quantitatively identified ultrabright axial FLAIR lesions are significantly associated with gadolinium enhancement.

Feasibility Study of a No-Insulation 1.5-T/600-mm All-REBCO Magnet for MRI Systems

This paper presents the design of a 1.5-T/600-mm all-REBCO magnetic resonance imaging (MRI) magnet comprising a stack of 118 no-insulation (NI) double-pancake coils. A 4.1-mm-wide and 0.1-mm-thick REBCO tape was used in this study. The preshim overall field homogeneity within a 50-cm diameter of spherical volume was estimated to be <25 ppm. According to the equivalent-circuit model of the NI magnet, the basic magnet performance was demonstrated analytically. The design results of the 1.5-T/600-mm all-REBCO MRI magnet with the NI technology indicate the possibility that the NI winding technology can be used in all-REBCO NI magnets for MRI systems. Consequently, the combination of the REBCO tape and the NI winding technique is expected to enable the development of 1.5-T MRI magnets that offer LHe-free operation and are self-protecting, highly compact, and mechanically robust.

Thermal annihilation of photo-induced radicals following dynamic nuclear polarization to produce transportable frozen hyperpolarized 13C-substrates

Hyperpolarization via dynamic nuclear polarization (DNP) is pivotal for boosting magnetic resonance imaging (MRI) sensitivity and dissolution DNP can be used to perform in vivo real-time 13C MRI. The type of applications is however limited by the relatively fast decay time of the hyperpolarized spin state together with the constraint of having to polarize the 13C spins in a dedicated apparatus nearby but separated from the MRI magnet. We herein demonstrate that by polarizing 13C with photo-induced radicals, which can be subsequently annihilated using a thermalization process that maintains the sample temperature below its melting point, hyperpolarized 13C-substrates can be extracted from the DNP apparatus in the solid form, while maintaining the enhanced 13C polarization. The melting procedure necessary to transform the frozen solid into an injectable solution containing the hyperpolarized 13C-substrates can therefore be performed ex situ, up to several hours after extraction and storage of the polarized solid.

Progress in the Development of Conduction-Cooled REBCO Magnets for Ultrahigh-Field MRI Systems

We started developing REBa 2 Cu 3 O 7-δ (REBCO) magnets for ultrahigh-field magnetic resonance imaging (MRI) systems in 2013. Our final targets are 9.4 T MRI systems for whole-body and brain imaging. In this paper, a conduction-cooled 1.5 T REBCO MRI magnet having a room-temperature bore of 396 mm was fabricated and tested in order to evaluate the magnetic field homogeneity and stability. The magnet was composed of 60 single pancakes whose inner diameter was 500 mm. The total conductor length was 10.3 km, and the total inductance was 12.4 H. The size of the homogeneous magnetic field region was 200 mm diameter spherical volume. The central magnetic field was as high as 1.5 T at 192.7 A, and the current density of single pancakes was 301 A/mm 2 . The magnet was cooled from room temperature to 4.7 K in 55 hours, and the temperature difference among the coils was 0.1 K or less during both initial cooling and excitation. The magnetic field inhomogeneity was 249.7 parts per million (ppm), and the Z 2 coefficient was largest at 579.6 ppm. The main reason for the error magnetic field was dimensional errors in the positions on the z-axis. The magnetic field inhomogeneity was improved to 4.1 ppm by passive shimming using iron pieces. The magnetic field stability was about 2 ppm/h because of a reduction in screening-current induced in the REBCO-coated conductors. Current sweep reversal improved the magnetic field stability from 2 ppm/h to 0.8 ppm/h.

Passive Shimming of MRI Static Magnetic Field Using Regularization of Truncated Singular Value Decomposition.

Purpose:To develop a new shimming calculation method, which can calculate iron piece placements rapidly to make the magnetic field homogeneous at intended homogeneity and then to make the shimming working time short.Materials and Methods:The shimming calculation yields magnetic moment (MM) distribution, which is calculated by the truncated singular value decomposition (SVD) from the measured magnetic field. The MM distribution is described by a superposition of eigenmodes obtained by SVD of a response matrix from the moment distributions to magnetic fields at the field of view (FOV). The homogeneity is regulated by a truncation number of the superposed eigenmodes. The magnetic moments are converted into iron volumes with the assumption of saturated magnetization and the iron pieces are placed according to the calculation results. Since the SVD calculation can be done in advance, the computational time at the shimming site is short.Results:Trial applications on a 0.5T magnetic resonance imaging (MRI) magnet were done using the new shimming calculation method, which was proved to work well. However, since the iron piece volumes had tolerances, the work was repeated until enough homogeneity was obtained. As a result, an intended homogeneity of 8.9 ppm (peak-to-peak) on 40 cm diameter spherical surface was successfully obtained from measured homogeneity of 543 ppm with short computational and working time.Conclusion:The test shimming work showed that the developed shimming calculation method with truncated SVD regularization is applicable to the shimming work on the MRI magnets.

Conductors for commercial MRI magnets beyond NbTi: requirements and challenges

Magnetic resonance imaging (MRI), a powerful medical diagnostic tool, is the largest commercial application of superconductivity. The superconducting magnet is the largest and most expensive component of an MRI system. The magnet configuration is determined by competing requirements including optimized functional performance, patient comfort, ease of siting in a hospital environment, minimum acquisition and lifecycle cost including service. In this paper, we analyze conductor requirements for commercial MRI magnets beyond traditional NbTi conductors, while avoiding links to a particular magnet configuration or design decisions. Potential conductor candidates include MgB2, ReBCO and BSCCO options. The analysis shows that no MRI-ready non-NbTi conductor is commercially available at the moment. For some conductors, MRI specifications will be difficult to achieve in principle. For others, cost is a key barrier. In some cases, the prospects for developing an MRI-ready conductor are more favorable, but significant developments are still needed. The key needs include the development of, or significant improvements in: (a) conductors specifically designed for MRI applications, with form-fit-and-function readily integratable into the present MRI magnet technology with minimum modifications. Preferably, similar conductors should be available from multiple vendors; (b) conductors with improved quench characteristics, i.e. the ability to carry significant current without damage while in the resistive state; (c) insulation which is compatible with manufacturing and refrigeration technologies; (d) dramatic increases in production and long-length quality control, including large-volume conductor manufacturing technology. In-situ MgB2 is, perhaps, the closest to meeting commercial and technical requirements to become suitable for commercial MRI. Conductor technology is an important, but not the only, issue in introduction of HTS/MgB2 conductor into commercial MRI magnets. These new conductors, even when they meet the above requirements, will likely require numerous modifications and developments in the associated magnet technology.

Evaluation of Overdamping Behavior of a Vibrating Conductive Cylinder in a Strong Magnetic Field

Magnetomechanically coupled problems of conductive structures are generally known, but the focus of interest has been on underdamping, not overdamping. The purpose of this study is to verify the applicability of unidirectional weak-coupling analysis to the overdamping behavior of a vibrating thin conductive plate in a strong magnetic field, which is a necessary procedure for designing magnetic resonance imaging (MRI) equipment. An impact vibration experiment was done with a thin conductive cylinder in the 3-T field of an MRI magnet. Magnetic modal damping ratios and accelerations of the cylinder were calculated by eddy current analysis and frequency response analysis. As a result, specified peaks of the acceleration curves were found to disappear and the corresponding eigenmodes were the same for both the experiment and analysis. Furthermore, relative errors of the acceleration amplitude decrease ratio were in the range of 9.2%–31.9%; hence the calculated acceleration was on the conservative side. Consequently, the analysis method was judged suitable as a practical calculation method for designing MRI equipment that considers the overdamping behavior.

A permanent MRI magnet for magic angle imaging having its field parallel to the poles

A novel design of open permanent magnet is presented, in which the magnetic field is oriented parallel to the planes of its poles. The paper describes the methods whereby such a magnet can be designed with a field homogeneity suitable for Magnetic Resonance Imaging (MRI). Its primary purpose is to take advantage of the Magic Angle effect in MRI of human extremities, particularly the knee joint, by being capable of rotating the direction of the main magnetic field B0 about two orthogonal axes around a stationary subject and achieve all possible angulations. The magnet comprises a parallel pair of identical profiled arrays of permanent magnets backed by a flat steel yoke such that access in lateral directions is practical. The paper describes the detailed optimization procedure from a target 150mm DSV to the achievement of a measured uniform field over a 130mm DSV. Actual performance data of the manufactured magnet, including shimming and a sample image, is presented. The overall magnet system mounting mechanism is presented, including two orthogonal axes of rotation of the magnet about its isocentre.

Influence of the heat-treatment conditions on various types of multifilamentary Nb-46.5%Ti superconducting wires

Unlike the NbTi superconducting wires used for high critical current density, NbTi wires for Magnetic resonance imaging (MRI) magnets have larger and fewer NbTi monofilaments and different cross sections, which show different superconducting properties. This study investigated the effects of varying the temperature, number of heat-treatment cycles, and total strain over a wide range for multifilamentary Nb-46.5%Ti wires on a mass production scale for use in MRI magnets. The heat-treatment conditions were optimized for an NbTi superconducting wire and the critical current density and the n-value were measured as functions of the final strain at temperatures of 4.2 K and 7 T. We noticed that the superconducting properties increased with increasing final strain of the multifilamentary NbTi wire. The microstructure and the effects of the size and the distribution of α-Ti precipitates on the individual heat-treatment steps were observed by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We, consequently, found the heat-treatment conditions that provided the highest superconducting performance for the two types of NbTi wires used in this study, and the results of the study are expected to very helpful in establishing not only the heat-treatment conditions but also important manufacturing parameters, such as the total strain, even as the design of NbTi wires for used in MRI magnets as changing with industrial demand.