A review of non-traditional approaches and emerging trends in superconducting magnets for MRI is presented. Novel technologies and concepts have arisen in response to new clinical imaging needs, changes in market cost structure, and the realities of newly developing markets. Among key trends are an increasing emphasis on patient comfort and the need for ?greener? magnets with reduced helium usage. The paper starts with a brief overview of the well-optimized conventional MR magnet technology that presently firmly occupies the dominant position in the imaging market up to 9.4?T. Non-traditional magnet geometries, with an emphasis on openness, are reviewed. The prospects of MgB2 and high-temperature superconductors for MRI applications are discussed. In many cases the introduction of novel technologies into a cost-conscious commercial market will be stimulated by growing needs for advanced customized procedures, and specialty scanners such as orthopedic or head imagers can lead the way due to the intrinsic advantages in their design. A review of ultrahigh-field MR is presented, including the largest 11.7?T Iseult magnet. Advanced cryogenics approaches with an emphasis on low-volume helium systems, including hermetically sealed self-contained cryostats requiring no user intervention, as well as future non-traditional non-helium cryogenics, are presented.

Novel technologies and configurations of superconducting magnets for MRI

Magnetic Resonance Imaging (MRI) is the largest commercial application of superconductivity. MRI is a powerful diagnostic tool that the medical community considers as a procedure of choice for visualization of soft tissue. The recent decade has marked substantial progress in MRI magnets and systems. The 3.0 tesla horizontal field and 1.0 tesla vertical field open whole-body MRI systems have become commercially available. The superconducting magnet is the largest and most expensive component of an MRI system. The magnet configuration is determined by numerous competing requirements including optimized functional performance, patient comfort, ease of siting in a hospital environment, minimum acquisition and lifecycle cost including service. The factors that drive the magnet requirements are increased center field, maximized uniformity volume, minimized field decay and stray field, magnet compactness, long helium refill period, and more. Advances in the cryogenic technology and magnet design practice provide means for improvements in magnet performance while meeting the market requirement for continuous system cost reduction.

Advances in Whole-Body MRI Magnets

Development of a superconducting bulk magnet for NMR and MRI.

Persistent current joints are crucial components of superconducting magnets—enabling the production of the high and ultra-stable magnetic fields required, for instance, for magnetic resonance measurements. At this critical juncture when persistent mode magnets containing commercial high temperature superconductors may soon become a reality, it is of value to take stock and evaluate current challenges faced in the field of jointing. This paper provides a review of progress made to date on the production and characterization of joints between the five major technological superconductors—NbTi, Nb3Sn, MgB2, BiSCCO and REBCO, including the materials that are used to make these joints.

https://iopscience.iop.org/article/10.1088/0953-2048/28/9/093001/pdf

Persistent current joints between technological superconductors

Following a dedicated R&D program, ASG Superconductors has recently developed techniques for designing and constructing open cryogen free MRI magnets, refrigerated by two double stage cryocoolers only. The magnet consists of two coils both made with six double pancakes, each double pancake being obtained reacting and winding 1600 m of multifilamentary, copper-stabilized MgB2 tape supplied by Columbus Superconductors. Here we report the thermal and electromagnetic characterization and the achieved targets of the first prototype, evaluated on a long term activity period. The MRI images, acquired starting from November 2006, further demonstrate the accomplishment of remarkable magnet performances. In parallel to the long term tests on the first prototype, ASG Superconductors has designed and constructed a second MRI magnet with improved characteristics. We present here the related test results and a comparison with the previous ones.

Construction and Operation of Cryogen Free ${\hbox{MgB}}_{2}$ Magnets for Open MRI Systems

Since the development of the first MgB2 tapes, ASG Superconductors started an R&D activity aimed to produce magnets using the MgB2 conductor. Reacted and wound magnesium diboride double pancake coils were designed, manufactured (using industrial equipment) and tested, in a cryogen free apparatus, by ASG Superconductors. Each double pancake coil was produced by winding a single length of MgB2, 1600 m long, multifilamentary stabilized tape. The successful achievement of the tests allowed us to produce and test the first coil made with 6 single double pancakes, then the second coil and finally the completion of a cryogen free magnet for MRI application. This paper reports the main features of the coil design and construction, the magnet and the test results.

Design, Construction and Tests of MgB2 Coils for the Development of a Cryogen Free Magnet

Feasibility of industrial production of MgB2 cables and magnets has been established, thus leading to MRI systems realization. Apart from continuing the development in performances of both cable and magnet, a further important step consists in applying superconductive junctions to windings, to obtain a better field stability. In 2006 a technique to obtain some tens of Ampere in persistent mode operation in a joined MgB2 cable was found. Since then, short windings were repetitively built to test the progress of the performances of the junctions. Among them, a single junction, five meter long windings with a diameter of 260 millimeter were put in persistent mode (i. e. with total resistance less than 10-14 Ohm) with 300 Ampere circulating at 20 Kelvin, self-field; also windings with two junctions and about one meter long with the same diameter were put in persistent mode with 200 Ampere circulating at 20 Kelvin, self-field.

Persistent Mode MgB $_{2}$ Short Windings

The recombination dipoles MBRD for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN are double-aperture superconducting magnets generating a central magnetic field of 4.5 T in a 105 mm diameter bore, directed in the same direction in both apertures. The integrated magnetic field is 35 T-m in a magnetic length of 7.78 m: with respect to the corresponding magnet presently installed in LHC, the aperture is larger, the length is smaller and the central field is higher. The project, currently underway, includes the fabrication by ASG Superconductors and testing at CERN of a short model (1.6 m long), a prototype, and a series of 4 + 2 spare dipoles. The short model was delivered to CERN and successfully tested in a vertical cryostat in summer 2020, reaching nominal current after three quenches in the second thermal cycle, validating most of the mechanical, thermal and electrical design and giving indications for improvements that have been implemented in the prototype, which was completed and delivered to CERN in October 2021. It was tested in October 2022 in its final cold mass, 14 m long, which also includes the two orbit correctors. This contribution reports on key results from the prototype tests and on the further activities related to the construction of the first magnets of the series.

S. Angius

Papers

Construction and Operation of Cryogen Free ${\hbox{MgB}}_{2}$ Magnets for Open MRI Systems

Design, Construction and Tests of MgB2 Coils for the Development of a Cryogen Free Magnet

Persistent Mode MgB $_{2}$ Short Windings

The MBRD Dipoles for the Luminosity Upgrade at the LHC: From Prototype Tests to the Series Production