Conductors for commercial MRI magnets beyond NbTi: requirements and challenges

Abstract

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.

Figure 1

Coil configurations for MRI magnets Abbreviation: ID-Length-J_avg, where ID is inner diameter of the Main coils (cm), Length is the length of all coils and J_avg is the average current density (Amp/mm²)

Figure 2

Width and height of the Main coils #1 vs magnet length, warm bore diameter and current density. “Length” here is specified as the axial distance between superconducting coils. The “length” does not include cryostat.

Figure 3

Peak magnetic field and radial field in the Main coils #1 vs magnet length, bore diameter and current density.

Figure 4

Conductor length requirements

Figure 5

Stored energy vs length

Figure 6

Operating current vs. N-value