Key designs of a short-bore and cryogen-free high temperature superconducting magnet system for 14 T whole-body MRI

Abstract

A 14 T whole-body magnetic resonance imaging (MRI) magnet is designed with Bi2223 high temperature superconducting (HTS) wire. Owing to the large critical current and large critical tensile strength of the wire type HT-NX at ultra-high field, the magnet is optimized in a compact shape with length of 1.9 m and outer diameter of 1.3 m. It is in the form of stacked double pancakes (DPs) with total wire consumption of 455 km. The DPs with various inner and outer radii enable the electromagnetic design to obtain highly homogeneous field of 1.5 ppm over 400 mm diameter of spherical volume (DSV). Hoopstress by winding process, thermal contraction and Lorenz force is calculated and below the wire strength specification. Considerable reduction of screening current effect in MRI magnet application is proved by a calculation example. Field homogeneity as it is influenced by various systematic tolerances and random geometric tolerances is numerically estimated, for further design optimization and design of shimming system. Iron room for passive shielding with dimension of 5 × 5 × 10 m is found to be with wall thickness of around 30 cm to effectively contain the 5-Gauss field to the wall surface. Cooling strategy for cryogen-free technique is proposed and the magnet coils temperature is calculated as below 8 K based on the estimation of joint Ohmic heating power of 1.0 W. Quench protection adopts circuit subdivision and dumping resistors to restrict the maximum voltage below 1.9 kV. No-insulation technique for the quench protection is also discussed. Some other wire candidates like REBCO and Nb3Sn for development of such ultra-high field MRI magnets are commented.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…Examining the history of 7 T magnet technology shows that the first generation of passively shielded magnets (length = 336 cm) was replaced by the first generation of actively shielded 7 T magnets (length = 255 cm), which were then superseded by the next generation of actively shielded magnets (length = 270 cm, weight less than 25 t, minimum room size 65 m 2 ), demonstrating how developments in magnet technology can easily move towards smaller, lighter and more clinically oriented systems. Pioneers from the MR research and superconducting materials science fields have already taken bold steps into higher altitudes, designing superconducting magnets customised for 14 T whole-body MRI [21,22]. One high-temperature superconductor (HTS)based design has a very compact configuration (total weight: 15 t, length: 1.9 m, outer diameter, 1.3 m, total HTS wire consumption of 455 km, weight of the HTS wires: 8.4 t, warm bore 80 cm) [21].…”

“…Pioneers from the MR research and superconducting materials science fields have already taken bold steps into higher altitudes, designing superconducting magnets customised for 14 T whole-body MRI [21,22]. One high-temperature superconductor (HTS)based design has a very compact configuration (total weight: 15 t, length: 1.9 m, outer diameter, 1.3 m, total HTS wire consumption of 455 km, weight of the HTS wires: 8.4 t, warm bore 80 cm) [21]. This compact magnet design would be even smaller and lighter than current commercial 7 T…”

A vision of 14 T MR for fundamental and clinical science

“…The most important element of the system is undoubtedly the magnet. Following a lengthy consultation and evaluation process a high temperature superconducting (HTS) magnet was selected [ 22 ] to be manufactured by Neoscan Solutions GmbH (Magdeburg, DE). In [ 22 ] it was proposed to use a first generation HTS wire, the ceramic (and rare-earth-free) superconducting compound consisting of bismuth strontium calcium copper oxide (BSCCO, pronounced ‘bisko’) embedded within a silver-containing matrix, which has a critical field of 28 T. Stable magnets of 25 T have previously been constructed using BSCCO tape [ 23 ].…”

“…Following a lengthy consultation and evaluation process a high temperature superconducting (HTS) magnet was selected [ 22 ] to be manufactured by Neoscan Solutions GmbH (Magdeburg, DE). In [ 22 ] it was proposed to use a first generation HTS wire, the ceramic (and rare-earth-free) superconducting compound consisting of bismuth strontium calcium copper oxide (BSCCO, pronounced ‘bisko’) embedded within a silver-containing matrix, which has a critical field of 28 T. Stable magnets of 25 T have previously been constructed using BSCCO tape [ 23 ]. As compared to a magnet based on a low temperature superconductor (LTS), an HTS magnet has the following generic advantages: HTS wire has a high current-carrying capacity, enabling a compact and relatively light magnet design.…”

“…The major disadvantage is probably the higher price of such a system. A detailed description of the magnet design is to be found in [ 22 ]. For reference a comparison of the salient characteristics with those of some existing LTS magnets is given in Table 2 .…”