Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-body, Actively Shielded, 3 T MRI Magnets Wound Using MgB2 Strands for Liquid Cryogen Free Operation

Abstract

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB₂ strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB₂ short (1.37 m length) MRI magnet based on the performance parameters of existing MgB₂ wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

Keywords: Magnetic Resonance Imaging; Superconducting magnet; conduction cooling.

Fig. 1.

|B| map at winding engineering current density of 94.251 A/mm² (max. on-axis field = 3T, max. field in the winding = 6.54 T. (a) 2D view, (b) 3D view.

Fig. 2.

10 ppm homogeneity in DSV = 0.5 m (2D view, only a quarter of geometry shown, x-coordinate in axial direction, y-coordinate in radial direction)

Fig. 3.

4.2 K critical current and n-value of 2G MgB₂ strand (0.84 mm OD) used in the modeling.

Fig. 4.

Schematics of the mechanical design (x = axis of rotation). Blue – superconducting winding, red – copper former, brown – stainless steel cryostat (wall thickness = 1 cm), green – G10 fiber-glass epoxy mechanical support (1 cm thick). Magnet length including cryogenics l = 1.4715 m, cryostat OD = 1.9877 m, cryostat warm bore ID = 0.77 m, d = 3 cm, g = 15 cm.

Fig. 5.

Full geometry of the MRI magnet: Magnet length l = 1.4715 m, magnet OD = 1.9877 m, magnet ID = 0.77 m.

Fig. 6.

Von Mises stress in Cu former and winding.

Fig. 7.

Temperature maps in the MRI winding around 10K. (a) 3D map, (b) 2D map.