Effect of Thermal Strain, Induced by Cryogenic Cooling, on a High Homogeneity Superconducting Magnet for MRI Applications

The heart of the modern whole body MRI scanners is a superconducting magnet producing the required magnetic field with high homogeneity. The superconducting magnet operates at 4.2 K (-268.95 °C) and thus is kept dipped in liquid helium to maintain its superconducting state. This cool down of the multi-coil magnet from room temperature to the LHe temperature generates thermal strains in the magnet structure which deforms the magnet. These deformations are found to affect the final magnetic field homogeneity and introduce artifacts in the MR images. We report our results on studies on the stresses induced in the bobbin and the coils consequent upon cooling the magnet to 4.2 K. The maximum von Mises stress in the coils is calculated to be 31.3 MPa while in the bobbin it comes out to be 60 MPa. We find that the cool down causes a relative movement of the coils which in turn degrades the field homogeneity from 5.5 ppm to 278 ppm. The centre field is found to increase by 60 G is caused by a reduction in the overall cross-section of the coils. The change in homogeneity is also analysed in terms of Legendre polynomials where we found that the relative displacements of the coils introduce odd order terms to the polynomial expansion terms which were not present in the original design.