Thermal shield of the zero-boil-off cryostat for a 1.5T magnetic resonance imaging magnet

Abstract

The image quality in a MRI magnet greatly depends on the mechanical, thermal and electromagnetic performance of the thermal shield in presence of any time varying field. A detailed analysis on the mechanical resonance frequency, temperature distribution and the eddy current distribution on a thermal shield of a zero-boil-off cryostat for 1.5T MRI magnet system is presented. Five combination of aluminum alloys using AA1100, AA5083-O, AA6061-T6 are considered for the analysis. The mechanical natural resonance frequency of shield is studied for its various length and the radial thickness varying in the range of 1–10 mm of the thermal shield. The resonance frequency of the AA1100 bore tube is found to varying in the range of 1.9–5.2 kHz. The overall temperature non-uniformity of 8.5 K is achieved in the binary shield made of AA1100 for the outer shell and the bore tube and AA6061-T6 for the endplates. The temperature non-uniformity achieved in the imaging length of 45 cm of the bore tube is 0.034 K. The eddy current induced along the imaging length of the bore tube is 1.7% of the peak eddy current onto the shield during a quench in the magnet. The highest eddy current of ~2490 A/cm2 is generated on the bore tube for the binary shield made of AA1100 for the outer shell and the bore tube and AA5083-O for the endplates.