Shielding of low-frequency external magnetic interferences in a 1.5T whole-body MRI scanner

An actively shielded whole-body MRI magnets are susceptible to low-frequency electromagnetic interferences generated by different sources, such as moving metal, subways vehicles passing by, electrical wires in walls, and power lines. The low-frequency fluctuations in the ambience magnetic field may cause a significant change in the B0 field and its resonance frequency. The fluctuation in the ambient field varies between 10 nT to 10 nT depending on the solar activity or moving metal objects in the vicinity of the MRI magnet. The imaging volume of the MRI magnet needs to be protected from any such external magnetic interferences. Various techniques are available for shielding such external disturbances in the presence of the low magnetic field, but it is difficult to shield the fluctuation using iron or metal shield at the high field region. These external magnetic disturbances significantly impact the homogeneity of the MRI magnet. In such cases, the best solution is usually to shield the primary coils by shorting one of the superconducting coils of the system. In this paper, a set of superconducting coils known as shielding coil or screening coil is designed to minimize the interference of any external magnetic disturbances for a 1.5 T whole-body superconducting MRI magnet by optimizing the screening factor, mutual geometrical factor, coupling factor, and overall homogeneity. These coils are self-activated and work passively in the MRI system. This paper briefly discusses the design and optimization of the superconducting shielding coil for a 1.5T whole-body MRI magnet system. (author)