FIELD STABILITY AND EXTERNAL INTERFERENCE SHIELDING OF 1.5 T CLINICAL MRI SCANNER

Abstract

Ministry of Electronics and Information Technology (MeitY), Government of India, has initiated a project to develop a 1.5 T superconducting MRI scanner in India. Inter University Accelerator Centre (IUAC), New Delhi, is primarily responsible for the development of a 1.5 T superconducting magnet and an ever-cooled cryostat for the MRI scanner. An actively shielded 1.5 T superconducting MRI magnet has been designed for a whole-body clinical scanner. The MRI magnet generates a 1.5 T magnetic field in a 45 cm Diametrical Spherical Volume (DSV) with ±5.5 ppm homogeneity. The magnet has been wound by using braided polyethylene terephthalate (PET) insulated wire in channel (WIC) Niobium Titanium (NbTi) (low temperature superconductor) wire on the bobbin. MRI magnet operates at 4.2 K (liquid helium) temperature, and its operating current is 450 A. This thesis presents a comprehensive study focused on advancing the development and understanding of critical components in a 1.5 T actively shielded superconducting MRI magnet. The research addresses both theoretical and practical aspects essential for optimizing MRI performance, with a particular emphasis on superconducting joints, Persistent Current Switch (PCS), Electromagnetic Interference Shielding (EIS) coils, and cryogenic components. The research provides a detailed analysis of various superconducting joint making techniques, discussing their advantages and disadvantages from an industrial perspective. Additionally, the study explores the solder matrix replacement method for different joints, including Cu-NbTi to Cu-NbTi and CuNi-NbTi to Cu-NbTi conductors. The reasons for superconducting joint failures are thoroughly analyzed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) techniques, offering insights into improving joint reliability A key achievement of this study is the development of a 4 K insert, which has enabled precise characterization of superconducting joints an essential component for ensuring the stability and efficiency of MRI magnets. The insert has demonstrated the ability to achieve near- x perfect superconducting conditions, with measured electrical resistance as low as 8 × 10⁻¹⁵ Ω at zero field between various conductor pairs. The research has also contributed significantly to the development and testing of a prototype PCS designed for a whole-body MRI magnet. This system is crucial for maintaining the magnet’s stability and operational efficiency. Through a comparative analysis of temperature profiles and switching behaviors across various thermo-foil heaters, the study identified optimal operating conditions. The design of an EIS coil involves careful consideration of various factors, including the coupling factor, mutual inductance, turns ratio, geometrical factors, and self-inductance. The design process includes defining specifications, initial design choices, simulation and optimization, final development, and rigorous testing to ensure the final coil design delivers accurate and reliable measurements. Additionally, the study explored the EIS coil's performance during quench scenarios, offering valuable insights for enhancing the safety and reliability of MRI systems under various operational conditions. Furthermore, the research extends to the characterization of power diodes operating at cryogenic temperatures, essential for their application in MRI systems. Detailed analysis of the V-I characteristics of power diodes, including HFR 120 and BYV 28–200, revealed significant behavioral changes at cryogenic temperatures (77 K and 4.2 K) compared to room temperature In summary, this thesis provides a rigorous and detailed exploration of the development, optimization, and testing of essential components for a 1.5 T MRI magnet.