The present invention relates generally to the cooling of superconducting magnets and more particularly to an apparatus and method for cooling a superconducting magnetic assembly such as a magnetic resonance imaging (MRI) assembly.
Various systems employ superconducting magnets to generate a strong, uniform magnetic field within which a subject, or as in the case of an MRI system a patient, is placed. Magnetic gradient coils and radio-frequency transmit and receive coils then influence gyromagnetic materials in the subject to provoke signals that can be used to form useful images. Superconducting magnetic assemblies or systems that use such coils include MRI systems, spectroscopy systems, magnetic energy storage systems, and superconducting generators.
FIG. 1 illustrates a partial cut-away view of a typical MRI system 100 of the related art which may include a superconducting magnet 122 embedded in a coil former 120, a thermal shield 114, gradient coils 108, an RF coil 106, a bucking coil 124, and an RF shield within a housing 102. The housing 102 includes a central bore 104 for the object (e.g., patient) to be placed therein.
The superconducting magnet 122 and bucking coil 124 are typically immersed in a cryostat bath (e.g., helium vessel) 116 containing a cryogen 118 creating a pool-boiling mode or state. The MRI system 100 further includes the thermal shield 114 and a vacuum region 112 created by a vacuum vessel 110 that further insulates the superconducting magnet 122 from the environment during operation. The superconducting magnet 122 also has a coil support structure (e.g., coil former 120) to support a coil winding, embedded within the helium vessel 118 for cooling. The helium vessel 118 is typically a pressure vessel located within the vacuum vessel 112 for thermal isolation and typically contains liquid helium to provide cooling for the superconducting magnet 122 so as to maintain a temperature of around 4.2 Kelvin for superconducting operation.
A significant cost for any system that employs superconducting magnets is for the provision of the cryogen bath. Helium, or a similar cryogen (e.g., neon), is needed both for initial start-up and operation of the superconducting magnet, and for keeping the magnet in a pool-boiling state. While thermodynamically efficient, a full helium bath, when employed in a superconducting magnet assembly, requires a relatively large volume of helium on the order of approximately 1,500 to 2,000 liters. Additionally, during shipping of an MRI system several hundreds of liters (e.g., 600-800 liters) of helium typically boil off from the system. The boiled-off cryogen must be replenished prior to operating the system. Helium is expensive per unit volume, is not always readily obtainable, and its cost is increasing.
A coldhead, or cryocooler, 130 supplies cooling power at cryogenic temperatures. In the case of a dual-stage coldhead, cooling power is provided directly at the cooling stages of the cooler. Typically, most cryocoolers used today in the MRI industry mainly work on the GM cooling principle. In an MRI application a cryocooler typically provides 50 W at 40 degrees Kelvin at the first stage and 1 to 1.5 W at 4 degrees Kelvin at the second stage.
Current coldhead configurations and designs have several drawbacks. Due to its proximity, the cryocooler is subject to a magnetic field imposed on it from the superconducting magnet of the MRI system. In order to prevent ghosting effects that may show up as artifacts during image acquisition, the cryocooler cold stages (specifically the moving piston within the cryocooler) must be magnetically shielded. This has proven to be costly. In addition, the motor drive part(s) of the cryocooler requires thick metallic magnetic shielding to ensure that the cryocooler can work properly without failure. This too is costly. The cryocooler typically is operated in a vertical, near-vertical, or horizontal orientation. While a vertical orientation is preferred due to cryocooler durability issues, this orientation causes room height issues with the design. Because the length of the cryocooler is substantial, when removing the cryocooler for maintenance (e.g., changeout) from its berth the length of the cryocooler must be added to the ceiling height above the MRI system to allow for servicing removal. As a result, the ceiling height is significantly higher for MRI systems having cryocoolers in the vertical, or near-vertical orientation.
Accordingly, there is an ongoing need for improving the overall design of superconducting magnet assemblies including the areas of maintenance and/or cooling.