Patent Application
20060266053
The present invention relates to a thermal shield and, more particularly, to a thermal shield for use with a superconducting magnet of, for example, a magnetic resonance imaging (MRI) system.
MRI systems are commonly used in medical imaging applications since MRI scans produce detailed images of soft tissues. MRI systems produce images by excitation of selected dipoles within a subject and receiving magnetic resonance signals emanating from the dipoles. In order to produce the excitation of selected dipoles within the subject, a powerful and uniform magnetic field is required. The powerful and uniform magnetic field may be produced by superconducting magnet coils.
Superconducting magnetic coils operate under cryogenic temperatures and therefore require robust cooling systems. The cooling systems typically require a cryogen or refrigerant, for example, liquid helium, in order to achieve cryogenic temperatures. However, cryogens are not abundant and often add significant cost to a cryostat portion of an MRI system. Thus, it is desirable to thermally isolate the superconducting magnet coils to the greatest extent possible to minimize cooling requirements.
In order to thermally isolate the superconducting magnet coils, thermal shields have been disposed around the superconducting magnet coils. Aluminum alloys have typically been employed for the thermal shields. Aluminum is considered advantageous for having properties including a low density and a light weight, while maintaining good strength and thermal conductivity characteristics. However, a disadvantage of aluminum is that aluminum has a high electrical conductivity. High electrical conductivity creates large mechanical stresses on the thermal shields when the magnet becomes normal or quenched. Additionally, aluminum thermal shields suffer from field instability that may reduce a quality of images obtained by the MRI system.
Two types of field instability, gradient and vibration induced field instability affect aluminum thermal shields by producing eddy currents in the thermal shield that reduce image quality. Vibration induced field instability is caused by vibration from a cooling engine (coldhead), environmental excitation and gradient pulse. Such vibration produces eddy currents that reduce image quality and are complex and costly to avoid. Gradient field instability is a result of magnetic fields generated during gradient pulse that may cause image artifacts.
Thus, it is desirable to design a thermal shield that improves upon existing art.
Exemplary embodiments of the invention include a thermal shield for a superconducting magnet. The thermal shield for a superconducting magnet includes a shield body having an annular shape. The shield body includes a material having a thermal conductivity greater than about 1000 W/m·K at about 70K.
Further exemplary embodiments of the invention include a shielding system for shielding and cooling a superconducting magnet coil via a cryogen. The shielding system includes a cryogen vessel, a thermal shield, and a vacuum vessel. The cryogen vessel contains the cryogen and is disposed proximate to the superconducting magnet coil to enclose the superconducting magnet coil. The thermal shield includes a shield body comprising a material having a thermal conductivity greater than about 1000 W/m·K at about 70K. The thermal shield is disposed proximate to the cryogen vessel to enclose the cryogen vessel. The vacuum vessel is disposed proximate to the thermal shield to enclose the thermal shield.
Still further exemplary embodiments of the invention include a thermal shield for a superconducting magnet. The thermal shield includes a shield body having an annular shape and a cladding. The cladding includes a material having a thermal conductivity greater than about 1000 W/m·K at about 70K. The cladding is disposed at a surface of the shield body.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
The superconducting magnet coil 12 may be any suitable superconducting coil known in the art. The helium vessel 14 is disposed proximate to the superconducting magnet coil 12 to enclose the superconducting magnet coil 12. The helium vessel 14 is filled with a cryogenic coolant, for example, liquid helium. The cryogenic coolant provides cooling to the superconducting magnet coil 12 to allow the superconducting magnet coil 12 to achieve superconductivity at cryogenic temperatures. In this exemplary embodiment, the helium vessel 14 has a shape of a hollow rectangular prism extended to form an annular shape, however, any suitable shape is envisioned.
The low temperature thermal shield 16 is disposed proximate to the helium vessel 14 to enclose the helium vessel 14. The high temperature thermal shield 18 is disposed proximate to the low temperature thermal shield 16 to enclose the low temperature thermal shield 16. The low and high temperature thermal shields 16 and 18 function to thermally isolate the superconducting magnet coil 12 to reduce the cooling requirements on the cryogenic coolant. A shape of the low and high temperature thermal shields 16 and 18 is substantially similar to that of the helium vessel 14. Although
The low and high temperature thermal shields 16 and 18 are each in thermal contact with a portion of a coldhead sleeve 24. In an exemplary embodiment, the low and high temperature thermal shields 16 and 18 may each be in physical contact with the coldhead sleeve 24. Alternatively, a thermal link 26 may provide thermal contact between the low and high temperature thermal shields 16 and 18 and the coldhead sleeve 24.
The coldhead sleeve 24 provides a means for cooling the low and high temperature thermal shields 16 and 18. In an exemplary embodiment, a cooling engine (not shown) provides cooling to the low and high temperature thermal shields 16 and 18 to cool the low and high temperature thermal shields 16 and 18 to a temperature of about 45K to about 70K, which varies in response to a conductance of a thermal shield. The cooling engine may be, for example, a stirling or pulse tube type, but is not limited to any particular engine. The shielding system 10 may alternatively include a plurality of coldhead sleeves 24, as shown in
The vacuum vessel 20 is disposed proximate to the high temperature thermal shield 18 to enclose the high temperature thermal shield 18 and maintain interior portions of the vacuum vessel 20 substantially at a vacuum with respect to regions exterior to the vacuum vessel 20. A shape of the vacuum vessel 20 is substantially similar to that of the low and high temperature thermal shields 16 and 18 and the helium vessel 14.
In an exemplary embodiment, a penetration 28 passes through the vacuum vessel 20, the high temperature thermal shield 18, the low temperature thermal shield 16, and the helium vessel 14. The penetration 28 provides a conduit to pass wires for electrical communication with the superconducting magnet coil 12 or instrumentation to monitor a characteristic of the superconducting magnet coil 12. The penetration 28 may include thermal links 26 providing thermal communication with the low and high temperature thermal shields 16 and 18.
In an exemplary embodiment, the low and high temperature thermal shields 16 and 18 are each a composite thermal shield 40 (see
In an exemplary embodiment, the first and second sidewalls 42 and 44 each include sub-members 50. Although FIG. shows two sub-members 50 for each of the first and second sidewalls 42 and 44, it should be understood that additional sub-members 50 may be employed. Each sub-member 50 forms an opposite end portion of the first and second sidewalls 42 and 44. Additionally, adjacent ends of each sub-member 50 are joined by a center support ring 54. The center support ring 54 extends around an interior portion of the composite thermal shield 40 to seal a joint between each sub-member 50.
Joints between each of the first to fourth sidewalls 42 to 48 are sealed by a corner support ring 58. Each corner support ring 58 extends around an interior portion of the composite thermal shield 40 to seal the joints between each of the first to fourth sidewalls 42 to 48.
The composite material used to make each of the first and fourth sidewalls 42 and 48 is chosen for its low density, high thermal conductivity and low electrical conductivity relative to aluminum and aluminum alloys. For example, thermal pyrolytic graphite (TPG) and pyrolitic boron nitride (PBN) may be used. The composite material is selected to have a thermal conductivity greater than about 1000 W/m·K at about 70K. Since aluminum has a thermal conductivity of about 300 W/m·K, thickness of a thermal shield may be reduced by three times and still achieve similar thermal performance to an aluminum thermal shield. The composite material is selected to have an electrical resistivity in a range of about 3 10-6 Ωm to about 3 10-3 Ωm. Since the composite material has a high electrical resistivity as compared to a conventional thermal shield, eddy currents induced in a thermal shield having the high electrical resistivity are negligible. Thus, vibration and gradient coil induced field instability are negligible. The composite material is selected to have density less than about 2.4 g/cm3 or about 10% less than aluminum, thereby decreasing weight of the thermal shield.
A surface of the composite thermal shield 40 that faces the helium vessel 14 must have emissivity control to reduce heat radiation from the composite thermal shield 40 to the helium vessel 14. To achieve the emissivity control, high conductive aluminum tape 70 may be applied to an inner surface of the composite thermal shield 40.
As stated above, any combination of composite thermal shields 40 and conventional thermal shields may be employed. Additionally, a conventional thermal shield may be disposed in contact and enclosing a composite thermal shield 40. Alternatively, a conventional thermal shield may be clad with TPG or PBN to provide improved performance. Furthermore, a thickness of TPG or PBN cladding may be varied to optimize thermal gradient and average shield temperature reductions.
In addition, while the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
