Abstract:
A cryostat comprises a cryogen vessel suspended within an outer vacuum container, the cryogen vessel is supported by an arrangement that includes at least one housing mounted on an exterior surface of the outer vacuum container and arranged to function as a floor mounting foot, for supporting weight of the cryogen vessel and the outer vacuum container, and at least two mounting points mounted within the housing(s). Each of at least two suspension elements (an upper suspension element and a lower suspension element) extends through a hole in the surface of the outer vacuum container, between the respective mounting point and a respective point on the cryogen vessel.

Description:
The present invention relates to arrangements for suspending an article within a vessel, and particularly relates to arrangements for suspending a cryogen vessel within an outer vacuum container. Such arrangements are useful in housing magnets for imaging systems such as magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) systems. The present invention, however, finds application in any applications which require a cryogen vessel to be suspended within a vacuum filled outer vessel. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
       FIG. 1  illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system. A number of coils of superconducting wire are wound onto a former  1 . The resulting assembly is housed inside a cryogen vessel  2  which is at least partly filled with a liquid cryogen  2   a  at its boiling point. The coils are thereby held at a temperature below their critical point. 
     Also illustrated in  FIG. 1  are an outer vacuum container  4  and thermal shield  3 . As is well known, these serve to thermally isolate the cryogen vessel  2 , typically containing a liquid cryogen  2   a , from the surrounding atmosphere. Solid insulation  5  may be placed inside the space between the outer vacuum container  4  and the thermal shield  3 . A central bore  4   a  is provided, of a certain dimension to allow access for a patient or other subject to be imaged. 
     Conventionally, a number of supporting elements  7  are connected between the cryogen vessel  2  and the outer vacuum container  4  to bear the weight of the cryogen vessel. These may be tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. The elements should have a very low thermal conductivity. This is important in order to minimise heat influx from the outer vacuum container  4 , which is typically at ambient temperature, to the cryogen vessel  2 . The suspension elements typically pass through holes in the thermal shield  3 . Similar, or alternative, suspension arrangements may be provided to retain the thermal shield  3 . 
     Copending United Kingdom patent application GB2426545 describes a particularly advantageous arrangement of support straps for use in such systems. 
     Typically, a floor mounted system will require mounting feet  6  or similar, attached to the outer vacuum container to support the weight of the system as a whole. 
     A disadvantage of the conventional arrangement results from the fact that the outer vacuum container  4  must carry all of the forces required to support the cryogen vessel  2  and its contents. The outer vacuum container  4  is the largest component in the illustrated system, and must be made of thick material in order to support such heavy loading. 
     The present invention aims to provide an alternative support arrangement, wherein the outer vacuum container need not be of thick material, since it does not bear the forces required to support the cryogen vessel, or other supported equipment.
         According to the present invention, therefore, suspension of the cryogen vessel is provided by tensile bands, tensile rods, straps, compression struts or any known element suitable for the purpose. In particular, these elements are connected between the cryogen vessel  2  and a housing provided on the outer surface of the outer vacuum container.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, and further, aims, advantages and characteristics of the present invention will become more apparent from the following description of certain embodiments thereof, along with the accompanying drawings, wherein: 
         FIG. 1  illustrates a cross-sectional view of a conventional solenoidal magnet arrangement for a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system; 
         FIG. 2  schematically illustrates the principle of the present invention; and 
         FIG. 3  shows a cutaway view of certain components of a suspension arrangement according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  schematically illustrates the principle of the present invention, wherein a housing is provided in the form of a foot  20 , providing floor mounting for the system, and supporting at least some of the weight of cryogen vessel and the outer vacuum container. In some embodiments, a single housing (foot) is provided along each side of the outer vacuum container  4 . In alternative embodiments, a plurality of housings (feet) may be placed at selected locations along each side of the outer vacuum container  4 . The locations will be selected so as to provide an acceptable combination of floor support for the system as a whole, and support for the cryogen vessel  2  within the outer vacuum container  4 . 
     As shown in  FIG. 2 , the support arrangement of the present invention typically involves a plurality of upper support elements  22  and a plurality of lower support elements  23 . The upper support elements  22  connect between a relatively low point  24  on the cryogen vessel and a relatively high point  26  in the housing  20 . The lower support elements  23  connect between a relatively high point  28  on the cryogen vessel and a relatively low point  30  in the housing  20 . 
     Any of the upper support elements  22  and lower support elements  23  may be arranged to be held under tension, or in compression, so as to prevent relative movement of the cryogen vessel and the outer vacuum container and to support at least some of the weight of the cryogen vessel. 
     The various support elements are arranged in calculated angular configurations, determined according to methods known to those skilled in the art to provide an acceptable combination of support and resistance to translation and rotation in all possible directions. Advantageously, the number of support elements used is kept to a minimum, since each support element  22 ,  23  represents a thermal conduction path from the relatively high-temperature outer vacuum container  4  to the much colder cryogen vessel  2 . 
     In certain embodiments, it may be found advantageous to locate suspension elements in planar configurations, with additional components provided as required to provide the required resistance to translation and rotation. 
     In preferred embodiments of the present invention, the suspension elements  22 ,  23  are each arranged to be as long as is practical, to increase their thermal resistance and tot hereby reduce the thermal influx carried by each suspension element. In practice, this will mean that lower suspension elements  23  are mounted  28  high on the cryogen vessel and are mounted  30  low on the housing  20 , while upper suspension elements  22  are mounted  24  low on the cryogen vessel and are mounted  26  high on the housing  20 . 
     While the arrangement shown in  FIG. 2  illustrates only the suspension elements required for suspending the cryogen vessel  2  within the outer vacuum container  4 , similar arrangements are preferably provided for supporting thermal shield(s)  3 . The shield(s)  3  weigh much less than the cryogen vessel  2  and its contents, so the advantages gained from the arrangement of the invention by avoiding load bearing on the outer vacuum container  4  are not so significant when applied to the shield(s). However, the arrangement of the present invention may allow longer, more thermally resistant suspension elements  22 ,  23  to be used, limiting the thermal influx to the shield(s). In addition, it may be simpler to arrange support of the shield(s)  3  according to the arrangement of the present invention in cases where the cryogen vessel  2  is supported according to the present invention, rather than providing two different types of suspension arrangement within a single system. Optionally, the shield(s) may be supported by the same suspension elements  22 ,  23  which support the cryogen vessel  2 . 
       FIG. 3  illustrates a partial cut-away of a housing  20  according to an embodiment of the present invention. The housing is arranged to function as a floor mounting foot, for supporting at least some of the weight of the cryogen vessel and the outer vacuum container. As illustrated, a cryogen vessel  2  is provided within an outer vacuum container  4 . Two thermal radiation shields  3   a ,  3   b  are provided and are located intermediate the outer vacuum container  4  and the cryogen vessel  2 . Housing  20  is provided on the outer surface of the outer vacuum container  4 . A hole  32  is provided in the wall of the outer vacuum container, enabling support elements such as tensile band  23   a  or rod  23   b  to pass from the inside of the outer vacuum container  4  to the inside of housing  20 . Since the housing  20  will be evacuated, it must be constructed and sealed to the outer vacuum container in a sufficiently airtight manner to maintain a hard vacuum. 
     In the illustrated embodiment, rod support element  23   b  is attached to the thermal shields  3   a ,  3   b , while tensile support band  23   a  is attached to the cryogen vessel  2 . Holes must be provided in each thermal radiation shield to allow support elements such as  23   a  to support the cryogen vessel. 
     Mounting point  30  provides mechanical support for rod support element  23   b . Other types of support member such as band  23   a  will require alternative mounting points, as appropriate. Mounting point  26 , for upper support members  22 , is illustrated, although no upper support members are shown in the figure in the interests of clarity. 
     According to the present invention, a number of housings  20  are provided, and support members are arranged such that the cryogen vessel may be supported by support members attached to a housing  20  mounted on the surface of the outer vacuum container. This allows the outer vacuum container itself to be constructed of thin, light material, since it does not need to bear the forces required to mechanically support the cryogen vessel. The housings  20  are likely constructed of thick, strong material but are relatively inexpensive and easy to fabricate, as compared to a structurally reinforced outer vacuum container, as in the conventional solution. 
     The present invention provides an advantageous thermally resistive suspension for a cryogen vessel and/or thermal shield(s) within an outer vacuum container, such as is used for housing superconducting magnets for nuclear magnetic resonance or magnetic resonance imaging systems. 
     According to the invention, loads supporting the cryogen vessel and/or thermal shield(s) are borne by a housing mounted on the outer surface of the outer vacuum container. The housing is arranged to function also as a floor mounting foot, a component which would be necessary in any case. This enables the outer vacuum container to be manufactured of sufficient strength to bear its own weight and external atmospheric pressure, where appropriate, without needing sufficient strength to support the cryogen vessel and/or thermal shield(s). In conventional arrangements, it has been found necessary to strengthen the outer vacuum container by way of thick walls, or strengthening arrangements such as buttresses, or doubler plates.