Abstract:
An assembly for thermal insulation of an MR magnet system during such a transport has a container for accommodating an MR magnet, the container being equipped with thermal insulation, and the container has an opening for accommodating a cooling unit. The assembly further has a protective cap, such that the opening is sealed in a reversible manner by the protective cap, and the protective cap is likewise equipped with thermal insulation.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an assembly for thermal insulation of a magnet in a magnetic resonance (MR) apparatus, in particular for thermal insulation at times during which the MR magnet is not used for MR tomography, e.g. during transportation thereof. 
     Description of the Prior Art 
     MR magnets in MR tomography apparatuses are typically designed as superconducting coil magnets. These are cooled by a coolant, typically helium, to superconducting temperatures. The coolant must be continuously cooled during operation, for which an electrically powered cooling unit (referred to as a “coldhead”) is provided. 
     MR tomography apparatuses can be transported to the target location with MR magnets that have not been cooled, and then first cooled to superconducting temperatures on site. Due to the time and the special equipment required for this, such on-site ramping up is complex and disadvantageous for the operator at the target location. For this reason, MR tomography apparatuses are often transported to the target location with MR magnets therein that are already cooled to superconducting temperatures. 
     If MR magnets are transported in an already-cooled state, a problem arises due to the cooling unit not operating during the transport. For this reason, it is impossible to prevent heating of the coolant during the transportation. Typical coolants (typically helium) vaporize during this heating, so that, depending on the extent of the heating, a more or less high loss of coolant occurs as a result of vaporization. The coolant losses can amount to as much as 50 liters/day. 
     The coolant reserve in an MR tomography apparatus is typically in the range of 1,200 liters. In order to prevent a complete evaporation of the coolant reserve, the transportation and installation at the target destination therefore must be completed within a maximum time period that must be strictly adhered to. Furthermore, the evaporated coolant must be replaced at the target destination, which results in additional logistical expenditures. Moreover, coolant, in particular helium, is relatively expensive, and thus a loss of coolant represents financial losses. For this reason, a reduction in the heating, and the thus resulting coolant losses, is desirable during such transport. 
     A cooling assembly having a cryostat is known from U.S. 2010/0041976 A1, in which inner and outer vacuum chambers are provided for the thermal insulation. The cryostat represents a heat bridge between the cooled interior and the environment. A ventilation structure enables the escape of vaporized coolant but the vaporized coolant is conducted by the ventilation structure such that the vessels and foils that are to be insulated are cooled by this vaporized coolant. By cooling the insulating structures, the thermal conductivity is reduced. 
     From U.S. 2012/0306492 A1 and U.S. 2012/0309630 A1, further assemblies are known for thermal insulation of cryostats. The assemblies are each based on a double-walled MR magnet container construction. The outer wall has an extended effective length for the thermal insulation. The inner wall is designed as a telescopic mechanism. 
     An MR tomography apparatus having superconducting magnets is known from U.S. Pat. No. 7,170,377 B2, with which reduced coolant losses occur. Vaporized coolant, normally helium, is cooled by a cooling unit, and re-condensed. The re-condensed coolant is returned to the cryostat. The cryostat is insulated by a radiation thermal shield and an additional super-insulating foil that acts against heating. The cooling unit, or the cryostat opening, in which it is disposed, is not likewise doubly insulated in its entirety, and therefore represents a relatively large heat bridge between the cooled interior and the environment. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to improve the thermal insulation for an MR magnet system. In particular, the thermal insulation is to be improved at times during which the MR magnet system is not in operation, e.g. during transportation. 
     This object is achieved in accordance with the invention by an assembly for thermal insulation of an MR magnet, having a container for accommodating an MR magnet, wherein the container is equipped with thermal insulation that insulates the interior of the container from its exterior environment, and wherein the container has an opening for receiving a cooling unit, and wherein the assembly furthermore has a protective cap that seals the opening in a manner that can be reversed, i.e. the degree of sealing can be lessened so as to depart from a complete seal by the protective cap, and the protective cap is likewise equipped with thermal insulation. 
     In an embodiment, the protective cap is placed in position without the cooling unit disposed in the opening. In this manner, particularly with large or impractically formed cooling units, e.g. those that extend outwardly to a large degree, an improved thermal insulation can be obtained, while simultaneously keeping the dimensions of the assembly small. The small dimensions are advantageous for the transportation of the assembly, for example. 
     In a further embodiment, the protective cap is placed on top of the cooling unit. In this manner, a particularly simple thermal insulation of the assembly is obtained, because the protective cap need only be placed in position, without having to remove and reinstall functional elements of the MR magnet system, in particular the cooling unit. 
     In another embodiment, the protective cap is placed over an intermediate sealing agent, which seals the space enclosed between the protective cap and the container against the environment outside of the assembly. The sealing agent also serves to reduce the transfer of heat from the external environment. In a particularly advantageous manner, the sealing agent enables the evacuation of the space enclosed between the protective cap and the container, by means of which, due to the low thermal conductivity of a vacuum, a particularly effective thermal insulation is obtained. 
     In a further embodiment, a venting opening is provided, which is connected by a pressure valve to a cooling system disposed in the interior of the container. The cooling system is the device for cooling by a coolant, typically by liquid helium. The venting opening allows a discharge of coolant gas, which is necessary for preventing a buildup of pressure within the assembly. The pressure valve limits the possible coolant pressure within the assembly thereby to a predefined maximal threshold value, above which it opens and allows coolant gas to escape through the venting opening. 
     In another embodiment, the venting opening is open in the space enclosed between the protective cap and the container. As a result, excess coolant gas does not escape into the environment, and contribute, due to its low temperature in relation to the temperature in the environment, to cooling of the protective cap and the components disposed therein, and thus connected thereto. As a result, the temperature gradient in the assembly components, which are in thermal contact with the interior of the container, is advantageously reduced, and thus the heat input is limited. Furthermore, these components can be made substantially of materials that have a thermal conductivity that decreases at reduced temperatures, such that the heat input is further reduced as a result. Both effects thus increase the thermal insulation of the assembly. 
     In a further embodiment, the protective cap and the container are designed such that coolant escaping through the venting opening in a gaseous state is able to escape at a reduced flow rate from the space enclosed between the protective cap and the container into the environment outside the assembly. As a result, the possible coolant gas pressure under the protective cap is limited in an advantageous manner, while at the same time a certain dwell time of the coolant gas under the protective cap remains ensured. In particular, by a sealing device designed in this manner, the necessity for an additional pressure valve between the environment and the space within the protective cap is eliminated. 
     As noted above, a seal is disposed between the protective cap and the container. In a further embodiment, this seal is designed such that coolant in the gaseous state is able to flow at a predefined flow rate through the seal. The flow rate is controlled in a suitable manner thereby, potentially in connection with the seal compacting pressure produced by the cap, with which the seal is compressed between the protective cap and the seal, with respect to the dwell time and the maximum pressure of the coolant gas beneath the protective cap. 
     In a further embodiment, the thermal insulation of the protective cap is formed as a vacuum insulation and/or as a multi-layered insulating foil. Thermal insulation thus is also provided for embodiments in which the space between the protective cap and the container is not evacuated for the purpose of thermal insulation. 
     In another embodiment, the venting opening is opened to the environment outside the assembly, and is connected by a venting pipe with the cooling system disposed in the interior of the container. Necessary venting is thereby provided for the excess coolant gas resulting from heating. At the same time, the pipe can be relatively small, in comparison to the quench pipes necessary for the operation of an MR tomography apparatus, i.e. they can exhibit a small cross-section, because only relatively small quantities of coolant gas need to be able to escape. As a result of the small size, a low thermal conductivity of the venting pipes, in particular in comparison with quench pipes, is ensured. 
     In another embodiment, the venting pipes are made of a material having a thermal conductivity that is equal to or lower than that of aluminum. 
     In another embodiment, a vacuum is generated in the space enclosed between the protective cap and the container. A particularly effective thermal insulation is thereby ensured, due to the low thermal conductivity of such a vacuum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section through a portion of an MR magnet container with a cooling unit according to the prior art. 
         FIG. 2  is a section through a portion of an MR magnet container with a cooling unit and protective cap in accordance with the invention. 
         FIG. 3  is a section through a portion of an MR magnet container in accordance with the invention, without a cooling unit and with a protective cap. 
         FIG. 4  is a section through a portion of an MR magnet container with a cooling unit and interior cooled with a protective cap in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An assembly for thermal insulation of MR magnets is depicted in  FIG. 1  in the form of a cutaway view. The MR magnets are disposed on a spool carrier  27 , and designed as coils. At least some of the coils are superconducting. Both shielding coils  26  and base field coils  28  and  29  are depicted. In order to cool the superconducting coils to the necessary low temperatures, these coils are surrounded by a coolant, typically helium. The coolant is located, together with the coils, in a coolant container  30 . 
     Because the coils are supplied with energy during the operation of the MR tomography apparatus, heat is generated during the operation. This causes a heating of the coolant in the coolant container  30 . In order to discharge the heat, and to maintain the necessary low temperatures, a cooling unit  23 , a so-called “coldhead” is provided. The cooling unit  23  serves to generate cooling energy, or to discharge heat from the coolant container  30  to the exterior. It can be designed in a two-step manner, and have a conventional structure. Coolant gas resulting from heating the coolant can be discharged through venting openings  20 ,  21  into the environment, in order to prevent excessive pressure in the coolant container  30 . The venting openings  20 ,  21  are sufficiently large enough to ensure a high enough flow rate in the case of an unintentional, abrupt occurrence of large quantities of coolant gas. An unintentional abrupt occurrence of large quantities of coolant gas in this manner can be triggered when the superconducting temperature is exceeded in one of the superconducting coils, which is referred to as a quench, for which reason the venting openings  20 ,  21  are also referred to as quench pipes. 
     The coolant container  30  is encompassed by a thermal insulation in the design of an actively cooled thermal shield  25 . Optionally, an additional, further shielding foil can be provided between the thermal shield  25  and the outer wall of the container  24 . The screening foil can be of a conventional type, e.g. a multi-layered insulating foil (MLI: Multi-Layer-Insulation) can be used. As a further thermal insulation, the container in which the coolant container  30 , including the MR coils, is disposed, is designed as a vacuum container  24 . It has a double wall in which a vacuum  12  is generated. The container in which the MR coils are disposed encompasses the bore  31  in which an object under examination, or a patient to be examined, respectively, is located during the MR imaging. Moreover, only the upper half of the cross-section of the vacuum container  24 , coolant container  30  and coil carrier  27 , including the MR coils, is depicted in the image. The container  24 , including the components disposed therein, encompasses the bore  31  in a continuous manner, from all sides, and is, in this regard, disposed cylindrically about the bore  31  in the conventional manner. The shape of the container  24 , including the components disposed therein, is not, however, significant with respect to the invention explained below. 
     A cryostat opening  15  is provided in the container  24 , through which the cooling unit  23  is disposed, such that it can be brought into contact with the coolant container  30 , or the coolant located therein, respectively. The cryostat opening  15  thus serves for the discharge of heat from the coolant container  30  to the environment, and thus represents a heat bridge, or gap, respectively, in the thermal insulation, through the container  24 . The cooling unit  23  is installed in a maintenance assembly  22 . Typically, the cooling unit  23  is powered by electricity, in order to bring about the discharge of heat from the coolant container  30  into the substantially warmer environment. It is apparent that, when the cooling unit  23  is not functioning, heat can be transferred from the environment through the cooling unit  23 , or the cryostat opening  15 , to the coolant container  30 . In this regard, the cryostat opening  15  comprises a relatively large heat bridge, which results in an undesired substantial heat input, e.g. during transportation. 
     The container  24 , with the MR coils disposed therein, and including the cooling unit  23 , is depicted anew in  FIG. 2 . Reference is made to the above description regarding the individual components. 
     In order to prevent an undesired heat input when the cooling unit  23  in the vacuum container  24 , or the coolant container  30 , is not active, a protective cap is placed over the cryostat opening  15 , including the cooling unit  23  disposed therein. The protective cap  32  has a wall that is impermeable to air, the bottom of which has a sealing surface  35  on the side facing toward the container  24 , that is placed on a seal  34 , which in turn lies on a sealing surface  36  of the container  24 . An airtight seal is created by the sealing surfaces  35 ,  36  and the seal  34  therebetween, between the container  24  and the protective cap  32 . For thermal insulation, a vacuum is generated in the space enclosed between the protective cap  32  and the container  24 . It is apparent from the depiction that the vacuum  12  encompasses the cryostat opening  15  on all sides, and thus thermally insulates the cryostat opening. Thus, a heat input through the cryostat opening  15  from the environment is prevented or at least limited. 
     Coolant gas is generated by the nevertheless resulting, slow heating of the coolant in the coolant container  30 , despite all of the insulating measures, including the protective cap  32 , causing an increase in pressure in the coolant container  30 . In order to counteract, or to limit, respectively, this increase in pressure, a venting pipe  33  is provided. The venting pipe  33  is connected at one end to the interior of the coolant container  30 , and open at the other end to the environment outside the container  24  and the protective cap  32 . The venting pipe  33  is equipped with a pressure valve  18 . The pressure valve  18  limits the excess pressure in the coolant container  30  at a predefined level, and allows excess coolant in the gaseous state to escape into the environment. The pressure valve  18  can be provided in addition to an already present venting valve, and allows for a higher pressure than the venting valve in the interior of the coolant container  30 . An embodiment without an additional pressure valve of this type, having only quench and venting valves, is also possible. 
     Because the protective cap  32  is not in place during the operation of the MR tomography apparatus, but instead is used primarily during transportation thereof, there is no reason to anticipate an abrupt increase in pressure in the coolant container  30 , e.g. in the case of a quench. In this regard, the quench pipes  20 ,  21  are not necessary, but rather, the relatively small venting pipe  33  is sufficient. Because of the small diameter of the venting pipe  33 , these also represent only a limited heat bridge. Accordingly, they are as small as possible, because they must be in contact with the environment, and thus a heat input from the environment via the walls of the venting pipe  33  cannot be prevented. 
     In order to further reduce a heat input via the venting pipe  33 , this can be designed such that the path for the coolant in the gaseous state flowing through the venting pipe is longer than its external length. In order to obtain the longest possible effective length in this manner, it can, for example, take the shape of a spiral. As a result, a longer dwell time for the vaporizing coolant in the venting pipe  33  is obtained, which in turn results in a better cooling of the pipe. 
     In order to further reduce a heat input via the venting pipe  33 , this can be formed of a material having a low thermal conductivity. For this purpose, aluminum, for example, could be used, which additionally has the advantageous property that with a decrease in temperature, the thermal conductivity decreases further. Other suitable materials are: stainless steel, Inconel, Kevlar or Teflon. 
     With conventional constructions of the vacuum container  24 , including the cooling unit  23  and maintenance assembly  22 , an airtight sealed connection between the components is provided, such that the vacuum  12  enclosed by the protective cap  32  is limited to the space between the protective cap  32  and the vacuum container  24 , as well as the maintenance assembly  22  and the cooling unit  23 . Depending on the construction, a further expansion of the vacuum  12  would be conceivable, which is of no significance to the invention. For the invention, it is only significant that the cryostat opening  15  is fully, or as fully as possible, surrounded by the vacuum  12  against the environment outside the protective cap  32  and the container  24  for the thermal insulation. 
     The container  24 , including the MR coils and other components, is again depicted in  FIG. 3 . With respect to these components, reference is made to the above description. The depicted embodiment differs with respect to the means for thermal insulation at the cryostat opening  15 . In this embodiment, a protective cap  42  is placed in the cryostat opening  15  instead of the cooling unit. The cooling unit itself is removed for this purpose. Because the protective cap  42  is primarily used for transportation purposes, the absence of a cooling unit is not important, since it does not function during transportation. 
     The protective cap  42  can thus be placed in the cryostat opening  15  in place of a cooling unit, and furthermore is located on the container  24  at a position comparable to the maintenance assembly  22  disposed there. It can be installed either in place of the maintenance assembly  22 , or in an alternative embodiment, can be formed by the maintenance assembly  22 , which for this purpose must be designed in a suitable manner, and the opening otherwise provided for the cooling unit must then be modified in a corresponding manner, for forming the protective function of the protective cap  42 . As an example, the maintenance assembly can be constructed such that it is airtight, or vacuum-tight from the start. In the opening in the maintenance assembly, in which the cooling unit is inserted, a correspondingly designed insert must then be inserted in place of the cooling unit, for transportation purposes. The alternative embodiment, in which the maintenance assembly forms the protective cap  42 , is structurally possible without further measures, and in this regard requires no further substantial explanation. 
     The protective cap  42 , whether it is used in place of the maintenance assembly or is formed by the maintenance assembly, encloses, together with the container  24 , a space in which a vacuum  12  is generated. For this purpose, the respective corresponding sealing surface  45  of the protective cap  42 , and the protective surface  46  of the vacuum container  24  are created such that they correspond to one another, or are connected to one another. In the case of the maintenance assembly, there can be a permanent connection between it and the vacuum container  4 . The vacuum  12  serves as the thermal insulation for the cryostat opening  15  with respect to the environment outside the protective cap  42  and the vacuum container  24 . 
     In order to enable the passage of coolant gas, resulting from the heating of the coolant in the coolant container  30 , a venting pipe  43  is provided in the protective cap  42 , which is connected to the environment at one end, and to the coolant container  30 , or the coolant located therein, respectively, at the other end. The venting pipe  43  is equipped with a pressure valve  18 , which limits the pressure in the coolant container  30  to a predefined level, and when this level is exceeded, allows coolant to escape. Excess coolant gas escapes into the environment through the pressure valve  18  and the venting pipe  43 . The pressure valve  18  can be provided in addition to an already present venting valve, and allow for a higher pressure than the venting valve in the interior of the coolant container  30 . An embodiment without an additional pressure valve of this type, having only a quench valve and a venting valve, is also possible. 
     The venting pipe  43  thus represents a direct connection between the environment and the coolant container  30 , which enables an undesired heat input. As explained above, the venting pipe  43  has a relatively small cross-section, such that accordingly, its thermal conductivity is relatively low. In a manner similar to that explained above, the venting pipe  43  can exhibit a greatest possible effective length, e.g. by a spiral-shaped design. In addition, as explained above, it can consist of materials having low thermal conductivity, e.g. aluminum, stainless steel, Inconel, Kevlar, or Teflon. A connection having a low thermal conductivity must be ensured between the protective cap  42 , or, if applicable, the maintenance assembly designed as a protective cap, and the cryostat opening  15 . 
     A further embodiment for a protective cap  17  is depicted in  FIG. 4 . The vacuum container  4 , in which the coolant container  5  and the MR coils are disposed, is only shown in part. A depiction of the MR coils is omitted. 
     The coolant container  5  is encompassed by a thermal shield  6 , which in turn is encased in an insulating foil  2 . The thermal shield  6  can be actively cooled by the cooling unit  7 , which is obtained by means of a connection, depicted as an s-shaped curve in the illustration, made of a material having good thermal conductivity, e.g. copper. The thermal shield  6  and the insulating foil  2  form a doubled thermal insulation inside the vacuum container  4 . The vacuum container comprises, as explained above, a vacuum wall, in which a vacuum  12  is generated for a substantial thermal insulation. 
     A cooling unit  7  is disposed in the cryostat opening  16 . The cooling unit  7  is further thermally insulated by insulation means  10 ,  11 , in order to reduce a heat input from the cooling unit  7  into the vacuum container  4 , or the coolant container  5 , respectively. The insulation means  10 ,  11  consist, accordingly, of materials having a low thermal conductivity. 
     The protective cap  17  is placed over the cryostat opening  16 , including the cooling unit  7  disposed therein. It consists of a material having a low thermal conductivity. If applicable, it is placed on a maintenance assembly, or otherwise, directly on the vacuum container  4 . The protective cap  17  has a double wall, formed by a warm outer wall  1  and a cold inner wall  3 . A vacuum  12  is generated within the double wall. In addition, there is an insulating foil  2  located inside the double wall. The insulating foil  2  can be designed as a multi-layered foil (MU), and its position must not necessarily be located inside the double wall. The insulating foil  2  forms a structure, together with the vacuum  12 , for thermal insulation, which encompasses the cryostat opening  16 , as depicted, and insulates against the environment. 
     In order to discharge excess coolant gas resulting from heat in a controlled manner, a vent  9  is provided, in which a pressure valve  18  is disposed. The pressure valve  18  allows coolant gas to escape through the vent  9  when a predefined pressure has been reached, which escapes through a venting opening  14  in the space enclosed by the protective cap  17  and the container  4 . Because the protective cap  17 , as explained above, is primarily used for transportation purposes, and thus an abrupt pressure increase in the coolant container  5 , e.g. in the case of a quench, cannot occur, the venting pipe  9  or the venting opening  14  can be designed to be relatively small. Normally, however, it is formed by the quench and venting pipe that is typically provided, in the typical dimensions. 
     Coolant gas discharged from the venting opening  14  is first located in the space enclosed by the protective cap  17  and the container  4 . Because it has a very low temperature with respect to the surrounding temperature, which is only higher than the temperature in the interior of the coolant container  5 , it results in a significant cooling of this space and the components disposed therein, and the cold inner wall  3  of the protective cap  17 . Of course, the warm outer wall  1  of the protective cap  17 , as well as all of the further components having thermal contact thereto are also cooled. 
     The protective cap  17  is placed on a seal  8 , which in turn is placed on the container  4 . The seal  8  is designed to firm a gas exit path  19  to allow a passage of coolant gas at a reduced flow rate into the environment outside the protective cap  17  and the vacuum container  4 . An excessive pressure increase beneath the protective cap  17  thus is prevented by the passage at a reduced flow rate. Additionally, it is ensured that the coolant gas exiting through the venting opening  14  remains for a certain time beneath the protective cap  17 . 
     The coolant gas remaining under the protective cap  17  cools, as explained above, the protective cap  17  as well as the cooling unit  7  and further components. In this manner, cooling of the cooling unit  7  is obtained, without the need for additional cooling measures, in that only the escaping coolant gas is used for this. The cooled cooling unit  7  obviously results in a lower heat input into the coolant container  5  than would be the case if the cooling unit were not cooled. Furthermore, the material for the protective cap  17  can be selected such that its thermal conductivity decreases when cooled. For this purpose, as an example, aluminum, stainless steel, Inconel, Kevlar or Teflon can be used. This reduction in thermal conductivity occurring, when cooled, in numerous materials, results in an additional increase in the thermal insulation performance of the protective cap  17 . 
     The protective cap  17  is mechanically fixed in place on the vacuum container  4 , or, if applicable, on an associated maintenance assembly, in a suitable manner. For this purpose, threaded rods are provided in the depicted embodiment, on which the protective cap  17  is placed, and fixed in place there by means of nuts  13 . Alternatively, the affixing of the protective cap  17  onto the vacuum container  4  can also occur by means of other attachment mechanisms, e.g. latches. For this attachment, it is important that it does not enable an excess heat transfer into the vacuum container  4 , and it must cause a suitable compression of the seal  8 , such that the desired through-flow rate of coolant gas through the seal  8  is ensured. The seal  8  for this purpose can be formed of a cotton-type or other suitable material or, for example, can have a waffle-like or arched, corrugated structure, or it can be formed by a mechanical construction of sealing elements, e.g. panels and baffles. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.