Abstract:
A zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging including a cryogen pressure vessel to contain a liquid cryogen reservoir to provide cryogenic temperatures to the magnet coils for superconducting operation; a vacuum vessel surrounding the pressure vessel and spaced therefrom; a first thermal shield surrounding and spaced from the pressure vessel; a second thermal shield surrounding and spaced from the first thermal shield and intermediate the vacuum vessel and the first shield; a cryocooler thermally connected by a first and a second thermal interface to the first and second thermal shields, respectively; a recondenser positioned in the space between the pressure vessel and the first thermal shield and thermally connected by a thermal interface to the cryocooler to recondense, back to liquid, cryogen gas provided from the pressure vessel; and means for returning the recondensed liquid cryogen the pressure vessel; wherein the second thermal shield surrounding the first thermal shield reduces a radiation heat load from the first thermal shield to the pressure vessel lowering boiloff of cryogen gas under conditions of failure or power off of the cryocooler.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present disclosure relates generally to superconductive magnets, and more particularly to a superconductive magnet having a cryocooler coldhead.  
         [0002]     Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense “boiled-off” helium. Superconducting magnets which recondense the helium gas back to liquid helium are often referred to as zero boiloff (ZBO) magnets.  
         [0003]     Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a constant magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet&#39;s bore where the object to be imaged is placed.  
         [0004]     Open magnets, including “C” shape and support-post magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.  
         [0005]     Cryogens such as liquid helium, however, are not abundant and therefore can significantly impact the cost of operation of the MRI system. As a result, a zero boil-off design has far better advantage over a lower boil-off design, since the former design consumes no helium during normal operation. In the current zero boil-off magnet design, the magnet assembly only has a single radiation thermal shield which is wrapped by multiple layers of superinsulation. A temperature on the thermal shield, depending on the thermal shield conductance thereof, is about 45° K. to 70° K. The radiation heat load from the thermal shield to the helium vessel attributes to 50% of the total head load.  
         [0006]     However, when the cryocooler coldhead extending through a penetration to the liquid-helium vessel is not operational due to power off, coldhead failure or transportation, the coldhead acts as a heat source and adds significant heat into the cryostat. The temperature on the single radiation thermal shield on such a zero boil-off design will climb up to about 100° K. to about 150° K. The increase in temperature depends on the thermal shield conductance, conductance of copper braids between a coldhead sleeve assembly and the thermal shield, and the radiation heat between the coldhead and the helium vessel, which attributes to most of the total head load and thus boil-off of the helium at a rate of about 1.4 liter/w.  
         [0007]     Accordingly, there is need in the art for an apparatus and method to reduce radiation heat load between the thermal shield and the helium vessel, conduction heat from the coldhead to the thermal shield, and conduction heat load between the penetration and the thermal shield when the coldhead is not operational.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     Embodiments of the invention provide for a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging including a cryogen pressure vessel to contain a liquid cryogen reservoir to provide cryogenic temperatures to the magnet coils for superconducting operation; a vacuum vessel surrounding the pressure vessel and spaced therefrom; a first thermal shield surrounding and spaced from the pressure vessel; a second thermal shield surrounding and spaced from the first thermal shield and intermediate the vacuum vessel and the first shield; a cryocooler thermally connected by a first and a second thermal interface to the first and second thermal shields, respectively; a recondenser positioned in the space between the pressure vessel and the first thermal shield and thermally connected by a thermal interface to the cryocooler to recondense, back to liquid, cryogen gas provided from the pressure vessel; and means for returning the recondensed liquid cryogen the pressure vessel; wherein the second thermal shield surrounding the first thermal shield reduces a radiation heat load from the first thermal shield to the pressure vessel lowering boiloff of cryogen gas under conditions of failure or power off of the cryocooler.  
         [0009]     Further embodiments of the invention provide a method to reduce boiloff rate of cryogen gas during a coldhead failure or power off condition in a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging. The method includes disposing a liquid cryogen in a cryogen pressure vessel to provide cryogenic temperatures to the magnet coils for superconducting operation; surrounding the pressure vessel with a vacuum vessel spaced from the pressure vessel; surrounding the pressure vessel with a first thermal shield spaced from the pressure vessel; surrounding the first thermal shield with a second thermal shield spaced from the first thermal shield, the second thermal shield intermediate the vacuum vessel and the first shield; thermally connecting a cryocooler by a first and a second thermal interface to the first and second thermal shields, respectively; positioning a recondenser in the space between the pressure vessel and the first thermal shield and thermally connected by a thermal interface to the cryocooler to recondense, back to liquid, cryogen gas provided from the pressure vessel; and returning the recondensed liquid cryogen to the pressure vessel; wherein the second thermal shield surrounding the first thermal shield reduces a radiation heat load from the first thermal shield to the pressure vessel lowering boiloff of cryogen gas under conditions of failure or power off of the cryocooler.  
         [0010]     Yet another embodiment of the invention provides for a zero boiloff liquid helium cooled recondensing superconducting magnet assembly suitable for magnetic resonance imaging including a helium pressure vessel to contain a liquid helium reservoir to provide cryogenic temperatures to the magnet resonance imaging magnet assembly for superconducting operation; a vacuum vessel surrounding the pressure vessel and spaced from the pressure vessel; a first thermal shield surrounding the pressure vessel and spaced from the pressure vessel; a second thermal shield surrounding the first thermal shield and spaced from the first thermal shield, the second thermal shield intermediate the vacuum vessel and the first shield; and a recondenser and a cryocooler for cooling the recondenser to recondense helium gas formed in the pressure vessel back to liquid helium, the cryocooler thermally connected by a first and a second thermal interface to the first and second thermal shields, respectively; wherein the second thermal shield surrounding the first thermal shield reduces a radiation heat load from the first thermal shield to the pressure vessel lowering boiloff of helium gas under conditions of failure or power off of the cryocooler.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
         [0012]      FIG. 1  is a cross section view of a portion of a MRI superconducting magnet having a conventional single radiation thermal shield;  
         [0013]      FIG. 2  is a cross section view of a MRI superconducting magnet shown in simplified form incorporating a plurality of radiation thermal shields, a double stage coldhead, a three stage coldhead, and thermally isolated penetration in accordance with an exemplary embodiment;  
         [0014]      FIG. 3  is a cross section view of the penetration of Figure  2  illustrating thermal links between penetration stations extending from the penetration and respective radiation thermal shields;  
         [0015]      FIG. 4  depicts a plumbing system employed with helium gas exhaust from a helium bath that passes through tubing disposed with respect to the plurality of radiation thermal shields, coldhead sleeves associated with the double and three stage coldheads, and thermally isolated penetration in accordance with an exemplary embodiment;  
         [0016]      FIG. 5  depicts one of the plurality of radiation thermal shields having copper tubing disposed on a circumferential periphery to cool the shields with sensible heat from the helium gas exhausted therethrough; and  
         [0017]      FIG. 6  depicts copper tubing mounted on an outer surface defining the coldhead sleeve to cool the sleeve with sensible heat from the helium gas exhausted therethrough. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring first to  FIG. 1 , a current MRI magnet system  10  includes helium pressure vessel  4  including a liquid cryogen such as helium surrounded by vacuum vessel  2  with thermally isolating radiation shield  6  interposed between the helium vessel and the vacuum vessel. A cryocooler  12  (which may be a Gifford-Mahon cryocooler) extends through vacuum vessel  2  within sleeve  8  such that the cold end of the cryocooler may be selectively positioned within the sleeve without destroying the vacuum within vacuum vessel  2 , and heat generated by motor  9  of the cryocooler is outside the vacuum vessel. External cryocooler sleeve ring  14  extends outside vacuum vessel  2 , and collar  19  and sleeve flange  15  enable the securing of outer cryocooler sleeve  13  to vacuum vessel  2 . Cryocooler  12  is installed in the cryocooler sleeve assembly  8 ,  18 ,  23  with matching transition flange  21  and secured with bolts  82  and associated washers.  
         [0019]     First stage heat station  16  of cryocooler  12  contacts copper first stage thermal sleeve or heat sink  18  which is thermally connected through braided copper flexible thermal couplings  22  and  24  and copper thermal blocks  26  and  28  on isolating radiation shield  6  to cool the radiation shield to a temperature of approximately 60° K. providing thermal isolation between helium vessel  4  and vacuum vessel  2 . Flexible couplings  22  and  24  also provide mechanical or vibration isolation between cryocooler  12  and radiation shield  6 .  
         [0020]     The bottom surface of second stage heat station  30  of cryocooler  12  contacts indium gasket  29  to efficiently provide a temperature of 4° K. to heat sink  11  positioned on the opposite side of the indium gasket. Indium gasket  29  provides good thermal contact between the cryocooler heat station  30  and heat sink  11 .  
         [0021]     Extending below, and thermally connected to, heat sink  11  is helium recondensing chamber  38 , made of high thermal conductivity material such as copper, which includes a plurality of substantially parallel heat transfer plates or surfaces  42  in thermal contact with heat sink  11  and forming passages between the surfaces of the plates for the passage of helium gas from helium pressure vessel  4 .  
         [0022]     Helium gas  40  forms above liquid helium surface level  44  of liquid helium supply  46  through the boiling of the liquid helium in providing cryogenic temperatures to MRI magnet system  10 . Helium gas  40  passes through gas passageway  52 , through the wall  53  of helium vessel  4 , and through helium gas passage  50  to the interior of the upper portion  41  of helium recondensing chamber or canister  38 . Heat transfer plates  42  within a recondenser  39  are cooled to 4° K. by second stage  30  of cryocooler  12 , such that helium gas  40  passing between the plates recondenses into liquid helium to collect in bottom region  48  of helium recondensing chamber  38 . The recondensed liquid helium then flows by gravity through helium return line  54  and liquid helium passage  58  in helium vessel  4  back to liquid helium supply  46 , it being noted that helium recondensing chamber  38  is positioned higher than liquid helium passageway  58  in helium vessel  4 .  
         [0023]     As a result, during operation of MRI magnet system  10  liquid helium  46  cools superconducting magnet coil assembly (shown generally as  60 ) to a superconducting temperature with the cooling indicated generally by arrow  62  in the manner well known in the MRI art, resulting in boiling of helium liquid  46  and production of helium gas  40  above helium surface level  44 . However, helium gas  40  instead of being vented to the surrounding atmosphere  37  as is common in many MRI equipments, flows through gas passageway  52  in wall  53  of helium pressure vessel  4 , and through helium gas passage  50  to the interior of helium recondensing chamber  38  to pass between cryocooler cooled heat transfer plates  42  to recondense back to liquid helium. The recondensed liquid helium drops to bottom region  48  of the helium recondensing chamber  38  where it collects and flows by gravity through helium return line  54  and liquid helium passageway  58  through helium vessel  4  back to liquid helium supply  46 , thus returning the recondensed helium gas back to the liquid helium supply as liquid helium.  
         [0024]     In addition to cooling radiation shield  6  by first stage  16  of cryocooler  12 , superinsulation  34  is provided in the space between radiation shield  6  and vacuum vessel  2  to further thermally isolate helium vessel  4  from vacuum vessel  2 . Superinsulation  35  is also provided between recondensing chamber  38  and helium vessel  4  to thermally isolate the recondensing chamber  38  during servicing of cryocooler  12  which warms up cryocooler sleeve  13 . Superinsulation  34  and  35  is aluminized Mylar multi-layer insulation used in the superconducting magnet industry.  
         [0025]     However, the above zero boil-off design with a single radiation thermal shield  6  allows a temperature thereof to increase to temperatures of about 100° K. to about 150° K. when the coldhead is not functioning, due to power off, coldhead failure or transportation. The coldhead acts as a heat source and adds significant heat into the cryostat when the coldhead is not functioning. Conductance of copper braids  22 ,  24  between the coldhead sleeve assembly and thermal shield  6  allow heat from the coldhead to heat the single radiation thermal shield  6 . The radiation heat between coldhead  12  and helium vessel  4  attributes to most of the total head load, thus allowing boil-off of the helium.  
         [0026]      FIG. 2  illustrates a MRI magnet system  100  having a plurality of radiation thermal shields in accordance with an exemplary embodiment. In theory, by adding more radiation thermal shields, the radiation heat load is reduced and less helium will boil-off. MRI magnet system  100  includes helium pressure vessel  104  including a liquid cryogen  160  such as helium surrounded by vacuum vessel  102  with thermally isolating radiation shields  106  and  107  interposed between the helium vessel and the vacuum vessel. A first cryocooler  112  (which may be a Gifford-Mahon cryocooler) extends through vacuum vessel  102  within a sleeve  108  such that the cold end of the cryocooler may be selectively positioned within the sleeve  108  without destroying the vacuum within vacuum vessel  102 , and heat generated by a motor (not shown) of the cryocooler  112  is outside the vacuum vessel. Cryocooler  112  is installed in a cryocooler sleeve assembly  120  similar to the cryocooler sleeve assembly  8 ,  18 ,  23  described with respect to  FIG. 1 . A compressor  121  is in operable communication with cryocooler  112  via line  123  for providing pressurized helium gas  140  to a cold end.  
         [0027]     Cryocooler  112  as illustrated may be a three stage coldhead having a first stage heat station  116  thermal contacting radiation shield  107  through braided copper flexible couplings  122  and  124 . Cryocooler  112  further includes a second stage heat station  130  thermal contacting radiation shield  106  through braided copper flexible couplings  222  and  224 . Lastly, a third stage of cryocooler  112  includes a recondensor  139  in fluid communication with cryogen liquid  160  in pressure vessel  104 .  
         [0028]     Still referring to  FIG. 2 , MRI magnet system  100  optionally includes a second cryocooler  212 . Cryocooler  212  as illustrated is a two stage cold head having a first stage heat station  216  thermal contacting radiation shield  107  through a corresponding set of braided copper flexible couplings  122  and  124 . Cryocooler  212  further includes a second stage heat station  130  thermal contacting radiation shield  106  through a corresponding set of braided copper flexible couplings  222  and  224 . It will be noted that the most probable type of failure of two or three stage coldheads involves compressor stoppage due to loss of helium, loss of compressor cooling, motor failure, or power outage.  
         [0029]     After the coldhead is off due to any of the reasons as stated above, the temperature on the high temperature thermal shield  107  will be close to the temperature on the single thermal shield design of  FIG. 1 . However, the temperature on the low temperature thermal shield  106  will be significantly lower. The radiation heat load between the low temperature thermal shield  106  to pressure vessel  104  will be reduced, and thus, the boil-off rate will be reduced relative to the single thermal shield design of  FIG. 1 .  
         [0030]     Since it is necessary to provide electrical energy to the main magnet coil and to various collection coils employed in MRI magnet system  100 , it is necessary that there be at least one penetration through the cryostat walls. A penetration  230  is shown thermally isolated from either of the cryocoolers  112 ,  212  in  FIG. 2 . Penetration  230  includes a thermal link with first and second radiation thermal shields  107  and  106  through braided copper flexible couplings  267  and  266 , respectively.  
         [0031]     Historically, the penetration heat station was attached to a coldhead sleeve, thus the coldhead would cool down the penetration during normal operation. However, if the coldhead fails to work properly or turns off, the coldhead would add significant heat to penetration. In exemplary embodiments depicted in  FIGS. 2 and 3 , the penetration heat station is attached to each thermal shield  106 ,  107  and remote from coldheads  112  and  212 . In this manner, penetration heat station is isolated from coldhead during a power off or failure condition of either coldhead  112 ,  212 .  
         [0032]     Referring now to  FIG. 3 , penetration  230  is shown more clearly with respect to thermal interfaces with radiation thermal shields  106  and  107 , via couplings  266 ,  266 , respectively. Penetration  230  extends through shields  106  and  107  while penetration  230  includes a first penetration heat station  276  defined by copper thermal blocks  226  and  228  in thermal communication with an outside surface defining penetration  230 . Penetration  230  includes a second penetration heat station  286  defined by copper thermal blocks  326  and  328  in thermal communication with an outside surface defining penetration  230 . Blocks  226  and  228  form a continuous annular ring disposed about penetration  230  that connects with braided copper flexible coupling, while blocks  326  and  328  also form a continuous annular ring disposed about penetration  230  that connects with braided copper flexible coupling  266  in an exemplary embodiment. Flexible couplings  266  and  267  also provide mechanical or vibration isolation between thermal shields  106  and  107 .  
         [0033]     Referring now to  FIG. 4 , a plumbing system  300  is schematically illustrated. Plumbing system  300  is configured to allow exhausted cryogen gas  140  carried sensible heat from pressure vessel  104  to cool down thermal shields  106 ,  107 , penetration  230  and coldhead sleeves associated with cryocoolers  112 ,  212 . More specifically, helium gas  140  from pressure vessel  104  generally indicated at  302  is diverted to pass through a coldhead sleeve  304 , through thermal shields  106 ,  107  at  306 , and pass through penetration  230  at  308 . Exhausted gas proceeds to a pressure relief valve  310  via line  312  and exits pressure relief valve  310  out to vent line  314 . In this manner, boiloff can be reduced by using the sensible heat in the exhausted gas  302  to cool the various components, thus reducing heat loads therebetween.  
         [0034]     Plumbing system includes tubing  320  in fluid communication with exhausted helium gas from pressure vessel  104  with specific reference to  FIGS. 2 and 4 .  FIG. 4  illustrates radiation thermal shield  106  with tubing  320  disposed about a circumferential periphery thereof. In an exemplary embodiment as shown, tubing  320  is copper while transition tubing from one component to another (e.g., one thermal shield  106  to another thermal shield  107 ) is made of a thermally non conductive material such as, stainless steel, or example. The stainless steel transition tubing connects copper tubing  320  between first and second thermal shields  107  and  106  in order to reduce the conduction heat load therethrough during normal operation of coldheads  112  and  212 .  
         [0035]     Referring now to  FIG. 6 , copper tubing  320  is mounted onto an outer surface of coldhead sleeve  120  and exhausted helium gas  140  will be allowed to pass through the tubing  320  mounted on the coldhead sleeve before it escapes to venting pipe  314 . The sensible heat contained therein cools down the sleeve or coldhead in the process.  
         [0036]     As disclosed, some embodiments of the invention may include some of the following advantages: reduction of helium boil-off during power off, coldhead failure and transportation by reducing radiation heat load using a plurality of radiation shields and thermally isolating penetration from a coldhead; and cooling of the thermal shields, coldhead sleeve and penetration by the sensible heat from the additional helium boil-off, while reducing conduction between the thermal shields during normal operation using a low conductive tubing to exhaust cryogen gas therebetween.  
         [0037]     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 or only 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.