Abstract:
A magnetic resonance imaging (MRI) system having an open superconducting magnet system ( 1 ) includes a number of horizontally oriented superconducting coils ( 3–10 ) in a cryogenic container ( 11 ). The cryogenic container contains a liquid cooling medium for cooling the superconducting coils which are located within the cryogenic container. At its top, the cryogenic container is provided with a recondensor ( 23 ) for continuously liquefying the cooling medium. The magnet system has a circuit for guiding the liquid cooling medium from the recondensor along at least part of the superconducting coils.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage entry of PCT Application No. PCT/IB02/05622 (Publication No. WO 03/054893), filed Dec. 18, 2002, the benefit of which is hereby claimed. 
     BACKGROUND 
     The invention relates to a magnetic resonance imaging (MRI) system having an open superconducting magnet system, which comprises a number of horizontally oriented superconducting coils and a cryogenic container for containing a liquid cooling medium for cooling the superconducting coils which are located within the cryogenic container, the cryogenic container being provided, at its top, with a recondensor for continuously liquifying cooling medium evaporating from the container. 
     An MRI system as mentioned in the opening paragraph is known from U.S. Pat. No. 6,011,456. This patent describes an open architecture recondensing superconducting magnet having a cryogenic container, being a helium vessel, for a superconducting MRI magnet in which the so-called “Zero-Boil-Off” (ZBO) technique is used for conservation of the helium coolant. The ZBO technique itself is a technique that aims to prevent loss of helium (or any other coolant) by means of a recondenser that re-liquefies the helium gas somewhere in the top of the helium vessel instead of letting it escape as a gas. In a magnet with a recondenser, the helium that evaporates cannot escape the helium vessel, because in the exit path (somewhere in the neck at the top of the magnet) it will encounter the cold surface of the recondenser, which causes the helium gas to liquefy. The recondensed helium then drips into the helium vessel again. So, there is continuous circulation of the helium. Typically, in an MRI magnet according to the invention, the heat leak is of the order of 1 W, causing an evaporation of 1.4 litres/hour of liquid helium. As a result, about 14 litres/hour of 4.2 K helium gas try to escape the helium vessel, which is quite a large amount compared to conventional MRI magnets not having an open structure. So strictly speaking, the term “zero-boil-off” is not correct. 
     A further detail of the ZBO technique is that uncontrolled operation of the cryocooler may lead to underpressure in the helium vessel which is undesirable. The solution consists in controlling the pressure by means of a heater at the bottom of the helium vessel. In fact the heater spoils the excess cooling capacity of the recondensor. This ensures a constant circulation of helium, regardless of the quality of the cryostat. 
     As indicated, the recondensor should prevent the evaporated helium from leaving the helium vessel and does so to a large extent. However in practice some loss of helium is inevitable during service actions, failures of the cryogenic system or the existence of small undetectable helium leaks through which gas can escape the helium vessel. In other words, the helium loss averaged over a long period of time is very small but not really zero. 
     An important quality factor of the helium vessel in this respect is the effective volume, which is defined as the difference between the maximum fill ratio of the cooling medium and the minimum fill ratio of the cooling medium between which the magnet is allowed to operate. For example, if the maximum fill ratio is 95% of the total volume of the vessel and the minimum fill ratio is 15%, the magnet can be filled with helium to 95% and will have to be re-filled before the fill ratio drops below 15%. In this case the effective volume is 80% of the total volume of the helium vessel. Typically, for an open magnetic resonance imaging (MRI) superconducting magnet system of medical imaging systems according to the state of the art, the maximum fill ratio and the minimum fill ratio would be relatively close to each other, for example respectively 95% and 85%, in which case the effective volume is only 10%, even if the magnet is equipped with a recondenser (ZBO technique). A large effective volume is therefore interesting because it will increase the helium re-fill interval. 
     SUMMARY 
     It is an object of the invention to provide a medical imaging system the cryogenic container of which needs a re-fill of the fluid cooling medium at increased intervals or even not at all anymore during the economical lifetime of such a system. 
     In order to achieve said object, an MRI system according to the invention is characterized in that the MRI system comprises a circuit for guiding the liquid cooling medium from the recondensor along at least part of the superconducting coils. The cooling medium within the circuit should be capable of sufficiently cooling the superconducting coils. This does not necessarily mean that the superconducting coil is directly in contact with the cooling medium. Alternatively, sufficient cooling of the superconducting coils can also take place, for example, by using a thermal conductive intermediate material, for instance of the circuit itself, between the cooling medium and the superconducting coils. Since the circuit only necessitates the presence of the liquid cooling medium in the circuit, V min  can be decreased drastically, thus correspondingly increasing the effective volume. For example in this manner, the invention enables the effective volume to be increased by a factor of 8, leading to an 8 times greater re-fill interval. In practice this can be the difference between re-filling every 2 years, or every 16 years. In the latter case the magnet is not expected to be re-filled at all because 16 years exceeds the economical lifetime of an MRI medical imaging system. 
     A particular embodiment of an MRI system according to the invention is characterized in that the circuit comprises at least one local reservoir for the liquid cooling medium, which is located close to an associated superconducting coil for cooling said associated superconducting coil. In this manner, a very efficient increase of the effective volume is achieved. 
     A further embodiment of an MRI system according to the invention is characterized in that the local reservoir comprises an overflow edge for the liquid cooling medium, which is located at or above a lower side of the associated superconducting coil. In this manner it is ensured that at least the lower part of the associated superconducting coil is immersed in the fluid cooling medium. Due to the very good thermal conductive properties of superconducting coils, immersing only the lower part of said superconducting coil in the fluid cooling medium is sufficient for maintaining the associated superconducting coil as a whole at the required reduced temperature. 
     A yet further embodiment of an MRI system according to the invention is characterized in that the local reservoir is at least partly ring-shaped. In this manner, the shape of the local reservoir is adapted to the general shape of the associated superconducting coil. It is not strictly necessary within the spirit of the invention that this particular ring shape encloses a full circle. Due to the good thermal conductivity properties, it also can be sufficient to limit the ring shape to for instance 15 degrees of the associated superconducting coil, thereby further decreasing the minimum fill ratio at which the magnet is allowed to operate. 
     A particular embodiment of an MRI system according to the invention is characterized in that the local reservoir comprises a winding body for the associated superconducting coil. Said winding body is for instance part of a so called coil former. The number of additional structural components within the cryogenic reservoir is reduced, because the winding body not only serves as a body around which the coil is wound during the manufacture of the superconducting coil, but also as a wall or restriction for the reservoir. 
     A similar advantage applies in a further embodiment of an MRI system according to the invention, wherein the local reservoir comprises a positioning body for the associated superconducting coil. Such a positioning body, which is for instance also part of a so called coil former, is normally present in the cryogenic reservoir anyway for avoiding unwanted deformation of the superconducting coils due to generated Lorentz forces. 
     A yet further embodiment of an MRI system according to the invention is characterized in that the circuit comprises downwardly sloping guides for guiding the liquid cooling medium from a first superconducting coil to a second superconducting coil which is positioned below the first superconducting coil. In this manner, an optimal routing of the fluid cooling medium is achieved. The fluid cooling medium is guided along the successive superconducting coils, with a minimal need for fluid cooling medium to be present between the superconducting coils. This is especially advantageous if the respective superconducting coils are not exactly positioned above each other. 
     A particular embodiment of an MRI system according to the invention is characterized in that the MRI system comprises sensor means for determining the level of the liquid cooling medium within the cryogenic container. In this manner, the fill ratio of the fluid cooling medium within the cryogenic container can be monitored. By extrapolating data derived from the sensor means, one can anticipate when, and if, a next re-fill would be necessary. 
     Hereinafter, the invention will be explained further by a description of a preferred embodiment of a magnetic resonance imaging (MRI) system according to the invention. For this description reference is made to the Figures, in which 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  diagrammatically shows a cross-section of an open superconducting magnet system of the MRI system according to the invention, however without features specific to the invention, 
         FIGS. 2A and 2B  diagrammatically show a cross-section of the magnet system according to  FIG. 1 , including features specific for the invention, and including a maximum level V max  and a minimum level V min  of helium between which the magnet system is allowed to operate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic cross-section of an open superconducting magnet system  1 , which forms part of a magnetic resonance imaging (MRI) system according to the invention.  FIG. 1  does not show the specific features of the invention. The remaining parts of the MRI system, including a frame, a patient support unit and a control unit, are not shown in  FIG. 1  and may be of a usual kind known to the skilled person. The magnetic field and the axis  2  of the magnet system  1  are oriented in the vertical direction. The magnetic field is generated by ring-shaped superconducting coils  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10  each lying in a horizontal plane. The two coils  3 ,  4  having the largest diameter constitute shield coils, which provide active shielding of the fringe field. Active shielding is the state-of-the-art method of reducing the fringe field of the magnet system  1  in order to minimize the volume occupied by the MRI system in the hospital. 
     In order to become superconducting, the superconducting coils  3 – 10  should be cooled. For such cooling the superconducting coils  3 – 10  are located within a cryogenic container  11 . The shape of the cryogenic container  11  corresponds to the specific shape of the magnet system  1 , which means that the cryogenic container  11  comprises a disc-shaped upper part  12  and a disc-shaped lower part  13 , both parts  12  and  13  being interconnected at one side by a post  14 . It is also possible within the scope of the invention to provide more posts, e.g. for mechanical stability, but only one post is shown in  FIG. 1 . Following these specific contours, a radiation shield  15  and a helium vessel  16  are present in the cryogenic container  11 . This results in the helium vessel  16  being common for both the upper part  12  and the lower part  13 . This is the most economical way, because if two separate helium vessels were used for the upper part  12  and the lower part  13 , many components would be needed twice, including the rather expensive cryocooler, as will become clear from the description given below. 
     In the particular design as shown in  FIG. 1 , there are four superconducting coils located in the upper part  12  of the cryogenic container  11  and four superconducting coils located in the lower part  13 , but this is not essential to the invention. A more homogeneous version of the magnet system may require more coils, whereas a non-shielded version of the magnet system may require fewer coils. In  FIG. 1  the coils of the upper and the lower part  12  and  13  are symmetric, however, this is not necessary within the context of the invention. 
     The areas  17 ,  18  represent the positions of further components of the magnet system  1 , such as gradient coils, RF coils, and a shim system, which have been recessed in recesses in the respective upper part  12  and lower part  13  of the cryogenic container  11 . In this manner, maximum space is achieved for the patient within the patient space  19  between the upper part  12  and the lower part  13 . 
     For cooling helium present within the helium vessel  16 , a cryocooler  20  is provided, which penetrates into the helium vessel  16  through a neck  21  on top of the magnet. The cryocooler  20  has two heat stations  22 ,  23 . The first heat station  22  is connected to the radiation shield  15  of the magnet system  1 . The second heat station  23  floats in the helium vessel  16  and acts as a recondenser. In this particular embodiment, an advanced two-stage cyocooler is used, of which the second heat station  23  or the second stage reaches a temperature below 4.2 K and is therefore capable of recondensing helium, while the first heat station  22  or the first stage cools the radiation shield  16 . However, combining the functions of recondensing and cooling the radiation shield in a single cryocooler is not essential to the invention. Alternatively, one could use a helium liquefier and a separate means to cool the radiation shield, e.g. a separate cryocooler or a nitrogen coolant. 
     In  FIGS. 2A and 2B  the helium vessel  16  is shown without the surrounding cryogenic container  11  and the radiation shield  15 . To improve the intelligibility of the FIGURES no lines are shown (unlike  FIG. 1 ) for showing the rotating movement of several elements, such as the superconducting coils, within the helium vessel  16 . In addition to the elements shown in  FIG. 1 ,  FIGS. 2A and 2B  show winding bodies  24 ,  25 ,  26 ,  27 ,  28 ,  29 ,  30 ,  31  and coil formers  32 ,  33 ,  34 ,  35 ,  36 ,  37 ,  38 ,  39  for each superconducting coil  3 – 10 . The superconducting coils are wound around the winding bodies during production of the magnet system  1 . The coil formers are used to support the superconducting coils mechanically against Lorentz forces. The winding bodies and the coil formers would be present within the helium vessel even without applying the present invention since they are necessary anyway. Furthermore the helium present within the helium vessel  16  is shown using a grey scale. 
     In  FIG. 2A  the helium is shown at the maximum level  40  at which the magnet system  1  is allowed to operate, whereas in  FIG. 2B  the helium is shown at the minimum level  41  at which the magnet system  1  is allowed to operate. The difference between maximum and minimum filling is the so called effective volume that can easily reach values between 80% and 90% of the total volume of the helium vessel  16 . 
     As described above, a dynamic balance exists within the helium vessel between on the one hand the evaporation of helium and on the other hand the liquefaction of helium by the condensor. Helium liquefied by the condensor is guided along all superconducting coils by means of a circuit. This circuit starts right below the condensor by a downwardly sloping chute  42 , via which the helium can flow to a first ring-shaped local reservoir  63  having a U-shaped cross-section. The legs of the U-shape are formed by part of the helium vessel  16  itself and by a wall  43  located at the inner side of superconducting coil  3 , whereas the body of the U-shape is formed by a bottom  44 . On the left hand side of  FIG. 2A  an overflow edge  45  of wall  43  is shown, from which a downwardly sloping pipe  48  extends to just above superconducting coil  5 . Coil  5  is positioned within a second ring-shaped local reservoir  64  having a U-shaped cross-section. The legs of this U-shape are formed by a part of winding body  25  and wall  46 , having an overflow edge  47 . The body of the U-shape is formed by a part of coil former  33 . Helium overflowing overflow edge  47  will arrive at a third ring-shaped local reservoir  65  formed by winding body  27  for superconducting coil  7  and by parts  49 ,  50  of the helium vessel  16 . The upper edge  51  of winding body  27  should be considered to be an overflow edge for the helium. After having overflown this edge  51 , the helium reaches a fourth ring-shaped local reservoir  66  also having a U-shaped cross-section for superconducting coil  7 . Wall  53 , part  52  of helium vessel  16  and winding body  27  constitute this fourth local reservoir  66 . The height of wall  53  is smaller than the height of winding body  27 . For this reason, if the fourth reservoir  66  is fully filled with helium, helium will tend to overflow upper edge  54  of wall  53 . Next, due to gravity, helium will fall through the part of helium vessel  16  extending through post  14 , onto the upper part of a downwardly sloping chute  55 , from which the helium flows down into a fifth ring-shaped local reservoir  67 . This fifth local reservoir  67  serves to jointly cool superconducting coils  6 ,  8  and  10 . The fifth local reservoir  67  is formed by winding body  30  for superconducting coil  6 , which winding body  30  is at its upper end sealingly connected to the helium vessel  16 . Furthermore, the fifth local reservoir  67  is formed by bottom  56  which extends underneath the three superconducting coils  6 ,  8  and  10 , and by wall  57 . On the right hand side of  FIGS. 2A and 2B  it can be seen that the upper edge  58  of wall  57  is positioned slightly underneath the wall of the helium vessel  16 , allowing passage of helium therebetween onto downwardly sloping chute  59 , which guides the helium to a sixth ring-shaped local reservoir  68 . This local reservoir  68  is formed by part  60  of the helium vessel  16 , by coil former  39  and by winding body  31  which is extended by extension part  61 . After having overflown upper edge  62  of the extension part  61 , the helium arrives at the bottom  41  of the helium vessel  16 . At the bottom a heater (not shown) is present for controllably evaporating the helium in order to create a dynamic equilibrium with the recondensation of the helium by the cryocooler  20 . 
     In order to monitor the helium level in the helium vessel  16 , a level sensor  69 , known to the men skilled in the art, is provided on the inside of the helium vessel. This enables continuous monitoring of the helium level. Monitoring the helium level is advisable in order to detect problems at an early stage. A drop of the helium level indicates a failure (e.g. a gas leak) that will eventually lead to loss of helium from the magnet system  1 . 
     It will be clear from the above description and from  FIG. 2B  (in comparison to  FIG. 2A ), that in order to cool the superconducting coils to the required temperature it is sufficient if all local reservoirs  63 ,  64 ,  65 ,  66 ,  67 ,  68  are filled with helium. This is already achieved if only a relatively small part of the helium vessel  16  is filled with helium, since the local reservoirs have volumes which are substantially smaller than the total volume of the helium vessel  16 . Liquid helium, which overflows one of the local reservoirs, arrives at a lower local reservoir, where applicable, via guiding elements such as chutes or pipes. In this manner, a continuous flow of liquid helium is achieved from the condensor to the highest local reservoir, and further to the lowest local reservoir via the intermediate local reservoirs, so that it is ensured that all reservoirs remain filled despite local helium evaporation in each local reservoir. 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.