Abstract:
The present invention provides acryogenic vessel ( 200 ), in particular for use in a magnetic resonance examination system ( 110 ) to mount therein superconductive main coils ( 142, 144 ) of the magnetic resonance examination system ( 110 ), comprising an inner vessel ( 202 ), an outer 300K vessel ( 204 ), and a radiation shield ( 206 ), which is located between the inner vessel ( 202 ) and the outer 300K vessel ( 204 ) and which surrounds the inner vessel ( 202 ), whereby the radiation shield ( 206 ) has at least one dry-friction area ( 206 ), where dry-friction is generated upon deformation of the radiation shield ( 206 ). The present invention also provides a superconductive magnet ( 114 ) for a magnet resonance examination system ( 110 ) comprising a set of superconductive main coils ( 142, 144 ), which are arranged in the above cryogenic vessel ( 200 ). The present invention further provides a magnet resonance examination system ( 110 ) comprising the above superconductive magnet ( 122 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of magnetic resonance (MR) examination systems, in particular to the field of superconductive magnets for MR examination systems, still more particular to the field of cryogenic vessels for superconductive magnets for MR examination systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    A magnetic resonance (MR) examination system comprises a superconductive magnet with a cryogenic vessel, in which main magnet coils of the superconductive magnet are mounted. The cryogenic vessel typically comprises two vessels, which are mounted spaced apart to achieve thermal isolation, and a mounting structure, which is located within the two vessels, for mounting main magnet coils of the superconductive magnet. There are two ways to keep the main coils at superconducting temperature. In the first way the superconducting coils are in contact with liquid cryogen. In the second way the superconducting coils are directly cooled from a coldhead, e.g. via copper strands. In the first case, the mounting structure is typically provided as a third, inner vessel, which contains the cryogen, and the main coils are mounted inside the inner vessel. The cryogen is typically provided as liquid having a low boiling temperature, e.g. a boiling temperature of about 4.2 K in case of helium, which already evaporates when small amounts of heat enter. 
         [0003]      FIG. 1  shows a state of the art superconductive magnet. The superconductive magnet  10  comprises a cryogenic vessel  12  and a set of main magnet coils  14 ,  16 . The cryogenic vessel  12  comprises three vessels  22 ,  24 ,  26 , which are mounted spaced apart to achieve thermal isolation. The three vessels  22 ,  24 ,  26  are an inner vessel  22 , also referred to as 4K vessel when helium is used, a radiation shield  24 , which is provided as a vessel surrounding the inner vessel  22 , and an outer vessel  26 , also referred to as 300K vessel, surrounding the radiation shield  24 . The outer vessel  26  is usually made of stainless steel or aluminum and the radiation shield  24  of aluminum. The main magnet coils  14 ,  16  are mounted at an inner side of the inner vessel  22  along inner and outer cylindrical walls  28 ,  30  as inner and outer coils  14 ,  16 , respectively. The inner vessel  22  contains a cryogen, e.g. liquid helium, which cools the main magnets  14 ,  16  and also enables heat buffering. Such a superconductive magnet is e.g. known from U.S. Pat. No. 7,170,377 B2. 
         [0004]    Gradient switching induces dissipation in this superconductive magnet. This leads to boil-off of the cryogen in the cryogenic vessel. Former superconductive magnets simply blow the cryogen into the air when boil-off occurred. As a consequence cryogen had to be refilled frequently. State-of-the-art superconductive magnets have zero boil-off. Therefore, the dissipation in the magnet leads to an increase of pressure in the inner vessel. This phenomenon is called dynamic boil-off (DBO). 
         [0005]    In detail, dynamic boil-off is caused by lack of eddy current shielding of the radiation shield due to mechanical resonances. When a gradient coil is switched, its stray field changes. As a consequence, eddy currents are induced in the radiation shield. Due to the eddy currents, forces are applied to this shield and the shield might start moving, e.g. oscillating. Its behavior depends on mass, stiffness and geometry of the radiation shield. Accordingly, the dynamic boil-off is a function of the frequency of the gradient switching. The DBO transfer function shows peaks as indicated in  FIG. 2 , which are related to mechanical resonances. This refers to mechanical resonances of the superconductive magnet, in particular of the radiation shield. 
         [0006]    When the radiation shield would not move, which would imply infinite stiffness and infinite mechanical impedance, the attenuation of the stray field of the gradient coil would be monotonously increasing, i.e. only determined by the time constant of the shield. In that case,  FIG. 2  would not show any peaks. 
         [0007]    The peaks in the DBO graph of  FIG. 2  are caused by mechanical resonances, as already discussed above. The mechanical impedance at a resonance is low. The lower the damping, the higher the Q factor and the lower the mechanical impedance. Since internal material damping decreases with lower temperatures, the magnitude of the damping can be several orders lower for very low temperatures close to OK, as indicated in  FIG. 3 . Hence, also the impedance of the radiation shield is low, so that it moves easily with the applied magnetic field and can enter in resonance. 
         [0008]      FIG. 4  shows another state of the art superconductive magnet. The superconductive magnet  10  comprises in accordance with the typical cryogenic vessel  12  of  FIG. 1  and a set of main magnet coils  14 ,  16 . The cryogenic vessel  12  comprises two vessels  24 ,  26 , which are mounted spaced apart to achieve thermal isolation. The two vessels  24 ,  26 , refer to the radiation shield  24  and the outer vessel  26  as described above. The cryogenic vessel  12  further comprises a mounting structure  32 , which is located inside the radiation shield  24 . The main magnet coils  14 ,  16  are mounted to the mounting structure  32 . Cold heads, which are not shown in  FIG. 4 , are in contact with the main magnet coils  14 ,  16  to keep them at superconducting temperature. Accordingly, this superconductive magnet does not require the use of a cryogen. 
         [0009]    The superconductive magnet without cryogen does not suffer from boil-off. Nevertheless, also for this type of superconducting magnet it is important to reduce heat from gradient switching, i.e. from movements of the radiation shield due to due to mechanical resonances, since there is no buffering/cooling from the cryogen for locally generated heat. 
         [0010]    One approach is to reduce resonances in relevant frequency ranges based on geometry of the superconductive magnet and the cryogenic vessel. For example, thickness of the radiation shield can be increased to shift resonance frequencies into frequencies ranges, which are not relevant for the above problems like boil-off. As a consequence, superconductive magnet coils have a larger radius as well. This requires more superconductor material for the same magnetic field in an imaging volume of the MR examination system. A too thick radiation shield has the additional disadvantage that it might be destroyed due to the eddy current forces from a quench. A thinner radiation shield is also not an option. A too thin radiation shield does not conduct the heat sufficiently towards the cold head of the cryogenic system. 
         [0011]    Another approach is based on material properties, e.g. on the choice of material. The resonance frequency is determined by E/ρ, where E is the Young&#39;s modulus E and ρ refers to the specific mass. The radiation shield is usually made from aluminum. E/ρ is relatively high for aluminum compared to other materials suitable for the use in cryogenic vessels for superconductive magnets. Hence, a change of material cannot reasonably reduce the above problems. 
         [0012]    Another potential option that has proven not applicable is to add a visco-elastic layer to reduce the magnitude of the resonances of the cryogenic vessel. This is, however not feasible. First of all, the damping property of visco-elastic material is negligible at low temperature. Second, visco-elastic materials tend to generate gasses, which cause serious problems in a high-vacuum environment such as a superconducting magnet. 
         [0013]    Apart from the cryogen problems described above in respect to the boil-off, there is also a risk of a magnet quench. As a consequence additional measures have been taken in the superconductive magnet, which are expensive and can add costs of several thousands of Euros to a single superconductive magnet. 
         [0014]    The U.S. Pat. No. 6,038,867 shows a superconducting magnet with insulating blankets. The known superconducting magnet comprises a helium pressure vessel which contains a superconducting magnet coil assembly. The Helium pressure vessel is surrounded by a thermally insulating shield (between the He-vessel and a vacuum vessel. Additionally, thermal insulation blankets are disposed between the radiation shield and the vacuum vessel. Each thermal insulation blanket is formed as a plurality of thermally reflective (Al) sheets separated by low conductively metal spacer sheets. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the invention to provide a cryogenic vessel, a superconductive magnet for a magnetic resonance (MR) examination system comprising such a cryogenic vessel and a MR examination systems comprising such a superconductive magnet, which overcome the above problems. In particular, it is an object of the present invention to provide a cryogenic vessel which has reduced mechanical vibrations and resonances in the radiation shield. It is a further object of the present invention to reduce boil-off of cryogen used in the cryogenic vessel for cooling the main coils of the superconductive magnet. 
         [0016]    This object is achieved by a cryogenic vessel, in particular for use in a magnetic resonance examination system, to mount therein superconductive main coils of the magnetic resonance examination system, comprising an outer 300K vessel, and a radiation shield, which is located inside the outer 300K vessel, and an inner mounting structure for mounting the superconductive main coils, which is located within the radiation shield, whereby the radiation shield has at least one dry-friction area, where dry-friction is generated upon deformation of the radiation shield. 
         [0017]    This object is also achieved by a superconductive magnet for a magnet resonance examination system comprising a set of superconductive main coils, which are arranged in a cryogenic vessel as specified above. 
         [0018]    This object is also achieved by a magnet resonance examination system comprising a superconductive magnet as specified above. 
         [0019]    The at least one dry-friction area enables to increase the mechanical damping of vibrations due to gradient switching. The dry-friction achieves mechanical damping of the cryogenic vessel, in particular of the radiation shield. The mechanical impedance is improved and therefore the radiation shield of the cryogenic vessel is less susceptible for mechanical vibrations and resonances. The at least one dry-friction area can be used independently from other measures to reduce vibrations due to mechanical resonances of the radiation shield. Accordingly, the at least one dry-friction area can be added to the radiation shield to improve state of the art cryogenic vessels, or to replace other expensive means for reduction of mechanical resonances. Since the damping is improved with the design of the radiation shield itself, drawbacks of the use of damping materials are avoided such as low elasticity and release of gases at low temperatures. Heat, which is generated due to the dry-friction at the dry-friction area, can be easily cooled away by a cryo-cooling system, since a typical cold head has a performance of approximately 100 W at an operation temperature of 40K, which is the typical temperature of the radiation shield during operation. Accordingly, heat dissipation can be easily achieved. 
         [0020]    Preferably, the vibrations are reduced at resonance peaks of the radiation shield. Hence, dynamic boil off of cryogen due to gradient switching can be reduced. The at least one dry-friction area is preferably provided locally, for instance in an area where local vibrations occur and e.g. DBO has the highest impact, such as at the location of the main coils of the superconductive magnet. 
         [0021]    According to a preferred embodiment the at least one dry-friction area comprises at least two shield layers, which are stacked on each other in surface contact, whereby the at least two shield layers are locally connected to each other. The surface contact enables the dry friction in-between the local connections. Preferably, the at least two shield layers are pressed to each other to increase the dry friction. 
         [0022]    According to a preferred embodiment the at least two shield layers are locally connected to each other by spot welding. Accordingly, the at least two shield layers are mechanically attached to each other at welding spots, and a contact area between the at least two shield layers is provided with a surface contact. Therefore, the at least two shield layers can move relative to each other at their surface contact, when the radiation shield is deformed, e.g. due to vibrations. While moving relative to each other, the at least two shield layers experience dry friction and the mechanical resonances are damped. Preferably, spot welding is used to attach a local patch as second shield layer to the first shield layer. 
         [0023]    According to a preferred embodiment the at least two shield layers are locally connected to each other by rolling. Preferably, the outer layer is stretched and the inner layer is compressed due to the rolling. As a consequence the outer layer can automatically be pressed against the inner layer. Preferably, the two shield layers are locally connected by a combination of rolling and spot welding. Rolling can be used e.g. to provide an entire cylindrical wall of the radiation shield with two shield layers. Preferably, the inner and outer cylindrical walls are both provided entirely with two shield layers. 
         [0024]    According to a preferred embodiment at least one shield layer of the radiation shield is made of aluminum. Aluminum is a suitable material for the shield layer, since it has a high static damping coefficient, which enables an efficient damping of vibrations. In case both shield layers are made of aluminum, static damping coefficient of approximately μs=1.35 can be achieved. 
         [0025]    According to a preferred embodiment at least two shield layers of the radiation shield are made of different materials. The two layers can be provided so that they are in surface contact for a typical operation temperature of the radiation shield, which is approximately 40K. The thermal shrink can be used to provide the two shield layers being pressed together at the operation temperature. Preferably, one shield layer is made of copper and the other shield layer is made of aluminum. Another preferred material combination is aluminum and stainless steel, which can achieve a damping coefficient of approximately μs=0.61. 
         [0026]    According to a preferred embodiment at least two shield layers of the radiation shield have a different thickness. By choosing the thickness of the at least two shield layers individually, a suitable thickness combination can be chosen to provide an efficient damping. Preferably, the two shield layers of the radiation shield having a different thickness are made of different materials. By a choosing a suitable combination of materials and thickness, the two shield layers can be provided to enable an efficient damping of vibrations. Additionally, a choice of materials can be used which enables efficient provisioning of the radiation shield. 
         [0027]    According to a preferred embodiment the radiation shield has a uniform thickness in at least one cylindrical wall of the cryogenic vessel. This enables the provision of a radiation shield, which can be used without structural modification of the cryogenic vessel. Preferably, the total thickness of the radiation shield remains the same compared to state of the art cryogenic vessels. Hence, the radiation shield can easily be used to improve state of the art cryogenic vessels. 
         [0028]    According to a preferred embodiment the at least one dry-friction area comprises a local patch, which is attached to the radiation shield. This is preferably used in an area, where space is less constrained. Hence, the patch could be applied on a flange of the radiation shield. The dimensions of the radiation shield in a radial direction remain unaffected. The patch can be easily applied, e.g. by rolling and/or spot welding, so that the radiation shield with reduced vibrations be provided with only small additional effort. Hence, in case of cryogenic vessels for use of a cryogen, dynamic boil-off can be reduced. 
         [0029]    According to a preferred embodiment at least one dry-friction area is provided at one of the longitudinal ends of a cylindrical wall of the cryogenic vessel. At least one coil of the main magnet is typically located at one of the longitudinal ends, so that vibrations with high magnitude are generated in this area. With the dry-friction area provided at one of the longitudinal ends of a cylindrical wall, these vibrations can be efficiently reduced. Hence, in case of cryogenic vessels for use of a cryogen, dynamic boil-off can be reduced. 
         [0030]    According to a preferred embodiment at least one dry-friction area is provided at one flange of the cryogenic vessel. The flange of the cryogenic vessel provides enough space, so that the dry-friction area can easily be formed therein without violating space constraints. The superconductive magnet as well as the cryogenic vessel usually do not underlie constructive restrictions in their longitudinal direction, so that the two shield layers can easily be provided at the flanges of the cryogenic vessel. Furthermore, at least one coil of the main magnet is typically located close to a flange, so that vibrations with high magnitude are generated in this area. With the dry-friction area located in the flange, these vibrations can be efficiently reduced. 
         [0031]    According to a preferred embodiment the inner mounting structure is provided as an inner vessel for mounting the superconductive main coils therein, whereby the inner vessel is adapted for containing a cryogen. The main magnet coils are mounted within the inner vessel, so that they can be efficiently cooled by the cryogen. The cryogen, e.g. liquid helium, enables heat buffering, which facilitates the cooling of the main magnet coils. The main magnet coils can be mounted directly to inner and outer cylindrical walls of the inner vessel. 
         [0032]    According to a further preferred embodiment the inner mounting structure comprises a mounting frame, for mounting the inner and/or outer set of outer coils within the inner vessel. The mounting frame can be mounted to the inner cylindrical wall of the inner vessel or to the outer cylindrical wall of the inner vessel. When mounted to the inner cylindrical wall of the inner vessel, the mounting frame may extend towards the outer cylindrical wall, where the set of outer coils can be mounted at least partially to the mounting frame. When mounted to the outer cylindrical wall of the inner vessel, the mounting frame may extend towards the inner cylindrical wall, where the set of inner coils can be mounted at least partially to the mounting frame. Furthermore, the mounting frame can be mounted to both the inner and outer cylindrical walls and extend between the inner and outer cylindrical walls. With the mounting frame, the set of inner and/or the outer coils can be mounted to the mounting frame at the respective position at the inner and/or outer cylindrical wall, without being mounted thereto. Accordingly, the set of inner and/or the outer coils can be mounted entirely or at least partially to the mounting frame, so that a space for circulation of the cryogen can be provided between the set of inner and/or the outer coils and the inner and/or outer cylindrical wall, respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such an embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
           [0034]    In the drawings: 
           [0035]      FIG. 1  shows a superconductive magnet of a magnetic resonance examination system known in the art, 
           [0036]      FIG. 2  shows a dynamic boil-off transfer function over the frequency having peaks, which are related to mechanical resonances, 
           [0037]      FIG. 3  shows a decrease of internal material damping for low temperatures, 
           [0038]      FIG. 4  shows a different superconductive magnet of a magnetic resonance examination system known in the art, 
           [0039]      FIG. 5  shows a general setup of a magnetic resonance examination system in accordance with a preferred embodiment of the invention, 
           [0040]      FIG. 6  shows in detail a first embodiment of the main magnet of  FIG. 5  including its cryogenic vessel as a partial sectional view including its rotational axis of symmetry, 
           [0041]      FIG. 7  shows in detail a sectional view of two layers of the radiation shield of the superconductive magnet of  FIG. 6  with and without vibration, 
           [0042]      FIG. 8  shows in detail a second embodiment of the main magnet of  FIG. 5  including its cryogenic vessel as a partial sectional view including its rotational axis of symmetry, and 
           [0043]      FIG. 9  shows in detail a third embodiment of the main magnet of  FIG. 5  including its cryogenic vessel as a partial sectional view including its rotational axis of symmetry. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0044]      FIG. 5  shows a schematic illustration of a part of an embodiment of a magnetic resonance (MR) examination system  110  comprising an MR scanner  112 . The MR examination system  110  includes a main magnet  114  provided for generating a static magnetic field. The main magnet  114  has a central bore that provides an examination space  116  around a center axis  118  for a subject of interest  120 , usually a patient, to be positioned within. In an alternative embodiment a different type of MR imaging system providing an examination region within a main magnetic field is used. Further, the MR examination system  110  comprises a magnetic gradient coil system  122 , provided for generating gradient magnetic fields superimposed to the static magnetic field. The main magnet  122  is concentrically arranged within the bore of the main magnet  114 , as known in the art. 
         [0045]    Further, the MR examination system  110  includes a radio frequency (RF) antenna device  140  designed as a whole-body coil having a tubular body. The RF antenna device  140  is provided for applying an RF magnetic field to the examination space  116  during RF transmit phases to excite nuclei of the subject of interest  120 . The RF antenna device  140  is also provided to receive MR signal from the excited nuclei during RF receive phases. In a state of operation of the MR examination system  110 , RF transmit phases and RF receive phases are taking place in a consecutive manner. The RF antenna device  140  is arranged concentrically within the bore of the main magnet  114 . As is known in the art, a cylindrical metal RF screen  124  is arranged concentrically between the main magnet  122  and the RF antenna device  140 . 
         [0046]    Moreover, the MR examination system  110  comprises an MR image reconstruction unit  130  provided for reconstructing MR images from the acquired MR signals and an MR imaging system control unit  126  with a monitor unit  128  provided to control functions of the MR scanner  112 , as is commonly known in the art. Control lines  132  are installed between the MR imaging system control unit  126  and an RF transmitter unit  134  that is provided to feed RF power of an MR radio frequency to the RF antenna device  140  via an RF switching unit  136  during the RF transmit phases. The RF switching unit  136  in turn is also controlled by the MR imaging system control unit  126 , and another control line  138  is installed between the MR imaging system control unit  126  and the RF switching unit  136  to serve that purpose. During RF receive phase, the RF switching unit  136  directs the MR signals from the RF antenna device  140  to the MR image reconstruction unit  130  after pre-amplification. 
         [0047]    As can be seen in  FIGS. 6 and 8  in detail, the main magnet  114  comprises two sets of coils  142 ,  144 , a set of inner coils  142  and a set of outer coils  144 . The set of inner coils  142  comprises inner conductive loops  158 , which are cylindrically arranged in an inner part of the main magnet  114  and spaced apart along the z-axis of the main magnet  114 , and the set of outer coils  144  comprises a set of outer conductive loops  160 , which are arranged in parallel to each other and spaced apart along the z-axis of the main magnet  114 . 
         [0048]      FIGS. 6 and 7  refer to the main magnet  114  shown in  FIG. 5  according to a first embodiment. The main magnet  114  is a superconductive magnet and comprises a cryogenic vessel  200 , as can be seen in  FIG. 6 . The cryogenic vessel  200  comprises an inner vessel  202 , also referred to as 4K vessel, an outer 300K vessel  204  and a radiation shield  206 , which is located between the inner vessel  202  and the outer 300K vessel  204  and which surrounds the inner vessel  202 . The inner vessel  202 , the radiation shield  206 , and the outer 300K vessel  204  are spaced apart for thermal isolation purposes. In this embodiment, the inner vessel  202  is provided to contain liquid helium, which is used as cryogen. 
         [0049]    The inner vessel  202  in the first embodiment is used as inner mounting structure for mounting the inner and outer coils  142 ,  144  therein. The set of inner coils  142  is mounted at an inner cylindrical wall  214  within the inner vessel  202 , and the set of outer coils  144  is mounted at an outer cylindrical wall  216  within the inner vessel  202 . 
         [0050]    The radiation shield  206  comprises an inner and an outer shield layer  208 ,  210 , which are stacked on each other in surface contact. The inner and outer shield layers  208 ,  210 , are connected by rolling, whereby the outer layer  210  is stretched and the inner layer  208  is compressed. The entire radiation shield  206  including the inner and outer cylindrical walls  214 ,  216  and the flanges  218  of the cryogenic vessel  200  comprises the two shield layers  208 ,  210 . 
         [0051]    The two shield layers  208 ,  210  are locally connected to each other by spot welding, whereby the shield layers  208 ,  210  are mechanically attached to each other at welding spots  212 . The welding spots  212  are spaced apart from each other, as can be seen e.g. in  FIG. 6 , so that the shield layers  208 ,  210  can move relative to each other between the welding spots  212  to enable dry friction when the radiation shield  206  is deformed, e.g. due to vibrations caused by gradient switching. Hence, the entire radiation shield  206  in this embodiment is provided as dry friction area. The two shield layers  208 ,  210  are pressed onto each other to increase dry friction. 
         [0052]    The two shield layers  208 ,  210  of the radiation shield  206  are made of aluminum and have the same thickness in this embodiment, providing a radiation shield  206  with a uniform thickness. The total thickness of the radiation shield  206  corresponds to the thickness of typical radiation shields of state of the art cryogenic vessels  200 . 
         [0053]    In an alternative embodiment, the two shield layers  208 ,  210  of the radiation shield  206  have a different thickness. 
         [0054]    In an alternative embodiment, the second shield layer  210  is locally provided as a patch on the first shield layer  208  in the area of the flanges  218  of the cryogenic vessel  200  and locally connected to each other by spot welding. 
         [0055]      FIG. 8  refers to the main magnet  114  shown in  FIG. 5  according to a second embodiment. The main magnet  114  of the second embodiment is in major features identical to the main magnet  114  of the first embodiment, so that the details described above in respect to the first embodiment also apply to the main magnet  114  of the second embodiment. Differences between the main magnet  114  of the first and second embodiment are described below. Features not described below in detail are supposed to be identical to respective features of the first embodiment. 
         [0056]    The main magnet  114  of the second embodiment is a superconductive magnet and comprises a cryogenic vessel  200 , as can be seen in  FIG. 8 . The cryogenic vessel  200  comprises in accordance with the first embodiment an inner vessel  202 , an outer 300K vessel  204  and a radiation shield  206 , which is located between the inner vessel  202  and the outer 300K vessel  204 . According to the second embodiment, the inner vessel  202  is provided to contain liquid helium, which is used as cryogen. 
         [0057]    The inner vessel  202  in the second embodiment is used as inner mounting structure for mounting the inner and outer coils  142 ,  144  therein. The set of inner coils  142  is mounted at an inner cylindrical wall  214  within the inner vessel  202 . The inner vessel  202  comprises a mounting frame  220 , and the set of outer coils  144  is mounted at the mounting frame  220  within the inner vessel  202 . The mounting frame  220  in this embodiment is mounted to the inner cylindrical wall  214  of the inner vessel  202  and extends towards the outer cylindrical wall  216 , where the set of outer coils  144  is mounted to the mounting frame  220  without being in contact with the outer cylindrical wall  216 . In a modified embodiment, the mounting frame  220  is mounted to the outer cylindrical wall  216 , and the set of inner coils  142  is mounted at the mounting frame  220 . According to a further modified embodiment, the sets of inner and outer coils  142 ,  144  are both mounted on the mounting frame, whereby the mounting frame is either mounted at the inner cylindrical wall  214  of the inner vessel  202 , at the outer cylindrical wall  216  of the inner vessel  202 , or at the inner and outer cylindrical wall  214 ,  216 . 
         [0058]    According to the second embodiment, the radiation shield  206  comprises an inner and an outer shield layer  208 ,  210 , which are stacked on each other in surface contact. The inner and outer shield layers  208 ,  210 , are connected by rolling, whereby the outer layer  210  is stretched and the inner layer  208  is compressed. The entire radiation shield  206  including the inner and outer cylindrical walls  214 ,  216  and the flanges  218  of the cryogenic vessel  200  comprises the two shield layers  208 ,  210 . The two shield layers  208 ,  210  are locally connected to each other by spot welding, whereby the shield layers  208 ,  210  are mechanically attached to each other at welding spots  212 , as described above in respect to the first embodiment. 
         [0059]      FIG. 9  shows a main magnet  114  as shown in  FIG. 5  according to a second embodiment. Mayor components of the main magnet  114  of the first and second embodiment are identical, so that the same reference numbers are used. Detail of the main magnet  114  not discussed in respect to the second embodiment correspond to those of the main magnet  114  of the first embodiment. 
         [0060]    The main magnet  114  according to the second embodiment is a superconductive magnet having a cryogenic vessel  200 . The cryogenic vessel  200  comprises an outer 300K vessel  204  and a radiation shield  206 , which is located inside the outer 300K vessel  204 . The radiation shield  206  and the outer 300K vessel  204  are spaced apart for thermal isolation purposes. 
         [0061]    The structure of the radiation shield  206  according to the second embodiment is as described above in respect to the first embodiment. 
         [0062]    Within the radiation shield  206  is located an inner mounting structure  202  for mounting the inner and outer coils  142 ,  144 . The set of inner coils  142  is mounted at an inner cylindrical wall  214  of the cryogenic vessel  200 , and the set of outer coils  144  is mounted at an outer cylindrical wall  216  of the cryogenic vessel  200 . 
         [0063]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. 
       REFERENCE SYMBOL LIST 
       [0000]    
       
           10  superconductive magnet (state of the art) 
           12  cryogenic vessel (state of the art) 
           14 ,  16  main magnet coils (state of the art) 
           22  inner layer, 4K vessel (state of the art) 
           24  radiation shield (state of the art) 
           26  outer layer, 300K vessel (state of the art) 
           28  inner wall (state of the art) 
           30  outer wall (state of the art) 
           32  mounting structure 
           110  magnetic resonance (MR) examination system 
           112  magnetic resonance (MR) scanner 
           114  main magnet, superconductive magnet 
           116  RF examination space 
           118  center axis 
           120  subject of interest 
           122  magnetic gradient coil system 
           124  RF screen 
           126  MR imaging system control unit 
           128  monitor unit 
           130  MR image reconstruction unit 
           132  control line 
           134  RF transmitter unit 
           136  RF switching unit 
           138  control line 
           140  radio frequency (RF) antenna device 
           142  set of inner coils 
           144  set of outer coils 
           158  inner conductive loop 
           160  outer conductive loop 
           200  cryogenic vessel 
           202  inner mounting structure, inner vessel, 4K vessel 
           204  outer 300K vessel 
           206  radiation shield 
           208  inner shield layer 
           210  outer shield layer 
           212  welding spot 
           214  inner cylindrical wall 
           216  outer cylindrical wall 
           218  flange 
           220  mounting frame