Patent Publication Number: US-2006006866-A1

Title: Mri system with a conductive member having a damping effect for vibrations

Description:
The invention relates to a magnetic resonance imaging (M) system comprising an examination volume, a main magnet system for generating a magnetic field having a main field portion with a substantially constant magnetic field strength in the examination volume, a gradient magnet system for generating gradients of the main field portion, and a damping member which is mounted to a part of the MRI system susceptible to vibrations relative to the magnetic field during operation, said damping member comprising an electrically conductive member which is present in the magnetic field and in which eddy currents are generated as a result of said vibrations.  
      An MRI system of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 6,326,788. The known MRI system, which is used to make images of the entrails of a patient&#39;s body by means of a nuclear magnetic resonance method, comprises a cylindrical examination volume, which is enclosed by a cylindrical magnet housing in which the main magnet system and the gradient magnet system are accommodated. In order to limit the time necessary for an examination, the gradients of the main field portion of the magnetic field are altered at relatively high frequencies. For this purpose, the electric currents in the coils of the gradient magnet system are altered at high frequencies. The electrically conductive member of the known MRI system is a cylindrical closed metal plate, which is rigidly mounted to the gradient magnet system and is arranged between the gradient magnet system and the main magnet system. The conductive member is used as an eddy current shield which limits the generation of unwanted eddy currents in the conductive portions of the main magnet system under the influence of the alternating magnetic field of the currents in the gradient magnet system.  
      As a result of the electromagnetic interaction between the magnetic field of the main magnet system and the altering currents in the coils of the gradient magnet system, alternating Lorentz forces are exerted on said coils causing unwanted mechanical and acoustic vibrations of the gradient magnet system. In order to limit the transmission of said mechanical vibrations to the main magnet system and further to the other parts of the known MRI system, the gradient magnet system is resiliently mounted to the main magnet system. In this manner, particularly the transmission of mechanical vibrations with a relatively high frequency from the gradient magnet system to the main magnet system and other parts of the MRI system is limited. As a result of the fact that the gradient magnet system is resiliently mounted to the main magnet system, however, vibrations with a relatively low frequency of the gradient magnet system relative to the main magnet system occur, which lead to distortions of the gradients of the main field portion of the magnetic field and to distortions of the image generated by the MRI system. Although not explicitly described in U.S. Pat. No. 6,326,788, said low frequency vibrations of the gradient magnet system of the known MRI system are damped and thus limited by the presence of the conductive member in the magnetic field of the main magnet system, the conductive member thus additionally forming a damping member for vibrations of the gradient magnet system. Said damping effect of the conductive member is caused by additional eddy currents in the conductive member, which are generated as a result of the fact that the conductive member is rigidly mounted to the gradient magnet system and therefore vibrates relative to the magnetic field. The electromagnetic interaction between said additional eddy currents and the magnetic field lead to Lorentz forces, exerted on the conductive member, which damp the vibrations of the conductive member and, accordingly, also the vibrations of the gradient magnet system rigidly mounted to the conductive member. Although in this manner distortions of the main field portion of the magnetic field and distortions of the image caused by vibrations of the gradient magnet system can be limited to an acceptable level, a disadvantage of the known MRI system is that the eddy currents in the conductive member lead to unacceptable distortions of the main field portion of the magnetic field and to unacceptable distortions of the image generated by the MRI system.  
      It is an object of the present invention to provide a magnetic resonance imaging (MRI) system of the kind mentioned in the opening paragraph in which distortions of the main field portion of the main magnet system and distortions of the image caused by the eddy currents in the conductive member are limited to an acceptable level without limiting the vibration damping effect of the damping member.  
      In order to achieve this object, a magnetic resonance imaging ( 3 R) system according to the invention is characterized in that the conductive member is arranged in a secondary portion of the magnetic field at a distance from the main field portion, the secondary portion having a magnetic field strength which differs by more than 25% from the magnetic field strength of the main field portion. The invention is based on the insight that a relation exists between the degree, to which the magnetic field strength of the main field portion of the magnetic field is influenced by the presence of eddy currents in a position at a predetermined distance from the main field portion, and the magnetic field strength of the magnetic field in said position. As said distance increases, on the one hand, the difference between the magnetic field strength of the magnetic field in said position and the magnetic field strength of the main field portion will increase and, on the other hand, the influence of eddy currents present in said position on the magnetic field strength of the main field portion will strongly decrease. It was found that the level, at which the magnetic field strength of the main field portion is distorted by the presence of eddy currents in a predetermined position in the magnetic field, is acceptably low if the magnetic field strength of the magnetic field in said predetermined position differs by more than 25% from the magnetic field strength of the main field portion. As according to the invention the conductive member of the damping member is arranged in said secondary portion of the magnetic field where the magnetic field strength differs by more than 25% from the magnetic field strength of the main field portion, the distortion of the main field portion by the eddy currents generated in the conductive member will be acceptable. As in said secondary portion the magnetic field strength is up to 75%, or even 125% or more, of the magnetic field strength of the main field portion, the eddy currents generated by the magnetic field in the conductive member will be sufficiently strong to provide a sufficiently large damping effect.  
      A particular embodiment of an MRI system according to the invention is characterized in that the magnetic field strength of the secondary portion differs by more than 50% from the magnetic field strength of the main field portion. In positions in the magnetic field, where the magnetic field strength differs by more than 50% from the magnetic field strength of the main field portion, the distance to the main field portion is so large that the influence of eddy currents in said position on the magnetic field strength of the main field portion is negligible. Also in this embodiment, the eddy currents generated in the conductive member by the local magnetic field, which has a magnetic field strength of up to 50%, or even 150% or more, of the field strength of the main field portion, appears to be strong enough to provide a sufficiently large damping effect.  
      A particular embodiment of an MRI system according to the invention is characterized in that the conductive member is arranged in a stray field portion of the magnetic field. The stray field of the magnetic field of the main magnet system is usually present at a relatively large distance from the main field portion, so that in this particular embodiment the eddy currents in the conductive member have a negligible influence on the magnetic field strength of the main field portion. On the other hand, however, the magnetic field strength of the stray field is considerable, so that the eddy currents in the conductive member still provide a considerable damping effect.  
      A particular embodiment of an MRI system according to the invention is characterized in that the conductive member is mounted to the gradient magnet system. In this embodiment the damping member is used to damp vibrations of the gradient magnet system relative to the main magnet system and other parts of the MRI system. Vibrations of the gradient magnet system are caused by alternating Lorentz forces exerted on the gradient magnet system as a result of the electromagnetic interaction between the altering currents in the gradient magnet system and the magnetic field of the main magnet system. Said vibrations of the gradient magnet system constitute the main source for mechanical and acoustic vibrations of the MRI system, so that in this embodiment the damping member is effectively used.  
      A further embodiment of an MRI system according to the invention is characterized in that the main magnet system comprises a first substantially rotationally symmetrical portion and a second substantially rotationally symmetrical portion at a distance from the first portion, wherein the examination volume is present between the first and the second portion, and wherein the gradient magnet system comprises a first and a second portion arranged, respectively, in a central cavity of the first portion of the main magnet system and in a central cavity of the second portion of the main magnet system, a first and a second conductive member being mounted, respectively, to the first portion of the gradient magnet system and to the second portion of the gradient magnet system and being arranged, respectively, in a portion of the central cavity of the first portion of the main magnet system remote from the examination volume and in a portion of the central cavity of the second portion of the main magnet system remote from the examination volume. In this embodiment the MRI system is of the so-called open type, wherein the examination volume is open and easily accessible as a result of its position between said two portions of the main magnet system. Each portion of the gradient magnet system is mounted to a separate damping member, so that the vibrations of each individual portion of the gradient magnet system are effectively damped. The portions of said central cavities, which are remote from the examination volume, constitute a practical and efficient housing for the conductive members of the damping members in that sufficient space is present for the accommodation of the conductive members, in that sufficient distance to the examination volume is present to prevent unwanted distortions of the main field portion of the magnetic field by the eddy currents in the conductive members, and in that the magnetic field in said central cavities has a sufficiently large magnetic field strength to provide an adequate damping effect.  
      A yet further embodiment of an MRI system according to the invention is characterized in that the first and the second conductive member each comprise a substantially circular cylindrical metal plate which is concentrically arranged relative to, respectively, the first portion of the main magnet system and the second portion of the main magnet system. Usually the portions of the central cavities remote from the examination volume are circular cylindrical. In this yet further embodiment the volumes of said portions of the central cavities are optimally used for the accommodation of the conductive members as a result of the circular cylindrical shape of the conductive members. Since the magnetic field in the central cavities is oriented substantially parallel to the central axes of the central cavities, the magnetic field is oriented substantially parallel to the plates of the conductive members, as a result of which an effective damping effect is achieved.  
      A yet further embodiment of an MRI system according to the invention is characterized in that the first and the second conductive member each comprise a closed conductive metal winding having winding portions extending substantially parallel to a central axis of, respectively, the first portion of the main magnet system and the second portion of the main magnet system. Since the magnetic field in the central cavities is oriented substantially parallel to the central axes of the central cavities, the magnetic field is oriented substantially parallel to said winding portions of the conductive members, as a result of which an effective damping effect is achieved. A further advantage of the windings is that the shape of the windings can be optimized so as to minimize the influence of the eddy currents in the windings on the main field portion of the magnetic field.  
      A particular embodiment of an MRI system according to the invention is characterized in that the conductive member is mounted to a housing of the main magnet system. In this embodiment the damping member is used to damp vibrations of the housing of the main magnet system relative to the main magnet system and other parts of the MRI system. Vibrations of said housing are for example caused by vibrations which are transmitted from the gradient magnet system to said housing. Since the housing usually comprises relatively large portions having specific resonance frequencies for mechanical vibrations, mechanical vibrations of the housing at said resonance frequencies are easily intensified, and as a result constitute an important source of mechanical and acoustic vibrations of the MRI system if no measures are taken. In this particular embodiment the damping member provides an effective damping effect for said vibrations of the housing of the main magnet system.  
      A further embodiment of an MRI system according to the invention is characterized in that the main magnet system and the gradient magnet system are substantially circular cylindrical, wherein the gradient magnet system surrounds the examination volume and the main magnet system surrounds the gradient magnet system, the conductive member being mounted to an annular end wall of the housing of the main magnet system. In this embodiment the MRI system is of the so-called closed cylindrical type, wherein the examination volume is substantially completely surrounded by the gradient and main magnet systems. The annular end walls of the housing of the main magnet system constitute a practical and efficient part of said housing for mounting the conductive member of the damping member in that sufficient space is present for the accommodation of the conductive member, in that sufficient distance to the main field portion of the magnetic field in the examination volume is present to prevent unwanted distortions of the main field portion by the eddy currents in the conductive member, and in that the magnetic field at the location of said end walls has a sufficiently large magnetic field strength to provide an adequate damping effect.  
      A further embodiment of an MRI system according to the invention is characterized in that the main magnet system and the gradient magnet system are substantially circular cylindrical, wherein the gradient magnet system surrounds the examination volume and the main magnet system surrounds the gradient magnet system, the conductive member being mounted to a portion of a cylindrical outer wall of the housing of the main magnet system adjacent to an annular end wall of said housing. In this embodiment the MRI system is of the so-called closed cylindrical type, wherein the examination volume is substantially completely surrounded by the gradient and main magnet systems. Said portion of the cylindrical outer wall of the housing of the main magnet system constitutes a practical and efficient part of said housing for mounting the conductive member of the damping member in that sufficient space is present for the accommodation of the conductive member, in that sufficient distance to the main field portion of the magnetic field in the examination volume is present to prevent unwanted distortions of the main field portion by the eddy currents in the conductive member, and in that the magnetic field at the location of said portion of the cylindrical outer wall has a sufficiently large magnetic field strength to provide an adequate damping effect  
      A further embodiment of an MRI system according to the invention is characterized in that the conductive member is mounted to a support member which supports the housing of the main magnet system. The support member of the housing of the main magnet system usually constitutes a comparatively rigid part of the MRI system in which the level of mechanical vibrations is comparatively small. Since in this further embodiment the conductive member is mounted to the comparatively rigid support member, the damping member is used to damp mechanical vibrations of the magnetic field generating portions of the main magnet system relative to the support member and relative to the surroundings of the MRI system. Vibrations of said portions of the main magnet system are for example caused by vibrations which are transmitted from the gradient magnet system to said portions. Said vibrations are unwanted because they lead to distortions of the main field portion of the magnetic field. In this further embodiment the damping member provides an effective damping effect for said vibrations of the magnetic field generating portions of the main magnet system.  
      A further embodiment of an MRI system according to the invention is characterized in that the conductive member comprises a substantially flat metal plate. The flat metal plate provides a particularly effective damping effect. 
    
    
      In the following, embodiments of an MRI system in accordance with the invention will be described in detail with reference to the Figures, in which  
       FIG. 1  schematically shows a first embodiment of an MRI system in accordance with the invention,  
       FIG. 2  schematically shows a second embodiment of an MRI system in accordance with the invention, and  
       FIG. 3  schematically shows an alternative conductive member of an MRI system in accordance with the invention. 
    
    
       FIG. 1  schematically shows the main components of a first embodiment of an MRI system  1  in accordance with the invention. The MRI system  1  is of the so-called closed cylindrical type and is used to make images of the entrails of a patient&#39;s body by means of a nuclear magnetic resonance method. For this purpose the MRI system  1  comprises a main magnet system  3  comprising a substantially circular cylindrical housing  5  in which a cryogenic container  7  is present accommodating a number of annular superconducting electric coils  9 . In FIG. I the housing  5  is only partially drawn, so that a part of the superconducting coils  9  is visible. The MRI system  1  further comprises an examination volume  11  in which a patient to be examined can be positioned. In this open type MRI system  1 , the examination volume  11  is substantially completely surrounded by the main magnet system  3 . The MRI system  1  also comprises a gradient magnet system  13  comprising a substantially circular cylindrical housing  15  accommodating a number of electrical gradient coils not shown in  FIG. 1 . The gradient magnet system  13  is arranged between the main magnet system  3  and the examination volume  11 , so that the main magnet system  3  surrounds the gradient magnet system  13  and the gradient magnet system  13  surrounds the examination volume  11 . During operation the superconducting coils  9  of the main magnet system  3  are used to generate a magnetic field which has a main field portion  17  with a substantially constant magnetic field strength B 0  in a portion of the examination volume  11 . The gradient coils of the gradient magnet system  13  are used to generate altering gradients of the main field portion  17  in order to select a series of successive positions in the patient&#39;s body to be imaged in accordance with the nuclear magnetic resonance method used. The housing  5  of the main magnet system  3  is supported on a horizontal floor  19  by means of a support member, comprising four rigid feet  21  in the embodiment shown, while the housing  15  of the gradient magnet system  13  is mounted to an inner wall  23  of the housing  5  of the main magnet system  3  by means of mounting members not shown in  FIG. 1 .  
      A known problem of MRI systems is the presence of mechanical and acoustic vibrations in the MRI system. Said vibrations are mainly caused by alternating Lorentz forces which are exerted on the gradient coils of the gradient magnet system  13  as a result of electromagnetic interaction between the magnetic field of the main magnet system  3  and the altering currents in the gradient coils, which are necessary to generate the required gradients of the main field portion  17 . In order to limit the time necessary for an examination, said gradients and accordingly also the currents in the gradient coils are altered at relatively high frequencies. As a result, said Lorentz forces and the vibrations caused thereby have both low-frequency components and high-frequency components, which are transmitted from the gradient magnet system  13  to the main magnet system  3  and to other components of the MRI system  1 . The transmission of the high-frequency components of the vibrations is considerably limited by mounting the gradient magnet system  13  to the main magnet system  3  by means of elastic suspension members, not shown in  FIG. 1 . However, a small portion of the high-frequency components of the vibrations will be transmitted to the main magnet system  3 . The transmission of the low-frequency components of the vibrations cannot be avoided by means of said elastic suspension members. Accordingly, part of the vibrations of the gradient magnet system  13  will be transmitted to the main magnet system  3  and will cause unwanted mechanical vibrations of the housing  5  of the main magnet system  3  and of the magnetic field generating portions of the main magnet system  3 , in particular of the superconducting coils  9 . Since the housing  5  usually comprises relatively large parts having a number of specific resonance frequencies for mechanical vibrations, mechanical vibrations of the housing  5  at said resonance frequencies can be easily intensified and, as a result, constitute an important source of mechanical and acoustic vibrations of the MRI system I if no measures are be taken. Mechanical vibrations of the superconducting coils  9  are unwanted because they lead to distortions of the main field portion  17  and, as a result, to distortions of the image generated by the MRI system  1 .  
      In order to limit the vibrations of the housing  5 , the MRI system  1  comprises a first damping member  25  and a second damping member  27 . The first damping member  25  comprises a number of electrically conductive members, in this embodiment a number of substantially flat plates  29  made of a material having a relatively high electrical conductivity, preferably a metal such as copper or aluminium. The flat plates  29  are mounted to an annular end wall  31  of the housing  5  of the main magnet system  3 . It is noted that, for the sake of simplicity, only one flat plate  29  is shown in  FIG. 1 , but that in reality in this embodiment a plurality of such flat plates is circumferentially arranged along the annular end wall  31  at regular mutual distances. In the embodiment shown, similar flat plates, which are not visible in  FIG. 1 , are mounted to the annular end wall  33  of the housing  5  opposite to the end wall  31 . The second damping member  27  also comprises a number of electrically conductive members, in this embodiment a number of curved plates  35 ,  37  which are also made of a material having a relatively high electrical conductivity, preferably a metal such as copper or aluminium. The curved plates  35  and  37  are mounted, respectively, to a first portion  39  and to a second portion  41  of a cylindrical outer wall  43  of the housing  5  of the main magnet system  3 , said first portion  39  being adjacent to the annular end wall  31  of the housing  5  and said second portion  41  being adjacent to the other annular end wall  33  of the housing  5 . It is noted that, for the sake of simplicity, only one curved plate  35  and one curved plate  37  are shown in  FIG. 1 , but that in reality in this embodiment a plurality of such curved plates  35 ,  37  are circumferentially arranged along, respectively, said first portion  39  and said second portion  41  at regular mutual distances.  
      The vibration damping effect of the damping members  25  and  27  is obtained as follows. The plates  29 ,  35 ,  37  of the damping members  25 ,  27  are each present in a stray field portion of the magnetic field of the main magnet system  3 . As a result of the vibrations of the housing  5  the plates  29 ,  35 ,  37  will vibrate relative to the magnetic field of the main magnet system  3  because the plates  29 ,  35 ,  37  are mounted to the housing  5 . As a result of these movements of the plates  29 ,  35 ,  37  relative to the magnetic field, eddy currents will be induced in the plates  29 , 35 ,  37  by the magnetic field. These movements will be damped by the Lorentz forces which are exerted on the plates  29 ,  35 ,  37  as a result of the electromagnetic interaction between these eddy currents and the magnetic field of the main system  3 . Since the plates  29 ,  35 ,  37  are mounted to the housing  5 , the vibrations of the housing  5  will also be damped.  
      In order to limit the vibrations of the magnetic field generating portions of the main magnet system  3 , in particular of the superconducting coils  9 , two of the curved plates  35  and two of the curved plates  37  of the second damping member  27  are each rigidly mounted to one of the four rigid feet  21 . It is noted that in  FIG. 1  only one curved plate  35  and one curved plate  37 , each mounted to one of the feet  21 , are visible. Since said two curved plates  35  and said two curved plates  37  are rigidly mounted to the rigid feet  21 , the level of mechanical vibrations of said four plates  35 ,  37  is comparatively low. As a result, the eddy currents generated in said four plates  35 , 37  are mainly caused by movements of the magnetic field of the main magnet system  3  relative to said four plates  35 ,  37 , i.e. by the vibrations of the superconducting coils  9  relative to the rigid feet  21  and the floor  19 . Said vibrations of the superconducting coils  9  will be damped by electromagnetic reaction forces which are exerted on the superconducting coils  9  as a result of electromagnetic interaction between said eddy currents in said four plates  35 ,  37  and the magnetic field of the main magnet system  3 .  
      An advantage of the position of the flat plates  29  on the annular end walls  31  and  33  and the position of the curved plates  35 ,  37  on the first and second portions  39  and  41  of the cylindrical outer wall  43  is that said positions are at such distances from the main field portion  17  in the examination volume  11  that the eddy currents in the flat plates  29  and in the curved plates  35 ,  37  hardly influence the magnetic field strength B 0  of the main field portion  17 . As a result, said eddy currents do not lead to unacceptable distortions of the main field portion  17  and of the image generated by the MRI system  1 . On the other hand, the magnetic field at the locations of the plates  29 ,  35 , 37  is sufficiently strong and, as a result, the eddy currents induced in the plates  29 ,  35 ,  37  are sufficiently large to provide an adequate damping effect for the vibrations of the housing  5  and of the superconducting coils  9 . The invention is however not limited to embodiments wherein the conductive member of the damping member is arranged in a position as discussed before and as shown in  FIG. 1 . Other positions for the conductive member are also possible within the scope of the invention. In general it was found that the conductive member should be arranged in a secondary portion of the magnetic field of the main magnet system  3  at a distance from the main field portion  17 , for example in a stray field portion as in the embodiment of  FIG. 1 , wherein the magnetic field strength in said secondary portion should differ by more than 25% from the magnetic field strength B 0  of the main field portion  17 . In positions of the magnetic field where the magnetic field strength differs by more than 25% from the magnetic field strength B 0  of the main field portion  17 , the distance to the main field portion  17  appears to be so large that eddy currents present in such positions do not lead to unacceptable distortions of the main field portion  17 . Suitable positions for the conductive member in accordance with the invention can be determined by the skilled person from measurements or calculations of the magnetic field of the main magnet system  3 . In general a sufficiently large damping effect is obtained if the conductive member is arranged in a suitable position thus determined, because the magnetic field strength in such a position is up to 75% or even 125% or more of the magnetic field strength B 0  of the main field portion  17 . A particularly suitable position for the conductive member is a position in the magnetic field of the main field system  3  where the magnetic field strength differs by more than 50% from the magnetic field strength B 0  of the main field portion  17 . In such a position, the distance to the main field portion  17  is so large that the influence of eddy currents in said position on the magnetic field strength of the main field portion  17  is even negligible. On the other hand, an adequate damping effect is still obtained because the magnetic field strength in such a position is up to 50% or even 1 50% or more of the magnetic field strength B 0  of the main field portion  17 .  
      An advantage of the use of the flat plates  29  and of the curved plates  35 ,  37  as the conductive members of the damping members  25 ,  27  is that said plates  29 , 35 ,  37  provide a particularly effective damping effect. However, other kinds of conductive members can also be used instead, as will be discussed hereafter. In general, for a conductive member, an optimum damping effect is achieved if the conductive member has an electrical conductivity such that a time constant of the eddy currents in the conductive member is in a range wherein a period time of the mechanical vibrations to be damped is present. If the time constant of the eddy currents in the conductive member, i.e. the time during which the strength of an induced eddy current is approximately halved as a result of the electrical resistance of the conductive member, is in the range wherein the period time of the mechanical vibrations to be damped is present, the electromagnetic interaction between the magnetic field and said eddy currents provides an optimum damping force on the conductive member. The necessary time constant of the eddy currents is achieved by a suitable electrical conductivity of the conductive member, i.e. by a suitable combination of the material, the shape, and the dimensions of the conductive member.  
      In the embodiment of  FIG. 1  the conductive members of the damping members  25 ,  27  are mounted to the housing  5  in order to limit the vibrations of the housing  5  relative to the magnetic field of the main magnet system  3  and to limit the vibrations of the superconducting coils  9  relative to the rigid feet  21 . The invention also covers embodiments in which the conductive member of the damping member is mounted to another part of the I system susceptible to vibrations relative to the magnetic field during operation in order to damp said vibrations. In the second embodiment of an MRI system  101  according to the invention as shown in  FIG. 2 , for example, the conductive member of the damping member is mounted to the gradient magnet system in order to damp vibrations of the gradient magnet system relative to the main magnet system and the other parts of the MRI system  101 .  FIG. 2  only schematically shows the main components of the MRI system  101 , which is of the so-called open type. The MRI system  101  comprises a lower part  103  and an upper part  105  at a vertical distance from the lower part  103 , the lower part  103  and the upper part  105  being mutually connected by a vertical post  107 . Between the lower part  103  and the upper part  105  an open examination volume  109  is present, which is easily accessible. The lower part  103  and the upper part  105  respectively comprise a first substantially rotationally symmetrical portion  111  and a second substantially rotationally symmetrical portion  113  of a main magnet system  115  of the MRI system  101 . Each portion  111 ,  113  of the main magnet system  115  comprises a housing  117  in which a cryogenic container  119  is present accommodating a number of annular superconducting electric coils  121 . It is noted that in  FIG. 2  the housings  117  are only partially shown, so that parts of the superconducting coils  121  are visible. The lower part  103  and the upper part  105  further comprise, respectively, a first portion  123  and a second portion  125  of a gradient magnet system  127  of the MRI system  101 . Each portion  123 ,  125  of the gradient magnet system  127  has a substantially conical housing  129  accommodating a number of electrical gradient coils not shown in  FIG. 2 . The first portion  123  and the second portion  125  of the gradient magnet system  127  are arranged in a conical portion  131  of, respectively, a central cavity  133  provided in the first portion  111  of the main magnet system  115  and a central cavity  135  provided in the second portion  113  of the main magnet system  115 . During operation the superconducting coils  121  of the main magnet system  115  are used to generate a magnetic field which has a vertically oriented main field portion  137  with a substantially constant magnetic field strength B 0  in a portion of the examination volume  109 . The gradient coils of the gradient magnet system  127  are used to generate altering gradients of the main field portion  137  in accordance with the nuclear magnetic resonance method used.  
      Like the gradient magnet system  13  of the MRI system  1  shown in  FIG. 1 , the first portion  123  and the second portion  125  of the gradient magnet system  127  of the MRI system  101  are susceptible to mechanical vibrations which are caused, during operation, by alternating Lorentz forces exerted on the gradient coils of the gradient magnet system  127 . The first portion  123  and the second portion  125  of the gradient magnet system  127  are mounted in the respective conical portions  131  of the central cavities  133 ,  135  by means of elastic suspension members, not shown in  FIG. 2 . As a result the high-frequency components of the vibrations of said first and second portions  123 ,  125  are considerably limited. However, particularly the low-frequency components of the vibrations of said first and second portions  123 ,  125  are not adequately limited by said elastic suspension members, and would lead to unacceptable distortions of the gradients of the main field portion  137  and to unacceptable distortions of the image generated by the MRI system  101  if no further measures were taken.  
      In order to limit the vibrations of the first portion  123  and the second portion  125  of the gradient magnet system  127 , the MRI system  101  is provided with a damping member comprising a first conductive member  139  mounted to the first portion  123  of the gradient magnet system  127  and a second conductive member  141  mounted to the second portion  125  of the gradient magnet system  127 . In the embodiment shown, the first conductive member  139  and the second conductive member  141  each comprise a substantially circular cylindrical plate  143  made of a material having a relatively high electrical conductivity, preferably a metal such as copper or aluminium. It is noted that the plates  143  are only partially shown in  FIG. 2  for the sake of clarity, but that in reality the plates  143  constitute closed cylindrical bodies. The circular cylindrical plate  143  of the first conductive member  139  is concentrically arranged in a circular cylindrical portion  145  of the central cavity  133  provided in the first portion  111  of the main magnet system  115 , while the circular cylindrical plate  143  of the second conductive member  141  is concentrically arranged in a circular cylindrical portion  145  of the central cavity  135  provided in the second portion  113  of the main magnet system  115 , both circular cylindrical portions  145  being arranged remote from the examination volume  109 .  FIG. 2  also shows that the MRI system  101  is also provided with a number of electrically conductive plates  147 , which are mounted to the housings  117  of the first and second portions  111 ,  113  of the main magnet system  115  in order to damp vibrations of the housings  117 . Said plates  147  will not be further discussed here as their functions and effects are similar to the functions and effects of the plates  29 ,  35 ,  37  of the MRI system  1  as discussed before.  
      The portions of the magnetic field of the main magnet system  115  present in the cylindrical portions  145  of the central cavities  133 ,  135  have a sufficiently large magnetic field strength-to cause the conductive members  139 ,  141  to have an adequate damping effect. Since the cylindrical portions  145  of the central cavities  133 ,  135  are positioned remote from the examination volume  109 , a sufficiently large distance is present between the conductive members  139 ,  141  and the main field portion  137  to prevent the eddy currents in the conductive members  139 ,  141  from causing unacceptable distortions of the main field portion  137  and of the image generated by the MRI system  101 . Since in this manner each portion  123 ,  125  of the gradient magnet system  127  is individually mounted to a separate conductive member  139 ,  141 , the vibrations of each portion  123 ,  125  of the gradient magnet system  127  are individually and thus effectively damped. As a result of the circular cylindrical shape of the conductive members  139 ,  141  the available volumes of the circular cylindrical portions  145  of the central cavities  133 ,  135  are optimally used for the accommodation of the conductive members  139 ,  141 . The portions of the magnetic field of the main magnet system  115  in the central cavities  133 ,  135  are oriented in the vertical direction, i.e. substantially parallel to the central axes of the central cavities  133 ,  135  and substantially parallel to the surfaces of the circular cylindrical plates  143 . This orientation of the magnetic field relative to the plates  143  provides an optimum damping effect of the conductive members  139 ,  141 .  
      It is noted that instead of the circular cylindrical plates  143  other kinds of conductive members can be used instead of the conductive members  139 ,  141  such as, for example, a number of parallel flat plates which are oriented in the vertical direction in the circular cylindrical portions  145  of the central cavities  133 ,  135 . Another example of an alternative conductive member  149 , which can be used in the MRI system  101  instead of the conductive members  139 ,  141 , is schematically shown in  FIG. 3 . In  FIG. 3 a  part of the first portion  123  of the gradient magnet system  127  is shown. The alternative conductive member  149  comprises a carrier  150  which is made from an electrically non-conductive material, in particular a rigid synthetic material. The conductive member  149  further comprises a closed conductive metal winding  151 , in this embodiment made from a copper wire, which is rigidly mounted to the carrier  150 . The winding  151  has winding portions  153 ,  155 ,  157 ,  159  which extend substantially parallel to the central axis of the central cavity  133 , i.e. substantially parallel to the magnetic field in the central cavity, so that these winding portions  153 ,  155 ,  157 ,  159  provide an optimum damping effect of the conductive member  149 . In the embodiment shown in  FIG. 3  the winding  151  has a first relatively short loop  161  at a side facing the examination volume  109  and a second relatively long loop  163  at a side remote from the examination volume  109 , said first and second loops  161 ,  163  being mutually connected by crossing winding portions  165 ,  167 . As a result of this configuration of the winding  151 , the influence of the eddy currents in the winding  151  on the main field portion  137  is further reduced.  
      From the foregoing description of the embodiments it will be clear that the invention broadly covers the use of an eddy-current based damping member in an MRI system to damp the vibrations of a part of the MRI system and the magnetic field generating portions of the main magnet system relative to each other. Accordingly, the damping member can be used to damp the vibrations which said part has relative to the magnetic field, but can also be used to damp the vibrations which the magnetic field generating portions of the main magnet system have relative to a part of the MRI system which is hardly susceptible to vibrations. The damping member may be mounted to any part of the MRI system which is susceptible to vibrations relative to the magnetic field, provided that the conductive member of the damping member is arranged in a secondary portion of the magnetic field having a magnetic field strength which differs by more than 25% from the magnetic field strength of the main field portion. Thus, the damping member may, for example, also be mounted to frame parts or to other housing parts or system enclosing parts of the MRI system susceptible to vibrations relative to the magnetic field. In the MRI system  1 , for example, an additional damping member may be mounted to the gradient magnet system  13  to damp the vibrations of the gradient magnet system  13 . In order to prevent unacceptable distortions of the main field portion  17  by the eddy currents in such an additional damping member, the conductive member of said additional damping member may, for example, be arranged outside the examination volume  11  and be connected to the gradient magnet system  13  by additional rigid mounting members. In the case of a relatively long cylindrical examination volume  11 , the damping members may possibly be mounted near the annular end walls of the cylindrical housing of the gradient magnet system  13 . From the foregoing description of the embodiments it will also be clear that the invention is not limited to the specific embodiments of the damping members and conductive members shown in the Figures.