Abstract:
A generator of time-variable magnetic fields of a magnetic resonance device having an examination space for registering at least one area to be examined of an item being examined has the following features: the conductors of a gradient coil arrangement of the generator define an area at least partially surrounding the examination space, said area contains at least a partial area which is free from conductors of the gradient coil arrangement and which contains conductors of a radio frequency antenna of the generator, and a field flowback space, inter alia for fields of the radio frequency antenna, extends proceeding from the partial area away from the examination space and is delimited beyond said partial area by a radio frequency shield.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the German application No. 10352381.2, filed Nov. 10, 2003 and which is incorporated by reference herein in its entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to a generator of time-variable magnetic fields of a magnetic resonance device and to a magnetic resonance device having said generator.  
       BACKGROUND OF INVENTION  
       [0003]     Magnetic resonance is a known technology for, inter alia, obtaining images of the interior of an item being examined. In a magnetic resonance device a static basic magnetic field generated by a basic field magnet is overlaid with gradient fields switched at high speed and generated by a gradient coil system. The magnetic resonance device further includes a radio frequency (RF) antenna which radiates radio frequency signals into the item being examined in order to trigger magnetic resonance signals, and registers the triggered magnetic resonance signals on the basis of which magnetic resonance images are produced.  
         [0004]     A magnetic resonance device is known from DE 44 14 371 A1 in which a radio frequency shield is located between a radio frequency antenna and a gradient coil system of the magnetic resonance device and is embodied in such a way as to be permeable in the low-frequency range for the electromagnetic fields generated by the gradient coil system and impermeable in the radio frequency range for the fields generated by the radio frequency antenna. The radio frequency shield contains a first electrically conducting layer arrangement and a second such arrangement located opposite the first, mutually separated by a dielectric, with said layer arrangements containing strip conductors arranged side-by-side and mutually separated by electrically insulating slots, with the slots in the first layer arrangement being displaced relative to those in the second arrangement, and with adjacent strip conductors being interconnected in at least one layer arrangement via specifically arranged jumpers which conduct radio frequency currents and which contain, for example, capacitors.  
         [0005]     Possible embodiments of a radio frequency shield are known from, for instance, DE 34 45 724 A1, wherein, for example, shielding layers are arranged on both sides of the gradient coil system of the magnetic resonance device.  
         [0006]     The radio frequency antenna of the magnetic resonance device is embodied in the form of, for example, what is termed a birdcage antenna. A birdcage antenna for generating a homogeneous radio frequency field within a volume enclosed thereby is generally embodied in such a way that conductors are arranged mutually parallel and equidistantly spaced on a cylinder jacket and are interconnected by means of end rings. Tuning to the high-pass and low-pass filter ranges is therein effected by introducing capacitances in each of the conductors or the end rings between the conductors so that a homogeneous radio frequency field results in the event of resonance. Embodiments of a birdcage antenna of said type can be found in, for example, U.S. Pat. No. 4,680,548.  
       SUMMARY OF INVENTION  
       [0007]     It is further known from DE 42 32 884 A1 how, in a magnetic resonance device having a basic field magnet of pole-flange type, to locate a radio frequency shield, embodiable with capacitively bridged slots, between the gradient coil system mounted on the pole flanges and embodied in substantially planar form and the antenna system. It is further known from DE 42 32 884 A1 that the endeavor in the case of basic field magnets of pole-flange type is to minimize the pole-flange spacing so as to keep the weight of the basic field magnet low and achieve better basic magnetic field homogeneity. All the elements installed between the pole flanges, such as the gradient coil system, radio frequency shield, and antenna system, must therefore also be kept as flat as possible. On the other hand it is advantageous for the antenna system to be located as far away as possible from the radio frequency shield so that the antenna system can operate with a high degree of efficiency, especially when used as a receive antenna for magnetic resonance signals.  
         [0008]     An MR device is known from U.S. Pat. No. 4,864,241 A in which eddy currents are compensated. This is done with the aid of gradient coils that are divided in two and usually form a hollow-cylindrical unit. A likewise hollow-cylindrical RF antenna having a smaller radius is introduced into the gradient coil unit for generating RF fields. Disadvantages of a structural design of this type are that it requires a large amount of space and that an examination volume of the MR device is determined by the diameter of the RF antenna.  
         [0009]     An object of the invention is to provide a generator of time -variable magnetic fields of a magnetic resonance device and a magnetic resonance device having the generator so that the generator is as compact as possible while at the same time exhibiting a high degree of efficiency.  
         [0010]     Said object is achieved by the claims. Advantageous embodiments are described in the dependent claims.  
         [0011]     According to an embodiment of the invention, a generator of time-variable magnetic fields of a magnetic resonance device having an examination space for registering at least one area to be examined of an item being examined exhibits the following features: 
        the conductors of a gradient coil arrangement of the generator define an area at least partially surrounding the examination space,     said area contains at least a partial area which is free from conductors of the gradient coil arrangement and which contains conductors of a radio frequency antenna of the generator, and     a field flowback space, inter alia for fields of the radio frequency antenna, extends proceeding from the partial area away from the examination space and is delimited beyond said partial area by a radio frequency shield.        
 
         [0015]     Conventional solutions already have to provide sufficient space between the gradient coils of a non-actively shielded gradient coil system and the basic field magnet or, as the case may be, between the gradient coils and the associated shield coils of an actively shielded gradient coil system for closing field lines of the gradient fields that can be generated by means of the gradient coils. Said space not previously avail able for a flowback of a radio frequency field capable of being registered by means of the radio frequency antenna is utilized according to the invention and is hence doubly used, as it were, so that the combination of gradient coil system and radio frequency antenna is embodiable having less overall height compared to like conventional solutions and with a comparable degree of efficiency. In a first instance this enables the basic field magnet of a magnetic resonance device to be dimensioned smaller without altering the space in said magnetic resonance device provided for accommodating a patient, and thus to be substantially more economical. In a second instance, more space is achieved for accommodating a patient without altering the basic field magnet, said increased space inter alia enhancing patient comfort. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Further advantages, features, and specifics of the invention will emerge from the exemplary embodiments of the invention described below with the aid of the Figures.  
         [0017]      FIG. 1  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space for accommodating a patient and having a generator of time-variable magnetic fields, which generator contains a field flowback space having a bracing element,  
         [0018]      FIG. 2  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space for accommodating a patient and having a generator of time-variable magnetic fields, which generator contains a field flowback space and a shimming device located outside said field flowback space,  
         [0019]      FIG. 3  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space for accommodating a patient and having a generator of time-variable magnetic fields, which generator contains a field flowback space and a shimming device traversing said field flowback space, and  
         [0020]      FIG. 4  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space for accommodating a patient and having a generator of time-variable magnetic fields, which generator contains two field flowback spaces. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0021]     As an exemplary embodiment of the invention,  FIG. 1  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space  115  for accommodating a patient. Said magnetic resonance device contains a substantially hollow-cylindrical superconducting basic field magnet  110  for generating an as homogeneous as possible static basic magnetic field in said space  115  for accommodating a patient. Located in the cavity of said basic field magnet  110  is a generator  120  of time-variable magnetic fields which contains an actively shielded gradient coil system and a radio frequency antenna.  
         [0022]     The generator  120  contains two hollow-cylindrical areas  121  and  122  in which are located electrical conductors of a first and second transversal gradient coil and of a longitudinal gradient coil for generating gradient fields that can be switched at high speed. Extending between the areas  121  and  122  is a hollow-cylindrical free space  131  functioning as an opening for a hollow-cylindrical field flowback space  132  located behind it and forming an undercut in terms of said hollow-cylindrical free space  131 . An area of the generator  120  facing the space  115  for accommodating a patient, including the areas bordering the free space  131  and field flowback space  132 , is furthermore provided with a radio frequency shield  140 . Said radio frequency shield  140  contains, in a known manner, for example a plurality of layers of an electrically conducting foil and is embodied with capacitively bridged slots.  
         [0023]     The hollow-cylindrical free space  131  is bridged with conductor rods  151  for forming the radio frequency antenna. Said conductor rods  151  are conductively connected on a radio frequency basis to the radio frequency shield  140 . A part of the radio frequency shield  140  also forms a part of the current paths of the radio frequency antenna. The conductor rods  151  are interrupted approximately in the middle for intermediately connected resonance capacitors  152  for embodying a low-pass birdcage radio frequency antenna.  
         [0024]     Field lines of a radio frequency field that can be generated by means of the radio frequency antenna or, as the case may be, registered by means of the radio frequency antenna can herein close across the hollow-cylindrical free space  131  within the field flowback space  132 . Double use, as it were, is here made of the field flowback space  132  in that it allows both the field lines of the radio frequency field and the field lines of the gradient fields generated by the gradient coils to close within it. The radio frequency antenna&#39;s efficiency is greater the more longitudinally extended the field flowback space  132  is embodied. The longitudinal extension of said field flowback space  132  is limited solely by the need to prevent integral multiples of a λ/2 resonance in the longitudinal direction.  
         [0025]     The generator  120  furthermore contains a hollow-cylindrical area  123  in which are located conductors of the shield coils belonging to the gradient coils. The remaining area of the generator  120  is available for, inter alia, accommodating parts of a shimming system and/or of a cooling system of the generator  120 . In particular two hollow-cylindrical areas  124  and  125  of the remaining area are used for connecting the gradient coils to the associated shield coils.  
         [0026]     A bracing element is located within of the field flowback space  132  for reducing mechanical vibrations and oscillations, and hence noise, occurring while the magnetic resonance device is in operation. Said bracing element is embodied in such a way that, owing to its dielectric properties, it impacts minimally on the functioning of the radio frequency antenna, with the properties of air being ideal for this. The bracing element consists of a combination of a small constituent of fiber-glass reinforced plates  161  serving to provide bracing and a larger constituent of a foam material  162  characterized by low dielectric radio frequency loss, for example Rohacell. Favored by the occurrence in the field flowback space  132  of essentially only radially directed forces, the generator  120  is adequately braced by the plates  161  occupying approximately 15-20% of the volume of the field flowback space  132 . Dielectric loss is kept low owing to the remaining volume&#39;s being filled with the low-loss foam material  162 .  
         [0027]     As a further exemplary embodiment of the invention,  FIG. 2  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space  215  for accommodating a patient. The magnetic resonance device includes a superconducting basic field magnet  210  approximately hollow-cylindrical in shape for generating an as homogeneous as possible static basic magnetic field in the space  215  for accommodating a patient. A generator  220  of time-variable magnetic fields comprising an actively shielded gradient coil system and a radio frequency antenna is located in the cylinder-like cavity of the basic field magnet  210 .  
         [0028]     The generator  220  includes two hollow-cylindrical areas  221  and  222  in which are located electrical conductors of gradient coils of the gradient coil system. Extending between the areas  221  and  222  is a hollow-cylindrical free space  231  functioning as an opening for a hollow-cylindrical field flowback space  232  located behind it and forming an undercut in terms of said hollow-cylindrical free space  231 . The generator  220  furthermore includes a radio frequency shield  240 . The hollow-cylindrical free space  231  is bridged with conductor rods  251  for forming the radio frequency antenna. Said conductor rods  251  are conductively connected on a radio frequency basis to the radio frequency shield  240 . The conductor rods  251  are interrupted approximately in the middle for intermediately connected resonance capacitors  252 .  
         [0029]     The generator  220  furthermore contains a hollow-cylindrical area  223  in which are located conductors of the shield coils belonging to the gradient coils. The remaining area of the generator  220  is available for, inter alia, accommodating parts of a shimming system and/or of a cooling system of the generator  220 . In particular two hollow-cylindrical areas  224  and  225  of the remaining area are used for connecting the gradient coils to the associated shield coils.  
         [0030]     For accommodating a passive shimming device the generator  220  includes free spaces  271  that are distributed in the circumferential direction and continuous in the axial direction. One of the free spaces  271  is shown in the longitudinal section view with the shimming device introduced into it in longitudinal section. The shimming device includes a support element  275  fitted with shimming elements  277  made of a magnetic material. In another embodiment the shimming device can of course also include an active shim. As the shimming device is located outside the field flowback space  232 , there is advantageously no mutual interference between the radio frequency antenna and the shimming elements  277 . In contrast to  FIG. 1 , the field flowback space  232  shown in  FIG. 2  is filled not with a bracing element but with air. The description for  FIG. 1  otherwise applies analogously to  FIG. 2 .  
         [0031]     As a further exemplary embodiment of the invention,  FIG. 3  shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space  315  for accommodating a patient. The magnetic resonance device includes a superconducting basic field magnet  310  approximately hollow-cylindrical in shape for generating an as homogeneous as possible static basic magnetic field in the space  315  for accommodating a patient. A generator  320  of time-variable magnetic fields comprising an actively shielded gradient coil system and a radio frequency antenna is located in the cylinder-like cavity of the basic field magnet  310 .  
         [0032]     The generator  320  includes two hollow-cylindrical areas  321  and  322  in which are located electrical conductors of gradient coils of the gradient coil system. Extending between the areas  321  and  322  is a hollow-cylindrical free space  331  functioning as an opening for a hollow-cylindrical field flowback space  332  located behind it and forming an undercut in terms of said hollow-cylindrical free space  331 . The generator  320  furthermore includes a radio frequency shield  340 . The hollow-cylindrical free space  331  is bridged with conductor rods  351  for forming the radio frequency antenna of the magnetic resonance device. Said conductor rods  351  are conductively connected on a radio frequency basis to the radio frequency shield  340 . The conductor rods  351  are interrupted approximately in the middle for intermediately connected resonance capacitors  352 .  
         [0033]     The generator  320  furthermore contains a hollow-cylindrical area  323  in which are located conductors of the shield coils belonging to the gradient coils. The remaining area of the generator  320  is available for, inter alia, accommodating parts of a shimming system and/or of a cooling system of the generator  320 . In particular two hollow-cylindrical areas  324  and  325  of the remaining area are used for connecting the gradient coils to the associated shield coils.  
         [0034]     For accommodating a passive shimming device the generator  320  includes free spaces  371  that are distributed in the circumferential direction and continuous in the axial direction. One of the free spaces  371  is shown in the longitudinal section view with the shimming device introduced into it in longitudinal section. The shimming device includes a support element  375  fitted with shimming elements  377  made of a magnetic material. In another embodiment the shimming device can of course also include an active shim. As the shimming device penetrates the radio frequency shield  340  and is located partially within of the field flowback space  332 , properties of the radio frequency antenna are indeed influenced, especially in terms of tuning and degree of efficiency, but this influence is the less the closer the shimming elements  377  are located around the radio frequency shield  340 . To compensate for this, the field flowback space  332  shown in  FIG. 3  is advantageously larger in volume than the field flowback space  232  shown in  FIG. 2 , with the external dimensions of the generators  220  and  320  otherwise remaining the same. This is because the shimming devices are located only at positions distributed over the circumference and the spaces available between the shimming devices can thus be used for the field flowback. In contrast to  FIG. 1 , the field flowback space  332  shown in  FIG. 3  is in this case too filled not with a bracing element but with air. The description for  FIG. 1  otherwise applies analogously to  FIG. 3 .  
         [0035]     As a further exemplary embodiment of the invention,  FIG. 4  finally shows a longitudinal section through a top half of a magnetic resonance device having a tunnel-like space  415  for accommodating a patient. The magnetic resonance device includes a superconducting basic field magnet  410  substantially hollow-cylindrical in shape for generating an as homogeneous as possible static basic magnetic field in the space  415  for accommodating a patient. A generator  420  of time-variable magnetic fields comprising an actively shielded gradient coil system and a radio frequency antenna is located in the cylinder-like cavity of the basic field magnet  410 .  
         [0036]     The generator  420  includes three hollow-cylindrical areas  421 ,  422 , and  423  in which are located electrical conductors of the gradient coil system. Extending between the areas  421 ,  422 , and  423  are two hollow-cylindrical free spaces  431  and  433  functioning as openings for hollow-cylindrical field flowback spaces  432  and  434  located behind them and forming an undercut in terms of the respective hollow-cylindrical free space  431  and  433 . An area of the generator  420  facing the space  415  for accommodating a patient, including the areas bordering the free spaces  431  and  433  and the field flowback spaces  432  and  434 , is furthermore provided with a radio frequency shield  440 . The hollow-cylindrical free spaces  431  and  433  are bridged with conductor rods  451  for forming the radio frequency antenna. The conductor rods  451  are conductively connected on a radio frequency basis to the radio frequency shield  440 . A part of the radio frequency shield  440  also forms a part of the current paths of the radio frequency antenna. Field lines of a radio frequency field that can be generated by means of the radio frequency antenna or, as the case may be, registered by means of the radio frequency antenna can herein close across the hollow-cylindrical  431  and  433  within the field flowback spaces  432  and  434 . Double use, as it were, is here made of the field flowback spaces  432  and  434  in that they allow both the field lines of the radio frequency field and the field lines of the gradient fields generated by the gradient coils to close within them.  
         [0037]     The generator  420  furthermore contains a hollow-cylindrical area  424  in which are located conductors of the shield coils belonging to the gradient coils. The remaining area of the generator  420  is available for, inter alia, accommodating parts of a shimming system and/or of a cooling system of the generator  420 .  
         [0038]     In other types of embodiments it is in particular possible for embodiments having shimming devices located inside or outside a field flowback space and embodiments having a field flowback space filled with a substance other than air to be combined with the respective other embodiments described in the preceding.  
         [0039]     Attention is furthermore drawn to the applicant&#39;s subsequently published DE 103 13 229 for, inter alia, a more detailed description of advantages of a gradient coil system containing two spaced apart areas for conductors of gradient coils between which is located a radio frequency antenna, in contrast to conventional solutions and in terms of the distribution of conductors of gradient coils over said areas.