Abstract:
A shielded compartment for a magnetic resonance apparatus for shielding electromagnetic radio-frequency fields has at least one region that is formed of a foamed metal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to a shielded compartment for a magnetic resonance apparatus for shielding electromagnetic radio-frequency fields.  
           [0003]    2. Description of the Prior Art  
           [0004]    Magnetic resonance technology is a known technique for acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the magnetic resonance signals that are triggered, from which magnetic resonance images are produced.  
           [0005]    Magnetic resonance signals are electromagnetic signals in the radio-frequency range. Their exact wave range is dependent on the strength of the basic magnetic field. So that the reception, and thus the magnetic resonance images, remain as uninfluenced as possible by external radio-frequency disturbances, it is standard in magnetic resonance technology to arrange at least the actual basic field magnet of the basic field magnet system, a gradient coil system of the gradient system and antennas of the radio-frequency system in a shielded compartment. The shielded compartment forms a gap-free envelope for the components of the magnetic resonance apparatus with an attenuation, for example, of at least 90 dB for a frequency range from 10 through 200 MHz and is—apart from windows—formed of a steel plate or a copper foil. The shielded compartment, of course, also develops its shielding effect in the opposite direction from inside to outside. U.S. Pat. No. 4,651,099 discloses an embodiment of a shielded compartment. The shielded compartment is fashioned of ferromagnetic metal for an additional shielding of, for example, the stray field of the basic field magnet toward the outside.  
           [0006]    Since, for example, a gradient amplifier of the gradient system that supplies the gradient coil system and a control system of the radio-frequency system connected to the antennas are arranged outside the shielded compartment, corresponding connecting lines, for example between the gradient coil system and the gradient amplifier, are conducted via filters integrated into the shielded compartment, so that radio-frequency disturbances cannot proceed thereover into the shielded compartment. U.S. Pat. No. 6,218,836 discloses an embodiment of said filter.  
           [0007]    For generating gradient fields, corresponding currents are set in gradient coils of the gradient system. The amplitudes of the required currents thereby amount to up to several 100 A. The current rise and decay rates can be up to several 100 kA/s. Given an existing basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to mechanical oscillations of the gradient coil system act on these time-variable currents in the gradient coils. These oscillations are transmitted via various propagation paths onto the surface of the magnetic resonance apparatus. There, the mechanical oscillations are converted into acoustic oscillations that ultimately lead to unwanted noise.  
           [0008]    A number of passive and active noise-reduction measures have been disclosed for magnetic resonance apparatuses. The known passive noise-reduction measures at a magnetic resonance apparatus include, for example, attachment of noise-damping foamed materials in cladding parts toward the gradient coil system and/or an arrangement of flexible layers at and/or in the gradient coil system. U.S. Pat. No. 4,954,781, for example, describes such measures.  
           [0009]    It is also known to line an installation room of the magnetic resonance apparatus with soundproofing or sound-damping materials of, for example, mineral wool in order to at least damp a noise emission from the installation room toward the outside.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is to provide an improved shielded compartment that exhibits a widespread shielding effect, among other things given a simple structure.  
           [0011]    This object is inventively achieved in a shielded compartment for a magnetic resonance apparatus wherein at least one region of the shielded compartment is formed of a foamed metal for shielding electromagnetic radio-frequency fields.  
           [0012]    Because the foamed metal, particularly an aluminum foam, is ultralight, can be manufactured in complicated shaped and is mechanically easy to process, simplifications can be achieved in the planning and installation. Aluminum foam, among other things shaped plate-like, is offered under the trademark ALULIGHT® by Alulight International GmbH in Ranshofen, Austria.  
           [0013]    In an embodiment, the foamed metal is fashioned for shielding the electromagnetic radio-frequency fields as well as for shielding another quantity, for example sound and/or heat. With suitable fashioning, the foamed metal, particularly aluminum foam, exhibits properties that are capable of effectively attenuating electromagnetic radio-frequency fields as well as sound and/or heat. Compared to known shielded compartments of copper or steel plates that are additionally lined with soundproofing or sound-damping materials for shielding sound, the following advantages can be achieved: By eliminating additional soundproofing or sound-damping materials, a reduced outlay for materials as well as a simplified planning and installation are achieved. This in turn yields, among other things, a more economic cost. Given the same effectiveness, further, the shielding can be implemented with a thinner wall thickness.  
           [0014]    In an embodiment, the region is selected such that the further quantity to be shielded can be effectively shielded in at least one direction. As a result, for example, a sound transmission from the inside of the shielded compartment can be damped direction-selectively by means of a specific wall of the compartment formed of the foamed metal, so that, for example, permitted noise limits can be satisfied in a residential area lying in the direction of the wall.  
           [0015]    In another embodiment, the shielded compartment is fashioned to be self-supporting. Among the things thereby achieved is a high degree of independence from the installation environment, for example the implementation of masonry walls and/or the wall thickness of the installation room. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 shows a self-supporting shielded compartment composed of aluminum foam in accordance with the invention.  
         [0017]    [0017]FIG. 2 shows a shielded compartment of aluminum foam as lining of an installation space in accordance with the invention.  
         [0018]    [0018]FIG. 3 shows a self-supporting shielded compartment with walls of aluminum foam and floor and ceiling of copper or steel in accordance with the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    As an exemplary embodiment of the invention, FIG. 1 shows a perspective view of a self-supporting shielded compartment  10 . The shielded compartment  10  is thereby composed of plates  11  of aluminum foam. The plates  11  are joined to one another in self-supporting fashion such that the shielded compartment  10  has a prescribable attenuation of, for example, 90 dB with respect to radio-frequency emission in a prescribable frequency range of, for example, 10 through 200 MHz. In another embodiment, the illustrated shielded compartment  10  has an additional skeleton to which the plates  11  of aluminum foam are correspondingly secured in order to achieve the self-supporting structure.  
         [0020]    One of the plates  11  of aluminum foam surrounds a filter plate  13  for the passage of energy and control lines  15  and  16  that connect a gradient amplifier arranged outside the shielded compartment to a gradient coil system of a magnetic resonance apparatus arranged inside the shielded compartment. The filter plate  13  is thereby fashioned such that substantially no radio-frequency signals are transmitted via the energy and control lines  15  and  16  from the outside into the interior of the shielded compartment and, conversely, no radio-frequency are transmitted thereover from the interior toward the outside, at least in the prescribed frequency range.  
         [0021]    Since the shielded compartment  10  composed of plates  11  of aluminum foam exhibits a good shielding effect not only in view of the radio-frequency emission in the prescribed frequency range but also has a good thermally insulating effect, the interior of the shielded compartment  10  can be climate-controlled in a simple way regardless of the space that surrounds it. To that end, the shielded compartment  10  has a further filter plate  14  through which the connecting lines  17  and  18  pass that connect a part of a climate-control device arranged inside the shielded compartment  10  to a part  19  of the climate-control device arranged outside the shielded compartment  10 . The further filter plate  14  has properties corresponding to the filter plate  13 .  
         [0022]    Since, further, the shielded compartment  10  of plates  11  of aluminum foam exhibits a highly damping effect as to sound transmission between the exterior and the interior of the shielded compartment  10 , noise transmission of the magnetic resonance apparatus arranged in the shielded compartment  10  toward the outside can be effectively damped with the illustrated shielded compartment  10 .  
         [0023]    As a further exemplary embodiment of the invention, FIG. 2 shows a cross-section through a shielded compartment  20  of plates  21  of aluminum foam that are secured to limiting surfaces of an installation space for a magnetic resonance apparatus  23 . The plates  21  of aluminum foam of the shielded compartment  20  are an integral component part of the walls  27  and  28 , of a ceiling  29  and of a floor  26  of the installation space. Compared to the shielded compartment  10  of FIG. 1, the shielded compartment  20  of FIG. 2 is not fashioned self-supporting. Otherwise, the above description for shielded compartment  10  applies analogously to the shielded compartment  20 .  
         [0024]    As a further exemplary embodiment of the invention, FIG. 3 shows a longitudinal section through a self-supporting shielded compartment  30  wherein only the walls of the shielded compartment  30  are formed of plates  31  of aluminum foam. A floor and a ceiling of the shielded compartment  30  are formed of steel or copper plates  32  of the type known in the art that effectively shield only a radio-frequency emission in the prescribed frequency range. In comparison thereto, the walls of plates  31  of aluminum foam develop their shielding effect not only with respect to the radio-frequency emission but also with respect to a noise emission of a magnetic resonance apparatus  33  operated in the shielded compartment  30 . The noise emission of the magnetic resonance apparatus  33  is effectively damped in the horizontal direction. The above-described fashioning of a shielded compartment is especially advantageous when a quantity to be shielded is to be direction-selectively damped.  
         [0025]    In a further embodiment a partial shielding, for example only one wall, of aluminum foam can be formed. As a result a transmission of sound and/or heat is effectively damped by the partial shielding, and the partial shielding need not be a component of a radio-frequency shielded compartment.  
         [0026]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.