Abstract:
A magnetic resonance apparatus has a basic field magnet with an interior bore having a bulge therein. A mass augmentation device is disposed in the bulge, and is composed of a number of assembled individual segments. The mass augmentation device reduces the propagation of mechanical oscillations that are produced during operation of the magnetic resonance apparatus, and which otherwise would produce unwanted noise.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention concerns a magnetic resonance apparatus.  
         [0003]     2. Description of the Prior Art and Related Subject Matter  
         [0004]     Magnetic resonance technology is a known technology to, among other things, acquire images of the inside of a body of an examination subject. In a magnetic resonance device, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. Furthermore, the magnetic resonance device has a radio-frequency system that radiates radio-frequency energy into the examination subject to excite magnetic resonance signals, and acquires the magnetic resonance signals, on the basis of which magnetic resonance images are generated.  
         [0005]     To generate gradient fields, appropriate currents are set in the gradient coils of the gradient coil system. The amplitudes of the required currents are up to more than 100 A. The current rise and fall rates are up to more than 100 kA/s. The basic magnetic field interacts with these temporally changing currents in the gradient coil to produce Lorentz forces on the order of 1 T, which lead to oscillations of the gradient coil system. These oscillations are propagated via various propagation paths at the surface of the magnetic resonance device. The mechanical oscillations are there transduced into sound vibrations that subsequently lead to an undesired noise.  
         [0006]     From German OS 199 27 494, a tube-shaped gradient coil system for a magnetic resonance apparatus having rings at its ends similar to the tube cross-section and made from high-density materials. The rings are firmly connected with the gradient coil system for noise reduction.  
         [0007]     From German OS 101 56 770, discloses a magnetic resonance apparatus with a gradient coil system in which an electrically-conductive structure is arranged and fashioned such that, at least within the imaging volume of the magnetic resonance apparatus, the magnetic field caused by a gradient field due to induction effects is configured geometrically similar to the gradient field. In an embodiment, at least one part of the structure is barrel-shaped as a component of a basic field magnet. Among other things, the gradient coil system can be fashioned without secondary coils since, due to the similarity of the magnetic field caused by the structure, the undesirable consequences of the gradient fields can be nearly completely controlled by a pre-distortion.  
         [0008]     German PS 199 40 551 discloses a magnetic resonance apparatus with a basic field magnet with a two-part enclosure forming a bore. In the region of the enclosure in which the gradient coil system is attached, the enclosure is connected with a decoupling device that prevents the transfer of noise-generating oscillations from the gradient coil system to the enclosure of the basic field magnet.  
         [0009]     From the subsequently published German OS 102 28 829, a magnetic resonance apparatus is known that has a basic field magnet with a bore that exhibits a shoulder (step). The gradient subassembly is clamped to this shoulder by means of a clamping element, such that a decrease in oscillation and a noise reduction are achieved.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of the present invention is to provide an improved magnetic resonance apparatus that in particular exhibits low noise emission characteristics.  
         [0011]     This object is inventively achieved by a magnetic resonance apparatus having a basic field magnet with a bore that exhibits a bulge, and a mass augmentation that is attached in the bulge and that is composed of joined segments.  
         [0012]     By means of the mass augmentation, the mass of the overall system tending to oscillation is increased, such that the amplitude of vibrations (and with it the noise as well) is reduced. The components connected with one another also exhibit high overall rigidity, such that the noise emission behavior of the magnetic resonance apparatus is positively influenced not only with regard to the amplitude, but also with regard to the frequencies of noise peaks. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a longitudinal section through a magnetic resonance apparatus with a mass augmentation composed of segments, in accordance with the invention.  
         [0014]      FIG. 2  shows the mass augmentation from  FIG. 1 , illustrating the segments, in a perspective view. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     As an exemplary embodiment of the invention,  FIG. 1  shows a longitudinal section through a magnetic resonance apparatus. To generate an optimally homogenous static basic magnetic field within an imaging volume  15 , the magnetic resonance apparatus has a superconducting basic field magnet, the essentially hollow-cylindrical vacuum reservoir  10  of which is shown in  FIG. 1  as an electrically-conductive structure. A non-actively shielded, essentially hollow-cylindrical gradient coil system  20  is disposed in the hollow bore of the vacuum reservoir  10 . For clarity, further components of the magnetic resonance apparatus are not shown, for example, an antenna system to emit radio-frequency signals and to receive magnetic resonance signals.  
         [0016]     The gradient coils of the gradient coil system  20  are designed in cooperation with the vacuum reservoir  10  corresponding to the concept of the previously cited German OS 101 56 770, such that a field resulting in the imaging volume  15  from the magnetic field generated by the gradient coil and an eddy current field generated by the vacuum reservoir  10  are adjusted immediately after a current change in one of the gradient coils. The resulting field exhibits the desired spatial distribution, in the form of a strong linear gradient field. For this purpose, the hollow bore of the vacuum reservoir  10  is bulged substantially like a barrel in the middle region thereof to implement the concept of German OS 101 56 770. By means of the pre-distortion, an undistorted resulting field can be set in the imaging volume  15 .  
         [0017]     Particularly in the regions of the vacuum reservoir  10  forming the hollow bore, due to eddy currents induced during operation of the magnetic resonance apparatus, Lorentz forces act on these regions similar to the way they act on the gradient coils fed with current flowing through them, such that, without counter-measures, these regions of the vacuum reservoir  10  tend to oscillate and thereby cause noise. As a countermeasure, in the region of the bulge of the hollow bore between the vacuum reservoir  10  and the gradient coil system  20 , a mass augmentation  30  composed of segments  32  is introduced that, as a strategically designed and placed mass, reduces the oscillation amplitudes.  FIG. 2  shows the mass  30  of  FIG. 1  pre-assembled from the segments  32  in a perspective view. The material of the mass augmentation  30  exhibits a high density (greater than approximately 1500 kg/m 3 ) and low cost, and furthermore is simple to process.  
         [0018]     Since the mass augmentation  30  is to be inserted into the bore of the vacuum vessel  10  through an opening diameter that is smaller than the diameter of the assembled mass augmentation  30 , the segments  32  are fashioned with regard to their size, shape and number such that they can be individually inserted (disassembled) from the outside into the bore and combined (assembled) in the bore. The combined segments  32  thereby conform to the geometry of the volume to be filled.  
         [0019]     In an embodiment, the aforementioned segments  32  are produced by filling casting molds of suitable geometry with fluid epoxy resin that hardens (cures) into a thermoset plastic. Depending on whether a cold-hardening or warm-hardening casting resin is used, the curing ensues at room temperature or at elevated temperature. After hardening, casting resins exhibit a reaction contraction that, given large wall thicknesses, can lead to unwanted deformations and even to tears and bubbles in the component. To prevent these problems, a casting resin  34  containing filling material is used in accordance with the invention. The filling material content is between thirty and sixty volume percent. In particular, electrically insulating materials are suitable as filling materials, such as quartz, translucent fused quartz, aluminum oxide and/or barium sulfate is in the form of powder in the sub-millimeter range. Negative effects of the reaction contraction are further minimized by, before the casting, filling the hollow space of one of the casting molds up to a large portion with suitable palletized or powdered filling material  36  made from inert materials in the millimeter range. Upon casting, the casting resin  34  then fills only the intervening spaces (interspaces) and during the hardening produces a more secure bonding of the filling materials. After removal from the molds, the segments  32  exist in the desired form without further elaborate post-processing.  
         [0020]     The individual segments  32  are affixed both with one another and with the vacuum reservoir  10  and the gradient coil system  20 , by bonding or bracing, such that in operation of the magnetic resonance apparatus no additional unwanted noise development and no abrasion as a result of vibrations occur, and the overall rigidity of the connected components is advantageously increased.  
         [0021]     In another embodiment, the segments  32  are produced from semi-finished products by machining a blank. For example, fabric-based laminate or paper-based laminate based on epoxy resin, phenol resin, silicon resin, melamine resin, polyester resin and mixtures of these are suitable as materials. Cotton fabric, fiberglass mats or fiberglass fabrics can be used as filling and reinforcement materials of the fabric-based laminate.  
         [0022]     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.