Abstract:
A magnetic resonance apparatus has a gradient coil system that includes a gradient coil arrangement with a conductor and magnetostrictive material, and contains a basic field magnet system for generating a basic magnetic field. The magnetostrictive material is thereby arranged in the immediate proximity of the conductor in regions having the same course as a course of the conductor, to suppress noise-producing vibrations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to a magnetic resonance apparatus of the type having a gradient coil system that includes a gradient coil arrangement with a conductor and magnetostrictive material, and having a basic field magnet system for generating a basic magnetic field.  
           [0003]    2. Description of the Prior Art  
           [0004]    Magnetic resonance tomography is a known technique for producing images of the inside of the body of an examination subject. To this end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field in the magnetic resonance apparatus. The static field is generated by a basic field magnet system. The magnetic resonance apparatus also comprises a radio-frequency system that emits radio-frequency signals into the examination subject for producing magnetic resonance signals and that picks up the generated magnetic resonance signals, from which magnetic resonance images are produced.  
           [0005]    For generating gradient fields, suitable currents are to be set in the gradient coils. The amplitudes of the required currents amount to several 100 A. The current rise and decay rates amount to several 100 kA/s. Given the presence of a basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to oscillations of the gradient coil system act on these temporally variable currents in the gradient coils. These oscillations are forwarded to the surface of the apparatus via various propagation paths. At this surface, these mechanical oscillations are converted into acoustic oscillations that ultimately lead to noise that is disturbing.  
           [0006]    For reducing such noise, German OS 196 43 116, corresponding to U.S. Pat. No. 5,952,829, discloses that forces that oppose the Lorentz forces be generated by magnetostriction. Such opposing forces are thereby generated by the same gradient field that also produces the Lorentz forces. This published application proposes for this purpose given a hollow-cylindrical gradient coil system, to provide a magnetostrictive material system composed of planar or strip-shaped, magnetostrictive components, with the gradient coil system, preferably the in longitudinal direction, parallel to the principal axis of the hollow cylinder and/or in circumferential direction. It is primarily the bending vibrations of the hollow cylinder produced by transverse gradient coils that are responsible for the noise. For suppressing these vibrations, it is proposed in this published application that four strip-shaped, magnetostrictive longitudinal elements, that are equally spaced in the circumferential direction, be secured to the hollow-cylindrical gradient coil system. It is especially disadvantageous in this solution that the four strip-shaped longitudinal elements represent large eddy current surfaces. Moreover, a significant amount of magnetostrictive material must be utilized overall for an adequate noise reduction.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide a magnetic resonance apparatus of the type initially described that alleviates the aforementioned disadvantages of known systems of this type.  
           [0008]    This object is inventively achieved in a magnetic resonance apparatus wherein magnetostrictive material is arranged in immediate proximity to the conductor in areas having the same course as the course of the conductor of the gradient coil system. As a result, the magnetostrictive material is arranged at those locations at which it experiences a large change in length due to its spatial proximity to the conductor, and thus to a conductor-proximate magnetic field that is produced in the immediate proximity of the conductor by a current flowing in the conductor. The noise-reducing effect is therefore considerable. At the same time, a large-area arrangement of magnetostrictive material is avoided, so that only a small amount of eddy currents can still form in the magnetostrictive material.  
           [0009]    In an embodiment, the magnetostrictive material is arranged in areas wherein a section of the conductor has a longitudinal direction substantially perpendicular to the basic magnetic field. A “section” means an imaginary subdivision of the conductor in the longitudinal direction of the conductor, and the longitudinal direction of the section means the direction that a current in the conductor has in the middle of the section.  
           [0010]    As a result, the magnetostrictive material is arranged exclusively at those locations at which Lorentz forces act on the conductor, a high noise-reducing effect being thus achieved given a minimal use of material.  
           [0011]    In an embodiment, the magnetostrictive material is arranged in areas wherein a conductor-proximate magnetic field, that is produced in the immediate proximity of the conductor by the current therein, has a field component that is co-linear with the basic magnetic field. As a result, a designational length change of the magnetostrictive material is achieved in the direction of the basic magnetic field, so that, for example, a pre-polarization or pre-stressing of the material by the basic magnetic field can be employed.  
           [0012]    In another embodiment, the magnetostrictive material is arranged in regions opposite one another with respect to a cross-section of the conductor, such that the material acts with a positive magnetostrictive effect in one of the regions and acts with a negative (opposite) magnetostrictive effect in the region lying opposite. A positive magnetostrictive effect means that the material exhibits an increase in length in the direction of the magnetic field given an increase in a magnetic field permeating the material. Material experiencing a negative magnetostrictive effect exhibits a decrease in length given such an increase in the permeating magnetic field. The noise-reducing effect is intensified by the use of materials with positive and negative magnetostrictive behavior, respectively.  
           [0013]    In a further embodiment for a transverse gradient coil arrangement of a hollow-cylindrical gradient coil system with at least one sub-coil fashioned saddle-shaped, the magnetostrictive material is arranged such that the quantity of the material is maximum in the region of a saddleback or ridge line of the sub-coil, and decreases toward the edges of the sub-coil proceeding from the saddleback line. As a result, the banana-shaped bending vibration that is mainly responsible for noise in such a hollow-cylindrical gradient coil system is suppressed and, at the same time, an undesired oscillation in the circumferential direction of the hollow cylinder is prevented from occurring.  
           [0014]    In another embodiment, the magnetostrictive material is arranged finely distributed in an electrically non-conductive matrix. As a result, formation of eddy currents in the magnetostrictive material is nearly completely suppressed, and thus disturbing influences on results of a magnetic resonance examination are avoided.  
           [0015]    In another embodiment, the magnetostrictive material is fashioned such that a pre-polarization of the material by the basic magnetic field effects an operating range of the material in the predominantly linear region of a flux density length change characteristic of the material. As a result, a uniformly high, noise reducing effect of the magnetostrictive material is achieved independently of the size and direction of the current changing over time in the gradient coil arrangement. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic illustration of a hollow-cylindrical gradient coil system having a transversal gradient coil arrangement.  
         [0017]    [0017]FIG. 2 is a longitudinal section through a hollow-cylindrical gradient coil system in accordance with the invention having a transversal gradient coil arrangement.  
         [0018]    [0018]FIG. 3 is a plan view of a conductor loop of the transversal gradient coil arrangement from FIG. 2.  
         [0019]    [0019]FIG. 4 is a cross-section through a hollow-cylindrical gradient coil system in accordance with the invention having a longitudinal gradient coil arrangement.  
         [0020]    [0020]FIG. 5 is a cross-section through an arrangement of magnetostrictive material in a matrix in accordance with the invention.  
         [0021]    [0021]FIG. 6 shows a flux density/length change characteristic of magnetostrictive material. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    [0022]FIG. 1 shows a schematic view of a hollow-cylindrical gradient coil system  1   a  with a transversal gradient coil arrangement, as is known in the art. The transversal gradient coil arrangement comprises, for example, four sub-coils TS 1  through TS 4  fashioned as saddle coils. Five loop-shaped sections of a conductor L of the gradient coil arrangement are shown as an example for each sub-coil TS 1  through TS 4 . Additionally, a saddleback or ridge line  2  is shown in FIG. 1 for each sub-coil TS 1  through TS 4 . For clarity, illustration of other components of the gradient coil system  1   a , as well as of further gradient coil arrangements, secondary coils, cooling and shim devices, etc., is omitted.  
         [0023]    The hollow-cylindrical gradient coil system  1   a , for example, is built into a bore of a basic field magnet system of a magnetic resonance tomography apparatus. The basic field magnet generates a uniform, static basic magnetic field B 0  that essentially permeates the entire gradient coil system. For producing magnetic resonance images, a gradient coil current I is rapidly switched in the conductor L of the gradient coil arrangement. Given the presence of the basic magnetic field B 0 , Lorentz forces that lead to oscillations of the gradient coil system  1   a  and, proceeding therefrom, to noise, act on the conductor L with current flowing therein.  
         [0024]    As an exemplary embodiment of the invention, FIG. 2 shows a longitudinal section through a hollow-cylindrical gradient coil system  1   b . For clarity, only one transversal gradient coil arrangement composed of four saddle-shaped sub-coils is again shown, and only one loop-shaped section LS 1  through LS 4  of a conductor of the gradient coil arrangement is shown. The longitudinal section through the gradient coil system  1   b  is taken approximately along the saddleback line  2  (FIG. 1) of the sub-coils. It is assumed the conductor has a circular cross-section, accordingly, two circular section faces per conductor loop LS 1  through LS 4  are shown. The direction in which a gradient coil current I flows are opposite at the two section faces for each of the conductor loops LS 1  through LS 4 , this being indicated by the symbols ⊙ and {circle over (x)}. The time-variable gradient coil current I flows through the gradient coil arrangement. At the point in time shown in FIG. 2, let the gradient coil current I have a positive value, so that the Lorentz forces F L  that act on the conductor loops LS 1  through LS 4  as a consequence of the basic magnetic field Bo have the respective directions indicated by arrows at the illustrated section faces.  
         [0025]    For suppressing noise-producing oscillations, magnetostrictive material  3   n  and  3   p  is built into the gradient coil system  1   b  in the immediate proximity of the conductor. Without the magnetostrictive material  3   n  and  3   p , the gradient coil system  1   b  would be placed into a bending vibration as a consequence of the temporally varying gradient coil current  1 , this representing a principal source of noise. Due to a conductor-proximate magnetic field that is produced in the immediate proximity of the conductor loops LS 1  through LS 4  by the gradient coil current, the material  3   n  and  3   p  experiences a length change indicated by arrows that prevents this bending vibration. Further, magnetostrictive material  3   n  exhibiting a negative magnetostrictive effect is arranged at that side of the cross-section of the conductor loops LS 1  through LS 4  at which an intensification of the basic magnetic field Bo by a component +AB 0  of the conductor-proximate magnetic field occurs as a result of the gradient coil current, and magnetostrictive material  3   p  with exhibiting a positive magnetostrictive effect is arranged at the opposite side, at which a reduction attenuation of the basic magnetic field B 0 , by a component −ΔB 0  of the conductor-proximate magnetic field, occurs. The suppressing effect for bending vibrations is intensified by the utilization of material  3   n  with negative magnetostrictive behavior and material  3   p  with positive magnetostrictive behavior.  
         [0026]    The exclusive use of magnetostrictive materials with negative or positive magnetostrictive behavior is likewise possible in other embodiments. In other embodiments, oval, sickle-shaped and/or teardrop-shaped cross-sections are also possible for the magnetostrictive material instead of the cross-section shown in FIG. 2.  
         [0027]    As an example, FIG. 3 shows a plan view of the conductor loop LS 1  shown at the upper left in FIG. 2. It can be clearly seen that the magnetostrictive material  3   n  and  3   p  is arranged in regions wherein sections of the conductor loop LS 1  have a longitudinal direction largely perpendicular to the basic magnetic field Bo. Further, the material  3   n  and  3   p  exhibits its largest cross-section in the region of the saddleback line of the conductor loop LS 1 , this cross-section tapering more and more as the distance from the saddleback line increases. The above arrangement of the magnetostrictive material  3   n  and  3   p  prevents an additional, unwanted oscillation of the hollow-cylindrical gradient coil system  1   b  from being excited in the circumferential direction by magnetostriction. This would be the case, if magnetostrictive material  3   n  and  3   p  were arranged with a constant cross-section without a taper along the entire section of the conductor loop LS 1  directed perpendicularly to the basic magnetic field B 0 .  
         [0028]    As a further exemplary embodiment of the invention, FIG. 4 shows a cross-section through a hollow-cylindrical gradient coil system  1   c  with a longitudinal gradient coil arrangement. For clarity, only one annular conductor loop LSr of a conductor of the gradient coil arrangement is shown. At the illustrated point in time, let the gradient coil current I flowing in the annular conductor loop LSr be positive. Given the directions defined for the gradient coil current I and the basic magnetic field B 0 , the Lorentz forces FL act on the annular conductor loop LSr in the influencing direction shown with arrows. This effects a contraction of the annular conductor loop LSr. Due to the gradient coil current I varying over time, this would lead to a noise-producing circumferential vibration of the gradient coil system  1   c . Analogous to the example of a transversal gradient coil arrangement shown in FIG. 2, magnetostrictive material  3   n  and  3   p  of magnetostrictive behavior is introduced into the gradient coil system  1   c  in an annular arrangement at both sides of the annular conductor loop LSr for suppressing this circumferential vibration. The functioning of the magnetostrictive material  3   p  and  3   n  in FIG. 4 is analogous to that of FIG. 2.  
         [0029]    As an exemplary embodiment of the invention, FIG. 5 shows how the material  3   n  can be arranged finely distributed in an electrically non-conductive matrix  4 , for example for a cross-section of the magnetostrictive material  3   n  with negative magnetostrictive behavior from FIG. 2. For example, the material  3   n  is arranged as a powder in a matrix  4  of plastic.  
         [0030]    As an exemplary embodiment of the invention, FIG. 6 shows a flux density/length change characteristic  5 , for example for the magnetostrictive material  3   p  with positive magnetostrictive behavior. The flux density/length change characteristic  5  thereby characterizes the length change ΔI of the magnetostrictive material  3   p  in a spatial direction, preferably corresponding to a direction of a basic magnetic field B 0 , dependent on a magnetic flux density H of a magnetic field that permeates the material  3   p  in said direction. The flux density/length change characteristic  5  is selected such that the flux density of the basic magnetic field B 0  produces an operating point P in the linear region of the characteristic  5 , and a length change of the material  3   p  as a consequence of a conductor-proximate magnetic field that effects an intensification of the basic magnetic field Bo by a component +ΔB 0  or a reduction by a component −ΔB 0 , in the generally linear region of the characteristic  5 .  
         [0031]    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.