Abstract:
A gradient coil assembly is directly coupled throughout its entire axial length and circumferential area to the inner cylinder of the cryostat vacuum container enclosing a cylindrical superconducting MRI static field magnet. Any air space between the concentric gradient coil assembly and the cryostat is eliminated. This stiffer system produces lower velocities of the switched gradient coil assembly, which in turn produces lower noise levels in the patient opening of the gradient coil assembly and in the ambient environment. Alternatively, an annular space separates the gradient coil assembly from the MRI static field magnet assembly, and the gradient coil assembly is rigidly coupled to the inner cylinder of the MRI static field magnet assembly by discrete coupling rings. The annular space or chamber between the two assemblies is broken into smaller volumes or subchambers, which do not interconnect in the axial direction and thereby prevent axial propagation of noise generated in the chambers. Acoustic treatment of the surfaces within the chambers, including evacuation of that space, can reduce the amount of noise escaping to the ambient environment.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to magnetic resonance imaging (MRI), and more particularly to an MRI system construction that reduces noise generation related to creation of magnetic gradients in a static magnetic field. 
     Noise generation has always been a significant problem in MRI apparatuses where living subjects are imaged in the bore of an MRI magnet. High frequency switching of the currents in the gradient coils used in magnetic resonance imaging applications produces alternating Lorentz forces. Applied at high frequencies, resultant vibrations of the gradient coil assembly and any vibrations transmitted to the MRI static field magnet, produce acoustic pressure levels which can be harmful to the unprotected ears of the patient and operator. For fast acquisition of magnetic resonant imaging such as echo-planar imaging (EPI), with rise times in a range of 250-2000 micro seconds, the sound pressure level (SPL) can exceed 130 dB. 
     Generally in MRI systems, a static magnet, which may be a superconducting device, creates a substantially homogeneous static magnetic field within a region to be imaged. A cylindrical patient opening that accepts a patient to be examined is surrounded by the superconducting magnet that generates the homogeneous magnetic field. In recent developments, static field magnets are plate-like generally flat devices that oppose each other across the patient opening. Electromagnetic gradient coils are positioned near the static magnet structure. When the gradient coils are energized in predetermined sequences, the static homogeneous field in the imaging area is momentarily altered to produce a controlled gradient magnetic field in a selected direction. Generally, three gradient coils control three orthogonal coordinate directions. 
     The gradient coils are cylindrical when the static magnet coils are cylindrical. The gradient coils are plate like when the static magnet is formed of parallel plates. During the MRI measurements, requisite gradient magnetic fields are created by currents that are rapidly applied and removed from these gradient coils, respectively. Rapidly switching current in the gradient coils creates alternating forces, as stated, in a continuous string of acoustical noise bursts within the patient opening. Acoustic frequency is directly related to the switching frequency of the gradient coil currents. 
     Whereas the diagnostic advantages of using MRI techniques far outweigh the unpleasantness related to intense levels of noise, a quieter MRI apparatus is greatly to be desired for use with living subjects, both patients and operators. 
     SUMMARY OF THE INVENTION 
     Generally speaking, in accordance with the invention, a quieter MRI apparatus is provided. The invention includes an integrated MRI gradient coil/MRI static field magnet assembly which produces significantly lower noise when the gradient coil is excited in a conventional manner. The assembly includes a gradient coil assembly that is directly coupled to the inner cylinder of the cryostat vacuum container enclosing a cylindrical superconducting. MRI static field magnet. 
     In one embodiment, the gradient coil assembly is directly coupled throughout its entire axial length and circumferential area to the container of the MRI static field magnet. This coupling eliminates any air space between the concentric gradient coil assembly and the static field magnet. Such an air space can act as a resonant acoustic volume producing very high levels of noise when the gradient coil assembly is excited. The source of noise is eliminated by elimination of this air space. 
     Further, the integrated gradient coil assembly/static field magnet system assembly is stiffer than a conventional system where the gradient coil assembly is attached to the main magnet only at its two extreme axial ends. A stiffer system produces lower velocities of the switched gradient coil, which in turn produces lower noise levels in the patient opening of the gradient coil assembly and in the ambient environment. Mathematical analysis indicated, and actual tests verify, that elimination of the air space and the acoustic resonance volume between the gradient coils and the static field magnet assembly produces a significantly quieter system. The construction is implemented by shrink fitting the inner cylinder of the outer vacuum container of the MRI static field magnet directly onto the gradient coil assembly. 
     In an alternative embodiment in accordance with the invention, an annular space separates the gradient coil assembly from the MRI static field magnet assembly, and the gradient coil assembly is rigidly coupled to the inner cylinder of the MRI static field magnet assembly by discrete coupling rings. The annular space or chamber between the two assemblies is broken into smaller volumes or subchambers, which do not interconnect in the axial direction and thereby prevent axial propagation of noise generated in the chambers. Acoustic treatment of the surfaces within the chambers, including evacuation of that space, can reduce the amount of noise escaping to the ambient environment. 
     Accordingly, it is an object of the present invention to provide an improved gradient coil mounting for an MRI apparatus that operates at reduced noise levels. 
     It is a further object of the invention to provide an improved gradient coil mounting construction that is stiffer than a conventional gradient coil mounting. 
     It is yet another object of the invention to provide an improved gradient coil mounting construction that generates noise at higher acoustic mode resonant frequencies than a conventional gradient coil mounting with the same excitation. 
     Still other objects and advantages of the invention will be apparent from the specification. The invention accordingly comprises the features of construction, combinations of elements, arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, references is had to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 illustrates bending action in a gradient coil assembly in a conventional MR apparatus. 
     FIGS. 2 a-c  illustrate shapes of openings for receiving a patient in MR apparatuses wherein the present invention is suited. 
     FIG. 3 is a partial cross-section of an MR apparatus in accordance with the invention taken along the line  3 — 3  in any of the MR apparatuses illustrated in FIG.  2 . 
     FIG. 4 is similar to FIG.  3  and is a second embodiment of a MR apparatus in accordance with the invention. 
     FIG. 5 is similar to FIG.  3  and is a third embodiment of a MR apparatus in accordance with the invention. 
     FIG. 6 is a cross section of a gradient field main coil. 
     FIG. 7 illustrates an arrangement of coupling rings in accordance with the invention. 
     FIG. 8 illustrates another arrangement with coupling rings in accordance with the invention. 
     FIG. 9 illustrates still another arrangement with coupling rings in accordance with the invention. 
     FIG. 10 is a graph of calculated noise reduction for an MRI assembly constructed in accordance with the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a generally accepted view of the reversed bending action that occurs in a conventional cylindrical gradient coil system as it vibrates under X or Y coil excitation. The maximum mechanical displacements occur at the ends  5  and center  5 ′, with intermediate static nodes  4 . Low frequency noise is high based upon the axial length of the assembly. To the contrary, in accordance with the present invention, a similar gradient coil is constrained at its ends and center, thereby greatly reducing the physical displacements and also shortening the wavelength of the vibrations to produce higher frequency sounds that are more easily attenuated by their surroundings. 
     With respect to FIGS. 2 a-c , a table  10  on which a patient lies is translatably located in a known manner within the opening  12  of an MRI apparatus  14 . Generally, in most common use, are MRI apparatuses  14  having a circular opening  12  (FIG. 2 b ). In such apparatuses, the static field magnetic coils are cylindrical as are the gradient coils X, Y, Z. 
     An elliptical opening  12  (FIG. 2 a ) may provide smaller overall dimensions and permit use of a lower strength main magnet for the static field. The static and gradient field coils are appropriately shaped for efficient construction and operation with the elliptical opening. 
     In a construction as in FIG. 2 c , the static magnet windings are in the form of flat plates, above and below the table  10 , as are the gradient coils associated with each of the static magnets. 
     Configurations of MRI apparatuses are not limited to those illustrated in FIG. 2 a-c . Many variants are now available to meet specific uses and other contours are possible in future applications. For example, in some MRI apparatuses a patient is seated on a chair in the patient opening. Nevertheless the principles of the present invention are applicable in all MRI apparatuses where gradient coils are associated with static magnetic field coils. 
     The table  10  is movable into and out of the cavity (patient opening) provided for the patient in the MRI apparatus, that is, in FIGS. 2 a-c , the table moves in the directions in and out of the paper of that figure. Elevation of the table is also adjustable. 
     FIGS. 3-5 are cross-sections taken along the line  3 — 3  of FIGS. 2 a-c . FIGS. 3-5 are schematic representations and are drawn to no scale. FIGS. 3-5 are applicable to cross-sections for each of FIGS. 2 a-c , that is elliptic, circular and “flat plate” openings. 
     In FIG. 3, a static field main magnet coil  16  is submerged in liquid helium in a vessel  18 , and is cylindrical about the center line  20  of the MR apparatus  14 . The cryogenically cold helium vessel  18  is contained in a cryostat including an outer vessel  22 . A high level insulating vacuum is maintained in the space  24  between the helium vessel  18  and the cryostat vessel  22 . A thermal shield  26  within the vacuum space  24  reduces heat transfer to the outside ambient and improves the thermodynamic efficiency of the cryostat by reducing evaporation of liquid helium. Construction of the cryostat is conventional, is not a novel portion of the present invention, and is therefore not discussed in detail herein. Such construction is well known in the cryostat art. 
     A gradient field coil assembly  28  is positioned between the inner cylinder of the outer cryostat vessel  22  and the patient opening center line  20  and is separated from the vessel  22  by coupling rings  30  that fix the gradient coils assembly  28  relative to the cryostat  22  and main magnetic coils  16 . Spaces  32  are present between the coupling rings  30 . As is known to those skilled in the art, it is extremely important that the gradient coils in the assembly  28  have fixed positioning relative to each other and to the main magnet coils  16 , for accurate imaging. 
     It should be understood that the MRI apparatus is basically symmetrical above and below the center line  20 . Also the depth of the opening  12  extends to the left of the approximately central partition line  32 , so that the full length of a patient  34  may be accommodated in the MRI apparatus  14 , as is conventional in the art. 
     Generally, there are three sets of gradient coils in the gradient field coil assembly  28 , but there may be applications where only one or two gradient fields are energized. The inner electrical and magnetic construction of the gradient field assembly  28  may not be novel. Nevertheless, the gradient field coil assembly  28  may include a gradient field main coil  36 , a gradient shield coil  38 , and gradient coupling rings  40  that space the gradient shield coil  38  from the gradient field main coil  36 . Use of discrete and properly placed gradient coupling rings  40  rather than a conventional continuous composite cylinder of, e.g. glass/epoxy material substantially reduces the cost of the gradient fields coil assembly  28  without compromising magnetic characteristics. Noise characteristics are enhanced when the rings  40  isolate the space(s)  41  from the ambient. The coupling rings  40  may also be of a metallic non-magnetic material. 
     Where the opening  12  is cylindrical, the coupling rings  30 ,  40  are continuous cylindrical hoops. Thereby, the spaces  32 ,  41  are isolated one from the other and are closed off axially from the ambient environment. Thus, propagation of noise generated between the gradient coil assembly  28  and the cryostat vessel  22  and within the gradient coil assembly is restricted in propagating to the external ambient. 
     Conventionally, a cooling tube  42  operates on the gradient field main coil  36  and is separated from the patient by an inside cover  44  that provides an attractive appearance surface to the patient as well as isolation from the cold surfaces of the cooling tube. 
     Flexing of the gradient field coil assembly as illustrated in FIG. 1 due to cycling of the coil assembly  28  is substantially reduced by the stiffening provided by the coupling rings  30  at central and end locations where high deflections would be expected. 
     In a construction where the opening  12  is elliptical in cross section shape and the magnetic coils are similarly shaped, elliptical coupling rings  30  would provide continuous support between the gradient coils and the cryostat vessel  22 , and provide chambers  32  associated with the gradient coil assembly  28  that are axially separated and isolated one from the other. 
     Similarly, if the main magnets are flat plates. (FIG. 2 c ), the gradient field coil assemblies  28  are similarly shaped as plates. The spacers (couplers), are straight rather than curved and separate the static field magnet element  16  from the gradient field magnet assembly  28 . In so doing, the spacers create completely enclosed spaces between the two assemblies such that the benefits of the present invention are achieved in every configuration. 
     In alternative embodiments in accordance with the invention, there may be coupling rings  30  only at the longitudinal ends of the gradient field coil assembly  28 . Where coupling rings  30  are positioned between the axial ends of the coil assembly  28 , the number of such coupling rings and their axial spacing are not limited, and the spacing need not be uniform between coupling rings  30 . Additionally, at the axial ends of the coil assembly. 28 , the coupling rings  30  need not be flush with the coils as illustrated in FIG.  3 . The end coupling rings may extend, for example, beyond the gradient and shield coils  36 ,  38  or may be recessed relative to the gradient shield coils  38  and gradient main coil  36 . 
     These spaces  32  may be useful in the MRI apparatus  14 , for example, in shimming the magnets as is known in the art. However, where such spaces  32  are not required the construction of a quiet gradient coil assembly and support structure in accordance with the invention, as illustrated in FIG. 4, eliminates the space between the gradient assembly  28  and the cryostat vessel  22  by shrink fitting the cryostat assembly  22  to the gradient coil assembly  28 . Constructions in FIGS. 3 and 4 are similar except that the shrink fit connection of FIG. 4 replaces the couplings  30  FIG.  3 . 
     This shrink fit may be accomplished by heating the vessel  22  along the intended interface surface  46  so that the opening  12  enlarges. Then the gradient field coil assembly  28  is inserted in the opening and the combination is allowed to cool down to room temperature. The coil assembly  28  may also be cooled during the process. A thin, e.g. 1 to 5 mil, adhesive layer e.g. epoxy, may be used to fill irregularities in the opposed mating surfaces  46  during the process of joining and subsequently rigidizes to form a permanent connection. 
     The process of shrink fitting differs from the traditional method of assembling MRI systems where the gradient coil assembly and the MRI static field magnet are treated as separate entities, in some cases shipped separately to a hospital site, and then assembled there. 
     In the embodiments with coupling rings  30 , the coupling rings can be collapsible, that is, constructed of several pieces, so that the rings  30  may be disassembled when it is desirable to remove the gradient coils  28  from the MRI apparatus  14 . 
     In another alternative embodiment (FIG. 5) in accordance with the invention, coupling rings  48  at an axial end of the gradient field coil assembly  28  include a pumpout port  50  so that the internal spaces  32 ,  41  may be evacuated using a vacuum system (not shown). The vacuum reduces noise propagation within the gradient coil assembly  28  and spaces  32 . 
     Where intermediate coupling rings  30 ,  40  are used (as in FIG. 3) together with the end coupling rings  48  that include the vacuum ports  50 , each internal space  32 ,  41  may be provided with a separate vacuum port whereby vacuum pump out of many spaces may be effected. Alternatively, the internal coupling rings  30 ,  40  may each include a fine opening  52  that allows the pressure to equalize between all chambers within the gradient coil assembly  28  and yet allows little axial noise transmission within the gradient coil assembly. 
     In other alternative embodiments in accordance with the invention, spaces  32 ,  41  within and adjacent to the gradient coil assembly  28  are filled with noise absorbent matter, for example, instead of using the vacuum. 
     FIG. 6 illustrates a typical construction of a gradient field main coil  36  (a component in a gradient fields coil assembly  28 ) which includes a Z axis coil  54  embedded in a glass epoxy structure  56 . Adjacent to the Z axis coil  54  is the Y axis coil  58 , which in turn is adjacent to the X axis coil  60 . The cooling tube  42  circulates liquefied refrigerant or a gaseous coolant, for example, air. The space  62  between the structure  56  and the Y axis coil  58  is filled with a thin composite layer, for example, glass-epoxy, that hardens in place to rigidize the assembly. 
     An object of the present invention, that of reducing the noise generated by the gradient field coils assembly  28 , is achieved by stiffening the gradient coil assembly by direct attachment to the cryostat containing the main static field magnet at the axial ends and on the entire circumferential area. The coupling rings  30 ,  40  that accomplish this rigidizing attachment may be viewed as simple straight bars that have been rolled into a circular or elliptical shape for the embodiments of FIGS. 2 a  and  2   b.    
     This coupling ring concept is illustrated in FIG. 7, where the three coupling rings  30  of FIG. 3 are shown, for the sake of discussion, as three unrolled (starting at imaginary seam A—A) flat bars  30 . In order to effect further stiffening, sound reduction, and alteration in the frequency range of the produced sounds, other arrangements of rings may be provided for coupling between the gradient field coil assembly  28  and the outer cryostat element  22 . 
     FIG. 8 illustrates (in unrolled format as in FIG. 7) a pattern for the coupling that comprises three coupling rings  30 ′ at the axial ends and center of a gradient fields coil assembly  28 , connected together by a crossed X web  31 . When these members are “rolled up” for use in cylindrical or elliptical MRI apparatuses, the result is increased direct contact surface between the outer cryostat surface and the gradient field coil assembly. Then a larger plurality of closed chambers  32 ′ exists between the coupled elements to further reduce noise propagation. 
     The patterns comprising straight (circumferential) and diagonal coupling members, may be repeated more than once, with reduced dimensions between intersections, both in the circumferential and longitudinal directions so as to produce any desired waffle/egg-crate pattern of inner chambers  32 ′. (FIG. 9 schematically) The axially isolated chambers  32 ′ may be filled with sound absorbent material or evacuated as discussed above. Every chamber and subchamber may not require the same treatment to effectively control noise output. 
     The surfaces of the coupling members and web elements that face into the chambers may be treated, for example, roughened, coated, perforated, irregularly shaped, resilient, etc., so as to interfere with sound waves within the chambers  32 ′ and attenuate the noise escaping to the external ambient environment. 
     FIG. 10 illustrates a calculated noise reduction (Sound Pressure Level in decibels) over a frequency range of 500 Hz to 1200 Hz when a Z gradient coil in accordance with the invention is assumed to be excited by a sinusoidal waveform (600 amperes peak to peak) at 500 Hz to 1200 Hz at 25 Hz intervals in a 1.5 Tesla static field. 
     It should be understood that a MRI magnet as used with this invention is not limited to a superconductive device maintained in a cryostat. MRI apparatuses using conventional electromagnets or permanent magnets may also be constructed for quieter operation by incorporation of the mechanical features and acoustic concepts described above. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limited sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which might be said to fall therebetween.