Abstract:
The present invention provides an apparatus for reducing acoustic noise in a magnetic resonance imaging device including a suspension element including at least one resilient element and an active drivable element for applying a compensating force to reduce vibration transmission. The active drivable element is positioned so as to not directly support the weight of the gradient coil assembly, which avoids applying strong forces to relatively fragile active drivable elements, such as piezoelectric force transducers. Force signals for the active drivable element are derived in a feed-forward manner from the applied gradient waveform or from motion of the gradient coil assembly bracket. Alternatively, the active drivable element can be driven by signals derived from measured vibration or other motion of parts of the MRI magnet, gradient coils or rf coils.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/453,863, filed Mar. 12, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a magnetic resonance imaging (MRI) scanner and more particularly to a low acoustic noise MRI scanner. 
     MRI scanners, which are used in various fields such as medical diagnostics, typically create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field that makes unpaired nuclear spins, such as hydrogen atomic nuclei, responsive to radiofrequency excitation via the process of nuclear magnetic resonance (NMR). The gradient coil assembly imposes a series of pulsed, spatial-gradient magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during an imaging pulse sequence. The radiofrequency coil applies an excitation rf (radiofrequency) pulse that temporarily creates an oscillating transverse nuclear magnetization in the sample. This sample magnetization is then detected by the excitation rf coil or other rf coils. The resulting electrical signals are used by the computer to create magnetic resonance images. Typically, there is a radiofrequency coil and a gradient coil assembly within the magnet. 
     Magnets for MRI scanners include superconductive-coil magnets, resistive-coil magnets, and permanent magnets. Known superconductive magnet designs include cylindrical magnets and open magnets. Cylindrical magnets typically have an axially-directed static magnetic field. In MRI systems based on cylindrical magnets, the radiofrequency coil, the gradient coil assembly and the magnet are generally annularly-cylindrically shaped and are generally coaxially aligned, wherein the gradient coil assembly circumferentially surrounds the radiofrequency coil and wherein the magnet circumferentially surrounds the gradient coil assembly. Open magnets typically employ two spaced-apart magnetic assemblies (magnet poles) with the imaging subject inserted into the space between the assemblies. This scanner geometry allows access by medical personnel for surgery or other medical procedures during MRI imaging. The open space also helps the patient overcome feelings of claustrophobia that may be experienced in a cylindrical magnet design. 
     A gradient coil assembly comprises a set of windings that produce the desired gradient fields. Such an assembly for a human-size whole-body MRI scanner typically weighs about 1000 kg. The windings consist of wires or conductors formed by cutting or etching sheets of conducting material (e.g. copper) to form current paths to generate desired field patterns. The wires or conducting coils or plates are themselves typically held in place by fiberglass overwindings plus epoxy resin. 
     Generally, the various components of the MRI scanner represent sources and pathways of acoustic noise that can be objectionable to the patient being imaged and to the operator of the scanner. For example, the gradient coil assembly generates loud acoustic noises, which many medical patients find objectionable. The acoustic noises occur in the imaging region of the scanner as well as outside of the scanner. Known passive noise control techniques include locating the gradient coil assembly in a vacuum enclosure. 
     Large pulsed electrical currents, typically 200 A or more, with risetimes and durations typically in the submillisecond to millisecond range, are applied to the windings. Because these windings are located in strong static magnetic fields (e.g., 1.5 T for a typical clinical imager to much higher values for research systems), the currents interact with the static field and strong Lorentz forces are exerted on different parts of the gradient coil assembly. These forces in turn compress, expand, bend or otherwise distort the gradient coil assembly. It will be readily understood by those skilled in the art that the frequencies of the acoustic noise so generated will be in the audio range. Typically there are strong components of noise from 50 Hz and below to several kHz at the upper end of the frequency range. 
     Vibrations can be conveyed mechanically from the gradient coil assembly to the patient area, the cryostat or other external parts of the MRI scanner via the gradient coil assembly support frame. These transmitted vibrations can cause the external parts to vibrate and thereby produce acoustic noise, which will be heard by the MRI subject and the MRI operator. 
     One way to decrease the transmitted vibrations is to use a passive vibration isolation mounts for the gradient coil assembly. It is known in the mechanical arts area to design and use isolation mounts so that vibrations from machinery supported by the isolation mounts are not transmitted to surrounding structure that supports the isolation mounts. Conventional isolation mounts include those of the elastomeric type and those of the spring type. Such isolation mounts are designed such that the natural frequency of vibration of the mounts and the machinery is less than the important excitation frequencies of the machinery in order to provide effective vibration isolation. 
     In one approach to providing a vibration isolation mount, solid metal brackets are mounted on the gradient coil assembly and corresponding solid metal brackets are attached to the cryostat. The gradient coil assembly is positioned so that the brackets are aligned and elastomeric pads (for example, rubber) are positioned between each cryostat bracket and the corresponding gradient coil assembly bracket. With this configuration, the transmission of vibration from the gradient coil assembly to the cryostat is attenuated by the elastomeric pads. 
     Unfortunately, there is a limit to the degree of passive attenuation achievable by use of elastomeric pads or spring isolation mounts as described. Generally speaking, softer pads or springs produce greater attenuation. However, the pads or springs underneath the gradient coil assembly must be able to support the gradient coil assembly weight. Also, pads or springs that are too soft might permit excessive motion of the gradient coil assembly in response to Lorentz forces, in which case image quality could be adversely affected. Pad or spring stiffness is thus a tradeoff between keeping the gradient coil assembly precisely positioned, on the one hand, and attenuating vibration transmission on the other. 
     In another approach active vibration compensation is used to substantially improve the vibration isolation efficacy as described above. One example of this approach is disclosed in Roozen et al., U.S. Pat. No. 6,549,010, 2003.  FIG. 1  is a reproduction of part of this device, and shows a gradient carrier  18  supported by suspension elements  19  including a resilient element  22  and an active drivable element  21  connected in series with suspension element  22 . If forces cause a displacement of gradient carrier  18 , then drivable element  21  can be lengthened or contracted to counteract the displacement and thereby prevent any force being applied to suspension element  22 . Unfortunately, with this arrangement, active element  21  must be able to withstand the entire weight of the gradient carrier, which may be hundreds of pounds. Roozen et al. describe drivable element  21  as a piezo actuator. However, such actuators are fragile and could fracture in this configuration. 
     These approaches to reduce acoustic noise due to the various components in MRI scanners have been partially effective, but patients and technicians still find the noise in and about a MRI scanner to be problematic. What is needed is a lower noise MRI scanner that addresses the multiple sources and pathways of acoustic noise in and about the scanner. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for reducing acoustic noise in a magnetic resonance imaging device including a suspension element including at least one resilient element and an active drivable element for applying a compensating force to reduce vibration transmission. The active drivable element is positioned so as to not directly support the weight of the gradient coil assembly, which avoids applying strong forces to relatively fragile active drivable elements, such as piezoelectric force transducers. Force signals for the active drivable element are derived in a feed-forward manner from the applied gradient waveform or from motion of the gradient coil assembly bracket. Alternatively, the active drivable element can be driven by signals derived from measured vibration or other motion of parts of the MRI magnet, gradient coils or rf coils. 
     A first aspect of the invention is directed to an apparatus for reducing acoustic noise in a magnetic resonance imaging (MRI) device, the apparatus comprising: a gradient coil assembly of the MRI device coupled to a frame of the MRI device by suspension elements to reduce acoustic noise due to vibration transmission, each suspension element including at least one resilient element and an active drivable element for applying a compensating force to reduce vibration transmission, wherein the active drivable element is positioned so as to not directly support the weight of the gradient coil assembly. 
     A second aspect of the invention is directed to a method for reducing acoustic noise due to vibration transmission from a gradient coil assembly to a frame of a magnetic resonance imaging device, the method comprising the steps of: supporting a support portion of the gradient coil system relative to the frame using a resilient element; and actively compensating for vibrations of the gradient coil assembly by applying a force, via an active drivable element, from a point not between the support portion and the frame. 
     A third aspect of the invention is directed to an apparatus for reducing acoustic noise due to vibration transmission from a gradient coil assembly to a frame of a magnetic resonance imaging device, the apparatus comprising: means for resiliently supporting a support portion of the gradient coil system relative to the frame; and means for actively compensating for vibrations of the gradient coil assembly by applying a force from a point not between the support portion and the frame. 
     A fourth aspect of the invention is directed to a suspension element for reducing acoustic noise due to vibration transmission from a gradient coil assembly to a frame of a magnetic resonance imaging device, the apparatus comprising: a C-fixture including a first support for applying forces to a support portion of the frame and a second support aligned with the first support and immovable relative thereto; a first resilient element for supporting a support portion of the gradient coil assembly on the support portion of the frame; and an active drivable element positioned between a second resilient element and the second support and configured to apply a force to reduce vibration transmission. 
     The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional end view of a prior art gradient carrier and active control support structure; 
         FIG. 2  is a cross-sectional side view of an entire magnetic resonance imaging (MRI) device showing magnet, patient handling system and gradient coil assembly according to the invention; 
         FIG. 3  is a schematic cross-sectional end view of an MRI device with vibration cancellation based on  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional view of a detailed force cancellation fixture including an active drivable element; 
         FIG. 5  is a schematic cross-sectional view of a detailed force cancellation fixture where the force cancellation fixture is supported by a floor; and 
         FIG. 6  is a schematic cross-sectional view of a force cancellation fixture where the gradient coil assembly is supported from a floor. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to  FIG. 2  there is shown an illustrative MRI device  90  to which embodiments of the present invention are applicable. MRI device  90  is of a type useful in producing magnetic resonance (MR) images of a patient or subject. Throughout the figures, like numerals represent like elements.  FIGS. 2-6  show MRI device  90  based on a closed, cylindrical superconducting magnet assembly  200 . It is to be appreciated by one skilled in the art that the functions and descriptions of the present invention are equally applicable to an open magnet configuration. 
     Referring to  FIG. 2 , this type of magnet assembly comprises an inner surface referred to as a magnet warm bore  304  and a cryostat shell  100  disposed radially around the outer surface. The magnet assembly further comprises end cap seals  212 . When end cap seals  212  are secured against rubber gaskets  220  positioned between end cap seals  212  and cryostat shell  100 , and secured against other rubber gaskets  220  positioned between end cap seals  212  and patient tube  106 , an airtight space containing the gradient coil assembly  102  is created. 
     Typically, cryostat shell  100  encloses a superconductive magnet (not shown) that, as is well-known, includes several radially-aligned and longitudinally spaced-apart superconductive coils, each capable of carrying a large electric current. The superconductive coils produce a homogeneous, main static magnetic field, known as B 0 , typically in the range from 0.5 T to 8 T, aligned along the center axis  250 . Cryostat shell  100  is generally metallic, typically steel or stainless steel. 
     A patient or imaging subject (not shown) is positioned within a cylindrical imaging volume  201  surrounded by patient bore tube  104 . Bore tube  104  is typically made of electrically conducting material such as stainless steel. Gradient coil assembly  102  is disposed around in a spaced apart coaxial relationship therewith and generates time-dependent gradient magnetic field pulses in a known manner. Radially disposed around gradient coil assembly  102  is cryostat shell  100  including warm bore  304 . Cryostat shell  100  contains the magnet that produces the static magnetic field necessary for producing MRI images, as described above. 
     Also shown in  FIG. 2  is a schematic view of an active vibration isolation suspension element  300  that reduces vibration transmission from gradient coil assembly  102  to fixed cryostat frame  100 . Suspension  300  is positioned about or adjacent to gradient coil assembly rigid support portion  108  and cryostat shell  100  rigid support portion  112 . Each support portion  108  and  112  extends from its respective component. In one embodiment, each support portion  108 ,  112  is a mounting bracket that is coupled to their respective components. However, each support portion  108 ,  112  may be formed as an integral part of their respective portions or provided in any other fashion that provides adequate support. Suspension element  300  does not support the weight of gradient coil assembly  102 . 
     In this configuration, cryostat shell  100  serves as a supporting frame for the gradient coil assembly  102 . Other configurations considered in this specification are separate structures mounted on the floor, which serve as supporting frames for the gradient coil assembly  102 . We will refer to any of these supporting structures as a “frame.” 
     Referring to  FIG. 3 , a cross-sectional end view of the magnet arrangement in  FIG. 2  is shown with end caps  212  removed.  FIG. 3  also shows a back view of suspension elements  300  with their interiors cut away to show details of the forces  118  and  119  and passive vibration isolation supports  116 . In particular, in one embodiment, two sets of passive vibration isolation supports  116  are positioned near the bottom of gradient coil assembly  102  to support its weight. The transmission from gradient coil assembly  102  to frame  100  depends on the stiffness and damping of the elastomeric pads  110 . The excitation of gradient coil assembly  102  by embedded gradient windings has a broad spectrum extending to several kHz. (See R A Hedeen et al.  Magnetic Resonance in Medicine  37, 7-10, 1997.) This excitation has the effect of bending or compressing the bulk of the gradient coil assembly  102 . The surface of gradient coil assembly  102  undergoes microscopic motion that acts through elastomeric pads  110  to apply force to frame  100 . The amplitude of gradient coil rigid support portion  108  displacement δx  117  is typically a few microns. Some force F  118  will then be applied along line  5 - 5 ′ via the elastomeric pads  110  to frame support portion  112 , namely, F=k δx, where k is the spring constant of elastomeric pad  110 . A compensating force F c    119  of amplitude F c =−k δx applied to frame support portion  112  will result in no net force on and no vibration of the frame support portion  112 , and no consequent induced vibration of frame  100 . 
       FIG. 4  is a more detailed cross-sectional view of the suspension elements  300  that apply the compensating force described above. Each suspension element  300  includes at least one resilient element  110 ,  124  and an active drivable element  140  for applying the compensating force, described above, to reduce vibration transmission. Active drivable element  140  is positioned so as not to directly support the weight of gradient coil assembly  102 . In other words, active drivable element  140  is positioned to apply a force from a point not between the support portion  108  of gradient coil assembly  102  and frame  100 . In one embodiment, each suspension  300  includes a first support  150  for supporting frame support portion  112 , a second support  152  aligned with first support  150  and immovable relative thereto, a first resilient element  110  for supporting support portion  108  of gradient coil assembly  102  on support portion  112  of frame  100 . In one embodiment, shown in  FIGS. 4 and 6 , first support  150  and second support  152  are coupled to one another to form a C-shaped fixture (C-fixture)  130 . Each suspension element  300  may also include an adjustable pre-stressing mount  126  for adjusting the position of first support  150  relative to support portion  112  of frame  100 . Gradient coil assembly rigid support portion  108  is attached to or integral with gradient coil assembly  102 , and the cryostat rigid support portion  112  is attached to or integral with frame (cryostat shell)  100 . Each resilient element  110 ,  124  may include: at least one elastomeric pad and/or at least one spring. 
     Continuing with  FIG. 4 , active drivable element  140  (sometimes referred to as a “force transducer”) provides a compensating force as described above in order to substantially decrease vibration transmission from gradient coil assembly  102  to cryostat shell (frame)  100 . In one embodiment, a second resilient element  124  provides additional vibration isolation so that the suspension element  300  does not inadvertently bypass and vitiate the isolation provided by first resilient element  110 . The additional resilient element  124  may not be necessary if the suspension element  300  operates optimally. In one embodiment, active drivable element  140  may be, for example, at least one of a piezoelectric actuator, a magnetic actuator and a hydraulic actuator. Adjustable pre-stressing mount  126  (e.g. a bolt, screw or threaded rod) enables the pre-stressing of the suspension element  300  in order to enable negative as well as positive forces to be applied between the gradient coil rigid support portion  108  and the cryostat rigid support portion  112 . 
     In order to illustrate the operation of this arrangement, assume that the gradient coil assembly support portion  108  is experiencing a displacement downward. Then a downward force will be applied along axis  5 - 5 ′, via resilient elements  124  and  110 , to frame support portion  112 , tending to push support portion  112  downward. Given suspension element  300 , a relative force applied to squeeze gradient coil assembly support portion  108  and frame support portion  112  together will tend to counter the force. The magnitude of the squeezing force applied by transducer  140  can, in principle, be adjusted so that there is no net force on support portion  112 . The squeezing force described above will tend to produce motion of the gradient coil support portion  108  in the same direction as the original vibration and thereby increase gradient coil assembly  102  motion. However, the amplitude of that additional displacement will be about the same as the original motion. This is not a problem for MR imaging as long as the original motion is microns. The magnitude of the additional gradient coil support portion  108  motion produced by active drivable element  140  can be seen from a simple argument. The force F c    119  applied to stop frame support portion  112  motion is F c =−k δx, where δx  117  is the original gradient coil assembly support portion  108  displacement. Assuming that frame support portion  112  does not move significantly, the motion of the gradient coil assembly support portion  108  caused by this force is δx′=−F c /k, which is the same as δx. So the amplitude of motion of the gradient coil support portion  108  may double. (Note, support portion  112  motion even with passive vibration isolation is much smaller than the gradient coil assembly  102  motion—30 dB as measured in W A Edelstein et al.,  Magnetic Resonance Imaging  20, 155-163, 2002. It is this motion, that the present invention reduces further.) 
     The forces involved can be quantified. In one example, resilient member  110  may be made of rubber elastomer that is compressed about 1 cm by half the weight of the gradient coil assembly  102  (500 kg, 4900 N), which gives a spring constant of k=4.9×10 5  N/m. 10 μm of motion of the gradient coil rigid support portion  108  gives a force of only 10μ·k=4.9 N=1.1 lbf. Given that frame support portion  112  has to be very stiff to support gradient coil assembly  102  weight, this implies that the motion of the cryostat rigid support portion  112  is small. This is consistent with the vibration amplitude attenuation of 30 dB between gradient support portion  108  and frame support portion  112  found in W A Edelstein et al.,  Magnetic Resonance Imaging  20, 155-163, 2002. 
     The compensating force is preferably applied in a “feed forward” fashion based on a predetermined, electrical-signal-to-gradient assembly motion transfer function. One way to determine such a transfer function is to measure the motion of gradient support portion  108  and/or frame support portion  112  when a pulsed gradient signal is applied to a single axis. This transfer function would then be embodied as a processor  302  including a hardware and/or software signal filter which would then apply the correct time-dependent force via active drivable element  140  between gradient support portion  108  and frame support portion  112  to counteract the force applied to frame support portion  112  by motion of gradient support portion  108 . Transfer functions for other axes would be similarly obtained, and the results combined independently to reduce frame support portion  112  motion and consequent acoustic noise production. Using additional relative force fixtures similar to suspension element  300 , this process could also be applied to sideways motion or longitudinal motion along axis  250  of gradient coil assembly  102 . 
     The signal for the force compensation can be derived in other ways than feed forward from the input gradient pulse as described above. The signal could be obtained by sensing the motion of gradient support portion  108  and feeding forward, that is, deriving a transfer function, to apply to active drivable element  140 . In one embodiment, the signal could be derived from feedback from at least one of gradient coil assembly  102  and cryostat shell (frame)  100 . In this case, processor  302  configured to activate active drivable element  140  based on the feedback could be implemented to bring the motion of frame support portion  112  to zero. 
       FIG. 5  shows a further configuration for decreasing induced vibration of frame  100 . In this embodiment, gradient coil assembly  102  is still supported by the fixed cryostat shell or frame  100  via passive vibration isolation support  116 . However, an active drivable element  140  is supported from the floor via floor pedestal  129 , and active drivable element  140  has its position adjusted by using pre-stressing mount  127  (e.g. a bolt, screw or threaded rod) to apply pre-stress to active drivable element  140 . 
       FIG. 6  shows a further configuration for decreasing induced vibration of frame  100 . In this embodiment, gradient coil assembly  102  is supported by floor pedestal  129 . A C-fixture  130  is positioned so that active drivable element  140  does not support the weight of gradient coil assembly  102 . 
     In any of the above-described embodiment, more stages of resilient elements and metal could be used between gradient support portion  108  and frame support portion  112  and force compensation could be applied to intermediate metal stages to achieve further vibration isolation. In addition, more or fewer suspension elements  300  with force compensation could be used around the circumference of the ends of gradient coil assembly  102  to provide the desired stability and vibration isolation for gradient coil assembly  102 . 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.