Abstract:
A magnetic resonance has a scanner unit with an examination space for placing at least a region of an examination subject therein, a gradient coil unit movable in a displacement direction at least in the examination space, a component of the scanner unit surrounding the examination space, and at least one pillow that can be arranged between the gradient coil unit and the component and which has an internal pressure that can be controlled for fixing the gradient coil unit relative to the component.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention is directed to a magnetic resonance apparatus.  
           [0003]    2. Description of the Prior Art  
           [0004]    Magnetic resonance technology is a known technique for, among other things, 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. 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 that picks up the triggered magnetic resonance signals, on the basis of which magnetic resonance images are produced.  
           [0005]    The magnetic resonance apparatus has an examination space containing an imaging volume wherein a region of the examination subject to be imaged is to be positioned for producing magnetic resonance images of that region. The positioning of the region to be imaged in the imaging volume is possible by displacement of a movable support mechanism with the examination subject placed thereon.  
           [0006]    The gradient system of the magnetic resonance apparatus includes a gradient coil arrangement that must be rigidly connected to the basic field magnet. To that end, German OS 197 22 481 discloses a magnetic resonance apparatus wherein a basic field magnet has a first surface and a rigidly installed gradient coil system with a second surface, the two surfaces facing toward one another and being spaced from one another. A noise reduction device for damping the oscillations of the gradient coil system and/or for stiffening the gradient coil system is arranged in contact with both surfaces. In one embodiment, the noise reduction device is formed of at least one pillow. Such a pillow preferably comprises an envelope and a core or a filling. In order to achieve an especially good noise-damping effect, the pillow is elastic and/or resilient and/or flexible. In the operating state of the magnetic resonance apparatus, the pillow lies tightly against the first and the second surfaces but is not rigidly connected to these surfaces. As a result, the gradient coil system can be more easily detached for maintenance or replacement (which requires the rigid connection to be dismantled. The pillow has an air-impermeable outside skin that is formed of welded plastic film. The outside skin is composed of PVC film, polyethylene film or some other film that is airtight and weldable. A foamed fill composed of an open-pore cellular material, for example polyurethane foam, is contained in the pillow. The fill of cellular material insures good noise damping as well as providing an adequate elasticity of the pillow when the air pressure in the pillow is approximately the same as the ambient air pressure. The pillow has a connection that can be fashioned as a valve. The pillow is thicker than the distance between the first and second surfaces when the pillow is not introduced into the magnetic resonance apparatus and the valve is open. When, with the valve open, this pillow is introduced into the magnetic resonance apparatus, then the fill of cellular material presses the outside skin against the first and second surfaces, so the gap between these surfaces is completely filled. A pillow with an airtight outside skin and a fill of cellular material can be easily introduced into the gap between the first and second surfaces and be removed therefrom when the air contained in the pillow is pumped out to such an extent that it is compressed to a thinner thickness by the ambient air pressure. In an alternative embodiment of the noise reduction device, the pillow be pumped to a slight over-pressure and/or it can be filled with some other gas or a fluid and/or the inner fill of cellular material can be omitted.  
           [0007]    In addition to a rigidly installed gradient coil system, moreover, U.S. Pat. No. 5,185,576 discloses a local gradient coil unit that is combined with a local radio-frequency antenna. The local gradient coil unit with the integrated local radio-frequency antenna is designed for a specific region of the examination subject, for example for the head of a patient. As a result, the local gradient coil unit can be implemented with smaller dimensions compared to the rigidly installed gradient coil system, yielding advantages in view of—among other things—gradient intensities that can be achieved and power demands made on the gradient amplifier that feeds the gradient coil unit. The local gradient coil unit with the integrated local radio-frequency antenna can be secured to the support mechanism such that the local gradient coil unit does not move relative to the support mechanism even during operation of the magnetic resonance apparatus despite the forces that act on it.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a magnetic resonance apparatus wherein a locally introducible gradient coil unit can be flexibly fixed in the apparatus.  
           [0009]    The object is inventively achieved by in a magnetic resonance having a scanner unit with an examination space for placing at least a region of an examination subject therein, a gradient coil unit movable in a displacement direction at least in the examination space, a component of the scanner unit surrounding the examination space, and at least one pillow that can be arranged between the gradient coil unit and the component and which has an internal pressure that can be controlled for fixing the gradient coil unit relative to the component.  
           [0010]    The invention is based on the perception that the noise reduction device known, for example, from German OS 197 22 481 and fashioned as a pillow can also be advantageously employed in a magnetic resonance apparatus having a movable gradient coil unit (as opposed to the rigidly connected gradient coil unit with which the pillow is intended for use) for fixing such a movable gradient coil unit. The advantage of mechanical decoupling accompanied by noise reduction and reduction in the transmission of vibrations known from German OS 197 22 481 is retained, with, further advantages in conjunction with the movable gradient coil unit. Thus, the movable gradient coil unit can be remotely controlled rapidly and free of the need for actuation of a manual interlock, by means of an appropriate pump for controlling the internal pressure, allowing the movable coil unit to be fixed inside the examination space with infinite variation in the displacement direction, i.e. it can be fixed at arbitrary positions within the examination space. Fixing to what is always the same location, of course, is not precluded.  
           [0011]    Further, the fixing by means of the pillow is advantageous because—apart from an introduction of the pillow—neither existing components of the magnetic resonance apparatus that limit the examination space nor existing, movable gradient coil units need by modified for employing this type of fixing.  
           [0012]    In an embodiment, the pillow has an extent in the displacement direction roughly corresponding to the extent of the gradient coil unit. Particularly given a pillow that is fashioned for large-area placement and/or given utilization of a number of pillows due to the large seating surface, a high security against slippage of the gradient coil unit is achieved.  
           [0013]    In another embodiment, the magnetic resonance apparatus has a movable support mechanism that is displaceable on a guide mechanism of the magnetic resonance apparatus, and the movable gradient coil unit is displaceable on the same guide mechanism. In one version, the examination space has at least two openings disposed opposite one another, so that the support mechanism can be moved into the examination space proceeding from one opening and the movable gradient coil unit can be moved into the examination space proceeding from the other opening. A time-efficient utilization of the magnetic resonance apparatus can be achieved as a result, since the examination subject, for example a patient, can be placed on the support mechanism during the displacement of the gradient coil unit. After the patient has been placed thereon, the support mechanism together with the patient placed thereon is moved into the examination space, where the gradient coil unit is already waiting, correctly positioned and fixed. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]    The single Figure is a side sectional view of a magnetic resonance apparatus constructed in accordance with the principles of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    As an exemplary embodiment of the invention, the Figure shows a longitudinal section through a magnetic resonance apparatus having a movable gradient coil unit  60  displaceable in at least one travel direction  65 . The magnetic resonance apparatus has an essentially hollow-cylindrical, superconducting basic field magnet  10  for generating a static basic magnetic field. A gradient coil system  20  for generating gradient fields that is likewise essentially hollow-cylindrical is permanently installed in the cavity of the basic field magnet  10 . The permanently installed gradient coil system  20  has three gradient coils, shielding coils associated with the gradient coils, cooling devices and shim devices. An essentially hollow-cylindrical whole-body antenna  30  for emitting radio-frequency signals and for receiving magnetic resonance signals is permanently installed in the control cavity of the gradient coil system  20 . A central cavity of the whole-body antenna  30  essentially defines an examination space  40  in which at least a region of an examination subject, for example a patient can be placed.  
         [0016]    A displaceable support mechanism  50  makes it possible to introduce the examination subject placed on the support mechanism  50  into the examination space  40  from the left with reference to the longitudinal section shown in the Figure. The support mechanism  50  is seated so as to be displaceable on a guide mechanism  55 . At the foot end, the support mechanism  50 , the bearing mechanism  50  has a connection unit  52  to which—among other things—a conduit of a vacuum cushion can be connected. The patient is placed in a comfortable and stable position on a sheet on the cushion. Air is then extracted from the vacuum pillow (filled with styropore balls) via the conduit, so that the shape of the vacuum pillow stabilizes.  
         [0017]    The movable gradient coil unit  60  is displaceable at least in a part of the examination space  40 . For generating gradient fields, the gradient coil unit  60  has at least one gradient coil (up to three gradient coils) and, dependent on the use requirements, shielding coils belonging to the gradient coils, cooling devices and shim devices and/or is combined with a local radio-frequency antenna. Displacement of the gradient coil unit  60  can ensue manually as well with the assistance of a motorized drive. Like the support mechanism  50 , the movable gradient coil unit  60  is fashioned to be movable on the guide mechanism  55 , which is unproblematical particularly given a weight of the gradient coil unit  60  that is somewhat less than an approved load weight of the support mechanism  50 . Special measures have to be provided only given a weight of the gradient coil unit  60  of more than roughly 200 kg. The electrical connection lines and, if present, cooling supply lines  62  necessary for supplying the gradient coil unit  60  are conducted to the gradient coil unit  60  from the right.  
         [0018]    A pillow  70  or multiple pillows  70  distributed in circumferential direction is/are glued to the gradient coil unit  60  for fixing the gradient coil unit  60  at an arbitrary position within the whole-body antenna  30 . Each pillow  70  has a gastight outer skin  72  that is composed of two pieces of a PVC film that are welded to one another. A connection  76  fashioned with a valve  77  is introduced gastight at a location of the circumferential seam. The outer skin  72  surrounds a fill  74  of cellular material formed of an open-pore polyurethane foam that is planarly glued to the outer skin  72 .  
         [0019]    Given a displacement of the gradient coil unit  60 , each pillow  70  is evacuated and each valve  77  is closed. After the gradient coil unit  80  has been positioned as desired, the valve  77  is opened for fixing the gradient coil unit  60  against the whole-body antenna, so that roughly ambient air pressure prevails in the inside of each pillow  70 . The pillow  70  is dimensioned such that the outer skin  72  are pressed against the whole-body antenna  30  and the gradient coil unit  60  by the fill  74  of cellular material. The spacing between the whole-body antenna  30  and the gradient coil unit  60  is then completely filled, so that the gradient coil unit  60  is fixed and—at the same time—oscillations occurring during operation of the gradient coil unit  60  are effectively damped.  
         [0020]    Air is pumped out of each pillow  70  via the connection  76  for rendering the gradient coil unit  60  mobile. The ambient air pressure then compresses each pillow  70 , and each valve  77  is are closed so that the gradient coil unit  60  can again be moved. For the pumping, each connection  76  can be connected via a hose system  79  to the connection unit  52  at the connection point for the vacuum cushion. In a magnetic resonance apparatus that has no facilities for a vacuum cushion, a separate unit for pumping air can be connected to each pillow  70 .  
         [0021]    After setting a new position of the gradient coil unit  60  within the whole-body antenna  30 , each valve  77  is again opened, so that air flows into each pillow  70 . Each pillow  70  regains elasticity and thickness and thus fixes the gradient coil unit  60  in a press fit.  
         [0022]    In an alternative embodiment each pillow  70  is pumped to a slight over-pressure and/or each pillow  70  is filled with a gas other than air or with a liquid. It is also possible to omit the inner fill  74  of cellular material.  
         [0023]    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.