Abstract:
An apparatus for supporting a patient support bridge member in an MR system where a first end of the bridge is supported by an upright support member on a first side of an MR imaging bore and the bridge extends through the bore so that a second end of the bridge extends out a second side of the bore, the apparatus including a bracket mounted to an MR main magnet on the second side of the bore and extending upwardly to contact and support the bridge there above.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to MR patient support configurations and more specifically to a patient support that mounts to a main MR magnet and supports a patient table bridge. 
     An MR imaging system or scanner commonly includes a cryostat, which contains a powerful superconductive main magnet positioned around a main magnet bore. The superconductive magnet is maintained at an extremely cold temperature and produces a strong static magnetic field Bo along a bore axis within the main magnet. Other essential components of the MR system include an RF coil, or RF antenna, and a gradient coil assembly, which comprises a hollow cylindrical structure. The RF coil may be operated in a transmit mode, to generate MR signals in an imaging subject, or may be operated in a receive mode to detect the MR signals. 
     The gradient coils are electrically excited to impose X-, Y-, and Z-gradient time varying magnetic fields on the primary magnetic fields that are required for imaging purposes. In a common arrangement, each gradient field is produced by a pair or set of gradient coils, wherein each coil is wrapped around one of two cylindrical coil forms. The two coil forms are placed in coaxial relationship, and the coil forms and respective X-, Y- and Z-gradient coils collectively comprise a gradient coil assembly. Arrangements of this type are described, for example, in U.S. Pat. No. 5,570,021, issued Oct. 29, 1996 and commonly assigned herewith to the General Electric Company. Such arrangements are also described in U.S. Pat. No. 5,760,584, issued Jun. 2, 1998 and likewise assigned to the General Electric Company. Typically, the gradient coil assembly is mechanically supported within the cylindrical bore of the main magnet. The gradient coil, RF coil and main magnet together form an imaging area about the main magnet bore axis. 
     To support a patient within the main magnet bore during data acquisition, a patient support structure is provided that typically includes an upright support, a bridge, a cradle and a bridge support. The upright support is a rigid member that rests on a floor adjacent an MR imaging system within an imaging room and includes an upper end for receiving a first end of the bridge. The bridge includes a stiff member that extends between first and second ends where the first end is mounted to the rigid upright support and the second end extends into and through the imaging area so that its length is generally parallel to the bore axis. In many designs the bridge includes tracks for receiving cradle wheels and guiding the cradle through the imaging area. 
     The cradle is typically a relatively flimsy member having upper and lower oppositely facing surfaces and a length that is generally sufficient to support a patient. The cradle includes a plurality of wheels mounted to its lower surface and arranged so as to be receivable within the bridge tracks for guidance there along. The cradle is capable of movement along the bridge into various positions with respect to the imaging area including a loading position outside the imaging area and at least one imaging position where at least a portion of a patient disposed on the cradle is positioned within the imaging area. 
     As configured above, when a patient (especially a relatively heavy patient) is supported on the cradle and the cradle is fully extended on the bridge, the bridge has a tendency to deflect slightly downward thereby causing a patient misalignment. To overcome this problem many support configurations include a bridge support. To this end, an exemplary bridge support includes a rigid member that is typically mounted to the inside of the gradient coil and extends upwardly to and is secured to the bridge relatively closer to the second end of the bridge than the first end. Thus, when a patient is positioned within the imaging area, the cradle and bridge are supported by the upright support on the first end and by the bridge support, gradient coil and main magnet on the second end. 
     Unfortunately, when a gradient coil is excited to generate magnetic gradients, the gradients interact with structure about the coils and the coils tend to be mechanically displaced (i.e., vibrates). The mechanical structure used to support the gradient coil assembly within the main magnet bore provides a path for transferring or coupling the vibrations of the gradient coils to the main magnet structure. Generally, the main magnet is supported on the floor of the building in which the MR system is operated. Accordingly, the gradient generated vibrations are often directly coupled from the magnet to the floor, and then travel through the floor to vibrate structures throughout the building. As a result, gradient coil vibrations can couple acoustically to rooms outside of the MR scan room, i.e., a room which is specially constructed to house the MR system. 
     In addition to the problems associated with transmitting gradient vibrations to other facility equipment and space, the rigid bridge support also causes the gradient vibrations to be transmitted to the patient support cradle and a patient thereon. While gradient related patient vibration in early MR systems was relatively minimal and therefore could essentially be ignored, characteristics of newer MR systems have resulted in greater adverse effects. For instance, gradient technology has evolved to the point where relatively high gradient fields are employed during data generation and acquisition so that the magnitude of gradient related vibrations is relatively greater in newer systems. In addition, the actual mass of the imaging components (i.e., the main magnet, coils, shields, etc.) has been reduced appreciably such that even small gradient fields sometimes cause appreciable vibration. 
     Cradle vibration has two adverse side effects. First, whenever a patient is exposed to a new or unfamiliar medical process, the patient typically and understandably experiences anxiety and nervousness about the experience. This is especially true of MR imaging procedures where noise and essentially uncontrolled gradient movement and vibrations are transmitted to the patient. Anxiety often causes patients to move or flinch during acquisition which can cause image artifacts in images generated with collected data. Second, even where a patient manages to remain essentially still relative to a supporting cradle, where the cradle and patient vibrate together relative to the RF data receiving coils, resulting images are typically polluted by image artifacts. 
     One configuration that essentially isolates the gradient coils from the main magnet and thereby mitigates transmission of gradient vibrations to the main magnet and surrounding facility equipment and space is described in U.S. Pat. No. 6,160,399 (hereinafter “the &#39;399 patent”) which issued on Dec. 12, 2000 and is entitled “Apparatus For Supporting MR Gradient Coil Assembly”. According to the &#39;399 patent, two mounting assemblies are mounted to the main magnet and extend axially to the gradient coils, a separate mounting assembly disposed at either end of the main magnet. The mounting assemblies transmit a static force from the main magnet to the gradient coil assembly to hold the coil assembly in place within the main magnet bore in coaxial relationship with the bore and in spaced-apart relationship with an internal bore wall. At the same time, the two mounting assemblies act to dampen or attenuate the gradient coil vibrations, and thus oppose passage transmission of the vibrations through the mounting assemblies to the main magnet. The &#39;399 patent configuration provides no other path through which gradient coil vibrations can be transferred from the gradient coil assembly to the main magnet. 
     While addressing the problem of transmitting gradient vibrations from the MR configuration to facility equipment and space, unfortunately the mounting assemblies described in the &#39;399 patent do nothing to reduce vibrations to the patient support cradle. In fact, in some cases, by isolating the gradient coil from the main magnet, the end result may be to increase cradle and patient vibrations thereby increasing discomfort and reducing image quality. 
     SUMMARY OF THE INVENTION 
     It has been recognized that a patient support table or cradle can be sufficiently isolated from MR gradient coils by, instead of supporting the cradle on the coils, providing a bracket that mounts directly to the MR main magnet and supports the cradle while in an imaging area. This concept is particularly useful where the coil gradient assembly is isolated from the main magnet as in the case of the &#39;399 patent referenced above as the combined coil isolation system of the &#39;399 patent and the present invention increase patient comfort and reduce imaging artifacts appreciably. 
     An exemplary embodiment of the invention is to be used with an MR system having a main magnet, a gradient coil assembly and a patient support, the magnet having first and second oppositely facing surfaces and forming a bore that extends between the first and second surfaces along a bore axis, the coil assembly disposed within the bore about an imaging area, the support including a support member and a bridge having first and second ends, the support member supporting the first bridge end proximate the first surface, the bridge extending into and through the bore so that the second end is proximate the second surface and the bridge forms an essentially downwardly facing undersurface. The apparatus is for supporting the bridge and comprises a bracket disposed for fixable attachment to the main magnet such that the bracket extends toward the imaging area. The bracket forms at least one essentially upwardly facing support surface for receiving the bridge undersurface and supporting the bridge there above. In several embodiments the bracket is mountable to the second magnet surface. 
     In one embodiment the bracket includes first and second post members that extend essentially upwardly to distal ends, the distal ends forming first and second upwardly facing surfaces for supporting the bridge, respectively. Each post may include a stainless steel rod having a distal end and a phenolic head member. 
     In some embodiments the bridge has a width dimension perpendicular to the bore axis, the bracket has a length dimension greater than the width dimension and the bracket is securable to the main magnet at opposite ends of the length dimension so as to have an essentially horizontal orientation. 
     In most embodiments the bracket components are formed of a low magnetic flux material such as, for instance, a phenolic resin material. 
     These and other aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an exemplary MR imaging system configured in accordance with the present invention; 
     FIG. 2 is a partial cross-sectional view taken along the line  2 — 2  of FIG. 1; 
     FIG. 3 is a perspective view of the system end illustrated in FIG. 2; and 
     FIG. 4 is an exploded perspective view of the system end illustrated in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numbers and characters correspond to similar elements throughout the several views and, more specifically, referring to FIG. 1, an exemplary imaging system  10  constructed in accordance with the present invention is illustrated. System  10  generally includes two separate sub-configurations, a magnetic resonance (MR) imaging configuration or assembly  12  and a patient support system  14 . As known in the MR art, among other components, system  12  includes a large main magnet  16  and a gradient coil assembly  18 . 
     Main magnet  16  has first and second oppositely facing surfaces  48  and  50 , respectively, is generally annular in shape and forms a main magnet bore  20  formed about a horizontal bore axis  22  that extends between first surface  48  and second surface  50 . Main magnet  16  rests on the floor in an imaging room of a medical facility. Referring also to FIG. 4, main magnet  16  forms two threaded apertures  80  and  82  in a lower portion of second surface  50  that are used to mount a bracket member  69  to surface  50  in a manner to be described in more detail below. In the illustrated embodiment apertures  80  and  82  are symmetrically positioned with respect to a plane (not illustrated) that divides magnet  16  into lateral halves although many other configurations are contemplated. 
     Gradient coil assembly  18 , like main magnet  16 , is generally annular in shape and forms an imaging bore  24  about an imaging area  26 . Gradient coil  18  is mounted within main magnet  16  such that the main magnet bore  20  and coil bore  24  are concentrically aligned along axis  22 . Configurations for mounting assembly  18  within magnet  16  are well known in the art and therefore will not be described here in detail. One exemplary mounting system which is particularly useful in the context of the present invention is described in the &#39;399 patent referenced above which is incorporated herein by reference. Suffice it say here that the &#39;399 patent coil mounting configuration generally blocks gradient coil  18  vibration from transmission to the main magnet  16  while still providing a static force between the main magnet  16  and the coil  18  sufficient to minimize coil motion within area  26 . 
     Support system or assembly  14  includes an essentially upright or vertical rigid support member  30 , a bridge or bridge member  32 , a cradle member  34  and a bracket assembly  36 . Support member  30  has a bottom end  38  and a top end  39 . Bottom end  38  rests on the floor of an imaging area and top end  39  extends up therefrom. 
     Referring to FIGS. 1 and 2, bridge  32  is generally an arcuate member which includes oppositely facing top and under surfaces  40 ,  42 , respectively, and extends along a length dimension from a first end  44  to a second end  46 . In the illustrated embodiment, bridge  32  has width dimension W 1  (see FIG. 2) and is concave along top surface  40 . The first end  44  of bridge  32  is mounted to the top end  39  of support member  30  so that bridge top surface  40  faces upward. Support member  30  has a height dimension (not illustrated) such that, when bridge  32  is mounted to top end  39 , bridge  32  can extend through imaging area  26  and is supported above the lower most portion of coil assembly  18  (i.e., a gap is formed between bridge undersurface  42  and coil  18  (see FIG.  1 )). Support member  30  is positioned with respect to system  12  such that bridge second end  46  extends through bore  24  and protrudes past main magnet second surface  50 . 
     Bridge undersurface  42  forms post receiving recesses  110  and  112  proximate second end  46 . Recesses  110  and  112  are generally equispaced along the width W 1  of bridge  46  and are proximate lateral bridge edges (see FIG.  2 ). Operation of recesses  110  and  112  with posts is described in more detail below. Although not illustrated, bridge top surface  40  may form tracks along its length (i.e., parallel to axis  22 ) for receiving wheels on a bottom of cradle member  34  to support and guide cradle member  34  between various positions described below. 
     Cradle member  34  is typically formed of a relatively flimsy material having a top surface  52  and a bottom surface  54 . Top surface  52  forms a patient receiving surface such that a patient can rest on surface  52  relatively comfortably during a data acquisition session. A plurality of wheels collectively identified by numeral  56  are mounted to bottom surface  54  of cradle  34 . Cradle  34  rests on the top surface  40  of bridge  32  for movement therealong essential parallel to bore axis  22 . In FIG. 1, cradle  34  is illustrated in a patient loading position. After a patient is positioned on surface  52 , cradle  34  is typically repositioned such that cradle  34  resides generally within imaging area  26  in an imaging position (not illustrated). 
     Referring now to FIGS. 1 through 4, bracket  36  is generally mounted to the second surface  50  of main magnet  16  proximate a bottom magnet portion. Generally, when mounted to magnet  16 , bracket  36  extends upwardly from magnet  16  toward imaging area  26  and forms at least one surface that contacts and supports the undersurface  42  of bridge  32  so that second end  46  of bridge  32  is supported and undersurface  42  is isolated from coil assembly  18 . 
     To this end, the exemplary bracket  36  includes a “mustache” or yoke member  69  and first and second post members  70  and  72 , respectively. Yoke member  69  includes a central arcuate segment  64  and first and second end segments  60  and  62 , respectively. Arcuate segment  64  forms a top surface  66  and a bottom surface  68  and is concave along top surface  66 . End segments  60  and  62  are separated by central segment  64  and each extends from an adjacent end of segment  64  away from undersurface  68 . Segment  60  forms a mounting hole  74  while segment  62  forms a mounting hole  76 . Holes  74  and  76  are spaced apart with respect to each other such that holes  74  and  76  are alignable with threaded apertures  80  and  82  on main magnet  16  to facilitate mounting. 
     Two mounting bolts  84  and  86  are provided which, to mount yoke member  69  to main magnet  16 , pass through holes  76  and  74  and are received within threaded apertures  82  and  80 , respectively. When so mounted, top surface  66  of yoke member  69  faces generally upwardly toward imaging area  26  and, like apertures  80  and  82 , yoke  69  is symmetrically positioned with respect to a plane that divides magnet  16  into lateral halves. In the illustrated configuration yoke  69  has a length dimension L 1  that is greater than bridge width WI which provides additional stability to the supported bridge  32 . 
     As best seen in FIG. 2, two post receiving apertures  90  and  92  are formed in top surface  66  of yoke member  69  and are generally vertically aligned. Apertures  90  and  92  are symmetrically formed within surface  66  so that a separate aperture is proximate each of end segments  60  and  62 . 
     Post  70  is generally bolt shaped having a threaded shaft member (not separately numbered) and a head member  98  at a distal end. Post  70  is sized and threaded such that post  70  is threadably receivable within aperture  90  so that the distal end  98  extends generally upwardly and toward imaging area  26 . Similarly, post  72  is generally bolt shaped having a threaded shaft that is receivable within aperture  92  and a distal head end  99  that extends generally upwardly toward imaging area  26 . In the illustrated embodiment, the top surfaces  100  and  102  of post heads  98  and  99 , respectively, form essentially upwardly facing support surfaces for receiving the undersurface of bridge  32  (see FIG.  2 ). As illustrated, heads  98  and  99  are received within recesses  110  and  112 , respectively, and cooperate therewith to minimize lateral movement of bridge  32 . Being threaded, posts  70  and  72  may be adjusted within apertures  90  and  92  to raise or lower heads  98  and  99  thereby adjusting the height of bridge second end  46  to an optimal level. 
     Importantly, for the purposes of the present invention, yoke member  69  and posts  70  and  72  are each, in at least one embodiment of the invention, formed of a non-flux generating material. In some cases the material used to form members  69  and posts  70  and  72  is a phenolic material that has suitable flux properties and has a high damping coefficient. Phenolic materials are particularly suitable for the present application as they have generally ideal properties. To this end, phenolic materials inhibit flux generation and generally have relatively high damping coefficients so that components constructed of phenolic materials reduce vibration transmission. Nevertheless, phenolic components are stiff enough to impart sufficient static support for bridge second end  46  that bridge  32  will not oscillate within imaging area  26 . 
     Thus, referring again specifically to FIG. 1, it should be appreciated that the present invention generally isolates bridge  32  from gradient coil assembly  18 . In MR configurations where gradient coil assembly  18  is isolated from main magnet  16  (i.e., as in the case of U.S. Pat. No. 6,160,399), bridge  32  is relatively well isolated from coil  18  and therefore patient comfort and image quality are increased appreciably by employing the present invention. For example, while the bracket is described as being relatively stiff it should be appreciated that a complaint damping material may be provided within the bracket supporting system (e.g., between bracket  69  and surface  50  or between surfaces  102 ,  100  and the undersurface of the bridge or between any other two adjacent surfaces) to further mitigate or dampen vibrations. In addition, while the bracket  69  is described as being secured to surface  50 , it should be appreciated that bracket  69  may be secured to some other main magnet surface (e.g., an internal surface, etc.). 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention.