Abstract:
An open or split type MRI apparatus has two axially spaced magnet coil half sections separated and supported by a compact support structure. Only two diametrically opposed supports are needed to react the high axial and torsional loads produced or received by the MRI apparatus. One support is loaded under pure compression, and the other support is loaded under compression and tension.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates in general to structural supports for magnetic resonance imaging (MRI) apparatus and relates in particular to an open MRI apparatus having axially-spaced pairs of magnet coils supported by a pair of diametrically opposed supports.  
         DESCRIPTION OF PRIOR DEVELOPMENTS  
         [0002]    An MRI magnet is characterized as open when there is an accessible, room temperature, physical gap provided between a pair of superconducting magnet coils. An open MRI magnet is desirable as it improves patient comfort and accessibility as compared to closed MRI magnets which many patients consider uncomfortable and which limit patient access.  
           [0003]    In order to provide the desired openness and create an open gap around a patient imaging region, a pair of magnet coil assemblies can be separated into two axially-spaced half sections. The half sections of the magnet produce high attractive magnetic forces which must be reacted with a structural support system that separates and supports the half sections and prevents the magnet coils from collapsing upon one another.  
           [0004]    Typical axial forces for a 1.0 T MRI magnet are in the range of about 160,000 lbs. (711 KN). Prior MRI support structures reacted these axial forces by an arrangement of axial posts which interconnected the two magnet half sections. The numerous posts limited the openness of the magnet.  
           [0005]    An MRI support structure must not only axially separate and axially support the two magnet half sections during operation of the MRI apparatus, the support structure should also provide circumferential support to accommodate torsional or twisting forces which may be applied to the half sections during, for example, shipping, installation, mobile and normal operation. Moreover, it is desirable to provide such a support system which resists the transmission of floor-induced vibrations from the surrounding building structure to the MRI apparatus.  
           [0006]    Accordingly, a need exists for a robust support structure for an open MRI apparatus which reacts high axial loads produced during operation of the apparatus, without adversely affecting the openness of the gap defined between two MRI magnet half sections.  
           [0007]    A further need exists for such a structure which reacts tensile and compressive loads produced by a pair of MRI half sections as well as any torsional loads which may be applied to the half sections.  
           [0008]    Another need exists for a support structure for an open MRI magnet assembly which resists the transmission of vibrations from the surrounding building floor.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been developed to fulfill the needs noted above and therefore has as an object the provision of a support structure for axially separating and supporting a pair of superconducting magnet half sections without adversely affecting the openness of the patient gap defined between the two magnet half sections.  
           [0010]    A further object of the invention is the provision of such a support structure which uses only two dramatically opposed supports for connecting together two MRI half sections to provide a high degree of openness with a minimum of patient obstruction.  
           [0011]    Another object of the invention is to provide such a support structure which is compact in size.  
           [0012]    Another object of the invention is the provision of a pair of axially-extending support assemblies having dissimilar or unequal constructions, dissimilar or unequal cross sections and which react loads differently.  
           [0013]    Still another object of the invention is the provision of such a support structure which has very high bending, shear and torsional natural frequencies so as to make the MRI apparatus more tolerant to floor induced vibrations.  
           [0014]    These and other objects are met by the present invention which is directed to a support structure for interconnecting and supporting a pair of half magnet sections of an MRI apparatus. The support structure includes two axially-extending supports or columns located diametrically opposite one another on the outer circumferential periphery of a pair of MRI magnet coil half sections. The opposed supports have unequal structural configurations and different cross sections and accommodate different types of loading.  
           [0015]    The support with the larger cross section is advantageously constructed with a compressive load bearing member and one or more tension load bearing members. These compressive and tensile load bearing members react only a portion of the axial magnetic force produced between the two magnet half sections. The remainder of the load is reacted by a second, much smaller support member that reacts only compressive axial forces.  
           [0016]    The larger support is radially offset to one side of the magnet assembly to enable the open magnet to maintain a high degree of openness and to improve patient comfort and accessibility. In order to provide for the flow of cryogenic fluid, such as liquid helium, between the two magnet half sections, at least the larger support includes a hollow fluid flow portion.  
           [0017]    To further enhance the openness of the MRI system, the outer vacuum container can be formed with an inner and outer conical portion to further reduce encroachment of the structure into the region of the imaging gap.  
           [0018]    Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    In the Drawings:  
         [0020]    [0020]FIG. 1 is a schematic perspective view of a helium vessel of an MRI apparatus constructed in accordance with the present invention;  
         [0021]    [0021]FIG. 2A is an axial cross sectional view of an MRI apparatus constructed in accordance with the present invention taken along section line  2 A- 2 A of the representative helium vessel of FIG. 1. FIG. 2A is sectioned through the top half of a pair of diametrically opposed supports and is a mirror image of the lower half of the helium representative vessel of FIG. 1.  
         [0022]    [0022]FIG. 2B is an axial cross sectional view of a lower magnet half section similar to FIG. 2A, taken along sectional line  2 B- 2 B of FIG. 1 and is circumferentially rotated 90° from the upper magnet half section shown in FIG. 2A;  
         [0023]    [0023]FIG. 3 is a view in section taken through section line  3 - 3  of FIG. 2A showing details of the large support for the helium vessel of FIG. 1;  
         [0024]    [0024]FIG. 4 is a right side view of FIG. 2A viewed along line  4 - 4  of FIG. 2A;  
         [0025]    [0025]FIGS. 5A and 5B are axial cross sectional views through the helium vessel of FIGS. 2A and 2B and through the surrounding thermal shield and outer vacuum container, and respectively taken through the same sections as in FIGS. 2A and 2B;  
         [0026]    [0026]FIGS. 6A and 6B are respective cross sectional views of the thermal shield of FIGS. 5A and 5B, taken through the same respective sections as in FIGS. 5A and 5B;  
         [0027]    [0027]FIGS. 7A and 7B are respective axial cross sectional views of the outer vacuum container of FIGS. 5A and 5B, taken through the same respective sections as in FIGS. 5A and 5B;  
         [0028]    [0028]FIG. 8 is a top plan view of FIG. 5A, with the cryocooler removed for clarity;  
         [0029]    [0029]FIG. 9 is a top plan view of the helium vessel;  
         [0030]    [0030]FIG. 10 is a top plan view of the thermal shield; and  
         [0031]    [0031]FIG. 11 is a top plan view of the outer vacuum container. 
     
    
       [0032]    In the various views of the drawings like reference characters denote like or similar parts  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    The present invention will now be described in conjunction with the drawings, beginning with FIG. 1 which shows a helium vessel  10  constructed in accordance with the present invention. Helium vessel  10  includes a first or upper magnet half section  12  and a second or lower magnet half section  14 . Magnet half sections  12  and  14  are substantially mutually symmetrical and take the form of annular hollow members aligned coaxially with one another.  
         [0034]    The magnet half sections  12 ,  14  are axially connected and supported by a first axially-extending support  16  and a second axially-extending support  18 . Support  16  may take the form of a cylindrical post or pipe. Support  18  includes a radially outer portion  20  and a radially inner portion  22 . Supports  16  and  18  are disposed diametrically opposed to one another on opposite sides of the substantially cylindrical magnet half sections  12 ,  14 . An axial gap  24  is defined between the magnet half sections  12 ,  14  to provide an imaging region for a patient.  
         [0035]    A central opening  26  may be formed through each magnet half section  12 ,  14 . Each magnet half section includes an axially and radially inner annular main coil portion  28  and an axially and radially outer shield coil portion  30 . The main and shield coil portions  28 ,  30  are coaxially aligned with one another and define an axial step  32  between them.  
         [0036]    As seen in FIG. 2A, the first or upper half magnet section  12  houses a main magnet coil  34  and a shield coil  36 . The main coil  34  and one or more field coils  38  are mounted in the hollow annular main coil portion  28  and the shield coil  36  is mounted in the hollow shield coil portion  30  which is located axially and radially outwardly from the main coil  34 . Coolant, such as liquid helium, flows freely around and through the first or upper half magnet section  12 . The helium vessel is filled with liquid helium. Magnet heat losses cause the liquid helium to boil off into gaseous helium. A two stage G-M cryocooler with a recondenser recondenses the gaseous helium back into liquid helium. Recondensed liquid helium is introduced from the cryocooler  40  into the shield coil portion  30  from which the helium flows into the main coil portion  28  via a series of passages, ducts or flow openings  42 .  
         [0037]    Liquid helium is also channeled axially through the first support  16  via flow passage  44 . It is also possible to channel liquid helium through the radially inner portion  22  of the second support  18 . In this manner, liquid helium may be directed to flow from the first or upper magnet half section  12  through the support  16  or supports  16  and  18  and into the second or lower magnet half section  14  shown in FIG. 2B, and vice versa.  
         [0038]    As further seen in FIG. 2A, the first support  16  is formed as a hollow cylindrical pipe which is fixedly connected to the axially inner surface  46  of the shield coil portion  30  of both the first and second magnet half sections  12 ,  14 . A mounting plate  48  is welded or bolted to both the upper and lower shield coil portions  30  and to the opposite ends of the first support  16  to form a strong rigid first interconnection between the first and second magnet half sections  12 ,  14 .  
         [0039]    When the main magnet coils  34  are electrically energized, the first and second magnet half sections are strongly attracted to one another. The first support  16  reacts this attractive force in pure compression and thereby prevents collapse of the magnet half sections into one another.  
         [0040]    The second support  18  also reacts this axial load, but in a somewhat different fashion. That is, the second support  18  is actually formed of three separate columns or posts as seen in FIGS. 3 and 4. The first of these posts is a radially inner post  50  constructed substantially the same as the first support post  16 . Post  50  is, in this example, formed of the same hollow pipe as the first support  16  and located at the same radial distance from the opening  26  as the first support.  
         [0041]    Post  50 , like the first support  16 , reacts only compressive loads. A mounting plate  52  is welded or bolted to the axially inner surface  46  of the shield coil portion  30  to provide a rigid interconnection between the post  50  and the first or upper magnet half section  12 .  
         [0042]    The second support  18  further includes, in addition to post  50 , a pair of radially outer posts  54  which, as seen in FIGS. 3 and 4, are located radially outwardly and circumferentially offset equidistant from the radially inner post  50 . The outer posts  54  are welded to a mounting bracket  56  which, as seen in FIG. 9, extends radially outwardly of the magnet half sections, i.e., from the bottom of radially outer shield coil portion  30 . By locating the outer posts  54  radially outwardly of the helium vessel  10 , that is, radially outwardly of both the main coil portion  28  and the shield coil portion  30 , the openness of the gap  24  within the support structure is increased.  
         [0043]    It should be noted that the first support  16  and the second support  18 , including posts  50  and  54 , are connected in the same manner to the second or lower magnet section  14  as that shown in the first or upper magnet section  12  in FIG. 2A. When an axial compressive load is applied to the first support  16  and radially inner post  50  by the magnet coils, the supports  16 , and  50  are axially compressed. At the same time, an axial tensile force is applied to each of the radially outer posts  54  tending to stretch the posts  54 . This tension-compression dual support arrangement provides great strength and shear and torsional rigidity, yet maintains a high degree of openness.  
         [0044]    By circumferentially offsetting the two radially outer posts  54  on opposite sides of the radially inner post  50 , the three posts  50 ,  54 ,  54  of the second support  18  provide a support structure which also resists and reacts circumferential torsional and shear loads. Moreover, by locating the first and second supports diametrically opposite one another and radially outwardly from the inner magnet coil portions  28 , the support structure of the magnet half sections  12 ,  14  has very high bending, shear and torsional natural frequencies, which makes the MRI magnet assembly more tolerant to floor induced vibrations.  
         [0045]    The rigidity of the second support  18  can be further increased by interconnecting the posts  50 ,  54  with braces or shear panels. As further seen in FIG. 2A and FIG. 4, a brace plate  58  is welded to the mounting bracket  56  and to the radially outer magnet coil portion  30  to provide an even greater rigidity to the support structure which rigidly separates the magnet half sections  12 ,  14 . An identical brace plate is provided on the lower magnet half section which is symmetrical with the upper magnet half section about central radial plane  60 .  
         [0046]    It should be noted that the largest electromagnetic forces occur on the main coil  34  and shield coil  36 . For this reason, the magnet half sections  12 ,  14  are strongly reinforced in these areas. This strength is provided by a series of concentric cylindrical structural support members and a series of radially-extending circumferentially-spaced gusset plates as described below.  
         [0047]    The radially outer shield coil portions  30  are constructed of a radially outer cylinder  62  and a radially inner cylinder  64 . These cylinders are welded or rigidly connected to an annual, axially-inner flange  66  and to an annular, axially outer flange  68  so as to form a first annular chamber  70  for securely housing the shield coil  36 . The radially inner cylinder  64  extends from the shield coil portion  30  into the main coil portion  28  to increase the overall strength and rigidity of both magnet coil portions  28 ,  30 .  
         [0048]    Additional rigidity is provided to each of the magnet half sections  12 ,  14  by a series of radially-extending circumferentially-spaced gusset plates  72 . Plates  72  may be welded to the inner and outer flanges  66 ,  68  as well as to the inner cylinder  64 . From 8 to 32 (or more) gussets plates  72  may be used to reinforce the magnet half sections  12 ,  14  via arrangement in a spoke-like array as seen in FIG. 8.  
         [0049]    The radially inner magnet coil portions  28  are rigidly constructed with a radially outer cylinder  74 , the radially inner cylinder  64  and a central cylinder  76 . An annular, axially-outer plate  78  rigidly interconnects the central cylinder  76  and the radially inner cylinder  64 . An annular, axially inner plate  80  rigidly interconnects the radially outer cylinder  74  and the radially inner cylinder  64 .  
         [0050]    A frusto-conical ring  82  rigidly interconnects the axially inner plate  80  and the central cylinder  76  to help define a second annular chamber  84  which houses the secondary or field coils  38 . Ring  82  converges axially outwardly and radially inwardly from axial gap  24  to maximize the openness of the support structure. A third annular chamber  86  is defined between the annular flange  66 , the axially inner plate  80 , the radially outer cylinder  74  and the axially inner portion of the radially inner cylinder  64 . This third chamber  64  is particularly structurally robust to accommodate the main coil  34  and its high magnetic forces.  100571  As seen in FIGS. 5A, 5B and  8 , the helium vessel  10  is enclosed in a thermal shield  90  and an outer vacuum jacket or container  92 . The thermal shield  90  is cooled down by the first stage of the two stage G-M cryocooler  40 . The second stage of the cryocooler is used as a helium recondenser in order to reduce the system liquid helium boil-off.  
         [0051]    Preferably, the thermal shield  90  shown in FIGS. 6A and 6B is fabricated as a light compact construction made of a high thermal conductivity aluminum alloy such as aluminum alloy  1100 . The thermal shields  90  enshrouding each magnet half section  12 ,  14  are securely and rigidly interconnected with one another by a pair of small and large thermal shield posts or columns  94 ,  96  as shown in FIGS. 5A and 6A. The small thermal shield post  94  is cylindrical in section and the large thermal shield post  96  has a polygonal, hexagonal or six sided section as seen in FIG. 10, to achieve maximum compactness in cross section and thereby provide maximum openness. The six sided post  96  flares or diverges radially outwardly so as to limit physical and visual obstruction of the patient imaging region.  
         [0052]    Both the helium vessel support posts  16 ,  50 ,  54  and the thermal shield posts  94 ,  96  are enclosed in the outer vacuum container posts  100 ,  102 , as seen in FIG. 5A. The posts  100 ,  102  rigidly interconnect the two halves of the outer vacuum container  92  together. The upper half of the container  92  is shown in FIG. 7A and the lower half is shown in FIG. 7B. A top view of the outer vacuum container is shown in FIG. 11. Post  100  is cylindrical in shape and post  102  is six sided to compliment the six sided section of the thermal shield post  96 .  
         [0053]    Both the helium vessel  10  and the thermal shield  90  are enclosed in the outer vacuum container  92 . As further seen in FIGS. 7A and 7B, the upper half of the vacuum container shown in FIG. 7A and the lower half of the vacuum container shown in FIG. 7B are each constructed from a radially inner cylinder  106 , a radially inner cone  108 , an axially inner annular flange  110 , a radially outer cone  112 , a radially outer cylinder  114  and an axially outer flange  116 .  
         [0054]    The inner and outer cones  108 ,  112  increase the perceived openness of the magnet opening  24  from the perspective of a patient as compared to a uniform gap defined by an extension of annular flanges  110  completely across the gap  24 . Both the helium vessel assembly and the thermal shield are suspended from the outer vacuum container via a composite strap suspension system of known construction. The spacing between the outer vacuum container and the thermal shield is filled with multiple layers of insulating material, in order to keep the thermal losses of the system to a minimum. The vertical opening of the magnet is approximately 45-50 cm, which is sufficient for patient imaging.  
         [0055]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims.