Abstract:
In a diagnostic imaging apparatus, a stationary gantry ( 24 ) and a rotating gantry ( 22 ) are interfaced by a plurality of air bearing elements ( 40 ). Lower air bearing elements bear the weight of the rotating gantry ( 22 ) which induce air hammering phenomena at a characteristic vibration frequency. To counteract the air hammering, a damping assembly ( 44 ) is mounted to at least one lower bearing element ( 40 ). The damping assembly ( 44 ) includes a damping mass ( 46 ) and an elastomeric connector ( 48 ) that are tuned to a frequency near the air hammer frequency to absorb the vibrational energy and damp the air hammer vibrations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to medical imaging arts. In particular, it relates to a rotating gantry such as those found in 3 rd  and 4th generation CT scanners, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other systems, such as nuclear cameras that use rotating gantries, and is not limited to the aforementioned application.  
           [0002]    Typically, 3 rd  and 4 th  generation CT scanners are equipped with mechanical ball or roller bearing systems. Because there is physical contact between the bearings and the rotating gantry, there is friction and wear that occurs over usage of the scanner. Additionally, functional speeds of the rotating gantry are limited by mechanical the bearings.  
           [0003]    In an effort to overcome the limitations of mechanical bearing systems for such medical imagers, fluid bearing systems are being used. Some fluid bearing systems include porous bearing pads that fit snugly to bearing races of the rotating gantry. When the bearing system is charged, a micro-thin layer of fluid is ejected from the porous bearing pads between the pads and the bearing races. This provides a virtually frictionless support for the rotating gantry.  
           [0004]    In such a system, the bearing pads that bear the weight of the rotating gantry typically exhibit a phenomenon called air hammering. Because of the shape of the bearing pad, and the stress exerted on the bearing due to the weight of the gantry, minor pressure inconsistencies of the bearing can result in rotational wobbling of the bearing pads. Air hammering can lead to premature wear of the bearing pads around the edges, where they frequently come into contact with the race, premature wear of the race for the same reason, and excess noise from the scanner.  
           [0005]    In attempts to counteract the vibrational disturbances due to air hammering, previous systems have included conventional spring and damper means. A damper is attached to a rigid body, such as the stationary gantry. This damper is then attached to the bearing element. This is a cumbersome setup, requiring amounts of space that might not be available in a cramped gantry system. Additionally, such dampers are removed from the bearing elements if the bearing elements are removed from the stationary gantry.  
           [0006]    The present invention contemplates an improved apparatus and method, which overcomes the aforementioned limitations and others.  
         SUMMARY OF THE INVENTION  
         [0007]    According to one aspect of the present invention, a diagnostic imaging apparatus is provided. First and second gantries define an imaging region. A bearing system includes a plurality of bearing elements, at least one of which exhibits a mechanical disturbance. A mechanical damper is mounted with the at least one bearing element that dampens the mechanical vibrations.  
           [0008]    According to another aspect of the present invention, a method of diagnostic scanning is provided. A gantry is rotated about a subject in an examination region. The rotating gantry is supported by a plurality of bearing elements, at least one of which tends to vibrate at a characteristic frequency. The vibrations of the bearing element are dampened.  
           [0009]    According to another aspect of the present invention, a CT scanner is provided. A plurality of bearing pads are movably mounted to a stationary gantry for supporting a rotating gantry. A mass is mounted to at least one of the bearing elements by a resilient element. An x-ray source is mounted to one of the gantries. An array of detectors receives x-rays from the x-ray source. A reconstruction processor reconstructs outputs of the detector array into an electronic image representation.  
           [0010]    One advantage of the present invention resides in reduced wear of component parts of a fluid bearing.  
           [0011]    Another advantage resides in reduced noise of a fluid bearing system.  
           [0012]    Another advantage resides in increased rotational potential of the rotating gantry.  
           [0013]    Another advantage resides in a damper that does not require attachment to a fixed body.  
           [0014]    Numerous additional advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for the purpose of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0016]    [0016]FIG. 1 is a diagrammatic illustration of a computed tomography scanner, in accordance with the present invention;  
         [0017]    [0017]FIG. 2 is a perspective view of a lower air bearing element including an inertial damper, in accordance with the present invention;  
         [0018]    [0018]FIG. 3 is an alternate embodiment of the inertial damper of FIG. 2 with a single entity connector;  
         [0019]    [0019]FIG. 4 is a schematic representation of the air bearing element of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    With reference to FIG. 1, a CT scanner  10  includes a subject couch  12  for moving a subject disposed thereon into and out of an imaging region  14 . X-rays from an x-ray source  16  are shaped and collimated into a fan beam, pass through the imaging region  14  and are detected by a detector assembly  20  on the far side of the imaging region  14 . In the illustrated 3 rd  generation embodiment, the source  16  rotates concurrently with the detector assembly  20 , always remaining 180° around the imaging region  14  from the detector assembly  20  as it rotates around an axis A. Alternately, a stationary ring of individual detectors on a stationary gantry  22  can replace the detector array  20 , as in a 4 th  generation CT scanner.  
         [0021]    Intensities of detected x-rays are collected in a data memory  30  as a rotating gantry  24  rotates the x-ray source  16  about the subject. As the data is collected, a reconstruction processor  32  applies a convolution and backprojection algorithm, or other suitable reconstruction technique, to the collected data, forming an image representation. The image representation(s) are stored in an image memory  34 . A video processor  36  withdraws selected portions of the image representations and formats them for viewing on a human readable monitor  38  such as a CRT monitor, active matrix monitor, LCD display, or the like.  
         [0022]    The first, rotating gantry  24  is disposed within the second, stationary gantry  22 . The x-ray source  16  and the detector array  20  are mounted on the rotating gantry  24 . Radial air bearing elements  40  are attached to the stationary gantry  22  by ball joints and abut against a bearing race  42  of the rotating gantry  24 . As discussed in the background, the weight of the rotating gantry  24  compresses the air bearing between the gantry  24  and the lower bearing elements  40  such that the phenomenon of air hammering tends to occur. Inertial dampers  44  are attached to the lower bearing elements  40  to dampen the movement of the air pads.  
         [0023]    With reference to FIG. 2, the lower air bearing elements  40  exhibit air hammering, that is rotation indicated by the arrow B. The inertial damper  44  includes a damper mass  46  and a resilient, dampening connector  48 . The mass  46  is sized proportionately to the bearing element  40  to have the greatest dampening effect at the resonant frequency of the hammering disturbance. Additionally, the farther away from the axis of rotation of the disturbance, (in this case the connection of the bearing element  40  to the stationary gantry  22 ) the more effective the damper will be. Preferably, the damper  44  is positioned at an extremity of the air bearing element  40 , and can alternately be placed on a side of the bearing element  40  rather than the top as shown in FIGS. 1, 2, and  3 . The mass  46  is connected to the bearing element  40  by the resilient connector  48 . The connector  48  is made of high dampening elastomeric polymer. Suitable elastomers include, but are not limited to, Polyurethane and Urethane. The connector  48  taken in conjunction with the inertial mass  46  dampens the majority of the inherent air hammering.  
         [0024]    The connector  48  acts as both a spring and a damper. The damping frequency of the damper mass  46  and the connector  48  is tuned to a frequency slightly less than the instability frequency of the bearing element  40 . As the bearing element  40  starts to excite at its instability frequency, the bearing element  40  starts to excite the damper mass  46 . The connector  48  absorbs some of that energy, thus reducing the motion of the bearing element  40 . In the preferred embodiment, the connector includes a plurality of pins, each secured to the air bearing element  40  by a threaded connector. Alternately, and as shown in FIG. 3, the connector  48  can be a single elastomeric mass. It is to be understood that other connectors and adhesives are also contemplated.  
         [0025]    With reference to FIG. 4, the preferred embodiment can be seen in a mechanical schematic. The motion of the bearing element  40  and damper  44  system can be described by the following differential equations:  
         z   ¨     =       L     J   p            [         -     (         K   p     L     +       K   2        L       )          z     -       (         B   p     L     +       B   2        L       )          z   .       +       K   2          Lz   2       +       B   2        L                     z   .     2       +     T   1       ]             and             z   ¨     2     =         g   c     M          [       -       K   2          (       z   2     -   z     )         -       B   2          (         z   .     2     -   z     )         ]                             
 
         [0026]    where z is the position along one axis of the bearing element  40  and z 2  is the position along the same axis of the damper mass  46 , L is the distance between a center of rotation of the bearing element  40  and an attachment point of the damper  44 , J p  is the rotational displacement of the bearing element  40 , K p  is the rotational spring constant of the air bearing itself, K 2  is the linear spring constant of the elastomeric connector  48 , B P  is the damping coefficient of an attachment of the bearing element  40  to the stationary gantry  22 , B 2  is the damping coefficient of the connector  48 , T i  is the torque on the air bearing element  40 , g c  is the gravitational conversion constant and M is the mass of the damping mass  48 . From these equations, assuming that K p , J p , T i , L, and B P  are known or measured and fixed, appropriate values for B 2 , K 2  and M can be determined.  
         [0027]    The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.