Abstract:
In a diagnostic system, having a rotating gantry ( 24 ) and a stationary gantry ( 22 ), a bearing race ( 50 ) rotates with surface portions having varying linear velocities in accordance with distance from an axis (A) of rotation. Tapered roller bearings ( 46 ) interface the bearing race ( 50 ) and are conically shaped to velocity match the variable linear surface velocity race ( 50 ). The race ( 50 ) preferably includes two faces, which provide both axial and radial supporting surfaces for the bearings ( 46 ) to interface. The bearings ( 46 ) are disposed about the race ( 50 ) in pairs. A drive motor ( 52 ) is connected to one of the bearings ( 46 ) to rotate the gantry ( 24 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to medical imaging arts. In particular, it relates to a rotating gantry such as those found in 3 rd  and 4 th  generation CT scanners, and will be described with particular reference thereto. However, the invention will also find application in conjunction with nuclear cameras and other imaging systems with rotating bearings, and is not limited to the aforementioned application. 
     Typically, 3 rd  and 4 th  generation CT systems have rotating gantries and stationary gantries. The two gantries are interfaced by a bearing system that allows rotation of the first gantry relative to the second gantry. 
     A large ball bearing assembly, often a meter or more in diameter, has been used to provide the interface between the gantries. Large ball bearing assemblies are expensive and tend to be noisy. 
     In other systems, roller bearings have been used. Cylindrical rollers support the rotating gantry in both axial and radial directions. Typically, the rotating gantry has three bearing races, or tracks along which the bearings roll. A circumferential race allows the bearings to give the rotating gantry radial support (a normal force counteracting the force of gravity) while the second and third races allow bearings to give the rotating gantry lateral, that is, axial support. To prevent the rotating gantry from wobbling, the roller bearings press against the second and third races with significant opposing pressure. 
     While the gantry rotates with a constant angular velocity, portions of the gantry move with different radially dependent linear velocities. More specifically, portions of the second and third races more distant from the rotational axis of the gantry have a higher linear velocity than portions closer to the rotational axis. Stated differently, the linear velocity of any moving element is a function of radial position, as well as angular velocity of the gantry. 
     This is significant to, among other things, the second and third axial support bearing races. The outer edges of these two races move faster than the inner edges of the same races. Each cylindrical roller bearing that contacts the second and third races only rotate at a single speed. Thus, slippage occurs between the bearing races and the roller bearings, causing high friction and wearing both the bearing races and the bearings prematurely. Additionally, functional speeds of the gantry are limited, in order to balance the speed of the gantry and the wear that higher speeds incur on the races and the bearings. 
     The present invention contemplates an improved apparatus and method, which overcomes the aforementioned limitations and others. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a diagnostic imaging device is provided. First and second gantries are interfaced by a plurality of tapered roller bearings that provide support for the second gantry as it rotates. The bearings interface bearing races on the rotating gantry. 
     According to another aspect of the present invention, a method of diagnostic imaging is provided. A first, rotating gantry is supported with a plurality of tapered roller bearings attached to a second gantry. The first gantry is rotated concurrently with the roller bearings, races of the gantry being in contact with the roller bearings. 
     According to another aspect of the present invention, a roller bearing for use in conjunction with a computed tomography scanner is provided. The bearing includes an axle, a tapered conical body with a trapezoidal cross-section, the tapered side of the body being a contact surface of the bearing. A taper angle φ of the bearing is defined by 
       ϕ   =     arcsin   ⁡     (         d   o     -     d   1         2   ⁢           ⁢     L   c         )           
 
where d o  is the outer diameter of the bearing, d i  is the inner diameter of the bearing, and L c  is the length measured along an edge of the bearing. A polyurethane coating covers the bearing body.
 
     One advantage of the present invention resides in increased life of the roller bearings and bearing races. 
     Another advantage resides in a reduced total number of roller bearings required. 
     Another advantage is that rotating friction of the bearing system is reduced. 
     Another advantage is that a single size of roller bearing can be used. 
     Another advantage resides in fewer precision machined bearing races. 
     Another advantage resides in a smaller rotating gantry. 
     Another advantage resides in an integrated drive motor. 
     Another advantage resides in reduced cost over similar systems currently in production. 
     Yet another advantage resides in faster rotational speeds. 
     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 
       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. 
         FIG. 1  is a diagrammatic illustration of a computed tomography scanner, in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of the rotating gantry of FIG.  1  and tapered roller bearings, in accordance with the present invention; 
         FIG. 3  is a detailed view of the roller bearings of  FIG. 2  including a drive motor; 
         FIG. 4  is a geometrical representation of a bearing-race interface, in accordance with the present invention; 
         FIG. 5  is an alternate two-face race embodiment of the present invention; 
         FIG. 6  is an alternate three-face race embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 the stationary gantry  22  can replace the detector array  20 , as in a 4 th  generation CT scanner. 
     Intensities of detected x-rays are collected in a data memory  30  as a rotating gantry  24  rotates 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. 
     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  along with other associated electronics  44 , such as power supplies, data buffers, etc. 
     The rotating gantry  24  is supported within the stationary gantry  22  by a plurality of tapered roller bearings  46 . In the preferred embodiment, there are four sets of two bearings, making eight roller bearings total. Of course, the number of bearing pairs can be more or less, dependent upon other factors such as the weight of the gantry  24 , functional speeds, and the like. Each bearing  46  rotates freely about its own bearing axle, the axle being mechanically fastened to the stationary gantry  22 . 
     With reference to  FIG. 2 , the bearings  46  interface with a conical bearing race  50 , The race  50 , depicted attached to gantry  40 , provides surfaces angled with respect to the axis A so that the bearings  46 , as shown in  FIG. 2 , provide both radial (directions perpendicular to the axis A) support and axial (directions parallel to the axis A) support. 
     As discussed in the background, the race  50  moves with constant angular velocity, but portions of the race  50  farther from the axis A have higher linear velocities than portions closer to the axis A while the gantry  24  is rotating. The bearings  46  are tapered into conical shapes to compensate for the linear velocity deviation. When the gantry  24  rotates, each bearing  46  in contact with the gantry  24  also rotates. Being conical in shape, the bearings each have a wide or larger diameter end and a narrow or smaller diameter end. Like the gantry, the surface at wide end of the bearing moves with a higher linear velocity than the surface at the narrow end. The bearings  46  are shaped with a varying diameter that is proportional to the slope and radial altitude of the race  50  such that there is no slippage between the bearings  46  and the race  50  as they all rotate. 
     Preferably, the bearings  46  are constructed of a metal core, preferably stainless steel, and coated with a polymeric coating, preferably polyurethane. The coating is preferably more than a surface coat, and more akin to a tire on a tricycle wheel, or the like. The coating is thick enough to provide for smooth cushioning, but thin enough that it stiffly supports the rotating gantry. Although polyurethane is preferred, other coatings that provide adequate stiffness (preventing axial and radial movement of the gantry  24 ) while preventing metal-to-metal contact between the race  50  and the bearings  46  are contemplated. 
     With reference to  FIG. 3 , one of the bearings is attached to an external drive motor  52  and becomes a drive bearing  54 . The drive bearing  54  is coated with a substance that can be different from the other bearings  46  for improved friction with the bearing race  50 . Such a substance may be a hardened rubber or the like. 
     In order for the conical bearing race  50  and the tapered bearings  46  to be velocity matched, an angle of expansion that describes the growth of the diameter of a bearing  46  along the axle is found. With reference to  FIG. 4 , the angle φ is found to cause velocities V o  and V i  to match on both the race  50  and the bearings  46 . First, a ratio of race  50  diameter to bearing diameter is found, 
       R   =       D   o       d   o           
 
where D o  is the outer diameter of the race  50  and d o  is the outer diameter of the bearing  46  and R is the ratio of the two measurements. It follows that the angular speed of the bearing ω t  is found by
 
ω t =ω g R
 
where ω g  is the angular speed of the gantry in rpm. From geometry of the system it is known that 
         L   c     =         D   o     -     D   i         2   ⁢           ⁢   sin   ⁢           ⁢   α           
 
where L c  is the length of the contact surface of the roller bearing  46 , and D i  is the inner diameter of the bearing race  50 , and α is the angle of elevation of the bearing race  50 . Solving for D i ,
 
 D   i   =D   o −2 L   c  sin α.
 
     In order to velocity match the contact surfaces, the velocities V o  and V i  at the extremities of the bearing  46  and race  50  are found to match: 
         V   o     =         ω   t     ⁢       d   o     2       =       ω   g     ⁢         D   o     2     .             
 
Solving for ω t , 
         ω   t     =       ω   g     ⁢       D   o       d   o             
 
where d o  is the outer diameter of the bearing  46 . Similarly, 
         ω   t     =       ω   g     ⁢       D   i       d   i             
 
where d i  is the inner diameter of the roller bearing  46 . Combining the above two equations, it is found that 
         d   i     =       d   o     ⁢         D   i       D   o       .           
 
Finally the angle φ can be found by: 
       ϕ   =       arcsin   ⁡     (         d   o     -     d   1         2   ⁢           ⁢     L   c         )       .         
 
The bearing axial length L r  can be found by:
 
L r =L c  cos φ
 
but it is to be understood that the actual length of the roller bearing  46  can vary to be slightly longer or shorter, keeping the same angle φ.
 
     In an alternate embodiment, and with reference to  FIG. 5 , a race  60  is an inverted negative of the race  50 . This alternate race  60  still provides both axial and radial support for a gantry  62 . 
     In another alternate embodiment, and with reference to  FIG. 6 , a race  70  has three faces, tapered roller bearings  72  being adjacent the vertical sides of the race  70 . A cylindrical (non-tapered) roller bearing  74  is adjacent the horizontal face of the race  70  since the entire face is equidistant from the axis A and thus does not display a velocity mismatch phenomenon. In this embodiment, the flat bearings  74  provide a gantry  76  with radial support, while the tapered roller bearings  72  provide the gantry  76  with axial support. 
     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.