Abstract:
In an improved computed tomography scanner drive system and bearing configuration, a gantry disk ( 30 ) is sheaved about its perimeter ( 65 ) such that the gantry is operable as a driven pulley rotatable about an object to be scanned. A motor ( 46 ) assembles mounted to a stationary frame ( 33 ) includes a similar sheaved drive pulley ( 80 ). A belt ( 64 ) tensioned between the drive pulley ( 80 ) of the motor assembly and the driven pulley of the gantry disk ( 30 ) transfers rotational motion of the motor to drive the gantry rotationally about the object. In a preferred embodiment, the belt comprises a V-belt or poly-V-belt ( 64 ), and the bearing comprises a wire bearing ( 59 ) located proximal to the gantry center of mass. In this manner, the present invention provides a simple and effective technique for driving the gantry about the object, providing sufficiently accurate angular positioning in a reliable and cost effective drive system. Various embodiments of an improved bearing system are also disclosed.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 08/948,930 filed on Oct. 10, 1997 now U.S. Pat. No. 5,982,844 in the names of Andrew P. Tybinkowski, Michael J. Duffy and Gilbert W. McKenna, and assigned to the present assignee. 
    
    
     In addition the application is related to the following U.S. applications filed on Oct. 10, 1997 and commonly assigned with the present application, the contents of which are incorporated herein in their entirety by reference: 
     U.S. application Ser. No. 08/948,937, “Air Calibration Scan for Computed Tomography Scanner with Obstructing Objects,” invented by David A. Schafer, et al., 
     U.S. application Ser. No. 08/948,928, “Computed Tomography Scanning Apparatus and Method With Temperature Compensation for Dark Current Offsets,” invented by Christopher C. Ruth, et al., 
     U.S. Pat. No. 5,909,477, “Computed Tomography Scanning Target Detection Using Non-Parallel Slices,” invented by Christopher C. Ruth, et al., 
     U.S. Pat. No. 5,901,198, “Computed Tomography Scanning Target Detection Using Target Surface Normals,” invented by Christopher C. Ruth, et al., 
     U.S. Pat. No. 5,887,047, “Parallel Processing Architecture for Computed Tomography Scanning System Using Non-Parallel Slices,” invented by Christopher C. Ruth, et al., 
     U.S. Pat. No. 5,881,122, “Computed Tomography Scanning Apparatus and Method Generating Parallel Projections Using Non-Parallel Slices,” invented by Christopher C. Ruth, et al., 
     U.S. application Ser. No. 08/949,127, “Computed Tomography Scanning Apparatus and Method Using Adaptive Reconstruction Window,” invented by Bernard M. Gordon, et al., 
     U.S. application Ser. No. 08/948,450, “Area Detector Array for Computed Tomography Scanning System,” invented by David A. Schafer, et al., 
     U.S. application Ser. No. 08/948,692, “Closed Loop Air Conditioning System for a Computed Tomography Scanner,” invented by Eric Bailey, et al., 
     U.S. application Ser. No. 08/948,493, “Measurement and Control System for Controlling System Functions as a Function of Rotational Parameters of a Rotating Device,” invented by Geoffrey A. Legg, et al., 
     U.S. application Ser. No. 08/948,698, “Rotary Energy Shield for Computed Tomography Scanner,” invented by Andrew P. Tybinkowski, et al., 
     BACKGROUND OF THE INVENTION 
     In modern third generation computed tomography (CT) scanners, an X-ray source and detector array are secured on opposite sides of the central opening of an annular disk. The disk is mounted to a gantry support for rotation about a subject or object (positioned in the opening) to be scanned. During a scan, the source and detectors image the object disposed within the machine at incremental scan angles. In fourth generation CT scanners the detectors are fixed relative to the object or subject being scanned, and only the source is mounted on the rotating disk for rotation about the subject or object. In both types of systems a process referred to as reconstruction generates a series of two-dimensional images or slices of the object from the captured data. 
     For “fixed z-axis” scans (the “z-axis” being the axis of rotation of the disk), the disk and its components rotate about a stationary object or subject with the disk fixed at a specific Z-axis location. For “helical” scans, translational movement along the Z-axis is simultaneous provided between the object or subject and the rotating disk. In both fixed and translational scanning systems, precision in the angular velocity, or rotation rate, of the gantry disk is essential for minimization of reconstruction errors. 
     Timing belts, or cog belts, have been employed in the past to effect a high degree of precision in rotation rate. A standard timing belt is driven by a motor mounted to the stationary frame. Periodic lateral grooves transverse to the major axis of the belt mesh with teeth on a drive sprocket at the motor and a large driven sprocket mounted to the gantry disk. The driven sprocket must be large enough to avoid interference with the central aperture of the gantry and thus allow room for a object to pass therethrough. For this reason, extraordinarily-large timing belts are required in these systems. 
     A typical prior art scanner requires at least a six meter timing belt. Timing belts of such a large magnitude are very expensive, as they are difficult to manufacture and often must be custom built, and/or purchased in large quantities. Furthermore, the large driven sprockets are specialized and are therefore expensive, available at a cost of $4,000 to $6,000, depending on the diameter. Alignment between the drive sprocket and driven sprocket must be accurate to a high degree of precision, to avoid lateral walking of the belt relative to the sprockets. Timing belts tend to wear rapidly, and therefore must be replaced frequently, for example once per year for a medical scanner. Replacement is an involved procedure, requiring removal of the scanner system from operation for an extended period of time; perhaps a couple of days. This is due to the fact that in prior art configurations, the driven sprocket is positioned between the annular gantry and the fixed frame. Access to the timing belt for its removal and replacement therefore requires complete removal of the gantry from the frame. Positioning of the sprocket on the component side of the gantry is impractical, since the timing belt would interfere with the rotating gantry components. 
     A further disadvantage of timing belts in CT systems is their tendency to modulate the rotational speed of the gantry at the frequency of their teeth or cogs. The modulation causes artifacts in the resulting images which must be resolved or otherwise corrected by the image processing system. 
     In addition, mounting the disk for rotational movement requires some type of reliable support so that the disk reliably rotates with little or no lateral movement in the plane of rotation. In the typical prior art system, standard bearing arrangements, with highly machined races and balls, are expensive. Because of the weight and size of the disk the bearings tend to wear, and are difficult to replace. One solution to this problem has been to mount the disk for centerless rotation on rollers such as shown in U.S. Pat. No. 5,473,657 issued Dec. 5, 1995 in the name of Gilbert W. McKenna, and assigned to the present assignee. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a CT scanner drive assembly that mitigates and/or eliminates the shortcomings associated with prior art scanner drive assemblies described above. The apparatus of the invention comprises an annulus, preferably in the form of a disk, which is sheaved about its perimeter such that the annulus is operable as a driven pulley rotatable about an object to be scanned. Electronic components are preferably mounted to the annulus for performing a tomographic scan of the object. A motor includes a similarly sheaved drive pulley. A belt tensioned between the drive pulley of the motor and the driven pulley of the annulus transfers rotational motion of the motor to the annulus for driving the annulus rotationally about the object during a scan. 
     In a preferred embodiment, the belt comprises a V-belt or poly-V-belt. An adjustable tensioner draws the motor drive pulley toward or away from the annulus for adjusting the tension of the belt. The annulus preferably comprises a disk having first and second faces. By spacing the disk from the frame, components may be mounted on both faces of the disk, or through apertures in the disk, mitigating space limitations for mounting components to the disk, and balancing the disk center of mass near the disk plane. 
     In one preferred embodiment, a disk bearing is preferably located at or near the disk center of mass, and mounted to spacers rigidly coupled to the system frame. This configuration reduces the moment arm between the bearing and disk center of mass, improving the life of the bearing and allowing for use of less expensive, simpler bearings, for example Franke four-wire bearings of the type described in U.S. Pat. No. 5,071,264, incorporated herein by reference. 
     In another preferred embodiment, the annulus is mounted for rotation within the gantry frame wherein opposing grooves are formed in the periphery of the annulus and the inner periphery of the opening of the gantry frame, and are shaped to receive less expensive, simpler bearings, for example Franke four-wire bearings of the type described in U.S. Pat. No. 5,071,264. In one preferred embodiment, the bearing system includes a single set of wire races and spherical bearings disposed therebetween. The spherical bearings are all centered in the center plane of the disk. In another preferred embodiment, the bearing system includes a pair of sets of wire races and spherical bearings disposed therebetween. The spherical bearings of the two sets are respectively disposed in parallel planes, preferably on opposite sides of and equally spaced from the center plane of the disk. 
     In this manner, the present invention provides a simple and effective technique for driving the annulus about the object or subject to be scanned. The V-belts provide accurate timing—as they minimize slippage, and maximize efficient energy transfer. Further, V-belts offer the additional benefit of a long life time, on the order of five years, before replacement is necessary. Large V-belts are currently available commercially at a relatively low cost of approximately $100. 
     This configuration is well adapted for continuous operation in an airport setting for baggage scanning applications and systems of the type that use CT scanners and which run continuously for 18-20 hours daily. By conveniently locating the belt on an outer edge of the annulus or disk, maintenance of the belt is relatively straightforward and can be performed expeditiously, on the order of 1-2 hours, without the need for disassembling the entire gantry as in the prior art. In addition, V-belts are generally relatively flexible and can be mounted without the need for critical alignment tolerances of prior art timing belts. The flexibility of the V-belt, in combination with its longitudinal grooves provide a smooth interface for driving the disk in continuous motion, without modulating the rotational speed. The drive system therefore makes no contribution to image artifacts. 
     In addition, the use of the simpler bearings allows for easier servicing of the scanner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a perspective view of an outer console of a baggage scanner system of the type using a CT scanner constructed in accordance with the present invention. 
     FIG. 2 is a front perspective view of a scanner frame and gantry disk configuration in accordance with the present invention. 
     FIG. 3 is a rear perspective view of the frame and gantry disk configuration of FIG. 2 in accordance with the present invention. 
     FIG. 4 is a side cross-sectional view of a portion of the gantry and frame of FIGS. 2 and 3, illustrating the sheaved outer edge of the gantry disk and a preferred bearing configuration in accordance with the present invention. 
     FIG. 5 is a close-up cut-away side view of one preferred embodiment of the improved bearing configuration of the present invention. 
     FIGS. 6A and 6B are exploded perspective views of alternative embodiments of the motor and drive pulley tensioner apparatus in accordance with the present invention. 
     FIG. 7 is a close-up perspective view of the interface between the V-belt and the sheaved outer perimeter of the gantry disk in accordance with the present invention. 
     FIG. 8A is a side cross-sectional view of a portion of the annulus and gantry frame of another preferred embodiment of the improved bearing configuration of the present invention. 
     FIG. 8B is a close-up cut-away view of the improved bearing configuration of FIG.  8 A. 
     FIG. 9A is a side cross-sectional view of a portion of the annulus and gantry frame of a third preferred embodiment of the improved bearing configuration of the present invention. 
     FIG. 9B is a close-up cut-away view of the improved bearing configuration of FIG.  9 A. 
     FIG. 10 is a perspective view of a disk lock in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective illustration of outer console  100  of a baggage scanning system of the type using an X-ray computed tomography (CT) scanner. The console  100  comprises a plurality of panels  104  mounted to a rigid frame (see FIGS. 2 and 3) erected on a base  33 . The panels  104  are hinged to the frame or are otherwise removable to provide access to the inner components of the scanner. A conveyor  102  transports objects to be scanned, for example, airport baggage, into the scanning area. As is well known, where the CT scanner is employed as a medical scanner, a suitable patient table usually supports a patient within the CT scanner so that a select portion of the patient can be scanned. 
     FIG. 2 is a front perspective view of the primary components of a CT scanner in accordance with the present invention. A rigid vertical frame  32  is erected on a base  33 . The base  33  includes a plurality of height-adjustable feet  34  for leveling the system. 
     An annulus or disk  30  preferably formed of a light-weight, rigid material such as aluminum, magnesium-aluminum alloy or the like is rotatably mounted on the frame  32 . The annulus  30  may be solid or hollow, preferably substantially uniform in cross-section and mass throughout, and is generally radially symmetrical, preferably in the shape of a disk or drum. To ensure that the grain or crystal structure of the disk is structurally uniform, it is preferred that the disk be formed by a precision casting as a single unit, annealed and finished by machining. 
     An X-ray source tube or source  36  is positioned on the disk  30  for directing an X-ray beam along the plane of the disk  30  across aperture  35  substantially perpendicular to the axis of rotation  37 . Similarly, an X-ray detector array  40  is mounted on the disk  30  opposite the source  36  for detecting emitted X-rays  38 . Additional components, for example, a data acquisition system  42  for the detector array  40 , X-ray power supply cathode  41  and anode  43 , air conditioning or cooling systems  45  and related electronics are likewise mounted on both front and rear faces  71 ,  73  of the gantry disk  30 . The disk  30  is rotatably mounted to the vertical frame  32  at bearing  59 , the details of which are described below. 
     A motor  46  and an associated drive pulley  80  (see FIGS. 6A and 6B) coupled thereto drive a belt  64 . The belt  64  in turn is coupled to the outer perimeter of the gantry disk  30  for rotating the disk which operates as a driven pulley. The belt  64  preferably comprises a V-belt, for example a poly-V-belt, to confer various advantages described throughout the specification, including low cost, increased longevity, and reduced sensitivity to alignment. Such belts are commercially available from various vendors, for example Browning Inc., Gates Inc., Goodyear Inc., and Jason Inc. 
     The outer edge  65  of the disk  30  is sheaved to interface with the longitudinal grooves of the poly-V-belt  64 . The cross-sectional V-shaped geometry of the belt in combination with the large disk circumference serve to minimize belt slippage, maximizing accuracy in rotational disk positioning and rotation rate. Tension in the belt  64  is controlled by tensioner  66  which adjusts the distance between the motor drive pulley  80  (see FIG. 6) and driven disk  30 . Replacement of the belt in this configuration simply involves loosening of the belt  64  at tensioner  66  and removal and replacement of the belt  64  at the front face  71  of the disk. Removal of the disk  30  from frame  32  is unnecessary for belt service in the present configuration, and therefore the belt can be removed and replaced in a matter of minutes. 
     FIG. 6A is an exploded perspective view of a motor and drive pulley system and corresponding belt tensioner in accordance with the present invention. The motor  46  is coupled to the base  33  at pivot  112 . A taper bushing  84  mounts a drive sheave to the motor axle. A tensioner  66  mounted to the motor plate and adjustable by nut  67  adjusts the distance between the drive pulley  80  and the gantry disk  30 , thereby adjusting the tension of the belt  64 . The rod or tensioner  66  is threaded such that tightening of the nut  67  relative to the rod causes the motor  46  to pivot away from the gantry disk  30  thereby tensioning the belt  64 . For removing the belt  64  during servicing, the nut  67  is loosened, removing tension in the belt which can thereafter easily be removed at the front face of the gantry disk  30 . 
     FIG. 6B is a perspective view of an alternative belt tensioner configuration. In this embodiment, the motor  46  is mounted to a movable plate  81  which slides relative to a fixed plate  89 . A tension bolt  87  is adjustable for moving the motor  46  relative to the gantry disk  30 , thereby tensioning the belt  64 . 
     FIG. 3 is a rear perspective view of the gantry disk  30  and frame  32 . Gantry components mounted on the rear face  73  of the gantry disk  30  are visible in this view, for example, the rear portion of X-ray source  36 , and associated cooling systems  19 , along with power distribution assemblies, communication units, oil pumps, etc., hidden from view. To provide room for rotation of the rear-face components between the gantry disk  30  and the frame  32 , bearing  59  is distanced from the vertical frame by frame spacers or extenders  52 . Apertures  82  are provided in the gantry disk  30  to allow for mounting of components through the disk; for example X-ray source  36  passes through aperture  48  and extends from both disk faces  71 ,  73 . Additional apertures  82  allow for passage of signals, power cables, and cooling fluids between components on opposite faces of the gantry disk  30 . 
     Slip rings and corresponding brushes (not shown) transmit power signals and high-bandwidth data signals between components of the gantry disk  30  and frame  32 . Microwave transmitter/receiver pairs provide further communication of low-bandwidth control signals. The signals are transmitted to a processing unit  83  which converts the signals to images. Air conditioner system  45  provides for circulation of air and maintains system temperature. 
     FIG. 4 is a sectional side view of the relationship of the gantry disk  30 , bearing  59 , and vertical frame  32 . The vertical frame  32  supports the gantry  30  system in an upright position, substantially perpendicular to the floor. Frame spacers, or extenders  52  relocate the position of the gantry bearing  59  a distance d from the frame  32  such that the various gantry components are mountable on the rear face of the gantry disk  30  without interfering with the vertical frame  32  during disk rotation. Ring frame  54  serves as a mount for the bearing  59 . 
     The interface between the longitudinal sheaves  50  on the outer perimeter of the gantry disk  30  and the mating longitudinal grooves on the poly-V-belt  64  is visible in the side view of FIG. 4. A close-up perspective view of this interface is shown in FIG.  7 . The poly-V-belt and sheave configuration serves to increase the surface area of the interface, thereby minimizing belt slippage. 
     Although the respective positions of the spacers  52  and bearing  59  could be reversed, with the spacers  52  mounted on the gantry disk  30  surface, and the bearing  59  mounted to the vertical frame  32 , such a configuration would increase the moment arm between the bearing and the center of mass of the disk, thereby increasing the radial load and trust load on the bearing. This would require a more robust and therefore more expensive bearing unit. By locating the bearing  59  near or at the center of mass of the gantry, the present invention allows for use of an inexpensive bearing configuration. This, in combination with the mounting of components on both sides of the gantry disk  30 , achieves dynamic balancing of the disk relative to the bearing, and reduces the cantilevered load on the bearing. 
     FIG. 5 is a close-up sectional view of the interface of bearing  59 , which is preferably configured to emulate the well-known Franke bearing interface, as disclosed in U.S. Pat. Nos. 4,797,008 and 5,071,264, incorporated herein by reference. A fixed outer bearing housing  61  mounts to the ring frame  54  by bolts  91 . Outer bearing wires  72  are deposited on each inside corner of bearing lip  77 , which serves to separate the bearing runs. An inner bearing housing, including first and second inner rings  60 ,  62  respectively, mounts to the gantry disk  30  by bolts  93 . The inner housing includes inner bearing wires  74  laid along the outer corners of the inner bearing housing as illustrated. Suspended between the outer and inner wire races of bearing wires  72 ,  74  are spherical ball bearings  75 , which glide across the wires with minimal resistance as the gantry disk  30  rotates. Side separators or ball spacers  76  prevent adjacent balls from contacting or otherwise interfering with each other. Preloading of the bearings is controlled by preload bolts  95 . 
     The bearing configuration of the present invention confers several advantages. The bearing/wire interface operates with less friction than traditional bearing races as the wires provide a smooth and efficient track for the bearings. No custom bearing housing is required, as the housing is provided by the inner surfaces of the races. The present bearing configuration requires 10 ft-lbs. of turning torque as opposed to the less efficient prior art designs requiring 50 ft-lbs. of turning torque, assuming a gantry disk of  6  feet in diameter, weighing 1500 lbs, allowing use of a smaller motor, for example a 0.5 horsepower, for rotating the gantry. Furthermore, this bearing configuration is light weight, operates quietly, and is relatively inexpensive. 
     An alternative bearing configuration is depicted in FIG. 8A, with a close-up cutaway side view of the bearing arrangement given in FIG.  8 B. In this configuration, the annulus, or disk  206 , is mounted for rotation within a circular gantry frame ring  202 , in turn mounted to a pivot shaft  201 . The pivot shaft  201  allows for pivoting of the entire frame at an angle relative to the translation axis through the center of the disk. The stationary mounting frame ring  202  is provided with a groove  209 A opposite a similar groove  209 B on the periphery of the rotatable annulus  206 . The opposed grooves  209 A,  209 B are shaped to receive bearings  209 , comprising wire races  212  and balls  214 , for example, the Franke bearings of the type described above. A pre-load ring  204  secured by bolts  218  secures the bearing  208  in place and urges the wire races  212  against the balls  214 . Cages  216  space the balls  214  relative to one another, as described above. An elastomer ring  210  may be mounted in either or both grooves to house the bearings and provide for more quiet operation during rotation of the disk  206 . A sheave  50  is preferably provided on the outer perimeter of the disk  206  for receiving a V-belt, conferring the advantages described above. 
     FIG.  9 A and FIG. 9B are a side view and a close-up cutaway view respectively of an alternative bearing configuration in accordance with the present invention. In this configuration, first and second pairs of opposed grooves  209 A,  209 B are formed on the stationary mounting frame  202  and the rotatable annulus  206  respectively. First and second bearings  208 A,  208 B are positioned in the grooves  209  to provide an interface between the frame and rotatable annulus. In this configuration, each bearing comprises first and second wire races  212 , between which balls  214  communicate. By spacing apart the bearings  208 A,  208 B on opposite sides of the center axis  217  this configuration provides for a more stable structure and therefore allows for reliable operation at increased rotation speeds. 
     The present invention may optionally further include a disk lock  92  (see FIG. 2) mounted to the rigid frame  32  for preventing rotation of the disk  30 , for example, during component installation or maintenance, or during shipping. In a preferred embodiment, a lock pad  93  is activated by engagement means and is urged against the drive belt  64 . As shown in the perspective close-up view of FIG. 10, the disk lock  92  includes a mount  91  fixed to the frame  32  by bolts  99 . A lock pad  93  is suspended below an extension  101  of the mount  91 , and is rotatably secured to a threaded adjustment bolt  96  interfacing with threaded hole  105  on the extension  101 . As the pad  93  position is adjusted via Allen wrench aperture  97 , guides  94  fixedly mounted to the lock pad  93  slide relative to holes  98  in the mount extension  101 . Once in position, a lock nut  103  secures the disk lock, preventing vertical movement. The lock pad  93  is preferably adapted to interface with the outer edge of the V-belt  64  as shown in FIG.  10 . 
     In an experimental apparatus, the gantry disk comprised a 6 ft. diameter aluminum disk weighing 1500 lbs. A commercially available poly-V-belt having 5 grooves, and commercially available at a cost of $150, was sufficient for rotating the gantry at 90 RPM, using a 1.5 horsepower motor, and delivering an angular rate accuracy better than 0.1%, exceeding the angular rate precision required for accurate scanning. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, while the embodiment shown in the drawings illustrate a CT Scanner of the third generation type, the invention can be used in CT Scanner of the fourth generation type.