Patent Number: 055747663
Section: summary

FIELD OF THE INVENTION The invention relates to x-ray beam limiters. It finds particular application in conjunction with x-ray sources for computed tomographic ("CT") scanners and will be described with particular reference thereto. It will be appreciated, however, that the invention may find further application in other areas where resistance to contamination, ease of manufacture, and physical size constraints are important in the design or selection of a bearing for such devices. DESCRIPTION OF THE RELATED ART In certain x-ray equipment, including x-ray equipment used in computed tomography applications, it is desirable to change the fan angle of the x-ray beam traversing a scan area. An apparatus and method for performing this function is described in U.S. Pat. No. 4,905,268 entitled Adjustable Off-Focal Aperture for X-Ray Tubes to Mattson, et al., incorporated by reference herein. As more fully described in Mattson, a beam limiting plate is rotatably mounted within the path of the x-ray beam such that the position of the plate and the corresponding fan beam width may be changed by rotating the plate. With particular reference to FIG. 3 of Mattson, the rotatable mounting was accomplished using ball bearings. Although ball bearing beam limiting devices have functioned acceptably, aspects of these devices may be improved. Particulate matter may become deposited between the ball bearings, causing the balls to become jammed, thus causing the device to stick. The device must be serviced to alleviate this condition. Various attempts have been made to alleviate the effects of such particulate contamination. One approach was to remove one of the ball bearings from the assembly to increase clearance, but this is at best a temporary solution. More recently, various bearing platings were used in order to eliminate the use of dry bearing lubricant. This approach ultimately did not prevent the accumulation of particles between the balls. Yet another approach was to reduce the entry of contaminants by surrounding the assembly with a physical barrier. This approach proved unsatisfactory, however, primarily because particulates from sources internal to the assembly can still contribute to contamination. Another disadvantage associated with the use of prior art bearings is the extensive labor required in the manufacturing process. In the prior art, approximately sixty balls were used within each bearing. Accordingly, the use of bearings other than ball bearings would simplify the manufacturing process. One alternative approach which eliminates the need for ball bearings is the use of conventional sleeve or cylinder bearings. Conventional techniques, however, require the use of one or more snap rings to hold the bearing in place. The snap rings require additional physical space within the bearing assembly and may also require additional machining to create grooves for the snap rings. For the foregoing reasons, there is a need for an x-ray beam limiting device which is exhibits increased reliability, is simpler to manufacture, and which requires a minimum of physical space and componentry. SUMMARY According to one aspect of the present invention, a sleeve bearing for use with an x-ray beam limiting device is provided. The sleeve bearing includes a disc characterized by a diameter and a radiused outer edge, the outer edge having a bearing surface. The bearing further comprises means for permitting the diameter of the disc to be reduced to a compressed diameter when a compressive force is applied to the disc and for urging the disc to return to substantially its original diameter when the compressive force is removed. The sleeve bearing also includes a bearing retaining race which has an inner diameter smaller than the uncompressed diameter of the disc. There is a circular groove within the inner diameter of the race. The groove has a diameter and radius sized to accept the disc and permit the disc to rotate. Thus, the disc can be inserted in the groove when a compressive force is applied to the disc. The disc is rotatably retained within the groove when the compressive force is removed. It will be appreciated that the foregoing eliminates the need for ball bearings as well as retaining rings associated with conventional sleeve bearings. It will further be appreciated that the invention obviates the need for bearing lubrication, reduces susceptibility to particulate contamination, and simplifies assembly. According to another aspect of the invention, the bearing surface comprises polytetrafluoroethylene, and the bearing race is stainless steel. In yet other aspects of the present invention the disc is made from a resin containing fiberglass or from a polyetherimide resin. In yet another aspect of the invention, the beam limiter further includes an actuator means for causing a predetermined angular rotation of the disc, spring means for urging the disc to return to a predetermined position, and an x-ray beam limiter mounted to the disc. In another aspect of the invention, the means which permit the diameter of the disc to vary when a compressive force is removed is an aperture within the disc and a slot extending from the aperture to the outer edge of the disc. According to another aspect of the present invention, the sleeve bearing retaining race has an inner diameter with a circular groove characterized by a diameter and a radius disposed therein. The sleeve bearing further includes a disc having a radiused outer edge, an aperture, and a slot extending between the aperture and the outer edge. The outer edge of the disc has a bearing surface, and the disc with the bearing surface applied to it has an uncompressed diameter which is less than or equal to the diameter of the groove and which is retained within the groove. The disc is thereby rotatably mounted within the retaining race when the compressive force is removed. In another aspect of the invention, the beam limiter comprises a bearing retaining race having an inner diameter and a groove disposed within the inner diameter. A rotating assembly includes a disc having a bearing surface applied to its outer diameter and which is retained within the groove so that the disc is rotatable about an axis orthogonal to the plane of the disc. A radiation attenuating means having at least first and second differently dimensioned radiation passing positions which cross at the axis of rotation is mounted to the disc. The disc further contains means for rotating the disc between at least first and second positions.