Patent Number: 055747663
Section: description

DESCRIPTION FIG. 1 shows a disc 18 which contains an aperture 20 and a slot 22 extending from the aperture 20 to the outer edge 28 of the disc 18. Mounting means such as screw holes 24a-d permit a beam limiting plate to be rigidly fastened to the disc 18 using fasteners such as screws. Mounting means such as a screw hole 26 provides a rigid mounting for an actuator pin. Mounting means such as screw hole 25 permits mounting of a return spring. In an uncompressed state, the disc 18 has a diameter D. The outer edge of the disc 18 has a radius R as shown in FIG. 2. The disc 18 is manufactured from a material which is not degraded by exposure to radiation such as a filament wound fiberglass epoxy resin matrix. An alternative material for the disc 18 is a polyetherimide resin with thirty percent (30%) glass reinforcement such as that sold as a shaped material under the trademark "Ultem.RTM." by Westlake Plastic Company of Lennl, PA. The radiused portion of the outer edge 28 of disc 18 is covered with a bearing material such as a woven fabric containing polytetrafluoroethylene fibers ("TFE," sold under the trademark "Teflon.RTM.") interwoven with bondable polyester (sold under the trademark "Dacron.RTM.") yarns. Suitable fiberglass disc material with fabric applied thereto is sold under the trademark Duralon.RTM. by Rexnord Corporation in Downers Grove, IL. The aperture 20 and the slot 22 in the disc 18 permit the diameter D of the disc 18 to be reduced to a compressed diameter D* when a compressive force is applied to the disc 18, such as by using conventional snap ring pliers inserted into the holes 24a and 24d. FIG. 3 shows a sleeve bearing assembly 30, which comprises the disc 18 and a bearing retaining race 32. With reference to FIG. 4, the bearing retaining race 32 is characterized by a cylindrical inner surface 34 having a diameter d. A groove 36 having a maximum diameter d* and radius r.sub.1 is disposed within the inner surface 34. Fastening means such as a screw holes 40a-c are disposed within one surface of retaining race 32. The retaining race, most particularly the area forming the groove 36, is preferably constructed from stainless steel. FIG. 5 shows a detail of the disc 18 inserted within the retaining race 32. The radius r.sub.1 and diameter d* of the groove 36 are machined to match the radius r and uncompressed diameter D (said diameter D including the thickness of the bearing material) of the disc 18. Satisfactory operation has been obtained wherein the uncompressed diameter D is slightly smaller than the groove 36 diameter d*, thereby providing a clearance in the range of 0.004 to 0.008 inches. Satisfactory operation has been obtained where the radius r of the disc 18 is smaller than the radius r.sub.1 of the groove 36, specifically with a radius r of 0.063 inches and a radius r.sub.1 up to 0.090 inches. With reference to FIG. 6, a support plate 42 comprises an elongated aperture 44 and a bore 46. The elongated aperture 44 defines an area through which x-rays can pass unobstructed though the support plate 42. The bore permits entry of an actuator 48. The support plate further contains fastening means such as countersunk screw holes 56a-c which permit the support plate 42 to be fastened to the sleeve bearing assembly 30. Actuator 48 is disposed against the actuator pin 50. The face of the support plate 42 is constructed of a dense, x-ray absorbing material or materials such as one or more layers of lead supported on a structurally stronger material which may also have high energy absorbing properties. An aperture 52 within the support plate 42 permits entry of an actuator pin 50. The outer edges of the aperture 52 define a range of motion for the actuator pin. With reference to FIG. 7, the support plate 42 is fastened to the sleeve bearing assembly 30 using conventional button head screws 58 inserted through apertures 56a-c and in communication with screw holes 40a-c in bearing assembly 30. The actuator pin 50 is screwed into the screw hole 26 in the disc 18 and is partially disposed within the aperture 52 in the support plate 42. A beam limiter 54 is fastened to bearing disc 18 using conventional flathead screws disposed through the beam limiter 54 and in communication with screw holes 24a-d in disc 18. The beam limiter is constructed of a dense, x-ray absorbing material, such as tungsten, tantalum, or reinforced lead alloys. With reference to FIG. 6 and also to U.S. Pat. No. 4,905,268, the beam limiter 54 contains an aperture 66 which defines a first or whole body scan radiation passing portion and a second or head scan radiation passing portion. Turning now to FIG. 8, a return means such as a return spring 60 is operatively disposed between the support plate 42 and disc 18 using fastening means such as screws 62a-b protruding through fastening points in each end of the spring 60 and in communication with the screw holes 25 and 64 disposed within the disc 18 and the support plate 42, respectively. To assemble, the actuator pin 50 is screwed into the disc 18. The spring 60 is attached to the disc 18, which disc 18 is then compressed to a reduced diameter, inserted within the groove 36 of the retaining race 32, and the compressive force is removed. The beam limiter 54 is then screwed into the face of the disc 18, and the actuator 48 is inserted into the bore 46 in the support plate 42. The support plate 42 is fastened to the retaining race 32. When used within a conventional CT system, the assembly is placed inside a lead shield to prevent x-ray leakage. In operation, the actuator 48 is depressed. The actuator 48 presses against the actuator pin 50, causing the bearing disc 18 and consequently the beam limiter 54 to rotate together as a rotating assembly within the bearing retaining race 32. The rotational motion is limited by the retaining pin 50 contacting the edge of aperture 52 which limits rotation to a position in which the head scan radiation passing portion of the beam limiter 54 is aligned with the elongated aperture 44 in the support plate 42. The rotational motion causes the spring 60 to become extended and thereby exert a force urging rotation of the disc 18 and consequently the beam limiter 54 to return their original positions, such rotation being prevented by the actuator 48. When the actuator 48 is retracted, rotational motion is again limited by the retaining pin 50 contacting the other side of the aperture 52 which limits rotation to a position in which the whole body scan portion of the beam limiter 54 is aligned with the elongated aperture 44 in the support plate 42. Thus, it can be seen that the instant invention provides a reliable, more easily assembled, and more compact x-ray beam limiter than was previously known. It will be appreciated that the invention has been described with reference to its 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 alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.