Patent Number: 
Section: description

Referring first to FIGS. 1 and 2, in computed tomography, a patient (not shown) to be examined is positioned in a scan circle 102 of a computer tomography (CT) scanner 100, parallel with a z-axis, and between an x-ray source 104 and a rectangular detector array 106. The x-ray source then projects a beam of energy, or x-rays 108, through the patient, to the detector array. By rotating the x-ray source about the z-axis and relative to the patient, radiation is projected through a portion of the patient to the detector array from a many different directions around the patient. An image of the scanned portion of the patient then is constructed from data provided by the detector array. The scanner 100 of FIGS. 1 and 2 employs a collimator 10 for shaping the cross-section of the beam 108 into a rectangular shape that matches the rectangular detector array 106. The collimator 10 ensures that only a preferred row of the detector array 106 is irradiated by the beam 108 and so that a patient being scanned is not subjected to an unnecessary dose of x-rays. Referring also to FIG. 3, the collimator 10 includes a plate-like body 12 defining at least one elongated slit 14 for allowing the x-ray beam to pass through the slit and be shaped by the collimator. As shown, the collimator 10 can be provided with a plurality of slits 14 of varied, but uniform widths, and the collimator can be included as part of an assembly 110 that allows for the. selection of one of the collimator slits 14 such that a desired beam width can be produced by the collimator 10. Details of the assembly 110 are disclosed in co-pending U.S. patent application Ser. No. 09/552,141, filed Apr. 19, 2000, now U.S. Pat. No. 6,301,334 issued Oct. 9, 2001, which is assigned to the assignee of the present application and incorporated into the present application by reference. As shown in FIG. 3, the collimator 10 also includes various mounting apertures 16 formed in the plate-like body 12 for mounting the collimator to the assembly 110. Referring also to FIGS. 4 through 6, the collimator 10 also includes a coating 18 covering a predetermined portion of a top surface 20 of the plate-like body 12. The coating 18 surrounds the collimating slits 14 and is comprised of an x-ray attenuating or absorbing material such a tungsten carbide. The plate-like body 12 is made of a suitable noncorrosive, more easily machined material such as stainless steel, aluminum or brass. As an example of a preferred embodiment of a collimator 10 constructed in accordance with the present disclosure, the plate-like body 12 is provided with a thickness of about 60/100 of an inch, while the coating 18 is provided with a thickness of at least about 1 millimeter. Referring to FIG. 7, a preferred method of applying the coating 18 is through a thermal spray process. For tungsten carbide an appropriate method is a plasma thermal spray process, which is basically the spraying of molten or heat softened tungsten carbide onto the top surface of the plate-like body to provide the coating. As shown, tungsten carbide in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot tungsten carbide impacts on the surface of the plate-like body 12 and rapidly cools to form the coating 18. This process carried out correctly is called a xe2x80x9ccold processxe2x80x9d as the temperature of the plate-like body 12 can be kept low during processing thereby avoiding damage, metallurgical changes and distortion to the body. The plasma gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localized ionization and a conductive path for a DC arc to form between cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionize to form a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current). When the plasma is stabilized ready for spraying the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Tungsten carbide powder is then fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of 25 to 150 mm. The plasma thermal spray process is most commonly used in normal atmospheric conditions. Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten, and plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spray processes. Referring to FIGS. 8A through 8D, a method according to the present disclosure of manufacturing the collimator 10 of FIG. 3 is progressively illustrated. As shown first in FIGS. 8A and 8B, the coating 18 is applied to a predetermined portion of the top surface 20 of the plate-like body 12 of the collimator 10. The collimating slits 14 are then machined through the coating 18 and the plate-like body 12 as illustrated in FIG. 8C. Preferably, wire electrical discharge machining (EDM) is used to machine the collimating slits 14. Wire EDM is a machining process for cutting metals using a thin wire electrode. Although not shown, electrical sparks between the metal collimator 10 and the thin wire electrode melts thin line-like portions of the coating 18 and the plate-like body 12 to form the collimating slits 14. Wire EDM is a preferred method since it can make high precision cuts on any conductive materials, can be as accurate as +/xe2x88x920.0001 inches, and is ideal for precision and delicate cuttingxe2x80x94as is required for x-ray collimating slits. Referring to FIG. 8D, after the collimating slits 14 are machined, the mounting apertures 16 are machined in the plate-like body 12 between an outer periphery 22 of the coating 18 and an outer periphery 24 of the body 12 using a less expensive method of machining. While this disclosure has been particularly shown and described with references to the collimator of FIGS. 3-8, it will be understood by those skilled in the art that various changes in form and in details may be made thereto without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, the novel features of a collimator as disclosed herein can be applied to a collimator having a single collimating slit, a curved collimator, or a post-patient collimator. In addition, the coating can comprise a suitable material other than tungsten carbide for attenuating and absorbing x-rays, such as a lead alloy. And the method of applying the coating is not limited to a plasma thermal spray process.