Patent Publication Number: US-2004052332-A1

Title: X-ray collimator and a method of making an x-ray collimator

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     [0001] Certain aspects of this invention were developed with support from the FAA (Federal Aviation Association). The U.S. Government may have rights in certain of these inventions. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1). Field of the Invention  
       [0003] This invention relates to an x-ray collimator and a method of making an x-ray collimator.  
       [0004] 2). Discussion of Related Art  
       [0005] X-ray technique-based nonintrusive inspection apparatus are often used for airport bag scanning for purposes of detecting contraband or explosives, or for medical imaging and diagnosis. Such a system usually has an x-ray source that radiates x-rays through a closed container or the body of a person, and a number of x-ray detector crystals that are used for detecting the intensity of the x-rays after passing through the container or the body of the person.  
       [0006] X-rays tend to scatter when passing through the container or the body of the person and from walls within the system, so that x-rays that emit toward a particular crystal may come from various different directions. X-ray collimators are usually mounted over the x-ray detector crystals. A collimator usually has a number of septa made of a material such as lead that substantially attenuates x-ray radiation. The septa are aligned with the x-ray source and collimate the x-rays so that x-rays detected by the x-ray detector crystals are primarily those x-rays being emitted directly from the x-ray source through the container or the body of the person.  
       [0007] The x-ray source is usually at a single location, and the x-ray detector crystals are spread out over a wider area. Each collimator preferably has septa with center lines that are aligned with the x-ray source in order to accurately collimate x-rays that are detected by all of the x-ray detector crystals. The manufacture of such a collimator is relatively cumbersome, and usually involves a complicated machining operation. The collimators may alternatively be formed in a cast, but such a cast may be intricate and expensive to manufacture, and it may be difficult to remove the collimators from the cast.  
       SUMMARY OF THE INVENTION  
       [0008] This invention relates to a method of making an x-ray collimator. A plurality of collimator sheets are formed. Each collimator sheet has a plurality of septa lands that are made of a material that substantially attenuates x-ray radiation. A respective region is defined between a respective pair of the septa lands which is free of the material. The collimator sheets are positioned adjacent to one another. Adjacent corresponding lands of the collimator sheets are positioned adjacent to one another.  
       [0009] Center lines of septa pieces defined by the septa lands preferably converge toward one another.  
       [0010] Center lines of x-ray passages formed by the regions preferably converge toward one another.  
       [0011] The material is preferably molybdenum.  
       [0012] The regions are preferably septa openings in the respective collimator sheet.  
       [0013] A mask may be formed on each collimator sheet, the mask having a plurality of mask openings formed therein. The collimator sheet may then be etched with the mask preventing etching of the septa lands while allowing etching of the septa openings through the mask openings.  
       [0014] The invention also provides an x-ray collimator. The x-ray collimator comprises a plurality of collimator sheets. Each collimator sheet has a plurality of septa lands made of a material that substantially attenuates x-ray radiation. A respective region is defined between a respective pair of the septa lands which is free of the material. The collimator sheets are positioned adjacent to one another so that adjacent corresponding septa lands of the collimator sheets are positioned adjacent to one another.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The invention is further described by way of example with reference to the accompanying drawings, wherein:  
     [0016]FIG. 1 is a top plan view of a collimator sheet having upper and lower masks formed thereon;  
     [0017]FIG. 2 is a cross-sectional side view of the collimator sheet and masks of FIG. 1;  
     [0018]FIG. 3 is a view similar to FIG. 1 after the collimator sheet is etched with the mask preventing etching of portions of the collimator sheet;  
     [0019]FIG. 4 is a cross-sectional side view of the collimator sheet and masks of FIG. 3;  
     [0020]FIG. 5 is a view similar to FIG. 3 after the masks have been removed;  
     [0021]FIG. 6 is a cross-sectional side view of x-ray detector crystals and an x-ray collimator on the x-ray detector crystals, the x-ray collimator having been made by stacking a plurality of collimator sheets on top of one another, according to an embodiment of the invention; and  
     [0022]FIG. 7 is an end view of a gantry assembly that includes a plurality of collimators such as in FIG. 6.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0023]FIGS. 1 through 6 illustrate a method of making an x-ray collimator. A plurality of collimator sheets are stacked on one another. Each sheet is etched to have a number of septa openings defined between respective septa lands. The septa lands of the sheets jointly form septa pieces having center lines that converge toward one another, so that extensions of the center lines meet at an x-ray source.  
     [0024] Referring first to FIGS. 1 and 2, one collimator sheet  20  is shown that is covered with upper and lower masks  22 . Each mask  22  has a plurality of elongated mask openings  24  formed therein. The mask openings  24  are formed adjacent to one another with a respective mask land  26  between a respective pair of the mask openings  24 . The mask openings  24  of the upper and lower masks  22  are aligned with one another. The collimator sheet  20  is exposed in the openings  24 .  
     [0025] As illustrated in FIGS. 3 and 4, the collimator sheet  20  is subsequently etched. The masks  22  prevent etching of portions of the collimator sheet  20  between the masks  22 . A respective septa opening  28  is etched where the mask openings  24  are. The septa openings  28  are separated from one another by septa lands  30  that are sandwiched between the mask lands  26 . As illustrated in FIG. 5, the masks ( 22  in FIGS. 1, 2, and  4 ) are subsequently removed to expose the collimator sheet  20 .  
     [0026] The collimator sheet  20  is made of molybdenum Other materials that may be used because of their ability to substantially attenuate x-ray radiation generally have atomic numbers above 23, such as tantalum, titanium, tungsten, tin, ruthenium, lead, and iron. For the present high-energy x-ray application, the material should preferably have an atomic number of at least 35. Some materials, such as molybdenum and ruthenium, may be easier to etch than other materials such as lead. Lead by itself may also lack the mechanical strength to be rotated at high speed, and may have to be alloyed with a material such as tin.  
     [0027]FIG. 6 illustrates an x-ray collimator  40  that is made by stacking a plurality of collimator sheets  20 A-D on top of one another. Each one of the sheets  20 A to  20 D is manufactured according to the method illustrated in FIGS. 1, 2, and  5 . Adjacent septa lands  30  of adjacent ones of the collimator sheets  20 A-D jointly form a respective septa piece  42 . The septa openings  28  that are located over one another jointly form respective x-ray passages  44 . Center lines  48  of the septa pieces  42  converge to a single point  50 . Similarly, center lines  52  of the x-ray passages  44  converge to the same point  50 . X-rays from an x-ray source located at the point  50  can be collimated by the septa pieces  42  toward individual x-ray crystals  54  below respective ones of the x-ray passages  44 .  
     [0028] It can thus be seen that a relatively uncomplicated and inexpensive method is provided for forming an x-ray collimator. Differing collimators can be made by simply modifying the masks  22 .  
     [0029]FIG. 7 illustrates a gantry assembly  60 , including a gantry  62 , an x-ray source  64 , a plurality of x-ray collimators  40 , and a plurality of blocks  66  of x-ray detector crystals. The gantry  62  has a gantry opening  70  through which a container can be transferred. The x-ray source  64  is secured to the gantry  62  above the gantry opening  70 , and the x-ray collimators  40  and blocks  66  are secured to the gantry  62  below the gantry opening  70 . Referring jointly to FIGS. 6 and 7, each one of the x-ray collimators  40  has a plurality of septa pieces  42  and x-ray passages  44 , with center lines  48  and  52  that converge toward the x-ray source  63 .  
     [0030] In use, a closed container is transferred on a conveyor belt through the gantry opening  70 . X-rays are emitted by the x-ray source  64  and transmit through the container toward the x-ray collimator s  40 . The x-rays are detected by the x-ray detector crystals of the blocks  66 . The gantry  62  may be rotatably mounted to a support frame. Rotation of the gantry  62  will result in CT-type scanning by radiating through the container from different sides.  
     [0031] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.