Abstract:
An anti-scatter grid for radiology imaging having an anti-scatter layer with a plurality of metallized partitions that enable X-rays emitted from a source located above the grid to pass and absorbing X-rays not derived directly from this source. The grid has at least one plate of expanded polymer material fixed on one surface of anti-scatter layer. The grid may be positioned with a frame.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
   This application claims the benefit of a priority under 35 SC 119(a)-(d) to French Patent Application No. 03 06139 filed May 22, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION  
   The invention concerns anti-scatter grids as used in radiology imaging and particularly in X-ray imaging. 
   A radiology imaging apparatus conventionally comprises a source of radiation, such as an X-ray source, and a means for forming the image, such as an image receptor, between which the object to be imaged is positioned. The beam of radiation emitted by the source passes through the object before reaching the receptor. It is partly absorbed by the inner structure of the object so that the intensity of the beam received by the receptor is attenuated. The global attenuation of the beam after passing through the object is directly related to absorption distribution within the object. 
   The image receptor comprises an optoelectronic detector or intensifying screen-film couple, sensitive to radiation intensity. Consequently, the image generated by the receptor corresponds in principle to the distribution of global ray attenuation subsequent to passing through inner structures of the object. 
   Part of the radiation emitted by the source is absorbed by the inner structure of the object, the other part is either transmitted (primary or direct radiation) or scattered (secondary or scatter radiation). The presence of scatter radiation leads to degradation of contrast in the image obtained and a reduced signal to noise ratio. This is of particular hindrance, in particular if it is desired to visualize details of the object. 
   One solution to this problem comprises inserting an “anti-scatter” grid between the object to be X-rayed and the image receptor. These grids are usually formed of a series of parallel strips or partitions of X-ray absorbing material. In so-called “focalized” grids (according to the terminology laid down by standard IEC 60627 on “X-ray imaging diagnostic equipment—Characteristics of anti-scatter grids for general use and mammography screening”) all the planes of the strips or partitions are oriented along planes passing through the focal point of radiation emitted by the source. Therefore, these grids allow direct radiation to pass and absorb scatter radiation. Focalized anti-scatter grids have contributed towards a considerable improvement in the contrast of images obtained. 
   In order to obtain good quality images it is desirable to provide grids having the finest possible structure so as not to disturb direct radiation. It is also desirable to control the orientation of the absorbing strips or partitions with precision. The precision with which the strips or partitions are orientated evidently depends upon the manufacturing technique used to produce the grid. However, it is found that during use of the grid it may undergo deformation, which substantially modifies strip orientation. The consequence is impaired precision of strip or partition orientation. This impairment is greater the narrower the thickness of the grid and its propensity to deform. 
   This problem is particularly raised in imaging devices with an overhanging grid, i.e., fixed on one side only. In this case it may undergo substantial bending stresses. 
   To overcome these disadvantages, grids have been proposed having an aluminium frame, the frame giving rigidity to the assembly. In addition, these grids are coated on each of their surfaces with plates in a composite carbon and resin material having a thickness of between 0.2 and 0.4 mm. 
   BRIEF DESCRIPTION OF THE INVENTION  
   An embodiment of the invention is directed to an anti-scatter grid comprising an anti-scatter layer having a plurality of metallized partitions, these partitions allowing radiation that are emitted from a source located above the grid to pass and absorbing those radiation which do not derive directly from this source and at least one plate in an expanded polymer material fixed to one surface of the anti-scatter layer. 
   An embodiment of the invention also directed to a method for fabricating an anti-scatter grid comprising: 
   forming an anti-scatter layer having a plurality of metallized partitions, these partitions enabling radiation to pass emitted by a source located over the grid and absorbing those radiation which do not derive directly from this source; and 
   fixing at least one plate of expanded polymer material on one surface of the anti-scatter layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     The invention and embodiments thereof will be better understood from the following description that is solely illustrative and non-restrictive and is to be read with reference to the appended figures in which: 
       FIG. 1  is a schematic view of an anti-scatter layer of a focalized grid; 
       FIG. 2  is a schematic view of the layers forming an anti-scatter grid according to an embodiment of the invention; 
       FIG. 3  is a schematic view of a frame intended to hold the grid-forming layers; 
       FIG. 4  is a schematic view of the positioning of two side parts of the frame; 
       FIG. 5  is a schematic view of the positioning of two other parts finalizing this frame; and 
       FIG. 6  is a schematic view of a crosspiece intended to hold in place the grid-forming layers. 
   

   DETAILED DESCRIPTION OF THE INVENTION  
   In  FIG. 1  anti-scatter layer  10  is formed of a planar substrate  12  of a polymer material, approximately 1 to 3 mm thick, comprising partitions defining cells  14 . As shown in  FIG. 1 , the thickness can typically be 1.7 mm. The inner walls of cells  14  are coated with an absorbing metal layer  16 . Anti-scatter layer  10  is focalized, which means that the cell walls are oriented along planes passing through the focal point of radiation emitted by a source of radiation. 
   As a result of the anti-scatter layer  10 , part of the direct radiation emitted by an X-ray source passes through the grid via substrate  12  while another part passes through the layer via cells  14 . On account of the low density of the polymer forming substrate  12 , the radiation passing through it is scarcely attenuated. 
   The inner walls of cells  14  coated with a metal layer  16  absorb scatter radiation arriving at anti-scatter layer  10  at too great an angle relative to the direction of focalization of one of cells  14 . 
   In  FIG. 2 , two plates  20  and  22  of expanded polymer material are arranged on each surface of anti-scatter layer  10 . The polymer material forming the plate should have sufficient rigidity to prevent grid deformation and sufficient homogeneity so as not to disturb the X-ray image through artifacts. Expanded polymer materials have the advantage of scarcely attenuating X-rays on account of their low surface density. The plate of expanded material also plays a protective role for the anti-scatter layer of the grid. 
   Both plates  20  and  22  may be formed of a hard polymethacrylimide (PMI) foam. This type of foam is manufactured, for example, by RÖHM GmbH under the trademark ROHACELL® or an expanded polyetherimide (this type of material is supplied for example by ALCAN AIREX AG under the trademark AIREX®). The plate is formed in a material having a density of between 20 and 70 kg/m 3 . ROHACELL® is available in this density range. In particular a density in the order of 30 kg/m 3  is available. The plates may have a thickness between 2 and 6 mm and the two plates may have the same thickness. 
   Plates  20  and  22 , respectively positioned on the surface of anti-scatter layer  10  are intended to be exposed to the rays emitted by the X-ray source, and on the surface of anti-scatter layer  10  located on the image detector side, can be identical. The thickness of the plates is on the order of 3 mm with an approximate density of 30 kg/m 3 . As shown in  FIG. 2 , there are two plates  20 ,  22  with a range of thickness between 2 and 4 mm. 
   Assembly of plates  20  and  22  of polymethacrylimide is made by bonding. The adhesive is preferably deposited on plates  20  and  22  and these plates are then superimposed on anti-scatter layer  10 .The adhesive may be distributed so that it only contacts a peripheral area of anti-scatter layer  10  which does not form an active part of the layer. Therefore the adhesive does not disturb radiation transmission through layer  10  and plates  20  and  22 . 
   Alternately, the adhesive may be placed so that it contacts the entire surfaces of anti-scatter layer  10  which improves the mechanical resistance of the assembly. In this case, an aerosol adhesive is preferred to provide a fine, homogeneous layer of adhesive. This bonding technique avoids filling the cells of the anti-scatter layer. 
   It is also possible to use a film adhesive. This type of adhesive is in the form of a film with or without a backing that can be deposited directly on a surface of each of plates  20  or  22  so that they can be assembled with anti-scatter layer  10 . Adhesive films have the advantage of providing a thin, homogeneous layer of constant thickness and therefore of obtaining constant radiation transmission over the entire assembly surface. 
     FIGS. 3 and 4  shows a frame  30  intended to be positioned around the assembly formed by the anti-scatter grid. The purpose of frame  10  is to rigidify and to protect the assembly. 
   In  FIG. 4 , positioning of the frame comprises a first step in which a crosspiece  38  is positioned on one of the longitudinal sides of the assembly formed by the superimposition of plates  20 ,  22  and anti-scatter layer  10 . The second step in frame positioning comprises placing two U-shaped  32  and  34  sections made of carbon composite material on the two opposite transverse sides of the assembly. The U-shaped sections encase the assembly and crosspiece  38 . As shown in  FIG. 4 , the thickness of the U-shaped portions of the sections  32  and  34  may be approximately 1.0 mm. As shown in  FIG. 5 , the legs of the U-shaped sections  32  and  34  may be approximately 5.0 to 10. mm. 
     FIG. 5  shows a third step comprising depositing a fine layer  36  (thickness on the order of 0.3 to 0.5 mm) of carbon composite material on the remaining longitudinal side of the assembly to finalize frame  30 . 
   The anti-scatter grid obtained ( FIG. 5 ) is particularly adapted for mammography screening applications. The longitudinal side coated with fine layer  36  is the side against which the patient leans, and the longitudinal side along which crosspiece  38  extends is the side on which the anti-scatter grid is held in place. With fine layer  36 , X-rays passing close to the patients&#39; ribcage are not hindered so as to obtain the most extensive mammography view possible. Crosspiece  38  is intended to fix the anti-scatter grid for a Potter-Bucky device. Crosspiece  38  limits vibrations of the anti-scatter grid should it be placed in movement. 
   The anti-scatter grid may also comprise one or more protection layers covering one or optionally both plates  20  and  22  of polymethacrylimide. The protection layer may be formed of a polymer material for example, a composite material containing carbon fibers, a lacquer or varnish. The protection layer is intended to protect the expanded polymethacrylimide plate against humidity and impact. The attenuation of X-rays by the protective layer should be the least possible. The protective layer is made of a polymer material for example having a thickness in the order of 0.1 mm that provides an acceptable attenuation of X-rays in the order of 1%. 
   The protection layer can be a polymer material, preferably a polyester (supplied for example by DUPONT DE NEMOURS under trademark MYLAR®) in polycarbonate (available from RÖHM GmbH for example under trademark EUROPLEX®), or in polymethylmethacrylate PMM (supplied for example by RÖHM GmbH under trademark PLEXIGLASS®). 
   The protection layer is preferably deposited on a surface of plate  22  oriented in an opposite direction to the X-ray source (i.e., towards the detector). The protective layer protects the grid against possible impacts during handling operations. However, plate  20  oriented towards the source may also be given a protection layer. 
   In one variant of embodiment of the invention, the assembly may be held in place by a crosspiece and not a frame. 
     FIG. 6  shows a crosspiece  38  intended to be positioned on one of the longitudinal sides of the assembly. Crosspiece  38  has a straight generally U-shaped section. The assembly, comprising the two plates  20  and  22  in expanded polymer material and anti-scatter layer  10 , is inserted between the two sides of the U. Crosspiece  38  is intended to rigidify and to protect the edge of the assembly. Crosspiece  38  is also used to fix the assembly to a Potter-Bucky. Fixations may be provided for this purpose on crosspiece  38 . The grid so fabricated is lighter than the grid in  FIG. 5 . 
   The plate of expanded material can rigidify the grid and maintain the anti-scatter layer in its initial form. Expanded materials offer a high bending strength-to-weight ratio. In addition, these materials have low surface density, which means they make practically no contribution towards grid deformation. 
   One skilled in the art may make or propose various modifications to the structure and/or way and/or function and/or result and/or steps of the disclosed embodiments and equivalents thereof without departing from the scope and extant of the invention.