Patent Application
20170206996
The present invention generally relates to grids used in radiation imaging including x-ray imaging.
The fields of medical and industrial radiography use the technique of directing beams of electromagnetic radiation toward an object (or part of the human body), so that the radiation passes through the object, to obtain an image of the interior of the object, that is otherwise difficult to access or view directly without cutting through the body or other object. Usually, the electromagnetic radiations used for imaging purposes are x-rays, which tend to scatter as they travel through the object to be imaged.
The scattered x-rays contribute to the degradation of the image of the object and more particularly to the degradation of the image contrast. The x-rays that travel through the object that are not scattered are referred to as primary transmissions and it is those transmissions that contribute the most useful information to the image. The various unscattered x-rays passing though the object are attenuated at differing levels by differing amounts and compositions of material within the object. The differences in x-ray attenuation along linear paths through the object produce an intensity pattern that comprises image information which is recorded by an image receptor.
The image receptor may be a screen having a layer of x-ray sensitive material or x-ray sensitive electronic medium. The resulting image produced by the image receptor is based on the differences in the intensity of primary x-ray transmissions detected by the receptor. To improve the image quality, the primary x-ray transmissions and any scattered x-rays that would reach the image receptor after having traveled though the body, are first passed through a grid before they are allowed to impinge onto the image receptor.
Anti-scatter grids are components of an x-ray imaging system that are placed between the imaged object and the image receptor. The purpose of the grid is to filter out any x-rays scattered by the object while permitting the unscattered (primary) x-rays to pass through. The efficiency of the grid is dependent on two factors, primary and the scatter transmission. Primary transmission should be as high as possible, while scatter transmission should be as close to zero as possible. It is understood that, quantitatively speaking, the scattered x-rays degrade the image contrast by a factor approximately equal to (1-SF) where SF is the scatter fraction of the total radiation transmitted through the body. The scatter fraction SF is defined as:
where S and P are the intensities of the scattered and primary radiations incident on the image receptor, respectively. Most conventional anti-scatter grids are considered linear grids as they are fabricated as a linear array of thin lead foil ribbons. Generally, the performance of an anti-scatter grid in this respect is given by the Contrast Improvement Factor (CIF):
where Cg and Co are the image contrasts with and without the grid, Ts and Tp are the transmissions of scatter and primary radiation by the grid, respectively. The ribbons are held in place by an x-ray transparent material that provides rigidity and that aligns the foils so that their surfaces converge to a focus line located at a specific distance above the top surface of the grid. X-ray transparent protective covers are usually provided over the top and bottom surfaces of the grid.
Primary transmission is an important factor because the lower the value the greater the radiation dose to a patient imaged with the grid. Primary transmission is determined by two factors, 1) the fraction of the x-ray incident surface occupied by the lead foils and 2) by the x-ray absorption of the supporting interspace material between the lead strips and by the protective covers. An ideal grid operated at its focus distance would have a primary transmission determined almost entirely by factor 1, with little absorption by the covers and interspace materials.
Scatter transmission depends on the thickness of the lead foils and on the geometry of the foils and the space between them. To a first order, the scatter transmission is reduced by increasing the thickness of the lead foil and by increasing the ratio of the foil height to the space between foils (grid ratio). Scatter transmission is further reduced by stacking two linear grids (crossed grids) with the foils in one grid orthogonal to that in the second grid although with conventionally designed aluminum interspace grids this reduces primary transmission thus requiring an increase in dose to the patient. Scatter transmission is also influenced by the attenuation properties of the interspace material. An explanation with diagrams is set forth in U.S Patent Publication No. 2013/0272505, published on Oct. 17, 2013.
The present invention provides an improved anti-scatter grid and a method for the manufacture of, a linear focused anti-scatter grid with greater primary transmission and smaller scatter transmission compared to conventionally manufactured linear grids.
In accordance with the present invention the grid comprises a series of walls formed of a material that is capable of absorbing such high energy radiation, the walls being so placed and aligned as to define the channels in such a manner as to focus any radiation passing through the channels on a single remote line or point. The walls absorb any scattered radiation, which travel transversely to the focused radiation, but may enter the grid with the focused radiation. The radiation absorbent walls are preferably supported by a frame, generally rectangular in outline, and in a preferred embodiment of the present invention by an interstitial structure, extending between and connecting the energy absorbing walls, and formed of a polymer; the polymer is preferably foamed to be at least semi-rigid, having sufficient rigidity to support and maintain the alignment of the energy absorbing walls, and having extremely low energy absorbing effect. Because the primary transmission of this invention is so high, the ultra high performance provided by a pair of crossed grids can be practical, with very low dose penalty. Alternatively, the liquid, uncured polymer can be mixed with glass microbubbles, hollow glass microspheres such as those manufactured by 3M, in lieu of foaming the polymer. Although the silicon in the glass walls has higher radiation absorbency than the usual polymers, as most of the volume is air the effect of the silicon is small.
The preferred embodiment of the grid of this invention can be formed with any practical grid ratio. Higher ratio grids made by conventional methods impose a radiation dose penalty due to the greater absorption in thicker interspaces. However the present invention has little radiation dose penalty from higher ratios because the foam material absorbs only a few percent over a wide range of x-ray energies. Such low absorptivity allows for the ultra high performance of a crossed grid with very low dose penalty. The preferred embodiment uses lead foils between 50 and 100 microns which produces much better scatter absorption than the thinner foils in conventional grids. More generally, the ribbons can have a thickness in the range of from 10 to 1000 microns
The radiation absorbent channel boundary can be designed to a desired or preferable state by changing the constituent elements, i.e., different atomic numbers of its elements, or the thickness or density of the absorbent layer, to better suit the absorption of x-rays of a specific range of photon energies. For example in an application using low energy x-rays such as in mammography, the absorbent layer may be only a few tens of microns thick and might include elements with atomic numbers as low as 29. Applications requiring more energetic x-rays, such as general medical radiography, may employ a thicker absorbent layer, which is preferably formed from heavy metal elements with atomic numbers above 65, such as Lead, Bismuth, Tungsten, or Tantalum.
The ribbons of metal need not necessarily be pure metal but may be a powdered material mixed with binding agents (e.g., polymers) to bind a relatively high concentration of heavy metal in the form of a fine powder, or as a compound mainly containing elements with atomic numbers greater than 28, or as an alloy. Depending on the application for which the anti-scatter grid is being used, the relatively high concentration of heavy metals may be in the range of 40% to 98% by weight. Because the channel boundaries are formed from foil under some degree of tension, some desirable, highly radiation-absorbent heavy metals, such as lead or bismuth or alloys thereof, will require reinforcement, such as by the addition of fibers or coatings (e.g., Mylar) to provide adequate tensile strength to a ribbon of the metal. This can include the use of glass fiber reinforced lead foil, or a lead foil wrapped in a thin braided weave of high tensile strength glass, nylon, polyester or other fiber materials. Alternatively, a thin tape may be adhered to the heavy metal ribbon formed, for example of Mylar or Kapton, two commercially available polymeric materials containing oxygen or nitrogen, respectively. As a further alternative, the heavy metal ribbon can be plated or otherwise bonded to a thin metal strip, preferably high tensile strength steel or stainless steel foils. The steel foil need only be 20-50 microns thick. As already described with respect to
One preferred embodiment of the focused grid of the present invention comprises an enclosed frame, comprising at least a pair of opposed side pieces, each supporting and positioning a ribbon of the material forming the grid walls. In one embodiment, an assembly frame base member comprising two parallel assembly frame members support parallel rows of small pins 6 on opposite sides of the assembly frame base. Each corresponding pair of pins in the two frame members provide support for a loop of a heavy metal foil, or ribbon, extending across the assembly frame base. There are preferably an equal number of pins in each row and pairs of pins on opposite rows are parallel to each other.
In another embodiment, each side piece is provided with a plurality of slots, or other openings, so spaced and disposed as to hold, preferably at each end, the material forming the walls defining the channels, in the proper alignment. The slots are so disposed relative to each other as to cause the wall materials held in the slots, to be in a configuration to focus any radiation impinging on one face of the grid to converge at a focus point, or line, beyond the opposing surface of the grid.
Preferably, the grid is formed of a series of interconnected and mating modules, each module being substantially identical to the other modules. In one such embodiment, each module is essentially a ribbon, or plate, of the radiation absorbent heavy metal material, held in a frame so as to maintain their juxtaposition relative to each other and to the radiation source and the imaging device. In another such embodiment, the ribbon, or plate, of the radiation absorbent heavy metal material is secured to one side of a suitably shaped support formed of a radiation transparent material, also preferably held in a suitable frame, as above.
In each preferred embodiment there is extending between, and defined by, the radiation absorbent walls, a substantially radiation transparent material, which most preferably, is only, or primarily, air, the material most transparent to x-rays. Alternatively, as a means of providing additional structural support and rigidity to the radiation absorbent walls, extending between and attached to at least one of the immediately adjacent pair of defining walls is a solid support material that is also substantially transparent to x-radiation, such as a hydrocarbon polymer or carboxylated hydrocarbon polymer, if the polymer is thick enough to completely fill the channel between the walls, the polymer is more preferably foamed to further increase primary radiation transmission. The grid design most preferably contains primarily air within the channels, so that transmission of primary radiation (Tp) through the grid is maximized, thus allowing the total radiation dose to the patient to be lower, as compared to conventional aluminum- or steel-supported grids.
A foam could consist of a polymer incorporating glass microbubbles; these are hollow glass microspheres such as those manufactured by 3M. Although the silicon in the glass walls is actually not ideal, as the walls are extremely thin, so that most of the volume of the microspheres is air, the effect of the silicon is small, and the possible vagaries of a foaming chemical reaction are avoided.
The thickness of the heavy metal, x-ray absorbent walls defining the channels and the depth of the channels (and thus the length, L, of the walls) can be varied to optimize primary transmission and reduce or eliminate transmission of the scattered radiation, for a given radiation energy.
One preferred method of the present invention comprises the steps of forming a preferred grid frame by forming the frame sides, by casting or molding, of for example, aluminum or steel or a high strength polymer. In the method of forming one preferred embodiment of the frame, high precision machining of the aluminum or steel or rigid polymer frame sides, produces a series of aligned slits on opposite sides of the frame. The planes containing the center lines of the pairs of opposed slits along the opposing frame sides, are so aligned and juxtaposed, as to converge at a line on the horizontal plane of the x-ray tube focus, as depicted in
The slits on opposite sides of the frame are precisely aligned so that slits on opposite sides are in the same planes orthogonal to the sides of the frame in which the slots are formed. The walls can be formed of thin ribbons of heavy metal foils held tightly in tension across the frame by the opposed slits. One embodiment is essentially a conventional linear grid where the metal foil ribbons form planes that extend from one edge of the frame to the other. In this embodiment the planes of all ribbons converge to a line through the x-ray focus.
A second related embodiment is based upon the first embodiment, except that a second similar frame is positioned over the first but with the slits and ribbons orthogonal to those of the first layer. This design results in what is effectively a crossed linear grid, which further reduces scatter radiation striking the imaging surface and results in a further improved image. The grid ratio is the ratio of channel depth to diameter, which can be 3:1 to 20:1, and is preferably between 5:1 and 16:1.
In another group of preferred embodiments, the focused grid of the present invention comprises an enclosed frame, comprising at least a pair of opposed side pieces, each supporting and positioning a ribbon of the material forming the grid walls, and each side piece provided with a plurality of slots, or other openings, so spaced and disposed as to hold, preferably at each end, the material forming the walls defining the channels, in the proper alignment. The slots are so disposed relative to each other as to cause the wall materials held in the slots, to be in a configuration to focus any radiation impinging on one face of the grid to converge at a focus point, or line, beyond the opposing surface of the grid.
Preferably, the grid is formed of a series of interconnected and mating modules, each module being substantially identical to the other modules. In one such embodiment, each module is essentially a ribbon, or plate, of the radiation absorbent heavy metal material, held in a frame so as to maintain their juxtaposition relative to each other and to the radiation source and the imaging device. In another such embodiment, the ribbon, or plate, of the radiation absorbent heavy metal material is secured to one side of a suitably shaped support formed of a radiation transparent material, also preferably held in a suitable frame, as above.
In each preferred embodiment there is extending between, and defined by, the radiation absorbent walls, a substantially radiation transparent material, which most preferably, is only, or primarily, air, the material most transparent to x-rays. Alternatively, as a means of providing additional structural support and rigidity to the radiation absorbent walls, extending between and attached to at least one of the immediately adjacent pair of defining walls is a solid support material that is also substantially transparent to x-radiation, such as a hydrocarbon polymer or carboxylated hydrocarbon polymer, if the polymer is thick enough to completely fill the channel between the walls, the polymer is more preferably foamed to further increase radiation transmission. The grid design most preferably contains primarily air within the channels, so that transmission of primary radiation (Tp) through the grid is maximized, thus allowing the radiation dose to the patient to be lower, as compared to conventional aluminum- or steel-supported grids.
The thickness of the heavy metal, x-ray absorbent walls defining the channels and the depth of the channels (and thus the length, L, of the walls) can be varied to optimize primary transmission and reduce or eliminate transmission of the scattered radiation, for a given radiation energy.
One preferred method of the present invention comprises the steps of forming a preferred grid frame by forming the frame sides, by casting or molding, of for example, aluminum or steel or a high strength polymer. In the method of forming one preferred embodiment of the frame, high precision machining of the aluminum or steel or rigid polymer frame sides, produces a series of aligned slits on opposite sides of the frame. The planes containing the center lines of the pairs of opposed slits along the opposing frame sides, are so aligned and juxtaposed, as to converge at a line on the horizontal plane of the x-ray tube focus, as depicted in
The slits on opposite sides of the frame are precisely aligned so that slits on opposite sides are in the same planes orthogonal to the sides of the frame in which the slots are formed. The walls can be formed of thin ribbons of heavy metal foils held tightly in tension across the frame by the opposed slits. One embodiment is essentially a conventional linear grid where the metal foil ribbons form planes that extend from one edge of the frame to the other. In this embodiment the planes of all ribbons converge to a line through the x-ray focus. A second related embodiment is based upon the first embodiment, except that a second similar frame is positioned over the first but with the slits and ribbons orthogonal to those of the first layer. This design results in what is effectively a crossed linear grid, which further reduces scatter radiation striking the imaging surface and results in a further improved image. The grid ratio is the ratio of channel depth to diameter, which can be 3:1 to 20:1, and is preferably between 5:1 and 16:1.
The method of producing the grid, in accordance with this invention results in an improved product and avoids the cost of expensive tensioning apparatus being required for the final grid.
The focused grid of the present invention as described above can be used as an anti-scatter grid for x-ray imaging useful, for example, in the fields of medical and/or industrial radiography. Referring to
The x-rays used for radiographic purposes usually include electromagnetic radiation having photon energies in the range of 10 keV to 1 MeV. For ease of explanation, the beam of radiation will henceforth be described as an x-ray beam in the range described. However, it should be understood that all of the embodiments of the claimed focused grid of the present invention may be operated and function as described using electromagnetic radiation having photon energies that fall outside of the range described above, with the boundaries or walls of the channels to be constructed from material that can absorb such scattered electromagnetic radiation as may be generated.
Continuing with the description of
When x-rays 18 pass through the object 12, they are attenuated by a combination of scattering and absorption. X-rays which have passed through the object 12 and are “focused x-rays” (meaning they also pass through the grid 16, following a focused path as described herein) and are also referred to as ‘primary x-rays’; the primary x-rays contribute to the formation of the image. That is, unscattered focused beams—having passed directly though the object 12—will mostly pass through the channels of the focused grid 16 of the present invention. Radiation, including x-ray radiation, which do not follow a focused path leaving the object being imaged are referred to as scattered, and scattered radiation 20 will intersect one of the metallic layers (or radiation absorbing layers) that define the channel boundaries, which are intended to absorb the scatter radiation to an extent depending on the composition and thickness of the boundary and the energy of the radiation. It must be noted that some focused x-rays, which pass in the plane of the foils will tend to be absorbed by those foils creating a shadow image of the foils in the resulting image. It is known to provide grid systems with a mechanism to move the grid during the x-ray exposure so that the image of the foils is reduced, if not eliminated, by the blurring resulting from the motion, without significantly reducing the resolution of the primary image.
The radiation absorbent channel boundary can be designed to a desired or preferable state by changing the constituent elements, i.e., different atomic numbers of its elements, or the thickness or density of the absorbent layer, to better suit the absorption of x-rays of a specific range of photon energies. For example in an application using low energy x-rays such as in mammography, the absorbent layer may be only a few tens of microns thick and might include elements with atomic numbers as low as 29. Applications requiring more energetic x-rays, such as general medical radiography, may employ a thicker absorbent layer, which is preferably formed from heavy metal elements with atomic numbers above 65, such as Lead, Bismuth, Tungsten, or Tantalum.
An x-ray transparent material (such as air, or a hydrocarbon polymer or other low molecular weight polymer which may also contain nitrogen or oxygen atoms, or even chlorine atoms) is a material through which an x-ray beam travels where the measurable intensity of the beam immediately prior to passing into the material is substantially equal to the measurable intensity of the beam immediately after exiting the material. Conversely, an x-ray absorbent material greatly reduces the amount of x-rays exiting such material compared to the strength of the x-rays that entered such material. X-rays passing through the object being imaged, and that are scattered, i.e., that do not follow a focused path through the channels of the focused grid, intersect and impinge upon the x-ray absorbent wall boundaries of the focused channels, and are thus absorbed by these walls.
The ribbons of metal need not necessarily be pure metal but may be a powdered material mixed with binding agents (e.g., polymers) to bind a relatively high concentration of heavy metal in the form of a fine powder, or as a compound mainly containing elements with atomic numbers greater than 28, or as an alloy. Depending on the application for which the anti-scatter grid is being used, the relatively high concentration of heavy metals may be in the range of 40% to 98% by weight. Because the channel boundaries are formed from foil under some degree of tension, some desirable, highly radiation-absorbent heavy metals, such as lead or bismuth or alloys thereof, will require reinforcement, such as by the addition of fibers or coatings (e.g., Mylar) to provide adequate tensile strength to a ribbon of the metal. This can include the use of glass fiber reinforced lead foil, or a lead foil wrapped in a thin braided weave of high tensile strength glass, nylon, polyester or other fiber materials. Alternatively, a thin tape may be adhered to the heavy metal ribbon formed, for example of Mylar or Kapton, two commercially available polymeric materials containing oxygen or nitrogen, respectively. The focused grid 16 of the present invention has a plurality of focused channels that allow unscattered primary x-ray beams having passed through the object 12 to impinge upon the image receptor 14, and thus form a clear, focused image.
A preferred embodiment of the finished anti-scatter grid is depicted in
As shown in the drawings, the fabrication of the preferred embodiment begins with an assembly frame base member 7 supporting two parallel assembly frame members 8, 9 for supporting parallel rows of small pins 6 on opposite sides of the assembly frame base 7 (
The pins 6 can be round in cross-section, or ovoidal, or polygonal. The critical dimension is the width of the pin 6, i.e., the dimension in the direction perpendicular to the top edge of the ribbon. This dimension determines the separation of the heavy metal wall surfaces, of the grid.
As it is preferred that the tops of the ribbons be parallel and in the same plane, the pins preferably increase in length as they are located farther from the central pin, in order to compensate for the increasing incline of the pins away from the center.
Although the difference in height above the frame edge will be relatively small as a result of the incline, the best results are achieved when the top edges of the ribbons are in the same plane, when tensioned by moving apart the two frames supporting the two sets of pins.
Prior to placement of the ribbon loops 11 on each pin 6, supporting blocks 10 are added to the assembly frame
Referring to
Referring to
After suitable tensioning, the support frame 9 is locked in position to maintain the desired tension, and the two part polyurethane mixture is introduced through the injection port 17 using a mixing nozzle 18 under pressure, see
Referring to
The grid is removed from the assembly form appearing as in
Prior to initial assembly the surfaces of the pin arrays, pins covers and other components of the assembly frame are coated with a suitable release agent to prevent adhesion of urethane foam and to facilitate cleaning for assembly of the next grid in production.
In the preferred embodiment depicted in
An advantage of the rigid grid of the present invention is that it maintains the linearity and juxtaposition of the grid ribbon surfaces without requiring additional hardware. In preparing the metal strips for tensioning and immersion into the rigid polymeric foam, lengths of the heavy metal ribbon are cut by a precision cutter to the exact same lengths. Each such precisely cut length is formed into a loop and the ends secured together to form the loop as shown in
A pair of series of pins 6, accurately aligned and in angularly juxtaposed relationships are held in a pair of opposing, parallel frame members 8, 9. Each frame member 8, 9 is provided with a series of holes 26, which are precisely formed and spaced to firmly hold the pins 6 in the desired juxtaposed relationships. The pins 6 are all precisely cut to the desired lengths and the diameter of each pin, and thus of the internal diameter of the hole, is equivalent to the desired foil spacing. The distances between the holes 26 are precisely the same and equivalent to the desired foil spacing plus twice the thickness of the ribbon walls 11. The holes 26 are formed to identical depths but angled such that the central axis of each of the holes, and thus of each of the pins 6 held therein, converge to the focus line, i.e., a line parallel to the surface of the grid midline that is located the focus distance above the grid surface. For example the central pins on each of frames 8, 9 are preferably perpendicular to the frame members. Typical general radiography grids, used in clinical radiography, are focused to 1000 or 1800 mm, representing the distance between the grid and source of the primary radiation.
Suitable polymeric foams include not only polyurethane foams, but also foams, for example, from polyimides, among other polymer materials.
In another group of preferred embodiments, the focused grid of the present invention comprises an enclosed frame, comprising at least a pair of opposed side pieces, each supporting and positioning a ribbon of the material forming the grid walls, and each side piece provided with a plurality of slots, or other openings, so spaced and disposed as to hold, preferably at each end, the material forming the walls defining the channels, in the proper alignment. The slots are so disposed relative to each other as to cause the wall materials held in the slots, to be in a configuration to focus any radiation impinging on one face of the grid to converge at a focus point, or line, beyond the opposing surface of the grid.
Referring now to the embodiments of
Referring now to
Referring now to
In one embodiment the slits are present only on two sides of the grid frame (
In this embodiment the scatter rejection capability would be similar to a conventionally fabricated linear grid with the same grid ratio, the same metal thickness and composition of channel walls, except that in accordance with this invention, the presence primarily of air in the channels between the metal ribbons ensures that the transmission of the primary radiation is substantially unimpeded and therefore superior.
In a second embodiment, shown in
In both embodiments the resulting grid is preferably covered on both top and bottom surfaces with thin Mylar polyester sheets to prevent entry of foreign materials, such as dust, into the open channel spaces, that might cause image shadows. Mylar film is substantially transparent to x-rays.
In another potentially less costly embodiment, the foil ribbons are made of lead or bismuth foil, possibly mixed in alloys also containing tin, antimony, indium or cadmium. In at least some of these cases, the resulting foil will not have sufficient tensile strength to be held in tension on the frame 24, and will require reinforcement with, e.g., a thin layer of Mylar or Kapton tape, such as on one or both surfaces of the foil. The films of Mylar or Kapton used for such tapes are usually about 1 mil in thickness. Alternatively a single steel reinforcing ribbon of the same thickness can be used. Alternatively, the heavy metal can be plated on high tensile strength steel or stainless steel foils rather than Mylar. The steel foil need only be 20-50 microns thick.
A preferred process for assembly of this reinforced foil is shown in
In the embodiment employing reinforced lead or alloy foils, it will not be possible to fold the ends of the ribbons creating a similar stop to fix the ribbon in position and to align it with the centerline of the bracket slit. Referring now to
In an example of this embodiment, a grid is prepared to reduce the scatter radiation for image receptors up to 43 cm×43 cm in size, where the x-ray source 10 focus to the image receptor 14 is a distance of 100 cm. The grid ribbons are constructed of tungsten foils 10 mm high (“L”) and 100 microns in thickness, and cut to a length of 44 cm. A length of 4 mm at each end is folded to produce the triangular stop 34. Brackets 44 are produced with slits 28 cut by wire electrical discharge machining (wire EDM) or laser cutting, to provide a slit width of 150 microns. The slits 28 would be spaced along the brackets with an angular alignment between center planes of 0.0573 degrees with respect to the x-ray focus and a depth of 10.5 mm. The brackets can be constructed of angle steel beams with L-shaped cross-sections with a thickness of 3 mm and web diameters of 11 mm. The grid frame is constructed of mild steel alloy with an inner open area of 45 cm×45 cm with a thickness of 3 mm and a depth of 15 mm. The heavy metal ribbons preferably should be arranged so as to be separated by a distance of about 1 mm.
One embodiment of the tensioning mechanism for the adjustment of the movable frame bracket 56, as shown in
In a further preferred example of this embodiment the outer grid frame of the first example is increased in depth to accommodate a second layer; The second layer contains a second set of brackets and ribbons essentially identical to that in the first layer, except that the slit separation angle would be increased to 0.0579 degrees with respect to the x-ray tube focus.
Continuing with the description of
The x-ray-transparent solid materials, forming the main body of the grids of this invention are preferably formed of a rigid polymer composed mainly of relatively low atomic number elements, (e.g., Hydrogen and Carbon, and possibly Oxygen, and Nitrogen) and have a physical density preferably less than 1.2 g/cm3 and are thus substantially transparent to x-rays. The x-ray transparency of these materials can be further enhanced by adding a foaming agent or micro bubbles to the polymer formulation during the molding process to further reduce the density, and thus increase the transparency of the final material.
The unscattered and focused beams may or may not have been attenuated when passing through the object being examined. X-rays which were scattered during the passage through the object being examined, do not follow a focused path through the channels of the focused grid, and thus intersect and impinge upon the x-ray absorbent boundaries of the focused channels and are thus absorbed by these layers. The absorbent layers are preferably formed of heavy metals such as Lead, Bismuth, Tungsten, or Tantalum. The layers of metal can also be made from low melting point alloys such as Low 117, Low 251 and Low 281, which are alloys of Bismuth with various combinations of Lead, Strontium, Cadmium and Indium. The layers of metal may not necessarily be pure metal, but may contain binding agents (e.g., polymers) to bind a relatively high concentration of heavy metal in the form of a fine powder or as a compound mainly containing elements with atomic numbers greater than 40. Depending on the application for which the antiscatter grid is being used, the relatively high concentration of heavy metals may be in the range of 40% to 98% by weight or volume of the absorbent layer.
Currently available grids are typically specified in terms of grid ratio, i.e., the ratio of channel depth to channel diameter or width. The same approach can be used for the focused grid of the present invention where the grid ratio is (L/W) (i.e., the ratio of channel depth, L, to channel width, W). A desirable set of dimensions for a grid—particularly a grid used generally for radiography purposes—is that the channel width, i.e., W, is approximately 1 mm. Thus, for a highly preferred range of grid ratios of 8:1 to 16:1, the channel depth will fall in the range of 8-16 mm.
An important performance characteristic of a grid is called the primary transmission P, which is defined by the following formula:
P=s/(s+t)e−μ(E)L
where t is the metal layer thickness as shown in
The primary transmission P represents the percentage of transmission that passes through the x-ray transparent material for a certain width, W, and depth, L, of the material and metal layer thickness, t. For a channel width, W, of 1.414 mm, the metal layer thickness would range from 0.0525 to 0.25 mm for primary transmissions, P, that range from 95% to 80% without the x-ray transparent material, respectively. For a channel made with a polymer material, the attenuation coefficient, μE, will vary with x-ray energy and with polymer density, which desirably should be less than 1.2 g/cm3. Considering the geometry and attenuation of the polymer material, the total primary transmission at 50 keV will range between 61% and 72% depending on metal thickness, t, and the density of the polymer material.
The various aspects, characteristics and architecture of the device and method of the present invention have been described in terms of the embodiments described herein. It will be readily understood that the embodiments disclosed herein do not at all limit the scope of the present invention. One of ordinary skill in the art to which this invention belongs can, after having read the disclosure, may readily implement the device and method of the present invention using other implementations that are different from those disclosed herein but which are well within the scope of the claimed invention, as defined by the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2015/041749 | 7/23/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62028161 | Jul 2014 | US |
