WO2017037043A1

WO2017037043A1 - Grid for selective transmission of x-ray radiation, and method of manufacturing such a grid

Info

Publication number: WO2017037043A1
Application number: PCT/EP2016/070378
Authority: WO
Inventors: Christiaan Kok; Harry Peter Marcel KLEIJNEN; Martinus Adam Petrus VAN DEN BOSCH; Peter van der STAP; Ezgi BERBER
Original assignee: Koninklijke Philips N.V.
Priority date: 2015-08-31
Filing date: 2016-08-30
Publication date: 2017-03-09

Abstract

A grid (1) for selective transmission of electromagnetic radiation and a method for manufacturing such grid is proposed. Therein, the grid (1) for selective transmission of electromagnetic radiation comprises a structural element (2) comprising a plurality of particles (19) comprising a radiation-absorbing material. The particles (19) are sintered together such that pores (21) are present between neighbouring particles (19). The pores (21) are at least partially filled with a polymer. The polymer increases the mechanical stability of the grid and improves the handling properties.

Description

Grid for selective transmission of x-ray radiation, and method of manufacturing such a grid

FIELD OF THE INVENTION

The present invention relates to a grid for selective transmission of X-ray radiation, to a method of manufacturing such grid and to a medical imaging device comprising such grid.

BACKGROUND OF THE INVENTION

Grids for selective transmission of X-ray radiation may be used for example in medical imaging devices such as computed tomography scanners (CT), standard X-ray scanners like C-arm, mammography, etcetera. These grids may also be used for selective transmission of photons in single photon emission computed tomography devices (SPECT) or for selective transmission of positrons in Positron Emission Tomography scanners (PET). Other devices, such as non-destructive X-ray testing devices, may also use such grids. The grid may be positioned between a source of electromagnetic radiation such as X-ray radiation and a radiation- sensitive detection device. For example, in a CT scanner, the source of electromagnetic radiation may be an X-ray tube whereas in SPECT/PET a radioactive isotope injected into a patient may form the source of electromagnetic radiation. The radiation- sensitive detection device may be any arbitrary radiation detector such as a CCD-device, a scintillator based detector, a direct converter etc. A grid may be used to selectively reduce the content of a certain kind of radiation that must not impinge onto the radiation-sensitive detection device. The radiation reduction is usually being realized by means of radiation absorption. In a CT scanner, the grid may be used to reduce the amount of scattered radiation that is generated in an illuminated object as such scattered radiation may deteriorate the medical image quality. As today's CT scanners often apply cone -beam geometry, hence illuminate a large volume of an object, the amount of scattered radiation is often superior to the amount of the medical information carrying non-scattered primary radiation. For example, scattered radiation can easily amount to up to 90 % or more of the overall radiation intensity, depending on the object.

Therefore, there is a large demand for grids that efficiently reduce scattered radiation. Grids that do fulfil this demand may be grids that have radiation absorbing structures in two dimensions that are called two-dimensional anti-scatter-grids (2D ASG). As such two-dimensional anti-scatter-grids may need to have transmission channels that are focussed to a focal spot of the radiation source that emits the primary radiation which shall be allowed to be transmitted through the grid, it may be time-consuming and costly to manufacture such grid.

WO 2008/007309 Al, describes a grid for selective transmission of electromagnetic radiation with structural elements built by selective laser sintering. Therein, a method for manufacturing a grid comprises the step of growing at least a structural element by means of selective laser sintering from a powder material, particularly a powder of an essentially radiation-opaque material. Selective laser sintering allows for a large design freedom. Having a structural element that is built by selective laser sintering, the grid can be a highly complex three-dimensional structure that is not easily achievable by conventional moulding or milling techniques.

WO 2010/016026 Al describes a grid for selective transmission of

electromagnetic radiation and a method for manufacturing such grid.

The mechanical stability and handling properties of conventional sintered grids may have to be further improved. Furthermore, the manufacturing of such sintered grids may have to be further simplified.

SUMMARY OF THE INVENTION

Accordingly, there may be a need for a grid for selective transmission of X-ray radiation and for a method of manufacturing such grid as well as for a medical imaging device using such grid wherein the mechanical stability and/or the handling properties of the grid are further improved. Additionally there may be a need for a structural element and for a method of manufacturing such element wherein the mechanical stability and/or the handling properties of such element are further improved.

These needs may be met by the subject-matter according to one of the independent claims. Advantageous embodiments of the present invention are described in the dependent claims.

According to a first aspect of the present invention a grid for selective transmission of electromagnetic radiation is proposed. The grid comprises a structural element comprising a plurality of particles comprising a radiation-absorbing material wherein the particles are sintered together such that pores are present between neighbouring particles and wherein the pores are at least partially filled with a polymer substantially free of X-ray radiation-absorbing material. The wording free of X-ray radiation-absorbing material is to be interpreted as free of material that substantially absorbs X-ray radiation. Examples of such materials are: metals, materials with high atomic number.

According to a second aspect of the present invention a method of manufacturing a grid for selective transmission of electromagnetic radiation is proposed. The method comprises:

-providing a structural element comprising a plurality of particles comprising a radiation- absorbing material wherein the particles are sintered together and pores are present between neighbouring particles;

-providing a liquid substantially free of X-ray radiation-absorbing material and comprising a liquid oligomer or a polymer dissolved in a solvent onto the outer surface of the structural element thereby penetrating the pores;

-removing excess liquid from the outer surface; and

-solidifying the liquid by at least one of a curing step and a drying step thereby either converting the liquid oligomer into a solid polymer or removing the solvent from the liquid.

According to a third aspect of the present invention, a medical imaging device such as a CT-scanner, X-ray C-arm system, X-ray mammography system, a SPECT-scanner or a PET-scanner comprising a grid according to the above second aspect of the present invention is proposed.

Further a structural element is proposed. The structural element comprises a plurality of particles comprising a metal wherein the particles are sintered together such that pores are present between neighbouring particles and wherein the pores are at least partially filled with a polymer. The metals may, for example, be non-magnetic metals in case this property is important for the application, e.g. near strong magnetic fields in order to prevent attraction forces. Magnetic metals may be used in other cases when other properties of the metal are important such as melting point, strength etcetera.

There may be applications in the medical domain which are not primarily designed for selective transmission of electromagnetic radiation such as MRI probes and similar non-magnetic applications. In addition to that, there may be other applications outside of the medical field, for instance in luggage inspection, and non-destructive testing. The other applications may include aerospace, space, nuclear energy or industrial counter weights where the selective transmission of electromagnetic radiation is not the primary purpose but where the mechanical strength and the smoothness of the surface are important. Further a method of manufacturing a structural element is proposed. The method comprises:

-providing a structural element comprising a plurality of particles comprising a metal wherein the particles are sintered together and pores are present between neighbouring particles; -providing a liquid comprising a liquid oligomer or a polymer dissolved in a solvent onto the outer surface of the structural element thereby penetrating the pores;

-removing excess liquid from the outer surface; and

A gist of the present invention may be seen as being based on the following idea:

A core of a grid for selective transmission of electromagnetic radiation may be provided as a structural element which is prepared by sintering particles to each other wherein the particles comprise a radiation-absorbing material. For this purpose, the well- known selective laser sintering (SLS) process, sometimes also referred to as direct metal laser sintering (DMLS), may be used. Thereby, complex two-dimensional or three-dimensional structures may be realized for the structural element.

However, after the known sintering process, pores of non-filled spaces remain between the sintered particles. It is the finding of the inventors of the present invention that such pores may deteriorate the mechanical stability and integrity of the structural element. Furthermore the laser sintered grids are porous and have a significant surface roughness which hampers the ability of e.g. a vacuum gripper to hold the grid in place during assembly in a medical imaging device or other device. An additional issue with the laser sintered parts is their fragility which is due to the properties of e.g. tungsten. Chipping off of small parts of the structural element due to accidental impact on the structural element is identified as a major problem. The inventors therefore propose to fill the pores with a polymer. Such filling may be achieved for instance by inserting a polymer dissolved in a solvent such that it may flow into the pores. Afterwards, the solvent evaporates such that the polymer remains in the pores. Alternatively a liquid comprising a liquid oligomer may be used to flow into the pores which afterwards is cured, thus forming a solidified polymer. An additional heating step may improve the capillary inflow of polymer into the pores. The mechanical stability of the entire grid is enhanced.

According to an embodiment of the invention the polymer is selected from the group consisting of epoxy resins, polyvinyl butyral (PVB), polyvinyl acetate (PVA) , polyvinyl alcohol (PVOH), ethyl vinyl acetate (EVA) and any mixture thereof. What is needed in case a polymer dissolved in a solvent is used is a tacky, glue-like polymer which is soluble in a low-viscous solvent which is preferably non-toxic (e.g. isopropanol). The mentioned polymers fulfil this requirement. To reduce the viscosity of the solution a relatively low molecular weight polymer resin has to be applied. For PVB, which is a preferred polymer, a broad range of compositions and molecular weights is offered.

In another embodiment an epoxy resin in the form of a liquid comprising a liquid oligomer is used as a starting point. In chemistry, an oligomer is a molecular complex that consists of a few (<5) monomer units, in contrast to a polymer, where the number of monomers is, in principle, not limited. The liquid may additionally comprise a curing agent and the curing agent will polymerize the liquid oligomer. Additionally a viscosity reduction agent (or diluting agent) may be added to reduce to viscosity of the liquid. This will improve the inflow of liquid into the pores of the structural element.

The grids are especially targeted for the medical field and are used for collimating X-rays or suppressing X-ray scattered radiation. It is further advantageous that the grid may have a surface structure with a reduced porosity which improves the ability of the vacuum grippers to hold the part. At the surface of the structural element, the polymer may help to smoothen the rough surface provided by the sintered particles thereby providing smooth wall surfaces for the structural element.

It is especially advantageous that the radiation-absorbing material is selected out of a group comprising molybdenum and tungsten. These materials show a high X-ray absorption.

In other words, the proposed concept may be seen as an improved method for precise and cost-effective manufacturing of for example two-dimensional anti-scatter-grids for X-ray and computed tomography detectors but also for other applications such as structural elements for applications mentioned above. The approach combines the use of prefabricated anti-scatter-grids, e.g. manufactured by laser sintering technology. The method provides a maximum design freedom and an optimization for X-ray absorption and mechanical stability as well as production speed and costs. The method could be also used for the fabrication of many other small but high precision devices where the combination of prefabricated laser sintered structures would require improved mechanical stiffness.

In the following, further possible features, details and advantages of embodiments of the present invention are mentioned. The structural element provided as a starting core for the grid may be provided in any two-dimensional or three-dimensional geometry which is suitably adapted for selectively transmitting electromagnetic radiation. For example, the structural element may have vertical walls which are slightly tilted such as to be directed to a focal point of a source for the electromagnetic radiation. Surfaces of the structural element are not necessarily plane and may be curved, e.g. spherically shaped. Particularly, a two-dimensional grid having focused channels may have a spatially rather complex structure. The channels may have a rectangular or hexagonal inner shape which requires channel walls having different angulations.

The particles from which the structural element is formed by sintering comprise a radiation-absorbing material, preferably an X-ray absorbing material. Therein, it may depend on the application and/or on the structure size, e.g. the thickness of radiation absorbing channel walls, whether the powder material formed by the particles can be considered as radiation-transparent or radiation-absorbing or radiation-opaque. Herein, the term radiation-transparent shall be defined as absorbing a, referred to a specific application, insignificant portion, e.g. less than 10%, of the incident radiation upon transition through the grid. The term radiation-absorbing shall be defined as absorbing a significant portion, e.g. more than 10%, and the term radiation-opaque shall be defined as absorbing essentially all, e.g. more than 90%, of the incident radiation upon transition through the grid. In

mammography applications, X-ray energies of about 20 keV may be used. For these energies, copper (Cu) can be considered as essentially radiation-opaque which means that grid walls fulfilling the requirements of certain geometry parameters like wall thickness (e.g. 20 μιη), channel height (e.g. 2 mm) etc. lead to absorption of the kind of radiation that is to be selectively absorbed so that a noticeable improvement of a quality parameter of the radiation detection occurs. A quality parameter may be the scatter-radiation-to-primary-radiation ratio (SPR), the signal-to-noise ratio (SNR) or the like. For CT applications in the range of e.g. 120 keV, molybdenum (Mo) or other refractory materials (e.g. tungsten) can be considered as essentially radiation-opaque but other materials like copper or titanium are likewise essentially radiation-opaque if the structure is made in the appropriate thickness.

Consequently, the material particles or powder may be considered as radiation-opaque if the resulting grid has satisfying selective radiation transmission properties. As the sintered structural element is directly made from a radiation-absorbing or radiation-opaque material, the required radiation- absorbing properties of the grid are inherent to the sintered structural element. For sintering the radiation-absorbing particles together, the well-known selective laser sintering (SLS) process may be used. In SLS, a powder material is sintered together using a fine laser beam of appropriate energy. The object to be made is sintered layer by layer and the resulting object is subsequently immersed in the powder material so that a next layer of powder material can be sintered on top of the already sintered structures. In this way, rather complex three-dimensional structures can be formed, e.g. having cavities, combinations of convex and concave structural elements, etc. Selective laser sintering allows for generating fine structures from e.g. molybdenum powder by selectively illuminating the top powder layer with a high-intensity laser beam. The grain size of the metal powder may be chosen according to the required structure size and surface roughness. Typical structure sizes (channel wall thickness) for e.g. CT grids are about 50μιη to 300 μιη such that grain sizes of about Ιμιη - 10 μιη may suffice. For PET/SPECT devices, typical structure sizes (channel wall thickness) may be about 100 to 1000 μιη so that grain sizes of about 5 to 50 μιη may suffice. For regular X-ray applications, typical structure sizes may be about 10 to 50 μιη so that grain sizes of about 0.1 to 5μιη may suffice. These numbers are only exemplary and shall not be understood as limiting.

The sintered structural element is now ready for further processing according to the method of the invention.

As an example, the polymer to be filled into the pores of the sintered structural element, can suitably be dissolved into a solvent resulting in a liquid with a viscosity low enough to easily flow into the pores of the structural element. Thereby, the liquid may flow into the pores or cavities of the sintered structural element and fill these pores up to nearly 100 %. Excess liquid is removed from the outer surface. After evaporation of the solvent the polymer will enhance the mechanical stability of the structural element.

The liquid will not only flow into pores deep inside the structural element but will also at least partially fill open pores at the surface of the structural element. Excess liquid is remove from the outer surface. Advantageously the polymer is further smoothened by heating the structural element in an oven during a predetermined period at a predetermined temperature. Thereby the surface roughness of the structural element is further reduced.

Alternatively a liquid comprising a liquid oligomer is provided onto the outer surface of the structural element thereby penetrating the pores. Excess liquid is removed from the outer surface. The liquid is solidified by a curing step thereby converting the liquid oligomer into a solid polymer. A curing agent is preferably added to the liquid to polymerize the oligomer. Preferably a viscosity reduction agent is added to the liquid to lower the viscosity in order to improve the inflow of liquid into the pores.

The structural element forming the core of the grid may have partial structures having different dimensions in different extension directions. For example, it may have vertical longitudinal walls having a wall thickness wherein the wall thickness is much smaller than the longitudinal extension of the wall and therefore forms a minimum structure dimension. For example, the wall thickness can be between 10 and 1000 μιη. Accordingly, the particles which are used to form such partial structures must have a particle size being substantially smaller than the minimum structure dimensions. In conventional grids being formed by selective laser sintering, very small particles are usually used for forming the partial structures in order to avoid large pores or voids within the partial structures. Particle sizes being smaller than 5 % of the minimum structure dimensions have been conventionally used. With the manufacturing method proposed herein, the size of the pores between neighbouring particles is much less critical than in the prior art as the pores may be subsequently be filled with a polymer. Accordingly, the structural element may be sintered using larger particles having sizes of e.g. 10% or more preferred up to 25% of the wall thickness which may substantially simplify the sintering process.

It shall be noted that the "maximum particle size" is referred to as the size the largest particles contained in a powder batch have. Usually, a powder batch has particles of different sizes. In conventional grid building techniques it may be preferred to use powder batches with mainly small particles to reduce the number and size of pores. However, a small portion of larger particles may not significantly deteriorate the overall result whereas to many large particles may lead to a very porous grid structure. With the method presented herein, powder batches having many large particles, wherein e.g. 90% of all particles are larger than 10% of the minimum structure dimensions of the grid, may be used without significant detrimental effect on the resulting grid.

Finally, some features and advantages of the present invention are repeated in another wording. An essential feature of the proposed manufacturing method may be seen in a post-processing of sintered geometries. The "rough" sintered feature may be dipped into a bath of a liquid comprising a polymer dissolved in a solvent or a liquid comprising a liquid oligomer to fill the still porous wall structure. The liquid would penetrate into the cavities or pores and therefore the mechanical stability will increase after solidification of the liquid and also the production efficiency of the sinter process may be improved. This may be because the grain size could be bigger and also the laser power could be used more efficiently and the laser focus could be bigger. So, the sintered structural element may be built with more rough grains and the pores in between be filled with polymer. Preferably with an additional heating step of the polymer the surface will be much smoother.

It has to be noted that aspects and embodiments of the present invention have been described with reference to different subject-matters. In particular, some embodiments have been described with reference to the method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination or features belonging to one type of subject-matter also any combination between features relating to different subject-matters, in particular between features of the apparatus type claims and features of the method type claims, is considered to be disclosed with this application.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will be further described with respect to specific embodiments as shown in the accompanying figures but to which the invention shall not be limited.

Fig. 1 shows an elevated perspective view of a grid structure including channels according to an embodiment of the present invention;

Fig. 2 schematically depicts a manufacturing method for a grid structure according to an embodiment of the present invention;

Fig. 3a show a photograph of an untreated structural element (right) and a treated structural element (left) both after being subject to a stress test according to Fig. 3B;

Fig. 3b shows the corresponding stress / strain graph of the two samples of Fig. 3A;

Figs. 4a and 4b show sectional views of walls within a grid structure according to an embodiment of the present invention;

Fig. 5 shows a perspective view of an example of a medical imaging device with a grid according to an embodiment of the present invention.

The drawings in the figures are only schematically and not to scale. Similar elements in the figures are referred to with similar reference signs. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An exemplary embodiment of a method of manufacturing a grid for selective transmission of X-ray radiation according to the invention will be described with reference to Figs. 1, 2 and 4.

A grid 1 comprises a 3-dimensional structural element 2 including vertical walls 3 arranged perpendicular to each other. As can be clearly seen in the enlarged portions of Fig. 1, the walls 3 form longitudinal channels 5 through which electromagnetic radiation can easily pass. However, radiation which is irradiated under an angle not parallel to the channels 5 will be absorbed within the walls 3 as the walls 3 comprises a radiation-absorbing material.

As schematically shown in Fig. 2, the structural element 2 can be built using a selective laser sintering technique. Therein, particles of a radiation-absorbing material are placed on a substrate 7. The substrate 7 is positioned on a table 9 which can be moved in the y-direction. Using a single laser and, optional, an arrangement for deflecting the laser beam or alternatively using a laser array 11, the particles may be sintered to each other at the location(s) of the focus of one or more laser beams. The laser array 11 may be controlled such that the location(s) of the focus of the one or more laser beams are scanned in x- and z- directions over the surface of the substrate in accordance with a 3-dimensional model 13 stored on a control unit 31 connected both to the laser array 11 and the table 9. After having scribed a first layer 15 of sintered particles, the table 9 can be moved downwards, the particles can be again evenly distributed over the surface of the already existing sintered structure and a second layer 17 of sintered particles can be generated using the laser array 11. Accordingly, the 3-dimensional model 13 stored in the control unit 31 may be reproduced by sintering particles layer-by-layer.

After having prepared the structural element 2 comprising a plurality of particles 19 comprising a radiation- absorbing material wherein the particles 19 are sintered together and pores 21 are present between neighbouring particles 19, a liquid comprising a liquid oligomer or a polymer dissolved in a solvent is provided onto the outer surface of the structural element thereby penetrating the pores 21 the pores e.g. by submerging (dipping) the structural element 2 into a bath of liquid as an exemplary embodiment. A suitable submerging time is about 5 min so that the liquid will penetrate in substantially all pores. Suitable polymers are e.g. epoxy resins, polyvinyl butyral (PVB), polyvinyl acetate (PVA), polyvinyl alcohol (PVOH), and ethyl vinyl acetate (EVA). PVB is the preferred polymer e.g. known under the commercial name Mowital B30H or B20H. The preferred solvents are ethanol or isopropanol. However it will also work with other tough polymers (glues) which can be dissolved in a volatile solvent. The preferred weight percentage of polymer B30H in the solvent ethanol is about 5%. However this may vary depending on the polymer and solvent used. The more determining factor is the viscosity of the solution. For a 5 weight % solution of Mowital B20H in isopropanol this will result in a viscosity of about 9 mPas at 20 degrees C and for a 5 weight % solution of Mowital B30H in isopropanol this will result in a viscosity of about 12 mPas at 20 degrees Celsius.

For liquid epoxy resin based embodiments, typical viscosity values are preferably lower than 350 mPas, more preferred below 200 mPas, where contact angles between epoxy and metal surfaces are preferably lower than 40 degrees, more preferred below 35 degrees. This provides suitable wetting conditions.

Due to the high porosity of the (laser) sintered material the liquid will penetrate into the walls and removing excess liquid from the outer surface can be achieved with e.g. a strong air flow. The liquid must not be too viscous because a high viscosity will prevent penetration of the liquid into the pores and will counteract removal of excess liquid. Removing the solvent from the penetrated polymer dissolved in a solvent is performed by a drying step. The drying step may be performed at room temperature, e.g. by letting the solvent evaporate during several hours or even overnight. Optionally, after the dipping step, the structural element may be put into an oven in which the residual solvent will evaporate rapidly and the polymer will melt. The liquid polymer will flow into the cracks of the walls. Besides that the structural elements become stronger and more resilient to external forces and chipping is reduced significantly the heating step in the oven also improves the geometrical accuracy of the outer walls. For the optional additional step of heating the structural element in an oven during a predetermined period at a temperature must preferably be set above the melting temperature range of the polymer. In case of Mowital B20H and B30H the melting (or softening) ranges are respectively 130-150 degrees Celsius and 140-160 degrees Celsius.

In another embodiment, a liquid comprising a liquid oligomer such as a Bisphenol A based epoxy resin mixed with a curing agent (multi-component pre-polymer) in a weight ratio of 10:3 is used. As liquid epoxy resin a mixture of bisphenol- A- (epichlorhydrin) and l,4-bis(2,3 epoxypropoxy)butane and Phenol-Formaldehyde Polymer Glycidyl Ether may be used. Examples of curing agents are 3-aminomethyl-3,5,5- trimethylcyclohexylamine or Poly(oxypropylene) diamine and mixtures thereof. In addition, a viscosity reduction agent (diluting agent) may be added such as 1,4-Butanediol diglycidyl ether. This improves the inflow of liquid into the pores. After the inflow the structural element is heated at 60 °-100° Celsius for 4-16 hours.

An important class of epoxy resins is formed from reacting epichlorhydrin with bisphenol A to form diglycidyl ethers of bisphenol A. The simplest resin of this class is formed from reacting two moles of epichlorhydrin with one mole of bisphenol A to form the bisphenol A diglycidyl ether (commonly abbreviated to DGEBA or BADGE). DGEBA resins are transparent colourless-to-pale-yellow liquids at room temperature, with viscosity typically in the range of 5-15 Pa.s at 25°C. Bisphenol F may also undergo epoxidation in a similar fashion to bisphenol A. Compared to DGEBA, bisphenol F epoxy resins have lower viscosity and a higher mean epoxy content per gramme, which (once cured) gives them increased chemical resistance.

Another large benefit of the heating step is the further decrease in porosity of the structure which further improves the ability of handling the structural element e.g. for the vacuum grippers to hold the part.

In Fig. 3a, a photograph is shown wherein at the right a grid is shown without the polymer PVB, and at the left with PVB. Both grids were exposed to the same force in a tensile tester at one edge. It can be clearly seen that the grid treated with PVB has far less damage at the edge and does not show chipping and only shows some minor bending while the grid without PVB shows pulverization and chipping and far more extensive damage. The chips on the table which are visible in the photo are all originating from the grid at the right. Similar results are achieved with cured epoxy resins as described before.

In Fig. 3b the left curve is the stress-strain curve for the grid treated with PVB. The curve does not show kinks which indicates that no walls collapsed or broke. The grid only bends and does not crack. The right curve of the grid without PVB treatment shows the typical kinks caused by breaking of material. It can be concluded that the PVB treatment substantially increases the strength of the grid.

In Figs. 4a and 4b, magnified sectional views of the walls 3, 3' included in the structural element 2 are shown. The walls may have a rectangular cross-section as shown in Fig. 4a or a wedge-like cross-section as shown in Fig. 4b. Particles 19 of radiation- absorbing material such as molybdenum or tungsten are sintered together. Pores 21 both at the inside of the wall 3, 3' as well as at its surface are filled with a polymer.

In Fig. 5, an example of a medical imaging device 200 is shown. Fig. 5 shows the main features of a CT scanner, namely an X-ray source 220, a radiation detector 210 and a patient couch 230. The CT scanner may rotate around the object to be observed and may acquire projection images by means of radiation detection using the detector 210. A grid as described above according to the invention can be used in the detector 210 to reduce the amount of scatter radiation generated in the object to be observed.

Finally, it should be noted that the terms "comprising", "including", etc. do not exclude other elements or steps and the terms "a" or "an" do not exclude a plurality of elements. Also, elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. A grid (1) for selective transmission of X-ray radiation comprising a structural element (2) comprising a plurality of particles (19) comprising a radiation-absorbing material wherein the particles (19) are sintered together such that pores (21) are present between neighbouring particles (19) and wherein the pores (21) are at least partially filled with a polymer substantially free of X-ray radiation-absorbing material.

2. The grid according to claim 1, wherein the polymer is selected from the group consisting of epoxy resins, polyvinyl butyral (PVB), polyvinyl acetate (PVA) , polyvinyl alcohol (PVOH), ethyl vinyl acetate (EVA) and any mixture thereof.

3. The grid according to claim 1 or 2, wherein the grid has a surface structure with a porosity which improves the ability of a vacuum gripper to hold the grid.

4. The grid according to one of claims 1 to 3, wherein the radiation-absorbing material is selected out of a group comprising molybdenum and tungsten.

5. A method of manufacturing a grid (1) for selective transmission of X-ray radiation the method comprising:

-providing a structural element (2) comprising a plurality of particles (19) comprising a radiation-absorbing material wherein the particles (19) are sintered together and pores (21) are present between neighbouring particles (9);

-providing a liquid substantially free of X-ray radiation-absorbing material and comprising a liquid oligomer or a polymer dissolved in a solvent onto the outer surface of the structural element thereby penetrating the pores (21);

-removing excess liquid from the outer surface; and

6. The method according to claim 5, wherein the liquid comprising a liquid oligomer additionally comprises at least one selected from a curing agent and a viscosity reduction agent.

7. The method according to claim 5 or 6, wherein the liquid oligomer is selected from at least one of the group of Bisphenol A and Bisphenol F based epoxy resins.

8. The method according to claim 5, wherein the liquid comprises a polymer selected from the group consisting of polyvinyl butyral (PVB), polyvinyl acetate (PVA), polyvinyl alcohol (PVOH), and ethyl vinyl acetate (EVA).

9. The method according to one of claims 5 to 8, wherein the liquid is inserted into the pores (21) by dipping the structural element into a bath of the liquid.

10. The method according to one of claims 5 to 9, with an additional step of

-heating the structural element in an oven during a predetermined period at a temperature above the melting temperature range of the polymer.

11. A medical imaging device (200) comprising a grid according to one of claims 1 to 4 or made with the method of manufacturing according to one of claims 5 to 10.