EP3584804A1

EP3584804A1 - Method for producing a grid-like beam collimator, grid-like beam collimator, radiation detector and medical imaging device

Info

Publication number: EP3584804A1
Application number: EP18178851.4A
Authority: EP
Inventors: Harald Geyer; Stefan Wirth; Jan Wrege
Original assignee: Siemens Healthcare GmbH
Current assignee: Siemens Healthcare GmbH
Priority date: 2018-06-20
Filing date: 2018-06-20
Publication date: 2019-12-25

Abstract

Method for producing a grid-like beam collimator (1) by printing, wherein a metal particles (5) containing suspension is printed in several stacked layers, whereby the used suspension additionally comprises bonding particles (6) made of or comprising a bonding material (8) with a melting temperature lower than the softening temperature of the metal particles (5), and that energy is introduced into the printed layer stack so that the melted bonding material (8) bonds the metal particles (5), whereupon the melted bonding material is cured for fixing the layer stack.

Description

The invention refers to a method for producing a grid-like beam collimator by printing, wherein a metal particles containing suspension is printed in several stacked layers.
Grid-like beam collimators, also referred to as scattered radiation grids, are used in X-ray systems like CT Systems (CT = computed tomography) or angiography systems to absorb scattered radiation. Usually stacks of thin metal plates bonded in a support mechanism were used for building such a collimator. The thin metal plates made of tungsten or molybdenum where inserted and glued in a plastic carrier. This collimators allow the suppression of scattered radiation in the phi direction, in other words in the gantry rotation direction of a CT system. As it is know, that a scattered beam correction is much more effective with the collimator acting in the phi direction and the z direction than with a simple phi collimator, particularly in dual source systems, a new type of collimators was developed. Among these collimators printed collimators are known, see for example DE 10 2012 206 546 A1 . A suspension or paste comprising a carrier medium with metal particles is screen printed in several layers to form a layer stack. As metal particles tungsten, molybdenum, tantal or copper are used. When printing layer after layer a three dimensional grid or web structure is built. By changing the sieve to smaller openings, it is also possible to adapt the webs of the grid to a focus, so that the openings or channels have a frustum form.
After printing the layer stack or web it is necessary to cast out the binder and to sinter the web structure for hardening it. During this sintering process the web structure is heated, so that the metal particle are firmly sintered together making the collimator or anti scatter grid stable.
This sintering process bears the problem, that the printed web structure or stack shrinks in an uncontrolled manner, while this shrinking is not equal in all parts of the web structure, so that it is not possible to predict the final thickness of the webs of the grid and the respective position at the sintered grid structure. A process stability is hard to achieve, the performance of the produced collimator or grid differs from one collimator to the other.
It is therefore an object of the invention to provide a method for producing a grid-like beam collimator which allows more precise production of a printed collimator.
According to the invention, this problem is solved by a method which is characterized in that the used suspension additionally comprises bonding particles made of or comprising a bonding material with a melting temperature lower than the softening temperature of the metal particles, and that energy is introduced into the printed layer stack so that the melted bonding material bonds the metal particles, whereupon the melted material is cured for fixing the layer stack.
The inventive method allows the production of a stable scatter grid or collimator without sintering the printed web structure. The suspension used according to the invention comprises, aside the metal particles, bonding particles made of or comprising a bonding material, which bonding material is used for bonding the metal particles, so that finally the printed web structure is a stiff and stable structure. The bonding particles are made of or comprise a bonding material, which has a melting temperature, which is way lower than the softening temperature of the metal particles, for example tungsten or molybdenum. The bonding material is therefore in a melted or liquid state at a very low or comparatively low temperature compared with the very high softening temperature of the metal particles, which in the prior art needs to be reached for sintering the web structure. As the bonding material is melted or liquid at this low temperature it can bewet the metal particles respectively flow between these particles so that the metal particles can be connected among each other at least in parts. When the melted bonding material is finally cured and hardened the metal particles are fixed in their position, so that the layer stack respectively the web structure is finally fixed and stable. As this stabilization is realized at a very low temperature, as, as explained above, the bonding material melts or softens at a low temperature, no sintering is necessary. As therefore the web structure is not heated to a very high sintering temperature, no uncontrolled shrinking will occur. The final structure still has the same geometry and, referred to its webs, the same web width and height also after the final curing step. Webs with a plane and smooth surface can be realized as they maintain their geometry and dimensions. The collimator produced according to the inventive method will therefore show very precise shape and dimensions, so that it can be produced with very high precision and process stability.
The bonding material is preferably a polymer. Among the polymers any polymer can be used, which exhibits a low melting temperature and which can be cured for fixing the layer stack. The term "polymer" also comprises any kind of glue or adhesive or binding agent fulfilling the respective properties.
According to a first embodiment of the invention the bonding particles may comprise a hollow shell filled with the melted material, which shall brakes when the energy is introduced allowing the melted material to contact the metal particles. As depicted above energy is introduced into the web or grid structure, so that the metal particles are bewetted by the melted or liquid bonding material. According to this first embodiment the bonding material is already in a melted or liquid state. The bonding particles comprise a hollow shell filled with the melted or liquid bonding material. These shell particles are mixed in the suspension, so that the bonding material is already in a melted or liquid state after the printing of the layer stack is finished. The energy introduced into the layer stack or web structure is used to break the shell and allowing the melted or liquid bonding material to flow between the metal particles and contact them.
The shell itself can be made of metal having a melting temperature lower than the melting temperature of the metal particles. For breaking this metal shell the energy may be introduced by heating the layer stack to the low melting temperature of the shell material, which is, as explained, way lower than the melting or softening temperature of the metal particles.
In an alternative, the shell material may also be a cured polymer. Also this polymer shell has a melting or softening temperature way lower than the melting or softening temperature of the metal particles. Therefore also in this case the energy for cracking the shells may be introduced by heating the layer stack.
In an alternative embodiment it is possible to introduce the shell cracking energy by applying external waves, especially ultrasonic waves to the layer stack. When these waves impinge on the shell, the shell cracks, so that the melted or liquid bonding material respectively polymer can flow between the metal particles and bewet them.
In an alternative embodiment plain bonding particles are mixed into the suspension. These bonding or polymer particles are cured, the bonding material therefore is in a cured state and not melted or liquid as explained to the first embodiment depicted above. It is therefore necessary to transfer the cured bonding particles into the melted or liquid state by energy introduction, which energy is preferably introduced by heating the layer stack. As the bonding particles respectively the bonding material has a softening or melting temperature way lower than the softening and melting temperature of the metal particles, also in this embodiment only a very soft heating of the layer stack is necessary, the temperature merely needs to be as high as the melting temperature of the bonding material, so that it becomes melted or liquid and can bewet the metal particles.
As explained above, for finally stabilizing the layer stack or web structure it is necessary to cure the melted or liquid bonding material after it has bewetted and contacted the metal particles for finally stabilizing and fixing the web structure. Depending on the bonding material used several curing methods can be used. It is possible to cure the melted bonding material by heating the layer stack. This heat curing may be used for example when the shell particles are cracked by ultrasonic waves, it may also be used when the shell particles are cracked by heating the structure, while the curing temperature may be for example a little bit higher than the cracking temperature.
In an alternative, it may be possible to cure the melted bonding material by maintaining an elevated temperature used for melting the bonding material. When the melted bonding material needs a comparatively longer time for curing at an elevated temperature, it is possible to simply hold an elevated temperature, which for example was used for cracking the shell, also for curing the bonding material.
A third alternative curing method is simply holding the layer stack at room temperature, for example when a shell was cracked by heat or wave introduction, and when the bonding material previously encapsulated in the shells cures at room temperature. Also when the bonding material is directly mixed in a solid state into the suspension and is melted by heat introduction, the curing can take place by simply cooling the web structure to room temperature.
Finally, a curing by UV radiation is possible, when the bonding material used is UV curable. It is only necessary to irradiate the UV light onto the web structure for curing the bonding material.
The inventive method allows the production of a grid-like beam collimator without any heat treatment for sintering the web structure. The danger of a temperature induced shrinking is completely avoided, allowing the production of collimators with a very high precision regarding the geometry and the dimensions. Due to this during the printing no measure need to be taken in view of any changes in the dimensions due to the shrinking, so that more collimators or grid structure can be printed on the same surface as compared to the sintering method according to the prior art.
It is also possible to print the web structure with different web thickness, so that especially thinner webs or walls are possible. It is possible that the outer webs or walls of the printed structure have a lower thickness, so that several printed structures can be placed next to each other with the outer webs or walls contacting each other, so that the effective thickness of these double-walled webs is the same as of the webs in the inner part of the respective grid structures.
Also the printing of slanting webs is possible, so that the webs or the collimator can be focused to an x-ray focus.
Furthermore, the invention refers to a grid-like beam collimator, comprising a grid structure with a number of crossing webs, the grid structure being made of metal particles which are bonded by a cured bonding material. This bonding material is preferably a polymer.
The bonding material can ab applied or distributed within the web structure by any of the above mentioned means during its production, so either by means of polymer filled shells or by using plain polymer particles in the pasty suspension.
Preferably the webs define passage channels in the shape of truncated pyramids, with the longitudinal axes of the channels being aligned to a common focus. The webs, which are preferably remarkably thinner than the channels to avoid shading of active pixels of the radiation active matrix of the radiation detector and to have a high dose application, are slanted or tapered, so that they confine openings or channels of frustum shape, which are aligned with their longitudinal axes to a common focus. This truncated pyramid geometry can be realized with very high precision, as no shrinking occurs according to the inventive way of manufacturing the web structure. During the printing process the cross section varying with the height of the webs respectively the channels in the shape of the truncated pyramids can be produced by replacing the printing screen used at least once or preferably several times with a successively changing printing geometry. In this embodiment the thickness of the webs varies over the height with the webs becoming thinner in the direction to the common focus. This also allows aligning the channels to a common focus.
The collimator can have an overall shape of a truncated pyramid with parallel upper and lower surfaces and slanted sides. Also the webs at the edge of the collimator can be slanted. This enables arranging several smaller collimators in a row for building a polygonal ring collimator for arranging in the gantry of a CT system. Certainly such a truncated collimator can also be used as a single collimator for example in a X-ray application comprising a movable C-bow or the like. Alternatively it can have a cuboid shape with parallel and vertical edge webs, which embodiment is preferably for single use of the collimator. This collimator having thicker edge webs is advantageous as it shows a higher stability when finally being hardened.
In an alternative the printed collimator can also be printed in a bent form or can be mechanically bent after printing for producing smaller collimators which can be arranged in a row for building a ring shaped collimator for a gantry of a CT system. Due to the large radius from the collimator ring to the central focus the mechanical deformation is small allowing to maintain the focus. For bending the printed but not yet hardened collimator one or more cylindrical drums can be used.
The invention also refers to a radiation detector, especially a X-ray detector, comprising a grid-like beam collimator produced according to the inventive method or as depicted above.
Finally the invention refers to a medical imaging system, especially a CT-system comprising a radiation detector as depicted above.
Additional details and advantages of the invention become evident from the embodiments discussed below as well as from the figures. The figures show:

Fig. 1 shows a principle sketch of a part of a printed layer stack in form of a cross-sectional view of a web, with bonding particles comprising a shell filled with a melted or liquid bonding material,
Fig. 2 the web of Fig. 1 after energy introduction and breaking of the shell,
Fig. 3 the web of Fig. 2 after curing the bonding material,
Fig. 4 a principal sketch of a printed layer stack in form of a cross-sectional view of a web with bonding particles directly mixed into the suspension,
Fig. 5 the web of Fig. 4 after energy introduction,
Fig. 6 the web of Fig. 5 after curing,
Fig. 7 shows a principal perspective sketch of an inventive grid-like beam collimator with webs aligned to a common focus,
Fig. 8 shows a principal sketch of an inventive grid-like beam collimator with webs varying in their thickness and aligned to a common focus,
Fig. 9 shows a principal sketch of an inventive grid-like beam collimator being bent,
Fig. 10 shows a flow chart explaining the steps of the inventive method,
Fig. 11 shows a medical imaging system in form of a CT system.

Fig. 1 shows an inventive grid-like beam collimator 1 in a partial view, which collimator comprises a web structure 2 which is realized by means of crossing webs 3. Fig. 1 shows a cross-sectional view of such a web 3. The web structure 2 is produced by printing, especially by serigraphy. A suspension is printed through a sieve to print respective layers, which are printed and therefore stacked above each other for creating a three dimensional web structure 2.
The suspension comprises a liquid carrier medium 4 in which metal particles 5 for example made of tungsten, molybdenum, tantal or copper. The suspension also comprises bonding particles 6, comprising a shell 7 made of metal with a very low melting or softening temperature, which is way lower than the melting or softening temperature of the metal particles 5. The shell 7 can also be made of a polymer which also has a very low melting or softening temperature. The shell is filled with a liquid bonding material 8, preferably a polymer. This bonding material 8 is, when the particles 6 are dispersed in the suspension, already liquid and remains in this liquid state during the printing process.
As all figures are principle sketches, the particles 5 and 6 are only shown in a principle way.
As Fig. 1 shows, the web 3 has a certain width x and height y, which is determined by the sieve used for printing the web structure 2.
When the printing process is finished, energy is introduced into the printed collimator 2 respective the web structure 2. This energy is used for cracking the shell 7 of the respective bonding particles 6. Fig. 2 shows the web structure 2 respectively the web 3 of Fig. 1 after this energy introduction. As shown in this principle sketch, the respective shells 7 of the bonding particle 6 are cracked. This opening of the shells 7 allows the liquid bonding material 8 to flow into the gaps or fissures between the respective metal particles 5. The liquid bonding material 8 can therefore bewet the metal particles 5 and can therefore connect the neighbouring metal particles 5, as it is shown in Fig. 2. The printed layer stack comprises a large number of gaps and fissures, although the particles 5 and 6 are mixed into a suspension liquid 4, due to the printing process. As Fig. 2 shows, the metal particles 5 are all interconnected by means of the melted or liquid bonding material 8, i.e. the polymer. The shells 7 maintain their shape, they only crack and do not collapse. Therefore, the web 3 maintains his geometry, especially his width x and height y.
The energy may be introduced by short heating of the web structure 2, so that each shell 7 partly melts. It can also be introduced by applying ultrasonic waves for cracking the shells 7.
After this step the still melted or liquid bonding material 8 needs to be cured for finally stiffening the web structure 2. The curing method for curing the melted or liquid bonding material 8 depends on the properties of the bonding material 8. It is possible to simply hold the web structure 2 on room temperature, if the bonding material 8 cures at room temperature. It is also possible to heat the web structure 2 to an ambient temperature for curing the bonding material 8. Furthermore, it is for example possible to apply UV radiation for curing the bonding material 8.
After curing the web structure 3 it is stiffened and very stable, due to the hardened, cured bonding material 8 connecting all particles of the web structure 2. The respective web maintains its dimensions x and y, no shrinking occurs. No sintering is necessary.
Figs. 4 to 6 show another embodiment of the inventive method respectively of an inventive grid-like beam collimator. Fig. 4 shows the collimator 1 as a principle sketch, also comprising a web structure 2, while in Fig. 1 a cross-sectional view of a web 3 is shown. Also this web structure 3 is printed by printing a suspension through a sieve (serigraphy), the web structure 2 is printed by stacking several layers above each other to reach a certain height y and a certain width x of each respective web 3.
In this embodiment the suspension comprises a liquid suspension carrier medium 4, in which metal particles 5 for example made of tungsten, molybdenum or the like are suspended. Furthermore, plain bonding particles 6 are suspended in the suspension. These bonding particles 6 are in a solid state, preferably the particles 6 are made of polymer, which has a relatively low melting temperature, compared to the extremely high melting or softening temperature of the metal particles 5.
As Fig. 4 shows the bonding particles 6 are more or less equally distributed between the metal particles 5.
After the printing, energy is introduced into the web structure 2, preferably by heating the web structure 2 to a temperature slightly above the melting temperature of the material of the bonding particles 6, so that they melt and become liquid and flow and distribute between the metal particles 5. As the printed layer stack is relative dense, no change of the dimensions, i.e. the width x and the height y occurs while the web structure 2 is heated for melting the bonding particles 6. The liquid bonding material 8 bewettens the neighbouring metal particles 5, so that they are all contacted among each other.
After this energy introduction step it is necessary to cure the bonding material 8. The curing method depends on the properties of the respective bonding material 8. It is possible to maintain the elevated temperature, which was used for melting the bonding particles 6, when the bonding material 8 slowly cures at this elevated temperature. It is also possible to reduce the temperature or even to cool the web structure to room temperature for curing and solidifying the bonding material.
As another alternative, it is possible to apply UV radiation for curing the bonding material.
Whichever curing method is used, the final web structure 2 is stiffened and stable and still shows its dimensions, so no shrinking occurs.
Fig. 7 shows a principal illustration of an inventive collimator 1. It comprises a grid-like web structure 2 having plane upper and lower sides and with crossing webs 3 (which are shown on the front side by the dashed lines), which are aligned to a common focus, as they are more and more slanted beginning from the center to the sides. The thickness of the webs remains the same over their height. As the webs define passage channels 9 also these channels are focussed, their longitudinal axes are aligned to the common focus. The collimator 1 has the overall shape of a truncated pyramide. This geometry can be produced with the printing process, whereby the printing screen needs to be changed once or several times due to the varying geometry over the height of the web structure 2.
Fig. 8 shows a side view of a collimator 1 of another inventive embodiment, which has also plane upper and lower sides. The web structure 2 with the webs 3 is also aligned to a common focus, but the thickness of the webs varies with the height of the webs. The webs become thinner in the direction to the focus the higher they are. Also this web geometry can be produced with the printing process with changing the printing screen.
Fig. 9 finally shows a collimator 1 with a web structure which is bent for aligning the web structure 2 respectively the webs 3 to a common focus. This bending can either be realized during the printing process, meaning that the webs structure is printed in the bent form. In an alternative the web structure 2 is printed with parallel upper and lower surfaces and is afterwards mechanically bent.
A flow chart for explaining the inventive method is shown in Fig. 10. In step S1 a suspension comprising a binder and metal particles is produced. The metal particles may be of molybdenum or tungsten for example. The viscosity of the suspension is chosen for a sieve printing method.
Additionally bonding particles are mixed into the suspension. The bonding particles may comprise a hollow shell which is filled with a melted or liquid bonding material, especially a liquid polymer. The shell can be made of metal or a cured polymer. Whatever shell material is used it has a melting temperature lower than the melting temperature of the metal particles. In an alternative plain bonding particle can be mixed into the suspension, with also these bonding particles being preferably made of a polymer. Whatever bonding particles respectively bonding material is used it has a melting temperature way lower than the melting temperature of the metal particles.
The pasty suspension is then, see step S2, printed through a sieve defining a web structure to produce a printed web structure respectively a collimator. The structure is built by printing several printed layers above each other for building a layer stack. If a focussed collimator shall be produced the sieve is frequently changed so that the printed webs become slanted. Thus the channels or openings defined by the slanted webs will have a frustum or truncated pyramid form with the longitudinal axes of the channels being all aligned to a common focus. As each channel is bounded by four webs it has a square cross section. It is also possible to vary the web thickness over the structure so that regions with smaller webs and regions with larger webs can be built.
After printing of the web structure energy is applied to the web structure to either crack the shells of the bonding particles to allow the liquid bonding material respectively the liquid polymer to flow into gaps or fissures in the printed web structure or to melt the plain bonding particles so that the melted bonding material can flow into the gaps or fissures of the web structure, see step S3. The energy can be applied by heating the web structure to a temperature where the shells or the plain bonding particles melt, it can also be introduced by applying ultrasonic waves to the web structure especially for cracking the shells. At the end of this energy introduction step the melted or liquid bonding material has bewetted the metal particles at least partially.
Finally, see step S4, the melted or liquid bonding material is cured to harden the webs. This hardening can be done for example by applying a curing means or by applying UV light or by heating the structure to a curing temperature or at room temperature. The needed method is chosen depending on the properties of the bonding material. As preferably a polymer is used as a bonding material the curing is performed preferably with a temperature variation.
Fig. 11 shows a principal sketch of a medical imaging device in form of a CT system 10. The CT system 10 is a dual source system comprising two radiation sources 11 and 12, e.g. X-ray sources, which are offset by an angle of 90°. If further comprises two radiation detectors 13 and 14, with the detectors being also offset by an angle of 90° and arranged opposite to the radiation sources 11 and 12. If the CT system is not a dual source systems only one radiation source with an opposite radiation detector is provided. The source-detector pairs are arranged on a gantry in a gantry housing 15.
Both detectors 13 and 14 each have one or more grid-like beam collimators 1 with one or more scattered radiation grids. The scattered radiation grids are produced as described above, they can be modular in structure and bring about a reduction of scattered radiation in both the phi direction and the z direction. The z direction here is considered to be the coordinate axis and phi is considered to be the rotational direction of the gantry.
Although the present invention has been described in detail with reference to the preferred embodiment, the present invention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention.

Claims

Method for producing a grid-like beam collimator (1) by printing, wherein a metal particles (5) containing suspension is printed in several stacked layers, characterized in that the used suspension additionally comprises bonding particles (6) made of or comprising a bonding material (8) with a melting temperature lower than the softening temperature of the metal particles (6), and that energy is introduced into the printed layer stack so that the melted bonding material (8) bonds the metal particles (5), whereupon the melted bonding material is cured for fixing the layer stack.
Method according to claim 1, characterized in that the bonding material (8) is a polymer.
Method according to claim 1 or 2, characterized in that the bonding particles (6) comprise a hollow shell (7) filled with the melted bonding material (8) are used, which shell (7) brakes when the energy is introduced allowing the melted bonding material (8) to contact the metal particles (6).
Method according to claim 3, characterized in that bonding particles with a shell (7) made of metal having a melting temperature lower than the melting temperature of the metal particles (5) or made of a cured polymer are used.
Method according to claim 3 or 4, characterized in that the energy is introduced by heating the layer stack or by applying external waves, especially ultrasonic waves to the layer stack.
Method according to claim 1 or 2, characterized in that the plain bonding particles (6) are mixed into the suspension.
Method according to claim 6, characterized in that the energy is introduced by heating the layer stack.
Method according to one of the preceding claims, characterized in that the melted bonding material (8) is cured by heating the layer stack or by maintaining an elevated temperature used for melting the bonding material or by holding the layer stack at room temperature or by UV radiation.
Grid-like beam collimator, comprising a grid structure (2) with a number of crossing webs (3), the grid structure (2) being made of metal particles (6) which are bonded by a cured bonding material (8).
Grid-like beam collimator according to claim 9, characterized in that the bonding material (8) is a polymer.
Grid-like beam collimator according to claim 9 or 10, characterized in that the webs (3) define passage channels (9) in the shape of truncated pyramids, with the longitudinal axes of the channels (9) being aligned to a common focus.
Grid-like beam collimator according to one of the claims 9 to 11, characterized in that it has the shape of a truncated pyramid.
Radiation detector comprising a grid-like beam collimator (1) according to one of the claims 9 to 12.
A medical imaging system comprising a radiation detector (13, 14) according to claim 13.
A medical system according to claim 14, characterized in that it is a CT system (10).