EP4092687A1

EP4092687A1 - Structured x-ray attenuators

Info

Publication number: EP4092687A1
Application number: EP21174092.3A
Authority: EP
Inventors: Gereon Vogtmeier; Andriy Yaroshenko; Thomas Koehler; Sven Peter PREVRHAL; David Bernd
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2021-05-17
Filing date: 2021-05-17
Publication date: 2022-11-23

Abstract

A method (100) of manufacturing a structured X-ray attenuator is disclosed that comprises printing (101) a mask structure (20) using an additive manufacturing process, and filling (102) trenches (25), holes and/or indentations of the mask structure with an X-ray attenuating material (27) to obtain a structured X-ray attenuator that locally absorbs X-rays. Walls (21) are printed in the mask to define the trenches and/or holes and/or indentations such that a grid or grating structure is formed. A further aspect relates to the structured X-ray attenuator, e.g. as thus manufactured.

Description

FIELD OF THE INVENTION

The present invention relates to the field of X-ray imaging and the manufacturing of structured X-ray attenuators for use in X-ray imaging, such as interference gratings and (e.g. anti-scatter) grids. More specifically, the invention relates to structured X-ray attenuators and methods of manufacturing such structured X-ray attenuators.

BACKGROUND OF THE INVENTION

In X-ray imaging, grating-based interferometry structures can be used to allow a wealth of information about an X-ray beam interaction with an object under study, e.g. a patient's body, to be recovered, in addition to the more conventional X-ray absorption information that has been studied and used extensively in the past. For example, this additional information can include phase information and small angle scattering information (e.g. known as dark field imaging). These new types of imaging modalities have, for example, been shown to have a significant potential for increasing the diagnostic accuracy of pulmonary disorders such as chronic obstructive pulmonary disease (COPD) and may offer advantages in many other practical applications.
Grating-based interferometry uses finely structured grating elements, e.g. having a spatial period in the range of a few micrometers, that need to be manufactured accurately to create the intended wave interference patterns that can be observed by an image detector. Not only are gratings used to create interfering waves, but the fringe patterns are typically also smaller than the achievable (or economically feasible) detector pixel size, such that gratings are also important to analyze the patterns, e.g. to allow the relevant fringe information to be (e.g. indirectly) resolved by the detector. A need exists for achieving such high spatial manufacturing resolution, a high accuracy and a good reproducibility in an efficient and profitable manner. Furthermore, to be able to image relatively large regions and/or volumes, e.g. anatomical structures in patients, the area of the gratings, over which the fine grating structures need to be provided, can be rather large. While many manufacturing processes are known for accurately producing very small-scale structures, e.g. in the field of semiconductor processing technology, scaling such techniques up to large areas is often not straightforward, cost efficient or even feasible. The adoption of interferometry-based imaging appears to be currently limited by the availability of large scale gratings at a reasonable cost, even in view of the potential diagnostic benefits. Therefore, a cost-effective and scalable manufacturing process would provide a clear advantage in further advancing research and application of this imaging technology.
For example, one of the known techniques for manufacturing x-ray interferometry gratings is the LIGA process (Lithography Galvanoformung Abforming: lithography, electroplating and casting), which uses synchrotron radiation for structuring a photoresist layer. This produces trenches on a thus structured substrate, which can be electroplated with a metal, typically gold, to locally absorb or attenuate x-rays. While this approach has been successfully used to manufacture pre-clinical and clinical prototypes, it is quite expensive and is not easily accessible.
The United States patent application no. US 2012/057677 relates to phase-contrast imaging. Focused gratings are used to reduce the creation of a trapezoid profile in a projection with a particular angle to the optical axis. A laser supported method is used in combination with a dedicating etching process for creating such focused grating structures. Trenches are created in a wafer substrate at a tilted angle with respect to the primary axis that is directed in the direction of the surface normal of the grating, i.e. typically arranged in the direction towards the source of radiation when installed.
It is noted that finely structured X-ray absorber structures for use in X-ray imaging are not necessarily limited to phase-contrast and/or dark-field imaging. Thus, an efficient manufacturing method may equally be applied to produce other types of (e.g. relatively large area, without limitation thereto) devices that interact with radiation on a small scale, e.g. having a nano-, micro- or millimeter scale structure. For example, anti-scatter grids share many of the characteristics of such large-area interferometry gratings, and may be produced in a substantially similar or the same manner.
3D printing techniques are known for manufacturing two-dimensional anti-scatter grids, in which selective laser sintering of tungsten is used. However, this approach is, at least presently, less suitable for achieving a small spatial grating period as required for the (e.g. "G2") gratings in dark-field and/or phase-contrast imaging. For example, the required wall thickness in such grating structure can be in the range of a few micrometer, e.g. well below 30 µm, whereas metal laser sintering technology appears to be unable to reach the required precision in manufacturing such small structures, e.g. due to limits in particle size of the tungsten grains and the optical properties of the sintering laser.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide in good manufacturing methods and systems for producing structured X-ray attenuators, such as X-ray grids and/or gratings, and the structured attenuators thus produced.
It is an advantage of embodiments of the present invention that large grating or grid structures, e.g. having an area in the range of 0.1 m² to 5 m² (without limitation thereto), e.g. in the range of 0.25 m² to 3 m², e.g. in the range of 0.4 m² to 1 m², can be produced at a high spatial manufacturing resolution, e.g. having a grating period of a few micrometers, e.g. in the range of 0.5 µm to 100 µm (without limitation thereto), e.g. in the range of 1 µm to 30 µm, e.g. in the range of 1 µm to 15 µm, e.g. in the range of 2 µm to 10 µm. These ranges are, however, merely illustrative. For example, the manufactured grid or grating structures may also have, in accordance with some embodiments, an area that could be substantially less than 0.1 m², e.g. for use in tiled systems (for example in computed tomography tile modules).
It is an advantage of embodiments of the present invention that the absorber structures can be manufactured with a high accuracy and with good reproducibility.
It is an advantage of embodiments of the present invention that a grating is provided that is suitable for use in X-ray interferometric imaging, e.g. phase-contrast and/or dark-field imaging.
It is an advantage of embodiments of the present invention that these structured X-ray absorbers can be manufactured at low cost and in an efficient manner.
It is an advantage of embodiments of the present invention that a manufacturing process for interferometry gratings or similar structures is provided that is easily scalable to high manufacturing volumes and/or high throughput.
It is an advantage of embodiments of the present invention that a manufacturing process is provided that does not rely on costly systems and/or systems with poor availability, e.g. such as synchrotron systems.
It is an advantage of embodiments of the present invention that a cheap structuring technology can be used to create high aspect ratio structures for a grating.
It is an advantage of embodiments of the present invention that the high spatial manufacturing resolution of nano and/or micro scale additive manufacturing techniques ("3D printing") can be applied to produce grating structures.
It is an advantage of embodiments of the present invention that the structured absorbers can be produced by 3D printing with a polymer material (e.g. a plastic) or a different (e.g. non X-ray absorbing) material, after which the required X-ray absorption properties can be obtained by applying a high atomic number (Z) material, e.g. a metal such as gold, to the printed structure, e.g. filling trenches in the polymer by electroplating (not necessarily excluding other methods).
It is an advantage of embodiments of the present invention that a printed structure can be provided with a coating to improve the adhesion of the attenuating material in a subsequent process step.
It is an advantage of embodiments of the present invention that a conductive material can be deposited in the 3D printing process, e.g. as a (e.g. a single) bulk material or as a separate surface material (e.g. in addition to another material forming the bulk of the printed structure), which can advantageously be used to form a seed layer for filling the trenches of the structure by electroplating.
It is an advantage of embodiments of the present invention that 3D printing techniques offer a high degree of customizability and/or design freedom, which can be leveraged to produce gratings or grids with a non-uniform, irregular and/or specifically shaped structure, e.g. to adapt the grating to a focused geometry. Furthermore, it is easy to adapt the process to produce different gratings without requiring extensive retooling or redesign, e.g. to manufacture gratings with different period or different focal properties.
It is an advantage of embodiments of the present invention that the X-ray attenuating material can be provided on both sides of the printed structure, e.g. both sides of the 3D print can be structured with trenches and filled with a suitable material. Thus, an advantageously large total height (and therefore high attenuation) and good structural integrity of the finished product (e.g. by providing the supporting substrate in a more central layer) can be easily achieved. Moreover, a good alignment between both sides can be ensured, e.g. by structuring both sides in the same additive manufacturing process.
It is an advantage of embodiments of the present invention that grating structures can be easily reinforced and/or stabilized, e.g. using bridge-like connections between the walls forming a trench. It is a further advantage that such bridges or similar stabilizing structures can be manufactured precisely, sparsely and/or with little material, thus minimizing the influence on the imaging process in use. Thus, a high filling rate of the X-ray absorbing material, when provided on the printed structure, can be achieved while simultaneously improving the grating's stability and/or rigidity. It is a further advantage that the stabilizing structures can be easily manufactured in a regular, irregular or even random pattern.
It is an advantage of embodiments that a focused grating can be manufactured without requiring a bending or heating process after manufacturing the basic structure of the grating, e.g. thus avoiding mechanical stresses and potential damage and/or inaccuracies.
It is an advantage of embodiments of the present invention that the high manufacturing quality that is achievable can imply or contribute to a high image quality in use.
A structured X-ray attenuator and method in accordance with embodiments of the present invention achieves the above objective.
In a first aspect, the present invention relates to a method of manufacturing a structured X-ray attenuator. The method comprises printing a mask structure using an additive manufacturing process. The mask structure comprises walls, in which these walls define (e.g. form; e.g. enclose) trenches and/or holes and/or indentations in the mask structure, such that a grid or grating structure is formed. The method further comprises filling the trenches, holes and/or indentations with an X-ray attenuating material to obtain a structured X-ray attenuator that locally absorbs X-rays.
In a method in accordance with embodiments of the present invention, the walls may comprise one or more sets of parallel walls.
In a method in accordance with embodiments of the present invention, the X-ray attenuating material as structured by filling the mask structure may be adapted for use in X-ray interferometric imaging and/or to reduce scatter when used in an X-ray imaging apparatus. For example, the structured X-ray attenuating material may form a grating for use in X-ray interferometric imaging, e.g. for creating sufficient coherence in an X-ray beam (e.g. a source grating "G0"), and/or for creating an X-ray interference pattern (e.g. a middle grating "G1"), and/or for analyzing the interference pattern into detectable patterns (e.g. a detector grating "G2"), in which G0, G1 and G2 refer to the three gratings used in a typical X-ray interferometric imaging setup (without limitation thereto), As another example, the structured X-ray attenuating material may form an anti-scatter grid.
In a method in accordance with embodiments of the present invention, the walls may be printed in a slanting pattern of varying slant angle across the area of the mask structure such that the structured X-ray attenuator forms a focused grating or grid adapted to a focused X-ray imaging geometry.
A method in accordance with embodiments of the present invention may comprise bending the mask structure and/or the structured X-ray attenuator, e.g. before or after said filling of the mask structure, such as to adapt the structured X-ray attenuator to a focused X-ray imaging geometry.
A method in accordance with embodiments of the present invention may comprise removing, e.g. by etching, the mask structure after said filling.
In a method in accordance with embodiments of the present invention, the mask structure may be printed on a substrate that is directly printed as part of the printing process or that is provided separately to be used as base platform for the printing process.
In a method in accordance with embodiments of the present invention, the printing may comprise printing the walls on (e.g. two subsets of the walls on respectively) both sides of said substrate.
In a method in accordance with embodiments of the present invention, said filling may comprise an electroplating process, and the substrate may comprise an electrically conductive surface, conductive lines and/or a conductive mesh onto which the walls are printed and which is used as an electrode in said electroplating process.
In a method in accordance with embodiments of the present invention, the or a material used to print said walls may comprise an electrical insulator, such that the X-ray attenuating material is deposited by the electroplating process as growing from the conductive surface, conductive lines and/or conductive mesh as a seed layer.
A method in accordance with embodiments of the present invention may comprise a treatment of the printed mask structure and/or a substrate onto which the mask structure is printed to improve the adhesion of the X-ray attenuating material.
In a method in accordance with embodiments of the present invention, the filling may comprise a mechanical filling process, in which the X-ray attenuating material is liquified or softened by temperature and/or pressure before pressing the mask structure into the X-ray attenuating material or pouring the liquified material into the mask structure as mold.
In a method in accordance with embodiments of the present invention, the filling may comprise pouring a suspension of microparticles into the mask structure and letting a solvent or carrier liquid of the suspension evaporate, such that the microparticles congeal and/or solidify to form the X-ray attenuating material.
In a method in accordance with embodiments of the present invention, the printing may comprise printing at least one mechanical connector forming a mechanical connection between adjacent walls to space the walls apart and/or to reinforce the walls during the filling process.
In a method in accordance with embodiments of the present invention, the at least one mechanical connector may comprise secondary supporting walls, beams and/or struts that form a connection between adjacent walls.
In a method in accordance with embodiments of the present invention, the secondary walls, beams and/or struts may be arranged such as not to form continuous aligned mechanical connections between more than two adjacent walls.
In a method in accordance with embodiments of the present invention, the struts may comprise elongate structures that interconnect adjacent walls and that are oriented at an angle in the range of 20° to 70° with respect to a floor surface onto which the walls are printed.
In a method in accordance with embodiments of the present invention, the mechanical connectors may be distributed randomly and/or without a recurring pattern over the area of the mask structure.
In a method in accordance with embodiments of the present invention, the printing may comprise printing said walls with gaps therein and/or passages therethrough, and filling said gaps and/or passages in said filling step to form structural connections between adjacent parts of the structured X-ray attenuator for structural integrity and stability.
In a method in accordance with embodiments of the present invention, a plurality of printing and filling steps may be repeated in an alternating sequence of printing and filling, such that in each iteration the structured X-ray absorber is built up to increasing heights.
In a second aspect, the present invention relates to a structured X-ray attenuator, e.g. for use as anti-scatter grid or interferometry grating in X-ray imaging, comprising a grid or grating formed from an X-ray attenuating material by filling trenches, holes and/or indentations in a mask structure printed using an additive manufacturing process.
The independent and dependent claims describe specific and preferred features of the invention. Features of the dependent claims can be combined with features of the independent claims and with features of other dependent claims as deemed appropriate, and not necessarily only as explicitly stated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a method in accordance with embodiments of the present invention.
Fig. 2 shows a printed mask structure comprising cross-wall structures spacing the primary walls apart and providing support, to illustrate embodiments of the present invention.
Fig. 3 shows a printed mask structure comprising slanted strut structures spacing the primary walls apart and providing support, to illustrate embodiments of the present invention.
Fig. 4 shows two examples of manufactured structured X-ray attenuators in accordance with embodiments of the present invention.
Fig. 5 illustrates a process step for filling a printed mask with an X-ray attenuating material, in accordance with embodiments of the present invention. Note that in another approach in accordance with embodiments, the printed mask can alternatively be filled with the attenuating material, e.g. when turned upside down.
Fig. 6 shows aspects of a method in accordance with embodiments of the present invention, in which wall structures are 3D printed on both sides of a substrate.
Fig. 7 shows aspects of a method in accordance with embodiments of the present invention, in which multiple printed masks are filled on top of each other in a stack.
Fig. 8 shows a photograph of an illustrative 3D-printed mask defining a grid structure, in accordance with embodiments of the present invention.
Fig. 9 shows a cross-section image of a printed mask structure, as used in embodiments of the present invention.
Fig. 10 shows a cross-sectional electron microscopy image of a printed mask structure, in accordance with embodiments of the present invention.

The drawings are schematic and not limiting. Elements in the drawings are not necessarily represented on scale. The present invention is not necessarily limited to the specific embodiments of the present invention as shown in the drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

Notwithstanding the exemplary embodiments described hereinbelow, is the present invention only limited by the attached claims. The attached claims are hereby explicitly incorporated in this detailed description, in which each claim, and each combination of claims as allowed for by the dependency structure defined by the claims, forms a separate embodiment of the present invention.
The word "comprise," as used in the claims, is not limited to the features, elements or steps as described thereafter, and does not exclude additional features, elements or steps. This therefore specifies the presence of the mentioned features without excluding a further presence or addition of one or more features.
In this detailed description, various specific details are presented. Embodiments of the present invention can be carried out without these specific details. Furthermore, well-known features, elements and/or steps are not necessarily described in detail for the sake of clarity and conciseness of the present disclosure.
In a first aspect, the present invention relates to a method of manufacturing a structured X-ray attenuator.
Referring to Fig. 1, an illustrative method 100 in accordance with embodiments of the present invention is shown.
The method comprises printing 101 a mask structure using an additive manufacturing process. Such additive manufacturing, or 3D printing, process advantageously provides a high degree of design freedom and can accurately achieve a high spatial manufacturing resolution.
The mask structure comprises walls 21 forming (e.g. defining; e.g. enclosing) trenches 25, holes and/or indentations (in and/or in between said walls), such that a grid or grating structure is formed. The walls may be linear, e.g. forming at least one set of parallel walls. Thus a set of parallel walls may form a grating, e.g. a (negative mask of an) interferometric grating, or two sets of parallel walls, at an angle (e.g. 90°) with respect to each other may form a grid, e.g. a (negative mask) of an anti-scatter grid. Further sets of walls are not necessarily excluded, e.g. to form a triangular mesh etc. However, the, or at least some of the, walls may also be curved or non-linear in accordance with embodiments of the present invention.
In a subsequent step, the trenches 25, holes and/or indentations are filled 102 with an X-ray attenuating material 27, e.g. to obtain a grating or grid structure that locally absorbs X-rays to create an interference pattern (e.g. when used in an X-ray phase-contrast and/or dark-field imaging system) or to reduce scatter.
The X-ray absorbing material may, for example, comprise or consist of a high Z material, e.g. a metal or metal alloy, e.g. such as gold, silver or bismuth. Other materials are not necessarily excluded, such as nanomaterials with high X-ray absorption, for example as a first material (e.g. as a carrier material) in which a second material, e.g. the X-ray absorbing nanomaterial, is provided, e.g. in a gel-like emulsion (without limitation thereto).
It is to be noted that the walls form a negative structure for the structured X-ray attenuator, which is created by filling in an X-ray attenuating material in the trenches, holes and/or indentations. Thus, the walls may form lamellae separating parts of the X-ray attenuating material into the intended structure, e.g. a grating or grid. These parts are however not necessarily entirely disconnected, e.g. may still be connected by e.g. a relatively thin layer of X-ray attenuating material and/or junctions, e.g. between intersecting lines in a grid. However, these parts may also be entirely disconnected (at least when only considering the X-ray attenuating material), e.g. as isolated lines forming a grating.
This also implies that, e.g. to obtain a grid, e.g. an anti-scatter grid, the walls may also comprise or consist of relatively short segments or studs, in between which two or more sets of (e.g. parallel) trenches are created to form a connected 2D raster when filled in with the attenuating material (without limitation thereto). Alternatively, e.g. to form an alternative anti-scatter grid, the walls may extend over substantial length, e.g. by two sets of extended walls (e.g. the sets at right angles with respect to each other), such that structured X-ray absorption is provided by filling in holes or indentations in between these walls, e.g. such that the scatter attenuation is provided by X-ray attenuating pillars or pillar-like structures.
The walls may be upright and substantially rectangular walls, e.g. at 90° with respect to a primary plane (i.e. a floor surface) in which the mask structure extends, e.g. the plane (or surface - not necessarily planar) of a substrate. The walls may also be tilted, e.g. to accommodate a focusing X-ray geometry, e.g. to follow the direction toward a focal point. Thus, the walls may have a parallelogram or tilted rectangular profile. However, embodiments are not limited thereto. For example, the walls may be tapered, e.g. have a triangular or trapezoid profile, without limitation thereto. It is an advantage of 3D printing that the shape of the walls can be designed with a high degree of freedom. For example, after printing (and optionally also after filling), the structure may be bent, and this bending may be taken into account in the printing process by optimizing the shape of the walls, e.g. using a tapered or otherwise shaped profile to optimize the pressure distribution in the bending process. As already mentioned, the walls may alternatively be printed directly in a shape that is optimized for a focusing geometry, such that bending can be avoided entirely.
The method may comprise a step of drying the printed structure, e.g. passively or actively (e.g. by applying heat and/or air or gas flow). The method may comprise a step of cleaning the printed structure, e.g. to remove dust, loose material and/or other contaminants. Thus, the X-ray attenuating material may be filled 102 in after the drying and/or cleaning.
The walls 21 may thus be used as a mask structure for filling in with the X-ray attenuating material. The walls (and/or connectors discussed below) may also be stripped 104 away after the filling 102 process, e.g. using an etchant, such that only the X-ray attenuating material remains, as structured by the walls (and possibly some connector or connectors, such as a base substrate, bridges, beams, etc.). If a step of bending 103 the structure is included, the walls may be removed 104 before, or after, the bending 103.
However, if the printed material does not interfere substantially with the imaging process, it is not necessarily removed, e.g. to maintain its mechanically supporting function. Embodiments in which the walls (and/or mechanical connectors) are only removed partially, e.g. partially etched away, are not necessarily excluded. Such partial removal may reduce the height of the wall structures without complete removal, or may use a mask to remove the walls in some areas and not in others, or to a different degree in different areas. Thus, some of the mechanical support function of the walls and/or connectors may be preserved while reducing the influence of these structures on the imaging process in use, e.g. by tuning a trade-off between mechanical stability and image quality and/or required imaging dose.
Thus, the method provides a hybrid approach that combines 3D printing as structuring technology, which can be relatively cheap, very versatile and easily available, with a filling step to provide the X-ray attenuating material in the printed structure. Thus, limitations in 3D printing of metals or other strongly X-ray attenuating materials can be avoided while still obtaining the advantages of high-resolution 3D printing.
The (e.g. all) walls may be provided at an angle of 90° with respect to a primary plane 26 (e.g. the mask structure may be substantially planar, extending in the primary plane, and the walls may be provided at right angles with respect to this plane). As is known in other manufacturing techniques, the structure may be bent 103 in a subsequent step to adjust the grating to a focused geometry of an imaging device for which it is intended, e.g. to provide a curvature to the primary plane. This bending may be performed before or after filling the trenches, holes and/or indentations with the X-ray attenuating material.
However, it is an advantage of 3D printing techniques that the walls may also be directly manufactured in a slanting pattern, e.g. such as to vary the angle of the walls over the area of the mask structure to provide such focused grating directly. Thus, bending of the structure can be avoided, and problems due to mechanical stresses are also prevented. For example, near the center of the structure (area), the walls may be at a substantially right angle with respect to the primary plane, and this angle may deviate to more acute angles toward the edges, preferably in a continuous (or at least the stepwise approximation thereof) and monotonous manner.
It is also an advantage that the structure may remain substantially planar, e.g. provided on a planar substrate, while tilted walls, as printed, can still accommodate the focused grating or grid adapted to a focusing imaging geometry. However, embodiments in which the entire structure is printed as a curved element are not necessarily excluded, e.g. by directly printing a curved substrate onto which the walls are created.
For example, the focused structure, as obtained by bending the construct (e.g. after printing and/or filling, and/or optionally before or after stripping the mask material away) or as directly printed, refers to the grating, grid or other structured X-ray absorber (and/or the printed mask structure that forms a negative mask for the absorber) being adapted to an X-ray beam that spreads out in use of the structure in a predetermined imaging geometry. For example, the structure may be adapted to be placed at a predetermined distance from an X-ray focal point, e.g. of an X-ray tube, and the walls as printed and/or the X-ray absorbed as filled in in the gaps between these walls may be oriented, e.g. tilted, in the direction toward this focal point. For example, the walls may be substantially upright with respect to a primary plane in a central region, and tilted at decreasing angles with respect to the plane as distance to this central region increases toward one or more edges of the primary plane, e.g. such as to take the fan shape of the beam into account.
The mask structure may be printed by one or more materials, e.g. a polymer material and/or a plastic material. It is an advantage of such polymer or plastic materials, as suitable for additive manufacturing techniques, that a high manufacturing resolution can be achieved. For example, the walls may have a thickness in the range of 0.1 µm to 50 µm, e.g. in the range of 0.5 µm 15 µm, e.g. in the range of 1 µm to 10 µm, e.g. in the range of 2 µm to 5 µm. The walls may be spaced apart from their adjacent neighboring wall such as to form a grating or grid pattern having a spatial period (pitch) in the range of 1 µm to 100 µm, e.g. in the range of 2 µm to 50 µm, e.g. 3 µm to 25 µm. A grating structure formed by the negative space created by the trenches, holes and/or indentations, e.g. when filled by the X-ray absorbing material, may form an absorption grating for use in X-ray interferometric imaging, e.g. such as the G0 and G2 gratings in a conventional phase-contrast and/or dark-imaging setup. The height of the grating, e.g. the X-ray absorbing filler material and hence also of the walls, may be, e.g. preferably, at least 200 µm (embodiments not limited thereto). For example, a height of at least 200 µm when using gold as X-ray absorbing material may be preferred to achieve a sufficient attenuation across (the entirety of) a typical X-ray spectrum used in such imaging systems. However, the required height may differ for other materials. Furthermore, the walls may be printed to a lesser height nonetheless. For example, as described further hereinbelow, the printing and filling steps may optionally be repeated to build up the wall structures in steps, e.g. such that the height of the walls printed in each step may be less than 200 µm. It is also noted that methods in accordance with embodiments of the present invention may also be used to construct different types of structured X-ray absorber, e.g. such as an anti-scatter grid, which may have different requirements in terms of spatial period and/or height of the constructed structures.
Examples of materials that may be used for the additive manufacturing process include acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), acrylonitrile styrene acrylate (ASA), polyethylene terephthalate (PET) and variants thereof (e.g. PETG, PETE, PETT), polycarbonate (PC), polypropylene (PP), nylon, polyaryletherketones (PAEK) and polyetherimides (PEI), and similar high-performance 3D printing materials (e.g. PEEK, PEKK, ULTEM, ...), without limitation thereto.
The choice of printing material or materials may be adapted to take the required spatial manufacturing resolution into account. For example, for manufacturing an X-ray grating, a high spatial resolution may need to be achieved, whereas, for example, an anti-scatter grating may not need to achieve the same high resolution. Another parameter that may be taken into account is the filling process that is applied. For example, when electroplating is used, the electrical conductivity (or lack thereof) of the material may be taken into account, as explained in more detail further hereinbelow. When a mechanical filling is applied, e.g. using high temperatures and/or pressures (e.g. imprinting the 3D printed template with a liquid or softened X-ray absorbing material), the printing material(s) may be adapted, e.g. selected, to have the mechanical properties and/or temperature requirements to withstand the imprinting process. For example, ULTEM and similar materials may be sufficiently strong for such conditions. Photopolymers may also be suitable, e.g. in view of the grating period that may need to be achieved for interferometric grating structures, but may impose tighter constraints on the temperature range which can be used for the filling process, e.g. may limit the temperature range for liquid metal alloys when these are used in the filling process.
The walls 21 may be formed on a substrate 23 in the printing process. The substrate may form (e.g. part of) at least one support structure, e.g. at least one mechanical connector; to space apart and support the walls. The substrate may be provided as basis to print the structure on, or may be integrally formed in the printing process, e.g. directly printed along with the rest of the printed mask structure. The substrate may be removed from the printed structure after the printing process (e.g. after the filling process step), e.g. by a mechanical removal and/or etching. The substrate is, however, not necessarily removed. In other words, the substrate may be temporary carrier, but may also remain present in the finished product to reduce the number of process steps and/or to provide or improve rigidity and/or structural integrity of the manufactured device in use thereof.
The substrate 23 may comprise an electrically conductive surface 24, e.g. the substrate may be formed from a conductive material or a conductive material may be coated on, attached to or printed on a bulk material (e.g. a thin plate). For example, the substrate may be a glass or other suitable (e.g. non-conductive) material slab, onto which a conductive material, e.g. a metal, is coated or provided, e.g. as a thin conductive (e.g. metal) foil.
Preferably, the substrate is substantially transparent to X-ray radiation. While the conductive surface may absorb or scatter some radiation, it is noted that a good electrical conductivity can be achieved in even a very thin layer, such that X-ray attenuation and/or scattering can be kept advantageously low and/or insignificant.
The electrically conductive surface material may also be printed on a (e.g. an otherwise non-conductive) substrate, e.g. forming the conductive layer directly in the printing process instead of relying on a separate coating or foil adhesion. For example, the entire structure may be printed on the substrate with a conductive material, e.g. including a base layer to form the conductive surface. Alternatively, multiple materials may be used in the printing process, e.g. including a conductive material and an insulating material, or different materials with different electrical conductivity.
The electrically conductive surface material does not necessarily need to cover the entire surface of the substrate. For example, a pattern of interconnected conductive lines or traces may be formed (e.g. printed or provided in a separate preparation of the substrate) on the substrate, e.g. forming a conductive grid, mesh or (interconnected) stripes.
The substrate (and/or the conductive surface thereof) may be treated, before the printing process, to increase the adhesion of the printing material to form the walls 21 on the substrate, e.g. by increasing the surface roughness, porosity or other properties.
The method may also comprise applying a coating or other surface treatment to the entire printed mask structure (and/or the substrate) to improve adhesion of the X-ray attenuating material used in the filling process. Likewise, the 3D printing process may be adapted to increase surface roughness of the walls (and/or other elements of the printed structure), e.g. by purposefully including small dimples or other small-scale surface features (e.g. texturing the printed walls).
Additionally or alternatively, the material used for printing, or one of the materials used for printing may be an electrically conductive material. For example, additives may be mixed into a (e.g. common) printing material to improve the electrical conductivity, or a material may be selected that already has a sufficient conductivity. For example, additives may be used to increase the conductivity while substantially maintaining the compatibility with the printing process, e.g. without requiring substantial adjustments of printing parameters and/or the printing process.
The conductive surface 24 and/or the conductive material used for printing e.g. the walls and/or supporting mechanical connectors may advantageously form a seed layer or structure that can be used in the step of filling the trenches or other cavities with an X-ray attenuating material.
The method comprises filling 102 the trenches 25, holes and/or indentations with the X-ray attenuating material, in which this filling 102 may comprise electroplating. The X-ray absorbing material may, for example, comprise or consist of a high Z material, e.g. a metal or metal alloy, e.g. such as gold, silver or bismuth. Other materials are not necessarily excluded, such as nanomaterials with high X-ray absorption. In the electroplating, the conductive surface and/or printed conductive structures may be connected to form one electrode for the electroplating process.
In the printing step 101, the walls 21 may be constructed from an electrically insulating material. This may improve the homogeneity of growing the X-ray attenuating material in the trenches or cavities, starting from the conductive seed surface 24 on the substrate, e.g. reducing the risk of forming cavities in the deposited attenuating material. Thus, an electroplating process can be used that starts from the conductive surface and builds up the attenuator inside the printed trenches and/or cavities. Depending on the dimensions and/or other parameters, alternatively, it may however be advantageous to create electrically conductive walls to further aid the plating process, but this may need to be balanced against a risk of creating flaws, e.g. voids, in the filling process. The same applies to the connectors 22 between the walls, which may be conductive or not. For example, in one illustrative embodiment, a surface (on top of which the walls are constructed) of the substrate is electrically conductive, and the walls and connectors are electrically insulating. In another illustrative embodiment, the conductive surface of the substrate is combined with electrically conductive connectors, e.g. struts, to provide additional seed area, while the walls are electrically insulating. Since the connectors may be relatively thin and take up little volume, the risk of creating voids in the filling process may be reduced, while still improving the speed of deposition in an electroplating process. It a third illustrative embodiment, the conductive surface, walls and connectors are electrically conductive. However, this does not mean that all of these have necessarily a same electrical conductivity, since properties and/or performance of the filling process may be tuned by e.g. using walls (and/or connectors) which have a conductivity that is substantially different from zero but still substantially different from (e.g. lower than) the conductivity of the substrate surface.
The filling 102 the trenches 25, holes and/or indentations with the X-ray attenuating material may however also be performed with a different method than electroplating. For example, the filling 102 may comprise a mechanical filling process, in which pressure and/or temperature are used to soften the X-ray attenuating material and press it into the trenches, holes and/or indentations of the mask structure. In other words, the printed structure may be used as a mold or stencil, into which the X-ray attenuating material is deposited under elevated pressure and/or temperature. The elevated temperature may be adapted to soften the X-ray attenuating material, without necessarily liquifying the material, and the elevated pressure may be used to conform the shape of the softened material to the negative formed by the printed mask structure. It will be noted that if the X-ray attenuating material is sufficiently soft at room temperature, an elevated temperature may not be required, even though this may be less practical in view of stability of the device after manufacture. It is also noted that the material is not necessarily provided in a single blob, e.g. may be provided in smaller portions, such that these smaller portions merge and/or congeal together under temperature and/or pressure in the desired shape as defined by the printed mask.
An optional step of wetting the printed mask structure before the filling with the X-ray attenuating material may be used to improve the efficiency and/or quality of the filling process.
The filling may also be performed by liquifying the X-ray attenuating material, e.g. a suitable metal or metal alloy, and this liquid may then be imprinted by the mask structure, e.g. using the printed mask structure as a stencil, or cast into the printed mask structure, e.g. using the mask structure as a mold.
For example, Fig. 5 illustrates a process step in which the X-ray attenuating material is liquified 110, or at least softened, e.g. by raising its temperature, to provide the X-ray attenuating material as a liquid 29 or softened blob or slab. In this example, the printed mask structure (e.g. optionally after being wetted) is used as a stencil or mask, and pressed 111 into the liquid 29 (or softened blob/slab), such that the cavities (trenches, holes, indentations, ...) are filled with the X-ray attenuating material.
After congealing, excess X-ray attenuating material may be removed 112, e.g. by a mechanical process such as cutting, planning, grinding and/or polishing, for example at the height of the walls 21 (or less, e.g. the walls may be originally printed to be higher than the intended height of the structured attenuator, e.g. such that a uniform height can be accurately controlled by the mechanical removal 112 step). However, this mechanical removal may be optional, or the height may alternatively be cut off such that some planar material remains over the height of the walls, e.g. such that the excess material provides some structural support.
As a further example, the 3D printed mask structure may be filled 102 by a suspension of microparticles, e.g. that go directly into the trenches, holes and/or indentations. Thus, the X-ray attenuating material may is provided after solidifying, e.g. by allowing a solvent or carrier liquid of the suspension to evaporate (e.g. passively or assisted by drying techniques, e.g. increased air flow, elevated heat, lowered ambient pressure and/or other methods to improve evaporation.
The mask structure comprises walls 21 defining trenches 25, holes and/or indentations, such that a grid or grating structure is formed (i.e. the actual grating or grating is formed by the X-ray attenuating material after filling in these trenches and/or cavities; in other words, the walls form a negative mask of the grid or grating). The walls may for example be build up, by the printing process, to e.g. a height in the range of 150 µm to 1000 µm, e.g. typically in the range of 200 µm to 500 µm, e.g. about 400 µm or more, e.g. for constructing an interferometric grating. These ranges are however not limitative. For example, an anti-scatter grid may comprise walls that are higher.
Referring to Fig. 2, the printed mask structure 20 comprises at least one connector 22 for spacing the walls 21 apart, e.g. at predetermined distance or distances with respect to each other. In a simple embodiment, this connector may comprise or consist of (e.g. merely) one or more plates, e.g. the substrate 23 and/or other plate- or platform-like structure(s), onto which the walls are formed. Thus, the walls extend from such plate at an angle or angles that is/are orthogonal or relatively close to orthogonal to the plate, e.g. in the range of 60° to 90°, e.g. 70° to 90°, e.g. 80° to 90°, e.g. 85° to 90°. The walls may be formed on top of the plate, or on both sides of the plate, or a plurality of plates may be used in between which the walls are provided (walls possibly also extending from one or both sides at the top and/or bottom).
However, the at least one connector 22 may also comprise (in addition to one or more of such plates or as alternative) secondary walls connecting the primary set of walls (the walls defining the grating or grid pattern), e.g. as shown in Fig. 2. Thus, the printed mask structure may extend in a plane, or substantially in a plane (e.g. may be curved), in which the primary and secondary walls are arranged perpendicular (or at least at an angle of e.g. 60°) with respect to this plane or curved plane.
The secondary walls may form a second grating pattern in a different direction, e.g. forming a grid-like structure in combination with the primary walls. However, the secondary walls do not necessarily form a functional grating (in terms of X-ray optics), unless this is so desired. The secondary walls may also be arranged in a bricklaying or honeycomb pattern, e.g. such that the secondary walls are not continuously aligned between adjacent pairs of the primary walls, e.g. as shown in Fig. 2.
The secondary, supporting, walls, may have the same, or substantially the same, height as the primary walls forming the grating pattern (e.g. as shown in Fig. 2). However, embodiments are not limited thereto. For example, the secondary walls may have a height that is less than the height of the primary walls, e.g. a height in the range of 10% to 90% of the height of the primary walls, e.g. in the range of 15% to 75%, e.g. in the range of 20% to 50%. This allows the entire hole, trench or other cavity formed between each pair of adjacent primary walls to be (to a large or larger extent) filled in with an X-ray absorbing material, e.g. over and/or under the secondary walls. The secondary walls may be centered in the space between the bottom and the top of the primary walls, such that the absorber may be filled in both above and below the secondary wall. Such free-standing bridges may for example be printed as an overhanging structure, e.g. a bow. By centering the secondary walls (e.g. bridges), such that the difference in height with respect to the primary walls is substantially evenly divided at the top and bottom, a more symmetric distribution of the X-ray absorber can be achieved. However, these elements are not necessarily centered in the height dimension (e.g. simulations may be performed to determine an optimal height to create such supporting structures), and/or such bridging connectors may be provided between the same pair of adjacent walls at different heights, possibly even on top of each other, e.g. forming two or more bridge decks above each other.
Referring to Fig. 3, the at least one connector may also comprise struts connecting the primary walls that define the grating or grid pattern. These struts can be combined with the secondary walls and/or base plate(s) (e.g. substrate) mentioned hereinabove or can be used as alternative to connect and support the walls in a stable and precisely spaced apart configuration. The use of (e.g. elongate) struts may leave more space free to be filled in with an X-ray absorbing material between the walls, thus improving the fill rate and, consequently, improving the image quality in use. It is an advantage of 3D printing techniques that a high degree in design freedom is provided, e.g. that such relative thin and/or elongate struts can be easily formed, and even at sharp angles with respect to the walls.
The struts may be provided at substantially perpendicular angles to the walls (e.g. slender horizontal beams or bridge structures as discussed hereinabove), but may also be slanted between the walls, e.g. forming diagonal struts. The direction of these struts may vary, e.g. alternate between slanting from bottom-left to upper-right and from bottom-right to upper-left, for example forming spaced apart legs in a cross formation. Other variations of the strut orientations within a single trench (e.g. other than alternating crosswires) are not excluded.
Thus, a supporting framework between the walls can be provided by these struts, e.g. forming a sunray-like pattern of struts. It is an advantage that the struts may occupy little space in between the walls, such that a good fill rate of attenuating material between the walls can be achieved. Furthermore, by slanting the struts, the influence on the attenuation profile of the attenuating filler, when view in projection direction (e.g. in a projection direction orthogonal to the primary plane in which the attenuator device extends), may be kept small.
The secondary support walls (note that these "walls" may also consist of or comprise bough, beam or bowtie-like structures for the purpose of the present disclosure) and/or struts have the advantage that the construct is (e.g. mechanically) stabilized, e.g. to counter or reduce instability due to strong adhesion forces in the filling process (and/or thereafter) and/or deformation due to physical manipulation during or after the manufacturing process. Furthermore, if the primary wall material (e.g. a resist material or a similar conveniently removable material, e.g. an etchable material) is removed before use of the construct, the secondary walls and/or other connectors may provide a substantial contribution to the mechanical support and/or rigidity of the grating structure. For example, a different material may be used for these support structures, such that the primary walls may be selectively removed, e.g. etched away while the support structures remain. It is also noted that when the grating is in use exposed to a strong flux of X-ray radiation (and/or other unfavorable conditions, such as heat), which may for example be the case for a source grating that is positioned relatively close to the X-ray source, the printed material (or some of the materials which may be used in the printing process) may be become unstable under the load of radiation, heat and/or other conditions. In other words, even if a primary material that is printed to be used as mask for the grating (or similar structured absorber) is not removed, e.g. by etching, it may still be desirable to include such additional support structures to improve the stability.
The connectors, e.g. connective (secondary) walls, bridges and/or struts, may be distributed in a uniform pattern over the area of the printed mask structure, but may also be distributed ununiformly (e.g. unevenly or without recurring positional pattern), e.g. in accordance with a random distribution. Distributing the connectors in a random pattern (during design) may avoid or reduce potential Moire pattern artefacts. It is also noted that, during design, mechanical simulations can be performed to evaluate the structural integrity of the design even in view of a random distribution of the connectors. Thus, a design may be selected from a plurality of candidates with random distribution of the connectors that provides good mechanical properties, or a design may be optimized by shifting and/or adding additional connectors in regions where the structural integrity is determined to be insufficient by simulations.
It is also an advantage of the slanted struts, and/or of providing secondary walls or bridges at an acute (and/or obtuse) angle with respect to the primary walls and/or the substrate, that deposition of the X-ray attenuating material may be improved by supporting the growth and/or accumulation along these structures. For example, the struts may be oriented at an angle in the range of 20° to 70° (and/or 110° to 160°) with respect to a floor surface onto which the walls 21 are printed. Other illustrative ranges include 30° to 60°, 20° to 45°, 45° to 60°, and their obtuse complements. In use of an electroplating filling process, this advantage may even be strengthened by providing the struts or similar connective structures as electrically conductive elements (without limitation thereto). A surface treatment of these structures (not necessarily selectively limited to only the struts or other supporting structures) to improve adhesion of the attenuating filler material may also contribute to a good and efficient deposition of the X-ray attenuating material.
Furthermore, the printed mask structure may define support structures in its negative form (mask), e.g. by openings or holes through the (primary) walls. Thus, these further openings may form passages between adjacent trenches or cavities forming the mask for the grating (or similar structure). In filling the construct, the X-ray absorbing material may extend through such connecting passages to form connections between the grating lines (or similar structures) for (e.g. improved) mechanical stability and/or integrity. This may advantageously allow the printed material (or at least some of the printed material) to be removed after filling, e.g. by etching, while still providing a good mechanical stability. The use of different filling materials is also possible. For example, the printed mask structure may be filled with a layer of X-ray attenuating material, and then, at the height position where such mask openings are provided, filled with a different material that provides mechanical stability (while for example attenuating X-ray radiation substantially). The order and/or number of such layers of X-ray attenuating material and material for improved stability may vary. As an example, the top of the construct may comprise gaps in the walls, such that in a top layer the filling process can provide strong mechanical connections between adjacent grating lines (e.g. forming a grid that is relatively thin compared to the height of the grating lines as such); whereas in the bulk of the grating lines, the walls do not show such gaps. The bottom of the construct may be similarly reinforced, and/or may obtain stability from the substrate (e.g. that is not intended to be removed in the manufacturing process) and/or the conductive layer that may form a seed layer for an electroplating filling process (without limitation thereto - different filling techniques may equally be applied). Furthermore, more such reinforcing layers may be added in between the top and bottom to reinforce the structure at different heights. This approach is also not limited to gaps or openings in the walls at specific heights, e.g. passages may be formed through the walls at an angle with respect to the primary plane. Thus, a negative of the struts as discussed hereinabove may be formed (e.g. forming a sunray-like pattern of passages through the mask) to be filled with the X-ray attenuator during the filling process (or a different material, even though this might be less practical).
The method may comprise bending 103 the structured X-ray attenuator, e.g. to obtain a focused grating. For example, the (e.g. substantially planar) printed mask (e.g. including a planar substrate), e.g. in which the walls may be constructed to be perpendicular to a primary plane, may be bend to obtain a predetermined curvature, either before or after filling the trenches, holes and/or indentations printed therein with the X-ray attenuating material. However, the walls can also be printed directly at an angle that varies across the area of the attenuator, such that bending can be avoided while still obtaining the advantages of a focused grating. For example, the additive manufacturing process may be performed in steps of layer-wise material deposition (e.g. additive build-up of the printed structure in layers), such that the angle of the walls can be easily provided by small shifts in the position of the walls in each following layer.
Fig. 4 shows two examples of structures (e.g. grid or grating-like structures that can be adapted to / suitable for focused X-ray geometries) to illustrate different types of supporting bridge elements, as can be obtained by 3D printing technologies in a method in accordance with embodiments of the present invention. In a method to obtain the structures of these examples, a printed mask structure can be filled in with X-ray attenuating material, e.g. by electroplating. The printed mask material can then be stripped away, and the remaining structure can be bent in order to focus on the focal spot of an X-ray tube in use in an imaging system. It is noted that this step of bending can be avoided by directly printing the walls of the printed 3D mask structure in a slanting pattern. The example on the left corresponds to the example shown in Fig. 2, e.g. in which connective cross-walls for support are provided in a pattern similar to a bricklaying stretcher bond pattern (not excluding other pattern structures, e.g. 1/3 displacement between rows, etc.). The example on the right in Fig. 4 shows a curved (e.g. after bending, or directly constructed as such by the printing process) regular grid pattern. In this example, reinforcements are constructed by the X-ray attenuating material (or alternatively a separate material, e.g. metal, applied to the top layer in the filling process). This can be achieved by creating small gaps in the walls in the printing process that are (in this example) only present in the top layer, such that cross-connections are created in the filling process between adjacent lamellae of the constructed grid in this top layer. As mentioned, this approach is not limited to only the top layer, and can, for example, be repeated at several intermediate heights to further reinforce the structure. This approach may offer the advantage of allowing the mask material to be etched away, or otherwise removed, after filling, while still providing a strong and stable grating (or similar structure). Note however that the mask material can alternatively be allowed to remain in the finished product, e.g. if not interfering substantially with the imaging process. Thanks to the freedom in printing the mask, perforations in the mask walls can also be provided at an angle, such that, when filled, slanted connections between the lamellae are obtained. This may for example result in a structure that is similar to the example shown in Fig. 3, but with the stabilizing struts embodied in the X-ray attenuating material instead of the mask structure. It is also noted that struts or other support structures that are printed as material in the mask may e.g. offer support to the mask while being filled, while struts or other support structures formed by filling gaps or openings in, e.g. passages through, the mask walls may offer support to the finished product, e.g. in use in an imaging process. Therefore, both features may be advantageously combined in an embodiment in accordance with embodiments of the present invention.
The mask structure may also be created by printing the walls (and/or other supporting structures) on both sides of a substrate. For example, Fig. 6 illustrates a mask structure comprising a substrate 23, on which on both sides walls 21 are printed to form trenches between the walls. In accordance with embodiments of the present invention, the trenches (or other types of cavities) on both sides can be filled with the X-ray attenuating material to form a structured X-ray absorber, e.g. an X-ray interferometry grating. The walls may be optionally removed, e.g. by etching. This can be combined with other features as discussed herein, e.g. connector structures for improved mechanical stability during the filling and/or in the finished device. The substrate may be provided as a separate element (e.g. a planar thin substrate, or a pre-bent or curved thin substrate) onto which the walls are imprinted (e.g. in two steps, on either side), or may be directly printed as a central structural support layer in a single printing step (note that central does not necessarily imply exactly in, or even close to, the center, even though this may be the case). Likewise, the printed structure may be bent to form a focused structured X-ray absorber, e.g. a focused grating. This bending may be performed after the printing (e.g. before filling) or after the filling step. The mask structure may also be printed directly such as to create a focused structure, e.g. by slanting the walls and/or printing the substrate with curvature.
Multiple steps of printing 101 and filling 102 may be repeated, such that in each repetition the structured X-ray absorber is built up to increasing heights. Thus, higher grating or grid structures can be achieved. For example, after an initial step of printing 101 a mask structure and filling 102 this printed structure, a further mask structure may be printed on top of the filled mask, which is subsequently filled with the attenuator material. This further mask structure may equally comprise walls 21 forming trenches 25, holes and/or indentations, and optionally connectors (e.g. struts). In other words, features of the printed mask structure discussed hereinabove may equally apply to the further printed structure or structures provided in further iterations. This is schematically illustrated in Fig. 7. It is noted that a new substrate 23 may be added between layers to form a new (e.g. thin) basis for the next printing step, or the result of the previous filling step may be used as basis to print further structures on. As already mentioned, the initial (bottom) substrate may also be optional or only provided as a sacrificial platform (e.g. removed in a later step).
It is also noted that the further printed structures (printed mask structures of further printing steps) are not necessarily exactly identical the previously printed structure in a previous step. For example, a focusing pattern of slanting (or otherwise shaped) walls such as to point to a focal spot can be approximated by slight shifts of the wall positions (or other parameters) in each next iteration. In other words, while simple upright walls may printed in each step, a stepwise shift in each printing step may approximate the effect of slanted (or otherwise shaped, e.g. trapezoid, parallelogram or triangular) walls in a discretized/quantized manner. Obviously, a combination of non-rectangular wall profiles in each printing step and variations in the printed design between subsequent printing steps may also be used. For example, the printing technology and/or the filling process may perform less than optimal when sharper edges are printed with respect to the primary plane (e.g. of a bottom substrate or, e.g., generally the horizontal plane in the printing process), while the effect of sharper edges can still be obtained by introducing suitable shifts between printing steps, e.g. starting from a sturdy base obtained after a previous filling step.
The filling process, regardless of whether multiple iterations of printing and filling are applied, may be performed in-situ, e.g. such that the printed object does not need to be repositioned between the printing and filling step, nor (in such case) between iterations of a repetitive printing/filling process. This may advantageously avoid misalignment problems or difficulties in precisely aligning the object during different manufacturing steps, e.g. particularly when multiple printing steps are applied. In a second aspect, the present invention relates to a structured X-ray attenuator comprising a grid or grating formed from an X-ray attenuating material 27 by filling trenches 25, holes and/or indentations in a mask structure printed using an additive manufacturing process.
The structured X-ray attenuator may comprise the mask structure, or the mask structure may be absent or partially removed, e.g. by an etching process.
The structured X-ray attenuator may be adapted for creating an X-ray interference pattern and/or to reduce scatter in an X-ray imaging apparatus.
The structured X-ray attenuator may be obtained or obtainable by a method in accordance with embodiments of the present invention as discussed hereinabove. Other features, or details of the features described hereinabove, of a structured X-ray attenuator in accordance with embodiments of the present invention shall be clear in view of the description provided hereinabove relating to a method in accordance with embodiments of the present invention.
A structured X-ray attenuator in accordance with embodiments of the present invention may be characterized by globular inclusions in the mask structure (if remaining present in the device). In other words, the mask structure may have a volumetric texture of isolated, abutting (and/or intersecting, e.g. merged) spheres, globular elements (possibly elongated or imperfectly formed) and/or droplet shapes. For example, these globular inclusions may be indicative of a 3D printing process to deposit the mask structure in typical 3D printing techniques.
Fig. 8 shows a photograph of (part of) an illustrative 3D printed grid-like structure for use in manufacturing an X-ray interferometry grating, in accordance with embodiments of the present invention.
Fig. 9 shows a cross section image of an illustrative example of the printed mask structure, in which walls 21 form a comb-like structure, supported by a substrate 25. Note that, at this stage, the printed mask structure has not yet been filled. Imperfections of the printing process can clearly be detected, e.g. even at the indicated scale.
Fig. 10 shows a high-resolution electron microscopy image, in which the connected ball geometry produced by 3D printing can be clearly observed. It is to be noted that this image is only illustrative and included to demonstrate the globular imperfections typically implied by 3D printing processes. For example, by further optimizing the 3D printing process and/or materials, more continuous structures may be obtained than shown in the example. For example, finishing operations and/or post-processing steps, such as an etching step, may be used to flatten the (connected or at least partially merged) ball features, e.g. to obtain a more smooth and/or flat surface (e.g. interior surfaces inside the grating lines). However, in view of the state of the art, it is unlikely that the presence of such globular artefacts can be avoided entirely.
It is noted that the presence of such globular imperfections in the mask may also be observable in the X-ray absorbing material, e.g. can be detectable even if the mask is removed. For example, (interior surfaces of) the X-ray absorbing material may be characterized by indentations formed by droplets of the printed mask material. In other words, the X-ray attenuating material 27 of the structured X-ray attenuator may comprise lamellae, in which the surface of these lamellae (particularly on the sides toward adjacent lamellae) are not flat or not uniformly flat, e.g. textured by indentations and/or protrusions on a scale that is substantially smaller than the scale of the structure, e.g. less than 5% of its height, e.g. less than 1% of its height, e.g. less than 0.1% of its height.

Claims

A method (100) of manufacturing a structured X-ray attenuator, the method comprising:
- printing (101) a mask structure (20) using an additive manufacturing process, wherein said mask structure comprises walls (21), wherein said walls define trenches (25) and/or holes and/or indentations in the mask structure, such that a grid or grating structure is formed, and

- filling (102) the trenches (25), holes and/or indentations with an X-ray attenuating material (27) to obtain a structured X-ray attenuator that locally absorbs X-rays.
The method of claim 1, wherein the X-ray attenuating material (27) as structured by filling (102) the mask structure is adapted for use in X-ray interferometric imaging to create and/or analyze an X-ray interference pattern and/or is adapted to reduce scatter in an X-ray imaging apparatus.
The method of claim 1 or 2, wherein said walls (21) are printed (101) in a slanting pattern of varying slant angle across the area of the mask structure such that the structured X-ray attenuator forms a focused grating or grid adapted to a focused X-ray imaging geometry.
The method of any of the previous claims, comprising bending (103) the mask structure and/or the structured X-ray attenuator, before or after said filling (102), such as to adapt the structured X-ray attenuator to a focused X-ray imaging geometry.
The method of any of the previous claims, wherein said mask structure is printed (101) on a substrate (23) that is directly printed as part of the printing process or that is provided separately to be used as base platform for the printing process.
The method of claim 5, wherein said printing (101) comprises printing said walls on both sides of said substrate (23).
The method of any of the previous claims, wherein said filling (102) comprises an electroplating process, and wherein said substrate (23) comprises an electrically conductive surface (24), conductive lines and/or a conductive mesh onto which said walls (21) are printed (101) and which is used as an electrode in said electroplating process.
The method of claim 7, wherein a material used to print (101) said walls (21) comprises an electrical insulator, such that the X-ray attenuating material is deposited by the electroplating process as growing from the conductive surface (24), conductive lines and/or conductive mesh as a seed layer.
The method of any of the previous claims, wherein said filling (102) comprises a mechanical filling process,
in which the X-ray attenuating material is liquified (110) or softened by temperature and/or pressure before pressing (111) the mask structure into the X-ray attenuating material or pouring the liquified material into the mask structure as mold, or
in which said filling (102) comprises pouring a suspension of microparticles into the mask structure and letting a solvent or carrier liquid of the suspension evaporate, such that the microparticles congeal and/or solidify to form the X-ray attenuating material.
The method of any of the previous claims, wherein said printing (101) comprises printing at least one mechanical connector (22) forming a mechanical connection between adjacent walls to space the walls apart and/or to reinforce the walls during the filling (102) process.
The method of claim 10, wherein said at least one mechanical connector (22) comprises struts interconnecting adjacent walls, wherein said struts are elongated structures that are oriented at an angle in the range of 20° to 70° with respect to a floor surface onto which the walls (21) are printed.
The method of claim 10 or 11, wherein said mechanical connectors (22) are distributed randomly and/or without a recurring pattern over the area of the mask structure.
The method of any of the previous claims, wherein said printing (101) comprises printing said walls with gaps therein and/or passages therethrough, and filling said gaps and/or passages in said filling (102) to form structural connections between adjacent parts of the structured X-ray attenuator for structural integrity and stability.
The method of any of the previous claims, wherein a plurality of printing (101) and filling (102) steps are repeated, such that in each iteration the structured X-ray absorber is built up to increasing heights.
A structured X-ray attenuator, comprising a grid or grating formed from an X-ray attenuating material (27) by filling trenches (25), holes and/or indentations in a mask structure printed using an additive manufacturing process.